Accurate and continuous monitoring of eye movements using compact, low‐power‐consuming, and easily‐wearable sensors is necessary in personal and public health and safety, selected medical diagnosis techniques (point‐of‐care diagnostics), and personal entertainment systems. In this study, a highly sensitive, noninvasive, and skin‐attachable sensor made of a stable flexible piezoelectric thin film that is also free of hazardous elements to overcome the limitations of current computer‐vision‐based eye‐tracking systems and piezoelectric strain sensors is developed. The sensor fabricated from single‐crystalline III‐N thin film by a layer‐transfer technique is highly sensitive and can detect subtle movements of the eye. The flexible eye movement sensor converts the mechanical deformation (skin deflection by eye blinking and eyeball motion) with various frequencies and levels into electrical outputs. The sensor can detect abnormal eye flickering and conditions caused by fatigue and drowsiness, including overlong closure, hasty eye blinking, and half‐closed eyes. The abnormal eyeball motions, which may be the sign of several brain‐related diseases, can also be measured, as the sensor generates discernable output voltages from the direction of eyeball movements. This study provides a practical solution for continuous sensing of human eye blinking and eyeball motion as a critical part of personal healthcare, safety, and entertainment systems.

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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.

Whereas piezoelectric pressure sensors (PPSs) have been applied in the monitoring of human body movement and physiological parameters, they show inherent limitations in wearable applications, including toxicity, degradation, and brittleness. In this study, we develop safe, stable, and mechanically flexible composite thin films consisting of polyvinylidene fluoride (PVDF), BaTiO3 nanoparticles (BTO-NPs), and textured aluminum nitride (AlN) thin film for the demonstration of wearable PPS with enhanced output performance and biocompatibility. The PPS made of BTO-NP-embedded-PVDF and AlN film on Cu foil is attached to different parts of human body to measure different output voltages depending on the physiological and physical stimulus. The simple bending (from breathing, chewing, and swallowing), joint motions (at wrist, elbow, and finger), and low- (from eyeball movement) and high-pressure applications (by squat, lunge, and walking) are measured. Our PVDF+BTO-NP/AlN-PPS (PBA-PPS) device has the potential for personal safety, healthcare, and activity monitoring applications with easy wearability.

Demonstration of a mechanically flexible single-crystalline GaN substrate by direct epitaxial growth on metal foil with significantly reduced processing costs and versatile functionality on flexible electronics and photonics.

We introduce hybrid chemical vapor deposition (HybCVD) employing chloride, an elemental source, and hydride for the precursors of the group-III element, transition-metal element, and nitrogen, respectively, for obtaining ScAlN thin films to address the technical challenges in current techniques associated with film quality and manufacturability. For the new deposition technique, we numerically investigate the thermodynamics of precursors and vapor− solid reactions to evaluate the suitability of the precursors and controllability of the alloy composition depending on various precursors and deposition conditions. The AlCl 3 precursor is needed to provide a high precursor input while protecting the CVD system. An elemental Sc source precursor is capable of providing a high precursor input and enough driving force for ScN formation. For the film deposition, the Sc composition of the film slightly decreases with increasing V/III ratio and growth temperature, while its change by the mixture ratio of inert gas and hydrogen in the carrier gas is negligible. The Sc composition in the alloy can be accurately controlled by the mixture ratio of the cation precursors. HybCVD offers the potential for both the high material quality and low-cost manufacturing of transition-metal-alloyed III-N thin films to maximize their expected benefits for highly functional piezoelectric materials in a wide range of applications.

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