Adv. Studies Theor. Phys., Vol. 5, 2011, no. 3, 143 - 154

Columnar Growth of Nanocrystalline ZnO Thin

Films Prepared through RF Magnetron Sputtering

K. Saravanakumar

PG and Research Department of Physics Kongunadu Arts and Science College

Coimbatore -29, India

V. Ganesan, N.P. Lalla

UGC-DAE Consortium for Scientific Research Khandwa Road, Indore 452 017, India

C. Gopinathan and K. Mahalakshmi

PG and Research Department of Physics The Madura College, Madurai – 625 011, India

C. Sanjeeviraja∗

Department of Physics, Alagappa University, Karaikudi – 630 003, India

* Corresponding author dr.k.saravanakumar@gmail.com

Abstract Nanocrystalline zinc oxide thinfilms have been prepared on glass substrates at

different temperatures through planar RF-magnetron sputtering. The particle size reduction with respect to substrate temperature has been studied with x-ray

144 K. Saravanakumar et al diffraction (XRD) and atomic force microscopy (AFM). Cross-sectional transmission electron microscopic (TEM) studies employing dark-field imaging reveals columnar growth of ZnO nano crystals with highly textured growth along [001]. The samples exhibit strong NBE emission at 388 nm followed by a weak and broad visible emission around 420-480 nm. Keywords: ZnO, XRD, AFM, nano crystals 1. Introduction Zinc oxide (ZnO) is a II–VI n-type semiconductor with wurtzite structure with ions connected with four counter ions in the tetrahedral bond of sp3 hybridization [1]. It has been widely studied because of its unique properties such as wide bandgap of 3.3 eV and large excitonic binding energy of 60 meV at room temperature [2]. It provides multi-emissions ranging from ultraviolet to infrared regions and exists in. These properties have made this material for both scientific as well as industrial applications. Nanostructured ZnO has received broad attention due to its distinguished performance in electronics, optics and photonics. With the reduction of particle size novel electrical, mechanical and optical properties are introduced, which are largely believed to be the result of surface and quantum confinement effects. Studies on nanostructured ZnO thin films not only will benefit for understanding the fundamental phenomena in low dimensional systems, but also useful for developing new generation nanodevices, e.g, field effect transistor [3], gas sensor [4] and nanoresonator [5] with high performance.

ZnO nanostructured thin films have been prepared by various methods, including physical and chemical roots, on crystalline substrates e.g, metal coated silicon wafer [6] and zinc substrate [7] to nucleate and induce the crystallinity for formation of nanostructure. Keeping the economic point in view of commercial applications of ZnO in, such as field emission flat panel displays and antireflection window in solar cells, we have made an attempt to produce ZnO nanocrystalline films in the absence of oxygen atmosphere on glass substrates using RF magnetron sputtering. In this communications, we report the studies of effects of substrate temperature on the growth of highly textured nanocrystalline ZnO thin films grown on glass substrates. 2. Experimental details The films were prepared by planar RF-magnetron sputtering from ZnO target (99.99%, Super conductors, USA). The target had a diameter of 2 inches and was set

Columnar growth of nanocrystalline ZnO thin films 145 at a distance of 6 cm from the substrate. Corning 7059 glasses were used as the substrate. High purity Ar (99.999%) was used as sputtering gas. The base pressure in the chamber was maintained at 10-4 mbar. Substrate temperature was varied from room temperature (RT) 27 to 300ºC, and the RF power was maintained at 200W. The deposition usually lasted for 30 min. The structure and phase purity characterization of the films was carried out with a Rigaku X-ray diffractometer using Cu Kα radiation. TEM characterization was done by making cross-sectional sample and looking it in a Tecnai G2 20 TEM operating at 200KV to characterize the growth behaviour. The surface morphology of the films was characterized using Nanoscope-E AFM (Digital Instruments, USA). The photoluminescence (PL) measurements excited by the 325 nm line from the He-Cd laser were carried with the instrument CARY ECLIPSE at RT for studying the defect related emission. 3. Structural Studies 3.1 XRD analysis Figure 1 shows the XRD patterns of the ZnO thin films deposited at 27, 100, 200 and 300 oC substrate temperatures. The XRD profiles of films reveal onely an intense diffraction peak at 34.22º, which correspond to (002) of the wurtzite phase of ZnO. This indicates that the growth of ZnO films is highly oriented along c-axis. At a substrate temperature of 300ºC, a new peak appears with (103) orientation with less intensity. The appearance of this peak is due to the defects produced at higher substrate temperature because of the relocating of atoms during the deposition and some disturbances in the crystal lattice [8]. Using Scherrer formula, DXRD=0.94λ/(β cos θβ), where λ, θβ,and β are the x-ray wavelength, Bragg diffraction angle, and the line width at half maximum of the diffraction peak, respectively, the mean crystallite sizes (DXRD) are calculated and has given in table 1. These values show the crystallite size is decreasing with increasing of substrate temperature. Film stress (σfilm) was obtained from XRD using biaxial strain model and the formula, σfilm = -233 ε was used for calculating the stress parallel to the film surface, where ε = (Cfilm – Cbulk)/Cbulk, the strain in direction of the c-axis, i.e., perpendicular to the substrate surface [9] and Cbulk value was taken as 5.205 Å [10]. The calculated stress values are given in table 1. From the stress values it is inferred that the tensile stress is acting on the film and it is found decreasing when increases the substrate temperature up to 200ºC and then increasing drastically with further increases in temperature. The decrease in stress is due to the enhancement in the adatom mobility by increasing the substrate temperature. The increase in stress was due to the mechanical strain that was induced in the film when the film is cooled to room temperature from higher substrate temperature. The increase in stress further was due to the defects produced in the film at higher substrate temperature. Dislocation density (δ) was calculated

