Characteristics of Ni/Au/Ni/Au ohmic contact in a p-AlGaN/GaN semiconductor

Abstract

The low-resistance NiAu/Ni/Au ohmic contact in p-type AlGaN/GaN semiconductor was carefully investigated by electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) after two-step annealing at 550 and 750°C. It is shown that complicate double-direction diffusion and reaction occurred in the metal layer and underlying GaN layer. The four metal stacks of Ni/Au/Ni/Au turned into almost one layer and an intimate relationship established at NiAu/GaN boundary, which should play a primary role in ohmic contact to reduce the contact barrier. At the intimate interface, the metal layer close to the contact was enriched with Ga and Au, and the GaN upper layer was metalized by Au and Ni. Numerous Ni atoms were oxidized and formed dispersive NiO nanoclusters in the metal layer, which might have a hindering effect on upward migration of Ga atoms. Dislocations connected with the contact boundary absorbed interstitial atoms of Au or Ni may serve as channels for current carrier transportation. Thus, a low-resistance p-GaN ohmic contact can be obtained by the above combination of these microstructural characteristics.