Morphology and carrier mobility of high-B-content B _x Al _{1− x} N ternary alloys from an ab initio global search

The excellent properties of III-nitrides and their alloys have led to significant applications in optoelectronic devices. Boron, the lightest IIIA group element, makes it possible to extend the flexibility of III-nitride alloys. However, both B_xAl_1-xN and B_xGa_1-xN ternary alloys suffer from poor material quality during crystal growth, their B contents in experimental reports are no higher than 22%, and the underlying mechanism is still unclear. Herein, ab initio global calculation by particle swarm optimization combined with density functional theory is carried out to identify the ground structures of B_xAl_1-xN alloys with different B contents (x = 0.25, 0.5, and 0.75). Furthermore, the electronic properties and intrinsic carrier mobility are studied. For B_0.25Al_0.75N and B_0.75Al_0.25N, quasi-wurtzite and quasi-hexagonal structures are energetically favourable, respectively, indicating a wurtzite-to-hexagonal structural transition due to the three-coordinated B atoms being incorporated into the lattice. When the B content is 50%, B_0.5Al_0.5N shows a ten-membered ring structure with an indirect bandgap of 3.52 eV and strong anisotropy of mobility. Our results uncover the mechanism of the structural and electronic property evolution with B content and pave a route for the application of B-containing III-nitride alloys.