UV-Cured Reactive Mesogen-YInZnO Hybrid Materials as Semiconducting Channels in Thin-Film Transistors Using a Solution-Process

The Recently, amorphous oxide semiconductors (AOSs) have been researched extensively for a use as the channel layer in thin-film transistors (TFTs),^1–3 which exhibit superior electrical and optical characteristics to amorphous Si (a-Si) TFTs. a-Si TFTs have a relatively low mobility of approximately 0.5 cm²/Vs because of the angular distortion of the strongly directive sp³ orbital.¹ In contrast, AOSs have a different carrier-path mechanism that permits them to achieve high mobility, as demonstrated by Nomura et al.¹ They proposed a unique mechanism: direct overlapping between neighboring s orbitals that have insensitive angular distortion because of the chemical bonding between heavy-metal ions, such as the In in the InGaZnO (IGZO) system. Thus, despite the amorphous phase, AOSs have mobility that is almost comparable to that of the crystalline phase.⁴ Previously, Shin et al. studied using YInZnO (YIZO) for the TFT channels by replacing the Ga in the IGZO system with Y.⁵ Y has a lower standard electrode potential (SEP) (−2.372 eV) and a lower electronegativity (1.22) than those of Ga; therefore, Y can easily form chemical bonds with oxygen, thereby decreasing the oxygen vacancies.

Solution-processed TFTs have low electrical performance in terms of characteristics such as low mobility and high sub-threshold swing (S.S) because they contain many pores and pin-holes, which lead to electron trapping.^6–8 Therefore, reactive mesogen (RM) was added to the YIZO system to obtain high electrical performance by inducing the channel material to become positioned along the source-drain direction. When a hybrid YIZO film is irradiated with UV radiation, a uniaxial directivity of the channel material is formed because RM is sensitive to UV light.^9–11

In this study, we produced hybrid YIZO TFTs using a solution-process with and without UV irradiation and investigated the role of UV-irradiated RM in the hybrid YIZO system in attaining high electrical performance.

A 0.1 M YIZO solution was prepared using yttrium nitrate hexahydrate [Y(NO3)3·6H2O], indium nitrate hydrate [In(NO₃)·xH₂O], and zinc acetate dihydrate [Zn(CH₃COO)₂·2H₂O] in 2-methoxyethanol (2ME). The molar ratio of Y:In:Zn was fixed at 2:11:7. To ensure a stable and homogeneous solution, we added mono-ethanolamine (MEA) and acetic acid as stabilizers. The solution was stirred at 75°C for 2 h and then aged for 1 day. Prior to the deposition, parallel-aligned RM(RMS03–015, Merck) was added to the YIZO system making a weight ratio of RM and YIZO to be 1:9. We used [120 nm SiO₂] substrates fabricated on a p⁺-Si wafer, and we spin-coated the hybrid YIZO solution onto these substrates to form the channels of the TFTs. After the channel deposition, the samples were pre-annealed at 300°C for 5 min, and then annealed at 500°C for 2 h on a hot plate. To investigate the role of the aligned RM, the hybrid YIZO film was irradiated with UV for 20 min via a 436-nm filter and a polarizer in the source-drain direction. We deposited Al for the sources and drains of TFTs via evaporation using a 150-μm shadow mask.

Figures 1a and 1b show field-emission scanning electron microscope (FE-SEM) images of the hybrid YIZO films with and without UV irradiation, respectively. Figures 1c and 1d show an atomic force microscope (AFM) images. In the case of the hybrid YIZO film that was cured under UV irradiation, the crystal size is larger than that of the other film. This difference indicates that the UV irradiation affects the alignment of the film surface by reacting to the RM in the YIZO system. The surface roughness was measured using an AFM, and the corresponding values for the hybrid YIZO films with and without UV treatment were found to be 5.779 nm and 6.034 nm, respectively. This result implies that the application of UV radiation makes the film smoother.

