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Ab-initio Prediction of Ultra-Wide Band Gap B$_x$Al$_{1-x}$N Materials

Ultra-wide band gap (UWBG) materials are poised to play an important role in the future of power electronics. Devices made from UWBG materials are expected to operate at higher voltages, frequencies, and temperatures than current silicon and silicon carbide based devices; and can even lead to significant miniaturization of such devices. In the UWBG field, aluminum nitride and boron nitride have attracted great interest, however, the B$_x$Al$_{1-x}$N alloys are much less studied. In this article, using first-principles simulations combining density-functional theory and cluster expansion method we predict the crystal structure of B$_x$Al$_{1-x}$N alloys. We find 17 ground state structures of B$_x$Al$_{1-x}$N with formation energies between 0.11 and 0.25 eV/atom. All of these structures are found to be dynamically stable. The B$_x$Al$_{1-x}$N structures are found to have predominantly a tetrahedral bonding environment, however, some structures exhibit $sp^2$ bonds similar to hexagonal BN. This work expands our knowledge of the structures, energies, and bonding in B$_x$Al$_{1-x}$N aiding their synthesis, innovation of lateral or vertical devices, and discovery of compatible dielectric and Ohmic contact materials.