Growth and characterization of uniform ZnO films as piezoelectric materials using a hydrothermal growth technique

Abstract

ZnO nanostructures, especially in the form of dense arrays of nanorods or belts have the ability to efficiently convert mechanical energy to electrical energy. One of the drawbacks though for the exploitation of nanorod technology for commercial devices is the ability to make the electrical contacts to these nanostructured piezoelectric converting elements. Although technologies have been developed that provide solutions for electrical contact issues, metal contact on uniform thin films are much simpler, and can readily be implemented to commercial mass-produced applications. At the same time it is known that high piezoelectric coefficients ZnO uniform films with columnar grains having their c-axis perpendicular to the substrate are required. In this work, we investigate the growth of uniform ZnO films, using a low temperature, low cost hydrothermal process typically used for the fabrication of ZnO nanorods. Under appropriate conditions coalescence of the nanorods occur resulting in uniform films with a columnar structure. The study focuses on understanding the role of the growth factors in order to be able to fully control the proposed process. Moreover, the hydrothermal method is further exploited for the fabrication of uniform ZnO nanostructures on patterned substrates with Au interdigitated electrodes (IDE) using standard lithography as a proof-of-concept of the applicability of the method to standard microfabrication techniques. The piezoelectric films with the IDEs are electrically characterized in order to assess the electrical properties of the grown films. From this analysis, process conditions have been identified for the growth of uniform nanostructured ZnO films, suitable for piezoelectric microgenerators.