146 K. Saravanakumar et al using the formula, δ=1/(DXRD)2 and the values are given in table 1. It is observed that the δ value increases with increasing substrate temperature.

2.2 AFM analysis Figure 2 shows AFM images of ZnO thin film deposited under different substrate temperatures. The films are uniform, dense and well packed between particles. The films deposited at various substrate temperatures show the columnar structure and the particles are arranged uniformly. The average particle size (DAFM) of the films was calculated from AFM and are given in table 1. The value is decreasing with increasing of substrate temperature reaching a minimum value of 100 nm at a substrate temperature of 300°C. During the deposition there is a possibility of oxygen atoms bonded together and pumped out as O2 gas. It is also reported that the oxygen gets re-evaporated from the surface at higher substrate temperatures [11]. Because of the oxygen deficiency during the growth, excess zinc is produced which occupies the interstitial positions in the crystal lattice formed as native defects. It is known from literatures that ZnO has a tendency to grow with native defects such as zinc interstitial (Zni) sites and oxygen vacancies (VO)2. Here, interstitial zinc atoms migrating at the grain boundaries act as a barrier for the further growth of grain boundaries which in turn decreases the particle size. Density functional theory calculations show that intrinsic defects, most likely zinc interstitials, are migrating in the vicinity of grain boundaries [12]. More excess zinc interstitial sites have been created at higher substrate temperatures because of the lower ionization energy [13] and the grain size of particles are reduced further. Root mean square surface roughness (Rq) value was calculated for the films and is given in table 1. From the values, Rq was decreasing from 76.21 nm at 27°C to 3.14 nm at 200°C substrate temperature and further it was increased for the film deposited at 300°C substrate temperature. 2.3 TEM analysis Figures 3a and 3b are the cross-sectional bright-field and dark-field TEM micrographs of ZnO film on glass substrate deposited at 200°C. The thickness of the film obtained from the TEM image is approximately 285 nm. Columnar growth can be clearly observed for the film deposited on glass (amorphous) substrate. As seen from figure 3b, the shape of the grains in the upper region of the structure is convex, it indicates that the upper region is more stable and closer to equilibrium morphology due to atomic mobility and stress relaxation in the upper region is almost fully achieved than in the lower region [14]. The inset of figure 3b is selected area electron diffraction with electron beam parallel to the [2110] zone axis of ZnO. The indexed diffraction pattern confirms the hexagonal structure of ZnO thin film. The spots marked by x are the forbidden reflections and have appeared due to double diffraction. It is seen from the dark-field cross-sectional TEM micrograph as shown in figure 3b, the growth pattern of the film is columnar with alternate nonuniform

Columnar growth of nanocrystalline ZnO thin films 147 bright and dark columns. It is expected that the adjacent columns have grown with ±10° rotation with each other. The change in the angle of crystallite direction may be due to the segregation of zinc atoms at the grain boundaries. Initially small grains have been observed in the lower region of the columnar structure and on the development of film texture in different spatial dimensions. This could be the competition between the growth instability of the deposited atoms and the already stabilized atoms due to surface energy anisotropy. The grain size is increased linearly with the thickness. 3. Photoluminescence measurements The room-temperature photoluminescence spectra of nanocrystalline ZnO films prepared under different substrate temperatures are shown in figure 4. The samples exhibit strong near band-edge emission (NBE) at 388 nm followed by a weak and broad visible emission around 420-480 nm. NBE originates from free-excitons recombination through an exciton-exciton collision process (so called P line) or electron-hole plasma (EHP) [15], whereas the visible emission attributed to the deep level emission (DLE), originates from the exciton recombination in the localized states. From hydrogenic model, it is observed that Zni is a shallow donor and zinc-oxygen antisites (ZnO) and oxygen vacancy (VO) are deep donors that have higher ionization energies [16]. With our experimental conditions such as in zinc rich environment and from theoretical model [13] defect formation energies of Zni and VO could be low at 27°C deposition. The increase of substrate temperature may induce the formation of ZnO combined with Zni and can increase the DLE further. It is obvious that the intensity of the NBE emission decreases while the intensity of the DLE increases. Decreasing of NBE is due to the particle size decreases so that the surface to volume ratio has increased and in turn it may create more localized states and increases the DLE.

