Physical and electrical investigation of ohmic contacts to AlGaAs/GaAs heterostructures

The mechanism by which Ni‐Au‐Ge metallizations establish electrical contact to the two‐dimensional electron gas (2DEG) in modulation‐doped AlGaAs/GaAs heterostructures is investigated. Transmission electron microscopy was used to examine samples after electrical characterization by magnetoresistance measurements at cryogenic temperatures. We present a picture in which a 2DEG of reduced electron density exists under the deposited metallization. The success of the contacting procedure is described in terms of the magnitude of this density and the size, areal density, and penetration depth of a series of metallic spikes which establish the electrical link to the 2DEG. We suggest that the electrical behavior is not dominated by the current injection process at the spike/2DEG interface but is instead dictated by scattering from the array of antidots formed by the spikes and by a dependence of the 2DEG density on the size of the metallic pad. The implications of this picture for future nanostructure devices, featuring patterned ohmic metallization smaller than a micron, are discussed and preliminary results are reported. © 1994 American Institute of Physics.