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In this study, a very thin Ni/Au bilayer metal film was prepared by electron beam evaporation and thermal alloying to form ohmic contact on p-type GaN film. After thermal alloying, the currentvoltage (I-V) characteristic of Ni/Au contact on p-type GaN film exhibited ohmic behavior. The Ni/Au contacts showed a specific contact resistance of 1.7ϫ10 Ϫ2 ⍀ cm 2 at an alloying temperature of 450°C. In addition, the light transmittance of the Ni/Au ͑2 nm/6 nm͒ bilayer on p-type GaN was measured to be around 85% at 470 nm. These results suggest that a suitable metallization technology for the fabrication of light emitting devices can be achieved.

In this work indium tin oxide (ITO) films were prepared using electron beam evaporation to form Schottky contacts on n-type GaN films. The thermal stability of ITO on n-type GaN was also investigated by annealing the samples at various temperatures. In addition, current–voltage (I–V) measurements were taken to deduce the Schottky barrier heights. Owing to the large series resistance, the Norde method was used to plot the F(V)–V curves and the effective Schottky barrier heights were determined as well. The effective Schottky barrier heights were 0.68, 0.88, 0.94, and 0.95 eV for nonannealed, 400, 500, and 600 °C annealed samples, respectively. Results presented herein indicate that an increase of the barrier heights may be attributed to the formation of an interfacial layer at the ITO/GaN interface after annealing.

The characteristic effects of doping with impurities such as Si, Ge, Se, O, Mg, Be, and Zn on the electrical and optical properties of GaN-based materials are reviewed. In addition, the roles of unintentionally introduced impurities, such as C, H, and O, and grown-in defects, such as vacancy and antisite point defects, are also discussed. The doping process during epitaxial growth of GaN, AlGaN, InGaN, and their superlattice structures is described. Doping using the diffusion process and ion implantation techniques is also discussed. A p-n junction formed by Si implantation into p-type GaN is successfully fabricated. The results on crystal structure, electrical resistivity, carrier mobility, and optical spectra obtained by means of x-rays, low-temperature Hall measurements, and photoluminescence are also discussed.

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Nonalloyed Cr/ Au-based metal contacts to n-GaN have been demonstrated. The deposited Au/ Cr/ n-GaN contacts exhibited a specific contact resistance ͑ c ͒ of approximately 5.6 ϫ 10 −5 ⍀ cm 2. Although the nonalloyed Ti/ Al-based contacts to n-GaN can also exhibit a comparable c value, their thermal stability is inferior to the Cr/ Au-based contacts. This could be attributed to the fact that Al tends to ball up during thermal annealing. Thus, the surface morphology of most of the annealed Ti/ Al-based contacts was quite rough, and the contacts became rectified when they were annealed at a temperature below 700°C. However, the annealed Cr/ Au-based contacts exhibited an Ohmic characteristic and had a smooth surface when annealing temperatures did not exceed 700°C. In addition, the thermal stability could be further improved by inserting a Pt layer between the Cr and Au layers. This scheme could prevent the diffusion of Au into the Cr layer, thus preventing Au from reaching the Cr/ GaN interface where it could form a possible Ga-Au phase, which would degrade the Ohmic contacts.

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