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. (Ibaraki, Inc. Japan).(Ibaraki, In Japan). addition, In we addition, adopted we a TFTadopted structure a TFT with structure a bottom with gate a bottom and top gate contact and (draintop contact and source (drain electrodes)and source as electrodes) this structure as this is suitable structure for is ink-jet suitable printing. for ink-jet printing.

FigureFigure 1. 1.Schematic Schematic diagram diagram of of the the procedures procedures for for fabricating fabricating a a printed printed InGaZnO InGaZnO TFT. TFT. The The nozzle nozzle waswas ﬁlled filled with with the the InGaZnO InGaZnO solution, solution, forming forming a a patterned patterned layerlayer withwith aa thicknessthickness ofof 3030 toto 120120 nm.nm. Then, after annealing, the nozzle containing the solution of silver nanoparticles formed the 60 to 80 nm Then, after annealing, the nozzle containing the solution of silver nanoparticles formed the 60 to 80 source and drain electrodes on the semiconductor layers. nm source and drain electrodes on the semiconductor layers.

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Next, The InGaZnO solution was filtered through a percolating filter with a diameter of 0.2 µm and then inhaled by a 10 µm Sonoplot (Middleton, WI, USA) nozzle with capillary force for 5 s. The nozzle was then placed on the substrate surface. The width of the lines formed on the Si/SiO2 substrate was about 40 µm, so the spacing of the printed lines in the program needs to be set to 40 µm. The thickness of one printed layer is about 30 nm in this way. Subsequently, the printed InGaZnO channel layer needs to be cured at 100 ◦C for 10 min and annealed to remove any organic impurities present. We adopted the method of stepped annealing in an air atmosphere with a heating rate of 5 ◦C/min. The preannealing temperature of the patterned film was set to 120 ◦C for an annealing time of 1 h, with a transition annealing step at 450 ◦C for 1 h, and finally annealing for 1 h at 550 ◦C. The drain and source electrodes were prepared on the InGaZnO channel layer with a 210 V printing voltage, 2 mm/s printing speed, 40 µm distance between the tip and the channel layers. The width of printed lines was 10 µm, and the thickness of resulting electrodes was 60 to 80 nm with a 3 µm nozzle. The printed electrodes were then annealed at 200 ◦C for 30 min to remove the organics that had coated the silver nanoparticles.

2.2. Characterization The microstructures of patterned channel layers were observed using an optical microscope (Carl Zeiss, AxioScope A1, Oberkochen, Germany). The contact angle between the printing ink and substrates was acquired using a contact angle tester (KRüSS, DSA25, Hamburg, Germany). Output and transfer curves of TFTs were measured by a semiconductor parametric instrument (Keysight, B1500A, Santa Rosa, CA, USA). Patterned channel layers and drain/source electrodes were, respectively, prepared by inkjet printing instruments (Sonoplot-Microplotter II and SIJ-S050).

