Ge growth on GaAs(001) surfaces studied by reflectance anisotropy spectroscopy

Surface modifications induced by germanium deposition onto clean GaAs(001) substrates have been monitored by reflectance anisotropy spectroscopy (RAS) and low energy electron diffraction (LEED). The clean GaAs(001) surfaces onto which Ge has been evaporated were c(4 x 4), (2 x 4), and (4 x 1) reconstructed. Regardless of the initial surface reconstruction, after deposition of 0.5 monolayers of Ge and further annealing at 850 K, we have always observed a (1 x 2) LEED pattern and the same characteristic RAS spectrum. On the contrary, overlayer structures obtained at intermediate stages between the clean surface and this (1 x 2) phase depend upon the initial surface reconstruction. Modifying the (1 x 2) reconstructed surface by deposition of additional monolayers of Ge or exposure to atmosphere, we have separated the surface, interface, and bulk contributions to the RAS spectra. Finally, monitoring the characteristic linear-electro-optical feature appearing at E1 and E1 + Δ1 bulk critical points, we discuss how its change in amplitude and sign could be connected to a variation of the substrate doping induced by annealing.