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The specific contact resistance (rc) 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 rc 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 rc values at 4.2 K can be made to p+-GaAs using either TiPd or properly annealed PdGeTiPt contacts. The rc 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. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Micron-sized loops of high-mobility two-dimensional electron gas have been made on GaAs-AlGaAs heterostructures using a novel split-gate technique. Aharonov–Bohm oscillations of amplitude up to 20% of the device resistance have been observed at very low temperatures (T<100 mK), together with h/2e oscillations which appear to be due to interference between pairs of time-reversed paths near B=0. The h/e period is found to vary by ∼25% with magnetic field, possibly as a result of the formation of edge states. In the quantum Hall effect, plateaus in Rxx are seen at high B due to variations in carrier concentration across the ring, which may cause backscattering of some edge states. [ABSTRACT FROM AUTHOR]