1999 MRS Spring Meeting & Exhibit

Chairs

Baldassare DiBartolo
Dept of Physics
Boston College
Higgins Hall 257
Chestnut Hill, MA 02167-3811
617-552-3601

Adrian Kitai
Dept of Engineering Physics
McMaster Univ
Hamilton, ON L8S 4L7 CANADA
905-525-9140

Joanna McKittrick
Dept of Applied Mech & Engr Sci
Univ of California-San Diego
MS 0411
La Jolla, CA 92093-0411
619-534-5425

Kailash Mishra
Osram Sylvania Inc
Beverly, MA 01915
978-750-1575

Cees Ronda
Philips GmbH
Forschungslaboratorien
Aachen, D-52085 GERMANY
49-241-6003397

 

Proceedings published as Volume 560
of the Materials Research Society
Symposium Proceedings Series.

* Invited paper

SESSION E1/B1: JOINT SESSION:
INORGANIC LOW VOLTAGE PHOSPHORS
Monday Morning, April 5, 1999
Metropolitan I (A)

8:30 AM *E1.1/B1.1
PRODUCTION OF HIGH PERFORMANCE, MICRON-SIZED, SPHERICAL POWDERS FOR DISPLAY APPLICATIONS. James Caruso, Mark Hampden-Smith , Toivo Kodas, Klaus Kunze, Scott Pennino and Dan Skamser, Superior MicroPowders, Albuquerque, NM.

Superior MicroPowders (SMP) has recognized the need for improved powders for use in the display industry. These powders include metal, glass, and phosphor (photoluminescent and cathodoluminescent) powders with unique characteristics that provide a performance advantage in the device. SMP is producing spherical, micron-sized, controlled size distribution powders that can be deposited by conventional and advanced deposition techniques to produce densely packed powder layers. This presentation will focus on the characteristics and performance of phosphor powders used in emissive displays that have high brightness and superior powder packing characteristics. In particular, the low voltage cathodoluminensce of europium-doped yttria will be described in detail as an example of the attributes of these materials. The fundamental origin of the improved performance of SMP powders over existing materials will be discussed.

9:00 AM E1.2/B1.2
PERFORMING ACCURATE CATHODOLUMINESCENCE MEASUREMENTS ON PHOSPHOR POWDERS AND SCREENS. Lauren Shea and Robert Walko, Sandia National Laboratories, Albuquerque, NM.

Cathodoluminescence (CL) efficiency is an important property used to determine the potential of a phosphor material for use in flat-panel display applications such as field emission displays (FEDs). The CL efficiency of a phosphor is the ratio of its energy output to the energy input. Materials properties that influence CL efficiency include band gap energy, electrical/thermal conductivity, and the defect structure of the host lattice. Processing-dependent properties such as particle size, crystallite size, and concentration of impurities also effect phosphor efficiency. To fully understand these effects requires CL measurement accuracy. There are many experimental factors that influence accurate measurement and calculation of the CL efficiency of a phosphor. These include electron beam profile (uniform, Gaussian), current density, electron accelerating voltage, secondary electron collection, physical state of the sample (e.g., powder, pellet, screen, thin-film), and optical detection system. This paper will present some of the common sources of experimental error and provide suggestions for achieving improved accuracy of CL measurements. Results of round-robin CL characterization experiments will also be discussed.

9:15 AM E1.3/B1.3
LUMINESCENCE AND MATERIAL CHARACTERIZATION OF CO-DEPOSITED TUNGSTEN DOPED ZINC OXIDE PHOSPHOR. J.B. Sobti , P.M. Babuchna, V. Bhatia, M.H. Weichold, Texas A&M University, Department of Electrical Engineering, College Station, TX.

High visual quality in combination with lightweight and low power consumption requirements make field emission (FEDs) of great interest for future use. However, their development is still dependent on refinement of many recent technical advances such as: high efficiency cathodes, high aspect ratio insulating spacers, and intrinsic properties and characterization of phosphor for low voltage applications. This paper presents luminescent properties of a co-deposited zinc oxide phosphor for low voltage FEDs. Previous work by other researchers has demonstrated that zinc oxide, a II-VI semiconductor, is capable of producing red and green luminescence. We used diffusion techniques of layered structures with different metals in zinc oxide (ZnO) and found that doping ZnO with tungsten (W) produced peak intensity at 476 nm during photoluminescence studies. In order to study the efficiency of the phosphor and to determine the fundamental mechanism possible for emission of blue light, the films are being prepared by the co-deposition of W and ZnO using ion-mill supttering techniques. The films prepared in this manner have varying W concentration  and , and are being annealed at , ,  for 4, 8, and 12hrs. in the presence of argon and oxygen. For the material charcterization of the films, X-ray diffraction (XRD) has been preformed. The XRD spectra of these films show the presence of zinc tungstate (ZnWO₄), along with tungsten oxide. Since ZnWO₄ has been known to have a blue emission band near 480 nm, it is possible that ZnWO₄ is being formed as a result of annealing and is responsible for the blue light emission. The room temperature cathodoluminescence (CL) studies of the films prepared under various conditions have shown blue luminescence for the samples that are annealed at higher temperatures  and . These films have demonstrated cathodoluminescence at voltages as low as 300 V. Peak intensity has been found to be at 499 nm during these studies. The cold temperature cathodoluminescence and infra red (IR) spectra will be presented for the material characterization of the phosphor. Color space and the efficiency measurements (lumens/watts) will also be reported for luminesce studies. Along with these results, room temperature photoluminescence (PL) charaterization will be reported. Pl excitation will be achieved using mecury vapor acr lamp.

