Reconstruction of GaAs/AlAs (311) and (100) Interfaces: Raman Study

Abstract

Phonon spectra of GaAs/AlAs superlattices (SLs) grown on facet surfaces (311)A,B and (100) surface were studied using Raman spectroscopy. Thickness of GaAs layers in (311) direction was varied from 1 to 10 monolayers, thickness of AlAs barriers was 8 monolayers. Using selection rules for (311)A and B surfaces, different polarisation geometries of Raman scattering were applied to study LO and TO modes. The splitting of TO x (atoms displace perpendicular to the facets on (311)A surface) and TOy (atoms displace parallel to the facets) modes observed for (311)A surface. In the case of (311)B SLs no splitting was observed. The phonon anisotropy of (311)A SLs can be indirect evidence of anisotropic structure of formed on (311)A GaAs surface quantum objects. The strong difference in photoluminescence spectra of (311)A and (311)B SLs was also observed. In the Raman spectra of the SL containing GaAs submonolayers grown on (2x4) reconstructed (100) surface, the triplet Raman peak corresponding to scattering on GaAs-like confined LO phonons was observed. The triplet structure appears due to additional lateral confinement of LO phonons in formed quantum objects. The calculations of Raman spectra were carried out using the model of rigid ions and Wolkenstain bond polarisability model. According to the calculations, the lateral localisation takes place when the thickness of AlAs barriers in lateral directions is 2 monolayers or more. The theoretical Raman spectra of the islands grown (in the context of a known model) on the (001)-(2x4) reconstructed surface are in surprisingly good agreement with the experimental ones. The calculations show that 70% of the islands contains less than 18 Ga atoms, what is in very good agreement with the recent STM data.