Localized Trap-State Engineering of Subthreshold Swing in Amorphous In–Ga–O Thin-Film Transistors via Vertically Positioned Sn δ-Doping

Abstract Precise control of the subthreshold swing (SS) in pixel-driving thin-film transistors (TFTs) is critical for accurate gray-scale expression and stable analog current modulation in active-matrix displays. Here, we demonstrate deterministic SS engineering in amorphous indium–gallium oxide (a-IGO) TFTs by controlling the vertical position of a tin (Sn) δ-doped layer within the oxide channel using plasma-enhanced atomic layer deposition. Sn δ-doped layers were selectively inserted near the passivation side, at the middle of the channel, and near the gate-insulator interface, while the macroscopic structural and chemical properties of the channel films remained nearly identical. Despite this structural similarity, the electrical characteristics were systematically modulated solely by the spatial location of the Sn layer. As the Sn δ-doped layer approached the front-channel conduction path, the SS increased from 0.128 to 0.462 V/decade, accompanied by a positive threshold-voltage shift from 0.03 to 0.99 V and a decrease in field-effect mobility from 36.5 to 19.3 cm2/V·s. Energy-band analyses indicate that these position-dependent electrical variations originate from the different degrees of interaction between Sn2+-induced trap centers and the bending quasi-Fermi level during subthreshold operation. This spatial trap-interaction model was further supported by frequency-dependent capacitance–voltage measurements, which revealed more pronounced frequency dispersion and stretch-out behavior when the Sn δ-doped layer was located closer to the front channel. Importantly, the Sn δ-doped TFTs exhibited bias-stress reliability comparable to that of the pristine IGO reference, with threshold-voltage shifts no greater than 0.60 V under the tested conditions. The capture and emission kinetics of the states associated with the Sn2+-related SS modulation remain to be established. These results establish vertically positioned Sn δ-doping as an effective and reliable strategy for tailoring the SS of amorphous oxide TFTs, providing a useful device-design pathway for OLED pixel-driving circuits that require precise current modulation and improved low-gray-level uniformity.