Zinc oxide (ZnO) is a promising material for piezoelectric sensors and actuators owing to its abundance, low cost, ease of processing, and environmental sustainability. The piezoelectric behavior of ZnO is strongly linked to crystal orientation, with the c-axis of the wurtzite structure providing the highest strain-induced polarization. In polycrystalline ZnO films, aligning individual grains along c-axis (texture development) is essential for enhancing piezoelectric performance. Conventional approaches achieve this alignment using monocrystal substrates (for example ZnO or Al₂O₃), which are costly, or buffer layers, which add complexity to the deposition process.

This thesis investigates routes to improve the piezoelectric performance of ZnO thin films grown non-epitaxially (self-textured), at relatively low temperatures (≤ 250 °C), and under open-air conditions. Growth conditions were optimized to achieve self-textured ZnO films using a custom-built atmospheric-pressure spatial atomic layer deposition (AP-SALD) system. Systematic methodologies were applied to examine the effects of deposition parameters on film texture and piezoelectric properties. As a result, high-quality ZnO films with significantly improved piezoelectric amplitude and domain polarity were obtained. The results further revealed that structural alignment alone does not fully account for the observed enhancements. Additional material characterization and exploratory approaches, including doping strategies, were therefore employed to elucidate the interplay between texture, defects, and functional response.

This work presents the first reported use of AP-SALD for the deposition of piezoelectric films. It demonstrates that AP-SALD is an effective and practical technique for precisely controlling both the structural and functional properties of ZnO thin films. As an open-air, scalable, and cost-effective method, AP-SALD shows strong potential for industrial adoption, particularly in continuous manufacturing processes such as roll-to-roll (R2R). These advances expand the opportunities for the integration of ZnO piezoelectric films into large-area electronics, wearable technologies, flexible smart materials, and multifunctional surfaces.

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