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To investigate the role of the UV-irradiated RM in the YIZO system, X-ray photoelectron spectroscopy (XPS) measurements were performed. Figures 2a and 2b show the O 1s peaks of the hybrid YIZO films with and without UV irradiation, respectively. These spectra consist of three main peaks centered at 530.1 eV, 531.2 eV, and 532.1 eV, as determined via Gaussian fitting. These peaks correspond to stoichiometric oxygen in the hybrid YIZO films (O1), oxygen vacancies in the hybrid YIZO films (O2), and the oxygen loosely bonded at the surfaces of the hybrid YIZO films (O3).¹² The O2/O1 ratio indicates the relative number of oxygen vacancies, and the values of this ratio are 24.65% and 51.86% for the hybrid YIZO films with and without UV irradiation, respectively. These values can be explained by the fact that the oxygen vacancies, which are the origin of the low mobility, were decreased by the application of UV irradiation in the hybrid YIZO system because the UV-cured film affects the channel alignment and densifies the channel.^12,13

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We then measured the optical retardation of the hybrid YIZO films with and without UV irradiation to trace the aligned channel material, as shown in Figure 3. Optical-retardation measurements can evaluate and be used to determine the anisotropy of hybrid YIZO films with and without UV irradiation.^14,15 The films are not crystalline, but involved reactive mesogen can be aligned to a specific direction and this gives the film the anisotropy.¹⁸ The results of the optical-retardation measurements indicate that the retardation was increased by UV irradiation of the film. Consequently, the UV-cured films exhibited enhanced anisotropy, which can lead to high electrical performance of the hybrid TFTs.

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Figure 4 shows the transfer characteristics of the hybrid YIZO TFTs with and without UV irradiation. Both TFTs were prepared as n-channel TFTs. When no UV irradiation was applied to the hybrid YIZO TFT, the V_TH was shifted from 10.96 V to 5.51 V, and the mobility was decreased because of the large reduction in the on current and its random direction in the channel material. These results indicate that UV-irradiated RM affects the channel structure by inducing the electrons to move along the direction from the source to the drain. The saturated mobility (μ_SAT) of each hybrid YIZO TFT with and without UV irradiation were found to be 1.26 cm²/Vs and 0.15 cm²/Vs, respectively, and the values of the S.S for the hybrid YIZO TFTs with and without UV irradiation were found to be 0.88 V/decade and 0.94 V/decade, respectively. We obtained each value using the following equations:

where I_DS, V_GS, C, L, and W are the drain current, gate voltage, capacitor, channel length, and channel width of the hybrid YIZO TFTs, respectively.

Equations 1, 2, and 3 were used to determine the μ_SAT, V_TH, and S.S values, respectively.

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Figures 5a and 5b show the mechanism of the UV-cured hybrid YIZO TFT and that of hybrid YIZO TFT that was not UV treated, respectively. In the case of the UV-cured hybrid YIZO TFT, the UV-irradiated RM makes the electrons flow along the source-drain direction. Therefore, the value of V_TH, which is related to the charge trapping in the channel, gate insulator, or the interface between them, is low. Under the UV irradiation, the RM-included channel material was aligned along the source-drain direction, thereby allowing the electrons to pass through the channel more easily than in the other TFT. The S.S value is strongly related to the interface trap density.^16,17 If the S.S value is high, it implies that the interface trap density is also high, and it causes the degradation of certain aspects of the TFT performance, such as mobility. As the hybrid YIZO system was UV irradiated, the structure of the channel was densified, and the trap density was reduced compared with that of just-deposited hybrid YIZO TFT.

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In conclusion, Y can serve as a substitute for Ga in the IZO system (as a carrier suppressor) because of its low electronegativity and low SEP. When RM is added to this system under UV irradiation, the surface roughness is lowered because of the high film density. As a result, the S.S value becomes small because the electrons are less frequently trapped in the hybrid YIZO channel, the SiO₂ gate insulator, or the interface between them.