Substrate induced modifications in the structural and

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1Department of Physics, Indian Institute of Technology Delhi, New Delhi – 110 016, India. 2Material ... Proceeding of International Conference on Recent Trends in Applied Physics and Material Science. AIP Conf. ... also like to acknowledge Ms. Rekha Gupta, research ... P. Kumar, J. P. Singh, Y. Kumar, A. Gaur, H. K. Malik.
Substrate induced modifications in the structural and optical properties of zinc oxide thin films Parmod Kumar, Hitendra K. Malik, and K. Asokan Citation: AIP Conf. Proc. 1536, 497 (2013); doi: 10.1063/1.4810318 View online: http://dx.doi.org/10.1063/1.4810318 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1536&Issue=1 Published by the American Institute of Physics.

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Substrate Induced Modifications in the Structural and Optical Properties of Zinc Oxide Thin Films Parmod Kumar1,*, Hitendra K. Malik1 and K. Asokan2 1

2

Department of Physics, Indian Institute of Technology Delhi, New Delhi – 110 016, India Material Science Division, Inter University Accelerator Centre, New Delhi – 110 067, India [email protected]

Abstract. Zinc oxide thin films were grown on two different substrates (quartz and ITO coated glass) using RF sputtering technique. X-ray diffraction study confirms the formation of hexagonal wurtzite phase of the grown films. The optical properties of the thin films studied using UV-Vis spectroscopy technique show no change in the optical band gap of the grown films, however the enhancement in absorption edge is observed for the films grown on ITO coated glass substrate as compared to quartz. Keywords: ZnO thin films, sputtering deposition, XRD, optical spectra and substrates induced modifications. PACS: 77.55.hf, 81.15.Cd, 07.85.Nc, 74.25.nd

polycrystalline in nature, although, both the films possess the same hexagonal wurtzite structure.

INTRODUCTION The wide variety of applications in zinc oxide (ZnO) is fuelled and fanned due to its wide band gap and large exciton binding energy. Moreover, it possesses excellent hardness, chemical stability, optical and piezoelectric properties, which makes it applicable in ultraviolet (UV) lasers, solar cells, photocatalysts, field emitters, gas sensors, piezonanogenerators and diluted magnetic semiconductors for spintronic device applications. Besides these applications, zinc oxide thin films are highly used in optoelectronic devices as compared to other wide band gap semiconductor materials. Due to the significant role of ZnO in optical devices such as short wave length lasers, it is of fundamental interest to understand how the substrates affect the optical and structural properties of ZnO thin film. Defect formation in ZnO by varying substrates has attracted keen interest because of the luminescence properties that can be modified. Many studies have been carried out in order to improve the optical properties of ZnO thin films using the different deposition parameters and annealing conditions including in the presence of different gases. There is hardly any report on the modifications in the optical properties of ZnO by varying the substrates. In the present work, we have studied the substrate induced modifications in the structural and optical properties of ZnO thin films grown using RF magnetron sputtering technique. The films grown on quartz substrate was highly oriented whereas the film grown on ITO coated glass substrate was

EXPERIMENTAL ZnO thin films were grown on two different substrates namely quartz and ITO coated glass respectively using RF sputtering technique. The ZnO target was prepared by sintering, 99.99% pure ZnO at 1000 ºC for 24 hours in ambient air. The distance between the target and the substrate was kept fixed at 10 cm. The target was pre-sputtered for 10 min before deposition of the films in order to remove any impurity attached with the target material. Sputtering was carried out in Ar gas environment using 100 W RF power without substrate heating. The deposition of the films was carried out for ten minutes. Structural properties of the grown films were studied using Philips X’pert PRO diffractometer and the optical properties were studied using (HITACHI3300) UV-Vis spectrophotometer at room temperature, in the wavelength range of 200-800 nm.

RESULTS AND DISCUSSION Fig 1 shows the XRD -2 scan of ZnO thin films grown on the quartz and ITO coated glass substrates. XRD patterns were analyzed using Powder-X software which shows that both the samples are in single phase with hexagonal structure. It can be seen from the graph that the films grown on quartz are highly oriented (002) direction whereas films grown on ITO coated glass substrate are polycrystalline in nature. The crystallite size for both the samples was calculated

Proceeding of International Conference on Recent Trends in Applied Physics and Material Science AIP Conf. Proc. 1536, 497-498 (2013); doi: 10.1063/1.4810318 © 2013 AIP Publishing LLC 978-0-7354-1160-9/$30.00

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corresponding to (002) peak at 2T position (34. 4o) using Scherer’s formula D=0.9O/ECosT, where  is the wavelength of x-ray radiation,  is the full width at half maximum (FWHM) of the peaks at the diffracting angle 2.

In addition to change in absorbance peak of the films, there are substantial tails observed towards lower energy side of the absorbance spectra in fig. 3.

FIGURE 3. Tauc’s Plot for ZnO thin films grown on quartz and ITO coated glass substrate.

The tails may be due to the scattering of a range of particulate sizes or some type of Urbach tail effect because of inter-grain depletion regions [4].

FIGURE 1. XRD patterns of ZnO thin films grown on quartz and ITO coated glass substrates.

To understand the substrates dependency on optical absorption and band gap energy of ZnO thin films, UV Visible absorption spectra were recorded and the results obtained are shown in Fig 2. From the fig it is observed that the absorbance value is higher for the films deposited on ITO coated glass substrate than the film deposited on quartz substrate.

CONCLUSION ZnO thin films were grown on two different substrates. The films grown on quartz are highly oriented whereas for ITO coated glass, they are polycrystalline. There is no change in the band gap values of ZnO thin films on both the substrates however absorbance is larger in ITO coated glass compared to that of the quartz substrate.

ACKNOWLEDGMENTS One of the authors (PK) would like to acknowledge Indian Institute of Technology Delhi, New Delhi for funding and Prof S. Annaporni, Delhi University, Delhi for the experimental facilities. The author would also like to acknowledge Ms. Rekha Gupta, research scholar, Delhi University Delhi for the experimental support.

FIGURE 2. UV-Vis spectra of ZnO thin films.

However the absorption edge overlaps for both the films. The band gap energy (E g) of the thin film was calculated from the Tauc’s plot assuming (h)n = (h - Eg), Here,  is the absorption coefficient, h is the energy of the radiation (photon energy), Eg is the optical band-gap of the materials. Superscript (n) represents the nature of transition and has value of 2 for direct allowed transition [3] as shown in fig 3. The band gap of the films was found to be 3.3 eV. There is no change in the band gap of the films grown on different substrate. The difference in absorption value may be due to different nature of substrates.

REFERENCES 1. U. Ozgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho and H. Morkoc, J. Appl. Phys., 98 041301(1-104), (2005). 2. P. Kumar, J. P. Singh, Y. Kumar, A. Gaur, H. K. Malik and K. Asokan, Curr. Appl. Phys.12 1166-1172 (2012). 3. J. Tauc, Amrphous and Liquid Semiconductors, Plenum Press, London – 1971. 4 F. Urbach, Phys. Rev. 92 (1953) 1324 - 1324.

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