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Linköping Studies in Science and Technology
Dissertation No. 1378
Luminescence Properties of ZnO Nanostructures and Their Implementation as
White Light Emitting Diodes (LEDs)
Naveed ul Hassan Alvi
Physical Electronics and Nanotechnology Division
Department of Science and Technology (ITN)
Campus Norrköping, Linköping University
SE-60174 Norrköping Sweden
Linköping 2011
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Copyright © 2011 by Naveed ul Hassan Alvi
ISBN: 978-91-7393-139-7
ISSN 0345-7524
Printed by LiU-Tryck, Linköping University,
Linköping, Sweden
June, 2011
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Luminescence Properties of ZnO Nanostructures and Their Implementation as White Light Emitting Diodes (LEDs)
Naveed ul Hassan Alvi
Department of Science and Technology, Linköping University Sweden, 2011
Abstract:
In the past decade the global research interest in wide band gap semiconductors has been focused on zinc oxide (ZnO) due to its excellent and unique properties as a semiconductor material. The high electron mobility, high thermal conductivity, good transparency, wide and direct band gap (3.37 eV), large exciton binding energy (60 meV) at room temperature and easiness of growing it in the nanostructure form, has made it suitable for wide range of applications in optoelectronics, piezoelectric devices, transparent and spin electronics, lasing and chemical sensing.
In this thesis, luminescence properties of ZnO nanostructures (nanorods, nanotubes, nanowalls and nanoflowers) are investigated by different approaches for possible future application of these nanostructures as white light emitting diodes. ZnO nanostructures were grown by different growth techniques on different p-type substrates. Still it is a challenge for the researchers to produce a stable and reproducible high quality p-type ZnO and this seriously hinders the progress of ZnO homojunction LEDs. Therefore the excellent properties of ZnO can be utilized by constructing heterojunction with other p-type materials.
The first part of the thesis includes paper I-IV. In this part, the luminescence properties of ZnO nanorods grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) grown on the same substrate were investigated. The effect of the post-growth annealing of ZnO nanorods and nanotubes on the deep level emissions and color rendering properties were also investigated.
In paper I, ZnO nanorods were grown on p-type GaN and 4H-SiC substrates by low temperature aqueous chemical growth (ACG) method. The luminescence properties of the fabricated LEDs were investigated at room temperature by electroluminescence (EL) and photoluminescence (PL) measurements and consistency was found between both the measurements. The LEDs showed very bright emission that was a combination of three emission peaks in the violet-blue, green and orange-red regions in the visible spectrum.
In paper II, different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) were grown on p-GaN and the luminescence properties of these nanostructures based LEDs were comparatively investigated by EL and PL measurements. The nanowalls structures were found to be emitting the highest emission in the visible region, while the nanorods have the highest emissions in the UV region due to its good crystal quality. It was also estimated that the ZnO nanowalls structures have strong white light with the highest color rendering index (CRI) of 95 with correlated color temperature (CCT) of 6518 K.
In paper III, we have investigated the origin of the red emissions in ZnO by using post-growth annealing. The ZnO nanotubes were achieved on p-GaN and then annealed in different ambients (argon, air, oxygen and nitrogen) at 600 oC for 30 min. By comparative investigations of EL spectra of the LEDs it was found that more than one deep level defects
VI
are involved in the red emission from ZnO nanotubes/p-GaN LEDs. It was concluded that the red emission in ZnO can be attributed to oxygen interstitials (Oi) and oxygen vacancies (Vo) in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) and 690 nm (1.79 eV) to 750 nm (1.65 eV), respectively.
In paper IV, we have investigated the effect of post-growth annealing on the color rendering properties of ZnO nanorods based LEDs. ZnO nanorods were grown on p-GaN by using ACG method. The as grown nanorods were annealed in nitrogen, oxygen, argon, and air ambients at 600 oC for 30 min. The color rendering indices (CRIs) and correlated color temperatures (CCTs) were estimated from the spectra emitted by the LEDs. It was found that the annealing ambients especially air, oxygen, and nitrogen were found to be very effective. The LEDs based on nanorods annealed in nitrogen ambient, have excellent color rendering properties with CRIs and CCTs of 97 and 2363 K in the forward bias and 98 and 3157 K in the reverse bias.
In the 2nd part of the thesis, the junction temperature of n-ZnO nanorods based LEDs at the built-in potential was modeled and experiments were performed to validate the model. The LEDs were fabricated by ZnO nanorods grown on different p-type substrates (4H-SiC, GaN, and Si) by the ACG method. The model and experimental values of the temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared. It was found that the series resistance has the main contribution in the junction temperature of the fabricated devices.
In the 3rd part of the thesis, the influence of helium (He+) ion irradiation bombardment on luminescence properties of ZnO nanorods based LEDs were investigated. ZnO nanorods were grown by the vapor-liquid-solid (VLS) growth method. The fabricated LEDs were irradiated by using 2 MeV He+ ions with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2. It was observed that the He+ ions irradiation affects the near band edge emissions as well as the deep level emissions in ZnO. A blue shift about 0.0347 eV and 0.082 eV was observed in the PL spectra in the near band emission and green emission, respectively. EL measurements also showed a blue shift of 0.125 eV in the broad green emission after irradiation. He+ ion irradiation affects the color rendering properties and decreases the color rendering indices from 92 to 89.
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List of publications included in the thesis
Paper I Fabrication and characterization of high brightness light emitting diodes based on n-ZnO nanorods grown by low temperature chemical method on p-4H-SiC and p-GaN N. H. Alvi, M. Riaz, G. Tzamalis, O. Nur, and M. Willander Semiconductor Science and Technology 25, 065004 (2010) Paper II Fabrication and comparative optical characterization of n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN Light emitting diodes N. H. Alvi, Syed M. Usman Ali, S. Hussain, O. Nur, and M. Willander Scripta Materiala 64, 697 (2011) Paper III The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes N. H. Alvi, K. ul Hasan, O. Nur, and M. Willander Nanoscale Research Letters 6, 130 (2011) Paper IV The effect of the post-growth annealing on the color rendering properties of n-ZnO nanorods /p-GaN light emitting diodes N. H. Alvi, M. Willander, and O. Nur (Accepted in Lighting Research and Technology) DOI: 10.1177/1477153511398025 Paper V Junction temperature in n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si) heterojunction light emitting diode N. H. Alvi, M. Riaz, G. Tzamalis, O. Nur, and M. Willander Solid State Electronics 54, 536 (2010) Paper VI
Influence of helium-ion bombardment on optical properties of ZnO nanorods/p-GaN light emitting diodes
N. H. Alvi, S. Hussain, J. Jensen, O. Nur, and M. Willander (Submitted to Nanoscale Research Letters)
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List of publications not included in the thesis Paper I Buckling and elastic stability of vertical ZnO nanotubes and nanorods M. Riaz, A. Fulati, G. Amin, N. H. Alvi, O. Nur, and M. Willander J. Appl. Phys. 106, 034309(2009) Paper II A potentiometric intracellular glucose biosensor based on the immobilization of glucose oxidize on the ZnO nanoflakes A. Fulati, Syed M. Usman Ali, M. H. Asif, N. H. Alvi, M. Willander, Cecilia Brännmark, Peter Strålfors, Sara I. Börjesson, and Fredrik Elinder Sensor and Actuators B 150, 673 (2010) Paper III The effect of the post-growth annealing on the electroluminescence properties of n-ZnO nanorods/p-GaN light emitting diodes N. H. Alvi, M. Willander, and O. Nur Superlattices and Microstructures 47, 754 (2010) Paper IV The impact of ion irradiation on piezoelectric power generation from ZnO nanorods array N. H. Alvi, S. Hussain, O. Nur, and M. Willander (manuscript) Paper V A comparative study of the electrodeposition and the aqueous chemical growth techniques for the utilization of ZnO nanorods on p-GaN for white light emitting diodes S. Kishwar, K. ul Hasan, N. H. Alvi, P. Klason, O. Nur, and M. Willander Superlattices and Microstructures 49, 32 (2011) Paper VI Selective potentiometric determination of Uric Acid with functionalized Uricase on ZnO nanowires Syed M. Usman Ali, N.H. Alvi, Omer Nur, Magnus Willander, and Bengt Danielsson Sensor and Actuators B 152, 241 (2011) Paper VII Single Nanowire-based UV photo detectors for fast switching K. ul Hasan, N. H. Alvi, Jun Lu, O. Nur, and Magnus Willander Nanoscale Research Letters 6, 348 (2011) Paper VIII Rectifying characteristics and electrical transport behavior of ZnO nanorods/4H-SiC heterojunction LEDs N. H. Alvi, G. Amin, O. Nur, and M. Willander (Manuscript) Paper IX Single ZnO nanowire biosensor for detection of glucose interactions K. ul Hasan, N. H. Alvi, U. A. Shah, Jun Lu, O. Nur, and M. Willander
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Paper X Comparative optical characterization of n-ZnO nanorods grown by different growth methods N. H. Alvi, S. Hussain, O. Nur, and M. Willander (Manuscript) Paper XI Fabrication and characterization of white light emitting diode based on ZnO nanorods on p-Si
M. M. Rahman, P. Klason, H.A. Naveed, M. Willander
2008 8th IEEE Conference on Nanotechnology (2008: Arlington TX United States) p. 51 – 54 Proceedings IEEE-NANO. (2008)
Paper XII
Photonic nano-devices and coherent phenomena in some low dimensional systems
Magnus Willander, Y.E. Lozovik , S.P. Merkulova , Omer Nour , A. Wadeasa , P. Klason , B. Nargis , N.H. Alvi , S. Kishwar
214th Electrochemcial Society Meeting, Abstract #2034 Honolulu, Hawaii, USA (2008)
Paper XIII
Intrinsic white-light emission from zinc oxide nanorods heterojunctions on large-area substrates
M. Willander, O. Nur, S. Zaman, A. Zainelabdin, G. Amin, J. R. Sadaf, M. Q. Israr, N. Bano, I. Hussain and N. H. Alvi
(Proceedings of SPIE Volume 7940) DOI: 10.1117/12.879327.
Mediterranean Conference on Innovative Materials and Applications held in Beirut, Lebanon in March, 2011
Paper XVI
ZnO as an energy efficient material for white LEDs and UV LEDs
M. Willander, O. Nur, N. Bano, I. Hussain, A. Zainelabdin, S. Zaman, M. Q. Israr, and N. H. Alvi
(Manuscript)
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ACKNOWLEDGEMENT On this memorable night in my life when I am going to finish the writing of this thesis, first of all I bestowed the thanks before Allah Almighty who vigorate me with capability to complete this research work. In the way to the completion of this thesis, my family, teachers, colleagues, and friends all contributed in different ways. At this moment I am very thankful to all of them.
I would like to express my deep sense of gratitude to my supervisor Prof. Magnus Willander for his useful and valuable suggestions, inspiring guidance and consistent encouragement without which this thesis could have never been materialized. He always taught me how to face hard moments in research and life.
I also wish to record my sincere thanks to my co-supervisor Associate Prof. Omer Nour for his kind cooperation, valuable contribution, patience and guidance during my study and research work.
I am also thankful to the ex-research administrator Lise-Lotte Lönndahl Ragnar and our group research administrator Ann-Christin Norén for their administrative help during my studies and research work.
I am also thankful to Dr. Peter Klason, Dr. Georgios Tzamalis, Dr. Alim Fulati, Dr. Muhammad Riaz, Dr. Lili Yang, Dr. Pranciškus Vitta, Prof. Artūras Žukauskas, and Dr. Jens Jensen for their endless cooperation in my research work and nice company.
I am also thankful to all my teachers in my academic career who gave me light of knowledge, every possible help, and guidance which enabled me to be a PhD researcher which was my dream in life.
Words are lacking to express my obligations to Higher Education Commission (HEC) government of Pakistan for partial financial help in my research work. I am also very thankful to Dr. Atta ur Rehman (Ex-Chairman HEC), Dr. Javeed Laghari (Chairman HEC), Muhammad Ashfaq, project manager (HEC), Dr. Sohail Naqvi, and Dr. Yasir Jameel for their cooperation and good wishes.
I offer my sincerest wishes and warmest thanks to all my group members. Many thanks for your cooperation and nice company. It was always my pleasure to work with you all.
I am also thankful to my friend Zia ullah Khan and his family. They always treated me like a brother. Thank you for help, guidance and nice company.
My gratitude will remain incomplete if I do not mention that great contribution of my caring brothers Waseed ul Hassan Alvi, Waheed ul Hassan Alvi, Ameed ul Hassan Alvi and my sisters Aisha Bano Alvi, Fatima Alvi, Wahida Alvi, and Saima Alvi during whole my studies. I wish to express thanks for their love and affection for me in every aspect of life.
I would like to express my profound admiration and salute to my affectionate Father Dr. Khursheed Ahmad Alvi and my sweet mother who taught me the first word to speak, the first alphabet to write and the first step to take. Thanks for your prayers, encouragement and unforgettable sacrifices with patience throughout my career. I am also thankful to all my family, especially my uncles Moulana Nazir Ahmad Alvi and Moulana Abdur Rasheed Alvi for their prayers and encouragement.
I am also very thankful to my loving wife Rahat Alvi (Gul Bano). Thank you for all your help with reference and figure corrections and also for taking care of the house and for cooking delicious foods during my studies and research. I love you
Finally I would like to thank you for reading this thesis and I hope you will enjoy the reading.
Naveed ul Hassan Alvi
Norrköping, Friday, 13th May 2011
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Table of Contents:
Chapter 1 ................................................................................................................................................ 1
Introduction and motivation ................................................................................................................ 1
Chapter 2 ................................................................................................................................................ 7
Properties of ZnO ................................................................................................................................ 7
2.1 Basic properties of ZnO............................................................................................................. 7
2.2 The basic physical parameters for ZnO ..................................................................................... 9
2.5 Defects and luminescence in ZnO ........................................................................................... 14
2.6 Electrical properties of ZnO .................................................................................................... 17
2.7 Mechanical properties ............................................................................................................. 19
Chapter 3 .............................................................................................................................................. 27
Device applications of ZnO nanostructures ...................................................................................... 27
3.1 Light emitting diodes ............................................................................................................... 27
3.2 ZnO based UV photodetectors ................................................................................................ 35
3.3 ZnO based biosensors .............................................................................................................. 36
Chapter 4 .............................................................................................................................................. 43
Synthesis of ZnO nanostructures and fabrication of LEDs ............................................................... 43
4.1 Substrate preparation ............................................................................................................... 43
4.2 Growth of ZnO nanostructures ................................................................................................ 45
4.2.1 The synthesis of nanorods by vapor-liquid-solid method..................................................... 45
4.2.2 Synthesis of nanostructures by the aqueous chemical growth (ACG) method..................... 50
4.2.2.1 Synthesis of ZnO nanorods ............................................................................................... 50
4.2.2.2 Growth of ZnO nanotubes ................................................................................................. 52
4.2.2.3 Growth of ZnO nanowalls ................................................................................................. 52
4.2.2.4 Growth of ZnO nanoflowers ............................................................................................. 52
4.3 Bottom contacts deposition ..................................................................................................... 55
4.4 Spin coating of photo resist and plasma etching ..................................................................... 56
4.5 Top contacts deposition ........................................................................................................... 56
Chapter 5 .............................................................................................................................................. 61
Experimental and characterization techniques .................................................................................. 61
5.1 Scanning electron microscopy ................................................................................................. 61
5.2 Atomic force microscope ........................................................................................................ 64
5.3 X-ray diffraction ...................................................................................................................... 64
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5.4 Photoluminescence .................................................................................................................. 66
5.5 Electroluminescence ................................................................................................................ 69
5.6 Electrical characterization ....................................................................................................... 70
5.7 Annealing studies .................................................................................................................... 70
5.8 Color rendering extraction ....................................................................................................... 71
5.9 Irradiation studies .................................................................................................................... 72
References ..................................................................................................................................... 73
Chapter 6 .............................................................................................................................................. 75
Results ............................................................................................................................................... 75
6.1 Luminescence properties of ZnO nanostructures .................................................................... 75
6.2 Junction temperature of n-ZnO nanorods based LEDs ........................................................... 92
6.3 The influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods based LEDs .................................................................................................................... 98
Chapter 7 ............................................................................................................................................ 107
Conclusion and outlook ................................................................................................................... 107
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List of Figures Figure 2.1: The hexagonal wurtzite structure of ZnO. One unit cell of the crystal is out lined for clarity. 8 Figure 2.2: The zincblende (left) and rock salt (right) phases of ZnO. Only one unit cell is illustrated for
clarity. 8 Figure 2.3: The band structure of bulk wurtzite ZnO calculated by the LDA method (a) and self-interaction
corrected pseudopotential (SIC-PP) method (b). Reprinted with permission from ref [22]. 12 Figure 2.4: The band structure and symmetries of hexagonal ZnO with splitting of the valance and the
conduction bands in the vicinity of the fundamental band-gap. 13 Figure 2.5: Shows the PL spectrum of ZnO nanoflowers and EL spectrum of ZnO nanorods based LED at
room temperature [49]. 14 Figure 2.6: Schematic band diagram of some deep level emissions (DLE) in ZnO based on the full
potential linear muffin-tin orbital method and other reported data [84, 92]. 18 Figure 2. 7: A typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs [49]. 18 Figure 2.8: (Color online) Load vs displacement curve of the as grown ZnO nanorods from indentation
experiment, (a) buckled ZnO nanorods, and (b) bending flexibility of ZnO nanorods curve [106]. 20
Figure 2.9: (Color online) Load vs displacement curve of the etched ZnO nanotubes from the nanoindentation experiment, (a) buckled ZnO nanotubes, and (b) bending flexibility of ZnO nanotubes curve [106]. 20
Figure 3.1: The electroluminescence (EL) of the as grown ZnO nanorods on p-GaN substrates [15]. 31 Figure 3.2: The photoluminescence (PL) of the as grown ZnO nanorods on p-GaN substrates [15]. 31 Figure3.3: Anderson model energy band diagram of the n-ZnO/p-GaN heterojunction structure. 32 Figure 3.4: Shows typical I-V characteristic for different ZnO (nanostructures) /p-GaN LEDs [16]. 34 Figure 3.5: Shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [16]. 34 Figure 3.6: Photo-response of a single ZnO nanowire under pulsed illumination by a 365 nm wavelength
UV light with (a) Schottky contact on one side, and (b) ohmic contacts on both sides [66]. 36 Figure 3.7: Reproducibility and stability of the glucose-sensing microelectrodes [30]. 37 Figure 3.8: Time response of the ZnO nanowires/Uricase sensor electrodes in 100 µM uric acid solution (a)
without membrane, and (b) with membrane. The calibration curves for the uric acid sensor (c) with membrane, and (d) without membrane [31]. 38
Figure 4.1: Schematic illustraion of the device fabrication. 45 Figure 4.2: A schematic diagram of the vapor-liquid-solid technique. 46 Figure 4.3: The VLS growth presentation of ZnO nanorods. 46 Figure 4.4: SEM images for ZnO nanorods grown on different substrates, (a, b) p-GaN, (c-e) 4H-SiC, and
(f-h) Si under different growth parameters. 49 Figure 4.5: Shows the SEM images of ZnO nanorods with different diameters, (a) ~440 nm, (b) ~200 nm,
(c) ~150 nm, and (d) ~35 nm. 51 Figure 4.6: SEM images of (a) ZnO nanotubes, (b) ZnO nanowalls, and (d) ZnO nanoflowers. 53 Figure 4.7: SEM image of grown ZnO nanorods by the sol-gel method. 54 Figure 4.8: The SEM image of grown ZnO nanorods by Electro-Chemical deposition (ECD) method. 55 Figure 5.1: Schematic diagram of the scanning electron microscopy. 62 Figure 5.2: Typical SEM images of different ZnO nanostructures (a) nanorods, (b) nanoflowers, (c)
nanotubes, and (d) nanowalls. 63 Figure 5.3: A 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN. 64 Figure 5.4: Shows a schematic diagram of Bragg reflection from crystalline lattice planes having interplan
distace “d” between two lattice plane. 65 Figure 5.5: Display the θ-2θ XRD spectra of ZnO (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d)
nanotubes grown on p-GaN substrates, respectively. 66 Figure 5.6: A schematic diagram of the PL setup. 68
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Figure 5.7: A typical I-V characteristics for different ZnO (nanostructures)/p-GaN LEDs [3]. 68 Figure 6.1: (a) Room temperature photoluminescence spectrum for the ZnO nanorods on p-4H-SiC
substrate, and in (b) the room temperature photoluminescence spectrum for the ZnO nanorods on p-GaN substrate [1]. 77
Figure 6.2: (a)Electroluminescence spectrum for ZnO nanorods/p-4H-SiC LED, and in (b) electroluminescence spectrum for the ZnO nanorods/p-GaN LED [1]. 77
Figure 6.3: Room temperature photoluminescence spectrum for ZnO nanostructures (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes on p-GaN and (e) shows the combine PL spectra of all the four nanostructures [4]. 80
Figure 6.4: Displays the electroluminescence spectra for n-ZnO (nanostructures)/p-GaN LEDs, in (a) nanowalls, (b) nanoflowers (c) nanorods, and (d) nanotubes, (e) shows the EL spectra of all the nanostructures, and (f) shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [4]. 82
Figure 6.5: Typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs as indicated in the figure [4]. 82
Figure 6.6: Schematic band diagram of the DLE emissions in ZnO based on the full potential linear muffin-tin orbital method and the reported data and references in [6]. 85
Figure 6.7: Electroluminescence spectra of different LEDs at an injection current of 3 mA, under forward bias of 25 V and it shows the shift in emission peak after annealing in different ambients [6]. 85
Figure 6.8: The CIE 1931 x, y chromaticity space of ZnO nanotubes annealed in different ambients [6]. 88 Figure 6.9: Normalized spectral power distributions for the LEDs based on the as-grown n-ZnO nanorods
and after annealing in air, oxygen, and nitrogen ambients [11]. 88 Figure 6.10: The chromaticity coordinates of different LEDs (a) under forward bias and (b) under reverse
bias plotted on the CIE (1931) x,y, chromaticity diagram [11]. 91 Figure 6.11: The energy band diagram of the n-ZnO nanorods/p-4H-SiC heterostructure [13]. 93 Figure 6.12: (a), (b), and (c) illustrate the measured forward voltage versus temperature for the n-ZnO
nanorods/(p-4H-SiC, p-GaN, and p-Si) heterostructures at different values of the current respectively and (d), (e), (f) show the measured series resistance versus the junction temperature for n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si), respectively [13]. 96
Figure 6.13: (a, b) show the measured I-V characteristics of the n-ZnO nanorods/ p-4H-SiC, p-GaN at different temperatures, (c) shows the measured I-V characteristics of the n-ZnO nanorods/ p-Si LEDs and (d, e) shows the electroluminescence spectrum for ZnO (NRs)/p-4H-SiC and p-GaN LEDs [13]. 96
Figure 6.14: Room temperature photoluminescence spectra for ZnO nanorods (a) as grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) shows the PL spectra of all the samples together for comparison [18] 99
Figure 6.15: Display the electroluminescence spectra for n-ZnO nanorods/p-GaN LEDs, (a) as grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) shows the EL spectra of all the LEDs together for comparison [18]. 99
Figure 6.16: Display the CIE 1931 x, y chromaticity space, showing the chromaticity coordinates of LEDs under forward bias for ZnO NRs/p-GaN LEDs, (a) as grown ZnO NRs, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) all together for comparison [18]. 102
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List of Tables Table 2.1: Basic physical properties of ZnO at room temperature [1, 9-13] 10
Table 4.1: Different ohmic contacts schemes for p-type GaN 56
Table 4.2: Different ohmic contacts schemes for n-type ZnO 57
XVI
1
Chapter 1 Introduction and motivation
Nanotechnology has developed a bridge among all the fields of science and
technology. Materials and structures with low dimensions have excellent properties which
enable them to play a crucial role in the rapid progress of the fields of science. With these
amazing properties, one dimensional nanostructures have become the back bone of research in
all the fields of natural sciences.
