Optical properties of porous nanosized GaAs

The optical properties of porous GaAs layers obtained by electrochemical etching of single-crystal n-and p-GaAs(100) wafers are studied. It is shown that the shape of the nanocrystals, their mean diameter, and their surface states depend on the conductivity type of the initial crystal. A shift of the peaks corresponding to the main optical phonons to lower frequencies in the Raman spectra is observed. Surface-phonon frequencies determined from the Raman spectra coincide with those determined from the reflection spectra in the infrared spectral region. The forms of the spectral dependences of the reflection coefficient in the phonon-resonance region in bulk GaAs differ from those in porous GaAs. This is caused by the appearance of additional oscillators related to spatially confined lattice vibrations in GaAs nanocrystals. Atomic-force microscopy is used to study the surface morphology of porous GaAs samples formed on the n-GaAs substrates, and a nanosized surface profile is observed. Estimations made for the mean diameter of GaAs nanocrystals based on data from the Raman scattering, infrared spectroscopy, photoluminescence, and atomic-force microscopy yield results that are in good agreement with each other.