ICMCTF2007 Session C3-2: Optical Thin Films for Active Devices and Microsystems

Tuesday, April 24, 2007 1:30 PM in Room Tiki Pavilion

Tuesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	C3-2-1 Substitution of Impurity-Doped ZnO Thin Films for ITO in Transparent Electrode Applications T. Minami, T. Miyata (Kanazawa Institute of Technology, Japan) This paper surveys the present status and prospects for further development of substitute　materials, which contain a reduced amount or no indium, for indium-tin- oxide (ITO) in practical thin-film transparent electrode applications, such as liquid crystal displays. A promising alternative material in indium-free materials is impurity-doped ZnO, such as Al- and Ga-doped ZnO (AZO and GZO). In particular, the best material is AZO thin film due to its, low obtainable resistivity on the order of 10^-5Ωcm and inexpensive and non-toxic source materials. We believe that the current problems associated with substituting AZO or GZO for ITO, besides the stability of thinner these films than approximately 50 nm as well as the non-uniform distribution of resistivity resulting from dc magnetron sputtering deposition, could be resolved. We also describe the techniques that we are currently developing to overcome the latter problem: newly developed deposition methods such as magnetron sputtering (MSD) or vacuum arc plasma evaporation (VAPE) that enables the preparation of AZO and GZO films on large area substrates with a high deposition rate. However, the resistivity of all AZO and GZO thin films prepared with a thickness of 10-300 nm on glass substrates at a temperature below 200°C increased with exposure time when long-term tested in a high humidity environment (air at 90% relative humidity and 60°C) as well as with a decrease of film thickness. Test results showed that impurity-doped ZnO thin films prepared with a thicker film are suitable for use in applications such as flat panel displays and solar cells. Nevertheless, to improve the stability of AZO and GZO thin films prepared with a thickness below approximately 50 nm is remaining problem.
2:10 PM		C3-2-3 Microwave Shielding Properties of Ga-Doped ZnO Films Prepared by Reactive Plasma Deposition S. Kishimoto, T. Yamada, A. Miyake, H. Makino, T. Arimitsu, T. Morizane, T. Yamamoto (Kochi University of Technology, Japan) We report on the microwave response properties of Ga-doped ZnO (GZO) films on alkali-free glass substrates at 473 K with the area of 119 * 64 mm². The film thickness was varied from 500 to 5000 nm. The source of ZnO pellet was prepared by sintering ZnO powder containing a small amount of Ga₂O₃ powder (3 wt %). Polycrystalline GZO films were fabricated by reactive plasma deposition (RPD) using the DC-arc discharge technique with a traveling substrate. Growth rate was very high, 170 nm/min. The shielding effectiveness of the films at a frequency of 2.45 GHz was measured using HP8510B Vector Network Analyzer. Sheet resistance decreases monotonically with increasing film thickness: the maximum sheet resistance of 4.28 Ω/Sq. at a film thickness of 500 nm and the minimum sheet resistance of 0.32 Ω/Sq. at a film thickness of 5000 nm. Good microwave shielding effectiveness more than 30 dB with high transparency above 70 % has been observed and the maximum effectiveness was 43.6 dB for GZO films with a film thickness of 3000 nm and a resistivity of 1.8 *10^-4 Ωcm. In such a film, we found high transparency; optical transmittance of 73 % at a wavelength of 550 nm in the visible region. GZO films are eminently suitable for use in electromagnetic interference (EMI) shielding materials for see-through openings which require higher transparency.
