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Solution-derived YZO films were investigated as liquid crystal (LC) alignment layers modified by ion beam (IB) irradiation. Solution processing was adopted in place of the sputtering method for the deposition of YZO films as LC alignment layers. Uniform and homogeneous LC alignment was achieved to produce a high performance LC system. X-ray photoelectron spectroscopy analysis showed that surface reformation of YZO films by annealing and IB irradiation affects the uniformity of the LC alignment. Superior electro-optical characteristics of twisted nematic LC cells constructed from IB-irradiated YZO films were observed, which indicates that the proposed solution-derived YZO films have strong potential for use in the production of advanced LC displays.
© 2014 Optical Society of America
1. Introduction
The uniform alignment of liquid crystal (LC) molecules is considered a core technological requirement for the fabrication of liquid crystal displays (LCDs). Intensive research on the use of organic or inorganic films as LC alignment layers has been conducted to obtain uniform LC alignment using various alignment techniques [1–18].
The rubbing method is conventionally used to induce the alignment of LC molecules on a polyimide (PI) layer. However, this method has significant drawbacks due to the contact that occurs during the technique, including the accumulation of electrostatic charge and the generation of fine dust [3–9]. As a result, alternative methods of LC alignment based on non-contact processes have been investigated to resolve these shortcomings. These alternatives include the photoalignment technique [3,4], nanoimprint lithography [7], oblique vapor [8] and sputtering deposition [10,11], and ion beam (IB) [5, 6, 12–15] and plasma beam [16–18] irradiation.
Among these techniques, collimated IB irradiation of the substrate surface is a non-contact alignment process that uses Ar+ ion-induced plasma to control the manufacture of high-resolution displays in a continuous process. This technique has been used to align LC molecules on both organic and inorganic films and has been intensively investigated using optically transparent inorganic materials, such as diamond-like carbon [4–8], SiOx [15], SiNx [20], Ta2O5 [21] and La2O3 [27], as a replacement for conventional PI layers.
In this study, we demonstrate the use of a solution-derived YZO film treated with IB irradiation as a LC alignment layer. Solution processing is simpler, has a higher throughput, allows easier compositional modification, and costs less than vacuum deposition methods [22]. Among foreign element-doped ZnO films, Y-doped ZnO (YZO) has been of specific interest because of its superior optical and electrical properties [28–32]. Homogeneous LC alignment was achieved on solution-derived YZO films by adjusting the annealing temperature. The peak shifts and bonding energy transitions of the IB-irradiated YZO films were investigated by X-ray photoelectron spectroscopy (XPS). The main reason that yttrium and zinc oxide films were chosen is that they have a relatively large band gap and excellent breakdown voltage [33]. Additionally, the films demonstrate superior electro-optical (EO) characteristics when driving LCs. The alignment film properties and EO properties of twisted nematic (TN) LCDs were measured using various methods.
2. Experimental
YZO thin films were prepared by solution processing on ITO-coated glass substrates (Samsung Corning 1737: standard 32 × 22 × 1.1 mm3, sheet resistance 10 Ω □−1). Prior to deposition, the ITO-coated glass substrates were cleaned by ultrasonication in a solution of trichloroethylene, acetone, methanol, and deionized water for 10 min and then dried with N2 gas.
A YZO solution (0.1 M) was prepared using yttrium nitrate hexahydrate [Y(NO3)3•6H2O] (Sigma-Aldrich Co.) and zinc acetate dihydrate [Zn(CH3COO)2•2H2O] (Sigma-Aldrich Co.) dissolved in 2-methoxyethanol (2ME) with acetic acid (AA) and monoethanolamine (MEA) as stabilizers to obtain a homogeneous solution. The molar ratio of Y:Zn was fixed at 1:9. The solution was stirred for 5 h at 75°C and aged for at least 1 day.
The YZO solution was spin-coated on indium tin oxide (ITO)-coated glass, which was then heated at 200°C for 10 min to remove any residual solvent. The films were subsequently annealed at 100, 200, 300, 400, and 500°C in a furnace for 2 hours. All IB-treated YZO films were irradiated with Ar+ IB plasma (104 to 105 ions cm−2 at an IB current of 1.6–4.9 mA cm−2) for 2 min at an incident angle of 45° and an irradiation energy of 1800 eV using a DuoPIGatron-type IB system [19].
