Effects of thermal annealing on the microstructure of ZnO thin films grown on Si substrates

Abstract

ZnO thin films are of current interest because of their potential applications in optoelectronic devices, such as transparent conducting electrodes, gas sensors, varistors, solar cells, light-emitting diodes, and laser diodes, because they are large band-gap semiconductors with physical properties of low dielectric constants, large exiton binding energies, and excellent chemical stabilities. ZnO/Si heterostructures are of particular interest in the integration of optoelectronic devices due to the large excitonic binding energy of the ZnO thin film and the cheapness and the large size of the Si substrate. Potential applications of ZnO thin films in high-efficiency optoelectronic devices operating in the blue region of the spectrum have driven extensive efforts to grow high-quality ZnO thin films on Si substrates. However, the achievement of high-quality ZnO epilayers on Si substrates is very difficult due to the differences in the lattice constants and the thermal expansion coefficients of ZnO and Si, which deteriorate the crystal quality of the ZnO thin film and the performance of optoelectronic devices fabricated utilizing the ZnO layer. Since the optical properties of ZnO thin films are strongly affected by the microstructural properties of ZnO thin films and of ZnO/Si heterostructures, studies of such properties are necessary for fabricating high-quality optoelectronic devices utilizing ZnO/Si heterostructures. Therefore, systematic studies concerning the microstructural properties variations due to thermal treatment in ZnO thin films grown on Si substrates have been studied in this thesis. The initial formation mechanisms of the supersaturation region and the columnar grains were investigated by using X-ray diffraction pattern and transmission electron microscopy (TEM). ZnO thin films were grown on n-Si (001) substrates by using plasma-assisted molecular beam epitaxy. A cross-sectional bright-field TEM image showed that small ZnO columnar grains were embe...