Photoemission determination of dipole layer and VB-discontinuity formation during the MBE growth of GaAs on Ge(110)

Synchrotron radiation-excited soft x-ray photoemission is used to study the heterojunction formation of GaAs on Ge(110) by examining 3d-core levels, the valence band (VB), and the secondary electron distribution cutoff in a photoemission spectrum. Arsenic saturation of the Ge(110) surface prior to MBE growth of GaAs causes a work function increase of 0.26±0.045 eV and a decrease in Ge valence-band maximum (VBM) from the Fermi level by 0.96±0.09 eV. The increase in work function can be explained by the formation of a GeAsx layer. The large VBM decrease is partly due to a band bending of 0.61±0.03 eV and a chemisorption-induced charge transfer reaction. Using two different photoemission methods, the VB discontinuity at the interface is determined to be 0.23±0.08 and 0.26±0.07 eV. This value is different from the one determined for the inverse heterojunction where Ge is deposited on top of GaAs(110) at the same epitaxial growth temperature of 320 °C. The difference in electron affinity between the clean Ge(110) substrate and the GaAs(110) overlayer is Δχ=−0.01±0.08 eV. Therefore, Anderson’s electron affinity rule does not explain the measured VB discontinuity.