Microstructure and optical properties of submicron porous silicon thin films grown at low current densities

The mechanism of porous silicon (PS) film growth remains a debate among researchers. This study investigates the effect of current density (J) and anodization time (t) on the film thickness, optical properties, and microstructure of films grown at low current densities. The samples reported herein were generated from polished and unpolished 〈100〉 single crystal wafers, with resistivities of 0.1 and 0.6 Ω cm, respectively. Current densities of 1–10 mA/cm2 and anodization times ranging from 3 to 900 s were used. This work shows that film growth is at first linear but beyond a given anodization time (dependent on J), the film produced is not continuous, but is best described as a finite number of stacked sublayers with each sublayer having a unique microstructure, porosity, and optical response. This article also shows that as the PS film grows, second and third order peaks appear in the reflectance spectra. This data, together with modulations in the index of refraction spectra and visual evidence from cross-sectional scanning electron microscopy (SEM) images demonstrate that submicron PS thin films grown at low current densities have a multilayer structure. Above a certain thickness, the modulation in the properties due to the sublayers phases out and the films appear as if they were single layered films. SEM is used to show the progression, from cross-sectional imaging, from a layered or stacked PS film structure to an apparently homogeneous film with depth. When using low current densities, single layer films are only obtained at very short times.