ICMCTF2003 Session C7: Optical Thin Films for Active Devices

Friday, May 2, 2003 8:30 AM in Room Sunset

Friday Morning

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8:30 AM C7-1 EL and PL Properties of Y2O3-Based Oxide Phosphor Thin Films
T. Miyata, T. Minami (Kanazawa Institute of Technology, Japan)
We review newly developed oxide phosphors shown to be promising as a phosphor emitting layer of thin-film electroluminescent (TFEL) devices. In particular, this paper focuses on Y2O3-based oxide phosphors: a binary compound and various ternary compounds and multicomponent oxides, composed of Y2O3 and another binary compound such as Al2O3, Ga2O3, Gd2O3, In2O3, B2O3, GeO2 or SiO2. Thin films were prepared on thick BaTiO3 ceramic sheets by rf magnetron sputtering, pulsed laser deposition or a sol-gel process. All the deposited films were postannealed at 700 to 1100°C in an Ar gas atmosphere in order to improve the luminescent characteristics. EL characteristics were investigated using a thick ceramic-insulating-layer-type TFEL device structure. The obtained EL and PL characteristics of the resulting oxide phosphor thin films were strongly dependent on the preparation and postannealing conditions as well as the composition (Y2O3 content). In addition, the intensity of PL emission was always correlated to the maximum luminance obtained in TFEL devices fabricated using the phosphor thin films. For example, high luminances and luminous efficiencies could be obtained in Mn-activated Y2O3-based oxide phosphor TFEL devices. Luminances above 7000 cd/m2 (1 kHz voltage) and luminous efficiencies of approximately 10 lm/W (60 Hz voltage) were obtained in TFEL devices fabricated using a Y2O3:Mn, a ((Y2O3) 0.6-(GeO2) 0.4):Mn or a ((Y2O3) 0.5-(Ga2O3) 0.5):Mn thin film and driven by an ac sinusoidal wave voltage at the noted frequencies. The color of EL emissions from these TFEL devices was yellow: however, that from the ((Y2O3) X-(Ga2O3) 1-X):Mn TFEL devices changed from yellow to green as the composition (X) was decreased.
9:10 AM C7-3 Amorphous Carbon Nitride Thin Film as Electron Injection Layer in Organic LEDs
M. Cremona (PUC-Rio, Brazil); R. Reyes (Universidad Nacional de Ingenieria, Peru); C.A. Achete, P.J.G. Araújo, S.S.Jr. Camargo (Universidade Federal do Rio de Janeiro, Brazil)

Recently, there has been an increased interest in organic light emitting diodes (OLEDs) due to their potential applications to color flat panel displays and in new optoelectronic components. These devices are assembled using three organic molecular materials: an electron injection layer, the emitting one and finally the hole injection layer. However, in most cases the electron injection is more difficult to achieve than hole injection. In this work two different amorphous carbon nitride thin films cathode were used to decrease the electron injection barrier. The first kind were a-C:N thin films deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) onto the MTCD/Alq3 layer structure thermally deposited. Amorphous carbon films were obtained with 100% N2 partial pressures (total pressure P=8 Pa). Next, amorphous silicon carbon nitride thin films (a-SiC:N) have been deposited using a rf Magnetron Sputtering (13.56 MHz) system onto the same organic structure. In this case the films were deposited in reactive nitrogen-argon atmospheres. Also in this case, the deposition pressures was P = 8 Pa. In both cases a thick (150 nm) aluminum electrode were deposited onto the whole structure. The deposition process for the organic compounds is performed in high vacuum environment(6x10-6 Torr)on glass substrates coated with an hole injecting ITO transparent layer. A preliminary investigation was conducted on the properties of the OLED device with the two a-C intermediate layers. The relationship between the properties of the amorphous carbon deposited films on the electroluminescent characteristics of the different devices are investigated. The refractive index of the a-C film deposited, their conductivity and optical absorption, the OLED I-V curves, a preliminary photoluminescent and electroluminescent OLED results are presented and discussed.

This work is supported by CNPq (RENAMI), FAPERJ and FAPESP.

