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[Display omitted] Linear piezoelectric materials offer new opportunities for next-generation multifunctional micro/nano systems on chip aside from ferroelectrics. However, the advantage of quantitative pressure sensing of linear piezoelectric materials over ferroelectrics has not been demonstrated, while the thermal stability of linear piezoelectric zinc oxide (ZnO) is yet to be evaluated. In this work, pressure sensors based on linear piezoelectric undoped ZnO and ferroelectric vanadium doped ZnO (V-ZnO) films are studied side by side. Both films possess similar fiber textures oriented along the polar ZnO [0 0 2] direction, yet each of the V-ZnO grains consists of strip-like domains observed by SEM. Applying triangular-wave pressures, the linear piezoelectric ZnO sensor generates triangular-wave voltage signals that are one-to-one linearly correlated to the applied pressure, showing decisive advantages for quantitative pressure sensing compared to the 'sail-like' voltage signals of the ferroelectric V-ZnO sensor that is many-to-one correlated to the applied pressure. Superior thermal stability is demonstrated in the linear piezoelectric ZnO sensor showing stable pressure sensing properties up to 450 °C compared to the degraded sensing properties of the ferroelectric V-ZnO sensor at 150 °C. The thermally stable piezoelectric sensors for quantitative pressure sensing based on linear piezoelectric ZnO films serve as candidates for high-performance multifunctional micro/nano systems. [ABSTRACT FROM AUTHOR]

Manufacturing high‐quality zinc oxide (ZnO) devices demands control of the orientation of ZnO materials due to the spontaneous and piezoelectric polarity perpendicular to the c‐plane. However, flexible electronic and optoelectronic devices are mostly built on polymers or glass substrates which lack suitable epitaxy seeds for the orientation control. Applying cubic‐structure seeds, it was possible to fabricate polar c‐plane and nonpolar m‐plane aluminium‐doped zinc oxide (AZO) films epitaxially on flexible Hastelloy substrates through minimizing the lattice mismatch. The growth is predicted of c‐plane and m‐plane AZO on cubic buffers with lattice parameters of 3.94–4.63 Å and 5.20–5.60 Å, respectively. The ∼80 nm‐thick m‐plane AZO film has a resistivity of ∼11.43 ± 0.01 × 10−4 Ω cm, while the c‐plane AZO film shows a resistivity of ∼2.68 ± 0.02 × 10−4 Ω cm comparable to commercial indium tin oxide films. An abnormally higher carrier concentration in the c‐plane than in the m‐plane AZO film results from the electrical polarity along the c‐axis. The resistivity of the c‐plane AZO film drops to the order of 10−5 Ω cm at 500 K owing to the semiconducting behaviour. Epitaxial AZO films with low resistivities and controllable orientations on flexible substrates offer optimal transparent electrodes and epitaxy seeds for high‐performance flexible ZnO devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • We achieved epitaxial linear-piezoelectric ZnO films on flexible substrates. • We enhanced the linear-piezoelectric constant of ZnO with epitaxial strains. • We proved the trivial impacts of textures on the piezoelectric constant of ZnO. Ferroelectrics are popular for sensors, actuators and transducers. However, the energy barrier for reversing the polarization leads to extra energy losses, while the hysteresis causes phase-lags and non-linearities between input and output signals. The high piezoelectric constants of ferroelectrics dictate the figure-of-merit of piezoelectric devices. Enhancing the piezoelectric constant of linear-piezoelectrics potentially leads to next-generation piezoelectric devices free from issues associated with ferroelectrics. By epitaxial growth of undoped ZnO films on flexible Hastelloy substrates, we demonstrated an ~85% improvement in the piezoelectric constant, and more importantly maintained the linearity of the piezoelectric response. Strong out-of-plane and in-plane crystallographic alignments are detected in the epitaxial sample, yet modeling of the effective thin-film piezoelectric constant shows only a limited contribution of the optimized textures. The improvement in the piezoelectric response is ascribed mostly to the in-plane epitaxial strain resulted from the lattice mismatch of ZnO to the substrate. The epitaxial ZnO film on Hastelloy promisingly offer high-precision positioners with nano-/pico-meter resolutions. Enhancing the piezoelectric constant by strain engineering potentially removes the barrier for applications of linear-piezoelectrics and enriches the piezoelectric research community. [ABSTRACT FROM AUTHOR]