Photo-Patternable ZnO Thin Films Based on Cross ... - ACS Publications

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Photo-Patternable ZnO Thin Films Based on Cross-Linked Zinc Acrylate for Organic/Inorganic Hybrid Complementary Inverters Yong Jin Jeong,† Tae Kyu An,‡ Dong-Jin Yun,§ Lae Ho Kim,† Seonuk Park,† Yebyeol Kim,† Sooji Nam,∥ Keun Hyung Lee,⊥ Se Hyun Kim,*,# Jaeyoung Jang,*,¶ and Chan Eon Park*,† †

Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, North Gyeongsang 790-784, Republic of Korea ‡ Department of Polymer Science and Engineering, Korea National University of Transportation, Chungju, North Chungcheong, Republic of Korea § Analytical Science Laboratory, Samsung Advanced Institute of Technology (SAIT), Yongin, Gyeonggi 446-712, Republic of Korea ∥ Smart I/O Control Device Research Section, Electronics and Telecommunications Research Institute, Daejeon, 305-700, Republic of Korea ⊥ Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea # School of Chemical Engineering, Yeungnam University, Gyeongsan, North Gyeongsang 712-749, Republic of Korea ¶ Department of Energy Engineering, Hanyang University, Seoul, 133-791, Republic of Korea S Supporting Information *

ABSTRACT: Complementary inverters consisting of p-type organic and n-type metal oxide semiconductors have received considerable attention as key elements for realizing low-cost and large-area future electronics. Solution-processed ZnO thin-film transistors (TFTs) have great potential for use in hybrid complementary inverters as n-type load transistors because of the low cost of their fabrication process and natural abundance of active materials. The integration of a single ZnO TFT into an inverter requires the development of a simple patterning method as an alternative to conventional time-consuming and complicated photolithography techniques. In this study, we used a photocurable polymer precursor, zinc acrylate (or zinc diacrylate, ZDA), to conveniently fabricate photopatternable ZnO thin films for use as the active layers of n-type ZnO TFTs. UV-irradiated ZDA thin films became insoluble in developing solvent as the acrylate moiety photo-cross-linked; therefore, we were able to successfully photopattern solution-processed ZDA thin films using UV light. We studied the effects of addition of a tiny amount of indium dopant on the transistor characteristics of the photopatterned ZnO thin films and demonstrated lowvoltage operation of the ZnO TFTs within ±3 V by utilizing Al2O3/TiO2 laminate thin films or ion-gels as gate dielectrics. By combining the ZnO TFTs with p-type pentacene TFTs, we successfully fabricated organic/inorganic hybrid complementary inverters using solution-processed and photopatterned ZnO TFTs. KEYWORDS: oxide thin-film transistors, complementary inverters, ZnO, low-voltage operation, zinc acrylate, solution process

1. INTRODUCTION Thin-film transistors (TFTs) based on organic or metal oxide semiconductors have received great interest as low-cost alternatives to traditional silicon-based devices due to their large-area capability and relatively low processing temperatures.1−3 Practical circuit designs require the integration of pand n-type TFTs to fabricate complementary logic devices that have low power consumption and provide robust device operation.4,5 Organic TFTs are particularly useful for p-type components, while metal oxide TFTs are promising n-type counterparts.6−10 Recently, significant efforts have been devoted to developing high-performance solution-processed © XXXX American Chemical Society

metal oxide semiconductors, such as In2O3, InZnO, and InGaZnO.11−17 However, all these materials contain a considerable amount of indium, which is scarce and expensive, making widespread utilization of indium-containing materials in mass-produced electronic devices unlikely.18−20 It is therefore desirable to develop metal oxide TFTs based on resourcefriendly materials, such as ZnO, which is naturally abundant and cost-efficient.21 Received: January 8, 2016 Accepted: February 3, 2016

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DOI: 10.1021/acsami.6b00259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the experimental procedure used in this study. The chemical structure shown in the illustration is that of zinc acrylate (ZDA).

formed a typical wurzite crystalline structure. The transistor characteristics of the patterned ZnO thin film-based TFTs and the effects of tiny amounts of In doping on the resulting ZnO TFTs were also investigated. We demonstrated low-voltage operation of the ZnO TFTs within ±3 V by using two different device architectures: (1) TFTs utilizing high-capacitance Al2O3/TiO2 laminate dielectrics and (2) TFTs gated by iongels. Finally, we successfully fabricated low-voltage organic/ inorganic hybrid complementary inverters using solutionprocessed and photopatterned ZnO TFTs (n-type) and pentacene TFTs (p-type).

