Effects of Annealing on Structural and Optical Properties of ZnO Nanowires
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Effects of Annealing on Structural and Optical Properties of ZnO Nanowires Anas Mazady, Abdiel Rivera, and Mehdi Anwar Electrical and Computer Engineering, University of Connecticut, Storrs, CT 06269, Email: [email protected]
ABSTRACT We report, for the first time, effects of annealing of ZnO NWs grown on p-Si substrates. ZnO NWs are grown using metalorganic chemical vapor deposition (MOCVD) and thermal annealing was performed in situ under nitrogen ambient at different stages of the growth process. Increasing the annealing temperature of the ZnO seed epi-layer from 635 °C to 800 °C does not affect the morphology of the grown NWs. In contrast, annealing the NWs themselves at 800 °C results in a 48% decrease of the surface area to volume ratio of the grown NWs. The optical quality can be improved by annealing the seed layer at a higher temperature of 800 °C, although annealing the NWs themselves does not affect the defect density. INTRODUCTION ZnO is a promising material for optoelectronic applications, such as laser diodes, optical sensors, solar cells, light emitting diodes (LEDs), and quantum cascade lasers (QCLs) due to its large direct band gap energy of 3.37 eV and a large excitonic binding energy of 60 meV [1, 2]. ZnO nanowires (NWs) exhibit large spontaneous and strain-induced piezoelectric polarizations required for energy harvesting applications [3]. In addition, being chemically stable and biocompatible, ZnO NWs-based devices can be used for bio-sensing applications [4]. Growth of ZnO nanowires (NWs) has been reported on different substrates, with Si substrates being of particular interest in order to maintain CMOS process compatibility [5]. However, 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, as has been investigated in this paper, can reduce this built-in residual stress and hence can improve the crystalline quality and minimize defects of the grown ZnO NWs. Native defects such as Zn and O vacancies vary upon growth conditions depending on the formation energy. These defects can degrade the crystal and optical quality of the (film or NWs). Nevertheless, these defects can be suppressed by post annealing process depending on the temperature and atmosphere (under air, N, argon, etc). Borseth et al. [6] demonstrated suppression of native defect, such as, VZn- and VO+, by annealing under O-rich, Zn-rich, and under a mixture of both atmospheres. Chen et al. [7] used positron annihilation spectroscopy to study vacancy defects in ZnO as a function of annealing temperature. The annealing experiment showed that the VZn- defects suppress at 600 °C while annealing at temperatures above 1000 °C produces these defects again. Considering the short windows to passivate native defects on ZnO, we investigated the impact of annealing on ZnO NWs before and after the growth in order to improve the crystal and optical quality.
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EXPERIMENT ZnO NWs are
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