ICMCTF2010 Session CP: Symposium C Poster Session

Polymer dispersed liquid crystals (PDLCs) which emerged in late 1980s are important technology in the development of switchable windows, electro-optic shutters, and especially displays because of their many advantages, viz., slim shape, low weight, no alignment layer, no polarizer, easy fabrication and so on, compared with conventional liquid crystal displays (LCDs) [1]. Hence, PDLCs for a display should have high contrast ratio, low driving voltage, and fast response time as well as low shrinkage. In this work, we incorporated different types o f reinforcing materials (RMs) such as hydrophilic silica (aerosil 200), polyhedral oligomeric silsesquioxane (POSS), and poly (dimetyl siloxane) (PDMS) into the conventional PDLC system to increase electro-optical properties by the increment of phase separation, resulting from the increase in elastic modulus of polymer matrix and the reduction of compatibility with LC, and the enhancement of conversion rate of monomer to polymer [2]. When additional amounts of RM are increased to a certain point, t he reactive mixture is homogeneously mixed before polymerization process. However, RM could be aggregated at above the optimum contents of RM, leading to the deterioration of electro-optical, mechanical, and thermal properties. Generally, as the contents of RM are increased, electro-optical properties viz., the contrast ratio, driving voltage, and rising time are improved because of the increase in LC droplet size and the reduction of anchoring strength between polymer and LC. However, when the contents of RM are higher, these properties are decreased due to probably poor phase separation between polymer and LC, caused by the aggregation of RMs. POSS with small-sized particles (1–2 nm) modified by organic substituent shows excellent properties in RMs used in this work.

ZnO is a material of technological importance for, among other things, its practical and potential applications for short wavelength optoelectronic devices and transparent conductive oxide films, such as in UV-lasers, blue to UV light-emitting diodes and solar cells electrodes. This oxide is also used in glass stacking as a UV spectrum filter and can provide other promising technological applications thanks to its adjusting photoluminescence properties. The advantage of the magnetron sputtering technique is the achievement of polycrystalline ZnO films deposition on large flat glass area without intentional substrate heating. However, it results in residual stresses which can be detrimental when occurrence of spontaneous delamination, or under scratch during processing or in service. The control of their mechanical reliability can then be achieved by an in depth comprehension of the residual stresses build up or relaxation mechanisms occurring in the films in relation with the material structure.

By means of X-ray diffraction, we show in this communication that as-deposited ZnO films encapsulated or not by Si₃N₄ protective coatings are highly compressively stressed. Moreover, a transition of stress is observed as a function of the post-deposition annealing temperature. After a 800°C annealing, ZnO films are tensily stressed while ZnO films encapsulated by Si₃N₄ are stress-free. With the aid of in-situ X-ray diffraction under ambient and argon atmosphere, we argue that this thermally activated stress relaxation can be attributed to a variation of the chemical composition of the ZnO films. This work is done in the framework of an ANR project named Merethif.

Thin silver films are essential for applications that require high visible transparency along with good electrical conductivity, such as low-emissivity and solar control coatings. The nature of silver film formation, characterised by the coalescence of island-type structures, often necessitates a compromise between transparency and conductivity that could be mitigated by improvements in nucleation and growth mechanisms. For the applications outlined above which employ large-area dielectric substrates in continuous motion, it would be advantageous to influence film growth without resorting to conventional methods such as substrate heating or electrical biasing techniques.

Sputtering a silver target with ions from an inert gas plasma has differing effects on the nature and energy of particles bombarding both target and substrate as a function of gas species. The choice of inert gas affects the contribution of energetic reflected neutrals to the growth of the film, in terms of the energy delivered and the inclusion of inert gas into the coating. Bombardment of the target with ions of varying mass affects the sputter yield, and implantation at the substrate will also be influenced by mass-effects of ionic bombardment.

Glass substrates were pre-coated with zinc oxide before the deposition of thin silver films using DC and pulsed-DC magnetron plasmas generated from neon, argon, krypton and xenon source gases. The films were characterised in terms of their structural, optical and electrical properties by a variety of techniques including AFM, XRD, spectrophotometry and Hall-effect measurements. The results show the effect each source gas has on the growth and structure of silver coatings and the resulting optical and electrical performance of these films.

Hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs) is mainly used as the switching device in active matrix liquid crystal displays (AMLCDs). For the requirement for light-weight, portable, and unbreakable in consuming electronics, the products replacing the glass substrates with flexible ones, such as electronic paper, personal digital assistant (PDA), global positioning system (GPS), and etc. have rapidly attracted lots of attention in recent years. In this thesis, we investigated the stability of a-Si:H TFTs under mechanical strain with and without silicon nitride (SiNx:H) passivation. The process temperature of these flexibledevice, including the passivation layer, was well-controlled below 200℃, and the substrate was using stainless steel foil. The strain stress was applied cylindrically parallel to the active channel path of TFTs. The stability measurement was performed by DC gate bias stress and lasted up to 104 seconds. By using electrical parameter fitting, the Vth metastability mechanism was dominated by state creation effect. Our result indicated the device with passivation layer was improved, and the Vth had less variation under both outward and inward bending. By exerting 190℃ post-annealing process after SiNx:H deposited, the Vth was shifted left and the reliability of flexible TFTs became better than without post-annealing process. That's related to the passivating effect of hydrogen ion under passivation layer and post-annealing process.

Transparent conductive oxides (TCOs) have recently gained much attention in a variety of technologies in passive applications such as transparent electrodes for liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), and solar cells. The possibility to use TCOs in active applications such as transparent thin film transistors (TTFTs), UV sensors, and UV LEDs has also become a reality. High performance TFTs for active matrix display applications should have high electron mobility, high on/off current ratio, low threshold voltage, and low process temperature. However, these requirements cannot be fulfilled by conventional Si-based technologies. For this reason, many researchers have focused on oxide semiconductors including zinc oxide (ZnO), indium zinc oxide (IZO), zinc tin oxide (ZTO), and indium gallium zinc oxide (IGZO) as channel layers of TFTs. Especially, IZO thin films exhibit high electron mobility even when it is deposited at room temperature. It is also possible that IZO thin films are used both as channel layers and as source/drain (S/D) layers of TFTs.

NiO_x has been extensively used as complementary counter electrochromic (EC) layer in smart windows. Reactive sputter deposition for obtaining NiO_x layer using metallic Ni targets has been very often considered, however, the ferromagnetic property of metallic Ni failed to be applied by conventional magnetron enhancement, particularly when considering large-area deposition. To overcome this, 8 at% V is alloyed into Ni target for eliminating the ferromagnetism, and Ni(V)O_x films is deposited via the reactive magnetron sputter technique in this study. Microstructure and the related EC properties were investigated.

Experimental results shows that the sputtering erosion of the non-ferromagnetic Ni(V) target surface becomes controllable as opposed to the pure Ni target. The Ni(V)O_x film deposited at the working atmosphere of 1.33 Pa and oxygen flow ratio of 40% exhibits the highest optical transmission change (colored to bleached). The half device assembled with a 200 nm-thick film presents the shortest response time (2 s for coloring, 3 s for bleaching) and the greatest optical transmittance change in visible region (69.5% for bleached, 42.0% for colored). This study demonstrates that alloying 8 at% V as a de-ferromagnetiser in Ni target does not observably affect the electrochromic properties of deposits.

Transparent, nonvolatile and reversible resistance switching of Cu₂O, TiO₂, Cu₂O/ TiO₂ thin films were studied by current-voltage measurements using ITO electrode and conductive atomic force microscopy. Crystalline cuprous Cu₂O, TiO₂, and ITO were prepared by radio frequency magnetron sputtering at room temperature. The transparent devices with various electrode sizes were fabricated through shadow mask. The stacked-structure-RRAM has a transmittance (including the substrate) in the visible region. The correlation between resistance switching behavior and film thickness is discussed.

It has been reported that the various properties obtainable in transparent conducting impurity-doped ZnO thin films are affected by the deposition method, even when prepared under an optimized deposition condition. In particular, thin films deposited by conventional dc magnetron sputtering (dc-MS) with an oxide target exhibit non-uniform resistivity distribution and increase the obtainable resistivity in thin films that are deposited on moving large area substrates for practical applications. In this paper, we demonstrate a deposition technique newly developed for improving various properties of transparent conducting impurity-doped ZnO thin films prepared on low temperature glass substrates by dc-MS. The decrease of the obtainable lowest resistivity and the improvement of the spatial resistivity distribution on the substrate surface in impurity-doped ZnO thin film prepared by conventional dc-MS were successfully achieved by inserting a very thin buffer layer between the thin film and the glass substrate.

