- Email: [email protected]
Nematic liquid crystal alignment on the ion beam-exposed ZnO film
Thin Solid Films 519 (2010) 885–889
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Nematic liquid crystal alignment on the ion beam-exposed ZnO film Soo-Won Hwang, Joo-Hong Seo, Tae-Hoon Yoon, Jae Chang Kim ⁎ School of Electronics Engineering, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 November 2009 Received in revised form 17 August 2010 Accepted 17 August 2010 Available online 25 August 2010 Keywords: Liquid crystal Alignment Ion beam treatment Zinc oxide Transmittance Groove pattern
a b s t r a c t This paper investigates the nematic liquid crystal (NLC) alignment on ion beam-exposed zinc oxide (ZnO) films. The ZnO films are deposited by a radio frequency magnetron sputtering. During the deposition of ZnO film, we supplied sufficient oxygen gas for high resistivity and transmittance. The deposited films show a high transmittance of over 90% and high resistivity of over 1010 Ω cm. The ZnO films show a high deposition rate of 26.7 Å/min. Images obtained via scanning electron microscopy of the ZnO film surfaces, before and after the ion beam exposure, show that groove patterns are formed being to be parallel to the ion beam exposure direction. LC cells are fabricated with the ion beam-exposed ZnO films. The NLC molecules align parallel to the ion beam exposure direction. The electro-optic and response characteristics of fabricated cells show the possibility of application to liquid crystal displays. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystal (LC) alignment is an important issue in fabricating liquid crystal displays (LCDs). The LC alignment methods can be classified into two categories, namely the contact method and noncontact method. The non-contact methods, such as the oblique evaporation method [1,2], photo-alignment method [3,4], and ion beam alignment method [5–7] have been proposed to resolve the drawbacks of the contact method, i.e. the rubbing method, such as the generation of electrostatic charges, dust and scratches, and the nonuniformity of alignment in large-size substrates. In particular, the ion beam alignment method has attracted much attention recently due to its potential for achieving good uniformity in large-sized substrates, as well as having strong anchoring energy and high thermal stability, compared with photo-alignment and oblique evaporation approaches. Over the past several years, several inorganic materials, such as diamond like carbon [5,8], SiOx [9,10], SiC [11] and SiNx [5], have been studied as potential ion beam alignment materials. The UV stability of inorganic materials is better than that of organic materials. In addition, during the fabrication process of LCDs, the inorganic alignment layer and thin film transistors can be deposited simultaneously so that an effective manufacturing can be accomplished without the coating and baking which are required in the organic alignment process. However, it has been reported that inorganic films have lower transmittance than organic ones in general [9]. Also, as the deposition rates of inorganic films are low, it takes much time in
deposition. Therefore, inorganic materials that have higher transmittance and deposition rates are required to be searched. Recently, vertical alignment of LC molecules on zinc oxide (ZnO) nanolevel surface that shows good electro-optical characteristics was reported [12]. ZnO is a II–VI semiconductor with a wide bandgap of 3.36 eV which has been accepted as a good transparent material, and thin films of ZnO have transmittances of above 90% over the entire visiblewavelength range [12,13]. ZnO also has advantages in application to LCDs, such as high resistivity, low cost (ZnO is available in abundance in the earth), easy growth on any substrate, and high deposition rate. This work investigates the homogeneous alignment of LC molecules on ion beam-exposed ZnO films. We optimize the deposition parameters of ZnO film to obtain high transmittance and resistivity, and then ion beam is exposed on the ZnO film. In order to identify the LC alignment mechanism, the surface morphology of the ZnO film is checked before and after the ion beam exposure using a field emission-scanning electron microscope (FE-SEM, S4700, Hitachi, Japan, 0.5–30 kV). From the SEM images, a groove pattern was observed on the ZnO films surfaces that were exposed sufficiently to the ion beam. Using the fabricated LC cells in an anti-parallel configuration, the correlation between groove pattern and alignment state is investigated and the electro-optic and response characteristics are measured to check the possibility of application to LCDs. 2. Experimental details 2.1. Deposition of ZnO film
⁎ Corresponding author. E-mail address: [email protected] (J.C. Kim). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.105
ZnO films were deposited on the indium tin oxide (ITO) coated glass substrates using a radio frequency (RF)-magnetron sputtering system.
