Papers

ICMCTF2004 Session C7: Optical Thin Films for Active Devices

Tuesday, April 20, 2004 8:30 AM in Room Sunset

Tuesday Morning

Start	Invited?	Item
8:30 AM		C7-1 Conductive Granular Molecule in C-Au-S Film by a Combination of Plasma CVD and Sputtering for 2D Photonic Crystals S. Morita (Nagoya University, Japan) Carbon-gold-sulfide (C-Au-S) films formed by co-operation process of plasma CVD and sputtering were characterized as a large refractive index material as 3.5. For the film with 10 atomic % of Au, the X-ray diffraction did not show any peaks of Au cluster, ESCA spectra showed large chemical shift for atoms of C, Au and S, and refractive index showed almost same value as the film with a few atomic % of Au. By thermally treatment at 200° for 30 min, those characteristics were stable. From the refractive index analysis, the atoms are expected to be ionic-bonded to granular molecule with 0.4~0.6 nm size, which is conductive and is distributed in an insulating amorphous carbon base. The films can be applied as dielectric for a photonic crystal and quantum dots devices.
9:10 AM		C7-3 Thin Film Based Novel Wide Band Gap MgZnO Alloys via Composition and Process Tuning and their Advanced Optoelectronic Applications R.D. Vispute, S. Hullavarad, I. Takeuchi, S.M. Kanetkar, V.N. Kulmarni, T. Venkatesan (University of Maryland) Optical and electronic grade wide band gap thin films of Mg_xZn_1-xO with "x" varying from 0 to 1 have been grown under metastable phase space processing conditions using pulsed laser deposition technique. We have systematically studied the band gap alloying as a function of Mg using combinatorial synthesis and found that the band gap can be tuned from 3.3 to 7.8 eV for Mg_xZn_1-xO with x=0 to 1 extending the cutoff wavelength from UV-A (320-400 nm) to UV-B (280-320 nm) and UV-C (200-280 nm) regions. Results from TEM and scanning x-ray diffraction microanalysis indicated the structural transition from purely hexagonal to mixed (hexagonal and cubic) phase for x>0.35 with typical features of cubic nanocrystal in ZnO matrix, whereas a pure cubic ZnMgO phase formation above x=0.5. Both, wurtzite and cubic thin film alloys have been grown on sapphire, MgO, ZnO, SrTiO₃ buffered Si, quartz, glass, and polymeric substrates and optical and electronic devices have been fabricated. Critical issues of band gap engineering, chemical ordering, phase separation, growth optimization for homo and heteroepitaxy of new alloys will be discussed. Novelty of the band gap engineering in the ZnMgO films via composition and process tuning for the fabrication of new optical filters, quantum well structures, electron emitters, UV and solar blind detectors and arrays, optically pumped UV lasers, transparent FETs, metal-semiconductor heterostructures, and optical spintronic devices will be addressed.
9:30 AM		C7-4 Electrophosphorescence Emission in Organic LEDs Based on (Sm+Eu) Complexes R. Reyes (Universidad Nacional de Ingenieria, Peru); M. Cremona (Pontificia Universidade Catolica do Rio de Janeiro, Brazil); E.E.S. Teotonio (Universidade de Sao Paulo, Brazil); O.L. Malta (Universidade Federal Pernambuco -CCEN, Brazil); H.F. Brito (Universidade de Sao Paulo, Brazil); C. Legnani (Pontificia Universidade Catolica do Rio de Janeiro,, Brazil) In the last decades there was a huge development of the research activities to obtain integrated light sources based on organic materials. Due to their applications in many different areas and manufacturing simplicity, organic electroluminescent diodes (OLEDs) represent a promising research in the development of new optoelectronic and photonic devices. The emission wavelength of the OLEDs can be changed by using different combinations of organic materials. This work reports the preparation of a tunable OLED using two different trivalent rare earth (Sm³⁺ and Eu³⁺)ion complexes. The devices were assembled using a heterojunction between three organic molecular materials. The junction was formed by a 40 nm thickness film of 1-(3-methylphenyl)-1,2,3,4 tetrahydroquinoline-6-carboxyaldehyde-1,1^'-diphenylhydrazone (MTCD), as hole transporting layer (HTL), a 25 nm thick layer of rare earth organic complexes [RE(TTA)₂(TPPO)₂], working as light-emitting layer. Then a 45 nm thick layer of tris(8-hydroxyquinoline)aluminium (Alq₃) has been used as electron transporting layer (ETL). Finally, an electron injecting Al electrode (150 nm) was deposited onto the system. The devices were grown in high vacuum environment onto indium tin oxide (ITO) transparent substrates. Polarizing the ITO as anode and the Al as cathode it was possible to observe the electroluminescence (EL) of the device at room temperature with the characteristics narrow emission bands from Sm³⁺ and Eu³⁺ ions. Continuous EL tuning emission was obtained varying the applied bias voltage from 10 to 22 V, changing the color emission from red to orange-yellow. Moreover an EL broad band peaked about 575 nm was observed and identified as molecular electrophosphorescence from the TTA-ligand. OLED light emission was almost linear with the current and the I-V curves show a typical dependence as expected for a semiconductor diode.
