TiAlNiAu contacts for ultrathin AlN/GaN high electron mobility transistor structures

The morphology of ultrathin AlN/GaN high electron mobility transistors without or with TiAlNiAu Ohmic contacts, and annealed from 750 to 950 °C, has been investigated using transmission electron microscopy and associated analytical techniques. After annealing, the contact surface roughness was degraded due to intermixing and phase separation of the metal layers. TiN contact inclusions (CIs) that had penetrated through the AlN layers into the underlying GaN layers along threading dislocations, were observed in all annealed samples. The CI density increased with increasing annealing temperature but the lowest specific contact resistivity was obtained for structures annealed at 850 °C. Annealing at 950 °C caused cracking on the contact metal surface. The AlN layers remained intact in dislocation-free areas of all samples. The relationship between annealing temperature, interfacial structure and contact resistance is also discussed.