1. Introduction

In liquid crystal (LC) applications, various factors in the system can influence on the quality of device ^(1-3). Above all, the LC alignment layer is the most important factor in LC device system. In conventional LC applications, polyimide with the rubbing process is widely used ^(4-6). Using this, LC molecules are aligned to one direction uniformly and LC application can control the light. Therefore, the fabrication of LC alignment layer occupies a significantly important position. Although the rubbing process is generally conducted as alignment method in industrial area, there are several disadvantages which can reduce the quality of LC applications, such as generation of debris, charge accumulation, and local defects ⁽⁷⁾. To avoid this, alterna- tive LC alignment methods have gradually studied. Normally, disadvantages of the rubbing process are produced due to direct surface contact. Therefore, non-contact alignment methods are investigated, including photo-alignment ^(8-9), and ion-beam ⁽¹⁰⁾. Despite, photo-alignment method is widely used in industry and research, material of the alignment layer should be carefully chosen for a photo reaction. On the other hand, various organic and inorganic materials can be adopted in the ion-beam system. Normally, inorganic materials have a good electric conductivity with physicochemical strength. Therefore, these are adopted in various electronic device to enhance the electrical performance. Thus, using inorganic materials as an alignment layer, performance of LC application can be enhanced, including low threshold voltage and fast response time. Indium zinc oxide (IZO) this film has a good electrical performance with stability. Therefore, various researches have used this material for enhance performance ^(11-13). Furthermore, applying this to LC application can enhance the electrical performance. To fabricate the alignment layer, deposition of thin film on the substrate is required, and typically, sputtering or chemical vapor deposition is widely used as a deposition method. However, these methods requires high cost and large facility area which can be disadvantages. On the contrary, the solution-process, one of deposition methods, has several advantages, such as simplicity, low cost, and high controllability ⁽¹⁴⁾. Therefore, using the solution-process, LC alignment layer can be easily deposited on the substrate.

In this study, the characteristics of the ion-beam irradiated IZO films were demonstrated. The solution process was used for depositing the IZO film on the glass substrate and the ion-beam was conducted as an LC alignment method. Using polarizing optical microscopy (POM), the alignment performance of LC molecules on the IZO film was observed. Furthermore, the thermal stability of anti-parallel LC cell based on the IZO films was investigated. To verify the effect of the ion-beam irradiation on the IZO film, surface morphology and chemical composition were analyzed by field emission scanning microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS), respectively. Further- more, through these, it was tried to Fig. out the reason of uniform LC alignment. Additionally, collecting the response time of the LC cell, electro-optical (EO) performance was measured.

Fig. 1. Schematic of experimental procedure; (a) dissolving indium and zinc into 2ME, (b) stirrer process, (c) IZO solution, (d) spin-coating process, (e) curing process, (f) ion-beam irradiation, (g) ion-beam irradiated IZO film, and (h) fabrication of LC cells.

2. Experimental Procedure

Indium nitrate hydrate and zinc acetate dihydrate were dissolved in 20 ml 2-methoxyethanol (2ME) to make IZO solution (0.1 and 0.9 mol, respectively), and mono ethanolamine with acetic acid was conducted as stabilizer of solution (Fig. 1(a)). The solution was stirred at 420 rpm at 85℃ for 2h on a stirrer (Fig. 1(b)). After then stirred solution (Fig. 1(c)) was coated on glass substrate by spin-coater with 3000 rpm for 30 s (Fig. 1(d)). Then, IZO-coated glass was pre-baked at 100℃ on a hot plate for 10 min and cured at 300℃ in a furnace for 2 h (Fig. 1(e)). After curing, IZO-coated glass was placed in ion-beam chamber, and using Ar+ plasma, IZO film was treated with the ion-beam energy of 700, 1200, and 1700 eV for 2 min (Fig. 1(f)). After this, ion-beam treated IZO film was achieved (Fig. 1(g)).