Commercial applications of solution-processed ZnO TFTs in integrated circuits should satisfy at least two requirements: (1) the TFTs should be patternable using simple and convenient processes that can replace complicate photolithography, and (2) the operating voltages of the TFT devices must be low to enable robust and stable device operation with minimal power consumption.22,23 Complex lithography processes that include photoresist deposition and chemical or plasma etching are the predominant methods currently used for TFT patterning. These complicated and time-consuming processes increase production costs, involve toxic chemicals, and can damage or contaminate the surfaces of the active layers.24,25 Numerous studies have attempted to directly pattern solution-processed metal oxide thin films by adding photosensitive activators to the metal precursor solution to form organosiloxane-based sol−gel films or metallic chelate compounds.18,25,26 Mixing a photocurable Zn precursor with In or Ga precursors has recently been applied to obtain patterned IGZO thin films.24 The use of a photocurable metal precursor has many advantages over the use of photosensitive activators, including good film uniformity and facile material control.24 Although direct patternable IGZO thin films have been prepared using a photocurable Zn precursor, photopatternable resource-friendly ZnO thin films have not yet been fabricated. Photopatternable ZnO thin films can presumably be achieved using a single Zn precursor without additional photosensitive activators. This approach would enrich the current family of direct-patternable metal oxide semiconductors by providing another option for resourcefriendly ZnO semiconductors. It would also be necessary to study the transistor characteristics of the photopatterned ZnO thin films and to investigate the feasibility of low-voltage operation in complementary logic circuits. Here, we fabricated solution-processed photopatternable ZnO semiconductor thin films based on a single soluble Zn precursor, zinc acrylate (or zinc diacrylate, ZDA), for use in high-performance low-voltage ZnO TFTs and complementary inverters using ZnO TFTs and pentacene TFTs as n- and ptype components, respectively. Spin-coated ZDA thin films were conveniently photopatterned by exposure to UV light through a shadow mask and development in a solvent. Exposure to UV light induced the ZDA molecules to form cross-linked ZDA, which was insoluble in the developing solvent.27,28 After thermal annealing, the patterned ZnO films

2. EXPERIMENTAL SECTION Photopatternable ZnO precursor solution was prepared using a 7% ZDA (Aldrich Co.) precursor solution in 2-methoxyethanol (2ME). The precursor solution was stirred overnight and stored in a 60 °C oven. The ZDA patterning process steps are briefly summarized in Figure 1. A highly n-doped Si substrate coated with a 100 nm-thick SiO2 layer (SiO2/Si) was cleaned using a boiled acetone solution, rinsed multiple times with acetone, and then further cleaned with UV−ozone (UVO) treatment for 20 min. A 20 nm-thick precursor film was coated onto the SiO2/Si substrate through the spin-coating method (3000 rpm for 45 s) and then exposed to 254 nm UV light (power = 50 W, G15T8, Sankyo Denki) for 20 min through a shadow mask. UV-irradiated ZDA film was then dipped in isopropyl alcohol (IPA) for 30 s and washed for development. The ZnO crystal film was formed by thermally annealing the patterned film at 450 °C for 1 h and then subsequently cooling the film. The fabrication of ZnO TFT devices was completed by thermally evaporating 100 nm-thick Al source and drain (S/D) electrodes onto the substrate to achieve a topcontact configuration. Indium-doped ZnO TFTs were prepared by adding tiny amounts of indium(III) nitrate hydrate (In(NO3)3·xH2O, 99.99%, Aldrich Co.) to the precursor solution. The ratios of ZDA to In(NO3)3·xH2O were 7:1, 14:1, and 28:1. ZnO TFTs that operated at low voltages were fabricated by depositing 50 nm-thick Al2O3/TiO2 laminate films onto highly n-doped Si substrate using plasma-enhanced atomic-layer deposition (PEALD). This layer was used as the dielectric layer,29,30 and leakage current density was measured by fabricating 0.0314 cm2 area of Al electrodes (100 nm) onto the 50 nm-thick Al2O3/TiO2 substrates. The ZnO thin film and Al S/D electrodes were sequentially deposited using the process described above. Ion-gel gated ZnO TFTs were formed using an ion-gel prepared by mixing 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and poly(styrene-b-methyl methacrylate-b-styrene) (SMS) in ethyl acetate, following methods previously described,31 and dropping the mixture onto the patterned ZnO film and the B


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Figure 2. (a) Optical images of patterned photo-cross-linked zinc acrylate (pZA) film and magnified optical microscopy images of pZA film. (b) FTIR data obtained from the as-coated ZDA film or from the UV-treated pZA film.

