Combining Mixed-phase Domain Configuration and Multilayer Structure for High-performance Thin-film Piezoelectrics

Thin-film piezoelectrics are critical to miniaturized devices such as sensors and actuators, but the electromechanical response of thin films is (in general) severely restricted by substrate clamping. Ferroelectric thin films with mixed-phase domain configurations have been proposed as promising candidates for thin-film piezoelectrics [1]. In such materials, the coexistence of different types of domain structures indicates the similarity in free energy between different domain structures, and further facilitates the transition between different domain structures under applied fields. For example, a previous study demonstrated that mixed-phase PbTiO₃ thin films, which consist of coexisting c/a and a₁/a₂ domains, exhibit local transitions from a₁/a₂ to c/a domain structures with poling and, correspondingly, large local strains of ~1.2% [2].

Here, we present a systematic study of the electromechanical response of mixed-phase PbZr_0.2Ti_0.8O₃ thin films. By tuning the strain state of PbZr_0.2Ti_0.8O₃ thin films, the domain configuration can be manipulated from c, to c/a, and to a mixture of c/a and a₁/a₂. Compared to thin films with c or c/a domain configurations, significantly enhanced electromechanical response was observed in the mixed-phase films, with the effective piezoelectric coefficient d₃₃^eff ≈ 170 pm/V and the maximum field-induced strain S_max ≈ 1.25%. Furthermore, in situ second-harmonic generation measurements reveal the conversion from in-plane polarized domains to out-of-plane polarized domains, while in situ scanning-transmission-electron-microscopy studies reveal the formation of c/a domain structures inside a₁/a₂ regions under applied field. Therefore, the field-induced transition from a₁/a₂ to c/a domain structures underpins the enhanced electromechanical responses of mixed-phase PbZr_0.2Ti_0.8O₃ thin films.

To further maximize the response that is possible from the films, we explored multilayer structures based on PbZr_0.2Ti_0.8O₃/0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ (PMN-0.32PT)/PbZr_0.2Ti_0.8O₃ trilayer heterostructures. PMN-0.32PT was selected as the middle layer due to its high dielectric constant and easy reorientation and alignment of nanoscale-polar structures, thus preventing both significant voltage drop across the middle layer and significant alteration of the domain configuration in the bottom and top layers. As a result, the mixed-phase domain configuration is maintained in the trilayer heterostructures, and under the same fields the electromechanical responses of the trilayer heterostructures approach that of the mixed-phase PbZr_0.2Ti_0.8O₃ thin films. In addition, a significant improvement of the electrical breakdown strength (E_b) was seen (surpassing 2.2 MV/cm) for the trilayer heterostructures as compared to the single-layer mixed-phase PbZr_0.2Ti_0.8O₃ thin films (E_b ≈ 1.2 MV/cm). This enhancement is attributed to the band misalignment at the PbZr_0.2Ti_0.8O₃-PMN-0.32PT interfaces and hence the reduction of leakage current across the heterostructures. Therefore, the maximum field-induced strain S_max is significantly enhanced in the trilayer heterostructures (S_max ≈ 2.10%) as compared to the single-layer mixed-phase PbZr_0.2Ti_0.8O₃ thin films (S_max ≈ 1.25%).

All told, this study demonstrates that mixed-phase ferroelectric thin films (and multilayer heterostructures derived from the same) exhibit high potential as thin-film piezoelectric materials. At a higher level, the combination of domain-structure optimization and multilayer-structure design may be adopted as a universal strategy for developing thin-film piezoelectric materials with good comprehensive performance of d₃₃^eff and S_max.

References:
[1] J. X. Zhang et al., Nat. Nano. 6, 98 (2011)
[2] A. R. Damodaran et al., Adv. Mater. 29, 1702069 (2017)