Optical absorption evidence of a quantum size effect in porous silicon

Abstract

This study presents optical transmission measurements performed on free-standing homogeneous porous silicon (PS) films of different porosities and substrate doping levels. The absorption coefficient curves deduced from these measurements, taking into account the total quantity of matter in the PS film, exhibit significant blue shift (up to 500 meV). These shifts, well correlated with the crystallite size variations with porosity and substrate doping observed by electron microscopy and gas adsorption experiments, are attributed to quantum size effects in the silicon microcrystallites.

Nanocrystalline Porous Silicon: Structural, Optical, Electrical and Photovoltaic Properties

“…For direct transitions, the experimental Eg of the films is about 2.64 -2.7 eV closer to the 3.3 eV value reported for powdered NPS supported between two transparent substrates by Feng (Feng & Tsu, 1994). The direct transition gives an Eg approximately 1.88 -2 eV closer to those reported for NPS used in optoelectronic applications (Lugo et al, 1998;Peng et al, 1996;Sagnes et al, 1993). High concentrations (25%) of HF produce free standing NPS with larger Eg, which is distinctive of highly porous films, whereas, in low concentrations (15%), the less porous films show lower Eg values.…”

“…At high porosities, a blue shift of the optical absorption of NPS can be observed (Sagnes et al, 1993), i.e., the Eg and n can be modified. (Bisi et al, 2000).…”

Degradation mechanism of hydrogen-terminated porous silicon in the presence and in the absence of light

“…The amplification of the porosity induced PL intensity could be explained as the extraction of strong PL by light scattering from the sidewalls of the Si crystallites, 24 whereas the blue shift could be attributed to quantum size effects. 25,26 The surface adsorbed OH − groups could react with the photogenerated h + to generate surfacebound ·OH at more negative redox potential (+1.5V vs. NHE) than the free OH − groups in solution, 27,28 whereas the photogenerated electrons transfer to the surface and react with the ubiquitous oxygen molecules to yield ·O 2 − at more positive potential (-0.28 V vs. NHE) than oxygen molecules. 29 Combining this with the above-mentioned ESR and PL measurements, we believed that electrons could react with oxygen molecules to yield ·O 2 − , whereas the adsorbed OH − groups on the surface of H-PSi wafers could react with photogenerated holes to generate ·OH active species.…”

Small-angle X-ray scattering study of the microstructure of highly porous silicon

“…Details concerning sample preparation can be found in ref. (6). Three samples having a porosity of 57, 66 and 74% and a thickness of about 41, 44.5 and 41 pm, respectively, have been prepared.…”