Characterization of GaAs wire crystals grown on porous silicon by Raman scattering

We measured the Raman spectra of GaAs wirelike crystals grown on porous silicon (PS) using two different excitation radiations which probe the near surface and the bulk. The transverse optic and longitudinal optic vibrational bands appear redshifted and broadened when compared to bulk GaAs, and with shoulders on their low frequency sides. These features are attributed to a disorder-induced relaxation in the selection rules for Raman activity arising from point defects. In addition, the bands show a strong frequency and bandwidth dependence on excitation energy. Differences in penetration depth together with a gradient in defect density are invoked to account for the observations. This analysis yields a picture in which there is a predominantly amorphous GaAs region buried at the PS–GaAs interface followed by microcrystalline GaAs cylindrical structures that become less defective as they grow farther from the interface. The near surface tends to approach the low defect density of crystalline GaAs.