Research Article
Large-area (64 × 64 array) inkjet-printed high-performance metal oxide bilayer heterojunction thin film transistors and n-metal-oxide-semiconductor (NMOS) inverters

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Abstract

It is a big challenge to construct large-scale, high-resolution and high-performance inkjet-printed metal oxide thin film transistor (TFT) arrays with independent gates for the new printed displays. Here, a self-confined inkjet printing technology has been developed to construct large-area (64 × 64 array), high-resolution and high-performance metal oxide bilayer (In2O3/IGZO) heterojunction TFTs with independent bottom gates on transparent glass substrates. Inkjet printing In2O3 dot arrays with the diameters from 55 to 70 μm and the thickness of ∼10 nm were firstly deposited on UV/ozone treated AlOx dielectric layers, and then IGZO dots were selectively printed on the top of In2O3 dots by self-confined technology to form In2O3/IGZO heterojunction channels. When the inkjet-printed IO layers treated by UV/ozone for more than 30 min or oxygen plasma for 5 min prior to print IGZO thin films, the mobility of the resulting printed In2O3/IGZO heterojunction TFTs are correspondingly enhanced to be 18.80 and 28.44 cm2 V−1 s−1 with excellent on/off ratios (>108) and negligible hysteresis. Furthermore, the printed N-Metal-Oxide-Semiconductor (NMOS) inverter consisted of an In2O3/IGZO TFT and an IGZO TFT has been demonstrated, which show excellent performance with the voltage gain up to 112. The strategy demonstrated here can be considered as general approaches to realize a new generation of high-performance printed logic gates, circuits and display driving circuits.

Introduction

Metal oxide semiconductor thin film transistors (TFTs) have attracted considerable attention due to the high optical transparency, environmental stability, super low off currents and relatively high mobility compared to traditional amorphous silicon and organic TFTs, and have been regarded as one of the candidates for next-generation flat panel displays with high resolution, fast refreshing rate, large size [[1], [2], [3], [4], [5], [6]]. In order to meet the needs of display technology, many strategies have been adopted to improve the performance of metal oxide TFTs. In the past few years, it has been demonstrated that metal oxide TFTs based on metal oxide heterojunction channel layers show remarkable device performance [[7], [8], [9], [10], [11], [12]]. For example, Yang et al. [13] reported that the confined ITZO/IGZO heterojunction TFTs on Si/SiO2 exhibited a high mobility of 22.16 cm2 V−1 s−1 with high on/off ratios of 107. Sooji et al. [14] used ZnO/SnO2 bilayer heterostructure as active channels, and achieved high-performance metal oxide TFTs with the mobility as high as 15.4 cm2 V−1 s−1. Dongyoon [15] et al. deposited alternatively the layers of In2O3 and ZnO thin films to form the heterojunction channels, and obtained good-performance devices with the mobility up to 13 cm2 V−1 s−1 and excellent current on/off ratios (>108). However, the previous works focus on using spin coating technology to fabricate the metal oxide heterostructure channel TFTs with common gates on Si/SiO2 substrates, resulting in difficult for fabricating the driving circuits, and logic gates circuits. Recently, inkjet printing has emerged as an attractive technique to directly fabricate patterned active layer structures, which can save raw materials, as well as reduce the lithography process [16]. Compared to spin coating technology, inkjet printing is more suitable for manufacturing large-area, low-cost TFT devices with independent gates. Until now, few groups have reported the metal oxide heterojunction channel TFTs by inkjet printing. In 2015, Lee [17] et al. fabricated good-performance In2O3/ZTO heterojunction TFTs by inkjet printing, which showed a mobility of 8.6 cm2 V−1 s−1, a threshold voltage of 2.76 V, a subthreshold slope of 0.52 V dec−1, and on/off ratios of 106. However, these results were lower than expected since it is difficult to totally overlap between the two channel materials to form homogeneous heterojunction thin films by inkjet printing. Lately, Liang et al. [18] developed a self-confined technology to print well-aligned, common-gate In2O3/IGZO heterojunction channel TFTs with the maximum mobility of 14.3 cm2 V−1 s−1 on Si/SiO2 substrates. But the sizes of these printed In2O3/IGZO bilayer heterojunction thin films (the stripes with the length and width of more than 1000 and 110 μm) are too large to be suitable for constructing OLED display with high resolution. For example, the two transistors have to be made in a narrow area of ∼68 μm × 148 μm for a 55 inch OLED display with 4 K resolution when the an aperture ratio is 30 % [19]. Therefore, it is imperative to develop some novel approaches to scale down the sizes of printed heterojunction thin films and achieve large-area, high-performance heterojunction channel TFTs with independent gates to meet the need of the mass production of the large size displays [20].

