  ZnSnN2 has recently garnered increasing interest as a potential solar
absorber material due to its direct bandgap that is predicted to be tunable
from 1.0-2.1 eV based on cation disorder. One important challenge to the
further development of this material for photovoltaics (PV) is to reliably
synthesize films with carrier density less than or equal to 10^17
electrons/cm^3. In this work, we perform a systematic annealing study on
compositionally-graded Zn-Sn-N thin films to determine the effects on carrier
density and transport of such post-growth treatment. We find that annealing up
to 6 hr under an activated nitrogen atmosphere results in a reduction in
carrier density by ~80% for zinc-rich films, and by ~50% for stoichiometric
films. However, we also find that annealing reduces mobility as a function of
increasing annealing time. This result suggests that initial film disorder
hampers the benefits to film quality that should have been gained through
annealing. This finding highlights that carefully managed initial growth
conditions will be necessary to obtain PV-quality ZnSnN2 absorber films.
