Fabrication of Highly-Oriented Wide-Bandgap Oxide Thin Films on the Surface-Modified Polymer Substrates by Room-Temperature UV Laser/Light Processes

The combination of flexible transparent polymer substrates and wide-bandgap oxide semiconductors with superior electrical and/or magnetic properties is highly expected to be applied to the wearable power-generation or sensor devices. Most of the optoelectronics devices using the highly crystal oriented oxide semiconductors such as Ga₂O₃ and ZnO thin films are fabricated on glasses or inorganic single crystal substrates at high temperatures. On the other hand, thermoplastic polymers such as cyclic-olefin-polymers (COPs) used in this study have extremely low hygroscopicity, high transparency and relatively high heatproof temperatures (about 150°C), so they are the promising polymer substrates for fabrication of the wide-bandgap material devices used in various human friendly situations. For growth of the high-quality oxide thin films for devices onto the polymer substrates with irregularly roughened surfaces, we had better employ thin film techniques to control the nucleation and growth direction of oxides at low temperature as well as nanoscale-surface modification of the polymers.<br/>So far, we reported oriented crystal growth of wide-bandgap oxide thin films deposited on the physically and/or chemically surface-modified polymer substrates to be greatly improved.[1] Development of the advanced low-temperature growth techniques of high-quality wide-bandgap oxide thin films on the flexible polymers would open the new field in the application of oxide-polymer hybrid flexible and wearable devices. In this study, we investigated the oriented crystal growth of ZnO and Ga₂O₃thin films on the surface-modified COP polymer substrates by the room-temperature process using UV excimer laser or VUV excimer lamp irradiation.<br/>In the experiment, thermoplastic COP (ZF16-188, ZEON; <i>T</i>_g~163°C, t~188 μm) sheets were used as polymer substrates. Two kinds of surface modifications of COP substrates were carried out as follows: (1) The COP substrates were irradiated by excimer lamp equipped with vacuum ultraviolet light (λ = 172 nm) in air at room temperature. (2) buffer layer formation on the substrates by pulsed laser deposition (PLD) of an amorphous oxide thin film such as aluminum oxide. Then, ZnO semiconductor thin films were grown at room-temperature by PLD with a KrF excimer laser (λ=248 nm, d~20 ns) under O₂ gas of 10^–3 Pa. The amorphous gallium oxide thin films were also subsequently deposited by PLD, and crystal growth was introduced by irradiation of a non-condensing excimer laser from thin film side in air.<br/>As a result, it was found that the present ZnO thin film on pretreated had a highly oriented c-axis growth and crystallinity compared to that grown on the untreated COP substrate from XRD and RHEED measurements. Furthermore, the electric conductivity was also improved. Furthermore, the (-201) oriented growth of the Ga₂O₃ thin films on the highly oriented ZnO thin film were induced by excimer laser irradiation, although the Ga₂O₃ thin films grown on the untreated COP substrates were randomly direction grown without orientation.<br/>Therefore, surface modification of the polymer substrate was considered to enhance homogeneous nucleation and crystal orientation in the initial stages of oxide thin film growth, leading to the development of oriented crystal growth on polymer substrates at room temperature.<br/>[1] T. Oga et al., Jpn. J. Appl. Phys., <b>59</b>, 128001 (2020).