In this work, we used the inkjet-printed heterojunction (In2O3/IGZO) dots as the channel materials by the self-confined technology, and could further scale down the diameters of heterojunction dots to 55 μm with the thickness of 20 nm by tuning the ink concentrations and the hydrophobic/hydrophilic properties of the dielectric layers. The independent-gate metal oxide heterojunction (In2O3/IGZO) TFT arrays (64 × 64) with the channel length of 30 μm and width of 50 μm were obtained on a glass substrate. The performance of heterojunction TFTs can be greatly improved when the printed In2O3 dots are treated by UV/ozone or oxygen plasma prior to deposition of IGZO dots. For example, the average mobility of printed In2O3/IGZO TFTs is 17.68 cm2 V−1 s−1 with on/off ratios of 108, the threshold voltage of −2 to 3 V and negligible hysteresis when In2O3 films are illuminated by UV/ozone for 30 min. Moreover, the printed heterojunction TFTs show superior bias stress stability under positive gate bias stress (PBS) and negative gate bias stress (NBS) after passivated by PMMA thin films. The N-Metal-Oxide-Semiconductor (NMOS) inverter consisting of a heterojunction TFT and an IGZO TFT exhibits excellent performance with the voltage gain up to 112, and can work well at frequency of 3 kHz.

Section snippets

Preparation of the oxide precursor solutions

The semiconductor inks based on the indium oxide (IO) and indium gallium zinc oxide (IGZO) precursor solution were prepared by dissolving In(NO3)3·xH2O (99.99 %, Sigma-Aldrich), Zn(NO3)2·xH2O (98 %, Acros Organics), and Ga(NO3)3·xH2O (99.99 %, trace metal basis, Sigma-Aldrich), into high purity deionized water (DI water). The as-received chemicals were used without any further purification. For indium gallium zinc oxide (IGZO), the molar ratio of the precursor solution was 3:1:2 (In: Ga: Zn) or

Printed metal oxide semiconductor heterojunction

Inkjet printing has been widely used to printed high-precision patterns for fabrication of electrical devices because of high throughput and low-cost [21,22]. The depositing of inkjet droplets with uniform morphology is critical to enhance the performance of the devices [23,24]. In the heterojunction channel TFTs, the high-mobility semiconductor is usually as the front channel layer to provide high-speed carrier transport, and the semiconductor with low carrier concentrations is used as the

Conclusion

In conclusion, we demonstrated a novel strategy to achieve large-scale, high-performance IO/IGZO TFT arrays (64 × 64) with independent gate electrodes on a glass substrate by inkjet printing. The printed IO/IGZO TFTs showed excellent electrical properties with the average effective mobility of 17.68 cm2 V−1 s−1, high on/off ratios (more than 108), low threshold voltage (−2−3 V), and free hysteresis after optimizing the fabrication process. Furthermore, the printed IO/IGZO heterojunction TFTs

Acknowledgements

This work was financially supported by the National Key R&D Program of “Strategic Advanced Electronic Materials” (No. 2016YFB04011100), the Basic Research Program of Jiangsu Province (Nos. BK20161263, SBK2017041510), the Science and Technology Program of Guangdong Province (Nos. 2016B090906002, 2019B010924002), the Basic Research Program of Suzhou Institute of Nanotech and Nano-bionics (No. Y5AAY21001), the National Natural Science Foundation of China (Nos. 61750110517, 61805166), the

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