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Stepped Annealed Inkjet-Printed Ingazno Thin-Film Transistors

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Stepped Annealed Inkjet-Printed Ingazno Thin-Film Transistors coatings Article Stepped Annealed Inkjet-Printed InGaZnO Thin-Film Transistors Xingzhen Yan * , Kai Shi, Xuefeng Chu, Fan Yang, Yaodan Chi and Xiaotian Yang * Jilin Provincial Key Laboratory of Architectural Electricity & Comprehensive Energy Saving, Jilin Jianzhu University, 5088 Xincheng Street, Changchun 130118, China * Correspondence: [email protected] (X.Y.); [email protected] (X.Y.); Tel.: +86-431-8456-6327 (Xin.Y.) Received: 24 August 2019; Accepted: 26 September 2019; Published: 27 September 2019 Abstract: The preparation of thin-film transistors (TFTs) using ink-jet printing technology can reduce the complexity and material wastage of traditional TFT fabrication technologies. We prepared channel inks suitable for printing with different molar ratios of their constituent elements. Through the spin-coated and etching method, two different types of TFTs designated as depletion and enhancement mode were obtained simply by controlling the molar ratios of the InGaZnO channel elements. To overcome the problem of patterned films being prone to fracture during high-temperature annealing, a stepped annealing method is proposed to remove organic molecules from the channel layer and to improve the properties of the patterned films. The different interfaces between the insulation layers, channel layers, and drain/source electrodes were processed by argon plasma. This was done to improve the printing accuracy of the patterned InGaZnO channel layers, drain, and source electrodes, as well as to optimize the printing thickness of channel layers, reduce the defect density, and, ultimately, enhance the electrical performance of printed TFT devices. Keywords: thin-film transistor; ink-jet printing; plasma treatment; annealing 1. Introduction In recent years, thin-film transistors (TFTs) have been widely used in large-scale functional integrated circuits owing to their small size, low power consumption, and good thermal stability [1–5]. This has triggered many studies into transistor materials that can exhibit a good electrical performance with regard to their mobility and on–off current modulation [6]. In recent years, the main direction of this research has been oriented toward new structural designs and channel semiconductor active materials for organic transistors [7–9] and metal oxide transistors [2,10–13]. Metal oxide transistors have some advantages regarding environmental stability and large operating voltage compared with organic transistors and have thus become a promising class of TFT active layer materials [14,15]. And metal oxide TFTs have been reported to be used in display [16], nonvolatile memory [17], and photodetector fields [18]. Among these oxide semiconductor active layers, ZnO-based thin-film transistors, in particular, such as ZnO [19–21], InZnO [22–24], ZnSnO [25–27], and InGaZnO [28–30], have become the focus of research. Of these, amorphous oxides based on heavy metal oxides with the electronic configuration (n 1)d10ns0 (n 4) are particularly promising for use as high-performance TFT channel − ≥ materials [31–33]. Recently, Zhang et al. reported nitrate-complex solution-processed metal oxide TFTs by doping both the In2O3 semiconductor and Al2O3 gate dielectric with boron [34]. For the structures of the channel, a corrugated heterojunction channel structure was proposed and developed by Lee et al. to achieve high mobility, low leakage current, and stable InSnZnO/InGaZnO TFTs [35]. In particular, studies into the mobility, stability, and transmittance of amorphous InGaZnO channel layers as proposed by Hosono and his co-worker group have led to InGaZnO TFTs becoming the Coatings 2019, 9, 619; doi:10.3390/coatings9100619 www.mdpi.com/journal/coatings CoatingsCoatings2019 2019, 9, ,9 619, x FOR PEER REVIEW 22 of of 11 11 focus of this type of research [11]. The saturation mobility and on/off ratio (Ion/Ioff) of the conventional 2 −1 −1 7 primaryannealed focus (600 of°C) this solution-processed type of research [InGaZnO11]. The saturationTFTs were mobilityreported andas 6.41 on/ ocmff ratio·V ·s (Ion and/Ioff 3.9) of × the 10 , 2 1 1 conventionalrespectively annealed[36]. The InGaZnO (600 ◦C) solution-processed TFTs by combustion InGaZnO method TFTs (300 were °C) could reported achieve as 6.41 mobility cm V −of s2.82− 2 −1 −1 7 3 · · andcm 3.9·V ·s10 and, respectively Ion/Ioff of 4.0 [×36 10]. The[37].InGaZnO The carrier TFTs concentration by combustion of InGaZnO method can (300 be◦ controlledC) could achieve at a low × 2 1 1 3 mobilitylevel, which of 2.82 is important cm V s forand TFTsIon /toI operate.of 4.0 Mo10reover,[37]. The Ga carrierserves concentrationas the suppressor of InGaZnO and stabilizer can · − · − off × beto controlledsuppress the at generation a low level, of whichoxygen is vacancies important and for decreases TFTs to the operate. free electron Moreover, concentration, Ga serves because as the suppressorGa–O bonds and are stabilizer much stronger to suppress