3. Results and Discussion Different molar ratios of the elements of the InGaZnO in the channel layers were spun onto the Si/SiO2 substrates and then annealed at 550 ◦C (InGaZnO layers with different molar ratios were about 40 5 nm thickness and 0.8 0.2 nm roughness), lithographed, and then finally, metal aluminum ± ± was evaporated to create the drain and source electrodes on the channel layers. Figure2a–d shows the drain current versus the drain–source voltage (ISD–VSD) output characteristics of the InGaZnO TFTs. The curves show typical n-type transistor performance with a clear transition from linear to saturation behavior with the different element molar ratios of the InGaZnO channel layers. It can be found that the current in the saturation region decreases with the decrease of indium mole ratio. The reason is that the increase of indium in the InGaZnO channel layers can lead to a large number of oxygen vacancies and interstitial indium, which can reduce the resistivity of the films. So the carrier concentration of InGaZnO with a ratio of 6:1:3 cannot be controlled at a low level, which is not good for the TFT to control the current. For other TFTs with different element molar ratios, the increase of VSD produced a reverse inhibition current and caused the ISD to decrease in the saturation region. Figure2e exhibits the typical transfer curves ISD–VG at VSD = 30 V; it can be seen that the Ion/Ioff was significantly enhanced along with a gradual decrease of the molar proportion of indium because a smaller percentage of Indium in InGaZnO resulted in a decrease of the off current [38]. Although the increase of the proportion of indium enhances the mobility of the channel layer, the Ioff of the TFT still retains a relatively high value in the off state, which will affect the stability and the ability to resist noise 1/2 signal interference of the TFT devices. Figure2f plots ( ISD) versus VG at VSD = 30 V; the saturation mobility of 0.172, 0.037, 0.035, and 0.027 cm2 V 1 s 1 was derived from a linear fit to the plot of the · − · − square root of ISD versus VG with ratios of 6:1:3, 5:1:4, 3:1:6, and 2:1:7. The value of VT is estimated by extrapolating the linear portion of the curve. It was found that the VT of InGaZnO channel layer TFTs differs for different element molar ratios. In addition, the VT of the TFTs with element molar ratios of 6:1:3 and 5:1:4 was negative (V of 3.8 and 2.8 V, respectively), while the V of the transistors T − − T with ratios of 3:1:6 and 2:1:7 was positive (VT of 8.6 and 12 V, respectively). Appreciable drain current flows at a gate voltage of 0 V, as can be seen in Figure2a,b. Thus, depletion mode and enhancement Coatings 2019, 9, 619 4 of 11 mode TFTs can be obtained simply through controlling the molar ratios of the channel elements. There Coatings 2019, 9, x FOR PEER REVIEW 4 of 11 into the gate leakage remained on the magnitude of nanoampere for the TFTs with different element molardifferent ratios, element which molar will not ratios, affect which the test will results. not affect To obtain the test TFTs results. with better To obtain electrical TFTs performances, with better theelectrical elemental performances, molar ratio the of elemental 2:1:7 for In:Ga:Znmolar ratio in of the 2:1:7 InGaZnO for In:Ga:Zn channel in the layers InGaZnO was selected channel for layers the 4 following printing and discussions owing to the higher Ion/I value (~3.1 10 ). 4 was selected for the following printing and discussions owingoff to the higher× Ion/Ioff value (~3.1 × 10 ).

FigureFigure 2.2. (a,d) OutputOutput characteristicscharacteristics of InGaZnO thin-ﬁlmthin-film transistors (TFTs) withwith didifferentﬀerent molarmolar ratiosratios ofof thethe elements,elements, namelynamely In:Ga:ZnIn:Ga:Zn ratiosratios ofof 6:1:36:1:3 ((aa),), 5:1:45:1:4 ((bb),), 3:1:63:1:6 ((cc),), andand 2:1:72:1:7 ((d).). ( e,f) Transfer characteristicscharacteristics ofof thethe InGaZnOInGaZnO TFTs.TFTs. TheThe channelchannel length,length, width, and thicknessthickness of the above TFTs areare aboutabout 6060 µµm,m, 300300 µµm,m, andand 4040 nm,nm, respectively.respectively.

