High Performance Metal Oxide Field-Effect Transistors with a Reverse

Research Article www.acsami.org

High Performance Metal Oxide Field-Effect Transistors with a Reverse Offset Printed Cu Source/Drain Electrode Young Hun Han,† Ju-Yeon Won,† Hyun-Seok Yoo,‡ Jae-Hyun Kim,‡ Rino Choi,† and Jae Kyeong Jeong*,§ †

Department of Materials Science and Engineering, Inha University, Incheon 402-751, South Korea Advanced Research Institute, Dongjin Semichem Co. Ltd., Gyeonggi 463-400, South Korea § Department of Electronic Engineering, Hanyang University, Seoul 133-791, South Korea ‡

S Supporting Information *

ABSTRACT: Nonvacuum and photolithography-free copper (Cu) films were prepared by reverse offset printing. The mechanical, morphological, structural, and chemical properties of the Cu films annealed at different temperatures were examined in detail. The Ostwald ripening-induced coalescence and grain growth in the printing Cu films were enhanced with increasing annealing temperature in N2 ambient up to 400 °C. Simultaneously, unwanted chemical impurities such as oxygen, hydrogen, and carbon in the Cu films decreased as the annealing temperature increased. The high electrical conductivity (∼6.2 μΩ·cm) of the printing Cu films annealed at 400 °C is attributed to the enlargement of the grain size and reduction of the incorporation of impurities. A printing Cu film was adopted as a source/drain (S/D) electrode in solution processable zinc tin oxide (ZTO) field-effect transistors (FETs), where the ZTO film was prepared by simple spin-coating. The ZTO FETs fabricated at a contact annealing temperature of 250 °C exhibited a promising field-effect mobility of 2.6 cm2/(V s), a threshold voltage of 7.0 V, and an ION/OFF modulation ratio of 2 × 105. KEYWORDS: printing process, copper film, reverse offset, field-effect transistor, zinc tin oxide On the other hand, field-effect transistors (FETs) acting as either a switching element or an amplifier are a key ingredient in many types of electronics. Amorphous silicon, organic semiconductors, and transition metal oxide semiconductors (TMOS) have been investigated as a channel layer for printing electronics. Among them, TMOS are preferred because they offer a superior field-effect mobility and better electrical reliability.13−16 Therefore, the combination of a printing copper electrode and transition metal oxide is a technologically important subject. Nevertheless, the application of a printing Cu source/drain electrode in TMOS FETs has been rarely attempted. Woo et al. reported the fabrication of a zinc tin oxide (ZTO) FET with an inkjet-printed Cu source/drain (S/ D) contact.17 The large contact resistance between the PVPcapped Cu nanoparticle-derived Cu and ZTO resulted in a rather low field-effect mobility of approximately 0.2 cm2/(V s) and an insufficient ION/OFF modulation ratio of approximately105 in ZTO FETs. Very recently, a xenon flash lamp was utilized for fast photosintering of printed Cu films, which was adopted to fabricate amorphous indium gallium zinc oxide

1. INTRODUCTION Direct printing of conductive materials and semiconductors holds great promise for a range of electronic applications including RFID tags, flexible electronics, and information displays.1,2 An intriguing benefit is cost savings compared to traditional techniques based on expensive vacuum deposition and complicated photolithography processes. To date, the printing silver electrode has been the most intensively studied because of its low resistivity (1.59 μΩ·cm)3 and strong antioxidation properties.4 The high cost5 and low electromigration resistance6 of silver itself, however, are two critical drawbacks that hinder its practical implementation in consumer electronics because it negates the main advantage of the printing process. In this regard, copper, which is inexpensive and offers excellent electrical properties (1.68 μΩ·cm), is a good candidate electrode material.7−10 The strong oxidizing tendency of Cu nanoparticles in conductive inks and sintering processing, unlike the novel metal, is a critical issue for its use in electronics applications because the formation of a thin oxide layer on the Cu nanoparticles significantly deteriorates the electrical conductivity. For this reason, there have been many attempts to synthesize shell-type Cu nanoparticles using capping agents to prevent the oxidation and aggregation of nanoparticles.11,12 © 2015 American Chemical Society

