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In this paper, a 1.8 keV ion beam (IB) sputtered thin layer of aluminum-doped zinc oxide (AZO) with columnar AZO bumps covering the surface working as an alignment layer for the homogeneous alignment of liquid crystals (LC) is investigated. Bumpy AZO alignment layers in twisted nematic (TN) cells generated larger LC pre-tilt angles and thus enabled accelerated switching of LC, and the highly conductive bumpy AZO thin layers allowed super-fast release of accumulated charges, and led to low residual DC performance. These results indicate the promising applications of AZO bumps layer as alignment layer in LC devices.
© 2016 Optical Society of America
Corrections
22 July 2016: A correction was made to the author listing.
1. Introduction
Transparent conductive oxides (TCO) [1, 2] with crystalline structures, excellent optical transparency, and metal-like low direct current (DC) resistivity have drawn significant attention for their use in transparent electronics applications. Zinc oxide (ZnO), a candidate comparable with indium tin oxide (ITO), shows obvious advantages in terms of material cost, availability, and environmental impact, but has poor conductivity [3, 4]. ZnO crystal growth is sensitive to the counterions, complexing agents, substrates, and ionic strength, but fortunately, aluminum (Al) doping can markedly improve the conductivity of ZnO [4–6] as Al works as a donor when it is substitutionally incorporated on zinc lattice sites as reported, and addition of such metal cations can suppress oxygen vacancy formation to reduce charge carrier generation in ZnO films, which in turn improves the device stability. Various methods have been used to prepare aluminum-doped zinc oxide (AZO) crystals, such as vacuum evaporation [7], chemical vapor deposition (CVD) [8], sol-gel processes [9], and sputtering deposition [10]. The sol-gel method seems especially promising because of its simplicity and acceptable cost.
Nowadays, liquid crystals (LC) have been comprehensively used in modern electronic devices to adjust the light route, and one of the foremost applications is in liquid crystal displays (LCD). The initial alignment of LC essentially determines the electro-optic (E-O) performances of these LC devices, especially in LCD which essentially determines visual quality and indirectly affects user visual satisfaction. LC alignment can be induced by chemicals [11–13], interaction between LC and alignment layers [14–19], interplay between LC and doped particles [20, 21], and the morphology of the alignment layers of substrates [22–26]. Many methods have been developed to align LC, such as the rubbing method [27], the photoalignment method [28], the self-assembled monolayer method [29], and the ion beam (IB) spurting method [30]. Each method has pros and cons. IB spurting as a non-contact LC alignment method was first introduced in 2001 by IBM. IB alignment treatment solves some serious problems of the common commercial process of rubbing a polyimide (PI) layer: unlike the rubbing method, IB treatment yields an alignment layer free of debris, and does not produce electrostatic discharge or streaking [30]. IB treatment improves upon the alignment yielded by rubbing, without destruction of the film. The IB treatment promotes alignment of LC by selectively destroying unfavorably aligned atomic rings within the film’s random polymer network [31]. To overcome the shortcomings of the IB spurting method, a combined consideration of the material properties and the morphologies transformation of alignment layers during low power IB spurting process was made.
Conductive alignment layers are ideal to align ferro-electric LC (FLC) with significant advanced E-O performances [32, 33], and conductive metallic layers directly aligning LC in non-display devices is advanced in simplifying the construction of these non-display devices by eliminating the general alignment layers from the technical view [34]. Conductive alignment layers replacing general alignment layers in LCD (such as polyimide (PI)) to align LC draws widespread controversy as referred that conductive alignment layers may improve the E-O performances of LC sandwiched between them [35, 36], while on one hand, the direct contact between LC and conductive layer may induce the electrolysis or degradation of LC, and on the other hand, the conductive alignment layers could decrease the voltage holding ratio (VHR) [37] and may induce more complicated LCD fabrication process, which limit the application of conductive alignment layers for high resolution multi-pixel-LCD (AM-LCD). Despite these shortcomings of conductive alignment layers previously discussed, highly transparent and highly conductive alignment layers for LC devices deserve further attentive focusing as they could both significantly decrease the accumulated charges on the cells when the applied voltage is removed, and result in an excellent E-O performances during LC switching, especially when they cooperate with non-contact LC aligning methods. Herein an IB-spurted AZO layer is proposed as an LC alignment layer and the potentiality of the cells with LC sandwiched between AZO thin layers is evaluated. The layer is prepared by means of a solution-based process conducted at room temperature, followed by a low-temperature pre-baking process; this fabrication route is both convenient and environmentally friendly consistent with other reported particle beam methods of LC alignment [38–41]. AZO bumps are considered to form during the IB spurting process. Information regarding the chemistry and morphological changes of IB spurting induced AZO bump thin films was obtained by means of X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). Homogeneously aligned nematic liquid crystals (NLC) on films prepared using various IB acceleration energies were observed by means of polarized optical microscopy (POM), and the electro-optical performance of cells incorporating the AZO bump thin films was characterized and is discussed herein.