9:30 AM E1.4/B1.4
IMPROVING THE LOW-VOLTAGE LUMINESCENT PROPERTIES OF BLUE EMITTING YTTRIUM SILICATE. E. Bosze , J. McKittrick and G.A. Hirata, Materials Science Program and Dept of Applied Mechanics and Engineering Sciences, University of California-San Diego, LaJolla, CA.

Yttrium silicate activated with Ce³⁺ and co-activated with GD³⁺, (Y_1-n-mCe_nGd_m)₂SiO₅, is a potential blue emitting phosphor for low voltage display applications. Low voltage field emission displays (FED's) are currently under development and in production as the next generation of displays. The industry proposed operating voltages for FED's is between 200 and 8000 eV. At these low operating energies, the luminous efficiency of the phosphors is diminished and does not meet the requirements for this application. Additionally, the chromaticity coordinates need to be shifted to a more saturated color to increase the color gamet of the screen and obtain a higher white emission. To improve the chromaticity and efficiency of (Y_1-n-mCe_nGd_m)₂SiO₅, other 3⁺ rare earth ions, such as Ho³⁺, Pr³⁺, Er³⁺, and Tm³⁺ and a transition metal, Ti⁴⁺, have been added as dopants in this work. Cathodoluminescence (CL), between 500 and 5000 eV, and photoluminescence (PL), =358nm, measurements have been made while varying the activator and dopant concentrations. Results will be presented on how these rare earth ions affected the emission spectra of this blue phosphor.

9:45 AM E1.5/B1.5
MODELING OF CATHODOLUMINESCENCE PROPERTIES OF PULSED LASER-DEPOSITED EUROPIUM-ACTIVATED YTTRIUM OXIDE THIN FILM PHOSPHORS. Kyu-Gong Cho , D. Kumar, Rajiv K. Singh, P.H. Holloway, University of Florida, Department of Materials Science and Engineering, Gainesville, FL.

We have conducted a series of experiments and developed a modeling to understand photoluminescence (PL) and cathodoluminescence (CL) properties of pulsed laser deposited europium-activated yttrium oxide (Eu:Y₂O₃) thin film phsophors. The Eu:Y₂O₃ films were deposited on (100) silicon substrates using 248 nm KrF pulsed laser. The roughness and the film thickness were found to play an important role in determining PL and CL brightness of the Eu:Y₂O₃ films. Prior to film deposition, the substrate surfaces were roughened to different level to investigate the effects of the film roughness on the PL and CL properties of Eu:Y₂O₃ thin films. The effect of the film thickness on the cathodoluminescence was investigated using Eu:Y₂O₃ thin film films with different thicknesses. An increase film roughness has been shown to significantly improve the brightness of the Eu:Y₂O₃ phosphor films. A model was developed to explain the roughness and thickness effects. The model is based on the internal reflection of generated light as a function of surface roughness.
 

SESSION E2/B2: JOINT SESSION:
THIN FILM PHOSPHORS
Monday Morning, April 5, 1999
Metropolitan I (A)

10:15 AM *E2.1/B2.1
THIN-FILM OXIDE PHOSPHORS AS ELECTROLUMINESCENT MATERIALS. Tadatsugu Minami , Kanazawa Institute of Technology, Electron Device System Laboratory, Ishikawa, JAPAN.

