Comparison of Ni/Au, Pd/Au, and Cr/Au metallizations for ohmic contacts to p-GaN

Reactions between electron beam evaporated thin films of Ni/Au, Pd/Au, and Cr/Au on p-GaN with a carrier concentration of 9.8x10(16) cm(-3) were investigated in terms of their structural and electronic properties both as-deposited and following heat treatments up to 600 degrees C (furnace anneals) and 900 degrees C (RTA) in a flowing N-2 ambient. Auger electron spectroscopy (AES) depth profiles were used to study the interfacial reactions between the contact metals and the p-GaN. The electrical properties were studied using room temperature current-voltage (I-V) measurements and the predominant conduction mechanisms in each contact scheme were determined from temperature dependent I-V measurements. The metallization schemes consisted of a 500 Angstrom interfacial layer of Ni, Pd, or Cr followed by a 1000 Angstrom capping layer of Au. All schemes were shown to be rectifying as-deposited with increased ohmic character upon heat treatment. The Cr/Au contacts became ohmic upon heating to 900 degrees C for 15 seconds while the other schemes remained rectifying with lower breakdown voltages following heat treatment. The specific contact resistance of the Cr/Au contact was measured to be 4.3x10(-1) Omega cm(2). Both Ni and Cr have been shown to react with the underlying GaN above 400 degrees C while no evidence of a Pd:GaN reaction was seen. Pd forms a solid solution with the Au capping layer while both Ni and Cr tend to diffuse through the capping layer to the surface. Ail contacts were shown to have a combination of thermionic emission and thermionic field emission as their dominant conduction mechanism, depending on the magnitude of the applied reverse bias.