886
S.-W. Hwang et al. / Thin Solid Films 519 (2010) 885–889
The initial and working pressures of the vacuum chamber were below 6.67 × 10−4 and 1.33 Pa, respectively. The working pressure is controlled by high quality oxygen gas (99.999%) and argon gas (99.999%). The RF-power and deposition temperature were 70 W and 200 °C, respectively. The oxygen and argon gas ratio is controlled as they are most important parameter in determining the transmittance and resistivity of ZnO film. In LCDs, high transmittance and resistivity of the alignment layer are required for energy efficiency and high voltage holding ratio, respectively. After ZnO films were deposited on ITO coated glass substrates for various argon and oxygen gas ratios, to observe the defects of ZnO films, the absorption spectra of ZnO films were measured using a UV–VIS (Cary 5E UV–Vis-NIR Spectrophotometer, Varian, USA, 175–3300 nm). In insufficient oxygen supply, ZnO film has defects as oxygen vacancies and these defects reduce the resistivity and transmittance of ZnO film. Using the measured results, the oxygen and argon gas ratio for high transmittance and resistivity was found. The resistivity and transmittance of the ZnO films were measured using a 2point probe and an optical spectroscopy (Photal Otsuka Electronics, MCPD-3000), respectively. Also, to estimate the deposition rate, the film thickness is measured using a SEM. 2.2. Ion beam exposure on ZnO films Ion beam was exposed on the ZnO film surface for uniform homogeneous alignment of the LC molecules. The ion beam is supplied by a cold hollow cathode type with Ar+ ions as an ion source. The base and working pressures were 6.67 × 10−4 and 1.33 × 10−2 Pa, respectively. Argon gas flow was 7 sccm. The ion beam energy, incident angle and current density were fixed at 250 eV, 15° and 30 μA/cm2, respectively. The exposure time was varied from 0 to 120 s. Before and after the ion beam exposure, the surface morphology of ZnO films was measured using a SEM (S4700, Hitachi, Japan). The used operating voltage was 15.0 kV. A groove pattern has been observed to be parallel to the ion beam exposure direction on the ZnO film surface for the exposure time of over 60 s. 2.3. LC cell fabrication and measuring electro-optical characteristics of LC cells After the ion beam exposure on the ZnO films, LC cells were fabricated in an anti-parallel configuration (electrically controlled birefringence-mode) with a cell gap of 3.8 μm. The LC used here is ML-0223 (Δn = 0.0809, Δε = 3.9, Merck). We investigated the correlation between the groove pattern and alignment of LC molecules. Therefore, we checked the alignment states of the fabricated LC cells with a pair of crossed polarizers. The dispersion characteristics of the ZnO cells and conventional rubbed polyimide (PI) cell are measured for the study of alignment quality using an optical spectroscope. The LC cells are sandwiched between a pair of crossed polarizers, and the optic axes of the LC cells are oriented at 45° relative to the transmission axes of the polarizers. The dispersion characteristics of bright and dark states are measured without applied voltage and with an applied voltage of 20 V, respectively. The electro-optical characteristics and response times are measured for the possibility of application to LCDs, and these have been compared with the conventional rubbed PI cell.
Fig. 1. The absorption spectra of ZnO films for various O2/Ar gas ratios.
absorption is the stoichiometry of the ZnO which usually has a lot of oxygen vacancies in its lattice. We observed an absorption band at 550 nm in the case of no oxygen supply. The broad absorption is related to the oxygen vacancies in the grown ZnO crystals. With the increase of oxygen pressure, the concentration of the oxygen vacancy decreases as already discussed in the above for the UV absorption. We observed that the resistivities for various oxygen and argon gas ratios are correlated with the absorption spectra, as shown in Fig. 2. The resistivity of ZnO film was strongly dependent on the oxygen pressure. When the oxygen and argon gas ratio is over 10 sccm/ 50 sccm, the deposited ZnO films show a high resistivity of over 1010 Ω cm. This is because the density of defect decreases as the oxygen vacancies decrease with oxygen supply. We measured the transmittance of ZnO films deposited at various deposition temperatures and compared with that of PI film (AL-90101). The oxygen and argon gas flow rates were 25 and 50 sccm, respectively. Fig. 3 shows the measured spectral transmittance of the ZnO films and PI film with respect to bare glass. The transmittance of ZnO films was also enhanced with the increase of deposition temperature. The average transmittances of ZnO films at 25, 200, 300 °C and PI film are calculated to be 90.8, 94.8, 96.8 and 95.3%, respectively. The transmittances of ZnO films are similar or better than PI film at over 200 °C, as shown in Fig. 3. From this result, it is considered that the ZnO film is applicable to LCDs for better light efficiency. We also measured the thickness of the ZnO films for various deposition times using SEM and the deposition rates were calculated. From the measured thickness, we observed that the thickness was increased linearly with the increase of deposition time. When the deposition temperature, argon and oxygen flow rates are 200 °C, 50 and 25 sccm, the calculated deposition rate was 26.7 Å/min.
