High-transparency Ni/Au bilayer contacts to n-type GaN

Abstract

A unique metallization scheme has been developed for obtaining both Schottky and low-resistance Ohmic contacts to n-GaN. It has been demonstrated that the same metallization can be used to make both Schottky and Ohmic contacts to n-GaN using a Ni/Au bilayer composite with Ni in contact to GaN. Using this metallization, contacts with a specific contact resistivity, ρs, as low as 6.9×10−6 Ω cm2 for a doping level of 5.0×1017 cm−3 was obtained after annealing the sample for 10 s at 800 °C in a rapid thermal annealer. The presence of only (111)Au and (111)Ni peaks in the x-ray diffraction (XRD) pattern of as-deposited samples indicates that both metals participate to form epitaxial or highly textured layers on the basal GaN plane. When the contact layer is annealed, Au and Ni react with GaN creating interfacial phases. Both XRD and transmission electron microscopy confirm that Ni3Ga and Ni2Ga3 intermetallic phases together with Au and Ni based face-centered-cubic solid solutions, are formed during annealing. The high optical transmission achieved (in the range of 400–700 nm) through this contact after annealing suggests that it is, indeed, very useful for electro-optic device applications. The contacts also demonstrate exceptional thermal stability. Experimental data suggest that the formation of interfacial phases with a low work function is responsible for the low contact resistance of the system. The Ni–Au layer forms a robust composite enabling the contacts to have high-temperature applications. Unlike the Ni/Au Ohmic contact, the Ni/Au Schottky contact to n-GaN has a relatively large barrier height. Improved material quality and Schottky contact technology are needed to improve upon the reverse breakdown voltage.