Growth of c-plane and m-plane aluminium-doped zinc oxide thin films: epitaxy on flexible substrates with cubic-structure seeds

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.