Figure 3. AFM topography images of (a) the as-coated ZDA film, (b) the patterned pZA film with an etched region, and (c) the ZnO film after thermal annealing at 450 °C. The inset graph in (b) shows the corresponding AFM cross-sectional height profile of the pZA film.

μm (see the magnified optical microscopy images). UV exposure for 20 min was the optimal condition for obtaining clear patterned film. Further exposure failed to pattern ZDA films, because heated substrate under UV light might induce unwanted cross-linking of ZDA where the region should be protected from UV by the mask.24 In a previous report, vacuum-deposited ZDA films were effectively photopatterned through cross-linking of the acrylate group under UV irradiation without the need for photoinitiators.28 Certain acrylate monomers are known to be easily photo-cross-linked without photoinitiators when exposed to a photon energy resonant with the acrylate absorption peaks.27,28 In this study, we selected a solution-process technique as the preferred method for fabricating electronic devices. Stable ZDA film was confirmed on the basis of the FT-IR analysis, as shown in Figure 2b. The absorption peaks of the as-coated ZDA film were similar to the FT-IR peaks typical of ZDA.28,32 The vinyl CC stretching peak was positioned at 1641 cm−1; the = C− H bending peak was positioned at 981 cm−1, and the out-ofplane C−H peak related to the acrylate moiety was positioned at 830 cm−1. After exposure of the ZDA film to UV light, these three peaks disappeared because the vinyl CC bonds were opened and converted to C−C single bonds as a result of crosslinking. The photo-cross-linked zinc acrylate (pZA) film did not include CC double bonds but rather included crosslinked C−C bonds and was therefore not soluble in IPA. The solution-processed ZDA film results agreed well with previous studies of vacuum-deposited ZDA films;28 therefore, the solution-processed ZDA film appeared to have been effectively patterned under UV irradiation via photo-cross-linking reaction

predeposited Al S/D electrodes. A platinum gate electrode was positioned on the ion-gel dielectric. To fabricate organic/inorganic hybrid complementary inverters, pentacene (Aldrich Co.) layers, 50 nm-thick, were deposited onto the substrate as drive (p-type) transistors at a rate of 0.2 Å/s by using an organic molecular beam deposition system. Prior to pentacene deposition, the indium-doped ZnO thin films were patterned and thermally annealed to form load (n-type) transistors. The 100 nm-thick Au and Al source and drain (S/ D) electrodes on the pentacene and indium-doped ZnO films were thermally evaporated to achieve a top-contact configuration. All devices had channel width (W) of 1000 μm, and the channel length (L) of 150 μm. ZDA film was UV-exposed in a N2-rich glovebox, and the other fabrication steps were performed in ambient air (RH: 40% ± 10%). All electrical measurements were performed using a Keithley 4200 SCS in a N2-rich glovebox. The morphologies of the ZDA, pZA, and ZnO films were investigated using Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Elec. Co.), atomic force microscopy (AFM, Veeco DI Dimension 3100 with Nanoscope V), Xray photoemission spectroscopy (synchrotron X-ray beam source at the 4D beamline of the Pohang Accelerator Laboratory (PAL)), and ultraviolet−visible (UV−vis) absorption spectroscopy (Cary, Varian Co.). Crystalline ZnO films prepared with or without indium doping were analyzed on the basis of θ−2θ mode out-of-plane X-ray diffraction (XRD) patterns collected using a synchrotron X-ray beam source at the 5A beamline of the PAL.

3. RESULTS AND DISCUSSION ZDA film was patterned by exposing the as-coated film to 254 nm UV light through a shadow mask, followed by dipping and washing in IPA for film development. The resulting film, as captured in the optical image shown in Figure 2a, clearly had microresolution patterns with feature spacings of less than 40 C


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Figure 4. Comparative XPS spectra showing the O 1s peak of (a) the as-coated ZDA film, (b) pZA film, and (c) thermally annealed pZA-ZnO film.