ToTo fabricatefabricate TFTTFT devicesdevices viavia printing,printing, SiSi/SiO/SiO22 substrates were treated withwith argon plasma cleaning toto remove surface surface impurities impurities and and adsorbed adsorbed function functionalal groups, groups, and and to toimprove improve the the wetting wetting of the of theink inkonto onto the thesubstrate. substrate. Wetting Wetting properties properties played played an an important important role role in in the the final final pattern pattern of of the solutesolute depositdeposit [[48].48]. As shown in Figure3 3a,a, aa microscopemicroscope imageimage ofof the the contact contact angle angle between between the the InGaZnO InGaZnO printingprinting inkink andand thethe SiSi/SiO/SiO22 substrate without argon plasmaplasma treatmenttreatment showsshows aa largelarge angleangle ofof ~38.8~38.8°,◦, whichwhich meansmeans thatthat thethe wettingwetting ofof thethe inkink waswas notnot goodgood enoughenough to form a continuouscontinuous patterned line. MicroscopeMicroscope images of the contactcontact angle after an argon plasmaplasma treatment for either 10 or 3030 minmin areare shownshown inin FigureFigure3 3b,c,b,c, respectively. respectively. In In Figure Figure3 b,3b, it it can can be be observed observed that that the the contact contact angle angle between between the the InGaZnOInGaZnO inkink and and the the Si/ SiOSi/SiO2 substrate2 substrate is significantly is significantly reduced reduced (contact (contact angle of angle ~14.3 ◦of), which~14.3°), indicates which thatindicates the wetting that the of thewetting ink onto of the the ink substrate onto the has substrate been improved. has been However, improved. when However, the duration when of thethe plasmaduration process of the isplasma increased, process the contactis increased, angle the between contact the angle ink and betw substrateeen the ink does and not substrate obviously does change not (Figureobviously3c). change Thus, by(Figure argon 3c). plasma Thus, treating by argon the Siplasma/SiO2 treatingsubstrates the for Si/SiO 10 min,2 substrates patterned for InGaZnO 10 min, channelpatterned layers InGaZnO of diff channelerent sizes layers can beof different obtained sizes based can on be previous obtained designs, based ason shown previous in Figure designs,3d–f. as Forshown printing, in Figure we chose3d–f. For the printing, size of the we InGaZnO chose the channel size of the layer InGaZnO that is shown channel in layer Figure that3d; is using shown this in patternFigure 3d; we using could this increase pattern the we area could through increase which th carrierse area through from the which electrodes carriers were from injected the electrodes into the activewere injected layer with into a certainthe active size layer of the with channel a certain layer size and of width the channel/length ratiolayer of and the width/length channel. ratio of the channel.

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Figure 3.3. ContactContact angle angle between between the the ink andink theand substrate the substrate without without (a) and ( witha) and argon with plasma argon treatment plasma fortreatment 10 min for (b) 10 and min 30 min(b) and (c). 30 (d –minf) Microscope (c). (d–f) Microscope images of di imagesﬀerent of sized different printed sized channel printed patterns channel on thepatterns substrate on the treated substrate with treated plasma wi treatmentth plasma for treatment 10 min. for 10 min.

