Hot-plate alloying for ohmic contacts to GaAs

Abstract

The fabrication and optimization of ohmic contacts to GaAs prepared by heating the wafers on a hot plate is described. The method offers high throughput and is production adaptable. The apparatus consists of a hot plate constructed of a heat pipe with a high surface temperature uniformity (±2°C over 4-in diameter) located inside a glove box with an ambient controlled to 5 percent H 2 /95 percent N 2 forming gas. Specific resistance and morphology of AuGe/Ni/Au contacts were characterized as a function of hot-plate surface temperature. At the optimum alloy temperature, specific resistances of less than 10-6Ω-cm2were obtained repeatably for VPE GaAs with active layers of n ∼ 1017cm-3and thicknesses of ∼0.4 µm as well as for VPE GaAs with an n+surface layer greater than 1018cm-3. The contact surface morphologies were smoothest for those alloyed at temperatures below the optimum, and, for those alloyed at or above the optimum, they were increasingly less smooth for increasingly higher alloy temperatures. A general discussion of this method's potential application to high-volume GaAs processing is given.