Repeated bending durability evaluation of ZnO and Al-doped ZnO thin films grown on cyclo-olefin polymer for flexible oxide device applications

Abstract

ZnO and Al-doped ZnO (AZO) thin films were fabricated on cyclo-olefin polymer (COP) substrates with different thicknesses for flexible device applications, and their bending strength was evaluated using surface observations and two-terminal resistance measurements. The fabricated films were subjected to repeated bending endurance tests up to 10 000 cycles. As COP substrate thickness was decreased, surface distortion decreased, two-terminal resistance fluctuations decreased, cracks in SiO2 buffer layer suppressed, and bending strength improved. When COP substrate thickness was 50 μm, variation in two-terminal resistance was negligible, and an oxide thin film device with durability of 10 000 cycles was successfully fabricated. Crystallinity evaluation using X-ray diffraction and mechanical characterization using nanoindentation were performed. This study discusses ZnO and AZO thin films for flexible device applications in relation to their surface conditions, electrical properties, crystallinity, and mechanical properties in cyclic bending tests. The results indicate that these thin films are useful materials for flexible devices.