In the past decade, the global research interest in wide band gap semiconductors
has been significantly focused to zinc oxide (ZnO) due to its excellent properties as a
semiconductor material. The high electron mobility, high thermal conductivity, good
transparency, wide and direct band gap (3.37 eV), large exciton binding energy and easiness
of growing it in the nanostructure form by many different methods make ZnO suitable for
wide range of uses in optoelectronics, transparent electronics, lasing and sensing applications
[1-4]. In last decade, the number of publications on ZnO has increased annually and in 2007
ZnO has become the second most popular semiconductor after Si and its popularity is still
increasing with time [5].
Obtaining controllable, reliable, reproducible and high conductive p-type doping
in ZnO has proved to be very difficult task [6-9], due to the low formation energies for
intrinsic donor defects such as zinc interstitials (Zni) and oxygen vacancies (VO) which can
compensate the accepters. The efficiency of light emitting diodes can be limited by the low
carrier concentration and mobility of holes therefore the excellent properties of ZnO might be
best utilized by constructing heterojunctions with other semiconductors. Therefore the growth
of n-type ZnO on other p-type materials could provide an alternative way to realize ZnO
based p-n heterojunctions. In this way, the emission properties of LEDs can still be
determined by the excellent optical properties of ZnO. Various heterojunctions of ZnO thin
films have been achieved using various p-type materials like, GaN, AlGaN, Si, CdTe, GaAs,
and diamond [10-15]. The p-GaN is the best among the candidates for developing
heterojunction based LEDs with n-ZnO because it has many advantages over other p-type
materials. Both ZnO and GaN have the same wurtzite crystal structure, the same lattice
parameters (lattice mismatch is only 1.8%) and have almost the same band gap of 3.37 eV and
3.4 eV, respectively at room temperature.
2
There are also reports on the fabrication of more complex device structures. To
modify the device structure, some insulating or undoped layers were introduced between n-
ZnO nanorods and p-GaN but insertion of such layers in the device structures has changed the
emission spectra as compared to simple n-ZnO/p-GaN LEDs [16-21].
The n-ZnO/p-GaN LEDs have great potential to be a possible candidate for
white light source as they emits emission covering the whole visible spectrum with no need of
light conversion. There is a large variety of results that have been reported in the literature on
the emission spectra and heterojunction investigations for ZnO nanostructures/p-GaN LEDs.
The comprehensive investigations of the properties of n-ZnO/p-GaN LEDs are still great of
interest. The low cost grown ZnO nanostructures/p-GaN thin films LEDs are of special
interest. The ZnO nanorods and nanotubes based LEDs are more interesting as they have the
potential to improve light extraction [22].
In this research work, n-ZnO nanostructures (nanorods, nanotubes, nanoflowers,
and nanowalls) were grown by low cost aqueous chemical growth (ACG) and vapor liquid
solid (VLS) growth techniques on p type GaN, 4H-SiC and Si substrates to construct p-n
heterojunction LEDs. The luminescence properties and color quality of the fabricated LEDs
were investigated. A relation for junction temperature of the n-ZnO nanorods/p-GaN, p-
4HSiC, p-Si LEDs was also modeled and experiments were performed to validate the model.
The influence of helium (He+) ion irradiation on the luminescence properties of ZnO
nanorods/p-GaN LEDs was also investigated for nuclear and space application.
Now the world is seeking to replace the high energy consumption conventional
light bulbs with low energy consumption LEDs, and in this way there will be a decrease in the
energy consumption by around 20%. According to the recent analysis by the U.S. Department
of Energy (DOE), the estimated cumulative energy saving for replacing lighting with LEDs
for period spanning 2010-2030 is $ 120 billion at today’s energy prices and it will also reduce
the emission of carbon in the environment by 246 million metric ton [23].
However, the GaN is still leading the commercial LEDs in the market but ZnO
has also much potential to compete and over head the GaN based LEDs. But the problem is to
understand and control the origin of visible emissions which is still controversial after
investigations for decades. This thesis is mainly devoted to investigations about the
luminescence properties of ZnO nanostructures and the implementation of these
nanostructures as white LEDs. Annealing studies of ZnO nanotubes were also performed to
3
investigate the origin of different emissions in ZnO by using photoluminescence (PL) and
electroluminescence (EL) experiments at room temperature. The luminescence comparison
study of ZnO nanorods grown on different p-type substrates and different nanostructures on
the same substrate were also used to investigate the emissions in ZnO.
This thesis has been organized in this way; chapter 2 focuses on some basic
properties of ZnO and gives a brief discussion about luminescence, electrical and mechanical
properties of ZnO. Chapter 3 focuses on LEDs, UV detectors, and biosensing applications of
ZnO nanostructures. Chapter 4 describes the synthesis of different ZnO nanostructures by
using different growth techniques and fabrication of LEDs. Chapter 5 focuses on the
experimental and characterization techniques used in this research work. Chapter 6 describes
the results and finally in chapter 7 the thesis ends with concluding remarks.
4
References:
[1] A. Janotti and C. G. Van de Walle, Rep. Prog. Phys. 72, 126501 (2009)
[2] M. Willander et al., Nanotechnology, 20, 332001 (2009)
[3] Z. L. Wang, Materials Today 7, 26 (2004)
[4] U. Ozgur et al., J. Appl. Phys. 98, 1 (2005)
[5] Peter Klason, Zinc oxide bulk and nanorods, A study of optical and mechanical
properties, PhD thesis, University of Gothenburg, (2008)
[6] S. B. Ogale, Thin films and Heterostructures for Oxide Elelectronics (New York:
Springer) (2005)
[7] N. H. Nickel, and E. Terukov, (ed) Zinc Oxide—A Material for Micro- and
Optoelectronic Applications (Netherlands:Springer) (2005)
[8] U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J.
Cho, and H. Morkoc, J. Appl. Phys. 98, 041301 (2005)
[9] C. Jagadish and S. J. Pearton (ed) Zinic Oxide Bulk, Thin Films, and Nanostructures
(New York: Elsevier) (2006)
[10] C. X. Wang, G. W. Yang, H. W. Y. H. Han. J. F. Luo, C. X. Gao, and G. T. Zou, Appl.
Phys. Lett. 84, 2427 (2004)
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K. Chu, Appl. Phys. Lett. 88, 132104 (2006)
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V. Chukichev, and D. M. Bagnall, Appl. Phys. Lett. 83, 4719 (2003)
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M. Zhou, Chin. Phys. Lett. 22, 2298 (2005)
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Lett. 83, 2943 (2003)
[16] R. W. Chuang, R. X. Wu, L. W. Lai, and C. T. Lee, Appl. Phys. Lett. 91, 231113 (2007)
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Lett. 91, 091107 (2007)
[18] J. W. Sun, Y. M. Lu, Y. C. Liu, D. Z. Shen, Z. Z. Zhang, B. H. Li, J. Y. Zhang, B. Yao,
D. X. Zhao, and X. W. Fan, J. Phys. D: Appl. Phys. 41, 155103 (2008)
5
[19] L. Zhao, C. S. Xu, Y. X. Liu, C. L. Shao, X. H. Li, and Y. C. Liu, Appl. Phys. B 92, 185
(2008)
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Shiojiri, IEEE Photon. Technol. Lett. 20, 1772 (2008)
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6
7
Chapter 2 Properties of ZnO
In the past decade global research interest in wide band gap semiconductors has
been attracted towards zinc oxide (ZnO) due to its excellent properties as a s emiconductor
material. The high electron mobility, high thermal conductivity, good transparency, wide
direct band gap (3.37 eV), large exciton binding energy and easiness of growing it in the
nanostructure form make ZnO suitable for optoelectronics, transparent electronics, lasing,
sensing, and wide range of applications [1-4].
2.1 Basic properties of ZnO ZnO crystallize preferentially in the stable hexagonal wurtzite structure at room
temperature and normal atmospheric pressure as shown in figure 2.1. It has lattice parameters
a= 3.296 nm, c = 0.520 nm with a density of 5.60 g cm-3. The electronegativity values of O-2
and Zn+2 are 3.44 and 1.65, respectively resulting in very strong ionic bonding between Zn+2
and O-2. Its wurtzite structure is very simple to explain, where each oxygen ion is surrounded
tetrahedrally by four zinc ions, and vice versa, stacked alternatively along the c-axis. It is
clear that this kind of tetrahedral arrangement o f O -2 and Zn+2 in ZnO will form a non
central s ymmetric structure composed of two interpenetrating hexagonally closed packed
sub-lattices of zinc and oxygen that are displaced with respect to each other by an amount of
0.375 along the hexagonal axis. This is responsible for the piezoelectricity observed in ZnO.
It also plays a vital role in crystal growth, defect generation and etching.
Other basic characteristics of ZnO are the polar surfaces that are formed by
oppositely charged ions produced by positively charged Zn+ (0001) and negatively charged O-
(000ī) polar surfaces. It is responsible for the spontaneous polarization observed in ZnO. The
polar surfaces of ZnO have non-transferable and non-flowable ionic charges. The interaction
among the polar charges at the surface depends on their distribution, therefore the structure is
arranged in such a way to minimize the electrostatic energy, which is the main driving force
for growing polar surface dominated nanostructures. This effect results in a growth of various
ZnO nanostructures such as nanowires, nanosprings, nanocages, nanobelts, nanocombs,
nanorings, and nanohelices [1].
8
Figure 2.1: The hexagonal wurtzite structure of ZnO. One unit cell of the crystal is out lined
for clarity.
Figure 2.2: The zincblende (left) and rock salt (right) phases of ZnO. Only one unit cell is
illustrated for clarity.
9
The wurtzite ZnO has four common face terminations and these are polar Zn
terminated (0001), and the O terminated (000ī) along c-axis. The non-polar faces are (1120)
and (1010). The non-polar surfaces contain equal number of Zn and O atoms. The polar faces
are stable and have different chemical and physical properties. The O terminated face (000ī) has slightly different electronic structure than the other three faces [2]. Due to lack of center
of inversion in the wurtzite ZnO, the grown ZnO nanorods and nanotubes have two different
polar surfaces on the opposite sides of the crystal. These different polar surfaces are formed
due to the sudden termination of the Zn terminated (0001) surface with Zn cations outermost
and the O terminated (000ī) surface with O anion outermost [1-3]. In addition to the wurtzite
structure, ZnO can also crystallize in the cubic zinc-blende and the rock-salt (NaCl) structures
which are illustrated in figure 2.2 [1].
The growth of the zincblende ZnO is a challenge as zincblende ZnO is stable
only by growth in cubic structures [4-5]. The cubic rock salt structure exists only at high
presser (10 GPa) and cannot be epitaxially stabilized [6]. In the rock salt structures each Zn or
O atom has six nearest neighbor atoms but in the wurtzite and zincblende structure each Zn
and O atom has only four nearest neighbors. The zincblende has lower ionicity as compared
to the wurtzite structure and leads to lower carrier scattering and high doping efficiencies [7].
Theoretical calculations indicated that a fourth phase cubic cesium chloride, may be possible
at extremely high temperatures, however, this phase has not yet been experimentally observed
[8].
2.2 The basic physical parameters for ZnO The basic physical parameters of ZnO at room temperature are shown in table
2.1 [1, 9-13]. There is still uncertainty in the values of the thermal conductivity due to the
influence of defects in the material. A stable and reproducible p-type ZnO is still a challenge
and cannot be achieved and the hole mobility and its effective mass are still doubtful. The
values of the carrier mobility can surely be increased after achieving good control on the
defects in the material [14].
10
Properties of wurtzite ZnO
Lattice parameters at 300 K
a0 0.32495 nm
c0 0.52069 nm
a0/c0 1.602 (ideal hexagonal structure shows 1.633)
Density 5.606 g/cm3
Stable phase at 300 K Wurtzite
Melting point 1975 oC
Thermal conductivity 0.6, 0.13 1–1.2
Linear expansion coefficient (/oC) a0: 6.5 × 10-6 , c0: 3.0 × 10-6
Static dielectric constant 8.656
Refractive index 2.008, 2.029
Energy gap 3.4 eV, direct
Intrinsic carrier concentration <106 cm-3 (max n-type doping>1020 cm-3 electrons;
max p-type doping<1017 cm-3
Exciton binding energy 60 meV
Electron effective mass 0.24
Electron Hall mobility at 300 K for
low n-type conductivity 200 cm2/V s
Hole effective mass 0.59
Hole Hall mobility at 300 K for low
p-type conductivity 5–50 cm2/Vs
Bulk Young’s modulus E (GPa) 111.2 ± 4.7
Bulk hardness, H (GPa) 5.0 ± 0.1
Table 2.1: Basic physical properties of ZnO at room temperature [1, 9-13]
2.3 Electronic band structure of ZnO
The electronic band structure of a semiconductor is very important to be
understood for its utility in devices and for further improving the performance of these
devices [12, 15]. The electronic band structure gives understanding of the electron/hole states.
ZnO is a direct band gap semiconductor. Several theoretical approaches such as the Local
11
Density Approximation, the Green’s functional method and the first principles were used to
calculate the energy band diagram of wurtzite as well as zincblende and rocksalt polytypes of
ZnO [15-31]. In parallel to the theoretical efforts, a number of experimental techniques such
as X-ray induced photo absorption, photoemission spectroscopy, angle resolved photo-
electron spectroscopy, and low energy electron diffraction have been commonly employed to
understand the electronic states of wurtzite ZnO [32-45].
There was a good agreement between the theoretical and the experimental
results qualitatively for the wide spread of the valance band but quantitatively there was a
disagreement and the prediction about the Zn 3d states remained a challenge for the research
community. Recently it was found that by including the Zn 3d level effects in the calculations,
the researchers have achieved good agreements with experimental data [17, 21-22]. The first
theoretical calculation of the energy band of ZnO was reported in 1969 by U. Rössler and
later on many approaches have been introduced and the process of theoretical improvement
continued [15-31]. In 1995 D. Vogel et al. [22] have calculated the electronic band structure
in which they included the Zn 3d electrons effects, their results are shown in figure 2.3. It
shows that the lowest conduction band minima and highest valance band maxima are at the Γ
point k = 0, which clearly indicates that ZnO is a direct band gap material. In figure 2.3 (b),
there are ten bands at ~ -9 eV in the bottom while these bands are absent in figure 2.3 (a).
These bands in the bottom are due to Zn 3d levels, as the effect of these levels were included
in the calculation of the figure 2.3 (b), while in the figure 2.3 (a) these effects were not
included. In figure 2.3 (b), six bands between -5 eV-0 eV are representing the O 2p bonding
states. The bandgap measured in the figure 2.3 (a) is about 3 eV. The bandgap shrinks here
due to the fact that to simplify the calculations in the standard LDA method, the Zn 3d states
have been taken as core levels. While in the SIC-PP calculation in figure 2.3 (b), the bands are
considerably shifted down in energy and in response, the bandgap opened drastically. The
band gap calculated by this method was 3.37 eV, which is very close to the experimental
value that is 3.34 eV.
12
(b)
Figure 2.3: The band structure of bulk wurtzite ZnO calculated by the LDA method (a) and
self-interaction corrected pseudopotential (SIC-PP) method (b). Reprinted with permission
from ref [22].
The band structure and symmetry of the hexagonal ZnO structure are shown in
figure 2.4. Schematically it shows how the ZnO valance band splits experimentally to three
subbands A, B, and C in the vicinity of the bandgap of the ZnO due to the interaction of the
crystal field and spin orbit. There is Γ7 symmetry in the A, C subbands while B subband has
Γ9 symmetry. The transitions from A and B subbands to the conduction band are dipole spin
flip and can be allowed only when E c and from C subband to conduction band can be only
when E c [46-47].