Received: September 23, 2015 Accepted: December 30, 2015 Published: December 30, 2015 1156

DOI: 10.1021/acsami.5b08969 ACS Appl. Mater. Interfaces 2016, 8, 1156−1163

Research Article

ACS Applied Materials & Interfaces

spectra measurements of the Cu films. The chemical properties of the printed Cu films were evaluated by X-ray photoelectron spectroscopy (XPS, SIGMA PROBE, ThermoG, U.K.). For the TMOS FETs, PECVD-derived SiO2 (or SiNx) films (100 nm) on a heavily doped p-type silicon (Si) wafer were used as a gate dielectric, where the p-type Si itself acts as a gate electrode. As a semiconducting channel layer, solution-based ZTO film was prepared by spin-casting on a SiO2 (or SiNx)/Si substrate. The precursor solution was prepared by dissolving zinc nitrate hydrate (Zn(NO3)2· H2O, Aldrich) and tin chloride dihydrate (SnCl2·2H2O, Aldrich) in 5 mL of 2-methoxyethanol, which was then stirred for 4 h at 75 °C until it was completely dissolved. The concentration of the precursor was 0.3 M and the Zn/Sn molar ratio was 1:1.7. The stirred ZTO solution was filtered through a 0.2 μm syringe filter. Spin coating of the ZTO layer was performed at 1000 rpm for 15 s and then at 3500 rpm for 40 s on SiO2/Si substrates. Before the coating, the SiO2/Si substrates were cleaned sequentially with acetone, isopropyl alcohol, and deionized water for 10 min each. The resulting ZTO film was then dried at 150 °C for 5 min to evaporate the solvent followed by postdeposition annealing (PDA) at 500 °C for 1 h in air atmosphere. The Cu film as the S/D electrode was prepared by a reverse offset process. Cu ink was dispensed onto the surface of the blanket roll (TI7400ES, Teain Chemical) by a bar coater (no. 4, RDS). As the blanket was rolled over the cliché with various patterns, unnecessary ink was removed from the blanket and transferred onto the top of the cliché surface. The conventional glass cliché did not result in a good pattern transfer due to the poor adhesion between glass and Cu paste ink. By trial and error, the stainless steel material was found to have the proper adhesion with Cu ink. Thus, the 100 μm thick stainless steel foil was used as the cliché material. The remaining Cu ink, which was in the desired pattern on the blanket, was manually transferred onto the ZTO/SiO2/Si substrate (25 mm × 25 mm) in the air atmosphere. The scheme of this reverse offset process is depicted in Figure 1. The width

(a-IGZO) FETs. The intense and short time photosintering effectively prevents the Cu particles from oxidizing in air, which results in an improvement of the field-effect mobility of IGZO FETs (∼1.4 cm2/(V s)), surpassing previously reported results of ZTO FETs with a Cu contact.18 This paper reports the fabrication of high performance ZTO FETs with a printing Cu contact as an S/D electrode. Importantly, reverse offset printing, which has one of the highest resolutions and good manufacturing capability among the existing printing technologies,19−22 was used to form a Cu S/D electrode for TMOS FETs. The evolution of the morphological, structural, and chemical properties of the printing Cu film was examined as a function of the sintering temperature in the range from 200 to 400 °C. On the basis of the in-depth understanding between these physical/chemical properties and electrical characteristics, a low resistivity of approximately 6.2 μΩ·cm was achieved for the printed Cu film annealed at 400 °C. The adoption of the printing Cu film as the S/D in FETs with a solution processable ZTO channel rendered high performance transporting and switching properties.

