Characterization of nonalloyed ohmic contacts

Low-temperature-grown gallium-arsenide (LTG : GaAs), typically grown between 200°C and 300° C, has high resistivity and a high concentration of defects. Because of these unique properties, it has attracted significant interest for various applications. For example, low resistance nonalloyed contact structures for n-GaAs and p-GaAs have been demonstrated using LTG : GaAs as a cap layer, as a buffer layer for GaAs metal semiconductor field effect transistors, and as an active layer for high frequency optical switches and detectors. This work presents a comprehensive study of nonalloyed ohmic contacts operated between 40K and 673K and an analytical model for the experimental uncertainty of specific contact resistance derived from the transmission-line-model (TLM). The nonalloyed ohmic contact structure consists of a LTG : GaAs cap layer on an n+ GaAs layer. With at least comparable contact resistance to the traditional alloyed ohmic contact, the nonalloyed contact survives annealing at temperatures between 300°C and 400°C. A cryogenic study shows that the nonalloyed contact has high activation doping density (\u3e5 × 18 cm−3) in the n+ space charge region, low barrier height (\u3c0.7eV) at the metal/LTG : GaAs interface, and that the primary conduction mechanism is tunneling through the contact potential barrier. An analytical model, which relates the uncertainty of the contact resistance to the physical measurement parameters, is used to develop optimization criteria for designing the TLM test pattern. This model can therefore be applied to design contact measurement patterns