Surface reflectance anisotropy of indium-terminated GaAs(100) surfaces

The growth of thin indium-layers on the GaAs(100) As-rich (2 x 4)/c(2 x 8) surface has been investigated by reflectance anisotropy spectroscopy (RAS), LEED and AES. Clean surfaces of the (2 x 4)/c(2 x 8) reconstruction were prepared in UHV by thermal desorption of a protective arsenic layer deposited on homoepitaxially grown MBE layers. Room temperature deposition of indium on the (2 x 4)/c(2 x 8) surface and subsequent annealing at 450 degrees C leads to a 90 degrees rotation of symmetry in the LEED pattern at a threshold coverage of 0.5 monolayers, i.e. a change from the (2 x 4)/c(2 x 8) to the (4 x 2),/c(8 x 2) reconstruction. The RAS spectra show the evolution of a distinct negative feature at 1.8 eV, that shifts to 2.1 eV after annealing, corresponding to optical transitions attributed to In-dimers orientated along the [011]-direction. AES analysis shows a change in growth mode beyond 0.5 ML indicating saturation of all available adsorption sites at this coverage and RAS spectra show a contribution from additional disordered In. The AES spectra display no evidence of a surface exchange reaction between gallium and arsenic atoms, thus indicating a surface termination by In-dimers adsorbed on a layer of As