Contact resistances of NiGeAu, PdGeTiPt, and TiPd ohmic contacts to GaAs and their temperature dependence from 4.2 to 350 K

The specific contact resistance (r_c) of NiGeAu and PdGeTiPt ohmic contacts to n‐GaAs and TiPd and PdGeTiPt ohmic contacts to p⁺‐GaAs were determined as a function of temperature between 4.2 and 350 K. The low r_c obtained for some of the contacts at 4.2 K implies that much of the total contact resistance measured at 4.2 K in two‐dimensional electron gas structures lies across the n–n heterojunction(s) in series with the metal semiconductor junction. Although NiGeAu contacts have a lower contact resistance to n‐GaAs, PdGeTiPt contacts, which have much better edge definition, can be substituted for the NiGeAu when they are properly annealed. Also, contacts with low r_c values at 4.2 K can be made to p⁺‐GaAs using either TiPd or properly annealed PdGeTiPt contacts. The r_c versus temperature curves for the TiPd and alloyed NiGeAu contacts fit the field emission model. The other contacts have a larger temperature dependence suggesting that tunneling occurs via thermionic field emission directly through the barrier or via defect states.