Optical Absorption and Quantum Confinement in Porous Silicon Nanostructures Studied by Chemical Dissolution in HF Solutions and Photoconduction

Abstract

The progress of the chemical dissolution of porous silicon (PSi) formed from lightly-doped p-type silicon in ethanoic HF solutions was monitored by recording in situ the photocurrent from monochromatic illuminations, which was used as a measure of optical transmission. The relations between dissolution time, porosity, and absorption coefficient were established and the porosity-dependence of the absorption coefficient derived from ∼60% porosity to 100% porosity. The absorption results were discussed considering the Bruggeman model of effective medium approximation and other measurements from the literature, together with the effects of quantum confinement (QC) and surface states. The porosity and spectral dependences of the QC in the absorption spectra were clearly observed. QC in the blue spectral range (<500 nm) was found to require extremely high porosities (>85%), contrary to the red to green region, where QC was identified for the whole porosity range studied. Our procedure allows the continuous exploration of a wide range of porosities, without limitation on the high side, while preserving an ideal hydrogen-terminated PSi surface and very good structural integrity, as PSi is always kept in HF solution and never dried. The study also allows the determination of the dissolution rate of silicon in various HF-based solutions.

Photoetching of Porous Silicon Nanostructures in Hydrofluoric Acid Using Monochromatic Light

“…28 Immediately after PSi formation, the solution was replaced by (HF55 wt%: Ethanol) = 1.75:1 (i.e HF wt% = 35%). The chemical etching rate in HF 35% is very low, as shown in our previous work, 27 which means that chemical etching will not affect the photoetching process in our experiments.…”

“…In this case, R should not depend on x, nor would n and S (situation similar to chemical etching). 27 R would also not depend on t. It would be the rate R 0 of the photoetching reaction (Eq. 1) after the initial hole capture.…”

“…The photocurrent was measured during photoetching in the same way as in our previous works. 26,27 Briefly, −2 V was applied to the silicon back side with respect to a platinum electrode in the solution. Since silicon was reversed biased, only a very small current could flow in the dark and the photocurrent could be measured when light was illuminating PSi through the solution.…”

“…In this paper, we show how the photoetching of PSi (initially luminescent or not) can be monitored and its rate evaluated using a photoconduction technique and a proposed model. The photoconduction method was used previously to measure PSi optical constants 26,27 and study PSi chemical etching in HF. 27 It is based on the fact that the photocurrent of liquid/PSi/Si system, when illuminated from the liquid side, is proportional to the optical transmission through PSi, and thus, changes in PSi absorption during photoetching can be understood from the evolution of the photocurrent.…”

“…The photoconduction method was used previously to measure PSi optical constants 26,27 and study PSi chemical etching in HF. 27 It is based on the fact that the photocurrent of liquid/PSi/Si system, when illuminated from the liquid side, is proportional to the optical transmission through PSi, and thus, changes in PSi absorption during photoetching can be understood from the evolution of the photocurrent. The photoetching can now be well controlled and monitored, opening the door toward its usage for various post-PSi formation treatments.…”