  Zinc-based nitride CaZn2N2 films grown by molecular beam epitaxy (MBE) with a
plasma-assisted active nitrogen-radical source are promising candidates of
next-generation semiconductors for light-emitting diodes and solar cells. This
nitride compound has previously only been synthesized in a bulk form by
ultrahigh-pressure synthesis at 5 GPa. Three key factors have been found to
enable heteroepitaxial film growth: (i) precise tuning of the individual flux
rates of Ca and Zn, (ii) the use of GaN template layers on sapphire c-plane as
substrates, and (iii) the application of MBE with an active N-radical source.
Because other attempts at physical vapor deposition and thermal annealing
processes have not produced CaZn2N2 films of any phase, this rf-plasma-assisted
MBE technique represents a promising way to stabilize CaZn2N2 epitaxial films.
The estimated optical band gap is ~1.9 eV, which is consistent with the value
obtained from bulk samples. By unintentional carrier doping, n- and p-type
electronic conductions are attained with low carrier densities of the order of
1013 /cm3. These features represent clear advantages when compared with
Zn-based oxide semiconductors, which usually have much higher carrier densities
irrespective of their intentionally undoped state. The carrier mobilities at
room temperature are 4.3 cm2/(Vs) for electrons and 0.3 cm2/(Vs) for hole
carriers, which indicates that transport properties are limited by grain
boundary scattering, mainly because of the low-temperature growth at 250
{\deg}C, which realizes a high nitrogen chemical potential.
