Optical Properties and Crystallinity of ZnO Thin Films Grown on Porous Silicon by Using Plasma-assisted Molecular Beam Epitaxy

Zinc oxide (ZnO) thin films were grown on porous silicon (PS) at different growth temperature by using plasma-assisted molecular beam epitaxy (PA-MBE). From photoluminescence (PL), the asymmetric-shaped near-band-edge emission (NBE) peaks comprised of a free exciton (FX) and a longitudinal-optical phonon replica of the FX (FX-1LO) were observed from the ZnO thin films at room temperature (RT). The intensity ratio of the FX and the FX-1LO peaks decreased with increasing the temperature. The Huang-Rhys factor S of the ZnO thin films increased linearly from 0.576 to 1.519 with increasing the temperature, resulting from a decrease in the probability of exciton-phonon scattering. The crystal quality of the ZnO thin films was degraded with increasing the temperature. Nonetheless, the degradation of the crystal quality for the ZnO thin films grown on PS was much smaller than that of the ZnO thin films grown on Si substrates at higher temperatures.