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Photochemically Activated Flexible Metal-Oxide Transistors and Circuits Using Low Impurity Aqueous System Jae-Sang Heo, Jae-Hyun Kim, Jaekyun Kim, Myung-Gil Kim, Yong-Hoon Kim, and Sung Kyu Park
Abstract— High mobility flexible metal-oxide thin-film transistors and circuits have been fabricated on an ultrathin plastic substrate using environmentally benign aqueous solution system and low-temperature photochemical activation process (∼150 °C). Results show that the indium–gallium–zinc oxide (IGZO) thin-film transistors (TFTs) fabricated from nitrate-based precursors in aqueous solution outperform the devices from acetate-based precursors in alcohol solution. Here, IGZO TFTs and seven-stage ring oscillators are demonstrated on a 3∼5 µm-thick polyimide substrates with an average mobility of >6.9 cm2 /V-s, subthreshold slope of ∼0.14 V/decade, and oscillation frequency of ∼340 kHz corresponding to 210 ns of propagation delay per stage at a supply bias of 20 V. Index Terms— Aqueous solution, photochemical activation, sol-gel, indium-gallium-zinc oxide, thin-film transistor, circuit.
I. I NTRODUCTION
R
ECENTLY, binary to quaternary metal-oxide (MO) semiconductors such as zinc oxide, indium-galliumzinc oxide (IGZO), and zinc-tin oxide have been extensively studied as an active semiconducting layer in thin-film transistors (TFTs) for active-matrix organic light-emitting diode displays and transparent electronics [1]–[5]. The MO semiconductors are particularly attractive as they enable formation of dense solid films with decent stability via a relatively low process temperature using vacuum deposition methods [6], [7]. Furthermore, many researchers are also trying to develop solution-processed MO semiconductors which are amenable to industrial thin-film manufacturing techniques such as spin-casting, slot-die coating, and roll-to-roll printing [8]. Although, some of the solution-processed binary MO semiconductors such as ZnO have been successfully fabricated Manuscript received December 8, 2014; accepted December 11, 2014. Date of publication December 18, 2014; date of current version January 23, 2015. This work was supported in part by the Industrial Strategic Technology Development Program under Grant 10045269, in part by the Ministry of Trade, Industry and Energy (MOTIE)/Korea Evaluation Institute of Industrial Technology, and in part by the Technology Innovation Program through the MOTIE under Grant 10047756. The review of this letter was arranged by Editor A. Nathan. J.-S. Heo, J.-H. Kim, J. Kim, and S. K. Park are with the School of Electrical and Electronics Engineering, Chung-Ang University, Seoul 156-756, Korea (e-mail:
[email protected]). M.-G. Kim is with the Department of Chemistry, Chung-Ang University, Seoul 156-756, Korea. Y.-H. Kim is with the School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2014.2382136
with a low-temperature [9], most of the ternary and quaternary MO semiconductors still require high process temperature (>350 °C) to achieve a dense and stable solid structure with reasonable electrical performance and operational stability. Lots of efforts have been made to lower the processing temperature by applying various chemical and physical approaches including photochemical activation process which utilizes deep ultraviolet (DUV) irradiation for densification and condensation of MO semiconductor films [1], [10]–[12], allowing scalable and large throughput productions using cheap solution-processes. For a low-temperature sol-gel process, however, complete removal of chemical impurities such as carbon-related compounds becomes an important issue since the low process temperature may not fully deliver chemical reactions to remove the unwanted chemical impurities. For this reason, precursor systems with low carbon contents are highly demanded in low-temperature sol-gel processes while maintaining the electrical performance and device stability. Here, we report high performance flexible MO TFTs and circuits using photochemically activated IGZO films derived from an aqueous system with nitrate-based metal