Metal contacts to gallium nitride

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Abstract
We report measurements on the nature of aluminum and gold contacts to GaN. The GaN films were deposited onto the R‐plane of sapphire substrates by molecular beam epitaxy and are autodoped n‐type. Metal contacts were deposited by evaporation and were patterned photolithographically. Current‐voltage characterization shows that the as‐deposited aluminum contacts are ohmic while the as‐deposited gold contacts are rectifying. The gold contacts become ohmic after annealing at 575 °C, a result attributed to gold diffusion. The specific contact resistivity of the ohmic aluminum and gold contacts were found by transfer length measurements to be of device quality (10−7–10−8 Ω m2). The results of these studies suggest a direct correlation between barrier height and work function of the metal, consistent with the strong ionic character of GaN.