3. Results and discussions Absorption spectra of ZnO films deposited with various oxygen and argon gas ratios are shown in Fig. 1, which are found to be strongly dependent on the oxygen pressure. The absorption spectra of ZnO films have a narrow peak near a band edge at 375 nm (3.36 eV) and a broad absorption band around 550 nm (2.25 eV) [14]. With the increase of oxygen pressure, the intensity of the UV absorption increases remarkably. One important result of the UV
Fig. 2. The resistivity of ZnO films for various O2/Ar gas ratios.
S.-W. Hwang et al. / Thin Solid Films 519 (2010) 885–889
Fig. 3. The transmittance of ZnO films for various deposition temperatures, and PI film (AL-90101).
We investigated the change of surface morphology before and after ion beam exposure. Fig. 4 shows the SEM images of the ZnO surface for various ion beam exposure times. When the ion beam exposure time is below 30 s, the change of morphology before and after the ion beam exposure cannot be observed. On the ZnO film surface of the exposure time of over 60 s, a groove pattern has been observed to be parallel to the ion beam exposure direction. The period of the grooves is 20–30 nm and it is thought that the groove pattern is the key factor that causes the LC molecules to align parallel to the ion beam direction uniformly, because the distortion energy of the LC molecules above the alignment layer is minimized when the LC
887
molecules align parallel to the groove [15,16]. In addition, a groove pattern and isolated grains are observed in the case of 60 s. When the exposure time is 120 s, only the groove pattern is observed. In order to identify the effect of the grooves on the alignment of the LC molecules, the alignment states were investigated using the fabricated cells. Fig. 5 shows the transmittances of the LC cells fabricated with ZnO films of various ion beam exposure time and conventional rubbed PI film. The ion beam exposure and rubbing direction of the LC cells is oriented at 0 and 45° relative to the transmission axes of the polarizers, respectively. For the exposure time of below 30 s, the LC molecules are distributed randomly so that a uniform homogeneous state is not achieved. However, the LC molecules are aligned parallel to the ion beam exposure direction for the exposure time of above 60 s, so that uniform dark and bright states are obtained. From this result, it is confirmed that the LC molecules align uniformly parallel to the exposure direction of the ion beam only in the cell that has a groove pattern. We measured the pretilt angle of the ZnO cells using crystal rotation method. For the exposure time of 120 s, the measured pretilt angle was 2.17°. We investigated the alignment states using electro-optical method such as dispersion characteristics and contrast ratio (CR). Fig. 6 shows the dispersion characteristics of the ZnO cells and conventional rubbed PI cell in the (a) bright and (b) dark states. The cells are designed to have maximum transmittance at around 550 nm. Hence, the field off- state (bright state) and field on-state will show a yellowish green appearance and a black appearance, respectively. As shown in Fig. 6, the dispersion characteristics of the dark and bright states of ZnO cells which have groove pattern is similar to the conventional rubbed PI cell. However, for the exposure time of below 30 s, the dispersion characteristics of both the bright and dark states
Fig. 4. The SEM images of the surface morphology for the ion-beam exposure time of 0, 30, 60, and 120 s, respectively.
888
S.-W. Hwang et al. / Thin Solid Films 519 (2010) 885–889
Fig. 5. The pictures of the transmittance of the LC cells with ZnO film for the ion-beam exposure time of 0, 30, 60, and 120 s and conventional rubbed PI cell, respectively; the cells are sandwiched between a pair of crossed polarizers and the direction of ion-beam exposure is oriented at 0 and 45° relative to the transmission axes of the polarizers.
are not improved compared with the conventional rubbed PI cell. A high CR of an LC cell indicates that LC molecules align uniformly to the alignment direction. The CR of the ZnO cells that were exposed to the ion beam for 0, 30, 60 and 120 s, and the conventional rubbed PI cell were 21:1, 34:1, 120:1, 157:1 and 147:1, respectively. When the exposure time is below 30 s, the CR is bad compared with the conventional rubbed PI cell, which indicates that the LC molecules are not well aligned. For the exposure time of 60 and 120 s, the CRs have similar results with the rubbed PI cell. Fig. 7(a) shows the electro-optical characteristics of the ZnO cells and the conventional rubbed PI cell. The ZnO cell was fabricated using ZnO film exposed to the ion beam for 120 s at 250 eV. The threshold and operating voltages of the ZnO cell and conventional rubbed PI cell are the same, and 1.2 and 8 V, respectively. Fig. 7(b) shows the response time of the LC cells measured at applied voltages of 0 to 10 V rectangular wave form of 1 kHz, respectively. The turn-on time is defined as the transient time from 10 to 90% of the maximum transmittance, and the turn-off time is defined as the transient time from 90 to 10% of the maximum transmittance. The ion beam-exposed ZnO cell showed a turn-on time of 0.53 ms and a turn-off time of 8.7 ms, while the rubbed PI cell showed a turn-on time of 0.55 ms and a turn-off time of 8.4 ms, which are the same. 4. Conclusions
Fig. 6. The dispersion characteristics of the ZnO cells for various ion-beam exposure time and conventional rubbed PI cell (AL-90101) in (a) the bright and (b) dark states.
