Diffuse reflectance spectroscopy: An effective tool to probe the defect states in wide band gap semiconducting materials

Optical properties of widely used semiconducting oxides namely; TiO₂, ZnO and ZrO₂ were investigated using diffuse reflectance spectroscopy (DRS). Prior to the optical absorption measurements, the structural purity of these samples was examined using powder x-ray diffraction experiments carried out on Indus-2 synchrotron source. It is observed that all the studied samples are structurally pure. The DRS of all the studied samples show an extra peak much below fundamental band gap. In order to understand the origin of the said low energy peak, the theoretical optical absorption spectra for these samples have been simulated. The simulations were performed using density functional theory, considering, ideal as well as defected systems i.e. by considering vacancy at all possible sites (for TiO₂ we have considered the vacancy at Ti site and also at O site taking in to account surface and bulk effects). It is observed that the simulated optical spectra show very similar feature as that of experimental optical absorption for oxygen vacancy. Photoluminescence spectroscopy further supports the presence of defect states in the studied samples. Thus; it appears that the diffuse reflectance spectroscopy is a useful tool to probe the signature of defects present in the sample.