Raman scattering and luminescence polarization anisotropy in porous Si

Abstract

Polarization anisotropy of the phonon and luminescence bands are observed in porous Si of different porosity. The depolarization ratio r, defined as (( I xy − I xx )/( I xx + I xy )), of the t 2g phonon (521 cm −1) peak in crystalline Si decreases from 1 to 0.1 with increasing porosity. This is attributed to the increasing disorder in high porosity films. On annealing the 70% porosity sample at higher temperatures in air, the value of ρ decreases from 0.5 to 0.3 which is indicative of an increasing disorder in Si nanostructures. We believe that Si crystallites form a random network structure surrounded by a thick SiO 2 matrix upon annealing, and the Si phonon mode becomes depolarized as it is observed in amorphous Si. A small (5%) excitation wavelength dependence of ρ is detected. The luminescence polarization anisotropy is only detected (∼8%) at shorter excitation wavelengths. This indicates that the dipole created by the electron-hole pair gets randomly oriented possibly owing to the irregular shapes and sizes of the nanocrystallites; a phonon-mediated recombination of the exciton also depolarizes the emission.