Thermally stable non-gold Ohmic contacts to n-type GaAs. I. NiGe contact metal

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Abstract
‘‘Nongold’’ NiGe Ohmic contacts were developed by a lift-off and annealing technique that was extensively used to fabricate the conventional AuGeNi contacts. The optimum conditions to prepare thermally stable, low resistance NiGe Ohmic contacts were determined by changing the Ge concentrations of the NiGe contacts from 13 to 43 at. %, and the deposition sequences of the Ni and Ge layers. Contact resistances of ∼0.8 Ω mm were obtained for both two-layered Ni/Ge and three-layered Ni/Ge/Ni contacts, with Ge concentrations of ∼38 at. % after annealing at 600 °C. The thermal stability of the electrical properties during subsequent annealing at 400 °C after contact formation was found to be influenced by the microstructure at the GaAs/metal interface. The excellent stability was obtained only when the NiGe contacts formed high melting point NiGe compounds. The present result indicated that removal of Au from the AuGeNi contacts was not satisfactory enough to improve the thermal stability of the AuGeNi contacts after contact formation. A model for the current transport mechanism through the GaAs/NiGe interface was also proposed by correlating the electrical properties and the microstructure. This model explained well the dependences of the contact resistances on the Ge concentrations and the deposition sequence of the Ni and Ge layers.