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 due to passivity breakdown in initiated pore walls. Also, a decrease in pore density and a diminishing of nanocrystallites density on PS samples surface was observed. 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) effect in the PS structures. PL peak position was also shifted to higher frequencies and it was in good agreement with Raman red shift due to QC effect in finite size crystallites. Maximum PL intensity was observed for 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. We report that porosity is not necessarily increase in order of creation more pores and more nanocrystallites formation in PS structures. Therefore porosity can not the only effective parameter for determination of optical properties such as photoluminescence efficiency. The morphology and size of pores can also be outstanding parameters for changing optical behavior of PS nanostructures.