Amorphous oxide semiconductor thin‐film transistors (AOS TFTs) are ever‐increasingly utilized in displays. However, to bring high mobility and excellent stability together is a daunting challenge. Here, the carrier transport/relaxation bilayer stacked AOS TFTs are investigated to solve the mobility‐stability conflict. The charge transport layer (CTL) is made of amorphous In‐rich InSnZnO, which favors big average effective coordination number for all cations and more edge‐shared structures for better charge transport. Praseodymium‐doped InSnZnO is used as the charge relaxation layer (CRL), which substantially shortens the photoelectron lifetime as revealed by femtosecond transient absorption spectroscopy. The CTL and CRL with the thickness suitable for industrial production respectively afford minute potential barrier fluctuation for charge transport and fast relaxation for photo‐generated carriers, resulting in transistors with an ultrahigh mobility (75.5 cm2 V−1 s−1) and small negative‐bias‐illumination‐stress/positive‐bias‐temperature‐stress voltage shifts (−1.64/0.76 V). The design concept provides a promising route to address the mobility‐stability conflict for high‐end displays.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202300373

Addressing the Conflict between Mobility and Stability in Oxide Thin‐film Transistors

The presence of low‐frequency noise (LFN) in amorphous oxide semiconductor (AOS) thin‐film transistors (TFTs) is of utmost concern, prompting extensive investigations into the analysis of LFN. However, prior research endeavors have tended to overlook the significance of the sub‐gap density of states (DOS) in the LFN analysis, resulting in an incomplete comprehension. To bridge this knowledge gap, the influence of sub‐gap DOS is demonstrated on LFN in Si‐doped ZnSnO (SZTO) thin‐film transistors (TFTs) under various conditions. The SZTO TFTs is intentionally subjected to positive bias stress and hot carrier stress in order to control the sub‐gap DOS and investigate how this change affects the LFN characteristics. It is revealed that the non‐uniform energetic distribution of sub‐gap DOS induces bias‐dependent excess noise in the SZTO TFTs. Additionally, self‐recovery behavior after the HCS is observed, accompanied by a commensurate reduction in 1/f noise. These empirical observations provide evidence that the conventional correlated mobility fluctuation model used to explain LFN in AOS TFTs is insufficient and underscores the critical importance of considering subgap DOS when analyzing LFN of AOS TFTs.

Unveiled Influence of Sub‐gap Density of States on Low‐Frequency Noise in Si‐Doped ZnSnO TFTs: Does Correlated Mobility Fluctuation Model Suffice?

Herein, indium-doped tin oxide (TIO) thin films were prepared using the solution process. The structural characteristics, surface morphologies, and chemical compositions of the prepared films were systematically investigated as a function of indium-doping contents. The films were integrated as a channel layer into thin-film transistors (TFTs). Results show that indium can be used as a carrier suppressor as well as a mobility enhancer for SnO2. The optimal devices exhibited satisfactory performance, achieving field-effect mobility of 8.94 cm2/V·s, positive threshold voltage of 0.25 V, small subthreshold swing of 0.4 V/decade, and on-to-off current ratio of 1.5 × 106. The results of this study suggest that TIO-TFTs are very promising for use in next-generation displays. 

Mobility enhancement of tin oxide thin-film transistor by indium-doping

Amorphous InGaZnO (IGZO) films with high visible transmittance and wide bandgaps are good candidates to fabricate thin film transistors’ (TFTs) switching for flat-panel displays. The traditional sputtering and liquid solution deposition processes make it difficult to prepare ultrathin IGZO films with large area and low cost. Herein, we report a one-step printing process to successfully obtain centimeter scale, ultrathin, and low cost IGZO films by printing liquid Ga–In–Zn alloys. The printed IGZO films have near 100% transmittance of visible light and bandgaps above 4.17 eV. The printed IGZO films were further used to fabricate TFTs, which show n-type switching and display a good stability under visible light illumination. The fabricated TFTs show an on/off ratio above 103, a maximum field effect mobility of 14.25 cm2 V–1 s–1 and a minimum subthreshold swing of 0.46 V dec–1. With the Ga2O3 concentration in printed IGZO films increasing, the on/off ratios of TFTs increase, while the field effect mobility of TFTs decreases. This printing strategy provides a facile route to obtain IGZO nano films, which have great potentiality in fabricating high performance TFT switching for flat-panel displays. 

