ZnO Thin Films Grown on Porous Silicon by Plasma-Assisted Molecular Beam Epitaxy

ZnO thin films were grown on porous silicon by plasma-assisted molecular beam epitaxy. Thermal annealing was then carried out at various temperatures in the range from 500 to 700 degrees C for 10 min. Atomic force microscopy, scanning electron microscopy, X-ray diffraction, and photoluminescence were carried out to investigate the effects of the annealing temperature on the properties of the ZnO thin films. The ZnO thin films exhibit a mountain-range-like surface. With increasing annealing temperature, the grain size increased and the residual stress decreased. The activation energy of the free exciton (FX) emission from the ZnO thin films was found to be about 32 meV and the fitting parameters in Varshni's empirical equation were alpha = 2 x 10(-3) eV/K and beta = 1200 K. The photoluminescence (PL) intensity ratio of near-band-edge emission to deep-level emission of the ZnO thin films increased with increasing annealing temperature. (C) 2012 The Japan Society of Applied Physics