Effect of low substrate deposition temperature on the optical [good guide paper].pdf

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Effect of low substrate deposition temperature on the opticaland electrical properties of Al2O3 doped ZnO films fabricatedby ion beam sputter deposition

J. W. Seonga)

P & I Corp., Shinnae-Techotown #405, 485, Sangbong-Dong, Jungrang-Gu, Seoul 131-221, Koreaand Department of Ceramic Engineering, Yonsei University, Sudaemun-Gu, Shinchon-Dong 134,Seoul 120-749, Korea

K. H. Kim, Y. W. Beag, and S. K. KohP & I Corp., Shinnae-Techotown #405, 485, Sangbong-Dong, Jungrang-Gu, Seoul 131-221, Korea

K. H. Yoon Department of Ceramic Engineering, Yonsei University, Sudaemun-Gu, Shinchon-Dong 134,Seoul 120-749, Korea

Received 19 September 2003; accepted 14 March 2004; published 15 June 2004

Aluminum 2 wt % doped zinc oxide AZO films are prepared by ion beam sputtering method onglass substrates with very low substrate deposition temperature below 150 °C. The results of Halleffect measurements showed the decrease in resistivity (2.6103 –7.3104 cm) came froman increase in mobility and carrier concentration as substrate deposition temperature increased. Weconfirmed the decrease of resistivity, which resulted from the formation of oxygen deficiency(ZnO1 x) as substrate deposition temperature increased. The increase of Hall mobility and carrier

concentration was also influenced by increases in crystallinity and grain growth of AZO films assubstrate deposition temperature increased. The optical transmission of AZO increased from 70% to80% with increasing substrate deposition temperature. The surface morphology analysis by atomicforce microscopy showed the decrease of surface roughness might be related to the development of optical properties. © 2004 American Vacuum Society. DOI: 10.1116/1.1738654

I. INTRODUCTION

Extensive research directed on deposition techniques de-velopment and investigation of zinc oxide ZnO thin filmsproperties was performed during the last two decades. Dcand rf magnetron sputtering in different modifications, pulselaser deposition as well as electron beam evaporation wereused for ZnO films preparations on glass substrates.1– 3 Thisgrowing interest originated first of all from the circumstancethat ZnO is now considered a prospective material alterna-tive to indium tin oxide ITO in display manufacturing. ITOis well known to be one of the most expensive coating ma-terials in the thin film market.

It was shown that ZnO films with good optical and elec-trical properties could be deposited on glass substrates atroom temperature by dual-ion-beam sputtering.4 It was alsofound that the electrical resistivity of polycrystalline filmsranged depending on the oxygen percentage in the assistance

ion beam from 103

to 105

cm.Later, Al, Ga, F, or B doping was used to improve sheetresistance and transmittance for visible light.5– 8 The resistiv-ity of Al doped ZnO AZO films studied on a large scalehave been found to be 2104 to 5104 cm which iscomparable to that of ITO films. However, these results wereobtained under relatively high deposition temperatures from150 to 250 °C.

Rf magnetron sputtering in usual mode9 and off-axisone10 were used for AZO film fabrication. It was found inthis research that properties of the films are strongly depen-dent on Ar working pressure, deposition chamber, and sub-strate location with respect to target. Note in accordance with

this that ion beam sputter deposition IBSD could be real-ized at the better vacuum conditions. Consequently higherarrival energy particles coming to substrate could beachieved. Thus, the further improvement of AZO films prop-erties fabricated by using IBSD could be expected.

In this study AZO films were fabricated on glass sub-strates by using Ar ion beam sputtering of the high purityZnO target doped with Al2O3 . The effect of substrate heat-ing on the electrical, crystalline, and optical properties of thefilms has been investigated.

II. EXPERIMENTAL DETAILSExperiments were carried out using high vacuum system,

which consisted of a stainless steel chamber, cold hollow ionsource, target, and substrate holders and auxiliary devices.The AZO target Cerac Co., Ltd. was 6-in.-diam and 0.3-in.-thick sintered oxide ceramic disk 98 wt% ZnO and 2 wt%Al2O3, 99.99% purity mounted on a water-cooled holder.The chamber was evacuated by a turbo pump to a base pres-sure of 1106 Torr. A 1 keV Ar ion beam with a diam-eter about 50 mm was used for film deposition. Angle of incidence of bombarding ions was fixed at 50° from nor-

aAuthor to whom all correspondence should be addressed; also at: ShinnaeTechotown #405, 485, Sangbong-Dong, Jungrang-Gu, Seoul 131-221, Ko-rea; electronic mail: [email protected]

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mal to substrate. Movable Faraday cup was used to measureparameters of Ar ion beam. The pressure was raised to 1104 Torr during film deposition.

