  Nitride films are promising for advanced optoelectronic and electronic device
applications. However, some challenges continue to impede development of high
aluminum-containing devices. The two major difficulties are growth of high
crystalline quality films with aluminum-rich compositions, and efficiently
doping such films p-type. These problems have severely limited use of
aluminum-rich nitride films grown by molecular beam epitaxy. A way around these
problems is through use of a liquid-metal-enabled approach to molecular beam
epitaxy. Although the presence of a liquid metal layer at the growth front is
reminiscent of conventional liquid phase epitaxy, this approach is different in
its details. Conventional liquid epitaxy is a near-thermodynamic equilibrium
process which liquid-metal assisted molecular beam epitaxy is not. Growth of
aluminum-rich nitrides is primarily driven by the kinetics of the molecular
vapor fluxes, and the surface diffusion of adatoms through a liquid metal layer
before incorporation. This paper reports on growth of high crystalline quality
and highly doped aluminum-containing nitride films. Measured optical and
electrical characterization data show that the approach is viable for growth of
atomically smooth aluminum-containing nitride heterostructures. Extremely high
p-type doping of up to $6 \times 10$$^{17}$ cm$^{-3}$ and n-type doping of up
to $1 \times 10$$^{20}$ cm$^{-3}$ in Al$_{0.7}$Ga$_{0.3}$N films was achieved.
Use of these metal-rich conditions is expected to have a significant impact on
high efficiency and high power optoelectronic and electronic devices that
require both high crystalline quality and highly doped (Al,Ga)N films.