We investigated the NLC alignment using ion beam-exposed ZnO films. For high transmittance and resistivity of ZnO films, we eliminated the defects such as oxygen vacancies in ZnO lattice formed during the oxygen supply. For the exposure time of over 60 s, a groove pattern was observed on the ZnO film surface to be parallel to the ion beam exposure direction and LC molecules aligned uniformly parallel to the grooves. We investigated the electro-optic and response characteristics of the ion beam-exposed ZnO cells and conventional rubbed PI cell, and the results show the possibility of application of this method to LCDs.
S.-W. Hwang et al. / Thin Solid Films 519 (2010) 885–889
889
References [1] J.L. Janning, Appl. Phys. Lett. 21 (1972) 173. [2] L.A. Goodman, J.T. McGinn, C.H. Anderson, F. Digernimo, IEEE Trans. Electron Devices ED-24 (1977) 795. [3] W. Gibbons, P. Shannon, S. Sun, B. Swetlin, Nature 351 (1991) 49. [4] Y. Wu, Y. Demachi, O. Tsutsumi, A. Kanazawa, T. Shiono, T. Ikeda, Macromolecules 31 (1998) 349. [5] P. Chaudhari, J. Lacey, J. Doyle, E. Galligan, S.C. Lien, A. Callegari, G. Hougham, N.D. Lang, P.S. Andry, R. John, K.H. Yang, M. Lu, C. Cai, J. Speidell, S. Purushothaman, J. Ritsko, M. Samant, J. Stöhr, Y. Nakagawa, Y. Katoh, Y. Saitoh, K. Sakai, H. Satoh, S. Odahara, H. Nakano, J. Nakagaki, Y. Shiota, Nature 411 (2001) 56. [6] J.P. Doyle, P. Chaudhari, L. Lacey, E.A. Galligan, S.C. Lien, A.C. Callegari, N.D. Lang, M. Lu, Y. Nakagawa, H. Nakano, N. Okazaki, S. Odahara, Y. Katoh, Y. Saitoh, K. Sakai, H. Satoh, Y. Shiota, Nucl. Instrum. Meth. Phys. Res. B 206 (2003) 467. [7] J.S. Gwag, C.G. Jhun, J.C. Kim, T.-H. Yoon, G.D. Lee, S.J. Cho, J. Appl. Phys. 96 (2004) 257. [8] Y. Nakagawa, Y. Kato, Y. Saitoh, K. Sakai, H. Satoh, K. Wako, S. Odahara, N. Toshifumi, J. Nakagaki, H. Nakano, P. Chaudhari, J. Lacey, J. Doyle, E. Galligan, S.C.A. Lien, A. Callegari, G. Hougham, P.S. Andry, R. John, M. Lu, C. Cai, J.S.S. Purushothman, J. Ritsko, SID Symp. Dig. 32 (2001) 1346. [9] P.K. Son, J.H. Park, J.C. Kim, T.-H. Yoon, Thin Solid Films 515 (2007) 3102. [10] W.Y. Chou, Z.Y. Ho, F.C. Tang, Y.S. Mai, T.Y. Wu, H.L. Cheng, C.R. Sheu, C.C. Liao, K.H. Liu, Jpn. J. Appl. Phys. 44 (2005) L876. [11] P.K. Son, J.H. Park, J.C. Kim, T.-H. Yoon, S.J. Rho, B.K. Jeon, S.T. Shin, J.S. Kim, S.K. Lim, Appl. Phys. Lett. 91 (2007) 103513. [12] B.Y. Oh, W.K. Lee, Y.H. Kim, D.S. Seo, J. Appl. Phys. 105 (2009) 054506. [13] B. Sang, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 37 (1998) L206. [14] H.-J. Egelhaaf, D. Oelkrug, J. Cryst. Growth 161 (1996) 190. [15] D.W. Berreman, Phys. Rev. Lett. 28 (1972) 1683. [16] U. Wolff, W. Greubel, H. KrÜGer, Mol. Cryst. Liq. Cryst. 23 (1973) 187.
Fig. 7. The characteristics of (a) electro-optic and (b) response time of the LC cells.