Recently, thin-film oxide phosphors have attracted much attention as luminescent materials for field emission displays and thin-film electroluminescent (TFEL) displays because of the higher chemical and thermal stability of oxide phosphors compared with sulfide phosphors. In this paper, we introduce rare earth- or transition metal-activated oxide phosphor thin films prepared by magnetron sputtering and dip-coated solution deposition. In addition, TFEL devices using these oxide phosphor thin films as the emitting layer are investigated with a single insulating layer device structure using a thick ceramic sheet insulating layer. High-luminance multicolor emissions and high luminous efficiencies were obtained in TFEL devices using various oxide phosphors: binary compound, Ga₂O₃; ternary compound, Zn₂SiO₄ and ZnGa₂O₄; multicomponent oxides, CaO-Ga₂O₃ and Zn₂SiO₄-Zn₂GeO₄ systems. Various thin-film oxide phosphors are suitable for the emitting layer of full-color electroluminescent displays because their emissions can be produced in the primary colors without the use of color filters. Luminances above 200 cd/m2 were obtained in Ga₂O₃:Mn, CaGa₂O₄:Mn, ZnGa₂O₄:Mn and Zn₂Si_1-xGe_xO₄:Mn TFEL devices driven at 60 Hz: 809 and 592 cd/m2 in Zn₂Si_0.75Ge_0.25O₄:Mn and CaGa₂O₄:Mn TFEL devices, respectively. Maximum luminous efficiencies of 2.35, 1.7 and 1.2 lm/W were obtained in the Zn₂Si_0.7Ge_0.3O₄:Mn, Ga₂O₃:Mn and ZnGa₂O₄:Mn TFEL devices, respectively. In addition, the improvement in electroluminescent characteristics was obtained in oxide phosphor TFEL devices driven by voltages at a high frequency. A stable long-term operation as well as a high-luminance green emission were obtained in dip-coated and sputtered Ga₂O₃:Mn phosphor TFEL devices even under 10 kHz-high frequency driving.

10:45 AM E2.2/B2.2
IN-SITU AND EX-SITU ANNELAING STUDIES ON METAL-ORGANIC CHEMICAL VAPOR DEPOSITION OF ZnS:Mn FILMS FOR APPLICATIONS IN ELECTROLUMINESCENT DEVICES. Anna Topol , Karl Barth, Janice Lau, Harry Efstathiadis, Alain Kaloyeros, Center for Advanced Thin Film Technology and Physics Department, The University at Albany-SUNY, Albany, NY; Richard Tuenge, Chris King, Planar Systems, Inc., Beaverton, OR.

Deposition of ZnS:Mn (orange monochrome) with viable EL performance is of great interest to panel display technologies. Variety of different process techniques (including MBE, ALE, sputtering, and MOCVD) have been investigated for the growth of this material. Continuing work focuses on the improvement in the efficiency of the phosphor films. In this paper, we report on in-situ and ex-situ annealing studies performed on MOCVD ZnS:Mn films. Results will be presented on the influence of various annealing conditions on film characteristics (morphology, crystallinity, and composition) and EL performance (brightness, efficiency, decay time, and threshold voltage). Optimized MOCVD ZnS:Mn growth conditions will be reported along with corresponding structural and chemical characterization results. In particular, characterization data obtained by RBS, dynamic SIMS, XRD, SEM, AFM, XPS, and CSTEM will be discussed.

11:00 AM E2.3/B2.3
LUMINESCENT PROPERTY OF EPITAXIAL ZnO THIN FILMS GROWN ON R-PLANE SAPPHIRE SUBSTRATE. J. Liu , H. Shen and D.C. Morton, Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD; C.R. Gorla, S. Liang and Y. Lu, College of Engineering, Rutgers University, Piscataway, NJ.

High-efficiency thin film phosphors has been of great intesest in the development of modern display devices. Compared to conventional powder phosphors, high quality crystallized thin film phosphors will have advantages of high electrical and thermal conductivities, fast response time, high energy saturation limit, and high screen resolution. High-efficiency thin film phosphors are also ideal for flat panel cathodoluminescent displays since it is less likely to release residual particles and gases to contaminate the electon emitters. We report here the investigation of luminescent property of epitaxial ZnO films for their potential application as thin film phosphor. The ZnO films were grown from the R-plane (0112) sapphire substrate using MOCVD. High quality epitaxial structure was obtained under a growth temperature of 500C. The thickness of the films ranges only from 0.5 to 1.0 m. Although no post-deposition annealling was used, TEM results showed atomically sharp and semi-coherent interface between ZnO and R-Sapphire. Unlike ZnO films grown on C-Sapphire, the film is free of columnar boundaries. Low temperature PL also indicates the film is strain free. Cathodoluminescence of these ZnO films was studied in comparison with other ordinary ZnO films and ZnO powder. The luminescence from an unannealed epitaxial ZnO film was found to be even better than an annealed ordinary one of three times in thickness. The emission spectrum of these films range from 450-750 nm. Significant improvements in luminescene are expected for Zn rich and thicker films.

11:15 AM E2.4/B2.4
FABRICATION OF BRIGHT ZnS:Mn THIN FILMS BY ATOMIC LAYER EPITAXY, D. Ravichandran , S. Blomquist, D.C. Morton and D.Y.T. Chiu, Army Research Laboratory, Adelphi, MD.

