Photoemission study of GeAs(2̄01): A model for the As-stabilized Ge surface on GaAs/Ge heterojunctions

Abstract

We present a study of the electronic states of GeAs as a model for the ordered, anion-stabilized surface phases which form on Ge when it is grown epitaxially on GaAs substrates using MBE. Angle-resolved, normal emission photoelectron energy distribution curves for four different azimuthal orientations of the GeAs(2̄01) face (i.e., four different polarizations) have been measured using photon energies from hν=10 to 28 eV. The valence band structure plots (binding energy versus photon energy) show behavior typical of a layered compound. A large number of valence band states (about 12) are observed, and they show essentially no hν dispersion in normal emission and very little dispersion with polar angle. The electronic states are therefore localized both within and normal to the layer. The excitation of the Ge 3d and the As 3d core levels into conduction band states observed by measuring the core hole decay emission does not show any enhancement peaks as usually observed if empty surface states exist in the conduction band. This suggests that the GeAs(2̄01) surface may not have any surface states. Chemical bonding information was obtained by measuring the Ge 3d core level binding energy. The Ge is more tightly bound compared to the bulk Ge as indicated by a 3d core level chemical shift of 0.45±0.1 eV toward higher binding energy. 3d core threshold emission for both the Ge and the As is used together with the line shape of the valence band edge emission to determine a forbidden band gap is 0.4±0.3 eV. These results support the Bauer/Mikkelsen hypothesis that the Ge:As surface phase that floats on top of the MBE GaAs/Ge heterojunction is energetically more favorable than the corresponding clean Ge surface having unsatisfied dangling bonds. The data on GeAs(2̄01) leads to a picture of an As-terminated surface having highly directional oribtals and very low reactivity.