Search

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]