Structural analysis of interfacial layers in Ti/Ta/Al ohmic contacts to n-AlGaN

Detailed structure of the interfacial layers of Ti/Ta/Al ohmic contacts to n-type AlGaN/GaN/ sapphire are investigated by means of transmission electr on microscopy. High-resolution electron microscopy (HREM), optical diffract ograms, and computer simulations confirmed that TiN (similar to 10.0 nm) an d Ti3AlN (similar to1.4 nm) interfacial layers form at the interface betwee n the Ti layer and the Al0.35Ga0.65N substrate by a solid state reaction du ring annealing for 3 min in N-2 at 950 degreesC. The orientation relationsh ip between Ti3AlN and Al0.35Ga0.65N was found to be: [011](Ti3AlN)parallel to [2 (1) over bar(1) over bar0](Al0.35Ga0.65N) and (1 (1) over bar1)(Ti3Al N)parallel to (0 (0) over bar 01)(Al0.35Ga0.65N). The cubic Ti3AlN interfac ial layer, has a lattice parameter of 0.411+/-0.003 nm with the space group Pm3m matching that of Al0.35Ga0.65N A model of the atomic configurations o f the Ti3AlN/Al0.35Ga0.65N interface is proposed. This model is supported b y a good match between the simulated and the experimental HREM image of the Ti3AlN/Al0.35Ga0.65N interface. The formation of TiN and Ti3AlN interfacia l layers appears to be responsible for the onset of the ohmic contact behav ior in Ti/Ta/Al contacts. (C) 2000 American Institute of Physics. [S0021-89 79(00)04924-0].