The Microstructure of Ti/Al and TiN Ohmic Contacts to Gallium Nitride

We carried out TEM investigations on annealed Al/Ti/GaN and directly deposited TiN films on GaN. In the Al/Ti/GaN system, the annealing step gave rise to a ≈︂20 nm rough TiN film. The ohmic contact is made by TiN crystallites which are epitaxially related to GaN, the orientation relationship being (0001)GaN ∥ (111)TiN. In between, amorphous patches of 1 to 2 nm extension can be found. When the stoichiometric TiN was deposited directly on GaN, we obtained columnar TiN grains of 5 to 20 nm section which cross the whole film thickness and are rotated mostly around the [111] axis. The same epitaxial relationship is obtained and no amorphous patches are observed at the interface. When the GaN surface was locally terminated by {101-1} pyramidal features, the epitaxial relationship was slightly modified, the (0001)GaN atomic planes and {111}TiN made an angle of 5°. This is found to allow the accommodation of the local misorientation of the facets and to bring about near coincidence.