Flexible High-Temperature Polycrystalline Silicon Thin Film Transistor on Metal Foil With S/D Doped by Diffusion

This article presents results on flexible high-temperature polycrystalline silicon (FHTPS) thin-film transistor (TFT) fabricated on metal foils. Metal foils eliminate processing temperature constraints which are rather stringent on flexible TFT devices fabricated on plastics and allow high-temperature post-crystallization annealing (PCA). The source/drain (S/D) area of the devices was doped by diffusion with a spin-coated solid source at high temperatures and this process also functioned as PCA to improve the quality of the active undoped area. Solid-phase crystallized TFTs with diffused S/D at 800 °C demonstrate saturation mobility ( $\mu _{SAT}$ ) of 103 cm2/ $\text{V}\cdot \text{s}$ , an ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio of close to 106, a subthreshold swing (SS) of 498 mV/dec, and a threshold voltage ( ${V}_{TH}$ ) of 4.9 V. Technology computer-aided design (TCAD) simulations reveal that parasitic resistance accounts for most of the performance variations of devices diffused at different temperatures when the devices are turned on, and defect density is dominant in the subthreshold and the off regimes. The performance of the fabricated devices suffered only slight changes after 10 000 cycles of bending with 7 mm radius showing their mechanical robustness.