The cells were fabricated in an anti-parallel configuration with a cell gap of 60 μm to observe the pre-tilt angle, and twisted nematic (TN) cells were prepared with a cell gap of 2.5 μm to examine the optical transmission and response time characteristics before injecting the LCs (Δn = 0.0946, Δε = 10.7). The LC was injected into the empty LC cells in the isotropic state using capillary force.
Voltage transmittance (V-T) and response times were investigated using an LCD evaluation system (LCD-700; Otsuka Electronics, Japan). Images of the LC alignment state and cells were obtained using polarized optical microscopy (POM, Olympus BXP51, Japan). Physical and chemical modifications of the YZO films before and after IB irradiation were analyzed using an atomic force microscope (AFM; XE-Bio, Park Systems) and XPS (ES-CALAB 220i-XL, VG Scientific). EO characteristics of the TN cells were measured using an LCD evaluation system (LCMS-200, Sesim).
3. Results and Discussion
Figure 1(a) shows photomicrographs of LCs prepared in antiparallel cells with an annealing temperature of 500°C and IB irradiation of various intensities for an incident angle of 45° and an incident time of 2 min. At an IB irradiation energy of 1.2 keV, the LCs were randomly aligned, defects were present, and light leakage was observed. At an IB incident energy of 1.8 keV, the LCs were uniformly aligned, and thus, the image is totally black without defects. When the irradiation energy was sufficient, the photographic images appeared to be uniform, confirming that the LC molecules were aligned parallel to the IB exposure direction on the YZO surface. Otherwise, the images were not uniform but exhibited irregular bright and dark regions, indicating that the LC molecules were poorly aligned. As a result of these findings, the irradiation energy was fixed at 1.8 keV.
Fig. 1 (a) Photomicroscopic images of LC cells with YZO films annealed at T = 500°C and treated with IB irradiation at 0.0, 0.6, 1.2, 1.8, and 2.4 kV. (b) Photomicroscopic images of LC cells with YZO films annealed at 100, 200, 300, 400 and 500°C treated with IB irradiation at 1.8 kV. (c) The transmittance vs incident angle curves for YZO films annealed at 100, 200, 300, 400 and 500°C.
Figure 1(b) shows photomicrographs of the fabricated YZO films. At annealing temperatures greater than or equal to 200°C, the LC molecules were oriented uniformly and a homogenous LC alignment was observed. The LCs were well-aligned without local defects immediately after the LC cell was fabricated.
As shown in Fig. 1(c), transmittance profiles over a range of incident angles from −70° to + 70° were collected and compared to simulations. The oscillation of the transmittance was measured by LC cell rotation. As seen, the precise pre-tilt angle was measured with low error and verified by comparing the simulated graphs (blue lines) with the experimental graphs (red lines) based on simulated information for a given LC, cell gap and birefringence value using a TBA107 tilt-bias angle evaluation device. The plot and error data values indicate that the pre-tilt angle measurements were highly reliable. At an annealing temperature of 100°C, the deviation between the experimental and simulated data was high, which indicates that the LC orientation was non-uniform. The pre-tilt angle of LC molecules on the YZO layers was measured to be approximately 0.5 degrees. The transmittance profiles of the LC layers indicate that the pre-tilt angle was highly reliable and that uniform LC alignment was achieved. These results demonstrate that a regular pre-tilt angle produces uniform LC alignment in LCDs.
To determine the mechanism of LC alignment, physico-chemical analysis was performed using XPS and AFM, as LC alignment is commonly affected by anisotropic film properties that are determined by the chemical composition and topography of the film. The IB irradiation strongly induced surface modifications irrespective of the initial state of the organic/inorganic films. The surfaces were reformed to nanoscale depths by IB irradiation using accelerated Ar+ ions. During the reformation of the alignment layer surface, the anisotropic characteristics of the layer were preserved because of the selective destruction of unfavorably oriented bonds. The anisotropic characteristics of the alignment layers are predominantly parallel to the direction of the IB.
The XPS spectra for the Y3d, Zn2p, and O1s peaks were analyzed at the surface for various annealing temperatures, as shown in Fig. 2. All binding energies were referenced to the C1s signal at 284.6 eV.
