*Abstract:*
Ultra-wide-bandgap (UWBG) materials, with band gaps larger
than that of GaN (3.4 eV), are interesting both for their technological
potential and for the unusual physics that emerges from strong bonding,
large energy scales, and competing phases. In this talk, I will discuss how
we use first-principles simulations, many-body *GW* calculations, and
machine-learned force fields to understand how their structure, bonding,
and interfaces control their properties.
I will begin with B*x*Al1-*x*N alloys, in which the composition provides
a direct means to tune properties. Using cluster expansion and *GW*
calculations, we have predicted strongly distorted alloy structures, band
gaps ranging from 6.2 to 7.4 eV, a direct-to-indirect transition near *x*=0.25,
strongly bowed dielectric constants, and large breakdown fields. We also
find that their band alignment depends strongly on composition, polarity,
and local structural distortion. I will then turn to a second UWBG
platform, diamond/cubic-BN, where the key physics shifts from alloying to
interfacial bonding and charge confinement. We find that in diamond/c-BN
heterointerfaces, interfacial stoichiometry can switch the system between
type-I and type-II band alignments. B- and N-terminated interfaces also
host two-dimensional hole and electron gases, respectively. Finally, I will
show how machine-learned force fields can be used to probe phase
transformations between BN allotropes under varying stacking, pressure, and
temperature conditions, providing microscopic insight into growth of
high-purity c-BN.