Optical and microstructural characterization of porous silicon using photoluminescence, SEM and positron annihilation spectroscopy

We have studied the dependence of porous silicon morphology and porosity on fabrication conditions. N-type ( 100) silicon wafers with resistivity of 2-5 Omega cm were electrochemically etched at various current densities and anodization times. Surface morphology and the thickness of the samples were examined by scanning electron microscopy (SEM). Detailed information of the porous silicon layer morphology with variation of preparation conditions was obtained by positron annihilation spectroscopy (PAS): the depth-defect profile and open pore interconnectivity on the sample surface has been studied using a slow positron beam. Coincidence Doppler broadening spectroscopy (CDBS) was used to study the chemical environment of the samples. The presence of silicon micropores with diameter varying from 1.37 to 1.51 nm was determined by positron lifetime spectroscopy ( PALS). Visible luminescence from the samples was observed, which is considered to be a combination effect of quantum confinement and the effect of Si=O double bond formation near the SiO2/Si interface according to the results from photoluminescence (PL) and positron annihilation spectroscopy measurements. The work shows that the study of the positronium formed when a positron is implanted into the porous surface provides valuable information on the pore distribution and open pore interconnectivity, which suggests that positron annihilation spectroscopy is a useful tool in the porous silicon micropores' characterization.