2. EXPERIMENTAL SECTION The Cu ink with a mass density of 1.21 g/mL and a viscosity of 70 cP provided by Dongjin SemiChem Co. LTD consisted of Cu particles, an organic binder, additives, and solvent. Cu nanoparticles were synthesized via chemical reduction of Cu ions in toluene (C6H5CH3, Aldrich, 99.8%) under inert atmosphere. Oleic acid (C18H34O2, Aldrich, 99.5%) was used as a surface capping molecule, and hydrazine (NH2NH2, Junsei, 98%) was added as a reducing agent for Cu ion. Amounts of 16.6 g of Cu acetate (Cu(CO2CH3)2, Aldrich, 98%), 10 g of oleic acid, and 16.6 g of hydrazine were dissolved in 100 mL of toluene. Then the mixture solution was slowly heated to 120 °C with magnetic stirring. The reaction solution was kept at 120 °C for 30 min and then cooled down to room temperature. After synthesis, Cu nanoparticles were washed with ethanol in a centrifuge tube and vortexed for 1 min, which was followed by the separation via centrifugation. The obtained Cu nanoparticles and ethyl cellulose (48.0−49.5% ethoxyl substitution, 9−11 cP viscosity range, Dow Chemical), 3-aminopropyltriethoxy-silane (APS, C9H23NO3Si, Aldrich, 99%) were dissolved in isopropyl alcohol ((CH3)2CHOH, Kumho Chemical, 99.8%). To prevent surface oxidation of Cu nanoparticle, ethyl cellulose was used as a surface capping molecule. APS was added to the Cu ink as an adhesion promoter. The prepared Cu inks with a composition ratio of Cu/ethyl cellulose/APS/IPA = 44.1:5.5:3.0:47.4 wt % were mixed strongly with zirconia balls in a paste mixer (PDM300; Dae Hwa Tech, South Korea). The dispersed Cu nanoparticles in the Cu ink have a spherical shape where the diameter was approximately 100 nm (see Figure S1, Supporting Information). For the evaluation of the mechanical, structural, and chemical properties of the Cu films, the Cu ink was spread on a blanket and then transferred to the SiO2 (or SiNx)/Si substrate. The coated Cu film was dried at 50 °C for 10 min and then prebaked at 200 °C for 30 s in order to decompose the organic binder contained in it. It is noted that most of the Cu green-body film dried at 200 °C for 30 s in air was not oxidized due to the existence of organic capping layer surrounding the Cu nanoparticles. The surface oxidation of only ∼5 nm was observed for the dried Cu film (see Figure S2, Supporting Information). The sintering process was performed at different temperatures of 200, 250, 300, and 400 °C for 10 min under N2 ambient using rapid thermal annealing (RTA) where the heating rate was 60 °C/min. The surface topography and roughness of the printing Cu films were analyzed by scanning electron microscopy (SEM, Hitachi, S-4300). The structural properties of the sintered Cu films were examined by X-ray diffraction (XRD, X’Pert-PRO MRD, PANalytical). Fourier transform infrared (FTIR, Bruker, IFS66C/S & HYPERION 3000) spectroscopy was used for the IR absorption

Figure 1. Process flow showing the formation of the patterned Cu film by the reverse offset printing method. A three-dimensional crosssectional image of a fabricated ZTO FET and the optical image of the printed Cu film are also shown in the upper region.

(W) and length (L) of the channel region in the fabricated ZTO FETs were 1000 and 150 μm, respectively. The drying and prebaking conditions of the printed Cu films on the ZTO/SiO2/Si substrate were identical to those of the unpatterned Cu films on the SiO2 (or SiNx)/ Si substrate. The ZTO FETs were contact-annealed in N2 atmosphere for 10 min at temperatures ranging from 200 to 500 °C using rapid thermal annealing (RTA) where the ramping rate was 5 °C/min. The schematic three-dimensional structure of the fabricated ZTO FET and the optical image of the Cu film formed by a reverse offset process were also shown in Figure 1. The electrical characteristics of the FETs were measured using a Keithley 2636 source meter at room temperature. 1157


