Surface Reflectivity of GaAs(110)

Abstract

Electronic transitions between surface states in semiconductors can be detected by optical techniques. In particular the Differential Reflectivity spectra, that is the relative change of the overall reflectivity between the clean and the oxidized surface, is directly related to the dielectric function of the surface layer. Data for the GaAs(110) cleaved surface show electronic transitions at around 3.0 eV. In Fig. 1 (Rcl-Rox)/Rox vs photon energies is shown for two different oxygen exposures. At a very low coverage (corresponding to an exposure of 10⁴ L) an oscillating structure is obtained (asterisks, dotted line). This structure is related to a bulk-like Franz-Keldysh effect induced by the inner elctric field in the space-charge region. In fact a small amount of defect states near the Fermi level induced by oxygen adsorption is enough to drastically modify the band bending. For higher coverages approaching one monolayer (corresponding to 2x10⁶ L) a much larger contribution due to surface transitions appears (open circles, full line in Fig.1).