Acknowledgment This work was supported by the Second Phase BK21 Program of the Ministry of Education & Human Resources Development, Korea.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Nematic liquid crystal alignment on the ion beam-exposed ZnO film Soo-Won Hwang, Joo-Hong Seo, Tae-Hoon Yoon, Jae Chang Kim ⁎ School of Electronics Engineering, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 November 2009 Received in revised form 17 August 2010 Accepted 17 August 2010 Available online 25 August 2010 Keywords: Liquid crystal Alignment Ion beam treatment Zinc oxide Transmittance Groove pattern
a b s t r a c t This paper investigates the nematic liquid crystal (NLC) alignment on ion beam-exposed zinc oxide (ZnO) films. The ZnO films are deposited by a radio frequency magnetron sputtering. During the deposition of ZnO film, we supplied sufficient oxygen gas for high resistivity and transmittance. The deposited films show a high transmittance of over 90% and high resistivity of over 1010 Ω cm. The ZnO films show a high deposition rate of 26.7 Å/min. Images obtained via scanning electron microscopy of the ZnO film surfaces, before and after the ion beam exposure, show that groove patterns are formed being to be parallel to the ion beam exposure direction. LC cells are fabricated with the ion beam-exposed ZnO films. The NLC molecules align parallel to the ion beam exposure direction. The electro-optic and response characteristics of fabricated cells show the possibility of application to liquid crystal displays. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystal (LC) alignment is an important issue in fabricating liquid crystal displays (LCDs). The LC alignment methods can be classified into two categories, namely the contact method and noncontact method. The non-contact methods, such as the oblique evaporation method [1,2], photo-alignment method [3,4], and ion beam alignment method [5–7] have been proposed to resolve the drawbacks of the contact method, i.e. the rubbing method, such as the generation of electrostatic charges, dust and scratches, and the nonuniformity of alignment in large-size substrates. In particular, the ion beam alignment method has attracted much attention recently due to its potential for achieving good uniformity in large-sized substrates, as well as having strong anchoring energy and high thermal stability, compared with photo-alignment and oblique evaporation approaches. Over the past several years, several inorganic materials, such as diamond like carbon [5,8], SiOx [9,10], SiC [11] and SiNx [5], have been studied as potential ion beam alignment materials. The UV stability of inorganic materials is better than that of organic materials. In addition, during the fabrication process of LCDs, the inorganic alignment layer and thin film transistors can be deposited simultaneously so that an effective manufacturing can be accomplished without the coating and baking which are required in the organic alignment process. However, it has been reported that inorganic films have lower transmittance than organic ones in general [9]. Also, as the deposition rates of inorganic films are low, it takes much time in
deposition. Therefore, inorganic materials that have higher transmittance and deposition rates are required to be searched. Recently, vertical alignment of LC molecules on zinc oxide (ZnO) nanolevel surface that shows good electro-optical characteristics was reported [12]. ZnO is a II–VI semiconductor with a wide bandgap of 3.36 eV which has been accepted as a good transparent material, and thin films of ZnO have transmittances of above 90% over the entire visiblewavelength range [12,13]. ZnO also has advantages in application to LCDs, such as high resistivity, low cost (ZnO is available in abundance in the earth), easy growth on any substrate, and high deposition rate. This work investigates the homogeneous alignment of LC molecules on ion beam-exposed ZnO films. We optimize the deposition parameters of ZnO film to obtain high transmittance and resistivity, and then ion beam is exposed on the ZnO film. In order to identify the LC alignment mechanism, the surface morphology of the ZnO film is checked before and after the ion beam exposure using a field emission-scanning electron microscope (FE-SEM, S4700, Hitachi, Japan, 0.5–30 kV). From the SEM images, a groove pattern was observed on the ZnO films surfaces that were exposed sufficiently to the ion beam. Using the fabricated LC cells in an anti-parallel configuration, the correlation between groove pattern and alignment state is investigated and the electro-optic and response characteristics are measured to check the possibility of application to LCDs. 2. Experimental details 2.1. Deposition of ZnO film
⁎ Corresponding author. E-mail address: [email protected] (J.C. Kim). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.105
ZnO films were deposited on the indium tin oxide (ITO) coated glass substrates using a radio frequency (RF)-magnetron sputtering system.
