Electrical and structural properties of Ti/W/Au ohmic contacts on n-type GaN

The electrical and structural properties of Ti/W/Au ohmic contacts to moderately doped n-GaN (4.07 × 10¹⁸ cm^-3) have been investigated. The as-deposited and annealing contacts at temperatures below 750°C exhibit non-linear behaviour. However, the contact showed ohmic behaviour after annealing at a temperature in excess of 850°C. Specific contact resistance as low as 8.4 × 10^-6 Ω cm² is obtained from the Ti(12 nm)/W(20 nm)/Au(50 nm) contact annealed at 900°C for 1 min in N₂ ambient. It is observed that annealing results in a large reduction (by ∼160 meV) in the Schottky barrier height of the contacts, compared to the as-deposited one. The atomic force microscopy results showed that the surface morphology of the contact annealed at 900°C is fairly smooth with a RMS roughness of 3.8 nm. Based on the Auger electron spectroscopy and glancing angle x-ray diffraction results, possible explanations are given to describe the annealing temperature dependence of the specific contact resistance of the contacts.