13
Figure 2.4: The band structure and symmetries of hexagonal ZnO with splitting of the valance
and the conduction bands in the vicinity of the fundamental band-gap.
2.4 Optical properties of ZnO
The optical properties of a s emiconductor are dependent on both the intrinsic
and the extrinsic defects in the crystal structure. The investigations of the optical properties of
ZnO has a long history that started in the 1960s [48] and recently it has become very attractive
among wide band gap materials due to its direct wide band gap (3.37 eV) with large exciton
energy (60 meV) at room temperature. The efficient radiative recombinations has made ZnO
promising for its applications in optoelectronics. The optical properties of ZnO, bulk and
nanostructures have been investigated extensively by luminescence techniques at low and
room temperatures. The photoluminescence (PL) spectra of ZnO nanoflowers and
electroluminescence (EL) spectra of ZnO nanorods/p-GaN LED at room temperature are
shown in figure 2.5 (a, b). The ultra-violet (UV) emission band and a broad emission band are
observed. The UV emission band is commonly attributed to transition recombinations of free
excitons in the near band-edge of ZnO. An exciton is a pair of excited electron and hole that
are bound together by their Coulomb attraction. There are two classes of excitons. The
14
excitons can be free to move through the crystal or can be bound to neutral or charged donors
and accepters [49]. The broad emission band in the visible region (420 nm - 750 nm) is
attributed to deep level defects in ZnO. There are many different deep level defects in the
crystal structure of ZnO and they affect the optical and electrical properties of ZnO. Their
details are explained in the next section.
400 500 600 700 800 9000
5000
10000
15000
20000
25000 450 nm
540 nm
400 n
mEL In
tens
ity (a
.u.)
Wavelength (nm)300 400 500 600 700
0
2000
4000
6000
8000
10000
12000376 nm
525 nm
PL In
tens
ity (a
.u.)
Wavelength (nm)
(EL spectra)(PL spectra)
Figure 2.5: Shows the PL spectrum of ZnO nanoflowers and EL spectrum of ZnO nanorods
based LED at room temperature [49].
2.5 Defects and luminescence in ZnO
For any luminescent material, it is very important to investigate the origin of its
luminescent centers and it is a key topic in optoelectronics. The electrical and optical
properties of a semiconductor material can be controlled and modified by controlling the
quantity and nature of the defects in it. These defects can be introduced during the growth
process or by post-growth treatments such as annealing or ion implantation. It is very crucial
to understand the behavior of these defects in ZnO. Both extrinsic and intrinsic luminescence
properties of ZnO based on e xtrinsic and intrinsic defects are under debate since 1960s.
Especially the origin of intrinsic luminescence in ZnO is still controversial due to native point
defects. The ZnO has donor and accepter energy levels below and above the conduction and
valance bands, respectively and these are responsible for the near-band edge emissions. ZnO
has also deep energy levels in the band gap with different energies and these deep levels are
responsible for the emissions in the whole visible region from 400-750 nm. The origin of
15
these deep level emissions have a considerable interest and is still under debate and many
people suggested different origins of these deep level emissions [1, 12, 50-66].
There are three common types of defects such as, line defects, point defects and
complex defects. Line defects belong to the rows of atoms such as dislocations, while point
defects belong to the isolated atoms in localized regions and the composition of more than one
point defect form the complex defects. There are intrinsic and extrinsic point defects and both
contribute to the luminescence properties in ZnO [12-1]. If foreign atoms such as impurities
are involved in the defects then these defects are called extrinsic point defects. If the defects
only consist of host atom, then these defects are called intrinsic atoms. Intrinsic optical
recombinations take place between the electrons in the conduction band and holes in the
valance band [12]. The deep level emission (DLE) band in ZnO has been previously attributed
to different intrinsic defects in the crystal structure of ZnO such as oxygen vacancies (VO) [4-
8], oxygen interstitial (Oi) [56-59], zinc vacancies (VZn) [60-63], zinc interstitial (Zni) [64-65]
and oxygen anti-site (OZn) and zinc anti-site (ZnO) [66]. The extrinsic defects such as
substitution Cu and Li [67, 58] are also suggested to be involved in deep level emissions.
The vacancy defects are formed when a host atom C is missing in the crystal and
it is denoted by VC. In ZnO, oxygen vacancy (VO) and zinc vacancies (VZn) are the two most
common defects. Vacancy is an intrinsic defect. The single ionized oxygen vacancies in ZnO
are responsible for the green emission in ZnO. The oxygen vacancy has lower formation
energy than the zinc interstitial and dominates in zinc rich growth conditions. The red
luminescence from ZnO is attributed to doubly ionized oxygen vacancies [68]. The origin of
the green emission in ZnO is the most controversial and many hypotheses have been proposed
for this emission [50, 69-76]. Zinc vacancies were thoroughly investigated and suggested by
many researchers to be the source of the green emission appeared at 2.4 - 2.6 eV below the
conduction band in ZnO [77-78, 63]. Many researchers also suggested oxygen vacancies as
the source of green emission in ZnO [79-80, 87, 55]. There are also reports that oxygen
interstitials and extrinsic deep levels such as Cu are sources of the green emission in ZnO [81,
46, 12]. Recently it has been investigated that more than one deep levels are involved in the
green emission in ZnO. It is found that VO and VZn both contribute to the green emission [69,
82-83]. The blue emission in ZnO belongs to zinc vacancies. The blue emission is attributed
to the recombination between zinc interstitial (Zni) energy level to VZn energy level and it is
approximately 2.84 eV (436 nm). It can be explained by the full potential linear muffin-tin
orbital method, which explains that the position of the VZn level is approximately located at
16
3.06 eV below the conduction band and the position of the Zni level is theoretically located at
0.22 eV below the conduction band [84].
The interstitial defects are formed when an excess atom D occupying an
interstitial site between the normal sites in the crystal structure and it is denoted by Di. In
ZnO, oxygen interstitial (Oi) and zinc interstitial (Zni) are the two common defects. These are
also intrinsic defects. The zinc interstitial defects are normally located at 0.22 eV below the
conduction band and play vital role in the visible emissions in ZnO by recombination between
Zni and different defects in the deep levels such as oxygen and zinc vacancies, oxygen
interstitials and produce green, red and blue emissions in ZnO [84]. Oxygen interstitials
defects are normally located at 2.28 eV below the conduction band and are responsible for the
orange-red emissions in ZnO [75-87].
The yellow emission in ZnO has also been attributed to oxygen interstitials [62,
87]. Recently the yellow emission was observed in ZnO nanorods grown at low temperature
by the aqueous chemical growth method and it was attributed to Oi and the Li impurities in
the growth material [58]. The yellow emission in the chemically grown ZnO nanorods was
also attributed to the presence of Zn(OH)2 that is attached to the surface of the nanorods.
Anti-site defects are formed when atoms occupy wrong lattice position. In ZnO,
the oxygen and zinc anti-site defects are formed when zinc occupies oxygen position or
oxygen occupies zinc position in the lattice. These defects can be introduced in ZnO by
irradiation or ion implantation treatments. The transitions at 1.52 eV and 1.77 eV above the
valance band are attributed to OZn related deep levels [66].
Some cluster defects are also present in ZnO that are formed by combination of
more than one point defect. The cluster defects can also be formed by a combination of point
and extrinsic defects such as VOZni cluster, which is formed by oxygen vacancy and zinc
interstitial and it was reported that it is situated at 2.16 eV below the conduction band [66].
Extrinsic defects also play a vital role in the luminescence from ZnO. The ultra-
violet (UV) emissions in ZnO at 3.35 eV are commonly related to the excitons bound to the
extrinsic defects such as Li, and Na accepters in ZnO [12]. The emission at 2.85 eV is due to
copper impurities in ZnO [67]. The yellow emission at 2.2 eV was observed in Li doped ZnO
thin film and Li related defects and this is located at 2.4 eV below the conduction band [89,
90]. Mn, Cu, Li, Fe, and OH are common extrinsic defects in ZnO and these defects are
17
involved in luminescence from ZnO. Defects with different energies produce the same color,
for example ZnO:Cu and ZnO:Co have different energies but both emit green color [46].
Hydrogen has also an interesting role in luminescence from ZnO. It is not a deep level defect
and lays at 0.03-0.05 eV below the conduction band [91]. Hydrogen is always positive in ZnO
and plays an important role as a donor and has low ionization energy, more details on this can
be found in [46, 12]. The luminescence defects and their possible transitions in ZnO indicate
that ZnO has a potential to emit excellent luminescence covering the whole visible region and
it has the potential to be used in the fabrication and development of white light emitting
diodes.
2.6 Electrical properties of ZnO
It is very crucial to understand the electrical properties of ZnO for applications
in nanoelectronics. The electrical behavior of undoped ZnO nanostructures is n-type and it is
widely believed that it is due to native defects such as oxygen vacancies and zinc interstitials
[93]. The electron mobility in undoped ZnO nanostructures is not constant and it depends on
growth method and it is approximately 120-440 cm2 V-1 s-1 at room temperature [12].
By doping, the highest reported carrier concentration is ~ 1020cm-3 and ~1019 cm-
3 for electron and holes, respectively [94]. However, such high levels of p-conductivity are not
stable or reproducible. The doping affects the carrier mobility in ZnO and doped ZnO has
lower mobility as compared to undoped ZnO. It is due to carrier scattering mechanism which
includes ionized impurity, non-ionized impurity, polar optical-phonon and acoustic phonon
scatterings. [12]. At room temperature the mobility of electrons is 200 cm2 V-1 s-1 and the hole
mobility is 5-50 cm2 V-1 s-1. The effective mass of electrons is 0.24m0 and the effective mass
of the holes is 0.59m0 and due to this difference in effective mass, the holes have very less
mobility as compared to electrons [11].
We have fabricated n-ZnO (nanostructures)/p-GaN LEDs [37]. The current
voltage (I-V) characterization of these fabricated LEDs is shown in figure 2.7 All the LEDs
show rectifying behavior as expected. Reasonable p-n heterojunctions are achieved and the
turn-on voltage of these LEDs was around 4 V . ZnO nanotubes based LED shows higher
current as compared to other nanostructures based LEDs and it is due to the large surface area
of the nanotubes as compared to other nanostructures.
18
Figure 2.6: Schematic band diagram of some deep level emissions (DLE) in ZnO based on
the full potential linear muffin-tin orbital method and other reported data [84, 92].
Figure 2. 7: A typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs [49].
-8 -6 -4 -2 0 2 4 6 8-0.10.00.10.20.30.40.50.60.70.80.91.0
-8 -6 -4 -2 0 2 4 6 81E-3
0.01
0.1
1
Curre
nt (m
A)
Voltage (V)
Curre
nt (m
A)
Voltage (V)
nanotubes nanorods nanoflowers nanowalls
19
2.7 Mechanical properties
ZnO has lack of center of symmetry and it gives rise to piezoelectric effect in
ZnO. By using this piezoelectric effect in ZnO, mechanical stress or strain can be converted
into electrical power. At the bottom scale, self powered energy is highly desired to operate
many devices. It is one of the main targets of nanotechnology to develop ultra small, self
powered nanosystems. Recently Z. L. Wang et al. have investigated ZnO nanowires for
producing electric power [95]. ZnO nanostructures based nanogenerators have potential to fill
the demand of ultra small self powered nanosystems. Therefore, it is very important to
investigate the mechanical characteristics of ZnO nanostructures. There are many reports on
the investigations of ZnO nanowires for mechanical buckling properties [96-105]. We have
investigated the buckling and elastic stability of vertical ZnO nanorods and nanotubes
quantitatively by nano-indentation technique [106]. Euler buckling model and shell
cylindrical model were used for the analysis of the mechanical properties. The critical load,
modulus of elasticity, and flexibility were measured. The critical load of nanorods was found
to be five times larger than that of nanotubes and nanotubes were found to be five time more
flexible than nanorods. The critical load, critical stress, strain, and Young modulus of
elasticity were measured.
20
Figure 2.8: (Color online) Load vs displacement curve of the as grown ZnO nanorods from
indentation experiment, (a) buckled ZnO nanorods, and (b) bending flexibility of ZnO
nanorods curve [106].
Figure 2.9: (Color online) Load vs displacement curve of the etched ZnO nanotubes from the
nanoindentation experiment, (a) buckled ZnO nanotubes, and (b) bending flexibility of ZnO
nanotubes curve [106].
21
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26
27
Chapter 3 Device applications of ZnO nanostructures ZnO is a large band gap semiconductor with excellent, optical, electrical,
chemical, piezoelectric and mechanical properties. It is a very attractive material for
applications in electronics, photonics, sensing and acoustics. ZnO is considered to be an
alternative to GaN for device applications due to many reasons such as low production cost
and excellent optical properties. Its high exciton binding energy is one of the properties that
make it superior over other semiconductors. It is an n-type material by un-intentional growth.
The achievement of stable and reproducible p-type ZnO is still a challenge for the research
community and it is the main obstacle in the fabrication and development of p-n
homojunction ZnO based optical devices. ZnO is also very attractive for transparent
electronics due to high transmittivity (for visible light). It is very attractive for acoustic wave
resonators, biosensors, gas sensors and solar cell device applications. ZnO has a rich variety
of nanostructures and these nanostructures are used in a variety of devices such as, LEDs [1-
23], bio-sensors [24-31], UV detectors [32-38], gas sensors [39-41] and nano-generators [42-
43] and bulk acoustic wave resonators (BAW) [44-47].
3.1 Light emitting diodes The artificial lighting has become the soul of modern human society. In USA
20% of the total generated electricity is used for general lighting. It is an ever-growing desire
of the society to develop efficient and environment friendly white light sources. Light
emitting diodes (LEDs) based light sources have potential to decrease electricity consumption
for light sources by 50 % and decrease the environmental pollution threats. The conventional
light sources are responsible for environmental pollution and add 1900 million tons of CO2 in
the environment each year. LEDs based light sources are more reliable than conventional light
sources. The lifetime of incandescent and fluorescent lamps is 1000 h and 10000 h,
respectively. The fluorescent lamps use phosphors for white light generation which decreases
the output power and increases the absorption within the lamp. The theoretical expected life
time for LEDs is 50000 h and they are strong candidates for future light sources [48-51]. ZnO is very attractive material for optical devices and has a large potential to be
used in light emitting diodes. ZnO has an important advantage in optoelectronic over other
semiconductors like GaN which is due to its high exciton binding energy (60 meV) at room
temperature. It is attractive for LEDs in the ultra-violet region due to large exciton binding
28
energy (60 meV) at room temperature and in the visible region due to deep level defects.
These deep level defects are responsible for emissions in the visible region from 420 nm to
750 nm. The achievement of ZnO p-n homojunction based LEDs is still very attractive but it
is dependent on the achievement of stable p-type ZnO. A stable and reproducible p-type ZnO
with acceptable electrical and optical properties is not achieved yet, that’s why p-n
homojunction LED is still a challenge. However some laboratories reported the fabrication of
ZnO based p-n homojunction LEDs but the electroluminescence from these fabricated LEDs
was too week [1-3], and the results were not reproduced by any other research group.
This problem motivated the research community to develop hybrid LEDs. The
excellent optical properties of ZnO can be utilized in the best way by constructing ZnO
heterojunctions with other p-type materials such as Si, SiC, GaN, AlGaN, Cu2O, GaAs,
diamond, ZnTe, CdTe and NiO or p-type organic materials [21, 52].
There are different designs for n-ZnO based hybrid single and double heterostructure LEDs
which have been reported such as:
(1) n-ZnO/p-GaN single heterostructure LED
(2) Inverted p-ZnO/n-GaN single heterostructure LED
(3) n-ZnO/p-SiC single heterostructure LED
(4) n-ZnO/p-AlGaN single heterostructure LED
(5) GaN/ZnO/GaN double-heterostructure LED
(6) MgZnO/ZnO/AlGaN/GaN double heterostructure LED
(7) Double heterostructure LED with CdZnO active layer
More details about these designs of ZnO based LEDs can be found in [20].
It is reported that in both single and double heterostructure LEDs interfacial
charges due to spontaneous electric polarization have significant impact on the operation of
these LEDs. Due to type II band alignment at the ZnO/III-N interface it is estimated that near
the heterostructure interface the polarization charge may provide an effective carrier
confinement. This may give tunneling radiative recombination of electrons and holes on
opposite sides of the interface of the LED and reduce the performance of the LEDs [20].
ZnO based LEDs are facing the following problems that can affect their
performance a lot [20].
(1) There is a considerable unbalance between the electrons and hole partial currents that
results in additional carrier losses on the contacts.
(2) These LEDs can have high series resistance that is not necessarily related to low
conductivity of the p-doped layers.
29
(3) Poor control on the emission spectra of these devices.
Some of the p-type materials are not suitable for constructing heterojunctions
with ZnO as the hetero-interface contains a large lattice mismatch and can strongly affect the
performance of the devices. Mostly, people prefer ZnO heterojunction with p-GaN or p-
AlGaN as these have similar physical properties to ZnO [9-14].
P-type GaN is a good choice for constructing heterojunction with ZnO because
the industry is already using GaN for the production of blue light emitting diodes and lasers
diodes. P-type GaN is preferred over other p-type materials due to the following similarities
with ZnO,
(1) Both have the same wurtzite crystal structure.
(2) Almost both have the same lattice parameters and the lattice mismatch is only 1.8%.
(3) At room temperature both ZnO and GaN have almost the same band gap of 3.37 eV and
3.4 eV, respectively.
There are also few reports that show that there can be imbalance between the
partial electron and hole currents in n-ZnO/p-GaN LEDs heterojunction. The imbalance in
electron and hole mobilities can be due to the difference in electron and hole mobilities and
difference in the heights of the potential barriers for electrons and holes in the space charge
regions at the heterojunction interface of the n-ZnO/p-GaN LEDs [20].
Some people reported the fabrication of more complex device structures. Some
insulating or undoped layers were introduced between n-ZnO nanorods and p-GaN to modify
the device structure but insertion of such layers in the device structures have changed the
emission spectra as compared to simple n-ZnO/p-GaN LEDs [53-58].
The n-ZnO/p-GaN LEDs have still great potential to be a possible candidate for
white light source as it emits emission covering the whole visible spectrum with no need of
light conversion. There is a large variety of results that have been reported in the literature on
the emission spectra and heterojunction investigations for ZnO nanostructures/p-GaN LEDs.
The comprehensive investigations of the properties of n-ZnO/p-GaN LEDs have still large of
interest. The low cost grown ZnO nanostructure/p-GaN thin films LEDs have special interest.
The ZnO nanorods and nanotubes based LEDs are more interesting as they have the
possibility to improve light extraction [59].
We have fabricated ZnO nanostructures/p-GaN and ZnO nanorods/p-4H-SiC
LEDs [15]. Figure 3.1 shows the electroluminescence (EL) of ZnO nanorods/p-GaN LEDs. It
was measured by photomultiplier detector at room temperature in the forward bias at a current
of 4 mA. The EL emission was so clear that it can be easily seen by the naked eye. The EL
30
spectrum of the fabricated LEDs shows three peaks that are approximately centered at 457 nm
(violet-blue emission), 531 nm (green emission), and 665 nm (orange-red emission). These
emissions can be attributed to different radiative recombinations due to deep level defects in
ZnO and in the Mg-doped p-type GaN substrate. At lower injection currents <2 mA the EL
emission cannot be detected which may be related to the presence of non-radiative
recombinations centers in the space charge region and provide a shunt path for the current
[60]. At higher injection currents >2 mA, these non-radiative recombination centers are
saturated and the injected carrier recombine radiatively and lead to the emission of photons.