9:50 AM		C7-5 Intelligent Window Coatings: Tungsten Doped VO₂ by Atmospheric Pressure CVD T.D. Manning, I.P. Parkin (University College London, United Kingdom) Thin films of tungsten doped vanadium(IV) oxide were prepared on glass substrates from the atmospheric pressure chemical vapor deposition of vanadium(IV) chloride, tungsten(VI) ethoxide and water at 500-600°C. The films were characterized by Raman microscopy, glancing angle X-ray diffraction (GAXRD), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering (RBS), scanning electron microscopy (SEM), and vis/IR reflectance-transmittance. The films showed a reduction in thermochromic transition temperatures from 68°C in VO₂ to 42°C in V_0.99W_0.01O₂ - approaching that required for commercial use as an intelligent window coating.
10:10 AM		C7-6 Electrochemical Studies of Pd Capped Samarium Hydride Thin Film Switchable Mirror P. Kumar, L. Malhotra (Department of Physics, Indian Institute of Technology Delhi, India) A 55 nm samarium film capped with a 15 nm palladium overlayer switched from a metallic reflecting to a semiconducting, transparent in visible state during ex-situ hydrogen loading via electrochemical means in 1 M KOH electrolytic aqueous solution at room temperature. The optical transmittance exhibits a hysteresis effects on loading and deloading of hydrogen in SmHx thin film between x = 2 and 3. The hysteresis is discussed in terms of strain (and consequently stress) at the interface between the dihydride and trihydride. The diffusion coefficients and switching kinetics are shown to depend on applied current density. The changes of anodic overvoltage with time during applied constant current discharge have been used to determine the electrochemical parameters the exchange current density io and the symmetry factor.
10:30 AM		C7-7 Luminescent Properties of Rare Earth-Activated Gd_1-XY_XVO₄ Phosphor Thin Films T. Minami, T. Miyata, Y. Suzuki (Kanazawa Institute of Technology, Japan) This paper describes the preparation of full-color emitting rare earth-activated Ga_1-XY_XVO₄ phosphor thin films. The thin films of multicomponent oxide phosphors activated with Tm, Eu and/or Er were deposited on thick BaTiO₃ ceramic sheets by r.f. magnetron sputtering using powder targets. A mixture of V₂O₅, Gd₂O₃, Y₂O₃ and dopant (Eu₂O₃, Tm₂2O₃ and/or Er₂O₃) powders calcined in an Ar atmosphere was used as the targets. All the deposited films were postannealed at about 900 to 1200°C in an air atmosphere in order to improve the luminescent characteristics. Electroluminescent (EL) characteristics were investigated using a thick ceramic-insulating-layer-type thin-film EL (TFEL) device structure. It was found that the highest luminous efficiency and luminance for blue emission were obtained in a GdVO₄:Tm phosphor thin film prepared with a Tm content (Tm/(Tm+Gd) atomic ratio) of 1 at.% and postannealed at 1100°C. For red emission, a YVO₄:Eu phosphor thin film prepared with a Eu content of 1 at.% and postannealed at 1100°C exhibited the highest photoluminescent (PL) intensity, luminous efficiency and luminance. In addition, the highest PL intensity for green emission was obtained in a YVO₄:Er phosphor thin film prepared with a Er content of 1 at.% and postannealed at 1100°C. As an example of multi-color emissions, (GdVO₄:Tm)_1-Z-(YVO₄:Eu)_Z phosphor thin films exhibited emissions that changed from blue to red as the YVO₄:Eu content or Z, of the targets was increased from 0 to 1. As another example, white emission could be obtained in Gd_1-XY_XVO₄:Tm,Eu,Er phosphor thin films prepared using (GdVO₄:Tm)_1-X-((YVO₄:Er)_1-Z-(YVO₄:Eu)_Z)_X powder targets: very high PL and cathodoluminescent intensities in white emission from postannealed Gd_1-XY_XVO₄:Tm,Eu,Er phosphor thin films prepared with the compositions X=0.2 and Z=0.5. In addition, a Gd_1-XY_XVO₄:Tm,Eu,Er TFEL device exhibited a white emission. The CIE chromaticity color coordinates for blue emission obtained from a Gd_1-XY_XVO₄:Tm,Eu,Er phosphor thin film were (x=0.394 ,y=0.305).