Next, the printed patterned InGaZnO channel layerslayers needed to be annealed to remove organic impurities. The The solution-processed solution-processed channel channel layers layers required required high-temperature high-temperature annealing annealing at 400 at 400to 600 to 600°C for◦C metal for metal oxide oxide TFT TFT devices devices [49,50]. [49,50 However,]. However, after after an an hour hour of of conventional conventional annealing annealing at at 550 ◦°CC in air atmosphere,atmosphere, the patternedpatterned channel layers were observed to be severely ruptured, as shownshown in Figure4 a.4a. Conventional Conventional annealing annealing increased increased from from room room temperature temperature directly directly to 550 to◦C 550 with °C the with heating the rateheating of 5 ◦rateC/min. of 5 This°C/min. was causedThis was by caused rapid curing by rapi ofd the curing surface of ofthe the surface patterned of the channel patterned layer, withoutchannel curinglayer, without the internal curing components; the internal thus, components; when the thus temperature, when the was temperature increased, thewas surface increased, cracked the surface owing tocracked gas generated owing to from gas thegenerated annealing from of thethe internalannealin solution.g of the Here,internal we soluti proposeon. aHere, stepped we propose annealing a methodstepped forannealing annealing method the channel for annealing layer that the includeschannel curinglayer that the includes layer for curing 1 h at athe temperature layer for 1 lowerh at a thantemperature the boiling lower point than of the the ink’s boiling solvent, point followed of the in byk's a transitionsolvent, followed annealing by temperature a transition of annealing 450 ◦C for 1temperature h, and subsequent of 450 °C annealing for 1 h, and at 550 subsequent◦C for 1 h.annealing After the at stepped 550 °C for annealing 1 h. After treatment, the stepped the annealing surface of thetreatment, patterned the channel surface layerof the was patterned unbroken, channel as shown layer in was Figure unbroken,4b. This as is requiredshown in for Figure the subsequent 4b. This is depositionrequired for of thethe drainsubsequent and source deposition electrodes of ontothe thedrain patterned and source InGaZnO electrodes channel onto layers. the Meanwhile, patterned theInGaZnO scanning channel electron layers. microscopy Meanwhile, (SEM, JSM-7610F,the scanning JEOL, electron Tokyo, microscopy Japan) images (SEM, and JSM-7610F, energy-dispersive JEOL, X-rayTokyo, spectroscopy Japan) images (EDS, and X-Maxenergy-dispersive 50, Oxford, X-ray Abingdon, spectroscopy UK) element (EDS, mapping X-Max 50, of Oxford, InGaZnO Abingdon, channel layerUK) element after traditional mapping annealing of InGaZnO and channel stepped layer annealing after traditional are shown annealing in Figure 4andc,d. stepped It was found annealing that indium,are shown gallium, in Figure and 4c,d. zinc It elementswas found are that evenly indium, distributed gallium, and in the zinc InGaZnO elements patterned are evenly films. distributed After annealing,in the InGaZnO the atomic patterned percentages films. After of annealing, indium, gallium, the atomic and percentages zinc were 0.179%, of indium, 0.077%, gallium, and and 0.744%, zinc respectively,were 0.179%, which 0.077%, were and similar 0.744%, to respectively, the proportion which of elements were similar in InGaZnO to the proportion printing ink. of elements in InGaZnOFor comparison, printing ink. we prepared two channel layers with different thicknesses by controlling the amountFor ofcomparison, ink printed we onto prepared the substrates. two channel We thenlayers printed with different drain and thicknesses source electrodes by controlling covering the theamount surface of ink of theprinted channel onto layerthe substrates. and controlled We then the printed channel drain length and to source 60 µm electrodes to build TFTcovering devices. the Insurface Figure of5 thea,b, channel the transition layer and from controlled the linear the to channel the saturation length regimeto 60 µm and to build a good TFT regulating devices. eInff Figureect on the5a,b, source–drain the transition current from the are linear observable to the saturation in the output regime characteristics and a good regulating of the TFTs effect with on the the InGaZnO source– patterneddrain current channel are observable layer thicknesses in the output of 30 and characteristics 120 nm under of the diff erentTFTs with gate voltages.the InGaZnO We found patterned that thechannel TFTs layer with thicknesses thinner channel of 30 and layers 120 could nm under achieve different more gate drain voltages. current We because, found in that the the thinner TFTs channel,with thinner a shorter channel path layers would could obviously achieve form more for drain carriers, current resulting because, in lower in the serial thinner resistance channel, and a consequentlyshorter path would higher obviously drain current form [for51]. carriers, The Ion /resultingIoff value in of lower the TFT serial with resistance the 30-nm-thick and consequently InGaZnO 4 channelhigher drain layer current (~2.0 10[51].) wasThe significantlyIon/Ioff value higherof the TFT than with that ofthe the 30-nm-thick TFT with the InGaZnO 120-nm-thick channel channel layer 4 3 × layer(~2.0 (×~1.9 10 ) was10 significantly) at VSD = 30 V,higher as shown than inthat Figure of the5c. TFT Furthermore, with the 120-nm-thick the VT of 12.3 channel V and the layer saturation (~1.9 × × 2 1 1 mobility103) at VSD of = 0.323 30 V,cm as shownV s inof Figure the TFT 5c. withFurthermore, the 30-nm-thick the VT of InGaZnO 12.3 V and channel the saturation layer and mobility that of the of · − · − TFT0.323 with cm2·V the−1·s 120-nm-thick−1 of the TFT channelwith the layer30-nm-thick (~11.2 V InGaZnO and 0.012 ch cmannel2 V layer1 s 1) and at V that= of30 the V wereTFT with derived the · − · − SD from120-nm-thick Figure5 d.channel Gallium layer serves (~11.2 as V the and suppressor 0.012 cm2·V and−1·s− the1) atstabilizer VSD = 30 V to were suppress derived the from generation Figure 5d. of oxygenGallium vacancies serves as becausethe suppressor Ga–O bondsand the are stabilizer much stronger to suppress than In–Othe generation and Zn–O of bonds oxygen in InGaZnOvacancies because Ga–O bonds are much stronger than In–O and Zn–O bonds in InGaZnO films [38]. However, the increasing film thickness will consequently introduce a large number of defects, such as

