Abstract
Vanadium doped ZnO (VZO) thin films were grown by RF magnetron sputtering, starting from a ZnO:V ceramic target. The crystal structure, chemical composition, electric and piezoelectric properties of the films were investigated either on the as-grown thin films or after a post-deposition rapid thermal annealing (RTA) treatment performed at 600 °C for different lengths of time (1 and 5 min) in an oxygen atmosphere. Substitutional doping of Zn2+ with V3+ and V5+ ions strongly deteriorated the hexagonal wurtzite ZnO structure of the as-grown thin films due to lattice distortion. The resulting slight amorphization led to a poor piezoelectric response and higher resistivity. After the RTA treatment, strong c-axis oriented VZO thin films were obtained, together with a partial conversion of the starting V3+ ions into V5+. The improvement of the crystal structure and the stronger polarity of both V3+ – O and V5+ – O chemical bonds, together with the corresponding easier rotation under the application of an external electric field, positively affected the piezoelectric response and increased conductivity. This was confirmed by closed-loop butterfly piezoelectric curves, by a maximum d33 piezoelectric coefficient of 85 pm·V−1, and also by ferroelectric switching domains with a well-defined polarization hysteresis curve, featuring a residual polarization of 12.5 μC∙cm−2.
Highlights
Vanadium doped Zinc oxide (ZnO) (VZO) thin films were grown by RF magnetron sputtering, starting from a ZnO:V ceramic target
The sputtering technique is of great interest since it allows for the growth of strongly textured ZnO thin films in a very uniform and controllable way on wide-area substrates of different natures[8,9,19]
The n-type nature of ZnO is advantageous for different applications, like the preparation of transparent conducting oxides (TCOs) and photoanodes for photovoltaic applications and solar cells[10,20]
Summary
Morphological and structural analyses of VZO thin films. Figure 1 shows the cross-section nanostructure of the as-grown VZO thin films. Besides improving the crystal structure of VZO thin films, it is found that another effect of the RTA treatment is the partial conversion of the starting V3+ species into V5+ ions This strongly influences the crystal structure of the annealed samples, as previously observed from the XRD characterization results, and is expected to influence the piezoelectric properties as well, as shown . Samples VZO-2% and VZO-2.5% show an increased amount of V5+ ions at the expense of V3+ ones, together with a remarkable improvement of the corresponding crystal structure Both these aspects are confirmed from XPS and XRD results, that evidenced the shift of the (002) diffraction peak toward higher 2θangles. It is found that V3+ ions do not suppress piezoelectricity at all, but on the contrary generally improve the piezoelectric response with respect to the pristine ZnO case
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
