Electrical, structural and surface morphological properties of thermally stable low-resistance W/Ti/Au multilayer ohmic contacts to n-type GaN

A W/Ti/Au multilayer scheme has been fabricated for achieving thermally stable low-resistance ohmic contact to n-type GaN (4.0 x 1018 cm-3). It is shown that the as-deposited W/Ti/Au contact exhibits near linear I-V behaviour. However, annealing at temperature below 800 deg. C the contacts exhibit non-linear behaviour. After annealing at a temperature in excess of 850 deg. C, the W/Ti/Au contact showed ohmic behaviour. The W/Ti/Au contact produced specific contact resistance as low as 6.7 x 10-6 Ω cm2 after annealing at 900 deg. C for 1 min in a N2 ambient. It is noted that the specific contact resistance decreases with increase in annealing temperature. It is also noted that annealing the contacts at 900 deg. C for 30 min causes insignificant degradation of the electrical and thermal properties. It is further shown that the overall surface morphology of the W/Ti/Au stayed fairly smooth even after annealing at 900 deg. C. The W/Ti/Au ohmic contact showed good edge sharpness after annealing at 900 deg. C for 30 min. Based on the Auger electron spectroscopy and glancing angle X-ray diffraction results, possible explanation for the annealing dependence of the specific contact resistance of the W/Ti/Au contacts are described and discussed.