2. Experimental
2.1 Materials and device fabrication
An AZO precursor solution was prepared by dissolving 0.001 mol zinc acetate (Zn(CH3CO2)2·2H2O, Aldrich Chemistry) and 0.001 mol aluminum nitrate (Al(NO3)3·9H2O, Aldrich Chemistry) in ethanol under ultrasonication. This solution was aged at room temperature for 24 h and then coated on substrates by means of spin coating (3000 rpm, 30 s). Substrates coated with AZO solution were prebaked at 80 °C for 10 min, annealed at 150 °C for 1 h, and then subjected to IB spurting of 0.6, 1.2, 1.8, or 2.4 keV for 2 min with a 45° incident angle. During IB spurting, the density of Ar plasma was maintained at 1014-1015 ions/cm2 and the IB current was maintained at around 1.0-1.2 mA/cm2. Antiparallel cells were fabricated using general glass substrates and indium tin oxide (ITO) glass substrates (Samsung Coring 1737; 32 × 22 × 1.1 mm3, sheet resistance 10 Ω/sq) with opposite IB spurting directions; cells with gaps of 60 and 5 μm were prepared for testing of alignment ability and residual DC property measurements, respectively. Twisted nematic (TN) cells for opto-electronic measurements were fabricated using ITO glass substrates, with the cell gap of 5 μm. All fabricated cells were injected with nematic liquid crystals (NLC, ne = 1.5702, n0 = 1.4756, and Δε = 10.7; Merck).
2.2 Characterizations
The binding energy of the AZO bumps was measured by means of X-ray photoelectron spectroscopy (XPS, ES-CALAB 220i-XL, VG Scientific). The surface morphologies of a pristine thin layer of AZO bumps and thin layers of AZO bumps subjected to IB spurting were examined by means of atomic force microscopy (AFM; Dimension 3100, Digital Instrument Co.) and field-emission scanning electron microscopy (FE-SEM, S-4200, Hitachi); the AFM results were also used to characterize surface roughness. The contact angles of layers with AZO bumps were characterized by using the sessile drop technique with water and diiodomethane droplets; measurements were conducted using an angle analyzer (Phoenix 300 Plus, SEO Co., Korea). LC alignment was characterized by means of polarizing optical microscope (POM, BXP 51, Olympus), and the pretilt angles of the LC were measured by means of a crystal rotation method (TBA 107, Autronic) in which the fluctuating transmittance was recorded while each anti-parallel cell was rotated latitudinally over the range of ± 70°. Voltage–transmittance and response time characteristics were evaluated using an LCD evaluation system (LCD-700, Otsuka Electronics), and the residual (direct current) DC behavior and the polar anchoring energy of LC on alignment layers were measured by means of a capacitance–voltage (C–V) hysteresis method, conducted using an LCR meter (Agilent 4284A).
3. Results and discussion
In XPS spectra, a slight signal transfer was observed after IB spurting, revealing the formation of AZO lattice. After various acceleration energies IB spurting (XPS spectra of AZO layers subjected to IB spurting of 1.2 and 1.8 keV are not shown here), the low-binding-energy O 1s signal was located at 530.07 eV, negatively shifted from the initial 539.25 eV before IB spurting; this was attributed to the lattice O in the wurtzite structure of a hexagonal Zn ion array [42,43]. The high-binding-energy O 1s signal at 532.15 eV, negatively shifted from the initial 540.65 eV before IB spurting, was attributed to chemisorbed oxygen species as a chemical activity of the ZnO thin layer as indicated in Fig. 1(a). Shifts in the Zn 2p1 and Zn 2p3 signals were also observed; these were located at 1045.4 and 1022 eV as indicated in Fig. 1(b), compared to the initial positions of 1054 and 1031 eV before IB spurting, respectively, thus indicating the successful oxidation of zinc [5].