Fig. 2 (a) Y3d peaks of the XPS spectrum of solution-derived YZO films annealed at 200, 300, 400 and 500°C before and after irradiation. (b) Zn2p peaks of the XPS spectrum of films annealed at 200, 300, 400 and 500°C treated with IB irradiation. (c) O1s peak of the XPS spectrum of films annealed at 200, 300, 400 and 500°C treated with IB irradiation.
Figure 2(a) shows the Y3d portions of the XPS spectra of 50-nm thick YZO film coatings on ITO glass annealed at 200, 300, 400 and 500°C before and after irradiation. In this analysis, the binding energies of the core levels were calibrated by setting the reference carbon 1s peak to 284.6 eV. For the Y2O3 film, the Y3d photoemission line was observed as a spin-orbit split doublet, with the oxidized Y3d5/2 and Y3d3/2 peaks at 157.2 and 159.2 eV, respectively [24]. However, in our YZO sample, the Y3d core levels appear at increased binding-energy positions (i.e., Y3d3/2 at 160.2 eV and Y3d5/2 at 158.2 eV). These discrepancies arise from differing Y-O bond lengths in Y2O3 and YZO crystal lattices. This result corroborates that the YZO contained no Y2O3 precipitates [33].
Through the low intensity of the Y3d peak, we confirmed that the YZO film was insufficiently formed at an annealing temperature of 200°C due to the high boiling point of the solvent. However, at an annealing temperature greater than 300°C, the Y3d peak intensities were consistent, showing that the YZO films were fully formed. These results indicate that the formation of solution-derived YZO films is dependent on the annealing temperature. Nevertheless, at an annealing temperature of 200°C, YZO films can be formed by IB irradiation, which causes selective destruction of the bonding structures of the YZO surfaces and oxidation. Therefore, the Y3d peak intensity increased considerably after IB irradiation. In the cases of annealing temperatures greater than 300°C, the Y3d peak intensities decreased due to the destruction of Y-O bonds after IB irradiation. However, even in this case, the Y3d peak intensities were not reduced below a certain level.
While the Y3d peak intensities changed as a function of the annealing temperature after IB irradiation, the Zn2p peak intensities were consistent regardless of the annealing temperature and IB irradiation, as shown in Fig. 2(b). This result indicates that IB irradiation has a greater effect on the Y-O bonds than on the Z-O bonds.
Figure 2(c) shows the O1s peaks of the XPS spectra of YZO films annealed at 200, 300, 400 and 500°C following IB irradiation. The relative change in the O1s spectrum illustrates the abundance of O–Y bonds and oxygen vacancies. The intensity of the O1s peaks could be adjusted by changing the annealing temperature of the YZO films. Similar to the Y3d peaks and in contrast to the Zn2p peaks, the O1s peaks decreased as a function of the annealing temperature upon IB irradiation. This result indicates that the Y-O bonds that caused the O1s peaks were more affected by IB irradiation than the Z-O bonds. In other words, IB irradiation may have destroyed the bonding structure of the YZO surfaces, leading to the preservation of the anisotropic characteristic. As the Y-O and Zn-O bonds on the YZO films were broken by IB irradiation, the number of delocalized electrons on the YZO surfaces increased and the remaining broken bonds caused anisotropic dipole polarization, which induced strong van der Waals forces [34]. The combination of anisotropic characteristics and van der Waals forces can induce uniform LC alignment [35].
Figure 3 shows the changes in the atomic area percentages of zinc, yttrium and oxygen before and after IB irradiation as a function of the annealing temperature. At annealing temperatures up to 300°C, the content of yttrium decreased, while the content of zinc was consistent. At annealing temperatures greater than 300°C, the YZO films were fully oxidized and gradually grew. However, the YZO films were not fully formed at an annealing temperature of 200°C due to the presence of residual solvent. For these reasons, the LC-mode cells were fabricated with IB-irradiated YZO films annealed above 300°C.
Fig. 3 The atomic area percentages of zinc, yttrium and oxygen from the XPS survey of the solution-derived YZO film as a function of the annealing temperature before and after IB irradiation.
