Reducing high carrier concentration in rocksalt-AlxSc1-xN with Mg acceptor doping

Abstract

Aluminum scandium nitride (AlxSc1-xN) is an emerging III-nitride semiconductor that has attracted significant interest in recent years in surface and bulk acoustic resonators for its high piezoelectric coefficient and applications in high-power electronic devices. AlxSc1-xN stabilizes in the rock salt phase for x < 0.52 when deposited directly on (001) MgO substrates and has been utilized as a semiconductor in single-crystalline TiN/AlxSc1-xN metal/semiconductor superlattices for thermionic energy conversion, optical hyperbolic metamaterials, and the fundamental studies on heat and current transport in materials. However, due to the presence of oxygen impurities and native defects, such as nitrogen vacancies, sputter-deposited rock salt-AlxSc1-xN exhibits a high carrier concentration in the (2-4) × 1020 cm-3 range that leads to its Ohmic tunneling contact with metals and prevents observation of thermionic emission. In this Letter, we demonstrate that magnesium (Mg) acts as an efficient hole-dopant in r-AlxSc1-xN, increases its resistivity, and reduces its carrier concentration as a function of Mg concentration to as low as 1.4 × 1018 cm-3. A combination of spectroscopy, microscopy, and first-principles modeling demonstrate (a) epitaxial 001 oriented AlxSc1-xN:Mg growth for the first 35-75 nm and subsequent pyramidal growth with multiple in-plane orientations, (b) MgxNy to form a uniform and homogeneous solid solution with r-AlxSc1-xN without any precipitation, phase separation, or secondary phase formation, and (c) Mg-defect states are located deep inside the valence and conduction bands that leave behind a pristine r-AlxSc1-xN bandgap and band edges. The demonstration of Mg-hole doping in r-AlxSc1-xN marks significant progress in r-AlxSc1-xN thin film and superlattice-based devices.