Thermally stable ohmic contacts to n-type GaAs. III. GeInW and NiInW contact metals

Abstract

Improvement in thermally stable, low-resistance ohmic contacts to n-type GaAs is reported for GeInW and NiInW contact metals. Coevaporation of In with Ge or In with Ni reduced the contact resistances by a factor of about 2 compared with those of the layered structures. The reduction is believed to be due to a uniform In distribution in the contact metals in the as-deposited state which resulted in an increased area of InxGa1-xAs phases in direct contact with the GaAs substrate. Annealing the coevaporated GeInW contacts for a short time at temperatures between 900 and 980°C resulted in a mean contact resistance of 0.5 Ω mm. Similar annealing of the coevaporated NiInW contacts at temperatures between 800 and 1000°C resulted in a contact resistance of 0.3 Ω mm. Additionally, the thermal stability of these ohmic contacts at 400°C after contact formation, which is required by subsequent integrated circuit process steps, was studied. Although a slight increase in the contact resistances was observed after annealing for 100 h at 400°C for the GeInW contacts, no change in the contact resistances was observed for the NiInW contacts after annealing for 100 h at 400°C and for 10 h at 500°C. This excellent thermal stability of the NiInW contacts is believed to be due to the formation of Ni3In intermetallic compounds which have high melting points. The present study suggests that in order to prepare thermally stable, low-resistance contacts it is desirable to deposit a metal which forms high melting point intermetallic compounds with In and which promotes formation of uniform InxGa1-xAs phases at the metal/GaAs interfaces. Further reduction in the measured contact resistances was achieved by reducing the sheet resistance of the contact metals.