Influence of applied current density on the nanostructural and light emitting properties of n-type porous silicon

Effects of current density on nanostructure and light emitting properties of porous silicon (PS) samples were investigated by field emission scanning electron microscope (FE-SEM), gravimetric method, Raman and photoluminescence (PL) spectroscopy. FE-SEM images have shown that below 60 mA/cm(2), macropore and mesopore arrays, exhibiting rough morphology, are formed together, whose pore diameter, pore depth and porosity are about 265-760 nm, 58-63 mu m and 44-61%, respectively. However, PS samples prepared above 60 mA/cm(2) display smooth and straight macropore arrays, with pore diameter ranging from 900-1250 nm, porosity of 61-80% and pore depth between 63-69 mu m. Raman analyses have shown that when the current density is increased from 10 mA/cm(2) to 100 mA/cm(2), Raman peaks of PS samples shift to lower wavenumbers by comparison to crystalline silicon (c-Si). The highest Raman peak shift is found to be 3.2 cm(-1) for PS sample, prepared at 90 mA/cm(2), which has the smallest nanocrystallite size, about 5.2 nm. This sample also shows a pronounced PL, with the highest blue shifting, of about 12 nm. Nanocrystalline silicon, with the smallest nanocrystallite size, confirmed by our Raman analyses using microcrystal model (MCM), should be responsible for both the highest Raman peak shift and PL blue shift due to quantum confinement effect (QCE).