We report a low-resistance thermally stable ohmic contact on p-type GaN using a promising contact scheme of Pd/Ni. Specific contact resistance as low as 5.7×10−5 Ω cm2 was obtained from the Pd (30 Å)/Ni (70 Å) contact annealed at 500 °C under an oxidizing ambient. NiO that formed at the surface prevented Pd atoms from outdiffusing, promoting the formation of Pd gallides, Ga2Pd5 and Ga5Pd. This reaction produces Ga vacancies below the contact, leading to enhancement of the thermal stability as well as reduction of the contact resistivity.
The role of two-step low-temperature GaN (LT-GaN) layers was investigated by cathodoluminescence, high resolution double crystal X-ray diffraction, transmission electron microscopy, atomic force microscopy, and current-voltage measurements. It was shown that the introduction of the LT-GaN layer prevents In from evaporating from InGaN during the high-temperature growth of p-GaN. The trasmission electron microscopic (TEM) results showed that the LT-GaN hampers dislocation propagation from the InGaN active layer into the p-GaN, leading to reduction in the dislocation density in the p-GaN. The use of the two-step LT-GaN resulted in an increase in the output power of light-emitting diodes and a decrease in the operating forward voltage.
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