Previously thin films of ZnS:Mn have been deposited by CVD, MBE and ALE process. In our work, ZnS:Mn thin-films have been fabricated by ALE and the photoluminescence has improved dramatically by Rapid Thermal annealing (RTA) under various gas conditons. The RTA processing distinguishes this work from previous results. ZnS is a classic phosphor when doped with transition metals like Mn or with rare earth ions. Thin films were coated on a 2"x2" glass substrate coated with transparent conducting layer ITO. The temperature of the chamber was maintained at 500C, which is sufficient to allow only a nominal monolayer of reactant to be retained at the substrate surface through physical adsorption. Grazing angle X-ray diffraction shows phase pure ZnS wurtzite structure with a preferred orientation along the 002 direction. Photoluminescence measurements show two strong broad bands at 502 and 440 nm with excitation at 328 nm. The films showed an emission in the blue-green region. SEM micrograph shows the particles were well crystallized with a grain size of 0.15 m. The films were annealed under O₂, N₂ and forming gas conditions to improve the emission intensity of the films. The effect of the annealing on morphology and luminesence will be presented.

11:30 AM E2.5/B2.5
STOICHIOMETRY, GRAIN SIZE, MORPHOLOGY, ADHESION, AND PHOSPHO-RESCENCE OF PULSED LASER DEPOSITED ZINC OXIDE NANOCLUSTER FILMS. L.N. Dinh , M.A. Schildbach, M. Balooch, W. McLean II, Lawrence Livermore National Laboratory, Livermore, CA.

The dependence of stoichiometry, grain size, phosphorescent colors, adhesion, and surface morphology of pulsed laser deposited zinc oxide films as a function of oxygen background pressure were investigated. Auger electron spectroscopy (AES) showed that stoichiometric ZnO films were obtained only at oxygen pressures > 150 mTorr. X-ray diffraction (XRD) revealed that pulsed laser deposited zinc oxide films were composed mainly of nanocrystals, the average grain size of which increased from a few nanometers to tens of nanometers as the oxygen pressure was increased up to 1 Torr. The surface morphology of the films as studied by secondary electron microscopy (SEM) also became more roughened as the grain size increased. Stoichiometrically deposited films exhibited blue luminescence under electron bombardment, while slightly sub-stoichiometric films glowed white under similar excitation. All zinc oxide films deposited in an oxygen background pressure up to 150 mTorr exhibited stronger adhesion to substrates than corresponding films synthesized by other techniques.

11:45 AM E2.6/B2.6
A SIMS STUDY OF ELECTROLUMINESCENT PHOSPHOR: SrS: Cu, Mn. Larry Wang , Lily H. Zhang, Charles Evans & Associates, Redwood City, CA; WuSheng Tong, YongBao Xing, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA.

Strontium sulfide (SrS) has recently emerged as an important phosphor host material in thin film electroluminescent displays (TFELD) [1]. The Cu and Mn are doped in SrS as color activators for blue and green light emission, respectively. The Y and Cl are added as co-dopants for charge compensation in SrS. There has been significant development recently in the growth and processing of SrS as EL phosphors [1,2]. However, in order for these materials to compete with the liquid crystal displays, the brightness has to be increased about 1.5x for SrS:Cu, 3x for SrS:Mn. Reliable and precise characterization techniques are needed to correlate film growth with device performance to better understand the basic mechanisms relating to EL performance. Due to its high sensitivity and depth profiling and imaging capabilities, SIMS was used in this study to determine the depth profile and lateral distribution of Cu and Mn as well as Y and Cl in MBE grown SrS films. Some of the SrS films were annealed in-situ in the MBE chamber, the diffusion behavior of Cu, Mn, Y was studied using SIMS depth profiles. The Mn ion images were obtained using SIMS to understand grain growth of SrS:Mn. These results of Cu and Mn concentration and their diffusion as well as the effectiveness of Y and Cl for charge compensation are related to device performance characteristics 1. S.S. Sun, E. Dickey, A Bright and Efficient New Blue TFEL Phosphor, SID, 1997, 301-304. 2. W. Tong, Y. Xin, M. Chaichimansour, J. Choi, T. Jones, W. Park, B.K. Wagner, C. J. Summers, S.S. Sun, Molecular Beam Epitaxy Growth of SrS:Cu, sunmitted to SID, 1998.
 

SESSION E3: INORGANIC PHOSPHORS
Monday Afternoon, April 5, 1999
Franciscan I (A)

1:30 PM *E3.1
METAL ACTIVATOR-FREE SILICATE PHOSPHORS. Michael J. Sailor , Will H. Green and Michael A. Ansell, Department of Chemistry and Biochemistry, University of California San Diego, CA.