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3. RESULTS AND DISCUSSION The adhesion strengths of the Cu films on the SiO2 (or SiNx)/ Si substrates were evaluated. None of the areas among the 6 × 6 parallel cuts on either the SiO2 and SiNx gate dielectric layers substrates were removed in the adhesion test using 3M tape, as shown in Figure 2a, which can be classified as 5B. Thus, the

from regions with a high interfacial curvature to a low interfacial curvature can be greatly enhanced with increasing temperature. To obtain insight into the coexistence of copper oxide (CuOx) in the printing Cu films, XRD analysis of the various Cu films was performed. Figure 2c shows the XRD patterns of the Cu films obtained at different annealing temperatures. Irrespective of the annealing temperature, no peak assignable to copper oxide was detected. The reflections at 43.5°, 50.7°, and 74.7° are indexed to the (111), (200), and (220) of Cu, respectively.25 Therefore, these samples were confirmed to be phase-pure copper without any oxide phase such as CuO, Cu2O, or Cu(OH)2. In addition, these characteristic peaks indicate the formation of a face-centered cubic (fcc) copper phase (JCPDS no. 04-0836). As the annealing temperature was increased from 200 to 400 °C, the intensity of the (111) reflection increased and its full-width at half-maximum (fwhm) Table 1. Full-Width at Half-Maximum (fwhm) of the Cu (111) Reflection and the Grain Sizes of the Cu Films as a Function of the Annealing Temperature Cu (111) annealing temp (°C) 200 250 300 400

fwhm (deg) 0.36 0.32 0.28 0.26

± ± ± ±

0.03 0.02 0.01 0.01

grain size (nm) 258 290 332 358

± ± ± ±

20 17 11 13

Figure 2. (a) Adhesion test results between the Cu film and gate dielectric film (SiO2 and SiNx) where 6 × 6 parallel cuts were removed on both SiO2 and SiNx gate dielectric layers using 3M tape. (b) Evolution of the surface morphologies of the various Cu films. (c) XRD patterns of the Cu films annealed at different temperatures.

decreased, as summarized in Table 1. The average grain size of the Cu films was estimated using Scherrer’s equation as follows:

printing Cu films were confirmed to have good mechanical properties on traditional gate dielectric layers. The excellent adhesion property of the Cu film and SiO2 (or SiNx)/Si substrate can be attributed to the synergic effect of a silane coupling agent in APS or hydroxide (OH) group in ethyl cellulose. The silane coupling agents consist of the organic and inorganic chemical reaction chain, which may promote the adhesion strength to the inorganic dielectric film because they act as a hybrid link; even a tiny amount of silane coupling agents under the detection limit of FTIR and XPS analysis remained in the printed Cu film.23 Also hydroxyl group (OH) in ethyl cellulose used as a binder is known to facilitate the chemical bonding between the Cu film and SiO2 (or SiNx)/Si substrate.24 Indeed, the existence of OH group in the printed Cu film was clearly confirmed in the FTIR data, which will be discussed below. The morphological and structural properties of the printing Cu films on the SiO2/Si substrate were evaluated as a function of the annealing temperature for 10 min in N2 atmosphere. Figure 2b shows the evolution of the surface morphologies for the various Cu films. The Cu film annealed at 200 °C had a spherical-like topological structure (Figure 2b). As the annealing temperature increased, the grain size in the Cu film was enhanced and the coalescence of various grains in the Cu film was simultaneously observed. The disperse grains statistically distributed with a smaller grain size are thermodynamically unstable due to the large interface grain boundary area. Therefore, the grain coarsening phenomenon as a result of Ostwald ripening is quite natural because the diffusion of mass

