Electronic Properties of Ultra-Wide Bandgap B$_x$Al$_{1-x}$N Computed from First-Principles Simulations

Ultra-wide bandgap (UWBG) materials such as AlN and BN hold great promise for future power electronics due to their exceptional properties. They exhibit large bandgaps, high breakdown fields, high thermal conductivity, and high mechanical strengths. AlN and BN have been extensively researched, however, their alloys, B$_x$Al$_{1-x}$N, are much less studied despite their ability to offer tunable properties by adjusting $x$. In this article, we predict the electronic properties of 17 recently predicted ground states of B$_x$Al$_{1-x}$N in the $x=0-1$ range using first-principles density functional theory and many-body perturbation theory within $GW$ approximation. All the B$_x$Al$_{1-x}$N structures are found to be UWBG materials and have bandgaps that vary linearly from that of wurtzite-phase ($w$) AlN (6.19 eV) to that of $w$-BN (7.47 eV). The bandstructures of B$_x$Al$_{1-x}$N show that a direct-to-indirect bandgap crossover occurs near $x = 0.25$. Furthermore, we find that B$_x$Al$_{1-x}$N alloys have much larger dielectric constants than the constituent bulk materials (AlN=$9.3~\varepsilon_0$ or BN=$7.3~\varepsilon_0$), with values reaching as high as $12.1~\varepsilon_0$. These alloys are found to exhibit large dielectric breakdown fields in the range 9--35 MV/cm with a linear dependence on $x$. This work provides the much needed advancement in the understanding of the properties of B$_x$Al$_{1-x}$N to aid their application in next-generation devices.