Nanocrystalline ZnO thin films grown on porous silicon by Sol-gel method and effects of post-annealing

Nanocrystalline ZnO thin films were grown on porous silicon (PS) by sol-gel spin-coating method. Atomic force microscopy (AFM), X-ray diffraction (XRD), Raman, and photoluminescence (PL) were carried out to investigate the structural and the optical properties of ZnO thin films on PS. The surface morphology of the ZnO thin films on PS exhibits an island structure. Nanosized craters were formed on the surfaces of the ZnO thin films by the post-annealing. In comparison with the ZnO thin films on Si, a higher and a narrower (002) diffraction peak was observed from the ZnO thin films on PS, and their residual stress was relaxed. By the post-annealing, the crystallinity of the ZnO thin films on PS was further enhanced, and the residual stress was further relaxed. A white light luminescence with blue, green, and red emission peaks was observed from the ZnO thin films on PS, and the intensities of the emission peaks became uniform with post-annealing.