The challenge in the improvement of the performance of fabricated devices is to
reduce the interface effects and to control the crystalline defects in ZnO. Further
investigations are still underway to control the defects concentration at the interface of the
heterojunction [60].
Figure 3.2 shows the photoluminescence (PL) of the as grown ZnO nanorods on
p-GaN substrates [15]. Photoluminescence measurements were performed at room
temperature. Laser lines with a wavelength of 266 nm from a laser diode pumped resonant
frequency doubling unit (MBD266) was used as an excitation source. The peaks are observed
at around 383 nm (band edge emission), 553 nm (green emission peak), and 693 nm (orange-
red emission), respectively. The observation of the band edge emissions at 383 nm are
attributed to free exciton near band edges and the presence of this peak show that our as
grown ZnO nanorods have good crystalline quality. By comparing the EL and the PL spectra
it is found that the PL spectrum is consistent with the EL spectrum and it is also clear that in
the EL spectra the violet-blue peak centered at 457 nm does not appear in the PL spectrum. It
might be suggested that it is from the substrate or from the heterojunction. Most probably it is
from the Mg-doped p-GaN substrate and is attributed to transitions from the conduction band
or shallow donors to deep Mg accepters.
31
Figure 3.1: The electroluminescence (EL) of the as grown ZnO nanorods on p-GaN substrates
[15].
Figure 3.2: The photoluminescence (PL) of the as grown ZnO nanorods on p-GaN substrates
[15].
It can be concluded that the EL emission from the fabricated LEDs is the
combination of emissions from the n-ZnO nanorods and the p-GaN substrate. It can also be
32
explained by the energy band diagram which was developed by using the Anderson model
[61] as shown in figure 3.3.
For the band diagram construction, the ZnO and GaN electron affinities (χ) and
band gap energies (Eg) were assumed to be (4.35 eV, 4.2 e V) and (3.37 eV, 3.4 eV),
respectively.
From the band diagram it can be seen clearly that the energetic barrier for
electrons is:
ΔEC = χ (ZnO) – χ (GaN) = 4.35 – 4.2 = 0.15 eV
and the energetic barrier for holes is:
ΔEV = Eg (ZnO) + ΔEC – Eg (GaN) = 3.37 + 0.15 – 3.4 = 0.12 eV
This means that the energy barrier for electrons and holes are approximately the same [60].
Figure3.3: Anderson model energy band diagram of the n-ZnO/p-GaN heterojunction
structure.
Recently it has been reported that the performance of the ZnO film/p-GaN LEDs
is decreased due to the formation of nonradiative centers at the interface [9, 11, and 13] and to
improve the performance of the LEDs, a wide band gap MgO has been introduced into ZnO
film/p-GaN LEDs [9]. It is also reported that intermediate layer of AlN is inserted into ZnO
film/p-GaN LEDs to improve the performance of fabricated LEDs. AlN has the largest direct
33
band gap of 6.2 eV among the III-nitride semiconductors with excellent physical and
chemical properties [62].
N-ZnO (nanostructures) /p-GaN based LEDs showed excellent current voltage
(I-V) characterization [16]. The I-V curves of the fabricated n-ZnO nanostructures/p-GaN
LEDs are shown in figure 3.4. All the LEDs showed rectifying behavior as expected. The I-V
curves indicate that reasonable p-n heterojunctions were achieved between the n-ZnO
nanostructures and the p-GaN substrate. The turn-on voltage of these fabricated
heterojunctions LEDs is around 4 V. It can be seen that ZnO nanotubes based LEDs have
higher current when compared to other nanostructures based LEDs and this is due to the large
surface area of the nanotubes as compared to the other nanostructures [16].
The CIE 1931 color space chromaticity diagram in the (x, y) coordinates system
for the n-ZnO nanostructures/p-GaN LEDs is shown in figure 3.5 [ 16]. The chromaticity
coordinates for n-ZnO nanowalls and nanoflowers are very close to Planckian locus (about >
1 MacAdam ellipse away). It can definitely be considered white light LEDs according to the
US standard ANSI_ANSLG C78, 377-2008 for solid state light sources which states that light
sources having chromaticity coordinates within three MacAdam ellipses from the Planckian
locus produce white light. The LEDs based on nanorods and nanotubes are more than three
MacAdam ellipses away from the Planckian locus and cannot be considered white light but
are very close to white light.
34
Figure 3.4: Shows typical I-V characteristic for different ZnO (nanostructures) /p-GaN LEDs
[16].
Figure 3.5: Shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs
[16].
-8 -6 -4 -2 0 2 4 6 8-0.10.00.10.20.30.40.50.60.70.80.91.0
Curr
ent (
mA)
Voltage (V)
nanotubes nanorods nanoflowers nanowalls
35
3.2 ZnO based UV photodetectors Recently ZnO based ultra-violet (UV) photo detection has attracted a great
attention due to the unique excellent structural, electrical and optical properties of ZnO. Due
to these properties it is very suitable material for the production and development of UV
detectors having high performance with high speed and high-detectivity. These detectors can
also be low cost, radiation resistant, chemically stable, and have the potential to be used in
civil, military, space, and nuclear applications [63].
In 1940, the first ultra-violet photo-response in ZnO thin films was observed
[64]. In 1980s, research on ZnO based photo detectors flourished gradually and in the
beginning, the properties of the fabricated UV detectors were not so good [65]. Different
techniques have been adopted to improve the efficiency of ZnO thin film based UV detectors.
Highly efficient ZnO based p-n and p-i-n junction and Schottky junction photo-detectors have
already been reported [63]. There are many reports on ZnO photo-detectors including
photoconductors, metal-semiconductor-metal (MSM) photo-detectors, Schottky photodiodes
and p-n junction photodiodes [63]. Recently we have fabricated single nanowires-based UV
photo-detectors for fast switching in our research group [66]. The Schottky photodiodes have
many advantages over the photoconductors and metal-semiconductor-metal photodiodes in
many ways as Schottky photodiodes have high quantum efficiency, high response speed, low
dark current, high UV/visible contrast, and have the possibility to operate at zero bias [67-68].
The first ZnO based Schottky diode was fabricated in 1986 [65]. Many research groups have
reported ZnO based Schottky photodiodes by different methods [32-34].
We have fabricated Schottky diodes by using single long (30 μm) ZnO
nanowires by applying Au and Pt electrodes across the single ZnO nanowires [66]. We also
compared these diodes with diode having ohmic contacts. The fabricated Schottky diodes
showed rectifying behavior with no reverse breakdown up to -5 V and under forward biasing
it showed high current. The fabricated schottky diode is stable up to 12 V and shows more
sensitivity, lower dark current and much faster switching under pulsed UV illumination as
shown in figure 3.6 (a, b) [66].
36
Figure 3.6: Photo-response of a single ZnO nanowire under pulsed illumination by a 365 nm
wavelength UV light with (a) Schottky contact on one side, and (b) ohmic contacts on both
sides [66].
3.3 ZnO based biosensors ZnO has become very attractive in the development and the application of
biosensors. Researchers working in the biosensing field have put their attention to ZnO due to
its excellent properties such as the bio-safety and non-toxicity. It is chemically stable and
electro-chemically active and it can easily be grown in the nanostructure forms at very low
cost. These promising properties have made ZnO favorite for detecting different biological
molecules. The use of ZnO nanostructures as biosensors is at an earlier stage and few reports
are available [24-31]. We have fabricated an improved intracellular glucose sensor based on
flake material which shows better sensitivity even for very small glucose concentrations as
compared to the earlier reported glucose sensors [30] and the sensitivity of the fabricated
nanoflakes based glucose sensors is 1.8 times higher than ZnO nanorods based glucose
sensors at the same conditions. ZnO nanoflakes grown on the tip of a borosilicate glass
capillary were used as a selective intracellular glucose sensor to measure the glucose
concentrations in human adipocytes and frog oocytes [30].
We have prepared glucose solutions with different glucose concentrations from
500 nM to 10 mM in a buffer (PBS pH 7.4) for the calibration of the extracellular
potentiometric response of the sensor electrode. The response time of the sensor electrode
became very fast when glucose was added. In 4 s it reached 95% of the steady state voltage.
The response of the sensor electrode in 50 μM has a linear relationship between the steady
state voltage and the glucose dependent electrochemical potential difference over the
complete glucose concentration [30].
For a single human adipocyte and for a single frog oocyte, the measured
intracellular glucose concentrations were approximately 60 ± 15 μM and 110 ± 20 μM,
37
respectively and these values were consistent with the reported data in the literature. The
reproducibility of the fabricated sensors is shown in figure 3.7 [30].
We have also fabricated sensitive, selective, stable and reproducible
potentiometric uric acid biosensor by immobilization of uricase on ZnO nanowires [31]. The
potentiometric response of the fabricated biosensor vs Ag/AgCl reference electrode was linear
over a relatively wide logarithmic concentration range (1 µM to 650 µM). The time response
of the fabricated uric acid biosensor in 100 μM is shown in figure 3.8 (a, b) and the
calibration curves for the uric acid sensor with membrane and without membrane are shown
in figure 3.8 (c, d). It was found that after applying a nafion membrane on the sensor the
linear range increased from 1µM to 1000 µM and the response time also increased from 6.25
s to > 9 s. The durability of the sensor also increased after applying the membrane. It was also
investigated that the normal concentrations of common interferents such as ascorbic acid
glucose have no effect on the sensor response [31].
Figure 3.7: Reproducibility and stability of the glucose-sensing microelectrodes [30].
38
0 5 10 15 20 25 30 35 40 45 50 55 60 650.000.020.040.060.080.100.120.14
EMF
[V]
Time [s]
Response time of sensor in 100uM uric acid solution with Membrane
0 5 10 15 20 25 30 35 40 45 50 55 60 650.000.020.040.060.080.100.120.14
EMF
[V]
Time [s ]
Response time of Sensor in 100 uM uric acid solution without membrane
-6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.00.06
0.08
0.10
0.12
0.14
0.16
0.18
EMF
[V]
Log [concentration]M-6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0
0.040.060.080.100.120.140.160.180.20
EMF
[V]
Log [Concentration]M
(c) (d)
(a) (b)
Figure 3.8: Time response of the ZnO nanowires/Uricase sensor electrodes in 100 µM uric
acid solution (a) without membrane, and (b) with membrane. The calibration curves for the
uric acid sensor (c) with membrane, and (d) without membrane [31].
39
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43
Chapter 4
Synthesis of ZnO nanostructures and fabrication of LEDs Zinc oxide is a unique material with versatile variety of nanostructures and can
be grown easily in many nanostructure forms such as nanorods, nanotubes, nanowalls,
nanoflowers, nanowires, nanobelts, nanorings, nanocages nanosprings, and nanohelixes by
using different approaches such as aqueous chemical growth (ACG), vapor-liquid-solid
(VLS), electro-chemical deposition (ECD), chemical vapor deposition (CVD), metal organic
chemical vapor deposition (MOCVD), physical vapor deposition (PVD), and chemical vapor
transport and condensation (CVTC) [1-21]. ZnO nanostructures used in this research work
were grown on the surface of different p-type substrates (4H-SiC, GaN, Si) by different low
and high temperature growth techniques. The device fabrication can be divided into five steps
as shown in figure 4.1, (1) substrate preparation, (2) growth of nanostructures, (3) bottom
contacts deposition, (4) spinning of photo resist and plasma etching, and finally (5) top
contacts deposition. Details of these fabrication steps are given below.
4.1 Substrate preparation
First of all the substrate preparation is very crucial for achieving high quality
and vertical aligned growth. Commercially purchased substrates from different companies
were cleaned to remove dirty particles, chemicals, and oxide layers from the surface. The
cleaning process follows the following steps; at first the samples (especially Si) were
immersed in DI-H2O: HF (9:1) solution for three minutes to remove native oxide layers from
the surface of the substrates. Then the substrates were immersed in acetone for sonication bath
for 5 minutes at 40 oC and this process was repeated in isopropanol and ethanol as well.
Between these steps the samples were washed with deionised water. After this process the
samples were cleaned with deionised water many time and then dried with nitrogen.
For the growth of ZnO nanorods on Si substrate by the vapor-liquid-solid (VLS)
method, the substrate needs special treatment. In the VLS growth method, a thin film of pure
Au (99.99%) with thickness of 2.0 nm-5.0 nm was deposited on the substrate in a low vacuum
metallization chamber. Au is used as a catalyst metal for growth of ZnO nanostructures and it
has high diffusion rate into the Si at high temperatures. At the start of the VLS growth
process, as the temperature increases to few hundred centigrades, the Au film diffuses into the
44
Si substrate and there leaves no more gold to be used as a catalyst for the growth of ZnO
nanostructures. This problem can be solved by introducing a thin diffusion barrier of SiO2
between the substrate and the Au film. To introduce thin film of native oxide of silicon on the
surface of silicon, at first the Si substrate was immersed for15 min in a preheated solution (at
70 oC) of hydrogen peroxide: ammonium hydroxide: deionized water with ratio of 1:1:5 to
remove the insoluble organic residues that are based on the oxidation desorption and
complexing with the solution. After removing the substrate from the solution, it was cleaned
with deionized water several times. There is a possibility of organic contamination during the
process and it can be removed by immersing the substrate in DI-H2O: HF (100:2) solution. In
the 2nd step the substrate was placed in DI-H2O: HCL: H2O2 solution at 70 oC for ten minutes
to remove metal ions and heavy metal ions. This process produces a high quality layer of
native oxide on the surface of silicon substrate. After that the substrates were cleaned with the
process described earlier [22].
45
Figure 4.1: Schematic illustraion of the device fabrication.
4.2 Growth of ZnO nanostructures
We grow ZnO nanostructures by using the following methods, (1) The vapor-
liquid-solid (VLS) method, (2) The aqueous chemical growth method (ACG), (3) sol gel
method, and (4) Electro-chemical deposition (ECD).
4.2.1 The synthesis of nanorods by vapor-liquid-solid method Recently, the vapor-liquid-solid technique has become very well known
technique for the growth of several types of ZnO nanostructures. In 1964, the theory of this
growth mechanism was proposed and established by R.S. Wagner and W.C. Ellis at the Bell
telephone laboratories [23]. For the growth of ZnO nanorods, using this process a thin film (2
nm - 8 nm) of Au was plated on the substrate in a high vacuum metallization chamber. After
that we used two methods to grow ZnO nanorods with different types of source materials. In
the first method, the source material was prepared by mixing graphite (99.99%) with ZnO
(99.99%) powder in 1:1 ratio.
46
Figure 4.2: A schematic diagram of the vapor-liquid-solid technique.
Figure 4.3: The VLS growth presentation of ZnO nanorods.
47
Graphite acts as a catalyst and it reduces the vaporization temperature of the
source material from ~1300 oC to ~800 oC. The source material was placed into a ceramic
boat and the substrate was placed just above the source material on the boat with face
downward to the source material. Argon gas was used as a carrier gas with a flow rate of 50-
80 sccm (standard cubic centimeters per minute). The growth temperature was 860 oC - 950 oC. The heating system takes 10-12 minutes to reach the growth temperature from room
temperature, but for cooling from growth temperature to room temperature takes about 5
hours. The growth time was from 10 minutes to 30 minutes.
In the 2nd procedure, the source material was pure zinc powder (99.99%). The
zinc powder was heated up under pure oxygen flow. The substrate was placed in the
downstream side on the boat with face down towards the bottom of the boat. It was 1-2 cm
away from the source material. Mixture of argon and oxygen gases with a ratio of 8:1 was
introduced in the quartz tube. The oxygen pressure affects the morphology of the
nanostructures significantly and it should be chosen carefully. Argon was applied as a carrier
gas and oxygen as reactant gas [20, 24-25]. The growth temperature was 600 oC - 750 oC.
The growth process was conducted in a horizontal quartz tube furnace system as
shown in figure 4.2. At first the boat with the source material and the substrate was placed in
the middle of the tube. The quartz tube ends were sealed with rubber rings system and cooling
water passed through the ends of the tubes to cool them during the growth process. Argon
(carrier) and oxygen (reactant) gasses enter from one end and exist from the other end of the
tube. The flow of the gasses in the tube was controlled by gas flow meters.
Around the quartz tube, there was a cylindrical heating chamber with six heating
elements to maintain the temperature constant along the tube in the heating chamber. The
temperature of the tube in the heating chamber was controlled by three heating sensors; one
controls the temperature of the central part and the other two for the ends of the tube in the
heating chamber. The heating sensors are controlled by a three digital display systems. The
growth mechanism of ZnO nanorods by this method is described schematically in figure 4.3.
At high temperature Zn, CO, CO2 gas vapors are produced and the carrying gas transferred
these to the Au droplet surface under the following reaction [26].
ZnO (s) + C(S) → Zn (vapor) + CO (g) at ~ 890 oC
CO (g)+ ZnO(S) → CO2 (g) + Zn (g) at ~ 890 oC
48
The zinc atoms have higher adsorption than CO, CO2 on the surface of Au
droplet. The possible adsorption of CO/CO2 molecules is the substrate- liquid interface [27,
28]. More details of the VLS growth mechanism can be found in [29, 30].
The growth of the nanostructures can be affected by different parameters such
as, growth temperature, flow rate of carrier gas and other gasses, growth time, substrates used,
the distance between source material and substrate, and catalyst metal (Au) thickness. The
ZnO nanorods with different diameters and lengths were grown, as shown in figure 4.4.
The growth temperature affects the density of the ZnO nanowires. At low
growth temperature ~ 800 oC the density of ZnO nanorods is less as compared to higher
growth temperature ~ 950 oC. It may be due to the less concentration of free Zn+2 and O+ at
lower temperature (~ 800 oC) as compared to higher temperature (~ 950 oC). Due to higher
concentration of free Zn+2 and O+ at higher temperatures, the super saturation of Au occurs
earlier and results in a dense ZnO nanorods growth [31]. The length of the nanorods increases
with growth time. We have grown ZnO nanorods from 1.2 µm to 40 µm in length.
Two things are very important for the substrates used for the growth of ZnO
nanostructures by the VLS method. First the substrate must be stable at the growth
temperature and the second is that the used substrate must have less lattice mismatch with
ZnO. GaN has a wurtzite crystal structure which has almost the same structure of ZnO, and
has very less lattice mismatch (1.9 %) [32]. Therefore growth of ZnO nanorods on GaN
produces very good alignment as shown in the scanning electron microscopy (SEM) images
in figure 4.4 (a). 4H-SiC has also wurtzite crystal structure and has a lattices mismatch of (5.5
%) [33] with ZnO, therefore ZnO nanorods have good alignment on 4H-SiC substrate, as
shown in figure 4.4 (b-d). The Si has diamond crystal structure and do not have a good lattice
mismatch (18.6%) [34] with ZnO, therefore the grown ZnO nanorods don’t show good
alignment and grow randomly as shown in figure 4.4 (f, h).
49
(a)
(d)(c)
(b)
Figure 4.4: SEM images for ZnO nanorods grown on different substrates, (a, b) p-GaN, (c-e)
4H-SiC, and (f-h) Si under different growth parameters.
