**4. Conclusions**

We extensively studied the vertical and lateral etch rate of AlN/Al1−xScxN in aqueous KOH solutions across etch temperature, KOH concentration and a broad range of scandium alloying. It was shown that the vertical etch rate declines steadily with increased levels of Sc alloying, declining from 124.6 ± 0.68 nm/s for AlN to 3.7 ± 0.063 nm/s for Al0.64Sc0.36N in 30 wt% KOH at 45 ◦C. By contrast, the resistance to lateral etching peaks at a mere 0.043 ± 0.002 nm/s when x = 0.125. This is orders of magnitude lower compared to the lateral etch rate of 1.99 ± 0.01 nm/s for AlN or 1.99 ± 0.17 nm/s for Al0.64Sc0.36N. We have also demonstrated that KOH wet etching of Al1−xScxN is mostly anisotropic, and that the etch profile can be predicted from the crystal structure coupled with a small-scale isotropic etching of the sidewall. A technique for fabricating a vertical sidewall by exposing the 1100 planes of sputtered Al1−xScxN was also demonstrated via etching an 800 nm thick Al0.875Sc0.125N film in 10 wt% KOH at 65 ◦C for 20 min. With this method, the fabrication of numerous MEMS devices such as LWRs, laser mirrors and UV-LEDs can be benefited. Future work will include detailed research on the activation energy for the lateral etching of AlN/Al1−xScxN using Arrhenius plots formed [32] from a series of design of experiments (DOE) using the Taguchi method [59].

**Author Contributions:** Conceptualization, Z.T. and R.H.O.III; methodology, Z.T., G.E. and R.H.O.III; validation, Z.T. and J.Z.; formal analysis, Z.T., G.E. and J.Z.; investigation, Z.T.; resources, G.E. and R.H.O.III; data curation, Z.T.; writing—original draft preparation, Z.T.; writing—review and editing, G.E. and R.H.O.III; visualization, Z.T.; supervision, R.H.O.III; project administration, G.E. and R.H.O.III; funding acquisition, G.E. and R.H.O.III All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded in part by the NSF CAREER Award (1944248). This work was carried out in part at the Singh Center for Nanotechnology at the University of Pennsylvania, a member of the National Nanotechnology Coordinated Infrastructure (NNCI) network, which is supported by the National Science Foundation (grant no. HR0011-21-9-0004). Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

**Data Availability Statement:** The data supporting the findings of this study are available from the corresponding author upon reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
