The dichotomous role of oxygen in the ohmic contact formation on p-type GaN grown by MBE and MOCVD

Abstract

Molecular beam epitaxy (MBE) is often considered as an ideal equipment for growing low-resistance p-type GaN (p-GaN). However, the Ohmic contact formation mechanism of p-GaN grown by MBE is still unclear. In this work, MOCVD and MBE-grown p-GaN were prepared for contrastive analysis. We reported for the first time the different variation tendency in their electrical properties after annealing in the same ambient. Surface analyzing instruments were used to investigate the interfacial properties of p-GaN. Experimental results revealed that the oxygen in the annealing ambient had a negative effect on the Ohmic contact formation of MBE p-GaN, and the physical mechanism underlying it was figured out. It is proved that the annealing conditions for preparing Ohmic contacts on MOCVD p-GaN is not suitable for that on MBE p-GaN. To further reduce the effect of oxygen, an optimized Ohmic contact scheme for MBE p-GaN based on the Ni-Pd contact layer was proposed, which successfully reduced the specific contact resistivity (ρc) by two orders, to 8.6 × 10−5 Ω•cm2, compared with the conventional Pd/Pt/Au system. This research provides valuable guidance for device fabrication based on GaN grown by MBE and lays an essential foundation for preparing optoelectronic devices.