Embodiments of an ohmic contact structure for a Group III nitride semiconductor device and methods of fabrication thereof are disclosed. In general, the ohmic contact structure has a root-mean-squared (RMS) surface roughness of less than 10 nanometers, and more preferably less than or equal to 7.5 nanometers, and more preferably less than or equal to 5 nanometers, and more preferably less than or equal to 2 nanometers, and even more preferably less than or equal to 1.5 nanometers.