50
4.2.2 Synthesis of nanostructures by the aqueous chemical growth (ACG)
method
4.2.2.1 Synthesis of ZnO nanorods The aqueous chemical growth (ACG) is the most common and simple method
for the growth of ZnO nanorods and it was described by Vayssieres et al. [35]. It is a low
temperature (<100 oC) growth technique. In this method zinc nitrate (Zn(NO3)2 6H2O) is
mixed with hexa-methylene-tetramine (HMT, C6H12N4). An equimolar concentration of HMT
and zinc nitrate (0.010- 0.075 mM) is usually used for growth. The substrate is placed in the
solution with the growth face of the substrate down towards the bottom of the solution
container and the solution container is placed in an oven at 50- 95 ºC for 2-5 hours. The
growth of ZnO nanorods proceeds through the following reactions [36]. At first the HMT
reacts with water and produces ammonia according to the following reaction:
(CH2)6 N4 + 6H2O → 6HCHO + 4NH3
The ammonia produced in the above reaction reacts with water and disassociates
into ammonium and hydroxide ions under the following reaction:
NH3 + H2O → NH4+ + OH-
The hydroxide ions produced in the above reaction react with zinc ions to grow
solid ZnO nanorods on the substrate according to the following reaction:
2OH- + Zn+2 → ZnO (s) + H2O
After the growth, the samples were cleaned with deionized water and dried in
air. The ZnO nanorods grown by the above method have less density and alignment. To
improve the quality of the grown ZnO nanostructures, the growth method is combined with
substrate preparation technique developed by Greene et al. [37]. In this method, ZnO seed
layer is usually spun coated on substrates to form a thin and uniform seed layer. The seed
solution can be prepared by two methods. In the first method 5 mM zinc acetate dihydrate is
diluted in pure ethanol. The ethanol needs to be at least 99% pure otherwise growth will not
lead to well aligned nanorods. The solution is shaken well until all of the zinc acetate crystals
are dissolved. The solution should look transparent; if it is not transparent then it needs more
pure ethanol. The second seed solution is introduced by Womelsdof et al. [38]. Using this
method, we mix 0.01M zinc acetate dihydrate in methanol. The solution should also be
51
transparent. This solution is heated to 60 oC under continuous stirring and we mix 0.03 M
KOH in methanol. We shake the solution until it looks transparent. We add the KOH
solution drop wise to the zinc acetate solution, still under stirring. The resulting solution
should be kept at 60 oC for 2h under stirring. The seed solution is applied to the substrate by
spin coating. In this way we cover the sample with the seed solution and spin it at 5000 rpm.
This process was repeated at least three times to get a uniform layer. Then the substrate is
heated in air at 250 ºC for 20 minutes. By using this treatment, well aligned ZnO nanorods
can be grown. The growth temperature, molar concentration of the HMT and the zinc nitrate,
and growth time affect the length, diameter and density of the grown ZnO nanorods. ZnO
nanorods with different diameters were grown by changing some growth parameters. SEM
images of ZnO nanorods with different diameters ~ 440, 200, 150 and 35 nm are shown in
figure 4.5 (a-d), respectively.
(a)
(d)(c)
(b)
Figure 4.5: Shows the SEM images of ZnO nanorods with different diameters, (a) ~440 nm,
(b) ~200 nm, (c) ~150 nm, and (d) ~35 nm.
52
4.2.2.2 Growth of ZnO nanotubes ZnO nano tubes were achieved from ZnO nanorods. First the ZnO nanorods
were grown by aqueous chemical growth method as described in section 4.2.2.1. The as
grown ZnO nanorods were etched along the growth direction by placing the samples in 5-7.5
molar KCl (potassium chloride) solution for 5-10 hours at 95 ºC. Figure 4.6 (a) shows the
SEM image of the ZnO nanotubes achieved by etching the ZnO nanorods. From the SEM
images, the mean inner and outer diameters of the obtained ZnO nanotubes were
approximately 360 nm and 400 nm, respectively. The Cl-1 ions in the solution of KCL can
easily be adsorbed on the top surface (0001) as compared to the most stable lateral walls
surface (1010) of ZnO. As the Cl-1 ions adsorb on (0001) surface, it decreases the positive
charge density of the surface and makes it less stable to be etched along the c-axis [39].
4.2.2.3 Growth of ZnO nanowalls The ZnO nanowalls structures were grown on p-GaN substrate. Aluminum with
a thickness of 10 nm was uniformly deposited on the substrate in a high vacuum evaporation
chamber then the substrate was spin coated with a ZnO seed solution three times. The spin
speed was 2000 rpm and the spinning time was 45s and the substrate was then annealed at 200
°C for 15 minutes. After that the substrate was inserted in the nutrient solution which
contained equimolar concentration (0.03 M) of zinc nitride hexa-hydrate [(Zn(NO3)26H2O)]
and hexa methylene tetramine (HMT) [(C6H12N4)]. The substrates are kept in the solution
with growth face downward to the bottom of the solution container. The solution was heated
at 90 °C for 2-5 hours to form layered hydroxide zinc acetate (LHZA),
Zn5(OH)8(CH3COO)2.2H2O, as a self template and then after that the LHZA films were
transformed into nanocrystalline ZnO nanowalls without morphological deformation by
heating at 150 °C in air [40-41]. The SEM image of the grown ZnO nanowalls is shown in
figure 4.6 (b).The thickness of the walls was estimated from the SEM images and it was
found to be around 50 nm.
4.2.2.4 Growth of ZnO nanoflowers ZnO nanoflowers were grown on p-GaN substrates by placing the substrates in
an aqueous solution of 0.025 M zinc nitrate and ammonium solution (NH4OH). The
ammonium solution was used to control the pH of the solution. The pH of the solution to
grow the nanoflowers was around 10.5. The substrate is placed in the solution with the growth
surface downward to the bottom of the solution container, and heated at 90 ºC for 3-6 hours
53
[42]. Figure 4.6 (c) shows the SEM image of the grown ZnO nanoflowers. The length of the
leaves of the flowers is about 2.2 μm and the diameter is about 500 nm.
(c)
(b)(a)
Figure 4.6: SEM images of (a) ZnO nanotubes, (b) ZnO nanowalls, and (d) ZnO nanoflowers.
4.2.3 Growth of ZnO nanorods by sol-gel method Sol-gel method is also very common technique for the growth of well aligned
ZnO nanorods at low temperature (<100 oC) [43]. In this method a sol-gel seed layers solution
was prepared by mixing zinc acetate (Zn(CH3COO)2 2H2O; 0.7 M) solution in the mixture
solution of 2-methoxyethanol and monoethanolamine (MEA) with equimolar ratio. Then the
resultant solution was stirred at 60 oC for 30 min to yield a clear and homogeneous solution.
Sol-gel solution was spun coated on the surface of the substrate with a spin speed of 3000 rpm
for one minute. This process was repeated three times. The samples were then annealed at 500
oC in air for 1 h [44]. After the annealing, the substrate was put in zinc nitrate
(Zn(NO3)26H2O) and hexa-methylene-tetramine (HMT, C6H12N4) solution prepared with
54
equimolar concentration for 4-8 hrs. The SEM image of resulting ZnO nanowires grown
through sol gel route is shown in figure (4.7).
Figure 4.7: SEM image of grown ZnO nanorods by the sol-gel method.
4.2.4 Synthesis of ZnO nanorods by the electro-chemical deposition (ECD)
Using this growth method, ZnO nanorods were grown on p-GaN substrate. The
substrate was spun coated with ZnO seed solution three times at a spin speed of 2000 rpm for
45s and then annealed at 200 °C for 15 minutes. Then it was immersed in equimolar solution
of zinc nitrate (Zn(NO3)26H2O) and hexa-methylene-tetramine (HMT, C6H12N4). The solution
was then saturated with pure oxygen by bubbling it for 1 hour before starting the growth
process. The substrate was used as working electrode while a platinum wire was used as a
counter electrode. A constant bias of 1.1 V was applied between the electrodes for 2 hours at
90 °C. More details can be found in [45]. SEM image of grown ZnO nanorods is shown in
figure (4.8).
55
Figure 4.8: The SEM image of grown ZnO nanorods by Electro-Chemical deposition (ECD)
method.
4.3 Bottom contacts deposition
The reliability and durability of ZnO based devices depend on the development
of metallization schemes with low specific contact resistance. The contacts with low
resistance between a semiconductor and a metal play a vital role in the electro-optical and
electrical characteristics and affects the life time of the devices. High contact resistance can
influence the performance of the device [46]. Ohmic contacts serve to inject and exit the
electrical current into the device and should ideally have symmetric and linear I-V relation. In
this research work different metal alloys were used to form the bottom contacts to the p type
GaN, 4-H-SiC, Si substrates. For the GaN, Pt/Ni/Au alloy with thickness of Pt, Ni, and Au
layers of 20 nm, 30 nm, and 80 nm, respectively were used. The sample was annealed at 350
ºC for 1 min in a flowing nitrogen (N2/Ar) gas atmosphere. This alloy gives a minimum
specific contacts resistance of 5.1×10-4 Ω-cm-2 [52]. Different ohmic contacts schemes for the
p-GaN are shown in table 4.1. For 4H-SiC, Ni/Al alloys with thickness of Ni and Al layer of
50 nm and 400 nm were used to form ohmic contacts. The samples were annealed in nitrogen
(N2/Ar) gas at 500 ºC for 2 minutes. This contact gives minimum specific contact resistance
of 7.9× 10-6 Ω-cm-2 [47]. Al layer of thickness 150 nm was used to form ohmic contact to the
p-Si substrates. Different ohmic contacts schemes for p-GaN are shown in table 4.1.
56
Table 4.1: Different ohmic contacts schemes for p-type GaN
Metallization Annealing temperature Lowest ρc (Ω- cm-2
) Ref
Pd/Au 500 oC 4.3×10-4 48
Pt/Au 750 oC 1.5×10-3 49
Ni/Pd/Au 550 oC 4.5×10-6 50
Ni/Au 750 oC 3.3×10-2 49
Pd/Pt/Au 600 oC 5.5×10-4 51
Pt/Ni/Au 350 oC 5.1×10-4 52
Pd/Ni/Au 450 oC 5.1×10-4 53
Ni/Au 500 oC (annealed under partial O2 ambient to form NiO)
1.0×10-4 54
4.4 Spin coating of photo resist and plasma etching
To fill the gaps between the nanorods and to isolate electrical contacts on the
ZnO nanorods from reaching the p-type substrate, an insulating photo-resist layer was spun
coated on the ZnO nanorods. It also helps to prevent the carrier cross talk among the
nanorods. After the deposition of the insulating photo-resist, the top of the ZnO nanorods
were exposed by using oxygen plasma ion etching.
4.5 Top contacts deposition
Non-alloyed Pt/Al metal system was used to form the ohmic contacts to the ZnO
nanostructures. The thickness of the Pt and the Al layers were 50 nm and 60 nm, respectively.
This contact gives a minimum specific contact resistance of 1.2×10-5 Ω -cm-2 [61]. Different
ohmic contacts schemes for n-ZnO are shown in table 4.2.
57
Table 4.2: Different ohmic contacts schemes for n-type ZnO
Metallization scheme Annealing temperature
Lowest ρc (Ω- cm-2
) Ref
Ti/Al/Pt/Au 200 oC 3.9×10-7 55, 56
Ti/Au 300 oC 2.0×10-4 57, 58
Ti/Au 600 oC 3.0×10-3 59
Ti/Au 500 oC 1.0×10-3 57, 58
Ti/Au None 4.3×10-5 60
Al/Pt None 2.0×10-6 61,62
Ti/Al/Pt/Au None 8.0×10-7 55, 56
Al None 4.0×10-4 61
Zn/Au 500 oC 2.36 ×10-5 63
In None 7 ×10-1 64
Re/Ti/Au 700 oC 1.7 ×10-7 65
Ru 700 3.2 ×10-5 66
Pt-Ga None 3.1 ×10-4 67
58
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61
Chapter 5 Experimental and characterization techniques
Different experimental and characterization techniques were used to investigate
the morphologycal, structural, optical, electrical and electro-optical properties of different
ZnO nanostructures and light emitting diodes (LEDs). We have used the following
characterization and experimental techniques:
(1) Scanning electron miscropy (SEM), (2) Atomic force microscopy (AFM),
(3) X-ray diffraction, (4) Photoluminescence (PL), (5) Electroluminescence (EL), (6)
Electrical characterization, (7) Annealing study, (8) Color rendering index extraction, and (9)
Irradiation study.
5.1 Scanning electron microscopy
Scanning electron microscopy (SEM) is an important tool to investigate the
surface and morophology of nanostructures. By using it we can estimate the diameter, length,
thickness, density, shape and orientation of the nanostructures. Figure 5.1 s hows the
schematic diagram of the SEM apparatus. An electron gun made of tungsten or LaB6 filament
is used to emmit electrons by heating and these emitted electrons are directed and focused on
the sample by using an anode and various electromagnetic lenses between the electron gun
and sample. These projected electrons eject secondery and back scattered electrons after
hitting the sample. These ejected secondary and back scattered electrons from the sample are
detected by detectors and these detectors tranfer these detected electrons into electronic signal
which is sent to a computer to display the image.
62
Figure 5.1: Schematic diagram of the scanning electron microscopy.
63
JEOL JSM-6301F and LEO 1550 scanning electron miscropes were used in this
research work. During the measurements the vaccuum pressure in the chamber was about 10-6
mbar and the electron gun voltage was 12 kV. The maximum resolution of the SEM was up to
10 nm. Figure 5.2 (a-d) shows the SEM images of ZnO nanostructures on p-GaN substrate
grown by low temperature techniques. The SEM image shows that well aligned hexagonal
vertical ZnO nanorods and nanotubes were grown with high density. The grown nanorods and
nanotubes cover the whole substrate uniformly. The grown nanorods and nanotubes had a
uniaxial orientation of (0001) with respect to the substrate. From the SEM images we
estimated that the mean diameters of our as grown ZnO nanorods was approximately 175 nm
and the length of the both nanorods and nanotubes is same (1.2 μm ). The nanotubes were
achieved by chemical etching of the nanorods. The average thickness of the nanowalls was
estimated to be around 50 nm and length of the nanoflower leaf was about 2.2 μm.
Figure 5.2: Typical SEM images of different ZnO nanostructures (a) nanorods, (b)
nanoflowers, (c) nanotubes, and (d) nanowalls.
64
5.2 Atomic force microscope
Atomic force microscope (AFM) is a high resolution scanning probe
microscopy. It is a useful tool used by the research community in material science to
investigate the surface profiles in three dimensions at the nanoscale level and it is also used in
manuplating atoms. AFM can not only scan the surface of a material at the nanoscale level but
it can also measure the force at nano-newton scale [1-2]. AFM is superior to SEM in many
aspects, It provides three dimensional image, in AFM the samples need no preparation to
improve the conductivity of the surface as it is needed for the SEM, therefore it is very useful
in the study of biological surfaces.
Figure 5.3 shows a 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN.
It shows that the ZnO nanorods are dense and grown uniformly on the substrate. The avearge
height of the nanorods is about 1µm and the average diameter is about 150 nm.
Figure 5.3: A 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN.
5.3 X-ray diffraction
The interatomic distance in solids is approximately one angstram (Å) and the
energy coresponds to this distance can be calculated as
E= hc/λ for λ= 1 Å
E ≈ 12×103 eV
65
If the structure of the solid is to be probed by electromagnetic waves then these
waves should have energy at least equal to the interatomic distance energies. X-rays have
approximately same energy (keV) as the interatomic distance energies of solids. X-ray
diffraction (XRD) is the most common characterization technique to investigate the
composition, quality, lattice parameters, orientation, defects, stress and strain in
semiconductor materials. After the discovery of X-rays in 1895, a new way was possible to
probe the crystalline materials at the atomic scale. In 1913 W.H and W.L. Bragg found that
the X-rays gave characteristics patterens after reflection from crystalline materials and these
patterens were different from those that were obtained from liquids. Later on the x-ray
diffraction patterens became an important tool for the material science research.
Figure 5.4 shows a schematic diagram of Bragg refelection from crystalline
planes having interplan distace d. The incident and reflected x-rays from the two planes are
also shown.
Bragg concluded that the path difference between the two x-rays diffracted from
two consective lattice planes is 2dsinθ and it leads to Bragg’s law, which states that the
condition for diffraction of X-rays for a crystalline material is:
nλ= 2dsinθ
Here θ is the angle of incidence and λ is the wavelength of the x-rays, n is an integer and it
is the order of refelection, and d is the distance between the lattice planes.
Figure 5.4: Shows a schematic diagram of Bragg reflection from crystalline lattice planes
having interplan distace “d” between two lattice plane.
66
Figure 5.5 (a-d) show the XRD patterns for different ZnO structures (nanowalls,
nanorods, nanoflowers and nanotubes), respectively. From the XRD spectrum, a strong (002)
peaks and weak (004) peaks indicate clearly that a hexagonal-shaped ZnO nanowalls,
nanotubes, nanorods and nanowalls were obtained along the c-axis. The (002) peak has a
strong and sharp intensity and narrower spectral width as compared to the other peaks
obtained in the XRD pattern. This indicate that the grown ZnO naostructures have a high-
quality wurtzite hexagonal ZnO phase [3-4].
Figure 5.5: Display the θ-2θ XRD spectra of ZnO (a) nanowalls, (b) nanorods, (c)
nanoflowers, and (d) nanotubes grown on p-GaN substrates, respectively.
5.4 Photoluminescence
Photoluminescene (PL) is a non-destructive technique used for the investigation
of extrinsic and intrinsic properties of semiconductors. In this technique the semiconductor
under investigation is excited optically and then the PL spectrum of the spontanious emission
from radiative recombinations in the semiconductor band gap is obtained. During the PL
process both radiative and nonradiative recombinations happen but in this research work we
have studied the radiative recombinations only. The PL thechnique can be used to dtermine
the bandgap, impurity levels, defects dectection and recombination process in the
semiconductor materials. More details can be found in [5]. In this research work, PL
measurements were performed at room temperature by using laser lines with a wavelength of
30 40 50 60 70 800
100000
200000
300000
400000
500000
600000
700000
ZnO (004)
ZnO (002)
Inte
nsity
(a.u
.)
2 Theta (deg.)30 40 50 60 70 80
0
500
1000
1500
2000
2500
ZnO (004)
ZnO (002)
Inte
nsity
(a.u
.)
2 Theta (deg.)
30 40 50 60 70 800
1000
2000
3000
4000
5000
6000
ZnO (002)
ZnO (002)
Inte
nsity
(a.u
.)
2 Theta (deg.)
(a) (b)
(c)
30 40 50 60 70 800
1000
2000
3000
4000
5000
6000
7000
ZnO (004)
ZnO (002)
Inte
nsity
(a.u
.)
2 Theta (deg.)
(d)
30 40 50 60 70 800
100000
200000
300000
400000
500000
600000
700000
ZnO (004)
ZnO (002)
Inte
nsity
(a.u
.)
2 Theta (deg.)30 40 50 60 70 80
0
500
1000
1500
2000
2500
ZnO (004)
ZnO (002)
Inte
nsity
(a.u
.)
2 Theta (deg.)
30 40 50 60 70 800
1000
2000
3000
4000
5000
6000
ZnO (002)
ZnO (002)
Inte
nsity
(a.u
.)
2 Theta (deg.)
(a) (b)
(c)
30 40 50 60 70 800
1000
2000
3000
4000
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7000
ZnO (004)
ZnO (002)
Inte
nsity
(a.u
.)
2 Theta (deg.)
(d)
67
266 nm from a diode laser pumped resonant frequency doubling unit (MBD266) as an
excitation source.
A schematic diagram of the PL setup used in this research work is shown in
figure 5.6. The PL measurement has three main steps,
(1) In the first step, the semiconductor is optically excitated to create electron-hole pairs.
Different kinds of lasers such as He-Cd laser and Ar+ laser with wave lengths of 325 nm
and 316 nm are comonnly used for the excitations in ZnO. The laser beam is then
projected on the semiconductor sample with the help of a setup as shown in the schematic
diagram.
(2) In the second step, the excited electron-hole pairs recombine radiatively and emit light.
(3) In the final step, the emitted light is detected and dispersed by a double grating
monochromator and photomultiplier detectors. The final spectrum is collected and
analyzed in a computer.
68
Figure 5.6: A schematic diagram of the PL setup.
Figure 5.7: A typical I-V characteristics for different ZnO (nanostructures)/p-GaN LEDs [3].
