Enhanced <i>p</i>-type GaN Ohmic contacts through strategic metal schemes and annealing

Abstract

To enhance the efficiency and performance of p-type GaN-based power devices (diode, MOSFET) and p-channel transistors (p-FET), forming an Ohmic contact with low specific contact resistance (ρc) at p-GaN can be an effective way. However, the specific contact resistance values of p-GaN remain limited to the range of mid-10−2Ω cm2 due to the low activation ratio of Mg dopants and high work function. Here, we propose an Ohmic contact method using a Pd-based metal stack to achieve lower specific contact resistance compared to conventional p-GaN contact metals such as Ni/Au or Pt/Au. A low specific contact resistance of 1.08 × 10−5Ω cm2 was demonstrated for p-GaN by annealing a Pd-based tri-layer metal contact in an oxygen-rich ambient with an optimized Pd thickness. The mechanism driving this low specific contact resistance was investigated using secondary ion mass spectroscopy, transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy, which indicated the mutual diffusion of Ni, Pd, and Ga atoms within the metal alloy. A NiO layer was formed on the top of the metal alloy, and Pd-Ga compounds were formed at the metal/p-GaN interface through mutual diffusion of atoms. This process increased the number of Ga vacancies in p-GaN, playing a crucial role in reducing its contact resistance.