Mechanism of low Ohmic contact resistance to p-type GaN by suppressed edge dislocations

In this paper, an excellent Ohmic contact to p-GaN with a low specific contact resistance (ρc) of 2.0 × 10−5 Ω·cm2 is demonstrated using a patterned sapphire substrate (PSS) and oxidized Ni/Au contacts. GaN epitaxy with high crystal quality on the PSS, confirmed by high-resolution x-ray diffraction, played a key role in the improved Ohmic contact to p-GaN. The edge dislocations were annihilated during the epitaxial process on the PSS to afford a low surface dislocation density, which was in accordance with the results of transmission electron microscopy and cathodoluminescence spectroscopy. Furthermore, a reduced Fermi level and enhanced activation efficiency of Mg with suppressed segregation around the dislocations were demonstrated by Kelvin probe force microscopy and contact Hall measurements, respectively. A GaN p-channel metal oxide semiconductor device fabricated on the PSS displayed a twofold higher forward current density and superior gate controllability compared with that fabricated on a conventional sapphire substrate.