Figure 5. (a) UV−vis absorption spectra of the ZDA precursor solution, as-coated ZDA film, pZA film, and ZnO film. The inset shows magnified spectra of as-coated ZDA film and pZA film. (b) Plot of (αhv)2 versus hv, used to determine the band gap of the ZnO films. (c) θ−2θ mode out-ofplane XRD profiles of the ZnO films prepared with or without indium doping (λ = 1.07 Å).

assumed that ZnO crystals formed after thermal annealing at 450 °C.37−39 To confirm the formation of ZnO crystals from ZDA, we performed XPS measurements. Figure S2a compares the XPS spectra of the as-coated ZDA, pZA, and thermally annealed films over a wide range of energies (0 to 1200 eV). Compared to XPS spectra of the as-coated ZDA and pZA, those of the annealed film revealed that the carbon 1s (C 1s) peak became smaller, whereas the oxygen 1s (O 1s) peak and zinc 2p (Zn 2p) peaks became larger. These results indicated that the organic carbon elements in the precursor were removed and that oxygen and zinc remained in the pZA film thermally annealed at 450 °C. A small quantity of carbon impurities still remained, despite thermal decomposition, which is an inevitable phenomenon that occurs in solution-processed metal oxide films. More insight into the formation of ZnO crystals was sought by analyzing the O 1s XPS spectra, as shown in Figure 4a. Deconvoluted O 1s peaks in these XPS spectra, including the metal oxide peak, M−O (O|), oxygen peak in M−OC bonds or near oxygen vacancy, M−OC (O∥), and metal hydroxide peak, M−OH (O∥|), of ZDA, pZA, and film thermally annealed at 450 °C were compared.15,40,41 Quantitative data including each binding energy (BE) and % area, as shown in Table S1, exhibited a little different results between ZDA and pZA films: % area of O∥ peak, which represents the relative extent of M−OC networks, decreased from 70.2 for ZDA to 67.4 for pZA, and % area of O| peak, which represents the relative extent of M−O networks,

of the acrylate moiety. A possible photo-cross-linking mechanism for ZDA was described in Figure S1. Acrylate groups could be photoinitiated by exposure to UV light according to two possible mechanisms including (1) the photocyclization mechanism and (2) the α-cleavage mechanism.33 Because α-cleavage leads to an active site for crosslinking,34 the latter (2) mechanism might be acceptable. Split acrylate groups in ZDA transformed to radical initiators, and these radical initiators attacked CC double bonds and underwent analogous α-cleavage reactions with successive cross-linking.34,35 The resulting pZA film was not solved in IPA, because secondary bonds between pZA and IPA could not overcome primary valence cross-links.36 Figure 3a shows AFM topography images of the as-coated ZDA films. Very smooth surfaces with an rms roughness (Rms) value of 0.318 nm and no noticeable impurities were observed. After photopatterning, the developed film also showed very smooth pZA surfaces with an Rms value of 0.411 nm (Figure 3b), confirming that UV exposure did not affect the surface morphology of the deposited film.28 Cross-sectional height profiles revealed that the thickness difference between the pZA surface and the etched SiO2 region was around 20 nm. Although the edge at the interface of the pZA/etched region was not quite sharp, it may be possible to obtain more clear patterns using a chrome-patterned quartz photomask in place of a shadow mask. Figure 3c shows the AFM image of a pZA film that was thermally annealed at 450 °C. The Rms of the surface of this thermally annealed pZA film was 2.03 nm. We D


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Figure 6. (a) Transfer characteristics (VD = 10 V) of ZnO TFTs prepared with or without indium doping (pZA-ZnO or pZA:In-ZnO). (b) Transfer characteristics (VD = ±10 V) and (c) output characteristics of the pentacene and pZA:In-ZnO TFTs. (d) VTCs and DC voltage gain (inset graph) of the inverter with various VDD values. The inset diagram shows the inverter structures based on zero drive load logic with the load and drive transistor. The channel length (L) and width (W) of the TFTs were 150 and 1000 μm, respectively.

