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Performance Enhancement of ZITO Thin Film Transistors via Graphene Bridge Layer by Sol-Gel Combustion Process Jianhua Zhang, Panpan Dong, Yana Gao, Chenhang Sheng, and Xifeng Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07148 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 20, 2015

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ACS Applied Materials & Interfaces

Performance Enhancement of ZITO Thin Film Transistors via Graphene Bridge Layer by Sol-Gel Combustion Process Jianhua Zhang, Panpan Dong, Yana Gao, Chenhang Sheng and Xifeng Li* Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, Shanghai 200072, China KEYWORDS: graphene, ZITO, bridge layer, sol-gel, thin film transistors

ACS Paragon Plus Environment

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ABSTRACT

In this article, we reported the stacked structure Zinc-Indium-Tin oxide (ZITO) thin-film transistors (TFTs) with Graphene nanosheets (GNSs) prepared by solution process. GNSs were used as bridge layer between dual-ZITO layers. The transmission of stacked ZITO/GNSs/ZITO films are more than 80% in the visible region and the resistivity of ZITO films with GNSs bridge layer decreased from 502.9 Ω•cm to 13.4 Ω•cm. The solution-processed TFT devices with GNSs bridge layer exhibited a desirable characteristic with a subthreshold slope of 0.25 V/dec and current on-off ratio of 107, and the saturation filed effect mobility is improved to 45.9 cm2V-1s-1, which exceeds the mobility values of the pristine ZITO TFTs by one order. These results demonstrate the solution-processed ZITO/GNSs/ZITO TFTs maybe make a further step to achieve high performance TFTs and show the potential for next-generation applications.


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1. INTRODUCTION Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) are attractive for application in active-matrix flat panel displays due to their superior performance, including high carrier mobility and high optical transparency, compared with conventional amorphous silicon TFTs.1 Among those AOS TFTs, zinc-indium-tin oxide (ZITO) TFTs were the most promising devices because of high electron mobility of more than 100 cm2V-1s-1, which is high enough to compete with that of low temperature ploy-silicon (LTPS) TFTs (50-100 cm2V-1s-1).2,3 As known to all, the AOS films can be manufactured by magnetron sputtering, pulsed laser deposition, etc, which all were based on vacuum processes; or by ink-jet, screen printing, slit-coating and spincoating which were based on wet solution process.4-8 The wet solution-based method is considered as a key part of next generation processing methods due to simplicity, high throughput, low-cost, uniformity, composition-controlled and large area manufacturing.8 However, it is still a tough challenge for solution processed AOS TFTs to achieve high performance in following parameters, such as high mobility, precipitous sub-threshold voltage swing (SS), high on/off ratio and low threshold voltage in comparison with vacuum-fabricated AOS TFTs.9-14 Plenty efforts had been made to improve the performances of solution processed AOS TFTs.1417

Fortunately, it had been reported that the injection of materials like nanotubes, nanowire and

graphene to AOS films can be a potential approach to improve the performance of AOS TFTs.15,18-22 Dai et al.15 reported that solution-based Indium Gallium Zinc Oxide (IGZO) TFTs with an addition of graphene nanosheets (GNSs) achieved a high performance and improved the mobility about 30 times higher than those of the pristine IGZO while off current had the same order of magnitude. This demonstrate that GNSs doping in the oxide thin films may improve the


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performance of AOS TFTs due to the excellent electrical properties of GNSs.23-24 AOS TFTs with GNSs stacked structure (AOS/GNSs/AOS) were possible to obtain unique performances, however, few investigations focus on the aspect. Therefore, the development of GNSs stacked structure AOS TFTs were meaningful and necessary. In this paper, ZITO/GNSs/ZITO stacked structure TFTs have been attempted to fabricate. The performance of both the ZITO TFTs and the stacked structure TFTs were studied comparatively. The origin of the improved device performance and the role of GNSs in the stacked structure TFTs were investigated intensively. 2. Experimental Section Precursor Solution Synthesis: An HfO2 solution (0.3 M) was synthesized by dissolving hafnium dichloride oxide octahydrate (HfOCl2·8H2O) in 2-methoxyethanol (2-MOE) solvent. The 0.3 M ZITO solution was synthesized by dissolving zinc acetate dehydrate (Zn(OC2H4)2·2H2O), indium chloride (InCl3), and tin chloride pentahydrate (SnCl4·5H2O) in 2MOE. The molar ratio of Zn: In: Sn was fixed at 1: 4: 4. Next, monoethanolamine (MEA) as a stabilizing agent was added into the mixed metal salt solutions, maintaining the molar ratio of monoethanolamine and total metal concentration in 2 : 1. The precursors stirred in the water bath of 70 oC for 3 h, and then aged for 12 h. The precursors were filtered through a 0.22 µm microfilter before spin-coating. The GNSs precursor solution was prepared by dissolving graphene nanosheets in 2-MOE, and stirred more than 72 hours by ultrasonic vibration technique until there were free of precipitation. The concentration of resultant solution is 0.01mol/L. Fabrication of films and TFTs: The stacked TFTs were fabricated using the bottom-gate topcontact configuration on Corning EXG glass substrate of 200 mm x 200 mm. First, a 50nm thickness ITO was sputtered and patterned as the gate electrodes. The resistivity of the