9:30 AM C7-4 Photovoltaic Effect in Nanocrystalline Pb(1-x)Fe(x)S-Single Crystal Silicon Heterojunctions
Rakesh K. Joshi (Indian Institute of Technology Delhi, India); S. Mohan, S.K. Agarwal (Solid State Physics Laboratory, Timarpur, India); H.K. Sehgal (Indian Institute of Technology Delhi, India)
Photovoltaic effect is observed in self-assembled heterojunctions obtained in p-type nanocrystalline Pb(1-x)Fe(x)S (x = 0.25, 0.33, 0.50) films and n-type single crystal Silicon substrates. The p-type ternary Pb(1-x)Fe(x)S films grown on the substrates by a solution growth technique were observed to be homogeneous and grow with basic lead sulfide structure. Average grain size in the nanocrystalline films was estimated by transmission electron microscopy and is observed to decrease from 25 nm to 17 nm with increase in iron concentration (x) in the films. The lattice parameter of the nanocrystalline ternary films was observed to decrease with increase in x. The films were observed to have direct optical band gap from 1.90 eV to 1.60 eV when the iron concentration (x) was increased from 0.25 to 0.50. The decrease in band gap with increase in iron concentration suggests the alloying of FeS (Eg = 0.04 eV) with the nanocrystalline PbS. The current-voltage (I-V) characteristics confirmed the formation of heterojunctions in all the samples. Increase in the value of forward current with increase in iron concentration (x) in the junctions can be attributed to the increase in surface area contact caused by decrease in the grain size of the films with x.
10:10 AM C7-6 Low Resistivity Transparent Conducting Oxide Thin Films Prepared by Pulsed Laser Deposition
A. Suzuki, H. Matsushita, T. Aoki, H. Agura (Osaka Sangyo University, Japan); M. Okuda (Osaka Prefecture University, Japan)

High-transparency, low-electrical-resisitivity indium-tin-oxide (ITO) films (7.2 times 10-5 ohm-cm) and Al-doped ZnO (AZO) films (9.27 times 10-5 ohm-cm) were prepared by pulsed laser deposition (PLD) using an ArF excimer laser (wavelength : 193 nm).

First, a remarkable feature of the method used in this study to fabricate the ITO films is to deposit films in magnetic field (1.24 T), which is caused by rare earth permanent magnet of NdFeB, applied perpendicular to plume. As a reason to explain these results, it is thought that electrons in the plume accelerated by Lorentz force play a role to ionize species such as atoms, molecules and clusters, to promote plasma densities and to increase carrier concentrations. Next, in order to improve a reproducibility of the method, the distances between the target and the substrate were varied from 80 mm to 10 mm. A purpose of this trial is to make full use of a thermal energy of the plume and to increase Hall mobilities through an improvement in crystallinity caused by exposing the substrate to instantaneously high-temperatures. As a result, the films with 10-5 ohm-cm were obtained in the probability of about 75 %.

Secondly, a similar method such as applying magnetic field to plume and shortening the distance between the target and the substrate was adopted to fabricate AZO films. As a result, we were preferentially able to fabricate the AZO films with the lowest-electrical-resistivity (9.27 times 10-5 ohm-cm) by optimizing various preparation conditions.