-8 -6 -4 -2 0 2 4 6 8-0.10.00.10.20.30.40.50.60.70.80.91.0
-8 -6 -4 -2 0 2 4 6 81E-3
0.01
0.1
1
Curre
nt (m
A)
Voltage (V)
Curre
nt (m
A)
Voltage (V)
nanotubes nanorods nanoflowers nanowalls
69
During recombination, the electron-hole pairs in the semiconductor can
recombine radiatively in many ways. It can be band-to-band transition or can be donor-
accepter recombinations in the band gap. The direct band gap semiconductors have high
possibility of band-to-band electron-hole recombinations. Therefore direct band-gap
semiconductors are attractive for LEDs. The band-to-band recombination rate is high at high
temperatures because of all shallow defects ionized at high temperatures, and at low
temperatures free-to-bound recombination rate dominates because at low temperature free
carriers bound to the defects. Both donors and accepters can also be found in many
semiconductors. In these semiconductors, some accepters capture electrons from the donors
and in consequence it contains ionized accepters and donors. These ionized donors and
accepters trap the carriers and introduce neutral donor and accepter excitons in the band gap.
There are free and bound excitons and both increase the transitions. In the donor-accepter pair
transitions, the electrons recombine with the holes in neutral hole centers. The free exciton PL
peaks in ZnO are due to recombinations of electrons and holes. Its transition energy is slightly
less than the band gap energy. The emission in bound excitons is due to recombination of
electrons and holes in the excitons bound to donors or accepters. Its transition energy is
slightly less than the free exciton recombination energy [6].
The excitons are very stable in ZnO at room temperature and give very strong
near band edge emission. In the radiative recombinations in ZnO, the excitonic emission
dominates the band-to-band transitions and the exciton binding energy is high compared to
other direct band gap semiconductors such as GaN. In the PL spectra at room temperature,
free excitons emission dominates but at low temperature bound excitons emission dominates
the excitonic emissions in ZnO. The free exciton PL peak is oftenly centered approximately
around 376-387 nm in ZnO. The broad emission peaks in the visible region from 420-750 nm
in ZnO belongs to deep level defects and these defects are present in the band gap at different
energies.
5.5 Electroluminescence
The electroluminescence (EL) is an electro-optical phenomenon with electrical
input and optical out put. In this phenomenon, an electrical current input is applied across the
material and it emits light. In this phenomenon, the emission spectrum is the result of
electron-hole radiative recombinations in the semiconductor. The electroluminescence is a
figure of merit for the LEDs. By improving the EL emission of the LEDs the efficiency can
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be improved. Different key factors such as charge injection, charge transport and charge
recombination contribute to the efficiency of the LEDs.
In this research work the electroluminescence of n-ZnO nanostructures/p-GaN,
p-4H-SiC LEDs is investigated for a possible future application of ZnO nanostructures in
white LEDs based lighting. The EL measurements were performed at room temperature. A
photomultiplier detector was used to detect the emitted light from the fabricated LEDs. The
fabricated LEDs were illuminated by applying a forward biasing injection current across the
electrodes by using a kiethely 2400 apparatus with the help of the metal probes. The spectra
were measured from the top contacts of the LEDs by detecting the light escaping from the
edge of the top contact electrode.
5.6 Electrical characterization
Semiconductor parameter analyzer (SPA) is a useful instrument for the electrical
characterization of different semiconductors based devices such as, LEDs, transisters,
Schotkey diode, logic gates, and so on. In this research work, it was used for electrical
characterization of the fabricated ZnO nanostructures based LEDs. The current versus voltage
(I vs V) measurements were performed for the fabricated LEDs. The I-V curves provide
different information about the LEDs such as its turn-on voltage, forward and reverse
currents, different current regims, break-down voltage, ideality factor, parallel and series
resistance of the fabricated LEDs.
Figure 5.7 [3] shows the I-V curve of the different n-ZnO nanostructures/p-GaN
LEDs. The I-V curves show rectifying behavior as expected from these LEDs. It indicates
clearly that reasonable p-n heterojunctions were achieved. The turn-on voltage of these
heterojunctions LEDs is around 4 V.
5.7 Annealing studies
Annealing is an important approach used by material researchers comunity to
change the material properties and to modify the defects in the material. In this research work,
we used post-growth annealing of ZnO nanrods and nanotubes. In this annealing study, ZnO
nanorods and nanotubes were annealed at 600 oC and then cooled down to room temperature.
The samples were kept at annealing temperature for 30 minutes. The post-growth annealing
was done in different ambients introduced in the annealing chamber. These ambients were
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argon, air, oxygen and nitrogen. These annealing ambients introduce a control on the intrinsic
defects and consequently the luminescence properties can be controlled.
In paper III, the origin of the red emission in ZnO was investigated by using the
post-growth annealing of ZnO nanotubes in different ambients and in paper IV, the post-
growth annealing technique was used to investigate the effect of different post-growth
annealing ambients on the color rendering properties of ZnO nanorods based LEDs.
5.8 Color rendering extraction
One of the most important requirement when using LEDs for indoor and
outdoor lighting applications is that the color rendering indices (CRIs) of these LEDs should
be high. Color rendering is the ability of a light source to render the true colors of the objects
in the enviorment. The Sun or the black body radiator are used as a reference and these have
maximum value of color rendering inex that is 100.
Color rendering indices (CRIs), correlated color temperature (CCTs),
chromaticity coordinates in CIE 1931 color space, and values of the set of fourteen test-color
samples were extracted from the emitted spectra of the fabricated LEDs by using a computer
software. These color parameters can be extracted from EL spectra or from the power spectra
(spectral power distributions of the radiation emitted by the LEDs). A photomultiplier
detector was used to measure the EL spectra of the the emitted light from the fabricated
LEDs. The fabricated LEDs were illuminated by applying a forward biasing injection current
across the electrodes by using the kiethely 2400 apparatus with the help of the metal probes.
The spectra were measured from the top contacts of the LEDs by detecting the light escaping
from the edge of the top contact electrode. In order to measure the spectral power
distributions of the fabricated LEDs, the electroluminescence was measured by using fiber
coupled CCD spectrometer Hamamatsu C7374. For the calibration of the spectrometer a
calibration lamp Bentham CL2 was used. Light can be precisely characterized by the power in
each wavelength in the visible spectrum. The measured spectral power distribution (SPD)
spectrum contains all the basic physical data necessary for the quantitative analysis of the
color. The chromaticity coordinates, color rendering indices and correlated color temperatures
were calculated from the spectra emitted by the LEDs [7].
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5.9 Irradiation studies
ZnO has unique and very attractive material properties for applications in
variety of devices such as, LEDs, bio-sensors, UV detectors, gas sensors and nano-generators
and so on. It is very interesting to investigate the effect of internal and external phenomenon
on the performance and reliability of these ZnO based devices. In this research work,
influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods was
investigated. When energetic particles (electrons or ions) hit a material they change the
properties of the targeted material and change the density of defects in the material [8]. The
purpose of this study was to investigate the reliability of ZnO nanorods based LEDs in space
and nuclear applications.
The ZnO nanorods and LEDs based on these nanorods were irradiated by using
2.0 MeV He+ ions with fluences of ~ 2×1013 ions cm-2 and ~ 4×1013 ions cm-2. All the
samples were irradiated at room temperature under normal incidence at the Tandem
Laboratory, Uppsala University Sweden. The ion beam flux was about 6×1010 ions s-1 cm-2
and the beam spot was roughly 1 cm2. The projected ion range was calculated by the
SRIM2008 code [9] to be 4.9 μm assuming a density for ZnO of 5.6 g/cm3, so the major part
of the whole ZnO nanorods were influenced by the beam.
73
References [1] http://en.wikipedia.org/wiki/Atomic_microscopy
[2] http://www.chembio.uoguelph.ca/educmat/chm729/afm/introdn.htm
[3] N. H. Alvi, S. M. Usman Ali, S. Hussain, O. Nur, and M. Willander, Scripta Materiala 64,
697 (2011)
[4] L. B. Freund, and S. Suresh (Eds.), Thin Film Materials, Cambridge University Press,
Cambridge, (2003)
[5] G. D. Gilliland, Mater. Sci. Eng. R18, 99 (1997)
[6] Peter Klason, Zinc oxide bulk and nanorods, A study of optical and mechanical properties,
PhD thesis, University of Gothenburg., (2008)
[7] N. H. Alvi, K. ul Hasan, M. Willander, and O. Nur, (Accepted in Lighting Research and
Technology) DOI: 10.1177/1477153511398025
[8] A. V. Krasheninnikov, and K. Nordlund, J. Appl. Phys. 107, 071301 (2010)
[9] SRIM (Stopping and Range of Ion in Matter) Software: http://www.srim.org.
74
75
Chapter 6 Results
Some of the results achieved in my research work are presented in this chapter.
The results are divided into three sections; in the first section, results from papers I-IV are
included. In this section a discussion about luminescence properties of ZnO nanorods grown
on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods,
nanotubes, nanoflowers and nanowalls) on the same substrates are included. The effect of
post-growth annealing on deep level emissions and color rendering index properties of ZnO
nanorods and nanotubes based LEDs are also investigated in this section.
In the second section, the junction temperature of n-ZnO nanorods based LEDs
at the built-in potential was modeled and experiments were performed to validate the model.
In the third section, the influence of helium (He+) ion irradiation on the luminescence
properties of ZnO nanorods based LEDs is presented.
6.1 Luminescence properties of ZnO nanostructures
In paper 1, the luminescence properties of ZnO nanorods grown on two different
substrates (p-4H-SiC, p-GaN) were investigated [1]. The purpose of the growth of ZnO
nanorods on two different substrates was to compare the luminescence from these ZnO
nanorods. The main purpose of this comparison was to know the contribution of the substrate,
the nanorods and the p-n junction in the luminescence spectra (especially EL spectra) of the
fabricated LEDs.
ZnO nanorods were grown by the aqueous chemical growth (ACG) technique.
This growth method has been described already in details in chapter 4. The luminescence
properties were carried out by photoluminescence (PL) measurements of the grown ZnO
nanorods and the electroluminescence (EL) measurements of the fabricated LEDs based on
these grown nanorods. Both measurements were performed at room temperature. Figure 6.1
(a, b) shows the PL spectra of ZnO nanorods grown on p-4H-SiC and p-GaN substrates,
respectively. Three emission peaks are observed. The band edge emissions are approximately
centered at 387 nm (3.20 eV), 383 nm (3.23 eV). A broad green and a orange-red deep level
emission peaks are approximately centered at 550 nm (2.25 eV) and 693 nm (1.78 eV),
76
respectively [1]. The band edge emissions at 387 nm (3.20 eV) and 383 nm (3.23 eV) in ZnO
nanorods show that ZnO nanorods are of good crystalline quality [2].
The EL spectra of the fabricated ZnO nanorods/p-4H-SiC, p-GaN LEDs at room
temperature is shown in figure 6.2 (a, b) [1]. A forward bias with injection current of 6 mA
and 4 mA was applied to the ZnO nanorods/p-4H-SiC, p-GaN LEDs, respectively. The EL
spectrum of the n-ZnO nanorods/p-4H-SiC showed violet-blue, green and orange-red
emissions that are approximately centered at 430 nm (2.88 eV), 531 nm (2.33 eV) and 665 nm
(1.86 eV), respectively. The EL spectrum for the ZnO nanorods/p-GaN LED showed violet-
blue, green, and orange-red, emissions that are approximately centered at 457 nm (2.71 eV),
531 nm (2.33 eV) and 665 nm (1.86 eV), respectively [1].
The comparison study of the EL and the PL spectra of these nanorods based
LEDs on different substrates helps to understand the luminescence properties of ZnO. It was
found that there is a consistency between the PL and the EL spectra of both LEDs but it can
be observed that the violet-blue peak in the EL spectrum is absent from the PL spectra. It
indicates that the violet-blue emission is not originating from the ZnO nanorods. It was also
found that the violet-blue emission in both LEDs has different position, the peak position for
n-ZnO nanorods/p-4H-SiC was approximately centered at 430 nm (2.88 eV) and for n-ZnO
nanorods/p-GaN LEDs it was centered at 457 nm (2.71 eV). This difference in peak position
is may be due to the effect of the substrate. Moreover p-GaN is magnesium doped and deep
magnesium accepters are commonly considered responsible for blue emission in GaN. ZnO
nanorods/p-4H-SiC LED also emits violet-blue emission and it suggests that this peak is
originating from the heterojunctions or from the substrate. Zhang et al. [3] also suggested that
this peak is originating from the heterojunction of ZnO nanorods/p-GaN LEDs. It can also be
seen clearly that violet-blue emission in ZnO nanorods/p-GaN LED has high EL intensity as
compared to n-ZnO nanorods/p-4H-SiC LED. This is also suggested that it is due to
difference of the substrate. In ZnO nanorods/p-GaN LEDs, p-GaN substrate also contributes
to the blue emission in the EL spectrum. The peak positions of the green and the orange-red
emissions are approximately centered at the same position in both LEDs. This suggests that
these emissions are from the ZnO nanorods and the substrates have no contribution or have
very little contribution to these emissions. The EL spectra of both LEDs showed very bright
white light as observed by the naked eyes during EL measurements. The measured
luminescence spectra is the result of the combination of three emission lines which are the
violet-blue, the green, and the orange-red peaks observed from both LEDs [1].
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Figure 6.1: (a) Room temperature photoluminescence spectrum for the ZnO nanorods on p-
4H-SiC substrate, and in (b) the room temperature photoluminescence spectrum for the ZnO
nanorods on p-GaN substrate [1].
Figure 6.2: (a) Electroluminescence spectrum for ZnO nanorods/p-4H-SiC LED, and in (b)
electroluminescence spectrum for the ZnO nanorods/p-GaN LED [1].
In Paper II, the luminescence properties of different ZnO nanostructures grown
on the same (p-GaN) substrate were investigated [4]. The purpose of this study was to
investigate the difference in luminescence properties of different ZnO nanostructures grown
on the same substrate and was to know which ZnO nanostructure has the best luminescence
properties to be used as possible candidate for ZnO based white LEDs. The different ZnO
nanostructures (nanowalls, nanorods, nanoflowers and nanotubes) were grown by the aqueous
chemical growth (ACG) technique. The electrical, optical and electro-optical characteristics
of these nanostructures were comparatively investigated. The luminescence properties were
investigated by measuring the PL and the EL spectra at room temperature [4].
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Figure 6.3 (a) [4] shows the PL spectra for ZnO nanowalls structure. The PL
emission peaks are centered approximately at 361 nm (3.43 eV), 378 nm (3.29 eV), and 490
nm (2.53 eV). The observation of the band-edge emission peak at 378 nm (3.29 eV) showed
that the as grown ZnO nanowalls have good crystalline quality [2]. The deep level emission
approximately centered at 490 nm (2.53 eV) can be attributed to oxygen vacancies (Vo)
related defects [5]. Figure 6.3 (b-d) [4] shows the PL spectrum for the ZnO nanoflowers,
nanorods, and nanotubes. The PL intensity peaks approximately centered at 376 nm (3.29 eV)
and a broad peak (covering the visible region from 400 nm to 700 nm) were observed in all
three nanostructures. The emission at 361 nm (3.43 eV) in the nanowalls structures can be
attributed to the band edge emissions in p-GaN substrate and the emission of this peak shows
that the commercially purchased p-GaN substrates were of high optical quality. It was
detected only in ZnO nanowalls as the nanowalls structures are not so dense on the substrate
therefore during the PL measurement the excitation laser beam reach the substrate easily [4].
From the comparison of the PL spectra, as shown in figure 6.3 (e) it was found
that among all grown ZnO nanostructures, ZnO nanorods have the highest excitonic band
edge PL emissions at 3.29 eV which indicates that ZnO nanorods have the best crystal quality
as compared to all other nanostructures. But in the visible region the nanowalls structures
have the higher PL intensity as compared to other nanostructures. This indicates that the
nanowalls structure have more density of deep level defects as compared to the other three
nanostructures [4].
Figure 6.4 (a-d) [4] shows the room temperature EL spectra for LEDs based on
the four different nanostructures. During the EL measurements, the LEDs were under forward
biasing of 25 V with injection current of 4 mA. Figure 6.4 (a) [4] shows the EL spectra of the
n-ZnO nanowalls/p-GaN LEDs and a violet, a violet-blue and a broad EL peaks covering the
visible region from 480 nm to 700 nm were observed. The EL peaks were approximately
centered at 420 nm (2.95 eV), 450 nm (2.75 eV) and a broad peak covering EL emissions
from 480 nm to 700 nm. The broad peak includes the green, yellow, orange and red
emissions. Figure 6.4 (b) [4] shows the EL spectrum for n-ZnO nanoflowers/p-GaN LED and
the violet, violet-blue and the broad peaks were observed. These observed peaks were
centered approximately at 400 nm (3.09 eV), 450 nm (2.75 eV) and a broad peak covering the
EL emission from 480nm to 700 nm. Figure 6.4 (c, d) [4] shows the EL spectra for the LEDs
based on n-ZnO nanorods, nanotubes, respectively. Both LEDs have the same spectra because
the ZnO nanotubes were achieved from ZnO nanorods by chemical etching. There is only
79
difference of the charge injection surface area and the ZnO nanotubes have more surface area
as compared to the ZnO nanorods. The electrons can also be injected from the inner side of
the hallow nanotube and this increases the emission intensity. A violet, a violet-blue and a
green EL peaks were observed in both LEDs and these peaks were centered approximately at
400 nm (3.09 eV), 450 nm (2.75 eV) and 540 nm (2.29 eV), respectively [4].
From the comparison of the PL spectra of all the LEDs it was found that the
nanorods have the highest PL intensity peak in the ultra-violet (UV) region when it was
compared to the PL intensity of the all other nanostructures. It means that ZnO nanorods have
good crystal quality and can be attractive for UV diodes. ZnO nanowalls structures have the
highest PL intensity emission in visible region when compared to other nanostructures. In
ZnO the deep levels defects are responsible for emissions in the visible range. This means that
ZnO nanowalls have the highest density of deep level defects as compared to all other
nanostructures [4].
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(a) (b)
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Figure 6.3: Room temperature photoluminescence spectrum for ZnO nanostructures (a)
nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes on p -GaN and (e) shows the
combine PL spectra of all the four nanostructures [4].
From the comparison of the EL spectra of the LEDs based on t hese different
nanostructures it was found that the nanorods and the nanotubes have identical EL spectra.
The nanotubes have the highest EL intensity in the visible region as expected because
nanotubes have more charge injection surface area under the contact as compared to other
nanostructures. From the comparison of the PL spectra it was found that the nano-walls
81
structures have the highest defect related emissions but have small charge injection surface
area under the contact as compared to other nanostructures. ZnO nanotubes are denser under
the contact as compared to nanowalls. It can also be seen from the I-V curves for all the LEDs
as shown in figure 6.5. The LEDs based on Z nO nanotubes showed the highest current as
compared to all other LEDs. The same results were obtained when we investigated the effect
of the post-growth annealing on the color rendering properties of ZnO nanorods and nanotube
based LEDs and ZnO nanotubes based LEDs showed higher EL emission intensity as
compared to ZnO nanorods based LEDs under the same injection of current [4].
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Figure 6.4: Displays the electroluminescence spectra for n-ZnO (nanostructures)/p-GaN
LEDs, in (a) nanowalls, (b) nanoflowers (c) nanorods, and (d) nanotubes, (e) shows the EL
spectra of all the nanostructures, and (f) shows the CIE 1931 x, y chromaticity space of ZnO
nanostructures based LEDs [4].
Figure 6.5: Typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs as
indicated in the figure [4].