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Coatings 2019, 9, x FOR PEER REVIEW 6 of 11 Coatings 2019, 9, x FOR PEER REVIEW 6 of 11 films [38]. However, the increasing film thickness will consequently introduce a large number of interstitialdefects,interstitial such indiumindium as interstitial andand zinczinc indium andand oxygenoxygen and zincvacancies.vacancies. and oxygen SuSuchch kindskinds vacancies. ofof defectsdefects Such willwill kinds relaterelate of defects toto anan increaseincrease will relate ofof the off current. So the Ion/Ioff ratio of the TFT with 120 nm InGaZnO channel layer decreased. The theto anoff increase current. of So the the o ffIoncurrent./Ioff ratio So of the theI onTFT/Ioff withratio 120 of the nm TFT InGaZnO with 120 channel nm InGaZnO layer decreased. channel layerThe thickerdecreased.thicker channelchannel The thicker formedformed channel longerlonger formed source–drainsource–drain longer transp source–draintransportort pathpath transport forfor carrierscarriers path and forand carriers waswas affectedaffected and was byby a ffdefectdefectected scattering,byscattering, defect scattering, whichwhich ledled which toto aa decreasedecrease led to a inin decrease field-effectfield-effect in field-e mobility.mobility.ffect mobility.

Figure 4. Microscope images of the patterned channel layer with traditional annealing (a) and a FigureFigure 4. Microscope imagesimages of of the the patterned patterned channel channel layer layer with with traditional traditional annealing annealing (a) and ( a) stepped and a stepped annealing treatment (b). SEM images and EDS element mapping of the patterned channel steppedannealing annealing treatment treatment (b). SEM (b images). SEM and images EDS and element EDS mappingelement ma of thepping patterned of the patterned channel layer channel with layer with traditional annealing (c) and a stepped annealing treatment (d). layertraditional with traditional annealing annealing (c) and a stepped (c) and annealinga stepped treatmentannealing (treatmentd). (d).

Figure 5. Output characteristics of TFTs with a patternedpatterned InGaZnO channel layer thickness of 30 nm Figure 5. Output characteristics of TFTs with a patterned InGaZnO channel layer thickness of 30 nm (a) andand 120120 nmnm ((bb).). ( (cc,,dd)) Transfer Transfer characteristics characteristics of of TFTs TFTs with didifferentﬀerent InGaZnO channel layer (a) and 120 nm (b). (c,d) Transfer characteristics of TFTs with different InGaZnO channel layer thicknesses. The channel length and width of the above TFTs areare aboutabout 6060 andand 300300 µµm.m. thicknesses. The channel length and width of the above TFTs are about 60 and 300 µm.