Fig. 1 XPS spectra of AZO layers subjected to IB spurting of 0, 0.6, and 2.4 keV: (a) O 1s, (b) Zn 2p1 and Zn 2p3.
The topological transformation induced by IB spurting of AZO thin layers was investigated by means of SEM analysis of a series of layers subjected to different IB acceleration energies as shown in Fig. 2. A layer without IB spurting treatment showed a nonuniform wrinkly topology, and no bumps were observed in a cross-sectional view. Treatment at 0.6 keV yielded a uniform and unwrinkled AZO thin layer; in a cross-sectional view, slight bumps were detected compared to those in the film without IB spurting treatment. With further increases of the IB acceleration energy, no further significant topology transformation was observed; however, cross-sectional views showed that the quantity and the size of the bumps increased with increasing energy. Topology changes in the AZO thin layers induced by IB spurting were detected by AFM. AZO bumps formed with increasing IB acceleration energies; for the IB acceleration energy of 1.8 keV, uniform AZO bumps were found covering the surface in a thin layer with columnar topology. Each bump was several nanometers in diameter. However, when IB acceleration energy was increased to 2.4 keV, larger AZO bumps appeared and an uneven surface was detected.
Fig. 2 SEM images and AFM images (2 µm × 2 µm) of AZO layers subjected to IB acceleration energy of (a) 0, (b) 0.6, (c) 1.2, (d) 1.8, and (e) 2.4 keV, respectively.
Combined with XPS analysis, AZO bumps were induced by the IB spurting method just as with the thermal annealing method and UV exposure method, although no significant bump topology was observed from SEM images. The invisibility of bumps in the AZO thin layers likely resulted from the extreme thinness of the AZO layers and collimation of the IB. In an IB sputtering process, the random sputtering of high-energy ions onto the AZO layers induced scattered AZO bumps, whereas the extreme thinness of the AZO layers spin-coated on the substrates restricted the growth of such bumps.
The roughness of AZO thin layers sputtered using IB of various acceleration energies was analyzed as shown in Fig. 3(a). IB sputtering considerably enhanced the layers’ roughness owing to the formation of the AZO bumps, and the slightly etching of the surface of thin layers and the corresponding induced chemical radicals. Contact angles of water and diiodomethane droplets on the various IB-spurted AZO layers were also measured [Fig. 3(b)]. The enhanced roughness observed from AFM images partly can also be confirmed from the observation of tunable contact angles, and according to the Wenzel or Cassie–Baxter models [44–46], increased surface roughness corresponds to the increase of surface wettability. When a droplet of water is dropped on the surface of a rough AZO layer, because air may be trapped between the thin layer and the droplet, the contact area is decreased in Figs. 3(b) and 3(c), and because of the opposite molecular polarity compared with water molecules, when a droplet of diiodomethane is dropped on the the surface of a rough AZO layer, the contact area is increased in Fig. 3(b). The AZO thin layer spurted at 1.8 keV was the roughest one at the nanoscale among the layers tested as shown in Fig. 3(a). By comparing the roughness of the various IB-treated layers and their wettability characterizations based on contact angle observations, we found that the wettability varied in close relation to the directly observed roughness change.
Fig. 3 (a) Roughness of AZO thin layer and IB spurted AZO thin layers; (b) contact angles of various AZO thin layer and IB spurted AZO thin layers; (c) schematic images of water droplets on smooth and rough substrate.
