Homogeneous luminescent stain etched porous silicon elaborated by a new multi-step stain etching method

This paper presents a new method to produce porous silicon which derived from the conventional stain etching (SE) method. But instead of one etching step that leads to formation of porous layer, the substrate is subjected to an initial etching step with a duration Δt0 followed by a number of supplementary short steps that differs from a layer to another. The duration of the initial step is just the necessary time to have a homogenous porous layer on the whole surface of the substrate. It was found that this duration is largely dependent of the doping type and level of the silicon substrate. The duration of supplementary steps was kept as short as possible to prevent the formation of bubbles on the silicon surface during silicon dissolution which leads generally to inhomogeneous porous layers. It is found from surface investigation by atomic force microscopy (AFM) that multistep stain etching (MS-SE) method allows to produce homogeneous porous silicon nanostructures compared to the conventional SE method. The chemical composition of the obtained porous layers has been evaluated using Fourier transform infrared spectroscopy (FTIR). Photoluminescence (PL) measurement shows that porous layers produced by SE and MS-SE methods have comparable spectra indicating that those layers are composed of nanocrystallites with comparable sizes. But the intensity of photoluminescence of layer elaborated by MS-SE method is higher than that elaborated by the SE method. Total reflectance characteristics show that the presented method allows the production of porous silicon layers with controllable thicknesses and optical properties. Results for porous silicon layers elaborated on heavily doped n-type silicon show that the reflectance can be reduced to values less than 3% in the major part of the spectrum.