Effective atmospheric-pressure plasma treatment toward high

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Functional Inorganic Materials and Devices

Effective atmospheric-pressure plasma treatment toward high performance solution-processed oxide thin-film transistors Jintaek Park, Jae-Eun Huh, Sung-Eun Lee, Junhee Lee, Won Hyung Lee, Keon-Hee Lim, and Youn Sang Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11111 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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ACS Applied Materials & Interfaces

Effective atmospheric-pressure plasma treatment toward high performance solution-processed oxide thin-film transistors Jintaek Park†,‡, Jae-Eun Huh†, Sung-Eun Lee†,‡, Junhee Lee†, Won Hyung Lee†, Keon-Hee Lim*,†,§ and Youn Sang Kim*,†,∥

†Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea ‡Samsung Display Company, Ltd, 181 Samsung-ro, Tangjeong-myeon, Asan-si, Chungcheongnam-Do, Republic of Korea. §Department of Mechanical Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States ∥Advanced Institute of Convergence Technology, 145 Gwanggyo-ro, Yeongtong-gu, Suwon 16229, Republic of Korea

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KEYWORDS: atmospheric-pressure plasma, oxide semiconductor, thin-film transistor, solution process, low temperature process


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ABSTRACT Solution-processed oxide semiconductors (OSs) have attracted much attention because they can simply, quickly and cheaply produce transparent channels on flexible substrates. However, despite such advantages, in the fabrication process of OS TFTs using the solution process, it is fatal problem that there are hardly any way to simply and effectively control important TFT parameters including turn-on voltage (Von) and on/off current ratio. For the practical application of solution-processed OS TFT, the approaches to simply and effectively control the parameters are urgently required. Here, we newly propose an atmospheric-pressure plasma (APP) treatment which can simply and effectively control the electrical properties in solution-processed InOx TFTs. Through exposure of APP, we successfully realized the change in important TFT parameters of solution-processed InOx TFT, Von from -11.4 V to -1.9 V and the on/off current ratio from ~103 to ~106, which still keep up the high field-effect mobility (> 20 cm2/Vs). Based on various analyses such as X-ray based analysis and UV-Visible spectroscopy, we identified that the APP treatment can effectively control oxygen vacancy and carrier concentration in solution-processed OS.


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1. Introduction Oxide semiconductors (OSs) have attracted great interest as a channel layer of thin-film transistor (TFT) in future display due to their good electrical and unique properties such as high optical transparency and flexibility.1-3 In particular, low-temperature solution-processed OSs have been widely investigated because they can be fabricated quickly and cheaply on a flexible substrate by advanced processes including ink-jet printing and the roll-to-roll process.4,5 Recently, high-performance solution-processed OS TFTs with high field-effect mobility can be realized through various novel methods such as sol-gel on chip, combustion, and ultraviolet (UV) radiation. 6-8 In particular, the aqueous route method has realized solution-processed indium oxide (InOx) TFTs with a remarkably high field-effect mobility (> 10 cm2/Vs) despite being fabricated with a low temperature (< 300 °C) process, which has shown new possibilities for practical application of solution-processed OS TFTs.9,10 However, despite the excellent electrical properties with high field-effect mobility, the application of a solution-processed OS TFTs still remains a difficult task as a practical technology due to the major disadvantages of current ratio. In conventional display technology, the Von and on/off current ratio are as important as the field-effect mobility of the TFT because they are strongly related to power consumption of the device in practical industrial application. The Von and on/off current ratio of TFT are determined in consideration of various factors such as thickness, trap density, gate metal type and carrier concentration of a semiconductor. Among them, in practical TFT applications, the Von and the on/off current ratio are mainly controlled by the carrier concentration of a semiconductor. For this reason, the control of carrier concentration of semiconductor is essential for the practical application of solutionprocessed OS TFTs.