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deposited-ITO was 43 Ω/□. After that, HfO2 dielectric layer was spin-coated on that substrate, and baked at 120 oC for 15 min. Then an annealing process was carried out for 1 h at 270 oC. Procedures above were repeated several times to obtain the desired thickness. Subsequently, thin films of ZITO and GNSs were spin-coated at a speed of 3000 rpm in ambient air onto the HfO2 dielectric layer, respectively. The active layer was formed as follow: a single-layer ZITO film was spin-coated at first, then one GNSs layer, and at last another single-layer ZITO film was spin-coated. And in the process each film from the active layer was baked at 150 oC, and then annealed at 270 oC under ambient air. The thickness of the as-coated ZITO/GNSs/ZITO films was about 50nm. Finally, the as-deposited ITO was patterned as source and drain electrodes. All the function layers were patterned by optical lithography and wet etching, and all the TFTs devices were annealed at 500 oC finally. The channel width (W) and length (L) of these TFTs were 20 and 4 µm, respectively. Characterization Methods: Surface morphology characteristics were checked with optical microscope (OM, Canon) and imaged by atomic force microscopy (AFM, Nanonavi, SPI-400). The composition and chemical states of ZITO or stacked ZITO/GNSs/ZITO films were examined by Raman spectroscopy (JY, LabRam-1B), UV-visible spectroscopy (Hitachi, U3900H), X-ray photoelectron spectroscopy (XPS, Thermofisher, WSCALAB). The Hall mobility, carrier concentration and conductivity were measured by Hall measurement system (Lakeshore, 8400). The electrical characteristics of TFT devices were performed using semiconductor parameter analyzer (Agilent, 4155C) with a probe station (LakeShore, TTP4). 3. Results and Discussion 3.1 Film Characteristics. The fabrication of ZITO/GNSs/ZITO TFTs were schematically illustrated in the figure 1. It was obvious that the GNSs could be easily distinguished at the


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channel region from the zoom image, compared with a pristine ZITO channel optical microscopy (OM) image on the left. And also it could be considered the graphene nanoflakes were homogenized distributed in the film since the distribution of GNSs in the OM image.

Figure 1. Schematic illustration of the fabrication process of the solution-based stacked ZITO/GNS/ZITO thin film transistors (TFTs) and OM images of the channel region of ZITO/ZITO and ZITO/GNSs/ZITO TFTs. Figure 2a and 2b show the surface AFM morphology images of pristine ZITO and stacked ZITO/GNSs/ZITO films, respectively. The surface morphology of both films was very smooth, and the root-mean-square (RMS) roughness values of those films are appropriate for device fabrication. The little protrusion in figure 2b could attribute to the existence of GNSs which thickness was about only 2.8 nm, and hardly effected the RMS value (RMS = 0.19 nm). Moreover, it also indicates that the GNSs had homogeneous distribution in the solution and films. The optical transmission spectra of the stacked ZITO/GNSs/ZITO and ZITO/ZITO films


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are shown in figure 2d, both types of films have high optical transparency in a widely range (300-900 nm), and the transmission is more than 80% in the visible range, these results were promising with regard to the applicability of these materials for transparent displays.


