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

Zinc oxide (ZnO) thin films were grown on porous silicon (PS) by plasma-assisted molecular beam epitaxy (PA-MBE). The thin films were annealed in various atmospheres such as argon, nitrogen, and vacuum, and their structural and optical properties were investigated by scanning electron microscopy, X-ray diffraction, and photoluminescence. The ZnO thin films grown on PS showed a mountain-range-like surface morphology, whereas those grown on Si showed a typical 3D island surface structure. The thin films grown on PS exhibited only one diffraction peak at 34°, whereas those grown on Si showed shoulders of the ZnO (002) diffraction peaks at around 33°; this implies an excellent c-axis preferred orientation and a better crystal quality when PS was used. Large crystals were partially formed at an annealing temperature of 700°C. The films annealed in a vacuum showed nanorod-like ZnO crystals, whereas those annealed in nitrogen and oxygen showed irregularly shaped crystals. It was confirmed that the structural and optical properties of the thin films were enhanced by the annealing process. In particular, relatively large changes in the full width at half maximum of the ZnO (002) diffraction peaks and UV emission peaks, indicating enhanced structural and optical properties, respectively, were observed when the thin films were annealed in argon.