High transconductance zinc oxide thin-film transistors on flexible plastic substrates

We report the fabrication and characterization on high-performance ZnO based TFTs on unheated plastic substrate. ZnO films were grown by pulsed laser deposition (PLD) on polyethylene napthalate (PEN) substrates. Top-gate ZnO-TFTs were fabricated by photolithography and wet chemical etching. The source and drain contacts were formed by lift-off of e-beam deposited Ti(20 nm)/Au(200 nm). An HfO$_{2}$ with thickness 100 nm was selected as the gate insulator, and top gate electrode Ti(20 nm)/Au(200 nm) was deposited by e-beam evaporation. We prepared a set of the structure with SiO$_{2}$/TiO$_{2}$ to investigate the characteristic changes that appear in the film characteristics in response to bending. From the $I_{D}-V_{DS}$ and the transfer characteristics which are affected by bending and return for the ZnO-TFT with SiO$_{2}$/TiO$_{2}$ buffers, the TFTs were bent to a curvature radius of 8.5 mm. The transconductance, $g_{m}$ is obtained 1.7 mS/mm on flat, 1.4 mS/mm on bending and 1.3 mS/mm on returning the film, respectively. The $I_{D}-V_{DS}$ characteristics were therefore not changed by bending. All of the devices exhibited a clear pinch-off behavior and a high on/off current ratio of $\sim $10$^{6}$. The threshold voltages, $V_{th}$ were not changed drastically. Furthermore, TFT structures were changed from a conventional top-gate type to a bottom-gate type. A high transconductance of 95.8 mS/mm was achieved in the bottom-gate type TFT by using Al$_{2}$O$_{3}$ oxide buffer.