Using the prepared IZO film, an anti-parallel cell with a 60 mm cell gap was fabricated for verifying the LC alignment characteristics, and twisted nematic (TN) cell was also fabricated to measure the EO performance. Capillary force was used to fill the empty cells with positive LCs at room temperature. A polarizing optical microscope (Olympus) was conducted to reveal the LC alignment. Fabricated anti-parallel cell was heated from 100℃ to 160℃ in increments of 20℃ each for 10 min to observe the thermal stability. Surface morphology of IZO film was observed by using FE-SEM and chemical composition was determined by XPS analysis. Finally, response time of TN cell was collected by an LC evaluation system.

3. Results and Discussion

Fig. 2 shows POM images of anti-parallel cell with IZO films at ion-beam energy of 700, 1200, and 1700 eV. In POM images, ‘A’ and ‘P’ indicate the analyzer and polarizer, respec- tively. When the LC molecules are uniformly aligned to one direction, fabricated LC cell controls the light correctly. So, between the analyzer and polarizer, dark image is observed in POM image because the controlled light is blocked by polarizer. Therefore, it can be evidence of the uniform LC alignment that achieving the constant dark color in POM image. Although at ion-beam energy of 700 eV showed several defects, regardless of ion-beam energy, POM images showed constant dark color. This indicates that the uniform LC alignment was achieved on ion-beam irradiated IZO films at all energies. Furthermore, because the fabricated LC cells were filled by positive LCs, this POM result also indicates the homogeneous LC alignment. Consequently, using IZO film with ion-beam at various irradiation energies, uniform and homogeneous LC alignment were achieved. The results of the thermal stability test is presented in Fig. 3, which is a simple method for evaluating LCD quality. The LC cell with the ion-beam irradiated IZO film was heated from 100℃ to 160℃ with increments of 20℃ for 10min, after then cooled to room temperature. At a heating temperature of 100℃, the POM images obtained of the films showed black color for all ion-beam energies, as was the case in Fig. 3. This indicated that the LC molecules remained uniform LC alignment on the ion-beam irradiated IZO films when heated to 100℃. After the films had been heated to 140℃, the state of LC alignment changed. While the films irradiated at 700 eV and 1200 eV continued to exhibit a homogeneous and uniform alignment of the LC molecules, the LC molecules were slighted misaligned in the other film. After heating at 160℃, the LC molecules were misaligned in all the films, that is, for all the ion-beam energies. Considering the conventional LC system with polyimide had a thermal budget up to near 100℃, these are significantly enhanced properties. Resultingly, conducting ion-beam irradiated IZO film as an LC alignment layer, high thermal budget performance was obtained.

Fig. 2. POM images of anti-cells based on the IZO films irradiated at ion-beam energies of (a) 700, (b) 1200, and (c) 1700 eV.

Fig. 3. Thermal stability POM images of anti-cells based on the IZO films irradiated at ion-beam energies of 700, 1200, and 1700 eV.

Normally, ion-beam irradiation on organic or inorganic film surface induces physical and chemical change due to accelerated ion penetration by strong ion-beam energy. Therefore, irradiated surface analysis should be performed. The surface morphology of the ion-beam irradiated IZO film on the glass substrate was investigated by using FE-SEM, as shown in Fig. 4. At ion-beam energies of 700 and 1200 eV, no special feature was observed on the surface. This indicates that these energies had smooth surfaces. After ion-beam energy increased to 1700 eV, nanostructures were appeared on the surface. This indicates that the IZO film surface was damaged by the strong ion-beam energy. From these FE-SEM images, it was revealed that over ion-beam energy of 1700 eV, the IZO film surface was significantly changed. Furthermore, comparing ion-beam energies of 700 and 1700 eV, changes in the surface could have affected defects in POM image.

Fig. 4. FE-SEM surface morphology images of anti-cells based on the IZO films irradiated at ion-beam energies of 700, 1200, and 1700 eV.

