Variable Angle and Spectral Ellipsometry Study of Porous Silicon: Internal Structure of the Layers

For a set of porous silicon on monocrystalline substrate samples, variable-angle and spectral ellipsometry has been carried out within 40…80° angle of incidence range (at 625 nm), and at the angles of 60°, 65° and 70° (within 1000…2000 nm range of wavelength). Angular dependencies of ellipsometric parameters were analysed according to several models of the reflective structure: 1) large-thickness single layer of porous material (which acts as semi-infinite medium in comparison to the wavelength); 2) two-layer model incorporating the same structure, just with additional finite layer on top of it. Experimental curves demonstrate better agreement namely with this two-layer model. Refractive indices of the media incorporated in the models were found, as well as the thickness of top finite layer. It was found that the porosity of the samples tends to be slightly higher near the surface than in the depth. Presence of surface layers of about 1 and 4 um thickness was revealed by spectral ellipsometry and scanning electron microscopy on a few of the samples. These layers demonstrate blurred boundary with the underlying porous material.