Effect of Growth Temperature and Annealing on ZnO
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Effect of Growth Temperature and Annealing on ZnO A. L. Cai, J. F. Muth*, M. J. Reed ECE Department, North Carolina State University H. L. Porter, C. Jin, J. Narayan Materials Science and Engineering Department ABSTRACT A series of epitaxial single crystal ZnO samples were grown by Pulsed Laser Deposition (PLD) on c-axis, double-side polished sapphire at temperatures ranging from 600oC to 800oC. A comprehensive optical study, consisting of cathodoluminescence (CL) and transmission spectroscopy measurements, was performed before and after annealing the samples in air and oxygen environments. The structural characteristics of the films were observed by x-ray diffraction and transmission electron microscopy (TEM). Using these measurements, the data revealed that annealing significantly improved the luminescence and exciton line-width of the better material grown at the higher end of the temperature range. In lower quality samples grown at the lower end of the temperature range, the optical quality of the material was degraded. In the samples grown at higher temperatures, annealing improved the optical qualities. An increase in the below band-gap, green band emission was also observed under some annealing conditions. I. INTRODUCTION Zinc Oxide (ZnO) has attracted substantial attention as a promising material for optoelectronic devices1. It is a material very similar to gallium nitride2 which has been successfully used to make high brightness LEDs and blue laser diodes. Both material systems have the wurtzitic crystal structure and almost the same in-plane lattice constant and band gap. ZnO has direct band gap energy of 3.3 eV at room temperature (RT) and a large excitonic binding energy of ~60 meV. High quality ZnO films grown on (0001) GaN template substrates have been achieved recently.3 Schottky diodes fabricated with high quality epitaxial ZnO films have been characterized4. However, due to the wide band gap and its low energy native point defects5 it has been difficult to obtain good p-type ZnO. And there are other reports of p-type doping of ZnO that suggest that it may be come a viable material for light emitters and other bipolar devices6. In this study we focus on the optical properties of ZnO as a function of initial growth temperature from 600–800oC and as a function of subsequent annealing conditions in air and oxygen. Particular emphasis is placed on how the exciton resonance changes under the different annealing conditions, since the exciton resonance provides a sensitive indicator of material quality. Mechanisms that broaden or shift the exciton resonance such as impurities, doping, strain and phonon interactions, in turn, broaden and shift the absorption band edge and influence the optical constants near the band gap7, 8, 9. In this paper we show that excitonic resonance is correlated with the quality of ZnO films and that the effects of annealing and associated changes *
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in the optical quality of zinc oxide thin films can be quantified by measuri
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