Improving Reproducibility of Sputter Deposited Ferroelectric Wurtzite Al0.6Sc0.4N Films using In-situ Optical Emission Spectrometry.

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.