Optimization of Annealing Conditions for ZnO-based Thin Films Grown Using MOCVD
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Optimization of Annealing Conditions for ZnO-based Thin Films Grown Using MOCVD Anas Mazady, Abdiel Rivera, and Mehdi Anwar Electrical and Computer Engineering, University of Connecticut, Storrs, CT 06269, Email: [email protected]
ABSTRACT In this work, effects of thermal annealing on the structural and optical properties of ZnO thin films grown on p-Si and GaN substrates using metalorganic chemical vapor deposition (MOCVD) are investigated. Annealing at 600 °C results in optimum crystal and optical qualities of the ZnO thin films on both substrates. Smaller lattice mismatch between grown ZnO epitaxial layer on GaN substrates results in better film morphology as compared to p-Si substrates. Higher annealing temperature along with a slower thermal ramp provides better crystal quality of ZnO thin films on both substrates. Annealing ZnO thin films at 700 °C with a slower thermal ramp results in better crystal quality as is evident from a 56% reduction in the full-width at half maximum (FWHM) of the (002) peak compared to the as-grown films. The optical quality also enhances with a slower annealing rate. The determination of the optimum annealing conditions for different substrates has important implications in fabricating optimized and efficient ZnO based electronics. INTRODUCTION ZnO, being a transparent conductive oxide, has drawn considerable interests in fabricating transparent electrodes for display devices, transparent electronics, and photovoltaic devices, to name a few [1]. A large direct bandgap energy of 3.37 eV makes it an ideal candidate for optoelectronic applications, such as, solar blind ultra-violet (UV) detectors, UV diodes, LEDs, among others. Reasonable smaller lattice mismatch of 2% with GaN also allows ZnO to be a good substrate material for GaN based devices, as stand-alone GaN substrates are not available yet [2]. Being resistant to radiation damage, compared with other semiconductors, makes ZnO a suitable candidate for space applications [3]. The large lattice mismatch of 40% and large difference in the thermal expansion coefficients of 87% between ZnO and Si substrates cause built-in residual stress in the grown ZnO NWs. Proper annealing conditions can reduce this built-in residual stress and hence can improve the crystalline quality and minimize defects of the grown ZnO NWs. Post annealing not only passivates native defects but also improves the near band-edge emission of ZnO films through reduction of non-radiative recombination centers in the films [4]. It also produced a smooth surface of the film required for most optoelectronic application. Sengupta et al. [5] investigated the annealing effects on ZnO film grown via sol-gel. They observed that the intensity of (002) peak gradually increases with increasing annealing temperature, giving rise to a smaller full width at half maxima (FWHM) and an average larger grain size. This phenomenon is attributed to the thermal annealing induced coalescence of small grains by grain boundary diffusion, resulting in larger grain size [6].
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