  High-Sc Al1-xScxN thin films are of tremendous interest because of their
attractive piezoelectric and ferroelectric properties, but overall film quality
and reproducibility are widely reported to suffer as x increases. In this
study, we correlate the structure and electrical properties of Al0.6Sc0.4N with
in-situ observations of glow discharge optical emission during growth. This
in-situ technique uses changes in the Ar(I) and N2(I) emission lines of the
glow discharge during growth to identify films that subsequently exhibit
unacceptable structural and electrical performance. We show that a steady
deposition throughout film growth produces ferroelectric Al0.6Sc0.4N with a
reversible 80 {\mu}C cm-1 polarization and 3.1 MV cm-1 coercive field. In other
films deposited using identical settings, fluctuations in both Ar(I) and N2(I)
line intensities correspond to decreased wurtzite phase purity, nm-scale
changes to the film microstructure, and a non-ferroelectric response. These
results illustrate the power of optical emission spectroscopy for tracking
changes when fabricating process-sensitive samples such as high-Sc Al1-xScxN
films.