precursors. Compared to carbon containing precursor system, the aqueous system lead to improved electrical performance which can be attributed to lower impurity components in the solution and preferred reaction with photo irradiation. II. E XPERIMENTAL For low-temperature photochemical activation, two types of solvent systems have been prepared; alcohol- and aqueousbased systems using 2-methoxyethanol (2-ME) and deionized water (DIW) as a solvent, respectively. In each solvent system, metal salt precursors of indium nitrate hydrate and gallium nitrate hydrate were dissolved with a fixed composition ratio. For a zinc precursor, zinc acetate dihydrate or zinc nitrate hydrate has been used. The molar concentrations of indium, gallium, and zinc precursors were fixed as 0.085 M, 0.0125 M, and 0.0275 M, respectively, based on our previous work [1]. For the fabrication of IGZO TFTs, a heavily doped Si wafer with 200 nm-thick thermally grown SiO2 was used as common gate electrode and gate dielectric, respectively. On the SiO2 , IGZO active layer was formed by spin coating and photochemical activation in N2 atmosphere for 2h using a DUV light source (low pressure mercury lamp with main
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HEO et al.: PHOTOCHEMICALLY ACTIVATED FLEXIBLE METAL-OXIDE TRANSISTORS AND CIRCUITS
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Fig. 1. (a) UV/vis absorption spectra of alcohol- and aqueous-based IGZO precursor solutions using zinc acetate dihydrate (ZA), zinc nitrate hydrate (ZN) as a zinc precursor, and (b) nitrogen and carbon impurity contents in the IGZO films using different precursor and solvent systems.
wavelength of 254 nm (90%) and 185 nm (10%)). The thickness of the active layer was around 9∼10 nm. The atomic compositions of the photo-annealed ZN-based IGZO films were analyzed by X-ray photoelectron spectroscopy (XPS). After patterning the IGZO, indium-zinc oxide layer was deposited by RF-magnetron sputtering as source/drain electrode and patterned by lift-off process. The channel width and length of the TFTs were 100 μm and 10 μm, respectively. For flexible device fabrication, a 3∼5 μm-thick polyimide (PI) film was used as a substrate. On a carrier glass substrate, PI solution (Polyzen 150PI & Picomax) was coated by spin coating. Afterwards, the film was pre-baked at 100 °C for 2 min and finally post-annealed at 300 °C for 1 hour. On the PI film, Cr gate electrode was sputter deposited and patterned. On the gate electrode, a 32 nm-thick Al2 O3 gate dielectric layer was deposited by atomic layer deposition process at a substrate temperature of 100 °C (pressure of 2 Torr). The active layer and source/drain electrode formation processes are identical to above described. III. R ESULTS AND D ISCUSSION The UV/vis absorption spectra of IGZO precursor solutions prepared with ZA and ZN precursors are shown in Fig. 1(a) for both alcohol- and aqueous-based solvent systems. From the results it can be found that the solutions with ZN precursor have more broad absorption in the UV region compared to the solutions with ZA precursor. The nitrate ion in more UV susceptible aqueous ZN precursor will be easily dissociated into volatile NOx species, which generates reactive oxygen radicals for final oxide lattice formation. In addition, at the final film formation stage, low impurity content is highly demanded. Therefore, we traced nitrogen and carbon contents in the photo-activated IGZO films depending on the precursor systems. Figure 1(b) shows the identified impurity contents obtained from XPS analysis. As shown here, the nitrogen content was noticeably decreased when using ZN precursor and DIW. It is known that presence of carbon sources with nitrate anions can induce nitrogen containing impurities due to incomplete decomposition [13], which explains the highest nitrogen content in the IGZO film fabricated from the ZA/2-ME precursor solution (4.67 at%).
Fig. 2. (a) Transfer, and (b) output characteristics of photochemically activated IGZO TFTs using ZA/2-ME and ZN/DIW precursor solutions, and (c) XPS O1s peaks of IGZO films from ZA/2-ME and ZN/DIW precursor solutions.