Films were deposited on commercial glass slides Corn-ing 2948, Corning Corp.. The substrate holder equippedwith a heater was placed parallel to the target surface. Thedistance between target and substrate holders was 15 cm.During a film deposition substrates temperature could be

kept in the range from 20 to 120 °C within an accuracy of 2 °C. Substrate temperature was measured by a thermo-couple attached to a substrate surface. A quartz crystal oscil-lator placed near the substrates monitored in situ depositionrate. The deposition rate was about 20 nm/min and films witha thickness of about 150 nm were fabricated. The films thick-ness homogeneity was within 5%. The conductance and thecarrier concentration of fabricated films were determined byusing four-point probe resistance and Hall effect measure-ments.

To examine a crystal structure of the films the x-ray dif-fraction XRD peaks were recorded in –2 geometry bySiemens Diffractometer D5000. A Ni-filtered Cu K

1.5418 Å source was used and the scanning range wasbetween 2 20° to 80° in these experiments. The films sur-face morphology and microstructure were investigated inscanning electron microscope SEM, S-4200FE, Hitachi Co.and by using atomic force microscopy AFM, SFM-BD2,Park Scientific Instruments. The film composition analysiswas conducted by x-ray photoelectron spectroscopy XPS

by using ESCA LAB 220-XL, VG Scientific. The XPS spec-tra were obtained with monochromatic Al K x-ray h 1486.6 eV. The position of the C 1s peak was taken as astandard 284.5 eV of binding energy. The films opticaltransmittance was characterized by UV-visible spectrometerHewlett Packard.

III. RESULTS AND DISCUSSION

A. Crystallinity, microstructure and surfacemorphology of AZO thin films

Figure 1 shows the XRD spectra of Al doped ZnO filmsdeposited at different substrate temperatures. The distinctpeak at 2 33.8° is seen in all spectra indicating that AZOfilms have c-axis orientation of the hexagonal phase. Theabove similar crystal structure was observed earlier for ZnOfilms.6 However, the only AZO films deposited at 120 °Crevealed a strong textured growth along c-axis.

It is also seen from Fig. 1 that at all temperatures, the002 AZO peak position shifts lower angles as comparedwith the normal ZnO powder value of 34.47°. Shifting thepeak about 0.4° towards larger 2 value observed for filmsfabricated by rf magnetron sputtering of ZnO target dopedwith 3 wt % Al2O3 and the shift value was found to dependon the dopant concentration.12–19 Authors suggested that if all Al atoms were substituted in Zn sites, the lengths of thec-axis should be shorter. However, increasing the AZO lat-tice parameters a and c with respect to ZnO was foundcontrary to these results when the films were prepared byspray pyrolysis at different dopant concentrations.9 Because

the c / a ratio was found to be the same for both doped andvirgin ZnO films,3 it was concluded that the sites availablefor Al in the ZnO lattice are probably very limited. In addi-tion, Al, Al3, or Al2O3 segregates at the grain boundaries.The peak position revealed in our measurements is very nearto ones found for the films fabricated by dc magnetron reac-tive sputtering 34.02° at 20°C11 and by dual ion beamdeposition 33.50°20 on quartz glass and glass substrates,correspondingly. Note also that similar to our measurements,the peak position was observed to be nearly independent ona substrate temperature change from 20 to 200 °C,11 whilethe peak intensity increasing was found to be seven times

when the temperature changed from 20 to 120 °C. Comparedto our data, other studies show a low increase of peak inten-sity such as 1.4 and 1.5 times.2,11

To derive the film stress film parallel to the film surface,the following formula has been used, which is valid for ahexagonal lattice:23

film233 Gpa the strain cfilmcbulk / cbulk ,

which is the tensile stress ( film) along the c-axis and woulddraw the c-axis out. Figure 2 appears that the full width athalf maximum FWHM decreased as the substrate tempera-ture increased and the stress decreased significantly from36.4 Gpa at 20 °C to 0.37 Gpa at 120 °C. It is likely to be of

intrinsic, rather than of thermal origin. This suggests that thestress should be generated during deposition due to the freez-ing of structural defects at low temperatures.11

It was concluded that the peak shifting could occur due tothe tensile stress along the c-axis.4,11,20 From these facts, wewill summarize that the stress value may be obviously de-pendent on such parameters as a deposition rate, gas captureor release, film thickness, etc., which could defer for differ-ent substrate temperatures and for various deposition tech-niques. Thus, additional experiments are needed to clarifyreasons of the peak shifting.