The sol-gel reaction of tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) with a variety of carboxylic acids leads to a solid glass within a few minutes to a few days. The as-prepared silicate glasses show no photoluminescence (PL) but a simple heat treatment in air between 200C and 500C produces a material that displays intense white PL between 450-600 nm. The external PL quantum yield (phi, measured under 365-nm excitation) ranges from 0.20 to 0.45 and the PL lifetime is < 10 ns. Glasses heated over 450C for more than 4 hr. additionally display bright green phosphorescence with a decay time of 0.5 s. The sol-gels could be dissolved in aqueous HF and one of the species extracted from this solution was a highly luminescent red oil. The PL of this crude extract is very similar to that of the original sol-gel strongly suggesting that this substance is responsible for the luminescence. The chromophore is proposed to be a polyaromatic species. A second class of luminescent materials was synthesized by the reaction of various carboxylic acids with amino-functional trialkoxysilanes such as 3-aminopropyltriethoxysilane (APTES). The transparent yellow-orange polysiloxane (aminosiloxane) that forms from the reaction of formic acid and APTES is highly luminescent (phi= 0.3-0.4) and is water soluble. Despite the similarity of their photophysical properties, the origin of the PL in aminosiloxanes is probably different from the tetraalkoxysilane-derived materials. The chromophore in 3-aminopropylsiloxanes is proposed to be a cyclic silyl imidate species. Similar polymers derived from N-(2-aminoethyl)-3-aminopropyltrimethoxysilane are deep red in color and display bright green fluorescence under blue excitation. Model compounds of the various aminosiloxanes have been synthsized and characterized in order to better understand the nature of the chromophore in these materials.

2:00 PM E3.2
RARE EARTH LUMINESCENCE IN GaN AND RELATED DEVICE PERFORMANCE. A. J. Steckl, R. Birkhahn and M. J. Garter, University of Cincinnati, Nanoelectronics Laboratory, Cincinnati, OH.

We review the recent breakthroughs made with the in-situ incorporation of rare earth (RE) elements into GaN films. GaN, being a wide bandgap semiconductor was expected to minimize the attenuation in near-infrared (IR) emission from the lowest excited state in the RE species for which it is a host (for ex. Er at 1.5 m, Pr at 1.3 m). We have obtained for the first time emission from higher excited RE states in GaN, resulting in light emission in the green (Er at 537, 558 nm) and red (Pr at 648, 650 nm) range of the visible light spectrum. This emission is suprisingly strong, being observable with the naked eye at room temperature. These films were produced by MBE with solid sources for Ga and the RE species and with a nitrogen plasma source. In this paper, we will first review the MBE growth of RE-doped GaN films on sapphire, Si and thick GaN film substrates. Next, we will describe the structural and compositional analysis of the GaN films, and their optical and electrical properties. We will then discuss the relationship between thin film growth conditions, RE incorporation level and corresponding emission intensity. We will also address the fabrication of various light emitting devices utilizing RE-doped GaN structures and rectifying metal contacts (both semitransparent Al and highly transparent and conducting In-Sn oxide (ITO). We will present recent device measurements (EL spectra, current-light, quantum efficiency) which indicate very promising characteristics. Finally, we will review the possible applications of RE-doped GaN in the fields of displays (ranging from flat panel TV to outdoor displays), lighting (from traffic lights to indoor white lights), instrument indicators, optical memory, etc. A.J. Steckl and R.H. Birkhahn, Visible Emission from Er-doped GaN Grown by Solid Source Molecular Beam Epitaxy, Appl. Phys. Lett., 73, 1700, Sept. 1998. R.H. Birkhahn and A.J. Steckl, Green Emission from Er-doped GaN Grown by Molecular Beam Epitaxy on Si Substrates, Appl. Phys. Lett. 73, 2143, Oct. 1998. A.J. Steckl, M.J. Garter, R.H. Birkhahn and J. Scofield, Green Electroluminescence from Er-doped GaN Schottky Barrier Diodes, Appl. Phys. Lett., 73, 2450, Oct. 1998.

2:15 PM E3.3
EFFECTS OF CO-DOPANTS ON THE MICROSTRUCUTRE AND EL PROPERTIES OF THE ZnS:Mn AND ZnS:Tb LUMINESCENT MATERIALS (This work is supported by the Phorsphor Technology Center of Excellence). Qing Zhai , Karen Waldrip, Jay Lewis, Jinghong Li, Kevin Jones, Paul Holloway, University of Florida, Dept. of Materials Science and Engineering, Gainesville, FL; Maggie Puga-Lambers, Mark Davidson, Univeristy of Florida, Microfebritech, Gainesville, FL.

Thin film of zinc sulfide (ZnS) doped with Mn or Tb, were deposited using magnetron sputter sources. Since the electroluminescent properties of the as-deposited films were poor, post-sputtering rapid thermal annealing (RTA) and the introduction of different co-dopants by thermal evaporation were carried out. The electroluminescent properties were measured using a trapezoidal waveform with 40 micro-seconds pulse width and 5 micro-seconds rise and fall times at 60Hz. Significant improvement on electroluminescent performance was observed on the samples with post-sputter processing. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were employed to characterize the microstructure of the investigated materials, including grain morphology, defect structure, as well as polytypes in the materials. Both inter-grain and intergrain distributions of the codopants were measured using energy dispersive X-ray spectra (EDX), and the distribution versus thickness was determined by secondary ion mass spectrometry (SIMS). A correlation between electroluminescent properties and the microstructure is obtained, and the mechanisms of the improvement in electroluminescent properties of the investigated materials due to the co-dopants will be discussed.