High-temperatureHigh-temperature annealingannealing inin airair willwill causecause manymany functionalfunctional groupsgroups toto gathergather onon thethe surfacesurface ofof thethe InGaZnOInGaZnO channelchannel layer.layer. ToTo analyzeanalyze thethe changeschanges ofof surfacesurface functionalfunctional groupsgroups beforebefore andand afterafter annealingannealing andand argonargon plasmaplasma treatment,treatment, wewe plottedplotted thethe transmissiontransmission infraredinfrared spectraspectra ofof InGaZnOInGaZnO

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High-temperature annealing in air will cause many functional groups to gather on the surface Coatings 2019, 9, x FOR PEER REVIEW 7 of 11 of the InGaZnO channel layer. To analyze the changes of surface functional groups before and after filmsannealing without and argonand with plasma stepped treatment, annealing we plotted and argo then transmission plasma treatment infrared as spectra shown of in InGaZnO Figure films6. In contrast,without andthe withabsorption stepped of annealing carboxyl and(1647 argon cm−1 plasma) and hydroxyl treatment (3337 as shown cm−1) infunctional Figure6. Ingroups contrast, are 1 1 obviouslythe absorption increased of carboxyl on the surface (1647 cm of− InGaZnO) and hydroxyl film after (3337 stepped cm− ) annealing functional as groups shown are in Figure obviously 6b. Theincreased functional on the groups’ surface ofadsorption InGaZnO filmonto afterthe steppedInGaZnO annealing channel assurface shown will in Figure lead 6tob. the The creation functional of acceptor-likegroups’ adsorption surface onto states the [52]. InGaZnO So if channelwe directly surface printed will leadthe drain to the and creation source of acceptor-likeelectrodes onto surface the surfacestates [52 of]. the So ifchannel we directly layers printed after thehigh-tempera drain and sourceture annealing, electrodes these onto functional the surface groups of the channel would introducelayers after defects high-temperature at the interface annealing, and affect these carrier functional injection. groups To solve would this introduceproblem, argon defects plasma at the treatmentinterface and was aff appliedect carrier to injection.the surface To solveof the this InGa problem,ZnO, the argon absorption plasma treatment of carboxyl was and applied hydroxyl to the functionalsurface of thegroups InGaZnO, adsorbed the on absorption the surface of carboxylwere significantly and hydroxyl reduced functional after 20 groups s of plasma adsorbed treatment, on the assurface shown were in Figure significantly 6c. In Figure reduced 7a–c, after TFTs 20 s ofwith plasma different treatment, plasma as processing shown in Figuretimes 6allc. exhibited In Figure7 harda–c, TFTs with different plasma processing times all exhibited hard saturation and operated as n-channel saturation and operated as n-channel enhancement mode devices. A positive VG was required to induceenhancement a conducting mode channel, devices. and A positive the channelVG was conduc requiredtivity increased to induce when a conducting the positive channel, gate bias and was the channel conductivity increased when the positive gate bias was increased. However, the value of increased. However, the value of Ion/Ioff, the saturation mobility, and the VT of the TFTs were affected byIon /theIoff ,duration the saturation of the plasma mobility, treatment and the ofVT theof interface the TFTs between were affected the patterned by the duration channel oflayer the and plasma the draintreatment and ofsource the interface electrodes. between Figure the 