METAL CONTACTS TO GALLIUM NITRIDE

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-degrees-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) OMEGA 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.