where k is a constant assumed to be 0.98, λ is the wavelength of the incident X-rays, β is the fwhm, and θ is the reflection angle. The XRD-derived average grain size for the Cu film at 200 °C was in the range of 23.8−27.8 nm, whereas those for annealing temperatures of 300 and 400 °C were 32.1−34.3 and 34.5−37.1 nm, respectively. This enhancement of the grain size with increasing annealing temperature is consistent with the morphology evolution observations resulting from the SEM analysis. It is noted that the peak intensity of the (111) reflection for the Cu films was rapidly enhanced with increasing annealing temperature, while that of the (220) reflection for the Cu films was relatively invariant, irrespective of the annealing temperature. This result can be understood because the (111) plane oriented parallel to the plane of the Cu film is favored in the fcc structure.26 The occurrence of the preferential crystallographic orientation of (111), which has the lowest surface energy, with increasing annealing temperature is interpreted as efficient thermodynamic stabilization in terms of the total grain boundary energy. The incorporation of chemical impurities such as carbon, nitrogen, and oxygen in the printing Cu films was analyzed in the FTIR spectra. Figure 3a shows the FTIR spectra of the Cu films with different annealing temperatures. The intense peaks at 3445 and 1635 cm−1 for the Cu film annealed at 200 °C are assigned to the O−H stretching mode and CC stretching mode, respectively.27,28 Also, the peaks at 2920 and 1382 cm−1 represent the C−H asymmetric stretching and bending modes, respectively.29,30 The CC and COC bonds can be assignable to come from the ethyl cellulose, whereas the OH

D=

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carbon and nitrogen incorporation in the printing Cu film were found to decrease with increasing annealing temperature. This indicates that the RTA processing for a short time (10 min) under the reducing ambient prevents significant oxidation of the Cu grains and incorporation of hydrocarbons and nitrogen in the Cu film. Therefore, the carrier transporting properties of the Cu film annealed at higher annealing temperatures are expected to be enhanced based on the SEM, XRD, FTIR, and XPS analysis results. Figure 4a shows the variations of the sheet resistance (Rs) and resistivity (ρ) of the printing Cu films. The RS and ρ values

Figure 3. (a) FTIR spectra and (b) Cu 2p3/2 XP spectra of the Cu films obtained at different annealing temperatures.

group in the Cu films can be attributed to the addition of IPA and ethyl cellulose. Obviously, the huge intensities of the carbon, oxygen, and hydrogen impurity-derived peaks indicate that the Cu film annealed at 200 °C may not be suitable for conductor applications. Interestingly, the intensities of these oxygen, hydrogen, and carbon-related peaks in the Cu films diminished monotonically with increasing annealing temperature. With a viewpoint of the adhesion property, the hydrocarbon and OH group remaining in the Cu film are likely to facilitate the chemical bonding between the Cu film and SiO2 (or SiNx)/Si substrate. This postulation is consistent with the fact that the adhesion strength between the Cu film and SiO2 (or SiNx)/Si substrate tends to decreased with increasing sintering temperatures. To obtain insight into the surface oxidation of individual grains in the Cu film, XPS analysis was performed. Figure 3b shows the peak fitting of the Cu 2p3/2 XP spectra of the different Cu films. The peaks at 932.2 and 933.7 eV were assigned to Cu and CuO, respectively.31,32 For all Cu films, the relative peak areas of the copper lattice were larger than 91%, where the portion of the oxidation-related CuO peak was less than 9%, demonstrating the high purity of the Cu films. It can also be seen that the CuO peak slightly increased from 3.8% at 200 °C to 8.4% at 400 °C, suggesting that the degree of surface oxidation of each Cu grain in the film can be enhanced at higher annealing temperatures, as summarized in Table 2.

Figure 4. (a) Sheet resistance (Rs) and (b) resistivity (ρ) values of the printing Cu films as a function of the sintering temperatures.

