Role of nanocrystalline ZnO coating on the stability of porous silicon formed on textured (1 0 0) Si

In this study, a colloid of nanocrystalline ZnO particles prepared by chemical route is sprayed on porous silicon layers. Porosity and thickness of PS layers were estimated by gravimetric analysis. Upon adsorption of ZnO colloids on PS films, oxidation of nanocrystalline Si causes shrinkage of the Si-core due to the breaking of Sisingle bondSi bonds resulting in a blue-shift in PL spectra. The PL blue-shift can also be related to Sisingle bondO species or due to defects and the silica networks on which OH groups are absorbed due to ZnO incorporation as also supported by our Fourier transform infrared (FTIR) and X-ray photoelectron (XPS) studies, respectively. From high resolution X-ray diffraction (HRXRD) studies, a better crystalline perfection and considerable reduction in stress/strain values were observed for PS/ZnO layers as compared to virgin PS layers. The changes in the chemical composition at the surface of PS upon adsorption of ZnO colloids as elucidated by FTIR and XPS studies could be responsible for different PL emission and lattice-mismatch characteristics. The improved stability properties of PS are attributed to the strong absorption/adsorption of ZnO into the highly porous vertical layers separating macroscopic domains of nanoporous silicon and the mechanism of light emission from PS/ZnO layers is discussed on the basis of proposed energy band gap diagram.