Tunable optoelectronic and ferroelectric properties in Sc-based III-nitrides

Abstract

Sc-based III-nitride alloys were studied using density functional theory with special quasi-random structure methodology. ScxAl1-xN and ScxGa1-xN alloys are found to be stable in hexagonal phases up to x ≈ 0.56 and x ≈ 0.66, respectively, above which rock-salt structures are more stable. Epitaxial strain stabilization can prevent spinodal decomposition up to x ≈ 0.4 (ScxAl1-xN on AlN or GaN) and x = 0.27 (ScxGa1-xN on GaN). The increase in Sc content expands the in-plane lattice parameter of ScxAl 1-xN and ScxGa1-xN alloys, leads to composition- and strain-tunable band gaps and polarization, and ultimately introduces ferroelectric functionality in ScxGa1-xN at x ≈ 0.625. A modified Becke-Johnson exchange-correlation potential was applied to study the electronic structures, which yielded band gaps comparable to those from hybrid functional calculations, yet in a much shorter computational time. The alloys were found to retain wide band gaps, which stay direct up to x = 0.25 (ScxAl1-xN) and x = 0.5 (ScxGa 1-xN). The band gaps decrease with increasing x for Sc xAl1-xN, in which the Sc-3d states dominate at the conduction band minimum and lead to flat electron dispersion at the Γ point. Conversely, the band gaps increase with increasing x for Sc xGa1-xN (up to x = 0.5), in which Sc-3d states do not contribute to the conduction band minimum at the Γ point. © 2013 AIP Publishing LLC.