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Figure 2. Films characteristics. (a)-(b): AFM images of pristine ZITO and stacked ZITO/GNSs/ZITO films, respectively. (c) Raman spectra of pristine ZITO and stacked ZITO/GNSs/ZITO films on glass substrate. (d) Optical transmittance spectra of pristine ZITO and stacked ZITO/GNSs/ZITO films. (e)-(f): XPS depth-profiling spectra of C1s of pristine ZITO and stacked ZITO/GNSs/ZITO films, respectively. (g)-(h): XPS fitting spectra of C1s of ZITO/GNSs/ZITO films with the etching time of 160s and 180s, respectively. Figure 2c shows the Raman spectroscopy of the stacked ZITO/GNSs/ZITO and ZITO/ZITO films that prepared on the glass substrate. The observed G peak at 1580 cm-1 corresponds to the high-frequency E2g phonon at Г, and D peak at 1335 cm-1 corresponds to the breathing modes of six-atom rings and require a defect for its activation and 2D peak, the second-order two-phonon mode, located at 2670 cm-1, separately, indicated the successfully synthesis of GNSs film. D peak comes from transverse optical phonons around the Brillouin zone corner K and active by double resonance. When the double resonance happens as an intravalley process, it turns to socalled D’ peak.25 Otherwise, the appearance of little sharp D and D+D’’ peaks which showed the deformation in graphene structure could attribute to the hydroxylation (or hydrogen-related) and oxidation at the edge of GNSs which turned them to specific functional graphene nanosheets.24,26-28


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Graphene has excellent electron mobility, and the functional GNSs could provide extra carriers in the semiconductor layer due to the modified group at the edge, which is favorable for improving the TFTs performance. For extensively understanding the differences of the active layer between ZITO/GNSs/ZITO and ZITO/ZITO TFTs, XPS depth-profiling analysis has been performed (Figure S1). Figure 2e-2f are the XPS depth-profiling illustration of C1s for both pristine ZITO/ZITO and stacked ZITO/GNSs/ZITO films (Figure S2). The surface C1s intensity of ZITO/GNSs/ZITO and pristine ZITO film are shown at figure S6, and it is clearly observed that the surface intensity of C1s from both types of films were almost the same, as it is considered the C comes from surface adsorption. In figure 2e, the change of C1s intensity in the ZITO/GNSs/ZITO film along with the increase of etching time clearly indicated the existence of GNSs layer and the position. When the etching time reached 160 s and 180 s, the intensity of C 1s suddenly increased, before or after these points, the C 1s intensity of ZITO/GNSs/ZITO films were similar to the C 1s intensity of ZITO/ZITO films. However, the C1s intensity in pristine ZITO film was relatively low and hardly varied along with etching time, and residual organic solvent was considered as the possible C source. The C 1s peak could be typically divided into several binding energy peaks located at 284.8 eV for C-C, 286.3 eV for C-OH, 287.3 eV for C=O and 288.7 eV for HOC=O bonds.28 In the figure 2g and 2h, the spectra of the C1s peaks of the ZITO/GNSs/ZITO film with the etching time of 160 s and 180 s were fitted with the four peaks which correspond to CC, C-OH, C=O and HO-C=O, respectively. The C-C peaks were clearly observed, which is complementary to the results of Raman spectra. In addition, the oxidation and hydroxylation at the edge of the GNSs also has been demonstrated from the C-OH, C=O and HO-C=O peaks.


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Since the functional GNSs were not zero band gap, they becomes increasingly metallic with concentration of charge carriers at zero energy gradually increases, and there appear several types of electron-like and hole-like carries.24 During the on state, gate voltage enables the injection of electrons into the active layer to form the channel, and the electrons travel through ZITO/GNSs/ZITO films, they inject into the graphene bridge layer and then exit to get into ZITO film repeatedly. It was considered that the GNSs have formed a fast transfer or conductive bridge layer in the channel area. 3.2 Transfer Characteristics. Accordingly, the addition of GNSs could make the TFT devices output higher current since electrons can utilize the conductive path.22The results are shown as below. Figure 3 plots the transfer characteristics of stacked ZITO/GNSs/ZITO and pristine ZITO TFTs. The transfer characteristics were measured under drain voltage (VDS) = 1 V while gate voltage (VGS) sweep from -5 to 20 V. The picture of the devices arrays was shown in figure S3. Both types of TFTs behaved as n-channel transistors and operated in the enhancement mode, which entered the turn-on state by applying a positive gate bias. This indicates that a number of excess electrons were accumulated in the active channel layer and formed the accumulation region.15 The saturation current under a gate voltage of 15 V improves from 6.77×10-5 A for pristine ZITO TFTs up to 3.96×10-4 A for the stacked ZITO/GNSs/ZITO TFTs, which reveals a better electrical performance by the introduction of GNSs bridge layer between ZITO films. However, the off-state current showed the same order of magnitude due to the isolation of GNSs as the GNSs fraction is fitted between dual ZITO films, which means during off state the deplete channel region disconnect the electron transport among these homogeneous distribution graphene nanoflakes.


