Record low sheet resistance in homoepitaxial N-polar AlN/GaN heterostructures using digital-alloy barrier by plasma-assisted molecular beam epitaxy

In this work, we demonstrate N-polar AlN/GaN heterostructures grown on free-standing GaN substrates by plasma-assisted molecular beam epitaxy, featuring an ultra-low sheet resistance at room temperature. An AlN/GaN digital-alloy barrier with high heterointerface quality is used to improve electron mobility by suppressing interface roughness and alloy disorder scatterings. Meanwhile, a Si-doped GaN layer is incorporated under the barrier to compensate hole traps and enhance the two-dimensional electron gas (2DEG) density. The homoepitaxial N-polar GaN heterostructure exhibits an atomically smooth surface with a root mean square roughness of 0.21 nm over a 5 × 5 μm2 scan area and an abrupt heterointerface in digital-alloy barrier, resolved by atomic force microscopy and scanning transmission electron microscopy, respectively. High electron mobility of 1729 cm2/(V s) and 2DEG carrier density of 1.95 × 1013 cm−2 are obtained with 10 periods of 0.5 nm AlN/1.5 nm GaN digital-alloy barrier, yielding a record low sheet resistance of 184 Ω/□ among all the reported homoepitaxial N-polar GaN heterostructures to date. This work establishes a promising platform for high-performance N-polar GaN-based electronic devices with low access region resistance.