The effect of thermal annealing on current–voltage properties of GaN light emitting diodes (LEDs) has been studied. At annealing temperatures above 700 °C, the p–n junction of the diodes became very leaky and Ga-contained metallic bubbles were observed on the surface of Ni/Au p-ohmic contact. Transmission electron microscopy and energy dispersive x-ray spectrometer studies revealed that these metallic bubbles resided directly on top of the threading dislocations in GaN and both Ni and Au were indiffused into the LED structure along the cores of the TDs. The conducting paths formed by the metal containing dislocation cores are believed to be the cause for the observed short circuit behavior of p–n junctions at high annealing temperatures.

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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.

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We have fabricated GaN-based light-emitting diodes (LEDs) using transparent indium tin oxide (ITO) for p-type contacts. The current-voltage (I-V) characteristics of the devices have been studied. When annealed at 700 C, the p-n junction of the diodes became very leaky, and even electrical short circuits have been observed. According to scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer analyses (EDS), it was found that indium (In) diffused into the LED structure with defects such as threading dislocations (TDs) or V-pits. The defects provide leakage paths to cause short circuits in p-n junctions at high annealing temperatures.