Electrical transport in the copper germanide-n-GaN system: Experiment and numerical model

Ultraviolet photoemission measurements of the copper germanide work function and numerical modeling of measured current-voltage data show that the Fermi level at the interface of Cu–Ge films on non-plasma-treated n-GaN cleaned by wet chemicals is pinned near 0.5eV below the conduction band edge, and that 300°C annealing lessens this pinning. Annealing Schottky diode structures at 400–600°C decreases the Cu–Ge∕n-GaN Schottky barrier height and increases electron tunneling through the barrier. Leakage currents are not dominated by edge effects, and are independent of measurement temperature, collectively indicating a tunneling transport mechanism for non-plasma-treated Schottky diodes. A plasma treatment of the GaN surface induces ∼0.5eV of downward near-surface band bending and increases surface oxidation, and these effects are responsible for low-resistance Ohmic behavior. Increased surface doping associated with plasma-treated material, when compared with non-plasma-treated n-GaN, causes greater tunneling due to a thinned depletion layer and reduces the Schottky barrier height through image-force barrier lowering and band gap narrowing. The combination of these two effects causes the I-V behavior of these Cu–Ge contacts to shift from rectifying to Ohmic.