Temperature dependences of the hole concentration and Hall mobility in Mg-doped GaN and Al0.08Ga0.92N grown by metalorganic vapor phase epitaxy were measured by the van der Pauw method over a wide temperature range from 100 to 500 K. Assuming that the effective mass of holes in Al0.08Ga0.92N is equal to that of GaN, the activation energy of the Mg shallow acceptor in Al0.08Ga0.92N is estimated to be about 35 meV deeper than that in GaN.

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In order to examine the possibility of preparing a nonreacted (nonalloyed) Ohmic contact to p-GaN, the effects of GaN surface treatments and work functions of the contact metals on the electrical properties between the metal contacts and p-GaN were investigated. A contamination layer consisting of GaOx and adsorbed carbons was found on the GaN substrate grown by metalorganic chemical vapor deposition. The contamination layer was not completely removed by sputtering the GaN surface with Ar and N ions where the ion densities were ∼10−2 μA/cm2. Although the contamination layer was partially removed by immersing in a buffered HF solution, little improvement of the electrical properties of the GaN/metal interfaces was obtained. Most of the contamination layer was removed by annealing the Ni and Ta contacts at temperatures close to 500 °C. These annealed contacts exhibited slightly enhanced current injection from the contact metal to the GaN. The present surface treatment study indicated that removal of the contamination layer did not significantly reduce the contact resistance. On the other hand, the resistance decreased exponentially with increasing the metal work functions, where Pt, Ni, Pd, Au, Cu, Ti, Al, Ta, and Ni/Au were deposited on the GaN. This result suggests that the Schottky barrier height at the p-GaN/metal interface might not be pinned at the GaN surface. The present study concluded that a contact metal with a large work function is desirable for nonreacted Ohmic contacts to p-GaN. However, these contacts did not provide the low contact resistance required for blue laser diodes.

We measured the Schottky barrier heights and specific contact resistivities of four different metals on p-type GaN. The Schottky barrier heights of Pt, Ni, Au, and Ti were obtained from the current-voltage characteristics to be 0.50, 0.50, 0.57, and 0.65 eV, respectively. The specific contact resistivities were 0.013, 0.015, 0.026, and 0.035 Ω⋅cm2, respectively. Our experimental results proved that the Schottky barrier heights and specific contact resistivities decrease with increase in metal work function as expected theoretically.

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