Improvement of the ohmic characteristics of Pd contacts to p-type GaN using an Ag interlayer

We have investigated the addition effect of an Ag interlayer (2 nm) at the Pd and GaN interface on the ohmic behaviour of a single Pd contact (110 nm). The Ag layer is broken up into nano-dots (11-22 nm in size) when annealed at temperatures of 330-530 degrees C. It is shown that the use of the Ag interlayer is effective in widening the temperature range for the ohmic formation of the Pd contact and improving the adhesion of the Pd contact to GaN. The improved ohmic behaviours are attributed to the reduction of Schottky barrier heights due to the shift of the surface Fermi level towards the valence-band edge and the formation of Ag nano-dots at the Pd/GaN interfaces.