Effects of silicon porosity on physical properties of ZnO films

We report on structural and optical properties of ZnO thin films deposited on different Si-based substrates presenting different porosities. ZnO layers were prepared by sol gel method and deposited on crystalline silicon (ZnO/Si), mesoporous silicon (ZnO/PS+) and nanoporous silicon (ZnO/PS−) by spin coating. Several techniques such as scanning electron microscope (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence spectroscopy (PL) and spectroscopic ellipsometry (SE) were used to study the influence of the pore size of porous silicon (PS) on physical properties of ZnO films. SEM images revealed the formation of ZnO granular nanoparticles on Si, PS− and PS+ substrates. We show by the XRD analysis that hexagonal crystallized (002) ZnO is mainly obtained for ZnO/PS− system causing by a strong absorption of the capillary effect and high adhesion to PS− surface. An intense PL related to ZnO and PS− was demonstrated for ZnO/PS− in UV and visible ranges. Optical properties of ZnO were determined and analyzed by SE using Tanguy dispersion model. For each sample, a specific optical model was carried out. SE confirms a good physical properties of ZnO/PS− comparing to ZnO/Si and ZnO/PS+. For example, the good crystallinity is characterized by low damping factor value (Γ). This value was found by SE to be low (29 meV) for the ZnO/PS−, while the damping factors of ZnO/Si and ZnO/PS+ are 47 meV and 70 meV, respectively. The amplitude of dielectric function of ZnO/PS− around 3.4 eV reveals an increase of grain size and crystallinity of ZnO layer.