Unpinned interface Fermi-level in Schottky contacts to n-GaAs capped with low-temperature-grown GaAs;: experiments and modeling using defect state distributions

Low-temperature-grown GaAs (LTG:GaAs) has been used as a cap layer in ex situ, low-resistance contact structures to n-GaAs, indicating that a chemically stable surface with well-controlled electrical properties can be realized using this cap. Recently, capacitance-voltage (C-V) measurements on Schottky contacts have provided a direct indication of an unpinned interface Fermi-level in structures consisting of n-GaAs layers capped with thin layers of LTG:GaAs. This' article describes experimental and modeling efforts to describe the near-interface energy band and Fermi-level behavior at metal/LTG: GaAs/n-GaAs interfaces. The C-V data are summarized, and the results of current-voltage measurements are presented to corroborate the initial observations. In simulation of the energy barrier, the defect bands in as-grown LTG:GaAs, for both unintentionally and Be-doped material, were described by models obtained by fitting to experimental data from scanning tunneling spectroscopy and other techniques. The near-interface energy band structure for the Schottky contacts to the LTG:GaAs-capped n-GaAs was computed by solving Poisson's equation using the resulting midgap state distribution model in the LTG:GaAs layer. This approach allows an accurate determination of the interface barrier height, phi(b), from measured C-V data and accounts for the net charge and state densities in the cap layer. The results of measurements using metals of different work functions and possible interpretations in terms of interface Fermi-level pinning models are presented. (C) 2003 American Institute of Physics.