Ohmic contacts to GaAs

New and improved schemes for making ohmic contact to epitaxially grown GaAs have been developed and will be presented in this paper. They have been successfully applied to microwave Schottky-barrier FET´s and Gunn diodes. These contacts were prepared by dc sputtering of thin Au-Ge and Au-Zn films on n- and p-type GaAs, respectively. By controlling the Ge concentration in the film, the usual balling up can be avoided and more uniform wetting obtained. Carrier concentration under the contacts was profiled by a new junction capacitance technique, and no high-resistance layer was detected under these contacts. Contact resistances of these contacts as well as ones fabricated by other techniques have been measured over a wide range of carrier concentration (1 × 10¹⁵< n < 5 × 10¹⁸/cm³; 3 × 10¹⁶< p < 2 × 10¹⁹/cm³). They range from 10^-4to 10^-6ohm-cm², depending on the carrier concentration of the material. These data can be explained by a simple model in which the surface concentration of GaAs is increased by the doping effects of Ge and Zn. The surface alloyed region of these contacts was extensively investigated and characterized by scanning electron microscopy, backscattering and channeling experiments, and electrical measurements. They indicate that Au penetrates exponentially into the bulk, and a disordered layer is introduced at the surface. These can be reduced if the alloying temperature and time are kept at a minimum.