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Figure 3. Transfer characteristics of pristine ZITO and stacked ZITO /GNSs/ZITO TFTs at VDS = 1 V. The GNSs/ZITO/ZITO and ZITO/ZITO/GNSs structure TFTs were also attempted to fabricate and their transfer characteristics are shown in figure S4 and figure S5, respectively. It is noted that the graphene nanoflakes directly contact with the surface of HfO2 dielectric in the GNSs/ZITO/ZITO structure TFTs, and the device performance deteriorated remarkably. This may be ascribed to the increasing number of the density of the interfacial trap states and defects between HfO2 and graphene nanoflakes interface which lead to more electrons to deplete and higher SS. When graphene nanoflakes directly contact with the source and drain electrodes in the ZITO/ZITO/GNSs structure TFTs, the drain current (Ids) could be observed as nearly a horizontal line which indicated the surface layer of active layer could be considered as a resistance which was caused by the high conductivity of graphene. The phenomena imply that the introduction of GNSs bridge layer between ZITO films be a promising way to improve the performance of the oxide TFTs. GNSs interlayer may be worked as a bridge between dual-layer ZITO films. And GNSs are good transport medium for both electron and hole, thus, the carriers in the active layer could be exchanged faster at the stacked films which could be confirmed by Hall measurement under Van der Pauw method. Hall


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measurement results of ZITO films are listed in the table 1. Carrier concentration of ZITO films increased almost two order when ZITO films introduced the GNSs bridge layer, meanwhile, the carriers mobility of ZITO films increases from 13.4 to 37.3 cm2V-1s-1 and the resistivity of ZITO films decreases from 502.9 to 13.4 Ω·cm. Table 1. Electrical characteristics of pristine ZITO and stacked ZITO/GNSs/ZITO films. Samples

µsat [cm2V1s-1]

Pristine ZITO

3.6

Stacked 45.9 ZITO/GNSs/ZITO

Ion/Ioff

SS [V·dec-1]

µhall [cm2V1s-1]

Carrier Resistivity concentration [Ω·cm] [1014cm-3]

6.0

~106

0.60

13.4

9.26

502.9

3.3

~107

0.25

37.3

125

13.4

Vth [V]

3.3 O 1s XPS spectrum. It was found that the threshold voltage of stacked ZITO/GNSs/ZITO TFTs exhibited a negative shift. The negative shift of threshold voltage in stacked ZITO/GNSs/ZITO TFTs indicated the increase of electron concentration in channel region, which would lead to a more negative threshold voltage to deplete the channel.29 The negative shift of threshold voltage was consistent with the results of Hall measurement, which can be confirmed further by O1s XPS analysis. In principle, the performance of oxide semiconductor materials strongly depends on the O1s bond. O1s chemical shift of stacked ZITO/GNSs/ZITO and pristine ZITO films was analyzed by XPS depth-profiling, as shown in the figure 4. The O 1s peak could be typically divided into three binding-energy peaks located around 530.21 eV for O2- binding with Zn, In, Sn (O-I); 531.36 eV for deficient oxygen (O-II) and 532.35 eV for -OH (O-III). It was observed that the areas of O-II peaks and O-III peaks increased in the stacked ZITO/GNSs/ZITO films with the increase of etching time to 160 s and 180 s, while those showed a stable trend at a relatively low level in the pristine ZITO films, as shown in the figure 4d, 4e, 4j and 4k, the etching time of 160


