INTERFACIAL ATOMIC COMPOSITION AND SCHOTTKY-BARRIER HEIGHTS AT THE AL/GAAS(001) INTERFACE

The influence of different interfacial chemical compositions on the Schottky barrier heights across the Al/GaAs(001) interface is studied using first-principles local density functional calculations. The barrier heights are calculated for seven different interfacial chemical compositions, including the chemically abrupt As-terminated and Ga-terminated interfaces, and also several other interfaces related by Al <-- --> Ga place exchange or containing As antisites. We find p-type barrier heights phi(p) that vary by 0.4 eV, demonstrating a significant influence of the interface composition on the resulting barrier heights. The barrier height variation is explained by the different chemical bonding at the interface in the various cases. The metal induced gap states (MIGS) of two structures with different barrier heights are compared in order to demonstrate why such states do not result in barrier heights independent of interfacial chemical composition. It is thus suggested that the reason an experimental value of phi(p) = 0.65 eV is generally found for Al/GaAs is not due to several possible interfacial structures all having their Fermi level pinned by MIGS at the same value, but rather due to the thermodynamic preference for certain structures over others with significantly different barriers. This proposal offers a potential explanation for recent photoemission experiments that find the Fermi level position in the pp varying by several tenths of a volt as a function of the initial surface structure of the GaAs substrate: some of the samples are likely to have interfacial compositions which are metastable with respect to structures that give the standard barrier heights.