The presence of oxygen in the annealing environment can exhibit a strong influence on the activation of p-GaN, as demonstrated by experiments described in this letter. We activated p-GaN at 600–900 °C in four environments: ultrahigh purity (UHP) N2 gettered to remove residual O2, UHP N2 without gettering, 99.5% UHP N2/0.5% UHP O2, and 90% UHP N2/10% UHP O2. The resistivity of the p-GaN was lowest when O2 was intentionally introduced during activation and was highest when extra care was taken to getter residual O2 from the annealing gas. The experiments also demonstrate that unintentionally incorporated O2 can be at high enough levels to influence the activation process.

Thin Ni films on GaN were annealed at temperatures between 400 and 900°C in N 2 , Ar, and forming gas and were analyzed using glancing angle x-ray diffraction and Auger depth profiling. The first indication of an interfacial reaction was found after an anneal at 600°C for 1 h, after which Ga was observed to be dissolved in the face-centered cubic Ni film. The extent of dissolution increased with continued annealing. After annealing at 750°C for 1 hr in either N 2 or Ar, greater intermixing occurred. The reaction product was either Ni 3 Ga or face-centered cubic Ni with dissolved Ga. Annealing at 900°C resulted in the formation of the B2 phase NiGa. It was clear from Auger depth profiles that the reacted film contained significantly more Ga than N and that N 2 gas was released to the annealing environment, even when the samples were annealed in N 2 gas at 1 atm. Thus, a trend of increasing Ga content in the reacted films was observed with increasing temperature. The observed reactions are consistent with the thermodynamics of the Ni-Ga-N system.

The oxidation of polycrystalline GaN powder and GaN epi layers in dry air has been investigated. Bulk 0-20 X-ray diffraction (XRD) revealed no evidence of oxide formation on the powder specimen exposed to temperatures of up to 750 °C for 25 h. However, when oxidized at temperatures of 900 °C or greater for 1 h or longer, an oxide was observed to form and was identified as the monoclinic 3-Ga203, the same oxide observed previously by the present investigators to form on epitaxial GaN films. Information regarding the oxidation kinetics was obtained in the present study by measuring the intensity of the f3-Ga203, (2 1 7) XRD peak as a function of oxidation time at various temperatures, and the initial stage of the oxidation was found to be limited by the rate of an interfacial reaction with an activation energy of --3 X 1 0 J/mol. An interfacial reaction-controlled mechanism was also observed to limit the rate of oxidation of GaN epi layers at 900 °C.

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The barrier heights of Re Schottky contacts to n-GaN were investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements. Both techniques indicate that the barrier height increases upon annealing at 500 °C for 10 min. After this anneal, a barrier height of 0.82 eV and ideality factor of 1.1 are obtained by I–V measurements performed at 150 °C. The C–V measurements performed at room temperature reveal a barrier height of 1.06 eV. These barrier heights are stable upon further short term annealing at temperatures as high as 700 °C. The Re Schottky contacts were also stable upon prolonged annealing for 24 h at 300 °C. The Re/n-GaN Schottky diode was chosen for study because of its anticipated thermodynamic stability against metallurgical reactions.

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Metallurgical reactions between Ni and GaN have been explored at temperatures between 400 and 900 'C in N 2 , Ar, and forming gas. A trend of increasing Ga content in the reacted films was observed with increasing temperature. The reactions are consistent with the thermodynamics of the Ni-Ga-N system. Changes in the film morphology on annealing were also examined. Metal island formation and a corresponding deep, non-uniform metal penetration into GaN were observed at high temperatures. The relevance of the observed nature of phase formation and morphology in these thin films to electrical properties of Ni/GaN and Au/Ni/GaN contacts is also considered.