magnified peaks of as-coated ZDA film and pZA film all indicated light absorption below 341 nm, similar to the result typical of ZnO precursor solution in 2ME.43 After making ZnO film after thermal annealing, the absorption peaks were redshifted compared to the peak obtained from precursor films (ZDA and pZA film). Because the ZnO crystals increased in size as they grew, their Eg values fell within the range typical of semiconductor band gap energies.44 The Eg of the annealed ZnO film was determined to be 3.15 eV based on plotting (αhv)2 versus hv and calculating the x-intercept extrapolated from the linear fit to the onset region of the plot (Figure 5b).45 The obtained Eg was smaller than the theoretical value of 3.35 eV, which we attributed to the presence of oxygen vacancies in the ZnO films with energetic states that acted as donors below the conduction band.43,46 In addition to the morphological studies conducted using AFM analysis, we also investigated X-ray diffraction (XRD) patterns of ZnO film to analyze the crystalline structure. Figure 5c shows the θ−2θ mode out-of-plane XRD patterns obtained from the ZnO films. The annealed ZnO films (pZA-ZnO) displayed intense peaks at 2θ = 22°, 23.7°, 25.1°, 32.7°, 38.5°, 42.7°, and 45.9° that corresponded to d-spacings of 0.280, 0.261, 0.247, 0.190, 0.163, 0.147, and 0.137 nm, respectively. These peaks were identical to those present in the XRD peak data representative of wurzite ZnO with a hexagonal structure, as reported in the corresponding JCPDS card (number 36-

increased from 15.7 for ZDA to 18.2 for pZA. The variation of % area in O∥ and O| peaks may result from the reduction of a few carbon−oxygen compounds and the increasing composition of oxygen bonded to Zn, as a result of photo-cross-linking according to the proposed possible mechanism in Figure S1. The O∥ peak further decreased after thermal annealing (Figure 4c) compared to the unheated ZDA and pZA films (Figure 4a,b), due to tremendous reduction in oxygen binding with carbon because the O2− ions were surrounded by Zn atoms.42 Consequently, the O| peak in the film thermally annealed at 450 °C increased dramatically and corresponded to the formation of ZnO film with M−O−M networks. Figure S2b,c compares the O 1s XPS spectra of thermally annealed ZnO films prepared with or without indium (14:1 ratio of Zn precursor to In precursor). The % area of the O| peak to the whole O 1s peak, which represents the relative extent of M−O−M networks, was 66.7 in both films. The addition of tiny amounts of the indium dopant did not affect M−O−M network formation. Semiconductors have an optical band gap (Eg) of 2−4 eV and absorb light that exceeds Eg in energy. UV−vis spectra of the films were collected to obtain information about the Eg level of the ZnO semiconductor. Figure 5a shows the UV−vis absorbance plot obtained from the samples, including the precursor solution, as-coated ZDA film, pZA film after photocross-linking, and ZnO film thermally annealed at 450 °C. The absorption peak of the ZDA precursor solution in 2ME and E


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ACS Applied Materials & Interfaces Table 1. Electrical CharacteRistics of the Various ZnO TFTs Examined in This Study maximum μ [cm2/(V s)] pZA:In-ZnO (SiO2 100 nm) pZA-ZnO (SiO2 100 nm) pZA:In-ZnO (Al2O3:TiO2 50 nm) pZA-ZnO (ion-gel) pZA:In-ZnO (ion-gel)

average μ [cm2/(V s)]

0.853 0.323 0.598 5.47 7.14

0.736 0.202 0.430 4.78 6.52

1451, a = 0.3249 nm, and c = 0.5206 nm).47 By comparing the XRD peaks of the pZA-ZnO film to the JCPDS card, as summarized in Table S2, the peaks of pZA-ZnO were found to correspond to the [100], [002], [101], [102], [110], [103], and [112] reflections, respectively. XRD analysis of ZnO films prepared with tiny amounts of indium doping was also conducted. Despite the presence of indium dopant in the films, the XRD pattern of the ZnO film prepared with indium (pZA:In-ZnO) also included peaks positioned at the same values as those observed in pZA-ZnO; however, the peak intensity and peak full width at half-maximum (FWHM) were reduced, indicating that the In-doped ZnO crystals were smaller in size than those of pure ZnO because grain size is inversely proportional to a peak’s FWHM (Scherrer’s formula).47,48 Therefore, the addition of tiny amounts of indium reduced ZnO crystal size, yielding the same results as obtained using other dopants.44,49 The AFM, XPS, UV−vis, and XRD results indicated that ZnO crystals were stably formed from photopatterned ZDA films. The electrical performances of the pZA-ZnO film were investigated by fabricating top-contact ZnO TFT devices and collecting their electrical measurements (a schematic TFT illustration is shown in Figure S3a). Figure 6a shows transfer characteristics of the ZnO TFTs. Field-effect mobility (μ) was calculated in the saturation regime (drain voltage, VD = 40 V) from the slope of square root of drain current (ID1/2) versus gate voltage (VG), obtained from Figure 6b, according to the following equation: ID =