FIG. 1. X-ray diffraction spectra of AZO films prepared at different substratetemperatures.

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Figure 3 shows SEM surface images of AZO films depos-ited at different substrate temperatures. The coalescence pro-cess of grains is seen from Fig. 3 to appear gradually with anincreasing of a substrate temperature. The grain size can bededuced from x-ray diffraction by using Scherrer formula l

0.9Cu K /cos (2 ) with l of grain size, Cu K

1.5418Å, 2 of FWHM.7 The calculated dependenceof the grain size on the substrate deposition temperature isshown in Fig. 4. The grain size of AZO films prepared in thisstudy at 20 °C is about 12 nm and is very similar to 10, 12,and 13 nm found for the films made by dual-ion-beam, 20 by

rf-magnetron sputtering4 and by midfrequency reactive mag-netron sputtering, respectively.15 Due to the crystallinity im-provement of the films seen from Figs. 1 and 2 the grain sizegrew 36 nm with substrate temperature increasing to 120 °C.The very similar values were estimated for AZO films depos-ited at 150°C about 31 nm15 and for the post-annealedfilms deposited at 20 °C from 20 to 45 nm with increasingannealing temperature from 272 to 390 °C.5 The grain sizeof 150 nm AZO films was estimated about 33 nm from XRDmeasurements, while the grain size revealed from SEM mi-crographs was about 20 times.12 Since thick films 3 m

were used for SEM it was suggested that the larger grains areformed via the aggregation of small grains or grain boundarymovement.

Figure 5 demonstrates the AFM images of the AZO filmdeposited on glass substrates at different temperatures. Thesurface of the films deposited at 20 °C appeared steep with arough topography, while the films grown at 120 °C showedround and uniform grains and a rather smooth surface.

The change of root mean square rms roughness of theAZO films as a function of substrate temperature can be seenin Fig. 6. The rms roughness decreased to the value about 0.5nm as substrate temperature increased up to 120 °C. The ef-fect of substrate heating on rms roughness was observed forAZO films deposited on Si and glass substrates.15 The valueof rms roughness was found in both studies to decrease by9.35 and 1.5 nm, correspondingly, with the increase of sub-strate temperature up to 300 °C. However, the highest Hallmobility and consequently the lowest resistivity of the filmswere measured at the deposition temperature about 150 °C,when the rms roughness was 3.4 nm.15

The improvement of surface roughness and quality of

AZO thin films was attributed to the increment of substratetemperature. The substrate heating leads to increasing of mo-bility or surface migration of condensed atoms. In the case of ion beam deposition, sputtered atoms were deposited on sub-strate surface with higher energy when compared with con-ventional deposition techniques such as vapor, spray pyroly-sis, or rf and dc magnetron deposition. Thus, sputtered atomsdue to ion beam bombardment provide an additional energyof surface atoms enhancing their surface mobility. Therefore,as it is seen from our results, the better AZO film propertiescan be achieved at relatively low substrate temperature.

B. Composition of the AZO films

The deposited AZO films have been found from XPSanalysis to be free from contaminants other than oxygen andcarbon. These contaminants are adsorbed partly during thedeposition from a residual gas environment and additionallydue to an air exposition while the films were transferred tothe XPS chamber. Figure 7 shows the O 1s peak in XPSspectrum of the films. It is seen from Fig. 6 that this peak canbe consistently fitted by Gaussian, centered at 530.150.15and 531.250.20 eV, respectively. The first of the peaksmentioned above corresponds to oxygen constituting theAZO film.11 The intensity of the second peak, which appearsdue to adsorbed oxygen decreases with increasing substrate

temperature. The second peak indicated the shoulder of O 1speak in XPS spectrum.

Ratios of oxygen-to-zinc concentrations in the ZAO films(CO /CZn) were calculated as ratios of areas under Zn 2 p

1024 eV and O 1s 530 eV peaks, respectively. A tempera-ture dependence of the concentration ratio is shown inFig. 8a. As it is seen from Fig. 8a, above 70 °C the ratio isless than 1 and decreases slightly with an increasing of thedeposition temperature, indicating an oxygen deficiency inthe AZO films. It was found also from analysis of XPSspectra that Al concentration in the films is about 3.9 at. %

FIG. 2. FWHM a and the residual stress b of AZO in dependence of thesubstrate temperature.