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s and 180 s correspond to the position of GNSs layer which was verified according to the figure 2f. Both types of films showed a relatively high content of O-II at the surface from the figure 4b and 4h. As hydrogen is more electronegative than metals, the oxygen atoms in the M-OH/C-OH species are less negatively charged than those in the oxide, leading to a shift toward a higher binding energy. O-II peaks are ascribed to deficient oxygen that generates electron carriers, and the larger areas of O-II in the oxide films indicate the higher carrier concentration of oxide films. XPS results manifest that introduction of interlayer GNSs in the ZITO films be conducive to improve the electrical properties of the ZITO films. Moreover, the ZITO films above and under the GNSs bridge layer still had a really low areas of O-II peaks and O-III peaks. That means the GNSs film could act as not only a bridge that connect the carrier transport paths between the ZITO films due to their high-conduction properties but also a carrier source which would provide extra electrons. The accumulated induced charge carriers flow between the semiconductors and the fast transported functioned GNSs, which means the effective channel resistance of carrier transport between the source to drain electrodes can be reduced.30-31 The shift of threshold voltage and the increase of carrier concentration confirmed the XPS analysis simultaneously.


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Figure 4. XPS depth-profiling spectra of O1s of pristine ZITO and ZITO/GNSs/ZITO films. (a)(f): spectra of the pristine ZITO film and fitting peaks. (g)-(l): spectra of the ZITO/GNSs/ZITO film and fitting peaks. ZO and GO represent pristine ZITO and ZITO/GNSs/ZITO, respectively. Since the GNSs act as transmission path between the channel region of ZITO TFTs, the efficient carrier transport tracks are further shortened, leading to higher mobility that increases


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from 3.6 to 45.9 cm2V-1s-1. In other words, the ZITO and GNSs are complementary to each other in the stacked thin films. The GNSs provide the fast carrier transportation paths and the ZITO fills the space around the GNSs and connects them to form a continuous composite thin film, leading to enhanced carrier concentration from 9.26×1014 to 1.25×1016 cm-3.18 Thus the stacked ZITO/GNSs/ZITO TFTs demonstrated more preferable performance as compared to pristine ZITO TFTs. And as shown in figure S5, the completed device on the 200 mm x 200 mm glass substrate, the similar results have been obtained during the test. The subthreshold slope (SS), a key parameter represents the defects at interface, can be calculated as follow:

SS = (

d log(I DS ) −1 ) dVGS (1)

And Nt at the interface could be determined by using the following equations: Nt = [

C SS log(e) − 1] i (kT / q ) q (2)

wherek, e, T, q and Ci are Boltzmann constant, the Euler’s constant, temperature, quantity of single electron and gate capacitance, separately. The SS values of GNSs/ZITO stacked TFTs and pristine ZITO TFTs calculated from equation (1) were 0.25 V/dec and 0.60 V/dec respectively. Based on the SS value and equation (2), we could have the Nt of ZITO/GNSs/ZITO stacked TFTs and pristine ZITO TFTs, expressed as 2.1×1012 cm-2 and 5.8×1012 cm-2. The SS value related to the Nt between the semiconductor and the gate dielectric. According to the calculated Nt, there was significantly different can be found between GNSs/ZITO stacked TFTs and pristine ZITO TFTs. The difference of SS between GNSs/ZITO stacked TFTs and pristine ZITO TFTs


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can be explained through the structural engineering and the smoother interface (RMS = 0.19 nm), which lead to remarkably reduced interface trap density. The distribution of structural defects could affect the electron path of accumulated electrons, and here the filler layer with dispersed GNSs which provided faster carrier transport tracks and shorten the channel length, also leading to reduced SS value.14

4. Conclusions In summary, we had successfully developed a solution-based ZITO/GNSs/ZITO stacked TFTs with excellent electronic properties, high transparency and smooth interface. According to the Raman spectra and X-ray photoelectron spectroscopy spectra, it was found that the oxidation and hydroxylation at the edge of GNSs which transformed the GNSs into specific functional graphene nanosheets after 500 oC annealing in atmosphere ambient. We demonstrated the stacked TFTs with a high field effect mobility of 45.9 cm2V-1s-1, the high on/off current ratio of ~107 and relatively low SS of 0.25 V/dec. Therefore, the solution-processed ZITO/GNSs/ZITO film may be a potential candidate as a solution-processable active layer for achieving excellent parameter comprehensively.


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ASSOCIATED CONTENT

Supporting Information. Additional XPS depth-profiling spectra of O 1s, XPS depth-profiling schematic illustration. Picture of the TFT devices arrays on Corning glass. Additional transfer characteristics. Additional XPS depth-profiling of C 1s. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no completing financial interest. ACKNOWLEDGMENT This work is supported by the Shanghai Science and Technology Commission under Grant 13520500200 and 14XD1401800.


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