Observations of phonon line broadening in the III-V semiconductors by surface reflection Raman scattering

Abstract

By means of surface reflection Raman scattering the dependence of the $q=0\mathrm{}$ TO- and LO-phonon line shapes on surface preparation in the compounds GaAs, InAs, GaSb, and InP has been investigated. Polished surfaces have broad phonon lines compared to etched, annealed, or cleaved surfaces. We show that phonon line broadening is a consequence of microscopic disorder generated during the polishing process. Temperature-dependence studies of phonon linewidths in polished and annealed GaAs surfaces demonstrate that the additional broadening observed in the polished surfaces is caused by a temperature-independent process. We conclude that the additional line broadening in the polished surfaces is caused mainly by inhomogeneous shifting of the phonon frequencies due to inhomogeneous strain near the sample surface. A secondary mechanism for phonon line broadening is increased phonon anharmonic decay in polished surfaces caused by local lowering of lattice symmetry in damaged surface layers. We observed that phonon line shapes in cleaved, etched, and 0.05-μm polished GaAs surfaces narrow as the optical skin depth of the exciting laser in increased; phonon lines in similarly prepared InAs surfaces broaden as the skin depth is increased. From this observation it is concluded that the magnitude of the strain in surface layers is peaked at ≅ 500 Å below the surface. The strain decreases beyond this depth and also relaxes to a lower value toward the surface.