A method for predicting thickness of the unoriented layer in ZnO film using piezoelectricity distribution in depth direction

The crystalline orientation of thin films gradually improves as the growth proceeds. Especially in the non-epitaxial growth, its initial layer is often unoriented. Because the unoriented layer degrades the performance of the device, the degree of crystalline orientation in depth direction is a very important issue. We propose a non-destructive method for predicting the thickness of the unoriented layer, making use of piezoelectricity distribution of films. An electromechanical resonator consisting of the single highly oriented layer excites a fundamental mode, but does not excite a second-overtone mode. The unoriented layer, on the other hand, exhibits little piezoelectric effect. A bilayer resonator consisting of the highly oriented layer on the unoriented layer excites a second-overtone mode because of the deference of piezoelectricity in these layers. In this study, the electromechanical resonance characteristics of Cu/ZnO(0 0 0 1)/Ti(0 0 0 1)/SiO₂, Cu/ZnO(0 0 0 1)/AZO(0 0 0 1)/Ti(0 0 0 1)/SiO₂ and Cu/ZnO(112̄0) /Al/SiO ₂ were experimentally observed. These results were compared with the theoretical estimations by a mechanical transmission line model to determine the thicknesses of the piezoelectrically inactive layers in the (0 0 0 1) or(112̄0) oriented ZnO films. The inactive layer thickness in(112̄0) the oriented ZnO film was in good agreement with the unoriented layer thickness observed by the cross-sectional transmission electron microscopy and electron diffraction.