Flexible transparent ZnO thin-film transistors by pulsed laser deposition

Full text: Flexible and transparent electronic devices such as a n electric circuit and a display have been attracting much attention recent years. In these electronic devices, thin film transistors (TFTs) are very important part for controlling the flow of electric current. Amorphous oxide semiconductors are focused as a material to a development of a transparent and flexible devices. Zinc-oxide (ZnO) based devices have been reported by our groups so far because it is a transparent electrode material of low cost and having extremely low toxicity. We reported fully transparent ZnO-TFTs by using Al-doped ZnO (AZO) low resistivity electrodes on glass substrates and a transconductance (𝘨_m) of 150 𝜇S/mm and ON/OFF ratio of 6.6×10⁶ were obtained. In this work, we report the fabrication and characterization of transparent ZnO-TFTs on flexible plastic substrates by pulsed laser deposition (PLD). A schematic diagram of a fabricated ZnO-TFT. SiO₂ thin-films as a buffer layer between ZnO-TFTs and polyethylene naphthalate (PEN) substrates were grown by electron beam physical vapor deposition. 40 nm thick ZnO channel layers and 100 nm thick HfO₂ gate insulators, 100 nm thick AZO electrodes of ZnO-TFTs were grown by PLD. A Nd:YAG laser (4^th harmonic, 266 nm) was used for ablation of the oxide material ceramic at room temperature. All fabrication process were under 100 °C. Figure 2 shows the photomicrograph of fabricated ZnO-TFTs and the devices are hard to identify visually by eyes. We characterized the fabricated transparent ZnO-TFTs. Figure 3 shows the typical output characteristics of a 3 𝜇m length gate (𝐿_𝐺) device. The device had a 𝘨_m of 252 𝜇S/mm and an ON/OFF ratio of 5.6×10⁵. From these results, we succeeded in fabricating the fully transparent ZnO-TFTs on PEN substrates and the characteristics can be comparable with the ZnO-TFTs on glass substrates. The ZnO-TFTs were also characterized in bending states. Figure 4 shows the characteristics of a bending ZnO-TFT. We started the measurement with the flat state, then the radius of a curvature is changed to 5 mm, and finally returned to the flat state again. The gm and ON/OFF ratio decreased after bending, however the 𝘨_m recovered after returning the state. (author)