Thermal Annealing Effect Between Ni Film and Mg-Doped GaN Layer

The effects of thermal annealing between Ni film and a p-type GaN layer have been investigated. The electrical and optical properties were measured by Hall effect, capacitance–voltage (C–V) and photoluminescence (PL) measurements. The samples activated with Ni film obtained higher effective carrier concentrations than those activated without Ni film. Effective carrier concentrations of 5×10¹⁵ and 1×10¹⁷ cm^-3 were achieved at an activating temperature of 400 °C without and with Ni film. The Ni film may act as a catalyst for the activation of Mg-doped GaN at a temperature less than 500 °C. At a temperature higher than 600 °C, the Ni film may react with the Mg-doped GaN. X-ray diffraction (XRD) analyses indicated that Ni film on Mg-doped GaN transforms to nickel oxide (NiO) and nickel nitride (Ni₃N) during thermal annealing in air. The peaks of the PL spectra at 15 K of the samples activated at 600 °C with and without Ni film were observed at approximately 3.2 and 2.9 eV. We suggest that Ni atoms not only enhance hydrogen desorption but also diffuse into the Mg-doped GaN layers to form Ni-compound materials. At a high annealing temperature, impurities such as Ni nitride, nitrogen vacancies or other defects may reduce the carrier mobility and provide an increase in the effective carrier concentrations in the surface region.