Homoepitaxial growth of GaN and AlGaN/GaN heterostructures by molecular beam epitaxy on freestanding HVPE gallium nitride for electronic device applications

We discuss the growth and characterization of homoepitaxial GaN layers and AlGaN/GaN high electron mobility transistor (HEMT) structures grown by plasma-assisted molecular beam epitaxy (MBE) on freestanding n-GaN substrates. The GaN substrates were fabricated by hydride vapor phase epitaxy and exhibit low dislocation densities of ∼107 cm−2. The best MBE-grown homoepitaxial epilayers on these substrates were grown in the gallium droplet regime. Root-mean-square roughnesses of these layers were 3.5-4.0 Å over 5×5 μm2 regions. AlGaN/GaN HEMT structures were grown on these substrates and exhibit room-temperature Hall mobilities of 1920 cm2/V s at an electron sheet density of 0.9×1013 cm−2. Electrical isolation of the two-dimensional electron gas from the conductive substrate was accomplished using a Be:GaN buffer. HEMT devices were photolithographically defined and DC and RF device characteristics were measured. Off-state breakdown voltages of 90 V, saturated drain currents of nearly 700 mA/mm, and gate leakage currents of 0.07 mA/mm were observed on unpassivated devices. Preliminary results on RF performance and device reliability are presented and discussed.