Current transport mechanism and I–V characteristics of titanium and indium contacts to p-type GaN

Several Mg-doped p-type GaN samples were grown on sapphire by horizontal MOCVD technique. Concentrations of Mg are in the 1019 cm−3 range. Free carrier concentrations are p=2–3×1017cm−3 and Hall mobility μH=11–13 cm2 (Vs)−1. Contacts for Hall probes are standard nickel–gold contacts. On such samples the titanium contacts were evaporated and the indium contacts were soldered. We study current transport for various combinations of metal contacts. Measured current voltage curves (I–V) show similar characteristics for various combinations of metals being dependent more on sample then on the choice of metal. The I–V characteristics showed symmetrical ‘s’-shape, identical in forward and reverse direction. Several expressions are considered in attempt to explain such behavior. It is concluded that the tunneling is dominant current transport process. It is expected that the surface layer properties are critical for the formation of ohmic contacts and the choice of metal is less important.