GaAs–Ge heterojunction interfaces: Cyclical behavior of band discontinuities

Abstract

The valence band discontinuity ΔEv of (110) and (100) oriented heterojunctions made by growth of GaAs onto Ge (GaAs–Ge) is found to decrease with time after interface formation (on a scale of tens of hours). The thin, abrupt heterojunction samples were prepared by molecular beam epitaxy techniques and characterized by x-ray photoemission spectroscopy. The final room temperature value of ΔEv(GaAs–Ge) is >0.2 eV lower than the ∼0.56 eV value for the stable Ge–GaAs (reverse growth sequence) interface. Annealing a sample for several minutes in vacuum at the growth temperature increases ΔEv(GaAs–Ge) to approximately the Ge–GaAs value. By control of anneal time and an accompanying As overpressure, ΔEv(GaAs–Ge) can be cycled by >0.2 eV. This variation in an interface electrostatic dipole is apparently related to a variable amount of electrically active As that is located at the heterojunction interface. The dipole formation mechanism involved may require the presence of the antiphase disorder that is expected for growth of GaAs on Ge.