Self-Aligned\nIonic Doping of TiO<sub>2</sub> Thin-Film Transistors for Enhanced\nCurrent Drivability via Postfabrication Superacid Treatment

Oxide semiconductor thin-film transistors\n(TFTs) have shown great potential in emerging applications such as\nflexible displays, radio-frequency identification tags, sensors, and\nback-end-of-line compatible transistors for monolithic 3D integration\nbeyond their well-established flat-plane display technology. To meet\nthe requirements of these appealing applications, high current drivability\nis essential, necessitating exploration in materials science and device\nengineering. In this work, we report for the first time on a simple\nsolution-based superacid (SA) treatment to enhance the current drivability\nof top-gate TiO<sub>2</sub> TFTs with a gate-offset structure. The\non-current of these transistors is limited by the relatively low mobility\nof TiO<sub>2</sub> due to its d-orbital conduction nature. It is found\nthat the on-current of TiO<sub>2</sub> TFTs is nearly doubled via\na quick dip in a SA solution at room temperature in ambient air. A\nseries of experiments, including comparative <i>I</i>–<i>V</i> measurements of TFTs with different treatments and gate\nstructures, <i>C</i>–<i>V</i> measurements,\nX-ray photoelectron spectroscopy, time-of-flight secondary ion mass\nspectrometry, and device simulation, were performed to uncover the\nunderlying reason for the current enhancement. It is believed that\nthe protons (H<sup>+</sup>) from SA are doped into the offset region\nof TiO<sub>2</sub> TFTs, forming an electron double layer and thus\nboosting the on-current, with the top gate serving as a self-aligned\nmask for ionic doping. Furthermore, the ionic size and the proportion\nof the offset region to the channel play crucial roles in the effectiveness\nof ionic doping, while the position of the incorporated ions, whether\nin the channel or dielectric, may result in distinct shifts in the\nturn-on voltage (<i>V</i><sub>ON</sub>) and affect the functionality\nof ionic doping. This study provides a pathway for enhancing the current\ndrivability of TiO<sub>2</sub> TFTs via selective ionic doping enabled\nby SA treatment and deepens our understanding of ion incorporation\nin electronic devices. This approach could be applicable to other\nmaterial systems and may also benefit TFTs with miniaturized dimensions,\nthus opening up unprecedented opportunities for TiO<sub>2</sub> TFTs\nin future applications requiring high current drivability.