OPTICAL-ABSORPTION IN POROUS SILICON OF HIGH-POROSITY

We report IR and absorption edge spectra of porous silicon (PS), covering a porosity range where the average size of the domains is comparable with the pore radius. There are two distinct IR absorption groups related to degree of specimen ''aging'' and porosity; these can be attributed to CH(i), H(C(j)H(i)) and to SiO(m), Si(OH(i)) bond vibrations. The last two dominate in high porosity specimens. The absorption edge spectra can be deconvoluted into a band-to-deep state absorption at congruent-to 1 eV and a parabolic band-to-band absorption, with energy gap E(g) = 1.6-2 eV and an absorption shoulder at hv less-than-or-equal-to E(g). The optical density of bands is significantly reduced for porosity above 70%. Optical parameters of an absorption shoulder correlate approximately with photoluminescence spectra and the strength of CH(i) and H(C(j)H(i)) vibrations.