Mechanism for ohmic contact formation of Ni∕Ag contacts on p-type GaN

The mechanism for ohmic contact formation of Ni∕Ag contacts on p-type GaN was investigated using synchrotron photoemission spectroscopy. A low contact resistivity of 6.6×10−5Ωcm2 was obtained from Ni(50Å)∕Ag(1200Å) contact after annealing at 500°C in O2 ambient. Ni out-diffused to form a NiO and Ag in-diffused into the contact interface during the oxidation annealing. Out-diffused Ga atoms from GaN could dissolve in the Ag layer to form Ag–Ga solid solutions, leaving Ga vacancies below the contact. Ga vacancies could increase the net hole concentration and reduce the surface band bending, resulting in the ohmic contact formation.