Correlation of interface composition and barrier height for model AuGeNi contacts to GaAs

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Abstract
Model contacts to GaAs that include nonalloyed layered structures of Au, Ge, and Ni in various combinations are used to establish a correlation between interface composition and large changes in barrier height φB. The interface Fermi level EiF and chemistry during initial contact formation were investigated by x-ray photoemission spectroscopy; the corresponding φB for the thick contact was obtained by current-voltage (I-V) measurement. The circumstances under which a thin (∼10 Å) Ge layer at the GaAs interface can produce φB =∼0.25–0.4 eV (as measured by I-V) are described. For all model contacts examined a φB range from ∼0.25 to 0.9 eV is observed. This result questions the usual assumption of a relatively fixed φB of ∼0.8 eV for the alloyed AuGeNi contact and offers an alternative explanation for the mechanism of ohmic contact formation. The conditions that define the exceptionally low φB contacts provide a guide for the design of nonalloyed tunnel ohmic contacts.