Thermopower modulation analyses of effective channel thickness for Zn-incorporated In₂O₃-based thin-film transistors

Abstract

Zn-incorporated In2O3 (IZO) thin-film transistors (TFTs) show high field-effect mobility (µFE ~40 cm2 V−1 s−1) when the IZO channel is amorphous, whereas a lower µFE (~10 cm2 V−1 s−1) is observed with polycrystalline channel. The reasons behind this difference in µFE remain unclear despite various studies on IZO TFTs. Here, we perform electric field thermopower modulation analysis on amorphous and polycrystalline IZO TFTs to measure the change in effective thickness. For amorphous channel, the effective thickness was zero when the effective gate voltage (Vg−Vth, Vg: gate voltage, Vth: threshold voltage) was zero. Furthermore, it gradually increased with Vg−Vth up to 1.6 nm, reflecting that conduction band bending occurred. By contrast, polycrystalline channel showed an initial effective thickness of 5 nm (≈ film thickness of IZO) and sharply decreased to ~1.7 nm. In amorphous channels, electron transport followed field effect theory, while factors like grain boundaries limited transport in polycrystalline channels.