μC iW (VG − Vth)2 2L

± ± ± ± ±

0.1 0.1 0.1 0.78 0.66

Vth [V] −4.31 14.92 0.93 1.08 0.74

± ± ± ± ±

0.48 2.89 0.23 0.04 0.02

Ion/off 4.0 × 105 1.2 × 105 ∼104 ∼105 ∼105

displayed higher drain currents (Figure S4a) than pZA-ZnO TFTs (Figure S4b). Addition of indium may have increased the mobility and current of the ZnO TFTs by significantly reducing the number of voids present in the film and inducing the ZnO to form a more closely-packed structure, despite the smaller crystal size, thereby increasing carrier density. This explanation is consistent with other reported results.44 The patterned ZnO TFTs prepared with or without indium doping showed reliable operation with a small Vth shift under a positive bias stress, as shown in Figure S5. We further investigated the electrical performance of pZA:In-ZnO TFTs according to the amount of indium doping. The resulting transfer characteristics and their μ values were compared in Figure S6 and Table S3, respectively. pZA:In-ZnO TFTs doped with a 7:1 ratio of Zn precursor to In precursor showed the best performance among 7:1, 14:1, 28:1, and 1:0 (pure ZnO TFTs). Although more indium doping improved electrical properties of ZnO TFTs, a clear pattern was not obtained over the film with the 14:1 ratio. As shown in Figure S7, a blurred edge appeared in 10:1 Zn/In precursor films and pattern quality was worse by increasing In amount, because In precursor was inactive for a UV-exposure. To confirm that our photopatterned ZnO TFTs could operate properly in combination with their p-type counterparts, we fabricated organic/inorganic hybrid complementary inverters by employing pentacene TFTs as the p-type component. Pentacene TFTs showed typical p-type transfer (Figure 6b) and output characteristics (Figure 6c) with comparable μ values (0.718 cm2/(V s)) to those of pZA:In-ZnO TFTs. Due to the balanced performance of p- and n-type TFTs, their hybrid inverters exhibited good voltage transfer characteristics (VTCs) under various supply voltages (VDD), as shown in Figure 6d. Because the load transistor (pZA:In-ZnO TFTs) exhibited slightly higher on-state currents than the drive transistor (pentacene TFTs), the switching threshold voltages (VM) shifted toward the lower VIN value. We believe that the shift in VM from the center of the voltage swing can be improved by device optimization with various kinds of organic semiconductors. The inverter voltage gains (inset of Figure 6d), defined as dVOUT/dVIN, reached the highest value of 22 with a VDD of 40 V. To the best of our knowledge, this is the first demonstration of organic/inorganic hybrid complementary inverters based on solution-processed and photopatterned metal oxide TFTs. The above ZnO TFTs and complementary inverters required high operating voltages (∼40 V) due to the low capacitance of the SiO2 gate dielectric. Because TFTs with low operating voltages can be achieved using high-capacitance dielectrics, we prepared photopatterned ZnO films (1) deposited on Al2O3/ TiO2 laminate films and (2) covered with an ion-gel dielectric based on poly(styrene-b-methyl methacrylate-b-styrene) (SMS), a triblock copolymer, to produce ZnO TFTs that operated at low voltages. Recently, Al2O3/TiO2 laminate films deposited by PEALD exhibited superior dielectric properties and their organic TFTs showed enhanced electrical perform-