In contrast, the lowest nitrogen content of 3.07 at% was observed for ZN/DIW precursor, which has minimal carbon source in the solution. The carbon contents, however, were low and almost identical in all samples (∼0.1 at%). The In/Ga ratio is important in determining the electrical properties of IGZO films [14]. From the XPS analysis, it was estimated that the In/Ga ratio of 6.8 from precursor solutions were consistent for IGZO films obtained from ZN/DIW and ZN/2-ME precursor solutions. Figure 2(a) and 2(b) show the transfer and output curves of photochemically activated IGZO TFTs fabricated using ZN/DIW and ZA/2-ME as precursor and solvent system. The devices fabricated from the ZN/DIW system showed enhanced saturation field-effect mobility of 3.01 cm2 /V-s compared to ZA/2-ME system having an average mobility of 2.35 cm2 /V-s. Summarized electrical properties shown in Table I more clearly illustrates the enhancement of electrical performance by using ZN precursor and aqueous solvent system. This enhanced electrical performance can be attributed better M-O-M network formation by using the aqueous solvent system. As shown in Fig. 2c, the IGZO films from an aqueous solvent system had a larger portion of M-O-M oxygen binding states (71.0∼73.5%) compared to those from an alcohol solvent system (69.4%). In addition, facile and carbonaceous contamination-free conversion of aqueous ZN precursor can be also responsible for the improved electrical performance. Compared to acetate in ZA precursor, the nitrate in ZN precursor is more easily decomposed without potential carbon contamination [15]. Also, the IGZO films derived from the aqueous route are likely to have less amount of residual impurities originated from the solvent molecules, such as 2-ME. Therefore, better M-O-M network formation and less nitrogen or carbon-related impurities are expected from the ZN/DIW system, improving the electrical performance. Moreover, in the chemical reaction aspects, the large dielectric constant of water more effectively screens the charges between cations (such as Zn2+ , − In3+ , and Ga3+ ) and anions (such as NO− 3 and CH3 COO ), and thus weakens the columbic force between them [16], [17].
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TABLE I E LECTRICAL P ROPERTIES OF P HOTOCHEMICALLY A CTIVATED IGZO TFTs
and photochemical activation (DUV) process. Easy decomposition of ZN precursors due to more broad absorption of the irradiated photons and the fewer amounts of residual impurities in aqueous system may lead to desired M-O-M lattice. These results show that good quality flexible MO devices and circuits can be fabricated with environmentally benign low impurity solvent system via photon energy delivered green process. R EFERENCES
Fig. 3. (a) Transfer characteristics and an optical micrograph (inset) of flexible IGZO TFTs, (b) field-effect mobility distribution and a schematic device structure of IGZO TFTs fabricated on PI film (inset), (c) output signal from a 7-stage ring oscillator operating with a supply voltage (VDD ) of 20 V, and (d) comparison of simulation and experimental data of 7-stage ring oscillators using Al2 O3 as a gate dielectric layer and an optical image of a 7-stage ring oscillator (inset).
Using the aqueous precursor solution with ZN precursor, flexible IGZO TFTs and 7-stage ring oscillators were fabricated on ultrathin spin-on PI substrates as shown in Fig. 3(a). With the Al2 O3 gate dielectric layer (Ci = 134 nF/cm2 ), IGZO TFTs with an average field-effect mobility of 6.95 cm2 /V-s and subthreshold slope of ∼0.14 V/decade were achieved. Figure 3(a) and (b) show typical transfer characteristics and statistical data for the field-effect mobility, respectively. The output signal from a 7-stage ring oscillator with an output buffer operating with a supply voltage (VDD ) of 20 V was measured and shown in Fig. 3(c). The β-ratio of the inverter in the ring oscillator was 10 with an overlap distance between the gate and source/drain electrodes of 5 μm. Figure 3(d) shows the comparison of the simulation (AIM-SPICE, ref. [18]) and experimental results of 7-stage ring oscillators, and it was observed that the simulation results show much higher oscillation frequencies comparing to the experimental data. IV. C ONCLUSIONS In summary, high performance IGZO TFTs were fabricated by green process using nitrate precursor (ZN) aqueous system
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