Effect of rapid thermal annealing on Ti/Al-GaN contacts

Non-intentionally-doped wurtzite GaN epitaxial layers used in this study were grown on (0001) sapphire by metal organic chemical vapour deposition technique. Ti/Al(24nm/90nm) contacts were deposited by ion beam sputtering. Effect of rapid thermal annealing on Ti/Al-GaN contacts was investigated by I-V measurements and AES depth profiles. Surface morphologies were characterized by AFM. Schottky barrier height and specific contact resistivity decreased at first and then increased as annealing temperature increased. The lowest specific contact resistivity was obtained at 600°C, i.e.3.04 × 10^-4Ω·cm². However, the ideality factors increased with increase of annealing temperatures. Auger depth profile analysis showed that Ti had diffused into annealed GaN samples, a heavily n-doped layer formed at Ti/GaN interface. The tunneling current mechanism played a role in increase of ideality factors after annealing. The changes of surface morphologies at different annealing temperatures were characterized by AFM. The root mean square roughness of the as-grown sample was 2.9Å. After 600°C annealing, the root mean square roughness increased to 41.5Å. These results showed that thermal annealing-induced changes near the surface region of contacts were significant. The thermal stability of contacts needs further study. Ohmic contacts formed at 600°C are due to lower barrier height and tunneling current transport.