Initial stages of Schottky barrier formation: Temperature effects

Recent experiments on the formation of Schottky barriers reveal a temperature-dependent pinning of the Fermi level (EF) which cannot be explained in terms of the current published models. Al, Au, Ag, In, and Sn are evaporated on room temperature (RT) and low temperature (LT=80 K) cleaved n- and p-GaAs(110). We find a substantial decrease in surface metal clustering at LT, leading to more homogeneous films, especially with Al, In, and Sn. Interface mixing and chemical reaction are also partly inhibited. Cancellation of the semiconductor surface relaxation is observed for Al and Sn. From the electronic point of view, low temperature has opposite effects on the pinning rate of EF on n- and p-GaAs. Pinning on n-GaAs as a function of coverage is dramatically retarded. Pinning on p-GaAs remains faster than on n-GaAs, as fast as at RT, faster for Al and In. These asymmetric rates make it impossible to explain the initial stages of Schottky barrier formation for all substrates with a single mechanism. They suggest that several independent but concomitant mechanisms might be at work in the pinning process.