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In vacuum-processed OS TFTs such as indium gallium zinc oxide (IGZO) TFT used in display technology, gallium atom in IGZO has been used to control the carrier concentration of OSs because it has a stronger bonding with oxygen than indium or zinc.11,12 In general, the addition of gallium atoms to OSs lowers the mobility of the TFTs. Since the vacuum-processed OS TFT has a sufficiently high field-effect mobility to compensate for the decrease in mobility, the decrease is not so fatal when applying vacuum-processed OS TFTs to display technology. However, the addition of gallium in the solution-processed OS TFT cannot be a simple solution. Although solution-processed OS TFTs with a high field-effect mobility have recently been developed with various methods, they still show much lower field-effect mobility in comparison to vacuum process OS TFTs because they have many defects due to the limitations of a solution process. Accordingly, the decrease in field-effect mobility by the addition of gallium can be fatal because the solution-processed OS TFTs fail to achieve enough high mobility that is required for application in practical display technology.13,14 As another method, conventional plasma treatment in vacuum conditions has been studied for controlling the carrier concentration of OSs, since oxygen plasma treatment controls the oxygen vacancy associated with the carrier concentration in OS. Several studies have demonstrated that conventional vacuum plasma treatment improves the electrical properties of TFTs with Von and on/off current ratio depending on the carrier concentration.15-17 Unfortunately, conventional oxygen plasma treatment essentially requires the vacuum conditions to activate the plasma state, which makes it difficult to apply the conventional plasma treatment to solution-process OSs because the introduction of a vacuum process strongly loses the advantages of the solution process, such as the formation in ambient conditions. Accordingly, there is a growing demand for a technique capable of adjusting


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the carrier concentration suitable for application to high-performance solution-processed OSs in a practical industry. Herein, we newly propose an effective atmospheric-pressure plasma (APP) treatment for simple and effective control of the carrier concentrations and important TFT parameters of solution-processed OS TFTs, such as Von and on/off current ratio. We have successfully implemented the improvements in important TFT parameters, Von from -11.4 V to -1.9 V and the on/off current ratio from ~103 to ~106, which still keep up the high field-effect mobility (> 20 cm2/Vs), by introducing the APP treatment into solution-processed InOx TFTs fabricated at low temperature condition (~ 250 °C). Based on various analyses such as X-ray photoelectron spectroscopy (XPS), electrical characterization, and UV-Visible spectroscopy, we identified that the APP treatment can effectively control oxygen vacancy and carrier concentration in solutionprocessed OS. To the best of our knowledge, this is the first introduction of the APP treatment to faithfully control the electrical properties of the solution-processed OS TFTs while maintaining the advantages of the solution process. These results show that the APP treatment has a great potential for significantly modulating the electrical properties of solution-processed OS TFTs.

2. RESULTS AND DISCUSSION Figure 1 shows the schematic of the solution-process OS TFTs fabrication process. InOx films were prepared by a spin coating process with precursor solutions (0.1, 0.15, 0.2, 0.25, and 0.3 M) and an annealing process of 250 °C with a tube furnace. They showed various thicknesses of 4.9, 6.5, 8.7, 10.6, and 12.9 nm, respectively (Figure S1). Then, the prepared InOx thin films are posttreated by the APP treatment. For an effective APP treatment, several key variables including film thickness, substrate temperature, plasma source power and treatment time must be carefully


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considered. Considering the changes in the electrical characteristics of the InOx TFTs, due to the thinner thickness of the InOx films, the greater change in electrical properties of the InOx TFT was induced by the APP process (Figure S2). However, at 10 nm or less, the effected changes were not significantly different. Accordingly, in this study, a thickness of less than 10 nm thin film coated with 0.15 M InOx precursor solution was mainly focused because it exhibits the highest mobility while suppressing the decrease of mobility by the APP treatment. Also, in view of the several key variables of the electrical characteristics of the InOx TFTs depending on the APP treatment, experimental conditions including the substrate temperature (150, 200, and 250 °C), plasma source power (500, 600, and 700 W), and APP treatment time (1, 3, 5, and 7 min) were changed. All of these variables affected the electrical properties of the InOx TFTs, but the effect of APP treatment time was the most obvious (Figures S3, S4). Thus, to effectively analyze the effect of APP treatment, the APP treatment time was controlled as a main variable under optimized conditions with substrate temperature (200 °C) and APP source power (700 W). Figure 2 and Figure S5 show the chemical changes in the InOx film depending on the APP treatment. The XPS analyses show a consistent tendency for a change in the O 1s spectrum of InOx films depending on the APP treatment time. For further analysis, the O 1s spectrum was fitted by the Gaussian-Lorentzian function. The O 1s spectrum of InOx thin films could be divided into three peaks centered at 532, 531.15, and 529.8 eV which were assigned to the oxygen bond of the hydroxide (O-H bond), the oxygen vacancy (Ovac), and metal oxide without oxygen vacancy (M-O bond), respectively.8,18 The Ovac/(Ovac + M-O bond) area ratios for APP-untreated and APP-treated InOx thin films are 0.342 (untreated), 0.305 (3 min), and 0.267 (7 min) (Table S1). It can be confirmed that the oxygen vacancy of InOx is reduced by the increase of APP treatment time, which means that APP effectively affects the chemical