In summary, highly textured ZnO films exhibiting columnar structure have been grown successfully on amorphous glass substrates at various substrate temperatures. The results from XRD and AFM, showing the particle size decreasing with increases in substrate temperature. Dark-field cross-sectional TEM micrograph shows the growth pattern of the film is columnar. The change in the crystallite direction at grain boundaries was explained from the bright and dark columns. The NBE emission decreases while the nanoparticle size reduced further. Acknowledgements

The authors gratefully acknowledged UGC-DAE Consortium for scientific research, Indore for the financial support. Also, the authors wish to thank the

148 K. Saravanakumar et al Director, UGC-DAE CSR, Indore for providing the facilities to perform this work. Authors thank Mr.L.S. Sharath Chandra, Mr. Mohan Gangrade, Ms. DeeptiJain & Ms. Shermista for their help in AFM and TEM analysis. References [1] M. Tsukada M, H. Adachi, C. Satoko, Prog. in Surf. Sci. 14 (1983) 113. [2] Yoshitaka Nakano, Takeshi Morikawa, Takeshi Ohwaki and Yasunori Taga, Appl. Phys. Lett. 88 (2006) 172103. [3] M.S. Arnold, Ph. Avouris, Z.W. Pan and Z.L.Wang, J. Phys. Chem. B 107 (2003) 659. [4] E. Comini, G. Faglia, G. Sberveglieri, W.Z.Pan and Z.L.Wang, Appl. Phys. Lett. 81 (2002) 1869. [5] X. Bai, E.G. Wang, P.X. Gao and Z.L.Wang, Nano Lett. 3 (2003)1147. [6] J.W.P. Hsu, D.R. Tallant D R, R.L. Simpson, N.A. Missert and R.G. Copeland, Appl. Phys. Lett. 88 (2006) 252103. [7] G.H. Du, F. Xu, Z.Y. Yuan and G. Van Tendeloo, Appl. Phys. Lett. 88 (2006) 243101. [8] A. Umar, B. Karunagaran, E-K. Suh and Y.B. Hahu, Nanotechnology 17 (2006) 4072. [9] R. Cebulla, R. Wendt and K. Ellmer, J. Appl. Phys. 83 (1998) 1091. [10] V.A. Karpina et al., Cryst. Res. Technol. 39 (2004) 980. [11] S.M. Jejurikar, A.G. Banpurkar, A.V. Limaye, S.K. Date, S.I. Patil, K.P. Adhi, P. Misra, L.M. Kukreja and Ravi Bathe, J. Appl. Phys. 99 (2006) 014907. [12] Paul Erhart and Karsten Albe, Appl. Phys. Lett. 88 (2006) 201918. [13] S.B. Zhang S B, S.-H. Wei S –H and A. Zunger, Phys. Rev. B 63 (2001) 075205.

Columnar growth of nanocrystalline ZnO thin films 149 [14] J. Shin. W and J.Y. Lee, T.W. Kim, Y.S. No, W.J. Cho and W.K. Choi, J. Appl. Phys. 88 (2006) 091911. [15] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma and Y. Segawa, Solid State Communications 103 (1997) 459. [16] N.F. Foster, J. Vac. Sci. Technol. 6 (1969) 111.

Table 1. Structural and surface morphological properties of ZnO films.

Substrate temperature (ºC)

27 100 200 300

DXRD (nm) 34.36 27.45 17.18 13.74 σfilm (GPa) -2.55 -0.85 -1.07 -1.57 δ (1012/m2) 847.22 1327.13 3386.90 5290.97 DAFM (nm) 311 140 108 100 Rq (nm) 76.21 3.30 3.14 12.28

150 K. Saravanakumar et al

Figure 1. XRD diffraction patterns of ZnO films grown on glass substrate for various

temperatures.

C

F

Columnar gr

Figure 2. AF

rowth of na

FM images

anocrystallin

of ZnO thin300 ºC

ne ZnO thin

n films deposubstrate te

in films

osited at (a) mperatures

27, (b) 100.

0, (c) 200 an

151

nd (d)

152 K. Saravanakumar et al

Figure 3. Cross-sectional a) bright-field and b) dark-field TEM images of ZnO film grown at 200°C.

Columnar growth of nanocrystalline ZnO thin films 153 The inset in figure (b) is a corresponding electron diffraction pattern with electron beam parallel to the [2 1 10] zone axis of ZnO.

Figure 4. Photoluminescence spectra of ZnO thin films grown on glass substrate for various substrate temperatures.

154 K. Saravanakumar et al

Received: December, 2010