Flexible Piezoelectric Sensors Based on Single-Crystalline III-N Thin Films for Personal-Healthcare and Extreme-Environment Applications

Single-crystalline group III-nitride (III-N) thin films are very promising by their excellent piezoelectric properties for various industrial fields and biomedical areas due to their advantages. Piezoelectric pressure sensors (PPSs) using III-N thin films have the merits of remarkable biocompatibility, mechanical, chemical and thermal stability, and simple fabrication without extra electric poling. In order to fabricate the flexible PPSs (F-PPSs), well-known layer-transfer method was applied to III-N thin films. This dissertation mainly covers five major topics using III-N thin film based F-PPSs. The first topic introduces that F-PPS was used for epidermic sensor with attachment on the temple area to monitor the eye blinking and lateral eyeball movement with piezoelectric voltage generation. The second topic introduces arrayed format of F-PPSs, which is developing the first topic. Arrayable sensors were attached to 3 different temple areas (upper, middle, and lower temple) to monitor the several types of eyeball movements including lateral, vertical, diagonal, and rotational direction. The third topic introduces III-N thin film-based immunosensor for the detection of cortisol with various conditions. The functionalized F-PPS was used, then the controlled cortisol solutions and collected real sweats were applied to the sensor to evaluate the sensing performances, then the sensor was also tested with skin-attachable mode on forehead. The fourth topic introduces flexible composite thin films consisting of PVDF, BaTiO3 nanoparticles, and textured III-N thin film for the demonstration of wearable F-PPS with enhanced output performance and biocompatibility. Several body motions are tested and composite F-PPSs show excellent output potential and robustness. The fifth topic introduces gas pressure sensor at extremely high temperatures using F-PPSs. The sensor was attached on stainless steel diaphragm, then the operating condition of the sensor was controlled from low to high temperature with various gas pressures. Overall, this dissertation includes a complete set of results from materials, device manufacturing techniques, electromechanical researches, simulations and theoretical calculations to illustrate associated novel aspects in F-PPSs using single-crystalline III-N thin films. In addition, these studies can be further developed in the future.