A physical investigation using AFM was performed, as shown in Fig. 4(a). The root mean square (RMS) roughness values of as-coated and IB-irradiated YZO films were 2.19 and 10.41 nm, respectively. This morphological change was due to Ar+-induced plasma but does not appear to change the form or shape of the surface, and it is less contributive than chemical reformation to achieving a uniform LC alignment.
Fig. 4 (a) The surface morphology of as-coated and IB-irradiated YZO films annealed at T = 500°C. (b) UV-Vis transmittance spectra of solution-derived YZO films.
Transparency is the one of the most important factors for the application of inorganic films in LCD devices. In the present study, the optical transmittance at 250–850 nm was measured at room temperature for as-deposited YZO on glass. As shown in Fig. 4(b), the average transmittance of the YZO films over the wavelength range of 420–780 nm was measured to be 87.9%. The YZO films exhibited transparencies similar to those of as-prepared ITO glass and PI-coated ITO glass, which have transmittances of 83.23% and 83.52%, respectively. Therefore, YZO films can be used as LC alignment layers without the loss of transparency.
Figure 5(a) demonstrates that the transmittance of the LC cell strongly corresponds to the LC alignment. The voltage transmittance (V-T) characteristics are shown in Fig. 5(a) under a driving voltage of 0 to 5 V, which yielded threshold voltages of 1.97, 1.68, 1.49 and 0.93 V at 200, 300, 400 and 500°C, respectively. The threshold voltage of the YZO layer was 0.93 V at 500°C, whereas that of the rubbed PI was 2.00 V at a transmittance of 90%. These results indicate that at temperatures greater than or equal to 300°C, the YZO layer has the potential to operate at lower external voltages with lower power consumption.
Fig. 5 (a) V-T characteristics of TN cells with IB-irradiated solution-derived YZO films. (b) Response rise time curve of TN cells with IB-irradiated YZO films. (c) Response fall time curve of TN cells with IB-irradiated YZO films.
Figure 5(b) and 5(c) shows the response time (RT) characteristics of LCs homogeneously aligned in TN cells with YZO layers. The YZO layers annealed at temperatures greater than or equal to 200°C exhibited rise times of 14.58, 4.40, 2.88 and 2.26 ms and fall times of 11.22, 7.04, 3.88 and 3.10 ms at 200, 300, 400 and 500°C, respectively. The RT characteristics of TN cells annealed at 200°C show that the slope of the RT curve is not steep and saturated. The horizontal orientation during the rise time of the LC field (applied voltage) indicates the passage of time; the fall time of the electric field was reduced, representing the LC interaction with the membrane.
These improved EO characteristics of the TN cell with yttrium and zinc oxide layers are attributed to the higher dielectric constant (12 and 11) compared to PI (3.4). The capacitance of the YZO layer was greater than that of the PI layer, thereby increasing the effective electrical field [25]. These features improve the EO characteristics of the TN cells. The measured anchoring energy of the YZO layers, which is the conventional factor that governs the EO characteristics, was 1.3 × 10−3 J/m2, similar to that of the PI layers. This result indicates that the anchoring energy of the YZO layer has no effect on the deviation of the EO characteristics. The parameters for the EO properties are summarized in Table 1.
Tabel 1. Parameters for the EO properties of TN cells with IB-irradiated YZO films.
The EO characteristics of the aligned homogenous LCs in TN cells fabricated with YZO layers were comparable to those in cells fabricated with PI, and IB-irradiated YZO film surfaces show good potential for use as an alignment layer, with a low threshold voltage (YZO: 0.98 V, versus PI: 2.00 V) and a fast response time (YZO: 2.26 ms, versus PI: 6.16 ms).
4. Conclusion
In the present study, we investigated the homogeneous alignment of LCs on IB-irradiated YZO surfaces deposited by solution processing and evaluated the effect of annealing on the YZO films. The homogeneous LC alignment was achieved by annealing above 200°C. XPS analysis of the YZO films indicated that IB irradiation of the YZO surfaces changed the intensity of the Y3d and O1s peaks. Because the reformation of the Y-O bonding and modification of the YZO surfaces play important roles in the homogeneous alignment of LC molecules, it is important to note that LCs deposited on IB-irradiated YZO films exhibited faster response times than LCs deposited on rubbed PI, indicating that YZO films are suitable for use in LCD devices.
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