POROSITY-INDUCED MODIFICATION OF THE PHONON-SPECTRUM OF N-GAAS

Porous GaAs layers have been produced by anodic etching of (100)-orien ted crystalline substrates in a H2SO4 solution. Scanning electron micr oscope images showed the formation of submicron pores, the average dim ension of the remaining GaAs walls being of about 100 nm. Raman scatte ring by LO-phonon-plasmon coupled modes, inherent in as-grown crystals , was not observed in the porous layers. Proposed explanations are eit her the depletion of the GaAs skeleton due to the surface space-charge effect or the decoupling of the LO-phonon and the plasmon modes at th e relative large wavevectors transferred in nanostructures. A new Rama n scattering peak at 275 cm(-1), located between the bulk TO and LO fr equencies, has been observed in porous layers and attributed to a surf ace-related phonon.