10:50 AM		C7-8 Highly Reflective Thorium-based Mirrors J.E. Johnson, D.D. Allred, R.S Turley (Brigham Young University) Thin films of materials coated onto substrates can act as reflective mirrors of electromagnetic radiation in the extreme ultraviolet and soft x-ray regions. Since the late 1990s under the direction of Dr. David Allred, the BYU thin films research group has conducted reflectance experiments with uranium-coated mirrors. The scientific community identifies uranium as an element which theoretically would exhibit high reflectance in the soft x-ray and extreme ultraviolet range. Though uranium is still very promising, there are still certain major obstacles associated with its use. Uranium oxidizes fairly easily in its elemental form and has many oxidation states. Thorium is more reliable when oxidized, maintaining a +4 oxidation state. Through theoretical calculations, it appears that compounds of thorium, such as ThO₂, could be the most reflective over some wavelength ranges at low angles. If such is the case, highly reflective mirrors could be produced, enabling us to analyze the far reaches of the galaxy and beyond. These areas are still largely unknown and are rich in astrophysical diagnostics. The reflectance of thin films of thorium dioxide is being measured and analyzed to determine whether it matches up with theory.
11:10 AM		C7-9 Active Mg Doping Concentration Influenced by the Dopant Species in the Buffer Layer Y.L. Lai (National Cheng Kung University, Taiwan, R.O.C.); R. Chen (Genesis Photonics Inc., Taiwan, R.O.C.); C.P. Liu (National Chung Kung University, Taiwan, R.O.C.) For the optical applications, the development of an efficient p-type GaN with high doping concentration is necessary. Of which, magnesium is the most common dopant for p-type GaN, however, the active doping concentration is not directly proportional to the total Mg doping concentration. It has been reported that the active doping concentration can be influenced by the polarity of the underlined GaN phase. Here we present our results showing the polarity of the GaN phase can be varied by the dopant in the buffer layer, which then largely affect effective hole concentration. The samples are grown by metal-organic chemical vapor deposition. Sample A consists of a GaN buffer deposited on a sapphire substrate followed by a n-GaN doped layer with Si, then a seven periods of InGaN/GaN superlattices and finally a p-type GaN doped layers with Mg. The growth conditions of sample B are exactly the same as sample A except that the GaN buffer layer is intentionally doped with Mg. Photoluminescence and secondary ion mass spectroscopy were adapted to examine the emitting property and the concentration profile of each constituent species. In order to investigate the cause of the difference on the doping concentration, the surface morphology of the samples upon etching by H₃PO₄ and KOH is executed. Convergent-beam electron diffraction patterns are taken to confirm the supposition. On the other hand, by taking advantage of the etching behavior of GaN, we show that GaN quantum dot size can be effectively diminished and the shape evolution is examined.
11:30 AM		C7-10 Room-temperature Deposition of Metal Oxide Thin Films Prepared by Off-plane Filtered Cathodic Vacuum Arc B.K. Tay, Z. Zhao, G. Yu, S.P. Lau (Nanyang Technological University, Singapore) Metal oxide thin films, such as zirconium oxide, titanium oxide, hafnium oxide and aluminum oxide thin films, were prepared by off-plane filtered cathodic vacuum arc (FCVA) at room temperature, respectively. Characteristics of these thin films are analyzed by optical spectroscopy, X-ray diffractometer, X-ray photoelectronic spectroscopy and atomic force microscopy. Under optimum deposition conditions, thin films are amorphous with smooth surface and macroparticles-free. Moreover, in addition to the high transmittance especially in visible range, these metal oxide thin films exhibit better or comparable values in terms of refractive index and extinction coefficient as well as high packing density, compared with the same metal oxide thin films by other techniques. The properties possessed by metal oxide thin films imply the potential applications in optical coatings and for FCVA technique in deposition of metal oxide optical coatings.
11:50 AM		C7-11 Effect of Thickness of Nanostructured TiO₂ Deposit on Photocatalytic Activity During Photodegradation of Organic Gases G.J. Yang, C.J. Li, X.C. Huang, H.X. Wang (Xi'an Jiaotong University, P.R. China) Nanostructured and nano-porous TiO₂ photocatalyst in the form of thin film or coating presents good photocatalytic performance. Photocatalytic reaction takes generally place on the surface of the nano-sized TiO₂. However, owing to the existence of voids in the coating, light irradiation can be transmitted into subsurface layer of the coatings and subsequently photocatalytic reaction may not be limited to the surface of the coating resulting from the transportation of reactants along voids in the coating. Therefore, the coating thickness will influence photocatalytic activity of the coating. But there is no reasonable explanation for this phenomenon. In the present paper, a theoretic model is proposed to explain the effect of the thickness on the photocatalytic activity to aim at the optimization of the coating thickness. The nanostrucutred and nano-porous TiO₂ of different thickness from around 1 µm to 100 µm was deposited through flame spraying with liquid feedstock for experimental correlation with the proposed model. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction were used used to characterize the microstructure of the coatings. The photocatalytic activity of the coating of different thicknesses was evaluated by photodegradation of acetaldehyde under ultraviolet illumination. The microstructural characterization shows that the coating was composed of nano-sized spherical particles of the mean diameter from 10 to 20 nm. The photocatalytic activity of the coating was increased with an increase in the thickness up to around 6 µm. A little decrease was then observed with a further increase in the thickness up to 20 µm. No significant change in the photocatalytic performance was observed with the coatings thicker than 20 µm. Such change in the photocatalytic performance of the coating was correlated well with the simulation result estimated by the proposed model.