Electrochemical preparation of a rugate filter in silicon and its deviation from the ideal structure

Abstract

Porous silicon single layers are formed by anodization of silicon in a hydrofluoric acid solution at different current densities. An accurate estimation of the etch rates and refractive indices is performed. The refractive index-depth profile of a porous silicon rugate filter (PSRF) prepared by sinusoidally modulating the current density during etch is investigated. The produced PSRF is found to have a periodic pseudo-sinusoidal variation of refractive index with depth. The sinusoidal current density waveform that was employed generates a nonideal refractive index-depth profile with an asymmetric period. It is found that this deformed refractive index profile affects the quality of the characteristic resonant peak of the rugate filter. A simple modification of the current density waveform can be implemented, which allows one to achieve a true sinusoidal refractive index-depth profile characteristic of a rugate filter. The modified current density waveform allows the generation of a PSRF with a low index contrast and a sharp resonant band. The ability of these structures to act as chemical vapor sensors is tested and compared with the existing literature. The filters exhibit a red-shift in the photonic feature upon exposure to saturated ethanol vapor. The modified signal does not impose a significant change on the filter sensitivity to ethanol vapor.