Temperature dependence of the specific resistance in Ti/Al/Ni/Au contacts on n-type GaN

The temperature dependence of the specific resistance rho(c) in annealed Ti/Al/Ni/Au contacts on n-type GaN was monitored, obtaining information on the current transport mechanisms. After annealing at 600 degrees C, the contacts exhibited a rectifying behavior and became Ohmic only after high temperature processes (> 700 degrees C), with rho(c) in the low 10(-5) Omega cm(2) range. The results demonstrated that the current transport is ruled by two different mechanisms: thermoionic field emission occurs in the contacts annealed at 600 degrees C, whereas field emission dominates after higher temperature annealing. The significant physical parameters related to the current transport, i.e., the Schottky barrier height and the carrier concentration under the contact, could be determined. In particular, a reduction of the Schottky barrier from 1.21 eV after annealing at 600 degrees C to 0.81 eV at 800 degrees C was determined, accompanied by a strong increase of the carrier concentration, i.e., from 2x10(18) cm(-3) in the as-prepared sample to 4.6x10(19) cm(-3) in the annealed contacts. The electrical properties were correlated to the microstructure of the interfacial region, providing a scenario to explain the transition from Schottky to Ohmic behavior in annealed Ti/Al/Ni/Au contacts.