N-polar binary GaN/AlN single-crystal high-electron-mobility transistor structures on bulk AlN substrates by molecular beam epitaxy

We report the molecular beam epitaxial growth conditions to realize coherently strained nitrogen-polar gallium nitride quantum wells on single-crystal bulk aluminum nitride substrates. The structural, optical, and electronic properties of these binary N-polar GaN/AlN heterostructures are discussed. The sharpness of the GaN/AlN interface and the preservation of the polarity across the heterojunction is studied by electron microscopy. Photoluminescence measurements reveal two peaks: one at ∼3.6 eV corresponding to a GaN layer under compressive strain that produces a blue shift and the other at ∼6.0 eV from the epitaxial AlN buffer layer. A high-density polarization-induced 2D electron gas is formed in the ultrathin N-polar GaN quantum well, whose transport properties are measured to cryogenic temperatures.