MBE‐Grown ScAlN‐on‐Si Films: Enhancing In‐Plane Crystallinity for Extensional Mode BAW Resonators

Abstract Piezoelectric thin films are playing an increasingly important role in micro‐electromechanical systems (MEMS) as the expansion of 5G networks and the rise of Internet of Things (IoT) technologies fuel the need for smaller, more reliable, and energy‐efficient sensors and actuators. Alloying Aluminum Nitride (AlN) with Scandium (Sc) is a promising approach to enhance piezoelectric properties in wurtzite semiconductors. However, investigations on ScAlN piezo‐on‐Silicon (Si) have been largely focused on sputtered materials, which often limit resonators to operate in the out‐of‐plane mode, resulting in limited Q. In this study, the piezoelectric properties of ScAlN thin films are reported, which are epitaxially grown on AlN‐buffered Si (111) with enhanced in‐plane crystallinity and a high piezoelectric modulus d ₃₃ up to 25.7 pC/N for Sc composition of 30%. This enables us to demonstrate extensional mode ScAlN‐on‐Si bulk acoustic wave (BAW) resonators with an ultra‐high Q of ≈97k at 70.28 MHz, resulting in a frequency‐ Q product of ≈6.86 × 10 ¹² , indicating low energy loss and high frequency precision, making it ideal for emerging wireless technologies with extremely low latency demands.