Enhancing the Piezoelectric Properties of Ultra-Thin ZnO Films on Flexible Substrates Using Atmospheric Pressure Spatial ALD

Zinc oxide (ZnO), known for its abundance, low cost, ease of processing, and ecological benefits, holds significant promise as an alternative material for lead-free piezoelectric sensors and actuators. Despite these advantages, improving the piezoelectric properties of polycrystalline ZnO films deposited on inexpensive flexible substrates at low temperatures remains a challenge. This study addresses the critical question of whether it is feasible to enhance the piezoelectric properties of ZnO films deposited using a facile open-air method at relatively low temperatures without any epitaxial relationship. The significance of solving this problem lies in the potential to directly deposit enhanced piezoelectric ZnO films on flexible substrates using simple, cost-effective open-air methods. This advancement could significantly expand the application of ZnO piezoelectric films in large-area and roll-to-roll (R2R) systems, making them more accessible for diverse applications, including wearables, smart flexible materials, and various surface integrations.<br/>The primary objective of this research is to optimize the texture and piezoelectric properties of ZnO at temperatures below 250 °C on inexpensive flexible substrates using atmospheric-pressure spatial atomic layer deposition (AP-SALD).[1] The study leverages the known relationship between c-axis preferentially oriented wurtzite ZnO structures and superior piezoelectric properties, focusing on the critical role of growth temperature in controlling film texture. Utilizing AP-SALD, we employed diethylzinc as the metal precursor, deionized water as the co-reactant, and nitrogen as the purging gas. The deposition process was systematically studied over a temperature range of 60 to 300 °C, on p-doped Si (100) substrates with a native oxide layer. Structural properties were analyzed using XRD, SEM, and TEM while vertical inverse piezoelectric properties were assessed with PFM.<br/>Key findings indicate that growth temperature has a strong impact on film texture. High-quality piezoelectric ZnO films with a three-fold improvement in piezoelectric amplitude and a high degree of phase polarization could be obtained after optimization. Notably, the optimal piezoelectric response was achieved within the 210 to 270 °C temperature range, with a (002) preferred orientation and an effective d₃₃ piezoelectric coefficient of 4.1 pm/V. [2] Such higher piezoelectric response was observed in films of only around 200 nm thick. In conclusion, AP-SALD is demonstrated to be a viable and efficient method for controlling the structural and piezoelectric properties of thin ZnO films, enabling high-quality piezoelectric materials suitable for R2R applications. The study highlights the importance of texturization and thickness in enhancing the piezoelectric properties of self-textured ZnO films, advancing both fundamental understanding and practical applications in the field of piezoelectric energy-harvesting and sensing materials.<br/><br/>References<br/>1.- D. Munoz-Rojas et al., Spatial Atmospheric Atomic Layer Deposition: A New Laboratory and Industrial Tool for Low-Cost Photovoltaics. Mater. Horiz. 2014, 1 (3), 314–320.<br/>2.- H. Okcu et al. Enhancing Piezoelectric Properties of ultra-thin ZnO Films Using Atmospheric Pressure Spatial-ALD. ACS Appl. Mater. Interfaces 2024 [Submitted]