(1)

where Ci is the capacitance per unit area of the gate dielectrics (≈30 nF/cm2) and Vth is the threshold voltage. The electrical characteristics of these TFTs are summarized in Table 1. pZAZnO TFTs exhibited a maximum μ value of 0.323 cm2/(V s) after thermal annealing at 450 °C, which is comparable to the value obtained from conventional solution-processed ZnO TFTs without the need for additional treatments.50−52 Though we also measured transfer characteristics of ZnO TFTs after thermal annealing at 250 and 350 °C, they exhibited low onstate currents and on/off ratio (Figure S3b). The value of μ for the ZnO TFTs could potentially be improved through additional treatment, such as spray pyrolysis or metal doping, as described elsewhere.53−56 For example, we doped ZnO film with tiny amounts of indium and examined the effects of this doping on the semiconducting properties of the ZnO thin film by characterizing TFT performance. Because acceptable electrical properties of transistors should have at least 104 value of on/off ratio in transfer characteristics, we fabricated indium-doped ZnO (pZA:In-ZnO) TFTs after thermal annealing at 450 °C. Compared to undoped devices, pZA:In-ZnO TFTs with the 14:1 ratio of Zn precursor to In precursor showed higher mobility and on-state currents. These films exhibited a maximum μ value of 0.853 cm2/(V s) and F


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Figure 7. (a) Transfer characteristics (VD = 1 V) including ID and IG of low-voltage operating pZA:In-ZnO TFTs on Al2O3/TiO2 laminate dielectric layers. (b) Transfer characteristics (VD = ± 1 V) and (c) output characteristics of pentacene and pZA:In-ZnO TFTs on Al2O3/TiO2 laminate dielectric layers. The inset shows an optical microscopy image of low-voltage operating inverter based on the zero drive load logic with the load and drive transistor. (d) VTCs and DC voltage gain (inset graph) of the low-voltage operating inverter with various VDD values. Channel length (L) and width (W) of the TFTs were 150 and 1000 μm, respectively.

ances compared to those of Al2O3 dielectric films.30 The Al2O3/ TiO2 laminate films had a high dielectric constant (κ) with a value of 15.4, which yielded a Ci value of 272 nF/cm2, as calculated using the following equation: Ci =

κε0 d

indicating reliable low-voltage operation. In another strategy for operating high-performance TFTs under low voltage, ion-gel gated ZnO TFTs were fabricated by utilizing ion-gel materials consisting of [EMI][TFSI] and SMS, and their transfer characteristics are shown in Figure S11a. Ion-gel gated ZnO TFTs with/without indium doping exhibited n-type electrical performances within −1 to 2 V with reasonable ON/OFF current ratio of 105. IG values were around 10−7 A, comparable to other reported ion-gel gated TFTs.44,57,58 The saturation mobility of the ion-gel gated ZnO TFTs was extracted using the reported Ci value for ion-gels, which is 12.2 μF/cm2 at 1 Hz.31 From the slope of ID1/2 versus VG curve in the saturation regime (VD = 2 V), the average μ of ion-gel gated ZnO TFT without indium doping was 4.78 ± 0.78 cm2/(V s) (maximum μ = 5.47 cm2/(V s)), whereas indium doped samples (pZA:In-ZnO TFTs) exhibited an average mobility value of 6.52 ± 0.66 cm2/ (V s) (maximum μ = 7.14 cm2/(V s)). Figure S11c revealed that output characteristics of ion-gel gated ZnO TFTs were fully saturated, and the devices with indium doping exhibited high on-state currents exceeding 1 mA. The μ of the ion-gel gated ZnO TFTs was larger than those of devices prepared with an Al2O3/TiO2 dielectric layer, possibly due to the exceptionally high electron density as a result from high capacitance of ion-gel, which filled the disorder-induced carrier traps

(2)

where κ is the dielectric constant, ε0 is the permittivity in a vacuum, and d is the thickness (50 nm) of the dielectric layer. By virtue of the high-capacitance and low leakage current density of the Al2O3/TiO2 laminate films over 2 MV/cm, shown in Figure S8, pZA:In-ZnO TFTs that operated at low voltages were prepared with the device structure shown in Figure S9a. The transfer and output characteristics of the TFTs are presented in Figures 7a and S9b, respectively. The devices exhibited typical n-type transfer characteristics of low-voltage operation within the operating voltages of −2 to 3 V. The average μ and Vth values of 20 different devices, calculated from the slope of ID1/2 versus VG curve in Figure S10a, were 0.430 ± 0.1 cm2/(V s) (maximum μ = 0.598 cm2/(V s)) and 0.93 ± 0.23 V, respectively (μ distribution was summarized in Figure S10b). These devices showed low gate leakage currents (IG), negligible hysteresis during the forward and reverse sweeps, and a small Vth shift under a positive bias stress (Figure S10c), G


