Ge/GaAs(001) interface formation investigated by reflectance anisotropy spectroscopy

The formation of the Ge/GaAs(001) interface has been investigated following the transformation of an As-dimer terminated GaAs(001)(2 X 4) surface into a Ge-Ga-dimer terminated (1 X 2) reconstruction and the subsequent deposit ion up to 10 ML of Ge. The modification of the surface atomic geometry and the related electronic structure has been monitored by reflectance anisotro py spectroscopy (RAS) and low-energy electron diffraction. Experimental res ults are compared to density-functional-theory-local-density-approximation and tight-binding calculations of the surface structure and optical respons e, respectively. The comparison between calculated and measured RAS spectra allows us to show that the (2 X 4) structure transforms into a well-ordere d (1 X 2) passing through a disordered (2 X 4) phase while a previously pro posed intermediate (2 X 1) structure is ruled out. At higher Ge coverages, surface and Ge/GaAs-interface contributions to the optical spectra are sepa rated by surface modification through exposure to atmosphere. A interface c ontribution is identified between 1.5 eV and 2.5 eV, almost identical in li ne shape and amplitude to the RAS features on the Ge-Ga-dimer terminated Ga As surface. This finding demonstrates that the backbonds of the Ge-Ga-dimer s, present at the Ge-Ga-dimer terminated surface as well as at the Ge/GaAs interface, determine the optical anisotropy, whereas the Ge-Ga-dimer bond i tself does not contribute significantly. [S0163-1829(99)12315-4].