Asymmetries in atomic intermixing at Au/Ge and Ge/Au interfaces

Synchrotron-radiation photoemission was used to study room-temperature intermixing at interfaces prepared by Au evaporation onto amorphous Ge films (a-Ge) and Ge evaporated onto Au films. Our results show that both interfaces develop intermixed regions but that their formation is highly asymmetric. The Au/a-Ge interface is characterized by gradual development of a Au₃Ge-like mixture, the persistence of the mixture to high coverage, and the failure of the system to form a pure Au layer. In contrast, the inverted Ge/Au interface is characterized by weak surface interaction at low coverage, triggering of reaction at about 2 A{} to form the Au₃Ge-like mixture, a very limited coverage range over which the mixture develops (2--8 A{}), and the growth of a Ge layer containing Au in solution at higher coverage. We conclude that the evolution of Ge/Au interface evolution is similar to that of the transition-metal/semiconductor junctions where fully-reacted products form at the substrate interface, dilute solutions develop farther from the boundary, and the dimensions are controlled by kinetics. This Ge/Au behavior contrasts to what has been reported for Ge--transition-metal interfaces where a reaction barrier precludes intermixing.