Piezoelectric Response of ZnO Thin Films Grown By Plasma-Enhanced Atomic Layer Deposition
Authors: Pilz Julian , Abu Ali Taher , Schäffner Philipp , Stadlober Barbara , Coclite Anna Maria 
Published: February 2020

ZnO is a direct band gap semiconductor with attractive piezoelectrical, optical, and electrical properties, particularly appealing for a variety of functional devices. Especially the utilization of piezoelectric properties of ZnO nanostructures for transforming mechanical to electrical energy has attracted much research interest. For most of these so called nanogenerators, solution based deposition methods have been applied to create the desired nanostructures, often lacking a precise control of the deposition parameters. Atomic layer deposition, on the other hand, allows conformal and uniform deposition on high aspect ratio structures with Å-level thickness control.

In this study, we investigate the piezoelectric response of ZnO thin films on flexible substrates as a starting point for piezoelectric nanostructures. The films are grown by plasma-enhanced atomic layer deposition (PE-ALD) to thicknesses below 100 nm by adapting diethylzinc and O2-plasma as reactants. In comparison to thermal ALD (where diethylzinc and water are used as reactants), PE-ALD allows the deposition of films with higher resistivity, an important property to minimize the leakage of piezoelectric charges. Commercially available Polyethylenterephthalat (PET) coated with Indium Tin Oxide (ITO) serves as the flexible substrate and bottom electrode, respectively. The deposition of ZnO thin films is carried out at substrate temperatures between room temperature and 100 °C, as a change in preferential crystal orientation from (100) to (002) can be observed in this temperature range. [1] The macroscopic piezoelectric characterization is performed in a home-built stamp station, in which a defined periodic force is exerted onto the samples and the generated piezoelectric charges are measured. Out of this, the longitudinal piezoelectric coefficient d33
can be obtained. Preliminary results show d33
coefficients > 7 pC/N, which is comparable to literature results. [2] The piezoelectric characterization is made for the different samples to understand how the d33
coefficient changes for films deposited at different substrate temperatures and thus having different crystal orientation. Since the [002] is the polar axis in the ZnO wurtzite crystal structure, films with preferred orientation in this direction are therefore expected to show higher d33
coefficients.

The work lays the basis for developing functional piezoelectric generators and sensors in thin film form. However, the concepts can be easily transferred to depositions on lithographically defined templates in order to create nanostructured ZnO, which exhibits increased piezo response.

[1] J.Pilz, A. Perrotta, G. Leising, A. M. Coclite, ZnO thin films grown by plasma-enhanced atomic layer deposition: material properties within and outside the “ALD window”, submitted.

[2] M. Laurenti, S. Stassi, M. Lorenzoni, M. Fontana, G. Canavese, V. Cauda, C.F. Pirri, Evaluation of the piezoelectric properties and voltage generation of flexible zinc oxide thin films, Nanotechnology. 26 (2015) 215704. doi:10.1088/0957-4484/26/21/215704.