2:30 PM E3.4
THE EFFECT OF ELECTRIC FIELDS ON CATHODOLUMINESCENCE FROM PHOSPHORS. Carleton Seager , Sandia National Laboratories, Albuquerque, NM.

When external electric fields are applied to phosphors the cathodoluminescence (CL) at low beam energies is strongly affected. This experiment has been carried out on a variety of common phosphors used in cathode ray tube applications, and the electron beam energy, beam current, and electric field dependences of the CL are thoroughly characterized. It is found that the general features of these effects, particularly the strong polarity and beam energy dependence, are consistent with a model which assumes that the main effect of the electric fields is to alter the populations of electrons and holes at the phosphor surface. This in turn, modulates the non-radiative energy losses that strongly affect the low-beam-energy CL efficiency. Because the external fields are applied without any direct contact to the phosphor material, the large changes seen in the CL decay rapidly as the beam-created electrons and holes polarize, shielding the externally applied bias. The dependence of the time constant for this field relaxation on the electric field size and direction, the beam energy and current yields insight into the transport and trapping of electrons and holes. We demonstrate that these results have important implications for designing phosphors which might be efficient at low electron energies.

2:45 PM E3.5
PREPARATION AND LUMINESCENCE OF EUROPIUM DOPED ZINC SILICATE PHOSPHOR. Hongxi Zhang , Yan Zhou, Chan Hin Kam, Yee Loy Lam, Yuen Chuen Chan, School of Electrical and Electronic Engineering, Nanyang Technological Univ, SINGAPORE.

Europium doped zinc silicate phosphors in both powder and film forms have been prepared by a modified sol-gel method. X-ray diffraction, scanning and transmission electron microscopy, atomic force microscopy and FT-IR have been used to characterize the micro-structures of the powders and films. The results indicate that nano-sized powders of zinc silicate can be obtained by proper heat-treatment of the dry gel. Well-crystallized films have been deposited on bare silicon and barium titanate buffered silicon wafers. I has been found that by adjusting the concentration of the precursors and annealing parameters, the surface roughness of the films can be maintained to the range of 20 nanometers. Photoluminescence of the powders and films has been carried out under UV excitation and strong red emission has been observed. The results have shown that the annealing parameters have significant influence on the relative emission intensity and decay kinetics of the phosphors and intense emission can be obtained for those phosphors annealed to a temperature of 800 degree in air and oxygen and cooled down slowly. Decay kinetics measurement and fitting showed fast electron transfer from oxygen vacancies to luminescent centers. The paper will report the detailed experimental results together with analysis and discussions.

3:15 PM E3.6
OPTIMIZATION OF CERIUM DOPED GARNETS USING COMBINATORIAL CHEMISTRY FOR APPLICATION AS LUMINESCENT CONVERSION PHOSPHORS IN WHITE LEDS. Jennifer L. Wu , Univ of California-Santa Barbara, Dept of Chemical Engineering, Santa Barbara, CA; Martin Devenney, Earl Danielson, Damodara M. Poojary, Henry Weinberg, Symyx Technologies, Santa Clara, CA.

An area of considerable research interest is the development of visible light, down-converting phosphors for application in white light emitting diodes (LEDs). In such devices, a blue LED can act as the primary light source, exciting photoluminescence in a phosphor with subsequent broad band emission occurring at lower energies. Although a few phosphors exist for this application, their efficiency is low. We have developed a combinatorial approach to synthesize and screen potential inorganic phosphors for such an application and have identified improved phosphors for blue to yellow down conversion. Using solution chemistry techniques, solid state thin-film arrays (libraries) based on the garnet compound Y₃Al₅O₁₂:Ce³⁺ (YAG) were synthesized. Solution precursors were used to generate the array of thin film phosphor samples on a silicon wafer. After thermal processing, characterization of the samples was performed by exciting the phosphors with 465 nm light (FWHM 35 nm) to mimic commercial blue LED light and imaging the library simultaneously with a CCD camera. Libraries of a YAG-substituted host, (Y_1-xGd_x)_3-z(Al_1-yGa_y)₅O₁₂:Ce_z, where x and y = 0 to 1.0 and z = 0.03 in which Gd is substituted for Y and Ga for Al were both synthesized and characterized to demonstrate that the combinatorial approach can be used to rapidly screen potential phosphors for use as luminescence down convertors in white LEDs. X-ray powder diffraction patterns show that the thin films are of the crystalline garnet phase and that solid solutions are formed with the addition of Ga and Gd in the YAG matrix. Emission and excitation trends match those reported in the literature for traditionally prepared powder samples; as Gd content increases, emission red shifts and as Ga content increases emission and excitation blue shift. The optimal Ce³⁺ concentration is identified and within the YAG-substituted host, specific compositions are identified as promising blue to yellow phosphor candidates.

