Ohmic contact to AlGaN/GaN heterostructures on sapphire substrates

Abstract

Low ohmic contact resistance and smooth AlGaN/GaN surface morphology were obtained on a 2-inch c-plane sapphire substrate by using stepwise annealing at three different temperatures. Metallization was performed under deposition of a composite metal layer of Ti/Al/Ni/Au with thickness variations of 30 nm/100 nm/30 nm/100 nm (MS1), 15 nm/220 nm/40 nm/50 nm (MS2), 20 nm/120 nm/55 nm/45 nm (MS3), 15 nm/120 nm/40 nm/50 nm (MS5) and 20 nm/200 nm/55 nm/45 nm (MS6) and of Au/Ge/Au/Ge/Pt with thicknesses of 120 nm/60 nm/120 nm/60 nm/85 nm (MS4), respectively. After multi-layer metal stacking, a rapid thermal annealing (RTA) process was applied with a stepwise annealing temperature profile of 400°C for 180 s, 900°C for 30 s and 940°C for 30 s. We obtained a minimum specific contact resistance of ρc = 4.3 × 10−7 cm2 for MS1, ρc = 1.0 × 10−4 cm2 for MS2, ρc = 3.2 × 10−6 cm2 for MS3, ρc = 5.4 × 10−8 cm2 for MS5 and ρc = 1.1 × 10−7 cm2 for MS6.