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properties of solution-processed InOx thin films. Typically, in the OSs, oxygen vacancy acts as a shallow donor, one of the main factors controlling the carrier concentration of OSs and the electrical properties of OS TFTs. Actually, in InOx TFT transfer characteristics in the linear region with the gate-source voltage (Vgs) sweep from -30 to 70 V and drain voltage (Vd) of 10 V, it can be confirmed that physical parameters of the InOx TFT including Von and on/off current ratio were prominently changed according to the changes in APP treatment time (Figure 3). As APP treatment time is increased, the Von and on/off current ratio was significantly changed. The APP-treated TFTs are shown as Von of -10.7 V for 1 min, -5.8 V for 3 min, -4.8 V for 5 min, and -1.9 V for 7 min. Here, Von means Vgs value where drain current (Id) starts to increase. Also, the APP-treated TFTs are shown as an on/off current ratio of ~104 for 1 min, ~104 for 3 min, ~105 for 5 min, and ~106 for 7 min. Compared with the untreated InOx TFTs with Von of -11.4 V and on/off current ratio of ~103, the APP-treated InOx TFTs show the strongly improved Von and on/off current ratio values. Meanwhile, although the magnitude of the change is not large, field-effect mobility of InOx TFTs is slightly decreased by the APP treatment. The field-effect mobility of InOx TFTs with 26.1 cm2 V-1 s-1 decreased to 24.3 cm2 V-1 s-1 for 1 min, 23.7 cm2 V-1 s-1 for 3 min, 23.6 cm2 V-1 s-1 for 5 min, and 22.1 cm2 V-1 s-1 for 7 min. Importantly, the decrease in field-effect mobility of the InOx TFT by the APP method is considered to be relatively small in comparison with the decreasing behavior of the field effect mobility of the InOx TFT according to the doping method for improving the on/off current ratio and Von of the OS TFT. We speculate that the APP treatment has more effect on oxygen vacancies forming deep traps which is far from the conduction band minimum (CBM) than oxygen vacancies acting as a shallow donor state close to CBM, which strongly affects the


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change of field effect mobility.19 The changes of electrical properties coincide with the tendency to be induced by the change of oxygen vacancy contents, which implies that the reduction oxygen vacancy by the APP treatment effectively decrease the carrier concentration of the InOx thin film. A summary of statistical values of electrical characteristics obtained from the currentto-voltage characteristics are listed in Table 1. To verify the change in carrier concentration of InOx thin film according to the APP treatment, the behavior in activation energy and optical band gap of InOx according to the APP treatment was investigated. To extract the activation energy, the transfer characteristics of the InOx TFTs were obtained at 133, 173, 213, 253 and 298 K under a vacuum environment (Figure S6). Activation energy (Ea) can be obtained by field-effect mobility and ln(µFET)-1000/T plot depending on Vgs and temperature, as shown in Figure 4a-b. At the same Vgs condition (10 V), it is confirmed that the activation energy of InOx increases in accordance with the APP treatment processing time, which means that the Fermi level of InOx TFT is shifted by the APP treatment. It can be seen that the longer the plasma treatment time, the lower the Fermi level of InOx. Typically, in the n-type OS based TFT, when the Vgs increases, the localized states are filled with accumulated carriers and the Fermi level is closed with the mobility edge, which results in the decrease of the activation energy.20,21 Actually, at higher Vgs conditions, APP-treated InOx has similar activation energy in comparison to untreated InOx. Through the plasma treatment, it can be confirmed that the activation value of InOx at each Vgs moves almost perfectly parallel to the Vgs direction. The results show that the carrier concentration of InOx is decreased by the APP treatment without changing the localized states near the Fermi level. Generally, as the trend of optical band gap in InOx depending on the APP treatment is similar to that of the activation energy for OS TFTs, we analyze the change of optical band gap in InOx