886
S.-W. Hwang et al. / Thin Solid Films 519 (2010) 885–889
The initial and working pressures of the vacuum chamber were below 6.67 × 10−4 and 1.33 Pa, respectively. The working pressure is controlled by high quality oxygen gas (99.999%) and argon gas (99.999%). The RF-power and deposition temperature were 70 W and 200 °C, respectively. The oxygen and argon gas ratio is controlled as they are most important parameter in determining the transmittance and resistivity of ZnO film. In LCDs, high transmittance and resistivity of the alignment layer are required for energy efficiency and high voltage holding ratio, respectively. After ZnO films were deposited on ITO coated glass substrates for various argon and oxygen gas ratios, to observe the defects of ZnO films, the absorption spectra of ZnO films were measured using a UV–VIS (Cary 5E UV–Vis-NIR Spectrophotometer, Varian, USA, 175–3300 nm). In insufficient oxygen supply, ZnO film has defects as oxygen vacancies and these defects reduce the resistivity and transmittance of ZnO film. Using the measured results, the oxygen and argon gas ratio for high transmittance and resistivity was found. The resistivity and transmittance of the ZnO films were measured using a 2point probe and an optical spectroscopy (Photal Otsuka Electronics, MCPD-3000), respectively. Also, to estimate the deposition rate, the film thickness is measured using a SEM. 2.2. Ion beam exposure on ZnO films Ion beam was exposed on the ZnO film surface for uniform homogeneous alignment of the LC molecules. The ion beam is supplied by a cold hollow cathode type with Ar+ ions as an ion source. The base and working pressures were 6.67 × 10−4 and 1.33 × 10−2 Pa, respectively. Argon gas flow was 7 sccm. The ion beam energy, incident angle and current density were fixed at 250 eV, 15° and 30 μA/cm2, respectively. The exposure time was varied from 0 to 120 s. Before and after the ion beam exposure, the surface morphology of ZnO films was measured using a SEM (S4700, Hitachi, Japan). The used operating voltage was 15.0 kV. A groove pattern has been observed to be parallel to the ion beam exposure direction on the ZnO film surface for the exposure time of over 60 s. 2.3. LC cell fabrication and measuring electro-optical characteristics of LC cells After the ion beam exposure on the ZnO films, LC cells were fabricated in an anti-parallel configuration (electrically controlled birefringence-mode) with a cell gap of 3.8 μm. The LC used here is ML-0223 (Δn = 0.0809, Δε = 3.9, Merck). We investigated the correlation between the groove pattern and alignment of LC molecules. Therefore, we checked the alignment states of the fabricated LC cells with a pair of crossed polarizers. The dispersion characteristics of the ZnO cells and conventional rubbed polyimide (PI) cell are measured for the study of alignment quality using an optical spectroscope. The LC cells are sandwiched between a pair of crossed polarizers, and the optic axes of the LC cells are oriented at 45° relative to the transmission axes of the polarizers. The dispersion characteristics of bright and dark states are measured without applied voltage and with an applied voltage of 20 V, respectively. The electro-optical characteristics and response times are measured for the possibility of application to LCDs, and these have been compared with the conventional rubbed PI cell.
Fig. 1. The absorption spectra of ZnO films for various O2/Ar gas ratios.
absorption is the stoichiometry of the ZnO which usually has a lot of oxygen vacancies in its lattice. We observed an absorption band at 550 nm in the case of no oxygen supply. The broad absorption is related to the oxygen vacancies in the grown ZnO crystals. With the increase of oxygen pressure, the concentration of the oxygen vacancy decreases as already discussed in the above for the UV absorption. We observed that the resistivities for various oxygen and argon gas ratios are correlated with the absorption spectra, as shown in Fig. 2. The resistivity of ZnO film was strongly dependent on the oxygen pressure. When the oxygen and argon gas ratio is over 10 sccm/ 50 sccm, the deposited ZnO films show a high resistivity of over 1010 Ω cm. This is because the density of defect decreases as the oxygen vacancies decrease with oxygen supply. We measured the transmittance of ZnO films deposited at various deposition temperatures and compared with that of PI film (AL-90101). The oxygen and argon gas flow rates were 25 and 50 sccm, respectively. Fig. 3 shows the measured spectral transmittance of the ZnO films and PI film with respect to bare glass. The transmittance of ZnO films was also enhanced with the increase of deposition temperature. The average transmittances of ZnO films at 25, 200, 300 °C and PI film are calculated to be 90.8, 94.8, 96.8 and 95.3%, respectively. The transmittances of ZnO films are similar or better than PI film at over 200 °C, as shown in Fig. 3. From this result, it is considered that the ZnO film is applicable to LCDs for better light efficiency. We also measured the thickness of the ZnO films for various deposition times using SEM and the deposition rates were calculated. From the measured thickness, we observed that the thickness was increased linearly with the increase of deposition time. When the deposition temperature, argon and oxygen flow rates are 200 °C, 50 and 25 sccm, the calculated deposition rate was 26.7 Å/min.
