Piezoelectric Sensors Operating at Very High Temperatures and in Extreme Environments Made of Flexible Ultrawide‐Bandgap Single‐Crystalline AlN Thin Films

Extreme environments are often faced in energy, transportation, aerospace, and defense applications and pose a technical challenge in sensing. Piezoelectric sensor based on single‐crystalline AlN transducers is developed to address this challenge. The pressure sensor shows high sensitivities of 0.4–0.5 mV per psi up to 900 °C and output voltages from 73.3 to 143.2 mV for input gas pressure range of 50 to 200 psi at 800 °C. The sensitivity and output voltage also exhibit the dependence on temperature due to two origins. A decrease in elastic modulus (Young's modulus) of the diaphragm slightly enhances the sensitivity and the generation of free carriers degrades the voltage output beyond 800 °C, which also matches with theoretical estimation. The performance characteristics of the sensor are also compared with polycrystalline AlN and single‐crystalline GaN thin films to investigate the importance of single crystallinity on the piezoelectric effect and bandgap energy‐related free carrier generation in piezoelectric devices for high‐temperature operation. The operation of the sensor at 900 °C is amongst the highest for pressure sensors and the inherent properties of AlN including chemical and thermal stability and radiation resistance indicate this approach offers a new solution for sensing in extreme environments. Ultrawide‐bandgap flexible AlN thin film is fabricated into a piezoelectric sensor on a stainless steel diaphragm to measure pressures at high temperatures up to 900 °C. The sensor can be used to measure other physical parameters such as strain and vibrations in extreme conditions thanks to its chemical and thermal stability, corrosion resistance, and radiation hardness.