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depending on the APP treatment. The transmittance spectra of the InOx thin films on quartz substrate are shown in Figure 4c. As compared to untreated InOx, slight red shifts in the transmittance of APP treated InOx were observed. To investigate the phenomenon in more detail, the optical band gap is extracted by the Tauc relation22,23 as follows: (αh ν )m = A(hν− E g )

where hν is the photon energy and A is a constant. Although the m value of 1/2 is commonly used for non-direct semiconductor, in this study, we applied the m value of 2, which is used in direct semiconductor model, to examine the small changes of optical band gap in more detail. The variation of the (αhν)2 versus the photon energy in the absorption region is plotted for the samples in Figure 4d, and extrapolation of linear fit to the photon energy axis at (αhν)2 =0 gives the optical band gap. As a result, it can be seen that the longer the plasma treatment time, the smaller the optical band gap of InOx. According to the Burstein-Moss effect, because the doubly occupied state is limited by the Pauli principle and the optical transition should be vertical, extra energy is required to enable the valence electrons to move to higher energy states in the conduction band, which causes the increase of optical band gap according to the increase of carrier concentration.24-27 In this respect, it is deduced that the decrease in the optical band gap of InOx is induced by the decrease in the carrier concentration in InOx with the APP treatment, which agrees with the tendency of electrical characteristic change in InOx TFTs according to the APP treatment. Overall, analyses with the behavior in activation energy and optical band gap of InOx according to the APP treatment imply that the carrier concentration of InOx is decreased by the APP treatment, which is consistent with the tendency of oxygen vacancies of InOx to change with the APP treatment, and is confirmed by XPS analysis. This shows that possibility the APP


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treatment can control the carrier concentration of the OSs as the APP treatment controls the oxygen vacancy of the OSs. Generally, in the display industry, conventional APP is mainly used for surface treatment. In fact, it has been reported that the electrical properties of IGZO fabricated by a vacuum process are not greatly changed when treated with conventional APP. Otherwise, our results show that APP can significantly control the electrical properties of solution-processed OS TFTs. We may conclude that the APP has a significant effect on the solution-processed OSs because the density of solution-processed semiconductors is relatively low. Actually, the low density and thin thickness, ~ 10 nm, of solution-processed OS shows other advantages in application to flexible substrates.21,28 In this part, in-depth research is ongoing.

3. CONCLUSIONS In summary, we newly introduce the potential of an APP treatment that can simply and effectively control the carrier concentration of solution-processed OS thin film. With an effective APP treatment, we demonstrate that important electrical properties for power consumption, such as Von and the on/off current ratio, of solution-processed InOx TFT fabricated at low temperature (~ 250 °C) can be controlled in atmosphere conditions while maintaining the high field-effect mobility (> 20 cm2/Vs). Furthermore, because the Von converges to 0 V and at the same time the on/off current ratio is maximized (~106), this APP technique can provide significant advances toward industrial applications of solution-processed OS TFTs, while maintaining the advantages of the solution process.


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4. METHOD 4.1. Synthesis and Characterization of Precursor OS Solution: InOx precursor solutions (0.1, 0.15, 0.2, 0.25, and 0.3 M) were prepared by dissolving aluminum nitrate hydrate in 10 mL of 18.2 MΩ deionized water, and then the solution was stirred at 500 rpm for 3 days at 25 °C. The precursor solution was filtered through a 0.22 mm syringe filter PTFE, and was spin coated onto a heavily doped P-type Si wafer with 200 nm thick SiO2 gate dielectric layer at a speed of 5000 rpm for 30 s under a humidity of 10 %. Then the films were soft-baked at 250 °C for 60 s, and were annealed in a tube furnace under

Effective atmospheric-pressure plasma treatment toward high

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