7d,e patterned show th channele transfer layer characteristics and the drain of andInGaZnO source TFTs electrodes. with Figure7d,e show the transfer characteristics of InGaZnO TFTs with di fferent plasma processing times different plasma processing times at VSD = 30 V. As shown in Figure 7d, the value of Ion/Ioff of TFTs withoutat VSD = plasma30 V. As and shown with ina plasma Figure7 treatmentd, the value time of ofIon 20/Io andff of 40 TFTs s was without 1.0 × 10 plasma4, 1.0 × and 104, with and a4.6 plasma × 104, treatment time of 20 and 40 s was 1.0 104, 1.0 104, and 4.6 104, respectively. Furthermore, respectively. Furthermore, the defects at× the interface× between the× channel layers and the drain and sourcethe defects electrodes at the mainly interface affected between the the injection channel of layerscarriers. and The the modified drain and interface source was electrodes able to mainlyreduce affected the injection of carriers. The modified interface was able to reduce the power consumption the power consumption of the TFT devices. Therefore, we found that the value of VT of the TFTs treatedof the TFT with devices. 20 s of Therefore,plasma treatment we found decreased that the valuefrom 12.3 of V Ttoof 8.7 the V. TFTs However, treated an with excessive 20 s of plasma treatment decreased from 12.3 to 8.7 V. However, an excessive plasma processing time will increase processing time will increase the VT (~13.9 V with 40 s plasma treatment), as shown in Figure 7e. The variationthe VT (~13.9 of the V withpatterned 40 s plasma InGaZnO treatment), surface asroug shownhness in before Figure 7ande. The after variation different of theargon patterned plasma treatmentInGaZnO time surface was roughness analyzed beforeby atomic and force after micros differentcope argon in Figure plasma 7f. treatmentThe channel time layer was with analyzed small surfaceby atomic roughness force microscope could be inbenefici Figureal7 f.to Theform channel better contact layer with with small the surfacedrain/source roughness electrodes could and be improvebeneficial carrier to form injection better contact efficiency. with the The drain surface/source roughness electrodes of and the improve printed carrierInGaZnO injection channel efficiency. layer withoutThe surface plasma roughness treatment of the was printed 2.131 InGaZnOnm, and the channel roughness layer withoutof the layer plasma was treatment decreased was to 1.810 2.131 nm,nm afterand the20 s roughness of plasma oftreatment. the layer However, was decreased the surfac to 1.810e of the nm layer after treated 20 s of with plasma 40 s treatment. of plasma treatment However, wasthe surface slightly of damaged, the layer treatedand the with roughness 40 s of plasmaincrease treatmentd to 2.401 was nm, slightly thereby damaged, reducing and the theperformance roughness ofincreased the TFT to devices. 2.401 nm, The thereby value reducingof the saturation the performance mobility ofof thethe TFT TFTs devices. could also The valuebe improved of the saturation through optimizingmobility of thethe TFTsplasma could processing also be improved time (0 s: through 0.054 cm optimizing2·V−1·s−1; 20 the s: plasma0.071 cm processing2·V−1·s−1; and time 40 (0 s: 0.052 0.054 2 1 1 2 1 1 2 1 1 cm2 V−1 −s1 ; 20 s: 0.071 cm V s ; and 40 s: 0.052 cm V s ). The interface plasma treatment thus cm ·V· −·s· −). The interface plasma· − · −treatment thus played· a− key· − role in improving the performance of aplayed TFT device a key roleprepared in improving using printing the performance technology. of a TFT device prepared using printing technology.