10:50 AM C7-8 Electrochemically Induced Electrical and Optical Switching in SmHx Thin Films
P. Kumar (Indian Institute of Technology Delhi, India); L. Malhotra (Institute of Technology Delhi, India)
Discovery of optical and electrical switching in yttrium and lanthanum hydride thin films in 1996, has triggered large scale investigations of rare earth hydride thin films. Both light and heavy rare earths have been shown to exhibit this so called "switchable mirror" effect. Besides gas phase loading, solid as well as liquid electrolytes have been used as means of hydrogen loading. In this paper, we report on the ability of samarium films to quickly switch between the reflecting and transparent states electrochemically by altering the hydrogen content. Samarium films of typical thickness 55 nm deposited by vacuum evaporation and covered with Pd overlayer of thicknesses 5, 8 and 11nm were switched in a 1M KOH solution galvanostatically at room temperature. The lateral resistance of the films was found to increase, at the end of the hydrogen charging, to yield a metal to semiconductor transition. Our studies have shown that thickness of palladium overlayer plays the most crucial role in observing fast metal to semiconductor transition as well as to obtain a very high optical contrast. The transition from the metallic gray samarium film to golden greenish samarium hydride thin films was achieved in < 6 sec. This large change in transparency suggests possibilities of its use as an optical shutter as well as a hydrogen sensor material.
11:10 AM C7-9 The EMI Shielding Effect of IZO/Ag Alloy Multi-layered Thin Films
W.M. Kim (Korea Institute of Science and Technology, South Korea); D.Y. Ku, I.K. Lee (Hankuk Aviation University, South Korea); B. Cheong, T.S. Lee, K.S. Lee (Korea Institute of Science and Technology, South Korea)
A study was made to examine the electro-magnetic interference (EMI) shielding effect of the multi-layered thin films in which indium-zinc oxide (IZO) thin films and Ag or Ag alloy thin films were deposited alternately at room temperature using RF magnetron sputtering. The optical, the electrical and the morphological properties of the constituent layers were analyzed using an UV-visible photospectrometer, a 4-point probe and an AFM, respectively. The EMI shielding effect of the multi-layered thin films was also measured with an ASTM method. A detailed analysis showed that the control of the film morphologies, i.e. the surface roughnesses of the constituent layers were essential in order to increase the visible transmission and to decrease the sheet resistance of the multi-layered thin films. It was shown that properly optimized IZO/Ag alloy multi-layered thin films could yield a visible transmission of more than 70 per cent, a sheet resistance less than 1 Ω/, together with an EMI shielding effect larger than 45dB in the range from 30 to 1000 MHz.
11:30 AM C7-10 Microstructure and Photo-catalytic Performance of HVOF Sprayed TiO2 Coatings
G.-J. Yang (Xi'an Jiaotong University, PR China); C.-J. Li (Welding Research Institute, PR China); F. Han, S.-F. Mao (Xi'an Jiaotong University, PR China)
TiO2 is a promising photocatalytic material to be applied to water treatment and air purification. The utilization of TiO2 material in the form of coating is more desirable and compatible. Thermal spray process has been widely used to deposit coating at a large scale. During thermal spraying spray material at fully or partially melted condition is projected to a substrate followed by flattening and rapid cooling and solidification. The coating of a quenched microstructure is usually formed. Plasma spraying and conventional flame spraying have been applied to deposit TiO2 coatings. It was found that those coatings generally consisted of rutile TiO2 with only a small fraction of anatase TiO2 despite the crystal structure of starting powder. However, the anatase TiO2 is more effective as a photocatalyst than rutile TiO2 . The recent investigation revealed that high velocity oxy-fuel (HVOF) process could deposit TiO2 coating of a high anatase TiO2 content. Therefore, in the present study, the effect of HVOF spray conditions on the crystal structure of TiO2 coatings is investigated. The effect of the constitution the crystal structure in TiO2 coatings on the photocatalytic decomposition of phenol in the water is examined. The microstructure of the coating is characterized by XRD and SEM. The decomposing characteristics are evaluated by Ultra-Visible spectrum. It was found that the fuel gas flow has a significant influence on the structure of the coating. The coating contained up to over 50% anatase TiO2 estimated by XRD can be obtained under HVOF conditions despite the crystal structure of the starting powder. The high anatase TiO2 content is related to a better photocatalytic performance.
11:50 AM C7-11 The Characteristics of Thin Film Electroluminescent Displays Produced using Sol-gel Produced Tantalum Pentoxide and Zinc Sulphide
Y. Kavanagh, D.C. Cameron (Dublin City University and Institute of Technology (Carlow), Ireland)
An inverted double insulating thin film electroluminescent (TFEL) display has been fabricated using all sol-gel methods. Sol-gel deposition procedures in conjunction with sulphidation are used here to produce the active ZnS:Mn layer in the TFEL device. The silicon substrate which serves as the lower electrode, is coated with an high-permittivity insulating Ta2O5 layer followed by the active ZnS:Mn layer and then by another Ta2O5 layer. The top transparent electrode is ZnO:Al. All layers are deposited using sol-gel techniques; in addition, a sulphidation process is used to make the ZnS:Mn. This paper reports the effect of the crystal structure of the Ta2O5 on the luminescent properties of the device. The luminescent characteristics of the fabricated devices have been measured and a comparison is made with the characteristics of sol-gel TFEL devices using SiO2 insulators. The TFEL devices with the Ta2O5 insulators have shown higher stability and more reliability in their operation.
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