A very promising piezoelectric property of Ta2O5 thin films. II: Birefringence and piezoelectricity

Birefringent and piezoelectric properties of Ta2O5 ceramic thin films of monoclinic and trigonal structures were analyzed. The birefringence, observed by reflected polarized light microscopy, yields information on thin film microstructures, crystal shapes and sizes and on crystallographic orientations of grains of trigonal structure. Such an information was considered for investigating piezoelectric properties by laser Doppler vibrometry and by piezoresponse force microscopy. The vibration velocity was measured by applying an oscillating electric field between electrodes on both sides of a Ta2O5 film deposited on a Si substrate which was pasted on an isolating mica sheet. In this case, it is shown that the vibration velocity results were not only from a converse piezoelectric effect, proportional to the voltage, but also from the Coulomb force, proportional to the square of the voltage. A huge piezoelectric strain effect, up to 7.6%, is found in the case of Ta2O5 of trigonal structure. From an estimation of the electrical field through the Ta2O5 thin film, this strain likely corresponds to a very high longitudinal coefficient d33 of several thousand picometers. Results obtained by piezoresponse force microscopy show that trigonal grains exhibit a polarization at zero field, which is probably due to stress caused expansion in the transition monoclinic–trigonal, presented in a previous article (part I).

Image of cross-polarized optical microscopy showing grains of trigonal structure embedded in the monoclinic phase (on the left); (a) mounting of the sample for Laser Doppler Vibrometry, sample constituted of several layers and its equivalent electrical circuit; (b) longitudinal displacements due to converse piezoelectric and Coulomb effects and corresponding piezoelectric strain–Uapp.. hystereses.Figure optionsDownload as PowerPoint slideHighlights
► A new Ta2O5 trigonal phase is shown to be birefringent and piezoelectric.
► This phase is related to a reversible transition with a monoclinic phase.
► The piezoelectricity of this trigonal phase is of several thousands of pm/V.
► It is compared to piezoelectricity of the monoclinic phase of several tens of pm/V.