Formation of Low-Resistance Ohmic Contact by Damage-Proof Selective-Area Growth of Single-Crystal n +-GaN Using Plasma-Assisted Molecular Beam Epitaxy

Abstract

To achieve very low ohmic contact resistance, an n +-GaN layer was selectively deposited using plasma-assisted molecular beam epitaxy (PAMBE). During this process polycrystalline GaN grew on the patterned SiO2 region, which was subsequently removed by a heated KOH solution, resulting in damage to the n +-GaN surface. To prevent this damage, an additional SiO2 layer was selectively deposited only on the n +-GaN region. To optimize the fabrication process the KOH etching time and n +-GaN layer thickness were adjusted. This damage-proof scheme resulted in a specific contact resistance of 4.6 × 10−7 Ω cm2. In comparison, the resistance with the KOH etching damage was 4.9 × 10−6 Ω cm2 to 24 × 10−6 Ω cm2. The KOH etching produced a large number of pits (4.1 × 108 cm−2) and degraded the current transport. X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (SIMS) analysis indicated that KOH etching was very effective in removing the oxide from the GaN surface and that the O-H bonding at the GaN surface was likely responsible for the degraded contact performance. The optimum n +-GaN thickness was found to be 54 nm.