Cathodoluminescence and X-Ray Photoelectron Spectroscopy of ScN: Dopant, Defects and Band Structure

Abstract

Abstract Body: The rocksalt-structure semiconductor ScN is a relatively unexplored group IIIB transition metal nitride with outstanding physical properties that, combined with its III-nitride counterparts, has the potential to dramatically expand the range of their electrical, optical, and thermoelectric device applications. Recent works have focused on the degenerate nature of ScN by substitutional impurities ON and FN, which shift the direct (X–X) gap transition to higher energies via the Burstein–Moss effect. Here, we used cathodoluminescence spectroscopy (CLS) to observe optical signatures of both the midgap VN precursor to ON doping as well as signatures above the direct (X–X) bandgap corresponding to band-to-band transitions from four separate conduction bands near the Γ point with the valence band minimum, in agreement with calculated band structure diagrams. Thin film ScN grown by reactive magnetron sputtering displays mild degenerate doping by substitutional oxygen as indicated by elevated (X–X) transition energies and the presence of Sc–O bonding determined via x-ray photoelectron spectroscopy (XPS), while ScN grown by physical vapor transport exhibited the intrinsic, non-degenerate (X–X) bandgap predicted by theory. CLS reveals a sharp, sub-bandgap emission at 1.26 eV for sputter grown ScN on GaN, which we attribute to nitrogen vacancies (VN) based on surface sensitive CLS and XPS chemical trends. This finding is in strong agreement with theoretical calculations for VN predicting the formation of a defect energy level within the gap. Since O substitution into VN defects creates the shallow donor level responsible for degenerate doping, optical identification of this doping precursor can help understand the growth and processing that influence ScN doping. L.J.B., M.S.H., and S.S. acknowledge the support from the National Science Foundation, Grant No. DMR 18-00130. A.H. and J.H.E. acknowledge the support for ScN crystal growth from the National Science Foundation under Grant No. DMR-1508172. This research was also supported by the Air Force Office of Scientific Research through Project No. FA9550-RY17COR490 (J.S.C. and A.N.R.).