MBE growth, structural, optical, and thermal properties of AlBN

We report plasma-assisted molecular beam epitaxial growth of AlBN thin films on a nitrided c-plane Al2O3 substrate. The AlBN film epitaxially grows in rotational alignment with an out-of-plane/in-plane directions of AlBN [0001¯]/[101¯0]‖AlN nucleation layer [0001¯]/[101¯0]‖Al2O3[0001]/[112¯0]. The B composition of the AlBN layer is varied from 0% to 15% by varying the growth temperature, exploiting the reaction rate-controlled growth mechanism. X-ray diffraction and high-resolution transmission electron microscopy are used to determine the structural properties as a function of boron composition. A monotonic decrease in the c-lattice constant and a non-monotonic change in the a-lattice constant are observed with an increase in the B content in AlBN films grown on nitrided sapphire. While the control AlN film showed a bandgap of 6.1 eV, the AlBN films with ∼15% boron showed a bandgap of 5.9 eV. The AlBN films with 15% B exhibit a fivefold increase in the nonlinear second-harmonic generation intensity compared to AlN. AlBN films exhibit higher thermal conductivity than AlScN films with comparable alloy compositions, and at equal or smaller thicknesses. The findings indicate several opportunities for AlBN films in applications of deep UV optoelectronics, nonlinear photonics, and high-power electronics devices, especially in high-voltage and high-temperature environments.