Photoluminescence Of Porous Silicon Fabricated By Electrochemical Method

Investigations on porous silicon (PS) with strong room-temperature visible emission by Canham have stimulated much effort to explore its emission properties. PS film because of its unique microstructure, significant quantum effect, and the compatible process with Si IC industry has a great potential in a variety of optoelectronic device applications. Recently, much meaningful work has been done on the formation mechanism, the luminescence phenomena of PS, and its possible applications. Porous silicon films were fabricated by direct current (DC) electrochemical method under different conditions in this work. The microstructure and photoluminescence properties of PS in visible waveband were studied.Fluorescence spectrometer and scanning electron microscope (SEM) were employed to characterize the photoluminescence and surface morphology of samples. Fourier transform infrared spectroscopy (FTIR) was used to characterize its chemical information.Keeping other experiment conditions unchanged, we only change the current density. SEM images show that when we applied small current, the number of the hole formed on the Si is little. The wall between the holes is relatively thick. That is to say, Si line is wide. As the current density increased, the wall between holes becomes thin and Si line is slim."Hill-like"silicon mounds on the surface of PS were observed by AFM. As the current density increased, the silicon mounds became dense. This indicates that more silicon was etched. PL measurements show that there are three emissions located at 360 nm,380 nm and 470 nm, respectively, in our sample. The peak at 380 nm comes from the combination of the holes and electrons in the surface states of PS. The peak at 360 nm is thought to be related with SiO2 from the surface oxidation of PS. The signal at 470 nm is caused by the Si-C bond formed by the reaction between C breaking off from anode and Si. The change of current doesn't cause the position shift of 380 nm peak. There exist three significant absorption peaks in FTIR. They are located at 610 cm?1, 734 cm?1 and 1110 cm-1 respectively. The peak at 610 cm?1 is due to the stretching vibration of Si-Si bond. 734 cm?1 absorption is attributed to the vibration of Si-C (Si1?xCx,x<0.5). 1110 cm-1 is caused by the antisymmetric stretching vibration of Si-O-Si in SiO2 formed by the surface oxidation of PS. We also note that if the current is too large, the solution will polish the surface of PS. In our experiment, the optimal current density is 50 mA/cm2. Keeping the solution concentration and current density constant, only etching time was changed. AFM observation shows the increase of the porosity of PS with the time increased, which caused the surface states increase. The intensity of 380 nm peak increased significantly with the increase in etching time. Increasing the current further, the surface of PS was polished. When the etching time is 30 min, we can obtain the most intense emission.PS was placed in the atmosphere. As the aging time increase, the oxidation layer becomes thick. The microstructure of PS coated with an oxidation layer becomes stable. The defect density decreased. The combination related with the defects becomes small.Only the solution concentration is changed, while other experiment conditions unchanged. PL measurements indicate that the larger the solution concentration is, the more obvious etching effect shows. The rich surface states bring more intense emission. At the same time, there will be more and more C deciduous from anode, forming more Si-C.