A robust Ni/Au process and mechanism for p-type ohmic contact applied to GaN p-FETs

In this study, a new robust ohmic contact preparation process for GaN p-FETs application is developed on p-GaN/AlGaN/GaN, which excluded the commonly used descum step. A specific contact resistivity of approximately 2.0×10–3 Ω·cm2 is obtained based on Ni/Au metal stack. X-ray photoelectron spectroscopy (XPS) analysis revealed that the p-GaN surface underwent oxidation by O2 plasma in the descum step, forming Ga2O3 as a barrier layer between the metal and p-GaN. The resulting Ga2O3 could not be completely removed using a hydrochloric acid solution, resulting in a Schottky contact. Moreover, the Ni/Au ohmic contact mechanism was revealed for the first time by varying the O2 content in the annealing atmosphere. The X-ray transmission electron microscopy–energy-dispersive X-ray spectroscopy and XPS results proved that the Ni (III) oxide or Ni2O3 present at the metal/p-GaN interface and the Ga vacancies formed on the p-GaN surface played a crucial role in facilitating the formation of ohmic contacts.