-8 -6 -4 -2 0 2 4 6 8-0.10.00.10.20.30.40.50.60.70.80.91.0
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By comparing the EL spectra of ZnO nanorods/p-GaN LEDs in paper I and
paper II it was found that the dominating EL peak in paper I is the orange-red peak which is
approximately centered at 665 nm (1.86 eV) while in paper II the dominating EL peak is the
green peak which is approximately centered at 540 nm (2.29). We have investigated the cause
of this difference and it was found that during the fabrication of the LEDs in paper 1, the ZnO
nanorods were annealed in nitrogen ambient after the growth for the deposition of the bottom
contacts. The samples were annealed at a temperature of 350 oC in nitrogen ambient for 2
minutes. For annealing the bottom contacts, we use a vapor-liquid-solid (VLS) growth
chamber and it takes 4 minutes to reach 350 oC but it takes a minimum of three hours to cool
down. Later on from annealing studies we found that nitrogen ambient play an important role
in shifting the dominating peak from the green region towards the red region [6]. It will be
described in details in the annealing studies in the coming section. The EL spectra in paper V
also support this argument where ZnO nanorods/p-4H-SiC, p-GaN LEDs were annealed in
different ambients and due to this difference both LEDs show different dominating peak, The
ZnO nanorods/p-4H-SiC LED was annealed in nitrogen ambient and it gave a dominating EL
peak centered approximately at 674 nm and the ZnO nanorods/p-GaN LED was annealed in
argon ambient and it gave a dominating EL peak approximately centered at 550 nm.
Another difference in the EL spectrum of ZnO nanorods based LEDs is that the
violet emission at 400 nm is absent in paper I. It is also due to the annealing effect as we
investigated in post-growth annealing studies that after annealing the ZnO nanotubes in
nitrogen ambient the violet emission at 400 nm almost disappear as compared to the as grown
ZnO nanotubes based LEDs [6].
The color rendering indices (CRIs) and correlated color temperatures (CCTs) for
all the LEDs were also extracted and it was found from the comparison that the nanowalls
based LEDs have the highest color rendering index (CRI) with a value of 95 with a correlated
color temperature of 6518 K while the nanorods have the lowest CRI reaching 87 with a
correlated color temperature of 4807 K. Figure 6.3 (f) shows the CIE 1931 color space
chromaticity diagram in the (x, y) coordinates system. The chromaticity coordinates for LEDs
based on ZnO nanowalls, nanorods, nanoflowers, and nanotubes are (0.3131, 0.3245),
(0.3332, 3470), (0.3558, 0.3970), and (0.3555, 0.3935), respectively. The chromaticity
coordinates for the ZnO nanowalls and nanoflowers are very close to the Planckian locus,
actually about less than one MacAdam ellipses away and can be called white light according
to the US standard ANSI_ANSLG C78, 377-2008 for solid state lighting which indicates that
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the light sources with chromaticity coordinates less than three MacAdam ellipses from the
Planckian locus can be considered as white light. The chromaticity coordinates for the ZnO
nanorods and nanotubes are about 3 Mac-Adam ellipses away and are very close to white
light. It is clear from the chromaticity coordinates that the fabricated LEDs based on different
nanostructures are white light emitting diodes [4].
In paper III we have investigated the origin of the red emission in ZnO
nanotubes [6]. The origin of different deep level defects related emissions in ZnO is still
controversial and cannot be fully understood and a considerable interest exists in investigation
of the origin of deep level emissions in ZnO in general and particularly in ZnO nanostructures
as nanostructures have great potential to be used in opto-electronics. In this paper we have
investigated the origin of the red emission in ZnO by comparing the EL spectra from ZnO
nanotubes (annealed in different ambients)/p-GaN LEDs. Aqueous chemically achieved ZnO
nanotubes were annealed in argon, oxygen, air, and nitrogen ambients at 600 oC for 30
minutes [6]. For comparison, the ZnO nanotubes (as grown)/p-GaN LED was used as a
reference. Figure 6.7 shows the EL spectra for the LEDs based on ZnO nanotubes (as grown
and annealed in argon, air, oxygen, and nitrogen ambients). The EL spectra were measured by
applying a forward biasing of 25 V across the electrodes. The EL spectra of all the LEDs
show EL peaks in the violet, violet-blue and broad dominating peaks centered in green,
orange, orange-red and red regions. The violet and violet-blue peaks are centered
approximately at 400 nm (3.1 eV) and 452 nm (2.74 eV) in all LEDs, respectively. The broad
dominating green EL emission peak centered approximately at 536 nm (2.31 eV), was
observed in the as grown and annealed in argon ZnO nanotubes based LEDs. The broad
dominating orange and orange-red EL emission peaks approximately centered at 597 nm
(2.07 eV) and 618 nm (2.00 eV) were observed in ZnO nanotubes (annealed in air and oxygen
ambients) based LEDs, respectively. The broad dominating red EL emission peak centered
approximately at 705 nm (1.75 eV) was observed in the ZnO nanotubes (annealed in nitrogen
ambient) based LEDs [6].
85
Figure 6.6: Schematic band diagram of the DLE emissions in ZnO based on the full potential
linear muffin-tin orbital method and the reported data and references in [6].
300 400 500 600 700 800 9000
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Figure 6.7: Electroluminescence spectra of different LEDs at an injection current of 3 mA,
under forward bias of 25 V and it shows the shift in emission peak after annealing in different
ambients [6].
86
From the comparison of the EL spectra of all the LEDs it was investigated that
air, oxygen and nitrogen annealing ambients have strongly affected deep levels emission
bands in ZnO. It was observed from the electroluminescence investigation that more than one
deep level defects are involved in the red emission appearing between 620-750 nm in the
visible region [6].
It was concluded that the red emission in ZnO can be attributed to oxygen
interstitials (Oi) appearing in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV). The orange-
red peaks in the EL spectra of the LEDs based on ZnO nanotubes annealed in air and oxygen
are centered at 597 nm (2.07 eV) and 618 nm (2.00 eV), respectively [6]. These emissions are
attributed to oxygen interstitials (Oi) and it is believed that it is due to band transition from
zinc interstitial (Zni) to Oi defect levels in ZnO [7]. The position of the Oi level is located
approximately at 2.28 eV below the conduction band and the position of the Zni level is
theoretically located at 0.22 eV below the conduction band. It is expected that the band
transition from Zni to Oi level is approximately 2.06 eV [7]. This agrees well with the orange-
red peaks centered approximately at 2.00 eV and 2.07 eV [6].
The red emission in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV) in ZnO
is attributed to oxygen vacancies (VO). The red emission in the ZnO nanotube (annealed in
nitrogen)/p-GaN LED is centered at 705 nm (1.75 eV) and it can be attributed to oxygen
vacancies (VO). When the ZnO nanotubes are annealed in nitrogen ambient, the following
process of oxygen desorption may occur:
ZnO → VO + ZnZn + 1/2O2
In this process the zinc vacancies are filled with zinc when ZnO nanotubes were
annealed in the nitrogen ambient. Then the majority of the defects in the deep level of ZnO
are oxygen vacancies, created by the evaporation of oxygen [8]. The red emission in the ZnO
nanotubes (annealed in nitrogen ambient)/p-GaN LED centered at 706 nm (1.75 eV) may be
attributed to electron-hole recombinations from VO to the top of the valance band in ZnO. It
can be calculated by using the full-potential linear muffin-tin orbital method that the energy
level of the VO in ZnO is 1.62 e V below the conduction band [9]. Therefore the energy
difference from the VO energy level to the top of the valence band is approximately 1.75 eV.
This agrees well with the energy of the red emission centered at 1.75 eV [6].
87
By comparing the EL spectra of the ZnO nanotubes (annealed in oxygen and
nitrogen)/p-GaN LEDs, it was observed that the total red emission in ZnO from 620 nm (1.99
eV) to 750 nm (1.65 eV) is not from the same defect and it is a combination of the red
emission from Oi and VO deep level defects in ZnO. The red emission in the range of 620 nm
(1.99 eV) to 690 nm (1.79 eV) can be attributed to Oi deep level defects in ZnO. It can be
explained by comparing the EL spectra of the LEDs based on ZnO nanotubes annealed in
oxygen ambient and the LEDs based on the as grown ZnO nanotubes. It can be observed that
in the EL spectra for the LED based on ZnO nanotubes annealed in oxygen ambient, the red
emission intensity in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) is enhanced
significantly as compared to the red EL emission intensity in the LED based on the as grown
nanotube [6].
Similarly the red emission in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV)
can be attributed to VO deep level defects in ZnO. It can be explained by comparing the EL
spectra of the LEDs based on Z nO nanotubes annealed in nitrogen ambient and the LEDs
based on a s grown ZnO nanotubes. It can be observed that in the EL spectra for the LED
based on the ZnO nanotubes annealed in nitrogen ambient, the red emission intensity in the
range of 690 nm (1.79 eV) to 750 nm (1.65 eV) is enhanced significantly as compared to the
EL intensity of red emission in the LED based on the as grown ZnO nanotubes. It can be seen
clearly that in the EL spectra of ZnO nanotubes annealed in nitrogen ambient, the EL intensity
of the green, yellow, orange, and the red emission (from 620 nm to 690 nm) are decreased but
the EL intensity of the red emission (from 690 nm to 750 nm) has increased significantly as
compared to the as grown ZnO nanotubes based LEDs. So it is clear that the red emission
from 620 nm to 690 nm and from 690 nm to 750nm has different origins. The red emission in
the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) can be attributed to Oi and in the range of
690 nm (1.79 eV) to 750 nm (1.65 eV) can be attributed to VO [6].
The color rendering properties of ZnO nanotubes annealed in different ambients
are similar to the color rendering properties of ZnO nanorods annealed in different ambients.
The annealing ambients (argon, air, oxygen and nitrogen) showed the same effect on color
rendering properties for ZnO nanotubes and nanorods. Figure 6.8 shows the CIE 1931 color
space chromaticity diagram in the (x, y) coordinates system of the LEDs based on ZnO
nanotubes annealed in different ambients. The chromaticity coordinates are (0.3559, 0.3970),
(0.3557, 3934), (0.4300, 0.4348), (0.4800, 0.4486), and (0.4602, 0.3963) with correlated color
temperatures (CCTs) of 4802 K, 4795 K, 3353 K, 2713 K, and 2583 K for the LEDs based on
88
ZnO nanotubes as grown, annealed in argon, air, oxygen and nitrogen, in the forward bias,
respectively [6].
Figure 6.8: The CIE 1931 x, y chromaticity space of ZnO nanotubes annealed in different
ambients [6].
Figure 6.9: Normalized spectral power distributions for the LEDs based on the as-grown n-
ZnO nanorods and after annealing in air, oxygen, and nitrogen ambients [11].
89
In paper IV we have investigated the effect of post-growth annealing on the
color quality of light emitted from different n-ZnO nanorods/p-GaN LEDs. The post-growth
annealing is an effective tool to enhance the luminescence properties. The ZnO nanorods were
grown by the ACG method. It is a low temperature growth technique and it is expected that
the grown ZnO nanorods have lattice and surface defects and their crystallinity and optical
properties can be altered by post-growth annealing. [10].
The as grown ZnO nanorods were annealed in nitrogen, oxygen, argon, and air
ambient at 600 oC for 30 minutes. After annealing the ZnO nanorods in different ambients, the
LEDs were fabricated from these post-growth annealed ZnO nanorods. After the fabrication
of the LEDs the color rendering indices (CRIs) and correlated color temperatures (CCTs)
were extracted from the spectra emitted by these fabricated LEDs [11].
It was investigated that the post-growth annealing ambients have an affective
role in modifying the light color quality and especially the nitrogen ambient is very effective
for altering the CRIs and the CCTs of the fabricated LEDs. The LEDs based on ZnO nanorods
annealed in nitrogen ambient showed excellent color rendering properties with CRIs of 97 in
the forward bias and 98 in the reverse bias and have CCTs of 2363 K in the forward bias and
3157 K in the reverse bias [11].
Figure 6.9 show [11] the normalized spectral power distributions (SPD) of the
light emitted by the LEDs based on ZnO nanorods annealed in different ambients. A forward
bias was applied across the electrodes with injection current of 7 mA. The LED based on the
as grown ZnO nanorods was used as a reference for normalization. The reference LED had a
spectral radiant flux of ~ 0.447 mW [11].
We can analyze and characterize light from any source precisely by measuring
the power contained by each wavelength in the visible spectrum. The measured spectral
power distribution (SPD) contains all the basic physical information that is necessary for the
quantitative analyses of the light. Color parameters such as chromaticity coordinates, color
rendering indices, and correlated color temperatures of the light emitted by the different LEDs
were extracted from the spectra emitted by these LEDs [11].
Figure 6.10 (a) [11] shows the CIE 1931 color space chromaticity diagram in the
(x, y) coordinates system for all the LEDs when operated in the forward bias. The color
rendering indices and correlated color temperatures were extracted from the
90
electroluminescence (EL) spectra of the LEDs and we have discussed the EL spectra in details
in [12]. The chromaticity coordinates are (0.3559, 0.3970), (0.3870,4213), (0.4220, 0.4209),
(0.4885, 0.4296) and (0.4819,0.4029) with the correlated color temperatures (CCTs) of 4802
K, 4124 K, 3398 K, 2480 K, and 2363 K for LEDs based on the as grown n-ZnO NRs/p-GaN
and those annealed in argon, air, oxygen, and nitrogen ambient, respectively [11].
It was investigated that the chromaticity coordinates of the LEDs based on ZnO
nanorods annealed in oxygen and nitrogen are very close to the Planckian locus (~ 1
MacAdam ellipse away) and these LEDs can be called white LEDs and the LED based on
nanorods annealed in air is about three MacAdam ellipses away and it can also be considered
as white light. The LEDs based on the ZnO nanorods as grown and annealed in argon are
more than 3 MacAdam ellipses away from the Planckian locus. The US standard
ANSI_ANSLG C78, 377-2008 for solid state light sources explains that the light sources
having chromaticity coordinates within three MacAdam ellipses from the Planckian locus can
be considered to produce white light. If the distance between two colors in the CIE (1931) x, y
chromaticity diagram is less than three MacAdam ellipses then the human vision does not
notice much difference between them [11].
91
Figure 6.10: The chromaticity coordinates of different LEDs (a) under forward bias and (b)
under reverse bias plotted on the CIE (1931) x,y, chromaticity diagram [11].
92
Figure 6.10 (b) [11] shows the chromaticity coordinates on the CIE (1931) x, y
chromaticity diagram for the light emitted from different LEDs in reverse bias. The
chromaticity coordinates for the LEDs based on ZnO nanorods annealed in air, oxygen, and
nitrogen are very close to the Planckian locus and are about 1 MacAdam ellipse away. These
LEDs can be definitely called white LEDs according to the US standard ANSI_ANSLG C78,
377-2008. The LEDs based on the as-grown n-ZnO nanorods/p-GaN and annealed in argon
are more than three MacAdam ellipses away from the Planckian locus [11].
From the comparison of color measurements from all the LEDs it was found that
the annealing ambient is very altering the color rendering properties of the fabricated LEDs.
The CIE chromaticity coordinates of the LEDs based on the as grown ZnO nanorods were
away and could not be considered as white LEDs according to light standards but after
annealing the CIE chromaticity coordinates are shifted close to the Planckian locus and
become white LEDs according to light standards. The same behavior was observed in the CIE
color rendering (CRIs). The CRIs of the LEDs also increase after annealing. The CRIs are
dependent on the broadness and the intensity of the emission peaks in the visible region. After
annealing the ZnO nanorods in air, oxygen and nitrogen ambients, the intensities of the
emission peaks increase and this results in an increase of their CRIs. After annealing the ZnO
nanorods in argon ambient, the light emission intensities decrease but the broadness of the
light emission peak increases and this increases the CRI. The CRIs also increase after
annealing the ZnO nanorods in air, oxygen and nitrogen ambients. It is due to the fact that
after annealing in these ambients the deep level defects responsible for the orange and red
emission become more active as compared to the defects responsible for green emissions in
the deep levels of ZnO and as a result there is a red shift in the dominating peak, it has been
discussed in details in paper III. Due to this the emission, the spectra shifted close to the black
body radiator spectra and the black body spectra are the standard reference for white light
with CRIs of 100 [11].
6.2 Junction temperature of n-ZnO nanorods based LEDs
In paper V, The junction temperature of n-ZnO nanorods/(p-4H-SiC, p-GaN,
and p-Si) heterojunction light emitting diode (LED) at the built-in potential was modeled and
experiments were performed at various temperatures (15 ºC - 65 ºC) to validate the model
[13]. The junction temperature of the LEDs is a cr itical parameter that affects the internal
efficiency, the maximum output power and the reliability of LEDs. It also influences the
93
chromaticity and the color-rendering properties of LEDs [14]. As all the LEDs operate near
the built-in potential therefore it is interesting to investigate the temperature coefficient of the
forward voltage near the built-in potential (~Vo). The modeled and experimental values of the
temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared
[13].
First the derivation of the model will be explained. A relationship for the
temperature dependence of the forward voltage was modeled and then it was generalized for
n-ZnO nanorods/(p-GaN and p-Si) LEDs. The theoretical and experimental results at the
built-in potential (Vo) are compared and discussed [13].
Figure 6.11: The energy band diagram of the n-ZnO nanorods/p-4H-SiC heterostructure [13].
The relation for the temperature dependent forward voltage was derived from
the energy band diagram of n-ZnO nanorods/p-4H-SiC heterostructure as shown in Figure
6.11. The relation for the temperature dependent forward voltage was derived for n-ZnO
nanorods/p-4H-SiC heterojunction and then it was generalized for n-ZnO nanorods/(p-GaN
and n-ZnO nanorods/p-Si heterojunction LEDs [13].
Here it is assumed that there is an abrupt interface and any effects due to the
interface states are neglected [15]. From the energy band diagram the equilibrium energy can
be given as,
94
( ) ( ) ( ) nfCNVPfpgpo EEEEEeV χχ +−+−−+= (1)
The band gap energies of ZnO and 4H-SiC were taken as 3.37 eV and 3.25 eV,
respectively. Where, Vo is built-in potential, Ef is Fermi energy level, Egp, χP, EVP, are the
energy band gap, the electron affinity and effective densities of states in the valence band of
p-4H-SiC, and χN, ECN are electron affinity and effective densities of states in the conduction
band of n-ZnO. The difference in energy between the Fermi level and the band edges can be
taken from Boltzmann’s statistics assuming that all dopants are fully ionized [15]. Now
equation 1 can be written as,
CNVP
ADcgpO NN
NNe
kTeE
eE
V ln+∆
+= (2)
Let’s assume that all the LEDs are operated in the forward direction close to the built-in
voltage.
So Vf =Vo and we differentiate equation (2) with respect to temperature (T) and assume
dVf/dT= dVo/dT
we get,
++
∆=
CNVP
ADgpCf
NNNN
ekT
dTd
dTdE
edTEd
edTdV
ln11 (3a)
Where,
CNVP
AD
AD
CNVP
CNVP
AD
CNVP
AD
NNNN
dTd
NNNN
ekT
NNNN
ek
NNNN
ekT
dTd
+
=
lnln (3b)
As all dopants are fully ionized ND and NA are independent of temperature, Eq 3(b) can be
written as,
ek
NNNN
ek
NNNN
ekT
dTd
CNVP
AD
CNVP
AD 3lnln −
=
(4)
95
Van Vechten et al. [16] derived a formula for the temperature dependence of the band offsets
for semiconductor heterojunctions as;
( ) ( ) ( ) ( )
∆
−
∆
±=∆
dTBEd
dTAEd
dTBAEd CVCVC 005.077.1/
(5)
The temperature dependence of the energy band gap of a semiconductor can be given by
Varshni formula [17];
By differentiating the Varshni equation with respect to T, we get
( )( )2
21ββα
++
−=TeTT
dTdE
eg (6)
By substituting equation (4), (5) and (6) in equation (3a), finally we get,
( )( )
( )( )
−
+
++
+
++
−=ek
NNNN
ek
TeTT
TeTT
dTdV
CNVP
AD
pn
f 3ln277.0277.1 22 ββα
ββα
(7)
Equation (7) is a useful relation for the determination of the temperature
dependence of the forward voltage close to the built-in potential without including the
resistance of the device. By taking ND =1018 cm-3, 1016 cm-3 and 1016 cm-3 for p-4H-SiC, p-
GaN, and p-Si respectively and NA = 1017 cm-3 for ZnO. The calculated value of Vo is about
3.37 V for n-ZnO nanorods/p-4H-SiC heterojunction light emitting diode (LED). In eqn. (7)
the first and the second terms are the temperature dependence of the band gap energy. The
third term is the temperature dependence of the intrinsic carrier concentration and the fourth
term is the temperature dependence of the density of states (DOS) [13].