3. Results and discussions Absorption spectra of ZnO films deposited with various oxygen and argon gas ratios are shown in Fig. 1, which are found to be strongly dependent on the oxygen pressure. The absorption spectra of ZnO films have a narrow peak near a band edge at 375 nm (3.36 eV) and a broad absorption band around 550 nm (2.25 eV) [14]. With the increase of oxygen pressure, the intensity of the UV absorption increases remarkably. One important result of the UV
Fig. 2. The resistivity of ZnO films for various O2/Ar gas ratios.
S.-W. Hwang et al. / Thin Solid Films 519 (2010) 885–889
Fig. 3. The transmittance of ZnO films for various deposition temperatures, and PI film (AL-90101).
We investigated the change of surface morphology before and after ion beam exposure. Fig. 4 shows the SEM images of the ZnO surface for various ion beam exposure times. When the ion beam exposure time is below 30 s, the change of morphology before and after the ion beam exposure cannot be observed. On the ZnO film surface of the exposure time of over 60 s, a groove pattern has been observed to be parallel to the ion beam exposure direction. The period of the grooves is 20–30 nm and it is thought that the groove pattern is the key factor that causes the LC molecules to align parallel to the ion beam direction uniformly, because the distortion energy of the LC molecules above the alignment layer is minimized when the LC
887
molecules align parallel to the groove [15,16]. In addition, a groove pattern and isolated grains are observed in the case of 60 s. When the exposure time is 120 s, only the groove pattern is observed. In order to identify the effect of the grooves on the alignment of the LC molecules, the alignment states were investigated using the fabricated cells. Fig. 5 shows the transmittances of the LC cells fabricated with ZnO films of various ion beam exposure time and conventional rubbed PI film. The ion beam exposure and rubbing direction of the LC cells is oriented at 0 and 45° relative to the transmission axes of the polarizers, respectively. For the exposure time of below 30 s, the LC molecules are distributed randomly so that a uniform homogeneous state is not achieved. However, the LC molecules are aligned parallel to the ion beam exposure direction for the exposure time of above 60 s, so that uniform dark and bright states are obtained. From this result, it is confirmed that the LC molecules align uniformly parallel to the exposure direction of the ion beam only in the cell that has a groove pattern. We measured the pretilt angle of the ZnO cells using crystal rotation method. For the exposure time of 120 s, the measured pretilt angle was 2.17°. We investigated the alignment states using electro-optical method such as dispersion characteristics and contrast ratio (CR). Fig. 6 shows the dispersion characteristics of the ZnO cells and conventional rubbed PI cell in the (a) bright and (b) dark states. The cells are designed to have maximum transmittance at around 550 nm. Hence, the field off- state (bright state) and field on-state will show a yellowish green appearance and a black appearance, respectively. As shown in Fig. 6, the dispersion characteristics of the dark and bright states of ZnO cells which have groove pattern is similar to the conventional rubbed PI cell. However, for the exposure time of below 30 s, the dispersion characteristics of both the bright and dark states
Fig. 4. The SEM images of the surface morphology for the ion-beam exposure time of 0, 30, 60, and 120 s, respectively.
888
S.-W. Hwang et al. / Thin Solid Films 519 (2010) 885–889
Fig. 5. The pictures of the transmittance of the LC cells with ZnO film for the ion-beam exposure time of 0, 30, 60, and 120 s and conventional rubbed PI cell, respectively; the cells are sandwiched between a pair of crossed polarizers and the direction of ion-beam exposure is oriented at 0 and 45° relative to the transmission axes of the polarizers.
are not improved compared with the conventional rubbed PI cell. A high CR of an LC cell indicates that LC molecules align uniformly to the alignment direction. The CR of the ZnO cells that were exposed to the ion beam for 0, 30, 60 and 120 s, and the conventional rubbed PI cell were 21:1, 34:1, 120:1, 157:1 and 147:1, respectively. When the exposure time is below 30 s, the CR is bad compared with the conventional rubbed PI cell, which indicates that the LC molecules are not well aligned. For the exposure time of 60 and 120 s, the CRs have similar results with the rubbed PI cell. Fig. 7(a) shows the electro-optical characteristics of the ZnO cells and the conventional rubbed PI cell. The ZnO cell was fabricated using ZnO film exposed to the ion beam for 120 s at 250 eV. The threshold and operating voltages of the ZnO cell and conventional rubbed PI cell are the same, and 1.2 and 8 V, respectively. Fig. 7(b) shows the response time of the LC cells measured at applied voltages of 0 to 10 V rectangular wave form of 1 kHz, respectively. The turn-on time is defined as the transient time from 10 to 90% of the maximum transmittance, and the turn-off time is defined as the transient time from 90 to 10% of the maximum transmittance. The ion beam-exposed ZnO cell showed a turn-on time of 0.53 ms and a turn-off time of 8.7 ms, while the rubbed PI cell showed a turn-on time of 0.55 ms and a turn-off time of 8.4 ms, which are the same. 4. Conclusions
Fig. 6. The dispersion characteristics of the ZnO cells for various ion-beam exposure time and conventional rubbed PI cell (AL-90101) in (a) the bright and (b) dark states.