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ACS Applied Materials & Interfaces positioned at grain boundaries and oxygen vacancies in the film.59,60 We also fabricated hybrid complementary inverters using Al2O3/TiO2 dielectrics to investigate the feasibility of lowvoltage device operation. Pentacene and pZA:In-ZnO TFTs with Al2O3/TiO2 dielectric layers (Figure 7c inset) exhibited good transfer (Figure 7b) and output (Figure 7c) characteristics within the operation voltages of −5 and 3 V, respectively. As shown in Figure 6d, the hybrid inverters showed decent VTCs for VDD values ranging from 2 to 4 V with voltage gains as high as 6.5. Collectively, our work suggests that the use of ZDA is a convenient solution-based route to achieving photopatterned ZnO thin films and fabricating low-voltageoperating organic/inorganic hybrid complementary inverters.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +82-54-279-8298. Tel: +8254-279-2269 (C.E.P.). *E-mail: [email protected] (J.J.). *E-mail: [email protected] (S.H.K.). Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS We prepared patterned ZnO thin films by introducing photocurable ZDA as the Zn precursor and fabricated the corresponding TFT and complementary inverter devices with low operating voltages. Solutions of ZDA were spin-coated and conveniently photopatterned using UV irradiation to induce the cross-linking of ZDA without the need for photoresists or the addition of other photoinitiators. Patterned pZA thin films formed a typical wurzite crystalline structure after thermal annealing and served as the active layers of n-type TFT devices. ZnO TFTs prepared using SiO2 gate dielectrics exhibited a maximum field-effect mobility of 0.323 cm2/(V s) that increased to 0.853 cm2/(V s) with tiny amounts of indium doping. Low-voltage operation of the ZnO TFTs, within an operation voltage of 3 V, was obtained by utilizing two different kinds of high-κ dielectrics: Al2O3/TiO2 laminate films and iongels. The ion-gel gated ZnO TFTs showed remarkably high performance with negligible hysteresis, a maximum mobility of 7.14 cm2/(V s), and an on/off ratio of 105. Finally, we successfully prepared organic/inorganic hybrid complementary inverters with low operation voltages by using solutionprocessed and photopatterned ZnO TFTs and pentacene TFTs as n- and p-type TFTs, respectively.

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ration mobility distribution, transfer characteristics of Al2O3/TiO2-based pZA:In-ZnO under gate bias stress, schematic illustration, transfer characteristics, and output characteristic of the low-voltage-operating ion gel-gated pZA:In-ZnO TFTs. (PDF)

ACKNOWLEDGMENTS This work was supported by a grant from the National Research Foundation of Korea (NRF), funded by the Korean Government (MSIP) (2014R1A2A1A05004993). This work was also supported by Basic Science Research Program through from the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A02062369). The authors thank the Pohang Accelerator Laboratory for providing access to the 4D and 5A beamlines used in this study.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00259. Schematic description of passible photo-cross-linking mechanism for zinc acrylate, broad XPS spectra of the ascoated ZDA, pZA, and ZnO films, O 1s peak XPS spectra of ZnO films with/without In, deconvoluted O 1s peak results of ZDA, pZA, and ZnO, XRD data obtained from the JCPDS ZnO card (36-1451), schematic illustration of the top-contact ZnO TFTs, transfer characteristics of the pZA-ZnO TFTs according to annealing temperature, transfer and output characteristics of the ZnO TFTs prepared with or without indium doping under gate bias stress, transfer and electrical characteristics of the ZnO TFTs according to ratio of Zn precursor to In precursor, optical microscope image of patterned pZA:In film with the 10:1 ratio of Zn/In precursor, leakage current density versus electric field characteristics of the Al2O3/TiO2 dielectrics, schematic illustration and output characteristics of the low-voltage-operating pZA:In-ZnO TFTs on Al2O3/TiO2 laminated dielectric layers, ID1/2 vs VG curve of 20 Al2O3/TiO2-based pZA:In-ZnO and their satuH


Research Article

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