96
Figure 6.12: (a), (b), and (c) illustrate the measured forward voltage versus temperature for
the n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterostructures at different values of the
current respectively and (d), (e), (f) show the measured series resistance versus the junction
temperature for n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si), respectively [13].
Figure 6.13: (a, b) show the measured I-V characteristics of the n-ZnO nanorods/ p-4H-SiC,
p-GaN at different temperatures, (c) shows the measured I-V characteristics of the n-ZnO
nanorods/ p-Si LEDs and (d, e) shows the electroluminescence spectrum for ZnO (NRs)/p-
4H-SiC and p-GaN LEDs [13].
20 30 40 50 60 703.23.43.63.84.04.24.44.64.85.05.25.45.65.86.06.26.46.6
Forw
ard
volta
ge (v
)
Temperature (ºC)
0.1mA 0.2mA 0.3mA 0.4mA
20 30 40 50 60 701.01.52.02.53.03.54.04.5
Forw
ard
volta
ge (V
)
Temperature (ºC)
1mA 2mA 3mA 4mA
10 20 30 40 50 60 70456789
10111213141516171819
Forw
ard
volta
ge (V
)
Temperature (ºC)
1mA 2mA 3mA 4mA 5mA 6mA 7mA 8mA 9mA 10mA
20 30 40 50 60 701300
1400
1500
1600
1700
1800
Res
ista
nce
(K-o
hm)
Temperature (ºC)20 30 40 50 60 70
0.40.60.81.01.21.41.6
Resi
stan
ce (K
- oh
m)
Temperature (ºC)
20 30 40 50 60 700.30
0.35
0.40
0.45
0.50
0.55
Resi
stan
ce (K
- oh
m)
Temperature (ºC)
(a) (b) (c)
(d) (e)(f)
-5 0 5 10 15 20-0.0010.0000.0010.0020.0030.0040.0050.0060.0070.0080.009
Cur
rent
(A)
Voltage (V)
15ºC 25ºC 35ºC 45ºC 55ºC 65ºC
-6 -4 -2 0 2 4 6 8 10 12-0.20.00.20.40.60.81.01.21.41.61.82.0
Cur
rent
(mA)
Voltage (V)
15ºC 30ºC 45ºC 60ºC
400 500 600 700 800 900 1000
0
2000
4000
6000
8000
EL In
tens
ity (a
.u.)
Wavelength (nm)
(a) (b)
-2 -1 0 1 2-0.020.000.020.040.060.080.100.120.140.160.18
Cur
rent
(mA
)
Wavelength (nm)
(c)
300 400 500 600 700 800 900
0
5000
10000
15000
20000
EL In
tens
ity (a
.u.)
Wavelength (nm)
(d) (e)
97
To validate the reliability of the model of the junction temperature of n-ZnO
nanorods/p-4H-SiC heterojunction it was crucial to measure the junction temperature
experimentally. Figure 6.12 (a-c) shows the experimental results of the fabricated LEDs for
the forward voltage (Vf) versus temperature (T) at different constant currents. It was extracted
that the experimental value of the temperature coefficient for the forward voltage from 1mA
to 10 mA was -12mV/K to -49mV/K, respectively. Using equation (7) we calculated the
forward voltage dependence temperature dVf/dT and it was -0.950 mV/K, -0.66 mV/K and -
1.29 mV/K for n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs,
respectively. It was found that there was a difference between the experimental and the
theoretical results of the temperature coefficient of the forward voltage. This difference can
be due to the series resistance effect and this effect was not included in the model. But after
including the series resistance term in the model we found good agreement between the
theoretical and the experimental results. It was found that the series resistance has an
important role in the heterojunction temperature of LEDs. Therefore the theoretical model for
the temperature coefficient of the forward voltage after including the series resistance effect
will be [13],
( )( )
( )( )
−
+
+
++
+
+−=
dTdR
AJek
NNNN
ek
TeTT
TeTT
dTdV S
fCNVP
AD
pn
f 3ln277.0277.1 22 ββα
ββα (8)
The temperature coefficient of the forward voltage from eqn. (8) was calculated
and it gives a values of -11.27mV/K, -5.66 mV/K and -6.54 mV/K for n-ZnO nanorods/(p-
4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively. These values are very close to
the experimental values (-12 mV/K, -6 mV/K and -6.95 mV/K) of the temperature coefficient
of the forward voltage at 1mA. There is very less difference between the experimental and
theoretical values and it was less than 7 %. At higher currents the difference increases, which
indicates that the theoretical model is suitable for the lower range near to the built-in
potential. Figure 6.12 (d-f) shows the temperature dependence of the resistance of the n-ZnO
nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively. The dependence of
series resistance on temperature was measured by increasing temperature from 25 ºC to 65 ºC.
The series resistance dependence temperature term dTdRS was calculated from the slope of
the graph, and the value of dTdR
I S was calculated to be -10.32mV/K, -5 mV/K and -5.25
mV/K for n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively.
98
During the resistance measurements, n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si)
heterojunction LEDs show current of 0.1 μA, 0.2 mA, and 1mA, respectively. The n-ZnO
nanorods/p-4H-SiC LEDs have very small current due to high resistance of the device. The
cause of this high resistance needs more investigations [13].
Figure 6.13 (a, b) [13] shows the current (I) versus voltage (V) of the n-ZnO
nanorods/p-4H-SiC and p-GaN LEDs at different constant temperature. I-V curves showed
rectifying behavior as expected from these LEDs. It is observed that the slope of the I-V
curve is increasing with increasing the temperature. As the slope is inversely proportional to
the series resistance, this shows that the series resistance is decreasing with increasing the
temperature. Figure 6.13 (c) shows the I-V curve of the n-ZnO nanorods/p-Si LEDs. The I-V
curves clearly show a nonlinear increase of the current under the forward bias, which
indicates that reasonable pn heterojunction characteristics are achieved [13].
Figure 6.13(d) [13] shows the EL spectrum for n-ZnO nanorods/p-4H-SiC
LEDs. Three peaks approximately centered at 430 nm (violet-blue emission), 520 nm (green
emission) and 674 nm (orange-red emission) are observed. Figure 6.13 (e) shows the EL
spectrum for n-ZnO nanorods/p-GaN LEDs. The observed peaks are violet, violet-blue, and
green, respectively. At room temperature the peaks are approximately centered at 400 nm
(violet emission), 455 nm (blue) and 550 nm (green). The n-ZnO nanorods/p-Si LEDs
showed no EL emission [13].
6.3 The influence of helium (He+) ion irradiation on the luminescence
properties of ZnO nanorods based LEDs
In paper VI [18], the influence of helium (He+) ion irradiation on the
luminescence properties of ZnO nanorods based LEDs was investigated by PL and EL
measurements. When the energetic particles (electrons or ions) hit a material they change the
properties of the targeted material and can change the density of defects in the material [19].
The purpose of this study is to investigate the reliability of the ZnO nanorods based LEDs in
space and nuclear applications. ZnO nanorods were grown by the vapor-liquid-solid (VLS)
growth method and the details of this growth method can be found in chapter 4. The LEDs
based on these catalytically grown ZnO nanorods were irradiated by using 2 MeV He+ ions
with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2 [18].
99
300 400 500 600 700
0
2000
4000
6000
8000
650 nm530 nm
380 nm
PL In
tens
ity (a
.u.)
Wavelength (nm)
300 400 500 600 7000
2000
4000
6000
650 nm512 nm
375 nm
PL In
tens
ity (a
.u.)
Wavelength (nm)
(a)
(d)
(b)
(c)
300 400 500 600 7000
1000
2000
3000
4000
5000
650 nm512 nm
375 nm
PL In
tens
ity (a
.u.)
Wavelength (nm)
370 380 390
375 nm380 nm
Wavelength (nm)
300 400 500 600 7000
2000
4000
6000
8000
650 nm530 nm512 nm
375 nm
380 nm
PL In
tens
ity (a
.u.)
Wavelength (nm)
(a)(b)(c)
Figure 6.14: Room temperature photoluminescence spectra for ZnO nanorods (a) as grown,
(b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~
4×1013 ions/cm2, and (d) shows the PL spectra of all the samples together for comparison [18]
300 400 500 600 700 800 9000
500
1000
1500
2000
2500
3000
3500
4000
397 nm
456 nm 560 nm
EL In
tens
ity (a
.u.)
Wavelength (nm)300 400 500 600 700 800 9000
5001000150020002500300035004000
456 nm
560 nm
EL In
tens
ity (a
.u.)
Wavelength (nm)
300 400 500 600 700 800 9000
1000
2000
3000
4000
5000530 nm
456 nm
EL In
tens
ity (a
.u.)
Wavelength (nm)
(a)
(d)(c)
(b)
300 400 500 600 700 800 900
0
1000
2000
3000
4000
5000
397 nm
560 nm530 nm
456 nm
EL In
tens
ity (a
.u.)
Wavelength (nm)
(a)(b)(c)
Figure 6.15: Display the electroluminescence spectra for n-ZnO nanorods/p-GaN LEDs, (a) as
grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with
fluency of ~ 4×1013 ions/cm2, and (d) shows the EL spectra of all the LEDs together for
comparison [18].
100
Figure 6.14 [18] (a-c) display the photoluminescence spectra from the as grown
and irradiated ZnO nanorods. Figure 6.14 (a) displays the PL spectrum for the as grown ZnO
nanorods. Three PL emission intensity peaks are observed. The band edge emission (UV) and
the deep level emission (green and orange-red) peaks are observed and these peaks are
centered approximately at 380 nm (3.26 eV), and 530 nm (2.33 eV) and 650 ( 1.90 eV),
respectively. Figure 6.14 (b-c) displays the PL spectra for irradiated ZnO nanorods with
fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2, respectively. Three PL intensity peaks
are observed. The edge emission (UV) and the deep level (green and orange-red) emission
peaks are observed and these are centered approximately at 375 nm (3.30 eV), and 512 nm
(2.42 eV) and 650 nm (190 eV), respectively [18].
Comparison of the PL spectra of the as grown ZnO nanorods and the He+ ion
irradiated is shown in figure 6.14 (d). From the comparison it can be observed that the PL
emission intensity in the UV region decreases and the PL intensity in the visible region
increases after the He+ ion irradiation. The UV emission decreases after irradiation due to
degradation in the crystalline quality and the enhancement in the green emission is due to the
increase of radiative defects such as oxygen vacancies [20] while the orange- red emission is
less affected by the irradiation.
From the comparison of the PL spectra of the as grown and the He+ ion
irradiated ZnO nanorods it is also observed that after the He+ ion irradiation there is a blue
shift in the UV peak and it is about 0.0347 eV. This blue shift in the excitonic UV emission
peak can be attributed to the presence of homogeneous compressive strain in the ZnO nanorod
and it was produced by the irradiation of the ZnO nanorods by He+ ions. This compressive
strain increases the band gap and affects the optical properties of the material [21].
From the comparison it is also observed that there is also a blue shift in the
green region centered peak after He+ ion irradiation of the ZnO nanorods and this blue shift is
about 0.082 e V. This blue shift can be attributed to the fact that the He+ ion irradiation
produced more oxygen vacancy related defects as compared to oxygen interstitials defects in
the ZnO. The green emission in ZnO is the combination of the green emission due to oxygen
vacancies and oxygen interstitials. It is commonly observed that the as grown undoped ZnO
nanorods introduce green emission approximately centered at 2.33 eV and it is very close to
the energy of oxygen interstitials below the conduction band but after He+ irradiation oxygen
vacancies defects in the deep levels increase and irradiated ZnO nanorods have green
101
emission that is centered at approximately 2.42 eV and it is very close to the energy of oxygen
vacancies below the conduction band. There is no shift in the orange-red PL emission peak
centered at 665 nm (1.90 eV). It is due to the fact that the orange-red emission can be
attributed to the transition from zinc interstitial (Zni) to oxygen interstitial (Oi) defect levels in
ZnO [22]. As He+ ion irradiation affect the oxygen vacancies and oxygen interstitials are not
affected therefore orange-red emission remains unaffected. The details, why He+ ion
irradiation is not affecting the orange-red emission needs more investigation.
Figure 6.15 (a) [18] displays the EL spectra of LEDs based on the as grown ZnO
nanorods. Three EL emission peaks in the violet, violet-blue and green regions are observed.
These peaks are centered approximately at 397 nm (3.12 eV), 456 nm (2.71 eV) and 560 nm
(2.21 eV), respectively. Figure 6.15 (b, c) [18] show the EL spectra of the LEDs based on He+
ions irradiated ZnO nanorods. The EL spectra shows the violet emission peak that was
observed in the EL spectra of the LEDs based on the as grown nanorods and it almost
disappeared after the He+ ion irradiation. It disappears due to the poor crystalline quality after
the He+ ion irradiation. The centre of the green emission EL peak is blue shifted by about
0.125 eV as compared to the as grown nanorods based LED. The blue EL emission peak
looks stable and there is no change in its position as it is believed that it is from the p-GaN
substrate. It was also observed that the position of the green emission peak in the EL spectra
is red shifted as compared to the PL spectra and this red shift can be attributed to the heating
effect of the device [18].
102
(b)
(a)
(c)
(a)
(d)(c)
(b)
Figure 6.16: Display the CIE 1931 x, y chromaticity space, showing the chromaticity
coordinates of LEDs under forward bias for ZnO NRs/p-GaN LEDs, (a) as grown ZnO NRs,
(b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~
4×1013 ions/cm2, and (d) all together for comparison [18].
Figure 6.16 (a-c) [18] displays the CIE 1931 color space chromaticity diagram
in the (x, y) coordinates system. The chromaticity coordinates of the non-radiated and He+
ion radiated ZnO nanorods with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2 are
(0.3557, 0.3805), (0.3735, 4020), and (03594, 0.3983) with correlated color temperatures
(CCTs) of 4745 K, 4345 K, and 4708 K, respectively. It was observed that the chromaticity
coordinates for non-radiated LED are close to Planckian locus (about 2 Mac-Adam ellipses
away) and can be called white light according to the US standard ANSI_ANSLG C78, 377-
2008. The chromaticity coordinates of the He+ ion radiated LEDs are shifted slightly away
from the Planckian Locus and are very close to white light. They are about 4 M ac-Adam
ellipses away from the Planckian Locus. The color rendering indices are 92, 90, and 89 for the
non-radiated and He+ ion radiated f LEDs, respectively [18].
From the comparison of the PL and EL investigations it was observed that the
irradiation of ZnO nanorods with He+ ions has influenced the high energy defects in the ZnO
103
nanorods such as the defects which are responsible for ultra-violet (UV), violet and green
emission in ZnO. It was concluded that after irradiation the crystallinity in ZnO nanorods
decreases and as a result the PL intensity of UV emission decreases. The blue shift in UV and
green emission was also observed. The EL spectra show the same blue shift in the green
emission as observed from the PL spectra. The He+ ion irradiation also affects the color
rendering properties of the LEDs and decreases the color rendering indices from 92 to 89 [18].
104
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106
107
Chapter 7 Conclusion and outlook
This research work is mainly concentrated on the luminescence properties of
ZnO nanostructures and color quality of the light emitted from ZnO nanostructures based
LEDs. The investigation of the luminescence from ZnO is very crucial to understand the
origin of deep level emissions in ZnO. It is very important for the future possible application
of ZnO nanostructures in the development of white LEDs to replace conventional light
sources. Low cost production is a main attraction of any product and it is believed that ZnO
has much potential to introduce low cost LEDs in the market. In this research work different
ZnO nanostructures were grown by different low cost growth methods on different substrates.
In the first part of the thesis, the nanostructures were grown by low temperature
(<100 oC) aqueous chemical growth (ACG). The luminescence from ZnO nanorods/p-GaN, p-
4H-SiC heterojunction LEDs was investigated. The same ZnO nanostructure (nanorods) were
grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures
(nanorods, nanotubes, nanoflowers and nanowalls) were grown on same p-GaN substrate. The
luminescence spectra (EL and PL) from these different ZnO nanostructures and different p-
type substrate based LEDs were compared to understand the luminescence from these LEDs.
Radiative recombinations in ZnO give near band edge ultra-violet emission due
to the presence of free electron-hole exciton in ZnO and visible emission covering the whole
visible region. Zinc vacancies (VZn), oxygen vacancies (VO), zinc interstitials (Zni), oxygen
interstitials (Oi) related deep level defects were said to be involved in the deep level emissions
in ZnO. ZnO nanotubes were annealed in different ambients (argon, air, oxygen and nitrogen).
The EL spectra of the LEDs based on these annealed ZnO nanotubes were compared and
investigated to understand the origin of the red emission in ZnO and it was found that more
than one deep level (VO, Oi) are involved in red emission. It was also found that the quality of
light from ZnO based LEDs also improved after annealing ZnO nanorods and nanotubes in
different ambients, especially nitrogen ambient was found to be very effective.
It was found that different ZnO nanostructures have different density of deep
level defects that are responsible for light emission in the whole visible region. The density of
the structures and their surface area are also important, different nanostructures have different
108
surface area under the contact. The ZnO nanowalls have more density of deep level defects
but ZnO nanotubes have more charge injection surface area under the contact. The light from
ZnO nanotubes based LEDs is more intense as compared to nanowalls based LEDs.
In the second part of the thesis a relation for junction temperature of the ZnO
nanorods/p-GaN, p-4HSiC, p-Si LEDs was modeled and experiments were performed to
validate the model. It was found that the series resistance has the main contribution in the
junction temperature. There was a large difference between the theoretical and the
experimental values when series resistance was not included but after including the series
resistance, there was good agreement between the theoretical and the experimental values.
In third part of the thesis, influence of helium (He+) irradiation on luminescence
properties of ZnO nanorods/p-GaN LEDs were investigated. The ZnO nanorods were grown
by high temperature (~700 oC) vapor-liquid-solid (VLS) method. The EL and PL
luminescence spectra of as grown and irradiated samples were compared. It was found that
He+ ions irradiation influence the UV emission (near band edge) emissions as well as it also
affects the deep level emissions in ZnO. A blue shift in the PL peaks in the near band
emission and green emission was observed. The EL measurements also showed a blue shift in
the broad green emission after irradiation. The irradiation also affects the color rendering
indices and decreases these from 92 to 89.
ZnO based devices especially LEDs have much potential to become popular and
attractive in the market. Now the community is wishing to replace conventional light sources
with white LEDs, LEDs based on ZnO especially ZnO nanostructures have potential to
become the choice of the world community due to the white light quality and the light colors
emitted from the ZnO based LEDs can be easily controlled by post-growth annealing
ambients. ZnO based LEDs can also be attractive for the market due to low cost production.
The results in this thesis concerning the luminescence and color quality investigation of ZnO
nanostructures based LEDs can be useful for the fabrication and development of ZnO based
white LEDs for lighting.
Notes