We investigated the NLC alignment using ion beam-exposed ZnO films. For high transmittance and resistivity of ZnO films, we eliminated the defects such as oxygen vacancies in ZnO lattice formed during the oxygen supply. For the exposure time of over 60 s, a groove pattern was observed on the ZnO film surface to be parallel to the ion beam exposure direction and LC molecules aligned uniformly parallel to the grooves. We investigated the electro-optic and response characteristics of the ion beam-exposed ZnO cells and conventional rubbed PI cell, and the results show the possibility of application of this method to LCDs.
S.-W. Hwang et al. / Thin Solid Films 519 (2010) 885–889
889
References [1] J.L. Janning, Appl. Phys. Lett. 21 (1972) 173. [2] L.A. Goodman, J.T. McGinn, C.H. Anderson, F. Digernimo, IEEE Trans. Electron Devices ED-24 (1977) 795. [3] W. Gibbons, P. Shannon, S. Sun, B. Swetlin, Nature 351 (1991) 49. [4] Y. Wu, Y. Demachi, O. Tsutsumi, A. Kanazawa, T. Shiono, T. Ikeda, Macromolecules 31 (1998) 349. [5] P. Chaudhari, J. Lacey, J. Doyle, E. Galligan, S.C. Lien, A. Callegari, G. Hougham, N.D. Lang, P.S. Andry, R. John, K.H. Yang, M. Lu, C. Cai, J. Speidell, S. Purushothaman, J. Ritsko, M. Samant, J. Stöhr, Y. Nakagawa, Y. Katoh, Y. Saitoh, K. Sakai, H. Satoh, S. Odahara, H. Nakano, J. Nakagaki, Y. Shiota, Nature 411 (2001) 56. [6] J.P. Doyle, P. Chaudhari, L. Lacey, E.A. Galligan, S.C. Lien, A.C. Callegari, N.D. Lang, M. Lu, Y. Nakagawa, H. Nakano, N. Okazaki, S. Odahara, Y. Katoh, Y. Saitoh, K. Sakai, H. Satoh, Y. Shiota, Nucl. Instrum. Meth. Phys. Res. B 206 (2003) 467. [7] J.S. Gwag, C.G. Jhun, J.C. Kim, T.-H. Yoon, G.D. Lee, S.J. Cho, J. Appl. Phys. 96 (2004) 257. [8] Y. Nakagawa, Y. Kato, Y. Saitoh, K. Sakai, H. Satoh, K. Wako, S. Odahara, N. Toshifumi, J. Nakagaki, H. Nakano, P. Chaudhari, J. Lacey, J. Doyle, E. Galligan, S.C.A. Lien, A. Callegari, G. Hougham, P.S. Andry, R. John, M. Lu, C. Cai, J.S.S. Purushothman, J. Ritsko, SID Symp. Dig. 32 (2001) 1346. [9] P.K. Son, J.H. Park, J.C. Kim, T.-H. Yoon, Thin Solid Films 515 (2007) 3102. [10] W.Y. Chou, Z.Y. Ho, F.C. Tang, Y.S. Mai, T.Y. Wu, H.L. Cheng, C.R. Sheu, C.C. Liao, K.H. Liu, Jpn. J. Appl. Phys. 44 (2005) L876. [11] P.K. Son, J.H. Park, J.C. Kim, T.-H. Yoon, S.J. Rho, B.K. Jeon, S.T. Shin, J.S. Kim, S.K. Lim, Appl. Phys. Lett. 91 (2007) 103513. [12] B.Y. Oh, W.K. Lee, Y.H. Kim, D.S. Seo, J. Appl. Phys. 105 (2009) 054506. [13] B. Sang, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 37 (1998) L206. [14] H.-J. Egelhaaf, D. Oelkrug, J. Cryst. Growth 161 (1996) 190. [15] D.W. Berreman, Phys. Rev. Lett. 28 (1972) 1683. [16] U. Wolff, W. Greubel, H. KrÜGer, Mol. Cryst. Liq. Cryst. 23 (1973) 187.
Fig. 7. The characteristics of (a) electro-optic and (b) response time of the LC cells.
Acknowledgment This work was supported by the Second Phase BK21 Program of the Ministry of Education & Human Resources Development, Korea.