3:30 PM E3.7
OPTIMIZATION OF MIXED STORAGE PHOSPHORS OF THE TYPE (Ba,Sr)F_1+xBr_1-x: EU FOR DIGITAL X-RAY RADIOGRAPHY. M. Batentschuk, P. Hackenschmied, A. Winnacker , Institute of Material Science 6, University of Erlangen-Nürnberg, Erlangen, GERMANY; R. Fasbender, Agfa-Gevaert AG, Photo-Technik Werk, München, GERMANY.

Image plates on the basis of the storage phosphor BaFBr:Eu play an increasingly important role in digital X-ray radiography. It had been found out before that the sensitivity of the material can be improved by the addition of Sr and by changing the F/Br-ratio away from stoichiometry. In this paper we investigated - as another possibility to change the F/Br-ration - the incorporation of F via CsF. We have observed effects on the stimulation spectrum identical to those by other ways to vary the F/Br-ratio. Control experiments, adding CsJ instead of CsF, indicate that the effect of CsF-doping is not primarily due to a replacement of Ba²⁺ by Cs¹⁺ and subsequent charge compensation. A systematic study of the phosphor efficiency as a function of various dopants is presented. The corresponding changes in stimulation as well as luminescence spectra are reported and discussed on the basis of microscopic models of the defect centre complexes, which are introduced into the material either in the syntheses process or under X-ray irradiation as part of storage process.

3:45 PM E3.8
EFFECT OF CO-DOPING ON THE PHOTOLUMINESCENT BEHAVIOR AND ELECTRONIC STRUCTURE OF ZINC SILICATE PHOSPHORS. Hee Dong Park , Kee-Sun Sohn, Bonghyun Cho, Hyunju Chang, Korea Research Institute of Chemical Technology, Advanced Materials Division, Taejon, KOREA.

The main objective of present investigation is to improve the photoluminescent performance of zinc silicate phosphors with co-dopants. We have also investigated the electronic structure of those phosphors, using the first principles embedded-cluster DV-X alpha method. The candidate co-dopants adopted in the investigation are confined to substituents for the Zn site. The co-dopants incorporated into the phosphors are believed to alter the internal energy state so that the change in emission and decay time could be expected. For this sake, several experimental skills such as time resolved emission spectra, decay curves, and time resolved excitation spectra, were conducted to co-doped zinc silicate phosphors. Both Mg and Cr have a favourable influence on photoluminescent properties, in particular, Mg enhances the intensity of green emission and Cr shortens the decay time. The optical transition state calculation method was employed to calculate the green absorption peak of the phosphors. The calculated transition energy agrees with the experimentally observed value reasonably.

4:00 PM *E3.9 DISPLAY PHOSPHORS - THE LAST FOUR YEARS. Ronald O. Petersen , Motorola, Inc., Flat Panel Display Division, Tempe, AZ.

The last four years has seen a marked increase in flat panel display phosphor development. What achievements have we made? What obstacles remain?
 

SESSION E4: SYNTHESIS, PROCESSING AND CHARACTERIZATION
Tuesday Morning, April 6, 1999
Franciscan I (A)

8:30 AM E4.1
GROWTH OF EPITAXIAL THIN FILM La₃Al₅O₁₂ ON Gd₃Ga₅O₁₂ SUBSTRATES BY MOLECULAR BEAM EPITAXY AND IN-SITU DOPING WITH Dy. Sangsig Kim and Jeffrey O. White, Materials Research Laboratory, Univ of Illinois at Urbana-Champaign, Urbana, IL; Stephen G. Bishop, Microelectronics Laboratory, Univ of Illinois at Urbana-Champaign, Urbana, IL; James N. Eckstein, Dept of Physics, Univ of Illinois at Urbana-Champaign, Urbana, IL.

Undoped epitaxial La₃Al₅O₁₂ (LAG) films were grown on Gd₃Ga₅O₁₂ (GGG) substrates by molecular beam epitaxy (MBE) equipped with an ozone (O₃) source. A GGG substrate (lattice constant: 12.375  ) was used because of its lattice match with LAG (lattice constant: 12.398 ). A (111) substrate was tried because garnets deposited on this orientation by other techniques are very flat and hillock-free. After growth, the undoped LAG film was characterized by Rutherford backscattering and X-ray diffraction. In situ Dy doping of an epitaxial LAG film was performed to confirm that the high solubility observed in bulk samples is also obtained in MBE. The optical quality of the Dy:LAG film was determined by Raman scattering, optical transmission, photoluminescence (PL), and photoluminescence excitation (PLE). The absorption and luminescence bands associated with Dy³⁺ were observed in the spectral range from the ultraviolet (0.3 m) to the mid-infrared (around 3 m). Dy³⁺ ions may occupy a variety of sites in garnets, each characterized by slightly different crystal field symmetry. PL and PLE spectroscopy revealed all the different Dy³⁺ sites present in Dy:LAG.

