**Preface to "Piezoelectric Aluminium Scandium Nitride (AlScN) Thin Films: Material Development and Applications in Microdevices"**

Not so long ago, aluminium scandium nitride (AlScN) emerged as a material that possesses superior properties as compared to aluminium nitride (AlN). Substituting Al with Sc in AlN leads to a dramatic increase in the piezoelectric coefficient as well as in electromechanical coupling. This discovery finally allowed us to overcome the limitations of AlN thin films in various piezoelectric applications while still enabling us to benefit from all of the advantages of the parent material system, such as a high temperature stability, CMOS compatibility, and good mechanical properties.

AlScN allows for the enhanced performance of RF filters where bulk acoustic wave (BAW) or surface acoustic wave (SAW) resonators are used. Additionally, energy harvesting and sensing applications can benefit as well. Higher pyroelectric coefficients allow for new advances in, for example, infra-red (IR) detectors. Recent progress in growing this material by MOCVD and MBE has opened up new prospects in high-frequency and -power electronics, such as with high-electron-mobility transistors (HEMTs). Finally, AlScN is the first wurtzite III-nitride where ferroelectric switching was observed a few years ago, opening up another exciting research field with many possible applications in semiconductor memories; additionally, it enables the additional functionality of switching to applications where piezoelectric materials are already in use.

Recognizing the importance of this new material, internationally recognized research groups participated in this Special Issue focused on AlScN to provide the scientific community with a highly visible, multidisciplinary open access collection of the recent advances in understanding the material itself that will enable us to unlock its full potential in microdevices.

This Special Issue presents contributions addressing:


We hope that you will enjoy this collection!

**Agn˙e Zukauskait ˙e ˇ** *Editor*