Large Area, Ultrathin, and Transparent InGaZnO Films from Printing of Liquid Ga–In–Zn Alloys for Thin Film Transistors

Abstract Amorphous oxide semiconductors, especially indium gallium zinc oxide (IGZO), have been widely studied and obtained significant progress in flexible thin-film transistors (TFTs) due to the high carrier mobility and low deposition temperature. However, a further annealing step is generally required to activate electrical properties and improve the device performance, which limited their applications in flexible electronics. In this study, we achieved flexible TFTs and arrays using co-sputtered IGZO and indium tin oxide (ITO) as channels deposited at room temperature without post-annealing. It was found that better transistor switching properties could be effectively achieved by regulating the sputtering power of ITO in the co-sputtered deposition. The device performance is comparable to that of the conventional oxide TFTs with high annealing temperatures (⩾300 °C), exhibiting a high saturation mobility ( μ sat ) of 15.3 cm 2 V −1 s −1 , a small subthreshold swing ( SS ) of 0.21 V dec −1 , and a very high on–off ratio ( I on/off ) of 10 11 . In addition, a 12 × 12 flexible TFT array was achieved with uniform performance owing to the low-temperature processing advantage of this technique. The flexible TFTs exhibited robust mechanical flexibility with a minimum bending radius of 5 mm and bending cycles up to 1000. Furthermore, an inverter based on co-sputtered IGZO and ITO was demonstrated with the maximum gain of 22. All these achievements based on the proposed TFTs without post-annealing process are expected to promote the applications in advanced flexible displays and large-area integrated circuits. 

Room-temperature fabrication of flexible oxide TFTs by co-sputtering of IGZO and ITO

In this work, the source/drain (S/D) electrode process is considered to have a great impact on the mobility (μFE)–threshold (Vth) trade-off for high-mobility amorphous oxide semiconductor (AOS) thin-film transistors. High-mobility AOS materials have worse oxygen-fixation ability compared to InGaZnO. In this work, we show that this is the case for the Mo S/D electrode process on InSnZnO channels and has a great impact on the mobility-threshold trade-off. Oxygen vacancy gradients are observed in the depth profile X-ray photoelectron spectroscopy analysis on InSnZnO after Mo S/D deposition and show a bigger change in oxygen vacancies (VO) between the surface and inner layers and thicker compared to InGaZnO. This leads to an obvious negative shift in Vth or even no switching characteristic. Postprocessing such as oxygen-plasma treatment can make Vth positive but degrade the μFE. In situ replenishing oxygen via adopting ITO S/D electrodes deposited in an oxygen-containing atmosphere facilitates the achievement of excellent overall electrical characteristics, with μFE up to 87.1 cm2/Vs, Vth ∼ −0.64 V, and Ion/Ioff ∼ 2.3 × 108. 

Impact of the Source/Drain Electrode Process on the Mobility-Threshold Trade-Off for InSnZnO Thin-Film Transistors

In this article, this research demonstrates the influence of in-situ introduction of H2 into the working gas on the physical properties of post-annealed In2O3 thin films and the performance of associated devices. A gradual increase in the H2 ratio leads to improved film quality, as indicated by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscope analyses showing a reduction in defect states such as band-tail states and VO in the film, and a smoother surface morphology with the root mean square roughness approximately 0.446 nm. Furthermore, this hydrogen doping effect results in a distinct shift in the device’s threshold voltage toward the positive direction, and an improvement in the field-effect mobility and subthreshold swing. Consequently, a high-performance In2O3:H TFT is developed, exhibiting a field-effect mobility of 47.8 cm2/Vs, threshold voltage of −4.1 V and subthreshold swing of 0.25 V/dec. These findings highlight the potential of in-situ H doping as a promising approach to regulate In2O3-based TFTs.

Effect of In-Situ H Doping on the Electrical Properties of In2O3 Thin-Film Transistors

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Addressing the Conflict between Mobility and Stability in Oxide Thin‐film Transistors

Impact of the Source/Drain Electrode Process on the Mobility-Threshold Trade-Off for InSnZnO Thin-Film Transistors

Effect of In-Situ H Doping on the Electrical Properties of In2O3 Thin-Film Transistors