Enhancement of the Raman phonon spectra of porous silicon films by H + ion implantation

Abstract

The enhancement of first and second orders Raman phonon spectra of porous silicon (PS) layers generated by anodization of previously H + ion implanted c-Si wafers is reported. The investigated PS samples present a two-layer structure. The outer layer is ca. 1 μm thick and preserves the same crystal structure of the c-Si substrate with no peak shift and broadening. The inner layer, ca. 10 μm thick, has the usual structure of PS films where the LO Raman phonon wavenumber is ca. 10 cm −1 downshifted and broader than the LO Raman phonon of c-Si. The 520 cm −1 LO phonon of the outer layer can be 9–70 times more intense than the LO phonon of the c-Si substrate, depending on the sample preparation. Raman spectra excited at different wavelengths and reflectance measurements have shown that the enhancement mechanism is due to a resonance Raman effect involving electronic states responsible for an electronic absorption near 450 nm. The H + ion implantation has the effect of increasing the density of electronic states near 450 nm, causing not only the enhancement of the Raman phonon spectrum as well as the enhancement of H + ion implanted layer photoluminescence (PL).