Oxygen assisted ohmic contact formation mechanism to n-type GaAs

The presence of oxygen in the top W layer of NiGe(Au)W ohmic contacts to n-type GaAs is found to play a critical role in reducing their contact resistance. Contacts with sputtered W containing less than 1 at. % oxygen and formed by rapid thermal annealing (RTA) yield a contact resistance (R C) greater than 0.45 Ω mm. Contacts with a reactively sputtered or electron-beam evaporated metallic W oxide top layer, containing ∼25 at. % oxygen, yield RC's of less than 0.15 Ω mm. Auger depth profiles of the reacted contacts show a significant outdiffusion of Ga from the GaAs substrate in the presence of the oxygenated W but not in the oxygen-free contacts. A contact formation mechanism based on the gettering of Ga atoms by oxygen is proposed.