  Alloy scattering in random AlGaN alloys drastically reduces the electron
mobility and therefore the power-electronics figure of merit. As a result, Al
compositions greater than 75% are required to obtain even a two-fold increase
of the Baliga figure of merit compared to GaN. However, beyond approximately
80% Al composition, donors in AlGaN undergo the DX transition which makes
impurity doping increasingly more difficult. Moreover, the contact resistance
increases exponentially with increasing Al content, and integration with
dielectrics becomes difficult due to the upward shift of the conduction band.
Atomically thin superlattices of AlN and GaN, also known as digital alloys, are
known to grow experimentally under appropriate growth conditions. These
chemically ordered nanostructures could offer significantly enhanced figure of
merit compared to their random-alloy counterparts due to the absence of alloy
scattering, as well as better integration with contact metals and dielectrics.
In this work, we investigate the electronic structure and phonon-limited
electron mobility of atomically thin AlN/GaN digital-alloy superlattices using
first-principles calculations based on density-functional and many-body
perturbation theory. The band gap of the atomically thin superlattices reaches
4.8 eV, and the in-plane (out-of-plane) mobility is 369 (452) cm$^2$ V$^{-1}$
s$^{-1}$. Using the modified Baliga figure of merit that accounts for the
dopant ionization energy, we demonstrate that atomically thin AlN/GaN
superlattices with a monolayer sublattice periodicity have the highest modified
Baliga figure of merit among several technologically relevant ultra-wide
band-gap materials, including random AlGaN, $\beta$-Ga$_{2}$O$_{3}$, cBN, and
diamond.