JVST A - Vacuum, Surfaces, and Films



regardless of the substrate deposition temperature in Fig.8b. Thus, it could be concluded from XPS analysis that thecomposition of deposited films is a nearly stoichiometric ra-tio and coincides well with the target composition.

C. Electrical property of AZO thin films

Dependence of the resistivity, carrier concentration, andHall mobility of the AZO films versus the substrate tempera-ture are shown in Fig. 9. It is seen that the resistivity of thefilms decreases considerably exhibiting minimum resistivityof about 4104 cm at 120 °C. The similar minimum re-sistivity value and temperature behavior of resistivity andHall mobility was observed in earlier studies.12,15 Note thatthe change of the resistivity with the sustrate temperaturewas attributed to the escape of chemisorbed oxygen.15 It wassuggested that the formation of oxygen vacancies leads to thedecreasing of resistivity and increasing of carrier concentra-tion and Hall mobility.21 These conclusions confirmed wellour results of the AZO films composition analysis.

In addition, the carrier concentration and Hall mobilitywas shown to increase with the increase of substrate tem-perature. Such a behavior of carrier concentration was attrib-uted to the increase of free electron concentration. The tem-perature dependence of Hall mobility was found to obeySato’s relation16

Lq1/2 mkT 1/2 exp E b / kT , 1

where L is the average grain size, q is the electron charge,and m* denotes the effective mass of the electron assumedto be 0.35 m e), T and E b are temperature and the grain

FIG. 3. SEM micrographs of the AZO films prepared on a glass substrate: a 20 °C, b 70 °C, c 95 °C, and d 120 °C.

FIG. 4. Influence of substrate temperature on calculated grain size of AZOfilms.



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boundary potential barrier height, respectively. The plot of ln( T 1/2) versus 1/ T is represented in Fig. 10. The slope of this dependence is linear throughout the investigatedtemperature range indicating the existence of grain boundaryscattering.17 It could be concluded from this fact that thegrain boundary scattering is characterized by an increasing of

Hall mobility with an increasing of carrier concentration, upto approximately 1021 cm3.1

D. Optical property of AZO thin films

Figure 11 shows the transmittance of the AZO films de-posited on glass substrates at different temperatures. In avisible light region, the optical transmittance of AZO asshown in Fig. 10, exhibit lower values compared to the trans-mittance 90% of pure ZnO.19 Such a transparency behavior

was attributed to light scattering that occurred due to Al2O3segregation into grain boundaries by doping Al2O3 .22 It is

FIG. 5. AFM of AZO films were deposited according to substrate deposition temperature: a 20 °C, b 70 °C, c 95 °C, and d 120 °C.

FIG. 6. Root mean square rms roughness of AZO films as a function of substrate temperature.

FIG. 7. XPS data of typical O 1s according to substrate deposition tempera-ture: a 20 °C, b 70 °C, c 95 °C, and d 120 °C.





also seen from this figure that the transmittance improves

from 70% to 80% with an increasing of the substrate tem-perature from RT to 120 °C. This improvement of the trans-mittance is most likely related to the above-mentioned im-provement of the crystallinity and surface roughness of thefilms with an increasing of the deposition temperature.

IV. CONCLUSIONS

We deposited AZO films on glass substrates under a rangeof substrate deposition temperature by IBSD. We introducedthe ion beam sputtering deposition method and also changedthe substrate deposition temperature below 150 °C. By theadopting of ion beam sputtering deposition method and con-

trolling of substrate deposition temperature as a processingparameter, we can achieve high quality AZO films whencompared with other processing methods. We confirmed this

FIG. 8. Influence of substrate temperature on chemical composition of aoxygen and zinc atomic percentage and b aluminum atomic percentage.

FIG. 9. Electrical properties of ZAO films deposited at various substratetemperature --: Hall mobility, --: resistivity, --: carrier concentration.

FIG. 10. Plots of ln( T1/2) vs 103 /T of AZO films deposited at differentsubstrate temperatures.

FIG. 11. Optical transmittance of AZO films prepared according to differentsubstrate temperature.



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fact by an analysis of the electrical and optical properties of AZO films. As substrate deposition temperature increased,the crystallinity and electrical properties of AZO filmsshowed improvement. The optical properties of AZO filmsimproved due to the introduction of a method of the ionbeam sputtering deposition.

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Effect of low substrate deposition temperature on the optical [good guide paper].pdf