Environmental and thermal aging of Au/Ni/p-GaN ohmic contacts annealed in air

In recent years, Au/Ni/p-GaN ohmic contacts annealed in air have been studied extensively because they provide low specific contact resistances and high transparency. In this article we focus on the environmental and thermal degradation that we have observed in these contacts. When the contacts were not protected from the environment, degradation of the contacts always occurred over a period of days, and after sitting 30 days in the laboratory at room temperature, the current-voltage characteristics of the originally ohmic contacts were nonlinear and the contacts were significantly more resistive. To pinpoint the degradation mechanism, samples were stored at room temperature in dry nitrogen, dry oxygen, dry air, air saturated with water vapor, nitrogen gas saturated with water vapor, or vacuum. These experiments revealed that water vapor was the cause of the room temperature degradation. Since no change in the sheet resistance of the p-type GaN was observed upon aging, four point probe measurements and x-ray photoelectron spectroscopy depth profiles were carried out to determine the interaction between water vapor and the p-type NiO present in the annealed contact metallization. The measurements indicated that hydroxyl groups were incorporated in the NiO, leading to a reduction in its conductivity and presumably a decrease in its hole concentration. Aging studies at 200 degreesC further revealed poor thermal stability of the contacts not only in water vapor but also in nitrogen gas and vacuum, and this degradation was again linked to a degradation in the conductivity of the NiO component of the contact metallization. (C) 2002 American Institute of Physics.