Interface crystallography and stability in epitaxial metal (NiAl, CoAl)/III-V Semiconductor heterostructures

Abstract

The stability and crystallography of AlAs/NiAl/ and AlAs/CoAl interfaces in AlAs/NiAl, CoAl/ AlAs heterostructures grown by molecular beam epitaxy (MBE) are described. The thermal stability of an interface between a transition metal aluminide (TM-Al) film and an AlAs substrate is a result of the bulk thermodynamic stability of these materials in a closed system. The TM-Al film surface, however, is not stable in the arsenic vapor ambient of an MBE chamber. Excessive exposure to arsenic results first in the depletion of aluminum from the TM-Al film to form AlAs followed by the reaction of the TM with arsenic to form a TM arsenide. These reactions can be prevented or controlled by growing the AlAs overlayer under aluminum-rich conditions. The defect structures of the upper and lower interfaces also differ. Since the TM-Al phase with the CsCl structure has higher symmetry than AlAs with the zinc blende structure, a single-variant epitaxial film of TM-Al can be grown on {100} AlAs. The lower symmetry of the AlAs, however, dictates that an AlAs layer grown on the {100} surface of a TM-Al film will contain translation domain boundaries and inversion domain boundaries. The constraints imposed by the large differences in chemistry and structure present challenging obstacles to the realization of high quality epitaxial metal/compound semi-conductor heterostructures of arbitrary complexity.