Mechanism for Ohmic contact formation of oxidized Ni/Au on <i>p</i>-type GaN

The mechanism for Ohmic contact formation of oxidized Ni/Au on p-type GaN was investigated using three-dimensional secondary ion mass mapping and synchrotron photoemission spectroscopy. Annealing under O2 ambient caused the preferential outdiffusion of Ni to the contact surface to form NiO, leading to the final contact structure of NiO/Au/p-GaN. Ga atoms were dissolved in the Au contact layer and the oxygen atoms incorporated during annealing promoted the outdiffusion of Ga atoms from the GaN layer, leaving Ga vacancies below the contact. The drastic reduction of contact resistivity by the oxidation annealing could be attributed to the formation of Ga vacancies, which plays a role in increasing the net hole concentration and lowering the Fermi level position.