Electrical, chemical, and structural characterization of the interface formed between nickel/gold and palladium/gold ohmic contacts and cleaned p-type gallium nitride (0001) surfaces

Characterization of pre-treated, Mg-doped, p-type GaN(0001) surfaces and Ni/Au and Pd/Au contact structures sequentially deposited on these surfaces has been conducted using X-ray and ultra-violet photoelectron spectroscopies, low energy electron diffraction, atomic force microscopy, scanning electron microscopy, X-ray diffraction, and current-voltage measurements. The effects or several in-situ cleaning techniques, including vacuum annealing, high-temperature nitrogen plasma pre-treatment, and ammonia-based chemical vapor cleaning, on the chemical and electronic properties of the GaN surface have also been assessed. The last technique was the most effective. Stoichiometric surfaces without detectable carbon and an 87% reduction in the surface oxygen to 2 ± 1 at% were achieved. The band bending and electron affinity at the cleaned surface were within the ranges of 0.7–1.5eV and 2.5–3.2eV, respectively.; Ni/Au contact structures deposited on both vacuum annealed and chemical vapor cleaned p-type GaN surfaces and Pd/Au contacts deposited on a chemical vapor cleaned surface were significantly less rectifying than identical contact structures deposited on conventional HCl treated surfaces, as evidenced by larger currents at equivalent voltages in the former two structures. The room temperature specific contact resistivity of these contacts deposited on vacuum annealed surfaces increased from 0.14 ± 0.04 Ω•cm 2 to 0.30 ± 0.04 Ω•cm2 following annealing at 140°C–150°C for 40 ± 2min. Identical contact structures on chemical vapor cleaned surfaces performed consistently with no degradation following operation within the same ranges of temperature and time.; The formation of Au:Ga (90:10) and Au:Ni (90:10) solid solutions was indicated by the results of X-ray diffraction (ω-2&thetas;) studies for Ni/Au contacts on HCl-treated surfaces following a 450°C anneal. Similar studies showed no evidence of interfacial reactions between similarly annealed contacts and chemical vapor cleaned GaN surfaces. Pd/Au contact structures on chemical vapor cleaned surfaces demonstrated excellent high-temperature microstructural stability as evidenced by the absence of significant change in the surface roughness, which was measured to be 6.9 ± 0.4 nm after successive annealing through 700°C. Identical contact structures on HCl-treated surfaces exhibited poor high temperature microstructural stability, as indicated by a significant increase in the surface roughness to 11.8 ± 0.5 nm and 80.2 ± 0.5 nm following successive anneals at 600 and 700°C, respectively.