2:30 PM		C3-2-4 Structural, Electrical and Optical Properties of Sputter Deposited Ag-Doped ZnO Films D.R. Sahu, J.-L. Huang (National Cheng-Kung University, Taiwan) Zinc oxide films have been investigated for the use in optoelectronics devices, piezoelectric transducer, space detectors, transparent conductive oxides, gas sensors etc because of their various electrical and optical properties in combination with direct wide band gap, large excitation energy, aboundace in nature and nontoxicity. We have prepared silver doped ZnO films by simultaneous rf magnetron sputtering of ZnO and dc sputtering of Ag on glass substrate. The dependency of coating parameter such as dopant content and substrate temperature on the properties of the as grown films are investigated. Several analytical tools such as X-ray diffraction, spectrophotometer, atomic force microscopy, scanning electron microscopy and four point probe were used to explore the possible changes in electrical and optical properties. The as grown film have a preferred orientation in the (002) direction. As the amounts of the Ag dopant were increased, the crystalinity as well as the transmittance and optical band gap were decreased while the electrical resistivity increased. However, as the substrate temperature was increased, the crystalinity and the transmittance were increased. A small amount of Ag (<1at %) lowered the resistivity by ~ 30% with only a slight decrease in the visible transparency. Details of the preparation process and the enhanced electrical conductivity due to Ag doping will be presented during presentation.
2:50 PM		C3-2-6 AZO Transparent Conducting Thin Films Deposited by Magnetron Sputtering using Targets Prepared Under Various Conditions T. Kuboi, T. Miyata, T. Minami (Kanazawa Institute of Technology, Japan) This paper describes electrical, optical, and structural properties in transparent conducting Al-doped ZnO (AZO) thin films prepared by different magnetron sputtering deposition (MSD) techniques, using sintered oxide targets prepared under various conditions. Although AZO thin films have recently attracted much attention as promising substitutes for ITO thin films, there are severe problems that prohibit their application as transparent electrodes in flat-panel displays such as liquid crystal displays. For example, a considerable spatial distribution of resistivity corresponding to the pattern of the target erosion area was created when AZO thin films were prepared on a low-temperature substrate by dc MSD using an oxide target. It was also found that the resulting electrical, optical and structural properties were considerably dependent on the MSD techniques employed as well as the target used in the deposition. In particular, the obtained resistivity and its resulting spatial distribution in AZO thin films prepared at a temperature below 200°C by MSD were considerably affected not only by the oxygen and Al content in the AZO target used, but also the target density. A lowered resistivity, as well as an improved resistivity distribution, were obtained in AZO thin films prepared using a target with lower oxygen content. In addition, the decrease of resistivity in AZO thin films was attributed to increases of both the carrier concentration and Hall mobility. Thus the target preparation conditions were optimized to obtain lower resistivity and more uniform resistivity distribution of AZO thin films prepared by MSD techniques. As a result, a resistivity as low as 3X10^-4Ωcm, and an almost uniform resistivity distribution, were obtained in AZO thin films prepared on substrates at a temperature below 200°C by MSD techniques using a newly developed target with low oxygen content.
3:10 PM		C3-2-7 Room Temperature Synthesis of Highly Oriented Nanocrystalline Anatase TiO₂ Thin Films by DC Magnetron Sputtering P. Singh, A. Kumar, D. Kaur (IIT Roorkee, India) We report the systematic study of the process parameters that facilitate the growth of highly oriented nanocrystalline anatase TiO₂ thin films prepared at low temperature by dc magnetron sputtering. The films were successfully deposited on various substrates such as glass, sapphire and silicon. The nanocrystalline films were deposited at room temperature while loosely adhering nanoparticles were deposited at lower temperature. The influence of sputtering power, substrate temperature, nature pressure and flow rate of sputtering gas on the deposition rate, mean particle size, surface morphology, crystallographic orientation and optical properties of these films were studied in detail. XRD pattern of the films shows only the anatase phase with highly intense 101 reflection for the films deposited at constant discharge power of 150 W and sputtering pressure of 10 m Torr. HRTEM studies also confirm the presence of anatase phase. Field Emission Scanning Electron Microscope (FE-SEM) and Atomic Force Microscope (AFM) studies show the uniform grain growth with low surface roughness over a large area of these films. The optical measurement revealed the systematic band gap variation with crystallite size. The transmittance of these films was also found to be influenced by the surface roughness and film thickness.