Al- or Ga-doped ZnO (AZO or GZO) thin films were prepared with thicknesses of 30 to 200 nm by a MS apparatus with an oxide target. The basic sputter depositions were carried out on OA-10 glass substrates at a temperature of 200^oC in a pure Ar gas atmosphere at a pressure of 0.4 Pa with a dc power of 200 W. For example, in the preparation of transparent conducting AZO thin film by dc-MS with an AZO target, first a very thin AZO film, deposited by a MS apparatus with the same target under different deposition conditions, was inserted as the buffer layer on the glass substrate . T hen the transparent conducting AZO thin film was deposited to a desired total thickness by conventional dc-MS. T he deposition of the buffer layer was found to be necessary to sputter the target surface whose oxidiz ation was stronger than the target surface during conventional dc-MS deposition. Such a strongly oxidized target surface was attained with a target sputtered under an appropriate condition, for example, after the target was used in the sputtering for an appropriate time under an rf power supply or oxygen gas was introduc ed into the chamber. Since t he buffer layer ’s optimal thickness was found to be approximately 10 nm, the obtainable lowest resistivity and resulting spatial resistivity distribution were evaluated by varying the thickness from 2 to 30 nm. The lowest resistivity of 4 X10^-4 Ω cm was obtained in 100-nm-thick-AZO thin films inserted in an AZO buffer layer with a thickness of 10 nm, whereas the obtained resistivity of the AZO thin films inserted without the buffer layer exhibited 9 X10^-4 Ω cm.

We recently reported that intense blue emission in both PL and EL was observed from La₂O₃:Bi phosphor thin film. This paper describes the photoluminescent (PL) and electroluminescent (EL) characteristics in newly developed various activator (X)-co-doped La₂O₃:Bi (La₂O₃:Bi,X) phosphor thin films prepared by an rf magnetron sputtering (rf-MS) deposition method. The La₂O₃:Bi,X phosphor thin films were deposited on thick BaTiO₃ ceramic sheets by a combinatorial rf-MS deposition method using a powder target, which was a mixture of La₂O₃, Bi₂O₃, and oxide co-activator (Tm, Yb, Er, Eu, Tb, Dy, Ce, or Mn) powders calcined at 1200^oC in air. Sputter depositions were carried out under the following conditions: atmosphere, pure Ar gas; pressure, 6 Pa; and substrate temperature, 250-350^oC. The thickness of all the deposited oxide phosphor thin films was approximately 1.5 µm. After deposition, the phosphor thin films were postannealed in either an Ar gas or an air atmosphere for 60 minutes at temperature rang ing from 700 to 1100^oC. Multi-color PL emissions were observed from all La₂O₃:Bi,X phosphor thin films postannealed above approximately 800^oC, regardless of the postannealing atmosphere . However, the PL emission color was considerably affected by the kind and the content of the co-doped activator . For example, multi-color PL emission consisting of blue broad peak due to the 6s²-6s¹6p¹ transition in Bi³⁺ and some sharp peaks due to the transition in additional rare earth ions were observed from La₂O₃:Bi,Dy, La₂O₃:Bi,Er, and La₂O₃:Bi,Eu phosphor thin films under excitation at a wavelength of 302 nm. N ote that PL emission due to the transition in rare earth ions was observed under the Bi³⁺ excitation wavelength. On the other hand, the color and intensity of the obtained EL emission were also considerably affected by the kind and the content of the co-doped activators. The thin - film EL (TFEL) devices were fabricated with an AZO/ZnS/La₂O₃:Bi,X/BaTiO₃/Al structure. The ZnS thin film, deposited by rf-MS, was introduced to improve the excitation efficiency of the hot electrons in the La₂O₃:Bi,X thin-film emitting layer. The EL emission in the wide area of the visible range was obtained in TFEL devices fabricated using La₂O₃:Bi,Er, La₂O₃:Bi,Dy, or La₂O₃:Bi,Eu emitting layers. The CIE color coordinate s of the EL emission from La₂O₃:Bi,Er, La₂O₃:Bi,Dy, or La₂O₃:Bi,Eu TFEL devices were changed from (0.18, 0.24) to (0.29, 057), (0.18, 0.24) to (0.34, 042), or (0.18, 0.24) to (0.50, 041), respectively. N ote that the CIE color coordinates of the EL emissions were control led by the content of the co-activator.