Alignment of LC on thin films including AZO bumps was evaluated based on POM images and pretilt angle measurements; the extent of agreement between simulated (blue line) and experimentally measured transmittance response (red line) indicated whether LC alignment was uniform as shown in Fig. 4 and Table 1. LC were observed to be randomly aligned in layers subjected to the lower acceleration energy spurting conditions of 0.6 and 1.2 keV, yielding greater pretilt angles. When the IB acceleration energy was increased to 1.8 keV, LC aligned homogeneously and the pretilt angle decreased correspondingly. When the acceleration energy was increased further to 2.4 keV, a somewhat random alignment of LC was detected by means of POM. The LC alignment direction coincided with the direction of undamaged bonding after IB sputtering, which is parallel to the IB sputtering direction [47,48], and the homogeneous alignment of LC on the IB-spurted AZO thin layer could be attributed to surface anisotropy induced in the AZO thin layer by the spurting and the partial ablation and modification of chemical composition of AZO thin layer during plasma spurting process [49], as the roughness of thin layers has been referred in producing a rough anisotropic surface and the surface anisotropy [50] of a substrate appears to play a defining role in determining the direction of LC alignment.
Fig. 4 POM images and pretilt angle measurements of LC sandwiched between AZO thin layers subjected to IB spurting of (a) 0.6, (b) 1.2, (c) 1.8, and (d) 2.4 keV. Red arrows in the POM images indicate the direction of IB spurting; blue and red lines in pre-tile angle results respectively represent simulated transmittance response for uniformly aligned LC and experimentally measured response.
Table 1. The pre-tilt angles and the polar anchoring energies of LC sandwiched between rubbed PI alignment layers and AZO alignment layers subjected to IB-spurting of 0, 0.6, 1.2, 1.8, and 2.4 keV, respectively.
Switching behavior of LC on an AZO thin layer subjected to 1.8 keV IB spurting is presented in Figs. 5(a) and 5(b). Compared with a cell based on a commercially available rubbed PI layer, the cell based on the AZO thin layer subjected to 1.8 keV IB spurting showed a slightly higher threshold driving voltage and a significantly faster response. Specifically, the latter showed a rise time of 2.11 ms and a decay time of 6.75 ms, compared to the formers’ 2.43 rise time and 10.95 ms decay time from commercially available rubbed PI layers cell; this represents a 33.8% shorter total response time compared with commercially available rubbed PI layers cell, or switching performance improvement of one third. This accelerated switching performance of LC sandwiched between 1.8 keV IB spurted AZO thin layers may be due to the slightly larger anchoring energy generated by fluctuating AZO bumps [51–54]. Image sticking is a phenomenon in which a faded previous image is still visible, even though the frame has already been refreshed; it is caused by accumulated charges that remain despite the application of an out voltage [55]. The image sticking performance of the cells assembled with AZO alignment layers evaluated from their C–V hysteresis is shown in Fig. 5(c). Compared with the cell based on a rubbed thin layer of PI, the cell based on the thin layer of AZO bumps had less C–V hysteresis, which arose from the original high conductivity of the thin layer of AZO bumps. These highly conductive layers release their accumulated charges rapidly, thereby effectively decreasing the volume of residual charges. Thus, a weak C-V hysteresis performance of the AZO bump thin layer-based cell results.
Fig. 5 (a) Transmittance versus voltage and (b) transmittance versus time of TN cells fabricated with rubbed PI and AZO bump layers subjected to 1.8 keV IB spurting. (c) Comparison of residual DC in antiparallel cells fabricated from rubbed PI and AZO bump layers subjected to 1.8 keV IB spurting.
4. Conclusion
Thus, bumpy AZO thin layers were fabricated by means of a simple, time-saving IB spurting method conducted at low temperature. The chemical structures and morphologies of AZO thin layers were investigated by means of XPS, SEM, and AFM, and IB-induced AZO bumps were found to exist homogeneously on the scale of several nano-meters after 1.8 keV IB spurting. When the AZO thin layer subjected to 1.8 keV IB spurting was used as an alignment layer, homogeneous alignment of LC arose. Bumpy AZO alignment layers in TN cells generated larger LC pre-tilt angles and enabled accelerated switching of LC sandwiched between AZO thin layers. Because the bumpy AZO thin layers were highly conductive, which allowed super-fast release of accumulated charges, and led to low residual DC performance. The IB processed AZO thin layers show promise for application in the LCD industry, considering the speed and simplicity of this fabrication process.
Acknowledgments
Yang Liu thanks the China Scholarship Council (CSC, No. [2015]3022) for fellowship support.
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