for the Cu film annealed at 200 °C under N2 atmosphere were approximately 218 mΩ/sq and 43.6 μΩ·cm, respectively. A 3fold smaller RS value of 67.3 mΩ/sq was obtained for the Cu film annealed at 250 °C. The RS value for the Cu film annealed at 300 and 400 °C was further reduced to 46.0 and 31.7 mΩ/ sq, respectively, which can be translated to a ρ value of 9.8 and 6.2 μΩ·cm. The oxidation degree of the Cu film was shown to increase with increasing sintering temperature (see Cu 2p and O 1s XPS data), which hinders the transport of electron carriers under the given electrical field. Nevertheless, the ρ value of the Cu film decreased with increasing sintering temperatures, suggesting that the other factors such as the structural necking, grain growth, and purification play a critical role in determining the overall ρ value of the printed Cu films. On the basis of the morphological evolution from the small spherical shape (200 °C) to the larger coalescence grains (250 °C) in the Cu film, this improvement in the ρ values from 200 to 250 °C can be attributed to the better connectivity of electrical percolation pathway in the Cu film. The Cu films annealed at 300 and 400 °C appeared to have a similar enough networked morphology to exhibit the sufficient conduction pathway. Therefore, the superior electrical resistivity of the Cu film annealed at 400 °C to that at 300 °C can be understandable in terms of the enhanced densification of the Cu film itself. Besides, the concentration of unwanted impurities such as C, O, and H in the Cu films was reduced monotonically with increasing annealing temperature, which is partially responsible for this annealing temperature-dependent resistivity dependence. The ρ value of 6.2 μΩ·cm for the printing Cu film is comparable with or even lower than those (9.1−96 μΩ·cm) reported previously in the literature for solution-based Cu films.33−36 It should be noted that rapid thermal annealing is used in the metallization process in the production line of the active matrix of flat panel displays due to its good manufacturing capability and short tact time.

Table 2. Comparison of the Cu 2p Peak from the XPS Spectra of the Cu Films with Different Annealing Temperatures Cu 2p3/2 peak (eV) annealing temp (°C)

Cu (932.24 ± 0.02 eV)

CuO (933.74 ± 0.02 eV)

200 250 300 400

0.962 0.949 0.930 0.916

0.038 0.051 0.070 0.084

Generally, impurities such as carbon and nitrogen come from the Cu nanoparticle precursor and organic solvent and act as carrier scattering centers under the application of an electric field. Thus, they reduce the mean scattering time and electrical conductivity of the resulting Cu film. Figure S3 in Supporting Information shows the C 1s and N 1s XP spectra of the different Cu films. Electrically unwanted impurities such as 1159


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exhibited marginal performance with μFE, SS, VTH, and ION/OFF values of 0.86 cm2/(V s), 1.46 V/decade, 6.7 V, and 5 × 104, respectively (see Figure 5a). Improvements of the mobility and current modulation ratio were observed for the solution processed ZTO FETs with the printing Cu S/D electrode contact-annealed at 250 °C. The μFE, SS, and ION/OFF values were improved to 2.6 cm2/(V s), 1.4 V/decade, and 2 × 105, respectively, for the device contact-annealed at 250 °C (Figure 5b) except for the similar VTH of 7.0 V. The μFE value of 2.6 cm2/(V s) is 10 times higher than that (0.23 cm2/(V s)) reported previously for the solution processed ZTO TFTs with printed Cu contact.17 However, further increasing the contact annealing temperature up to 400 °C in N2 atmosphere resulted in a loss of the drain modulation capability of the resulting device, as shown in Figure 5c. The rapid increase (10−7 A) in the off-state drain current of the device at 400 °C can be attributed to either the thermal degradation of the ZTO film or the adverse Cu migration during the contact annealing. It is noted that the contact annealing for the ohmic contact between the oxide channel layer and S/D conductive electrode in the metal oxide TFTs is usually performed under air or O2 atmosphere. However, the air ambient annealing is undesirable due to the unintentional oxidation of the Cu film. For this reason, the contact annealing was performed under reducing N2 atmosphere as mentioned earlier. Therefore, the effect of the annealing atmosphere on the electrical property of the ZTO films was examined using Hall effect measurement. All the spincast ZTO films on the SiO2/Si substrates were subjected to the PDA at 500 °C under air atmosphere. Then, the sputtered ITO electrode (150 nm) was formed using a shadow mask for the van der Pauw configuration. These samples were finally annealed in the air and reducing N2 atmosphere at the temperatures ranging from 250 to 500 °C. It corresponds to the contact annealing for fabrication of the ZTO TFTs. In case of the air atmosphere, the free electron concentration (Ne) values of the ZTO films (∼1017 cm−3) were independent of the contact annealing temperatures, as shown in Figure 6a. On the

