Ohmic contacts to p-GaN were obtained after annealing Ni/Au and Ni/Au/Ni contacts in flowing O 2 . Both Ni-capped p-GaN/Ni/Au and uncapped Ni/Au contacts were shown to result in specific contact resistance of ϳ10 Ϫ4 ⍀ cm 2 . In both cases, transparent NiO and thin, porous Au films were formed. Ni/Au contacts exhibited optical transmittance at ϭ450 nm of Ͼ85%. Addition of the Ni cap layer was shown to increase the thermal stability of thin Ni/Au ohmic contacts and increase the 450 nm transmittance to 93%, while maintaining a low contact resistance. The ability of the capping layer to produce these improvements was discussed based on a total energy model of thin film morphology.

Immersion in a peroxide solution was found to increase electrical conductivity in p-GaN. Auger electron spectroscopy, current–voltage, and Hall measurements were used for characterization. All samples in this study were grown on the c plane of sapphire substrates by molecular beam epitaxy using solid Ga and ratio frequency plasma-activated N2 source with a hole concentration of 1.1×1017 cm−3. The samples were first cleaned in boiling aqua regia (3HCl+1HNO3) for 10 min, rinsed in de-ionized (DI) water, and blown dry with N2. Metal contacts (2 nm Ni/100 nm Ti/100 nm Au) were deposited with an electron beam evaporator (<10−5 Torr). After metal deposition, samples were immersed in a H2O2 solution (1H2O2:5H2O or 1H2O2:1H2O) for different time (30–300 s). This was followed by a DI water rinse, a N2 blow dry and a short dry in 80 °C air. For a treatment of 1H2O2:1H2O for 300 s, a ∼100% increase of conductivity was observed. Hall data showed that the carrier concentration increased by 70% after these treatments. X-ray photoelectron spectroscopy data showed Ga 3d and O 1s bond energies were increased by H2O2:H2O treatments. The increased conductivity was speculated to result from reduction of compensation of Mg acceptors.