than the In–O generation and Zn–O of oxygen bonds vacancies[38]. and decreases the free electron concentration,However, because current, Ga–O mature bonds preparation are much stronger processes than for In–O TFTs and rely Zn–O on bonds lithography [38]. to obtain patternedHowever, thin current, films [39–41], mature but preparation this requires processes many forcomplicated TFTs rely onpreparation lithography steps, to obtain such as patterned repeated thinexposure, films [ 39development,–41], but this and requires etching. many By complicated contrast, the preparation introduction steps, of inkjet such printing as repeated technology exposure, to development,prepare devices and etching.has the Byadvantage contrast, of the creating introduction a simple of inkjet process printing with technology less material to prepare waste devices that is hasenvironmentally the advantage offriendly creating and a simple enables process easy, large-scal with less materiale production waste [42–46]. that is environmentally The first report of friendly inkjet- andprinted enables metal easy, oxide large-scale TFTs was production given by Chang [42–46 et]. al. The [47]. first However, report of there inkjet-printed are some issues metal to oxide overcome, TFTs wassuch given as the by poor Chang precision et al. [47 ].in However,patterning there these are thin some films, issues the to annealing overcome, of such the aspatterned the poor films, precision and ininterface patterning defects these caused thin films, by thethe annealingadopted printing of the patterned method. films, In this and study, interface we, defects therefore, caused present by the a adoptedstepped printingannealing method. process In that this can study, form we, a therefore,complete presentand undamaged a stepped patterned annealing InGaZnO process that channel can formlayer a completeeven after and annealing undamaged treatment patterned at 550 InGaZnO °C. We channel also took layer advantage even after of annealing a solution treatment method atto 550regulate◦C. We the also type took of advantageTFT device ofcreated a solution by changing method the to regulate molar ratios the type of the of elements TFT device in createdthe channel by changingink via the the spin-coated molar ratios and of the etching elements method. in the And channel we inkinvestigated via the spin-coated the use of andplasma etching treatment method. to Andimprove we investigated the printing theaccuracy use of of plasma the channel treatment layer toand improve to reduce the the printing interface accuracy defect density of the channelbetween layerthe drain/source and to reduce electrodes the interface and defect the patterned density between channel the layer drain to /achievesource electrodes a reduction and of the the patterned threshold channelvoltage layer(VT). to achieve a reduction of the threshold voltage (VT). 2.2. Materials Materials and and Methods Methods 2.1.2.1. Preparation Preparation of of Composite Composite Films Films TheThe preparation preparation process process of of printed printed InGaZnO InGaZnO TFTs TFTs is is shown shown in in a a schematic schematic diagram diagram (Figure(Figure1 ).1). First, we used a Si wafer with a SiO thickness of 285 nm from HEFEI KEJING Materials Tech Co., First, we used a Si wafer with a SiO2 2 thickness of 285 nm from HEFEI KEJING Materials Tech Co., Ltd.Ltd. (Hefei, (Hefei, China) China) as as the the bottom bottom gate gate and and insulation insulation layer layer for for the the TFT TFT devices. devices. Additionally,Additionally, colloidal colloidal precursors were prepared from indium nitrate [In(NO ) ], gallium nitrate [Ga(NO ) ], and zinc acetate precursors were prepared from indium nitrate [In(NO3 33)3], gallium nitrate [Ga(NO3 3)3], and zinc acetate [Zn(CH COO) ] dissolved in 5 mL of methyl glycol to achieve InGaZnO solutions with indium, [Zn(CH3 3COO)22] dissolved in 5 mL of methyl glycol to achieve InGaZnO solutions with indium, gallium,gallium, and and zinc zinc molar molar ratios ratios of of 6:1:3, 6:1:3, 5:1:4, 5:1:4, 3:1:6, 3:1:6, and and 2:1:7. 2:1:7. These These mixtures mixtures were were each each separately separately injectedinjected into into a flaska flask heated heated by an by oil bathan oil (150 bath◦C) with(150 magnetic°C) withstirring. magnetic Then stirring. 1.2 mL Then of ethanolamine 1.2 mL of andethanolamine 300 µL of and glacial 300 acetic µL of acidglacial were acetic successively acid were successively injected into injected the precursor into the solutionprecursor to solution act as stabilizers.to act as stabilizers. The reaction The reaction mixtures mixtures were further were refluxedfurther refluxed at 150 ◦ Cat for150 1°C h for and 1 thenh and allowed then allowed to cool to tocool room to room temperature temperature and aged and foraged 24 for h. The24 h. silver The silver nanoparticle nanoparticle printing printing inks thatinks werethat were used used as the as rawthe materialsraw materials for the for drain the anddrain source and electrodessource electr wereodes purchased were purchased from SIJ from Technology, SIJ Technology, Inc.
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