The conducting Cu film can be applied to the various electronics devices such as photovoltaic cells, RFIDs, flexible electronics, and active matrix displays. In this study, a printing Cu film was adopted as the source/drain electrode for metal oxide field-effect transistors. As described in the Experimental Section, the Cu S/D electrode was formed additively by using reverse offset printing on a metal oxide/SiO2/Si substrate. It should be emphasized that a ZTO or IZO channel layer was also deposited by using a low cost solution-based process. Figure 5 shows the representative transfer and output characteristics of the solution processed ZTO FETs with the

Figure 5. Representative transfer and output characteristics of the solution processed ZTO FETs with the printing Cu S/D electrode contact-annealed at (a) 250 °C, (b) 400 °C, and (c) 500 °C under N2 atmosphere.

printing Cu S/D electrode at the contact annealing temperatures of 200, 250, and 400 °C under N2 atmosphere. The fieldeffect mobility (μFE) was determined from the incremental slope of the IDS1/2 vs VGS plot in the saturation region using the following equation:37 IDS =

⎛ WCi ⎞ ⎜ ⎟μ (V − VTH)2 VDS ⎝ 2L ⎠ FE GS

Figure 6. Free electron concentration (Ne) values of the ZTO films as a function of the contact annealing temperatures under (a) air atmosphere and (b) N2 atmosphere.

other hand, the Ne values of the ZTO films annealed under N2 atmosphere increased with increasing contact annealing temperature: these values for the ZTO films at the contact annealing temperature (≥400 °C) were >1019 cm−3 (Figure 6b). It suggests that the shallow donors such as the oxygen vacancy defect can be created during the thermal annealing under the reducing N2 atmosphere.37,38 Or the thermal desorption of the adsorbed oxygen species on the ZTO back surface would be responsible for the transition to the

(1)

where L is the channel length, W is the width, and Ci is the gate capacitance per unit area. VTH is defined as the gate voltage that induces a drain current of 1 nA at VDS = 5.1 V. The subthreshold gate swing (SS = dVGS/d log IDS [V/decade]) was extracted from the linear part of the log(IDS) vs VGS plot. The solution processed ZTO FETs with the printing Cu S/D electrode at a contact annealing temperature of 200 °C 1160


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Figure 7. Representative transfer characteristics of the ZTO FETs with the sputtered ITO S/D electrode contact-annealed at (a) 250 °C, (b) 400 °C, and (c) 500 °C under N2 atmosphere. Transfer characteristics of the ZTO FETs with the sputtered ITO S/D electrode contact-annealed at (d) 250 °C, (e) 400 °C, and (f) 500 °C under air atmosphere.

conducting state of the ZTO film because the thermodesorption of the oxygen species would release the free electron carriers into the ZTO channel layer.39 Therefore, the high drain current level in the negative VGS region can be attributed to the high Ne value of the ZTO channel layer, which results from the creation of oxygen vacancy defects as a result of the high temperature annealing (400−500 °C) in reducing N 2 atmosphere. Therefore, the optimal solution processed ZTO TFTs with the printing Cu S/D electrode can be obtained at an annealing temperature of 250 °C. It would be interesting to compare the device performances for the ZTO TFTs with the developed Cu and sputtered standard ITO contact. Figure 7 shows the transfer characteristics of the ZTO TFTs with the sputtered ITO contact, which were contact-annealed at 250, 400, and 500 °C under reducing N2 and air atmosphere. The device contact-annealed at 250 °C under N2 atmosphere exhibited μFE, SS, VTH, and ION/OFF values of 5.4 cm2/(V s), 1.06 V/decade, 2.5 V, and 1 × 107, respectively (Figure 7a). In contrast, the devices contact-annealed at higher temperature (≥400 °C) suffered from the loss in the drain current modulation capability (Figure 7b and Figure 7c), which can be also attributed to the creation of the shallow VO defects under reducing N2 atmosphere. The undesirable thermal effect into the ZTO semiconductor can be prevented by performing the contact annealing under air atmosphere (see Figure 7d−f). The ZTO TFTs with the ITO S/D exhibited comparable μFE (4.8−4.9 cm2/(V s)), SS (0.30−0.54 V/decade), VTH (2.2−2.6 V), and ION/OFF ratio (≥107) irrespective of the contact annealing temperatures under air atmosphere (also see Table S1 in Supporting Information). It is evident that the contactannealing (or sintering) temperature and atmosphere of the printed Cu electrode for the ZTO TFTs should be carefully chosen with a viewpoint of the antioxidation of Cu film and prevention of the VO creation in the ZTO semiconductor. The inferior performance (μFE of 2.6 cm2/(V s), ION/OFF of 2 × 105) of the ZTO TFTs with the printed Cu contact at the identical contact annealing condition (250 °C under N2 atmosphere) compared to those (μFE of 5.4 cm2/(V s), ION/OFF of 1 × 107) of the device with the sputtered ITO contact suggests that the inevitable diffusion of Cu atom beneath the S/D contact region

