Effects of annealing temperature on the structural and the optical properties of ZnO thin films grown on porous silicon by using plasma-assisted molecular beam epitaxy

Zinc oxide (ZnO) thin films were grown on Si and porous silicon (PS) by plasma-assisted molecular beam epitaxy (PA-MBE) and then an annealing process was carried out at various temperatures ranging from 500 to 700 °C for 10 min under an argon atmosphere. The effects of the PS and the annealing temperature on the structural and the optical properties of the ZnO thin films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The surface morphology of the ZnO thin films grown on PS exhibited a mountain range-like structure while a typical 3D island surface structure was observed from the ZnO thin films grown on Si. The grain size of the ZnO thin films increased as the annealing temperature was increased without a significant change in its shape. The ZnO thin films grown on PS showed only a ZnO (002) diffraction peak while several diffraction peaks indicating the existence of a large lattice mismatch were observed in the ZnO thin films grown on Si. The PL intensity ratio of the near-band-edge emission (NBE) to deep-level emission (DLE) for the ZnO thin films increased with increasing annealing temperature. The structural and the optical properties of the ZnO thin films were enhanced in the case of PS, and the properties were improved as the annealing temperature was increased to 700 °C.