Figure 6.6. TransmissionTransmission infrared infrared spectra spectra of InGaZnO of InGaZnO ﬁlms withoutfilms without annealing annealing (a), with ( steppeda), with annealing stepped annealing(b), and with (b), stepped and with annealing stepped andannealing argon and plasma argon treatment plasma (treatmentc). (c).

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Figure 7.7. Output characteristics without ( a)) and with 20 s (b) and 40 ss ((cc)) plasmaplasma treatmenttreatment onon thethe surfacesurface of thethe patternedpatterned channelchannel layers.layers. ( d,,e)) Transfer characteristics of TFTs withwith didifferentﬀerent plasmaplasma treatments.treatments. The The channel channel length, length, widt width,h, and and thickness thickness of ofthe the above above TFTs TFTs are areabout about 60 µm, 60 µ 300m, µm, 300 µandm, and30 nm, 30 nm,respectively. respectively. (f) (Atomicf) Atomic force force microscope microscope images images of of the the printed printed InGaZnO InGaZnO channel layerlayer withoutwithout andand withwith didifferenﬀerentt plasmaplasma treatmenttreatment time.time.

4. Conclusions 4. Conclusions We investigated a printing method to prepare InGaZnO TFT devices with a bottom gate and top We investigated a printing method to prepare InGaZnO TFT devices with a bottom gate and top contact (drain and source electrodes) structure and presented a stepped annealing method to prevent contact (drain and source electrodes) structure and presented a stepped annealing method to prevent the patterned channel layers from cracking and to improve the electrical properties of the channel the patterned channel layers from cracking and to improve the electrical properties of the channel layers when using a high annealing temperature. We prepared InGaZnO inks for printing with different layers when using a high annealing temperature. We prepared InGaZnO inks for printing with ratios of the constituent elements; further, the type of TFT could be regulated by changing the element different ratios of the constituent elements; further, the type of TFT could be regulated by changing ratios. By improving the interface between the insulation layer and the channel layer and between the the element ratios. By improving the interface between the insulation layer and the channel layer and channel layer and the drain and source electrodes, we optimized the printing thickness of the InGaZnO between the channel layer and the drain and source electrodes, we optimized the printing thickness channel layer and reduced the defect density between the interfaces to reduce the power consumption of the InGaZnO channel layer and reduced the defect density between the interfaces to reduce the ofpower the printed consumption TFT devices of the and printed thus improvedTFT devices their and electrical thus improved performance. their electrical The optimized performance. printed TFTThe 2 1 1 4 exhibited a VT of 8.7 V, a field-effect mobility of 0.071 cm V− s− , and an Ion/Ioff of 10 . The electrical optimized printed TFT exhibited a VT of 8.7 V, a field-effect· mobility· of 0.071 cm2·V−1·s−1, and an Ion/Ioff performances of InGaZnO TFTs fabricated by different methods are summarized in Table1. Compared of 104. The electrical performances of InGaZnO TFTs fabricated by different methods are summarized within Table the 1. current Compared TFTs, with our full-printedthe current TFTs, preparation our full-printed method stillpreparation has a certain method gap still in thehas electricala certain properties,gap in the butelectrical we will properties, improve thebut performance we will impr ofove the the TFTs performance by modifying of thethe interfaceTFTs by next,modifying especially the tointerface improve next, the especially problem that to improve silver nanoparticles the problem as that electrodes silver nanoparticles will diffuse into as theelectrodes channel will layer diffuse with theinto application the channel of layer voltage. with the application of voltage.

Table 1. Electrical parameters of InGaZnO TFTs fabricated by diﬀerent methods. Table 1. Electrical parameters of InGaZnO TFTs fabricated by different methods. Mobility Threshold Method Method Mobility (cm2·V−1·s−1) Threshold Voltage (V) On On/Off/Oﬀ Ratio Ratio Ref. Ref. (cm2 V 1 s 1) Voltage (V) Traditional annealing, spin- · − · − 6.41 −37.7 3.9 × 107 7 [36] Traditionalcoating annealing, and etching spin-coating and etching 6.41 37.7 3.9 10 [36] − × 3 Combustion annealing, spin-coatingspin- and etching 2.82 1.9 4.0 10 [37] − × 3 Inkjet-Printed2.82 6−1.9 7.9 1.0 4.0 10× 105 [53[37]] coating and etching × Our results 0.071 8.7 1.0 104 – Inkjet-Printed 6 7.9 1.0× × 105 [53] Our results 0.071 8.7 1.0 × 104 –

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Author Contributions: Conceptualization, X.Y. (Xingzhen Yan); methodology, X.Y. (Xingzhen Yan) and K.S.; formal analysis, X.Y. (Xingzhen Yan) and X.C.; investigation, F.Y.; writing—original draft preparation, X.Y. (Xingzhen Yan); writing—review and editing, X.Y. (Xingzhen Yan) and X.Y. (Xiaotian Yang); project administration, Y.C. and X.Y. (Xiaotian Yang); funding acquisition, X.Y. (Xingzhen Yan) and X.Y. (Xiaotian Yang). Funding: This work is supported by the National Key R&D Program of Strategic Advanced Electronic Materials of China (No. 2016YFB0401103), National Natural Science Foundation of China (No. 51672103), and Natural Science Foundation of Jilin Province, China (Nos. 20180520227JH, JJKH20180589KJ). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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