Correlation Between Surface Morphology and Optical Properties of Quasi- Columnar Porous Silicon Nanostructures

In the current work, the effect of surface morphology on light emission property and absorption behavior of quasi-columnar macro-porous silicon (PS) was investigated. PS structures with different morphology were synthesized using photo-electrochemical etching method by applying different etching current densities. SEM micrographs showed that empty macro-pores size and porosity of PS layers were increased by increasing the current density which is due to passivity breakdown in initiated pore walls. Also, a decrease in pores density and a diminishing of nanocrystallites density on PS samples surface was observed for the samples with higher etching current densities and porosity. We conclude that porosity is not necessarily increased in order of forming more pores and formation of more nanocrystallites in PS structures. By increasing etching current density, a red shift in Raman peak position of PS samples was observed which was due to the residual stress and quantum confinement (QC) effects on the PS structures. PL peak position was also shifted to higher frequencies which is in good agreement with Raman red shift due to QC effect on finite size crystallites. Maximum PL intensity was observed for the sample with higher pore density and smaller pore size. Although the PS structure with maximum pore size had the maximum porosity, but it showed a remarkable decrease in PL intensity because of trapping of irradiated light in quasi-columnar empty macropores. Therefore, porosity is not the only factor having effect on optical properties such as photoluminescence efficiency. The morphology and size of pores can also be outstanding parameters responsible for variation of optical behavior of PS nanostructures.