CHARACTERIZATION OF GAAS WIRE CRYSTALS GROWN ON POROUS SILICON BY RAMAN-SCATTERING

We measured the Raman spectra of GaAs wirelike crystals grown on porou s silicon (PS) using two different excitation radiations which probe t he near surface and the bulk. The transverse optic and longitudinal op tic vibrational bands appear redshifted and broadened when compared to bulk GaAs, and with shoulders on their low frequency sides. These fea tures 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 d efect density are invoked to account for the observations. This analys is yields a picture in which there is a predominantly amorphous GaAs r egion buried at the PS-GaAs interface followed by microcrystalline GaA s cylindrical structures that become less defective as they grow farth er from the interface. The near surface tends to approach the low defe ct density of crystalline GaAs. (C) 1997 American Institute of Physics .