For specific surface investigation, XPS analysis was used. All the XPS spectra were based on the C 1s peak located at 258 eV. Fig. 5(a) shows the XPS spectrum of In 3d at ion-beam energies of 700 (black line) and 1700 eV (red line). The XPS spectrum of In 3d consists of two peaks which are 3d5/2 and 3d3/2, located at 445.20 and 452.75 eV, respectively. As ion- beam energy increased, no binding energy shift was observed. However, it was observed that the intensity of XPS spectrum was decreased at ion-beam energy of 1700eV. The XPS spectrum of Zn2p is presented in Fig. 5(b) and this consists of two peaks located at 1022.70 and 1045.70 eV which are 2p3/2 and 2p1/2, respectively. Also, in this spectrum, no binding energy shift was observed and decrease of intensity was measured. These results indicate that the strong ion-beam irradiation changed the IZO film surface chemically. Furthermore, this means that the breakages of metal oxide bonds were induced by ion-beam irradiation. Breakage of metal-oxide bonds occurred dangling bonds with delocalized electrons on the surface. Due to this, surface became unstable which was non-stoichiometric surface. This increased surface anisotropic characteristics and induced strong van der Waals force which affected on uniform LC alignment ⁽¹⁵⁾. Consequently, the strong ion-beam irradiation remarkably changed the IZO film surface and due to this, uniform alignment of LC molecules on irradiated surface was achieved.

Fig. 5. (a) In 3d and (b) Zn 2p XPS spectra of the surface of the IZO films irradiated at 700 and 1700 eV.

Fig. 6(a) and (b) show the response time of the TN cell based on the IZO film at ion-beam energies of 700, 1200, and 1700 eV. The response time is composed of two specific time, rise and fall time. Rise time is the time it takes for the LC molecules to rise from horizontal state due to applied electric field. After applied electric field appeared, the LC molecules returns to horizontal state and the time taken at this time is called as fall time. The rise time of TN cell based on the IZO films was 4.656, 3.677, and 5.528 ms and fall time was 12.58, 11.093, and 9.801 ms at ion-beam energies of 700, 1200, and 1700 eV, respectively. The total response time was 17.236, 14.77, and 15.329 ms at ion-beam energies of 700, 1200, and 1700 eV, respectively. All response time values are represented in Fig. 6(c) by a graph as a function of the ion-beam irradiation energy. Through this, no significant tendency of the response time as the ion-beam energy increased was observed. However, the TN cells at all the ion-beam energies had faster response time than the conventional rubbed polyimide. From these results, it was revealed that the TN cell with the ion-beam irradiated IZO films had stable LC switching performance which is suitable for LC device.

Fig. 6. (a) Rise and (b) fall response time curves of the TN cell with the IZO films as a function of the ion-beam irradiation energies. (c) Rise, fall, and total response time values of the TN cell with the IZO film as a function of the ion-beam irradiation.

4. Conclusion

In here, it was successfully demonstrated that the LC alignment performance using the ion-beam irradiated IZO films as an alignment layer. Also, effect of ion-beam irradiation on surface was investigated. Regardless of ion-beam energy, uniform and homogeneous LC alignment was achieved, and this was revealed by dark images from POM analysis. Furthermore, it was revealed that the LC cell based on the IZO films at ion-beam energies of 1200 and 1700 eV had a high thermal stability up to 140℃. FE-SEM and XPS analysis the IZO film surface change due to ion-beam irradiation. From FE-SEM, surface morphology of the IZO film was observed, and nanostructures were observed at ion-beam energy of 1700 eV. Surface chemical composition was revealed by XPS analysis. As ion-beam energy increased, the intensity of XPS spectrum decreased which indicated the breakage of metal oxide bonds on the surface. The strong ion-beam irradi- ation broke metal oxide bonds on the surface and this occurred non-stoichiometric surface which increased anisotropic characteristics of the surface. Due to this, strong van der Waals force was induced, and uniform LC alignment was achieved. To evaluate the applicability of the IZO film for LC device, fabricating the TN cell, EO performance was measured. The response time was collected using the TN cell with the IZO films and it was confirmed that this had a good LC switching properties. Thus, the ion-beam irradiated IZO film is a sufficient as an LC alignment layer for enhanced LC applications.