Lowering in Resistivity and Ultra-Flattening of AZO Thin Films on Cyclo-Olefin Polymer Substrates via Substrate-Surface Modification for Flexible Optoelectronics

ZnO and Al-doped ZnO (AZO) thin films can achieve relatively low resistivity even on amorphous substrates such as oxide glasses. Therefore, in combination with flexible transparent polymer substrates, these conducting thin films are expected to be used in wearable or flexible optoelectronic devices. The thermoplastic cyclo-olefin polymer (COP) used in this study is promising as a flexible substrate for wearable optoelectronics due to its very low moisture absorption, high transparency, and relatively high grass transition temperature (about 150°C), in contrast there are concerns about its thermal properties and surface roughness in comparison with oxide substrates. It might be useful for the growth of highly crystalline oxide thin films on polymeric substrates to modify of the polymer substrate surface to control oxide nucleation and growth direction. It might be useful to modify the polymer substrate surface for growth of highly crystalline oxide thin films on polymer substrates, leading to control of oxide nucleation and growth direction at low temperatures applicable for polymer materials. So far, we reported the atomic-scale patterning on the polymer surfaces having 0.3 nm-high atomic-steps and ultra-flat terraces, by applying thermal nanoimprinting using atomically stepped sapphire (α-Al₂O₃) molds [1]. Significant decrease in the surface roughness and crystallinity improvement was found for the ZnO thin films deposited on the surface-morphology-controlled polymer substrates subject to nanoimprinting [2]. The insertion of an oxide buffer layer on the polymer substrate will be expected to promote oriented crystal growth of ZnO thin film deposited on it. In this study, we investigate the influence of the oxide buffer layer inserted on the ultra-flat polymer substrate upon the crystal growth and electrical property of ZnO thin films in pulsed laser deposition (PLD) at room-temperature.<br/>Firstly, the surface topography of COP sheets used as the substrates was modified by thermal nanoimprinting (2 MPa, 180°C, 5 min, 10³ Pa) using a sapphire mold with a 0.3 nm-high atomic step and terrace pattern, followed by PLD using a KrF excimer laser (λ=248 nm, d~20 ns) to form oxide buffer layers such as Al₂O₃. Subsequently, Al-doped ZnO (AZO; Al: 2 wt%) thin films (t~200 nm) were grown at RT by PLD under O₂ gas at 10^-3 Pa. The AZO thin films grown on the surface-pretreated COP substrates showed very smooth surface without irregularities. The root mean square roughness in 3 µm squares of the AZO thin film on the pretreated COP substrate was reduced from 1.3 nm for the sample on the untreated substrate down to 0.4 nm. The XRD analysis of the diffraction peak attributed to AZO (001) indicated that the crystallinity and growth orientation of AZO thin films on the surface-pretreated substrate were clearly improved compared to the sample grown on an untreated COP substrate. The electrical resistivity of the AZO thin films deposited on the surface-modified COP substrates was decreased to 2.8 x 10^-4 Ωcm, approximately 30% lower than the films on the untreated substrates. Thus, physical and chemical pretreatments of COP substrate surfaces improved surface smoothness and electrical properties of AZO thin films even on the polymer substrates.<br/><br/>[1] G. Tan et al., Nanotech.,<b> 27</b>, 295603 (2016).<br/>[2] T. Oga et al., Jpn. J. Appl. Phys., <b>59</b>, 128001 (2020).