adversely affected the electrical properties of the resulting TFTs. The depth profile concentration of Cu atom into the ZTO film on the SiO2/Si substrate was investigated by XPS analysis. The penetration of non-negligible Cu atom into the underlying ZTO film was observed for the ZTO film even at the contact annealing temperature of 250 °C as shown in Figure S4 in Supporting Information. This adverse migration of Cu atom into the ZTO channel layer deteriorated the performance of the ZTO TFTs with printed Cu contact compared to those with sputtered ITO contact. This result is consistent with the fact that the Cu atom in the metal oxide semiconductors such as IGZO, ZTO acts as either trap states or carrier scattering center.40,41 Finally it is noted that the hysteresis loop in the transfer characteristics of the ZTO TFTs with the Cu S/D electrode contact-annealed at 200 °C was clockwise, indicating that the electron carriers accumulated in the ZTO channel layer are likely to be trapped at the bulk trap states of ZTO semiconductor. Interestingly, this hysteresis phenomenon was negligible for the ZTO TFTs with the sputtered ITO contact as shown in Figure 7. The result suggests that the hysteresis for the ZTO TFTs with the Cu contact at 200 °C can be attributed to the migration of the impurities such as C, H, and N near the ZTO channel where they can be acting as the electron trap center.42 Indeed, the hysteresis was reduced substantially for the ZTO TFTs with the Cu contact at 250 °C because the chemical impurities such as C, H, and N in the Cu film were largely eliminated due to the elevated contact annealing temperature.

4. CONCLUSION The mechanical, structural, and electrical properties of Cu films prepared by reverse offset printing were investigated in detail as a function of the annealing temperature. The Ostwald ripeninginduced coalescence and simultaneous purification of the Cu films with increasing annealing temperature allowed the printed Cu film to possess an excellent electrical conductivity (∼6.2 μΩ·cm). This result is also due to the antioxidation of Cu nanoparticles in the Cu film, which was facilitated by a RTA process in a reducing N2 ambient. The feasibility of this 1161


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ACS Applied Materials & Interfaces promising Cu printing film was confirmed in the ZTO and IZO FETs, where the printing Cu film was used as the S/D electrode. The solution processed ZTO FETs with the printing Cu S/D electrode annealed at 250 °C exhibited values of μFE, SS, VTH, and ION/OFF to be 2.6 cm2/(V s), 1.4 V/decade, 7.0 V, and 2 × 105, respectively; this suggests that the low cost solution processed ZTO semiconductor can be used in conjunction with the printing Cu S/D electrode in practical FETs.

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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.5b08969. SEM image of Cu nanoparticles for the formation of Cu ink, the concentration depth profiles (Cu, O, and C) for the Cu green-body film, the C 1s and N 1s XP spectra for the sintered Cu films, the concentration depth profiles (Cu, Zn, Sn, and O) for the Cu/ZTO/SiO2/Si samples annealed under N2 atmosphere, and the electrical performance summary for the ZTO FETs with the sputtered ITO S/D electrode (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the industrial strategic technology development program funded by MKE/KEIT under Grant 10041808 and the research fund of Hanyang University (Grant HY-2015).

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REFERENCES

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High Performance Metal Oxide Field-Effect Transistors with a Reverse

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