8:45 AM E4.2
NOVEL SYNTHESIS OF SULPHIDE BASED LUMINESCENT COATINGS ON PARTICULATE MATERIALS BY PULSED LASER DEPOSITION. James Fitz-Gerald, Naval Research Laboratory, Washington, DC; Michael Ollinger, University of Florida, Dept of Materials Science and Engineering, Gainesville, FL; Philip Rack, Rochester Institute of Technology, Dept. of Microelectronic Engineering, Rochester, NY; Rajiv Singh , Unviversity of Florida, Dept of Materials Science and Engineering, Gainesville, FL; Alberto Pique, Douglas Chrisey, Naval Research Laboratory, Washington, DC.

Sulphide based phosphor materials have been routinely utilized in cathode ray tube displays (CRT), electroluminescent (EL) displays and have been explored for field emission display (FED) applications. Sulphide based materials yield high luminescent efficiencies in comparison to oxide base phosphors. However, due to inherent degradation mechanisms associated with sulphide based materials, these phosphors cannot be efficiently utilized in many field emission display technologies. In this paper we present an alternative use for these materials in the area of particulate coatings. Sulphide based coatings of ZnS:Ag and ZnS:Cu were deposited on particulate core materials with a modified pulsed laser deposition system in a hydrogen sulphide atmosphere at pressures ranging from 50-600 mTorr. A pulsed excimer laser (lambda=248nm), with a pulse duration of 25 ns was operated from 2-15 hz at energies between 2-5 J/cm². The core particulate materials (alumina, silica) were mechanically agitated continuously during deposition for coating uniformity. Characterization was performed by cathodoluminescence (CL), photo luminescence (PL), x-ray photo electron spectroscopy (XPS) and scanning electron microscopy (SEM).

9:00 AM E4.3
PHOTOLUMINESCENCE OF INORGANIC MATERIALS: RELATIONSHIPS WITH SYNTHETIC ROUTE AND SUBSEQUENT TREATMENTS. David Ajò and Giorgio Pozza, ICTIMA, C.N.R., Padova, ITALY; Marco Bettinelli and Adolfo Speghini, Dipart. Scientifico e Tecnologico, Università di Verona, Verona, ITALY.

Spectroscopical studies of crystalline or amorphous systems provide information of value in several fields, from optoelectronics to gemmology. This information concerns the oxidation state of a luminescent center, its chemical and geometrical environment and, in turn, the material story from synthesis - or mining - till the final product. The involved mechanisms are various in nature, e.g.:
- the luminescence spectra of some doped nanostructured oxides (1) appear to be significantly different from those of the bulk materials, due to enhanced cross relaxation of some excited states for some dopants, and to different average coordination for others.
- natural or synthetic oxides (2) such as corundum, spinels, silicates and titanates exhibit various kinds of optical behaviour due to the presence of non-intentional impurities - often in different oxidation states - coming from the raw materials, or to structural and morphological properties related to the synthetic route and/or to physico-chemical treatments.
Recent results concerning the above topics and related fields will be reported and discussed. (1 ) G. Tessari, M. Bettinelli, A. Speghini, D. Ajò, G. Pozza, L.E. Depero, B. Allieri and L. Sangaletti; ``Synthesis and Optical Properties of Nanosized Powders: Lanthanide-doped Y<sub>2</sub>O<sub>3</sub>'', Appl. Surface Sci., in press.
(2) S. Carbonin, D. Visonà, I. Rizzo, M.L. Favaro, G. Poza and D. Ajò; ``Optical Properties and Composition of Corundum and of Synthetic and Mineral Related Materials'', Inst. Phys. Conf. (1998) Ser. no.152, Section G, 827-830.

9:15 AM E4.4
SPUTTERING LUMINESCENT TA2ZN3O8 THIN FILMS. Philip D. Rack , Michael D. Potter, Advanced Vision Technologies, Inc., West Henrietta, NY; Andrew Woodard, Santosh Kurinec, Rochester Institute of Technology, Department of Microelectronic Engineering, Rochester, NY.

Thin films have been sputter deposited by rf magnetron sputtering from a stoichiometric Ta2Zn3O8 ceramic target (3ZnO+1Ta2O5). Negative ion re-sputtering effects have been observed when sputtering stoichiometric Ta2Zn3O8 target. These effects have been attributed to O- ion formation resulting from a Ta-to-O electron transfer process. Zinc deficient thin films were depositted as a result of the preferential re-sputtering of Zn versus Ta. The negative ion re-sputtering effects are exacerbated at higher powers and lower pressure. This observation is correlated to the oxygen ion transport through the dark space and the plasma, which ultimately controls the energy distribution of the oxygen particles that arrive at the substrate. Another ceramic target with excess ZnO (6ZnO+1Ta2O5) was also sputtered under similar conditions, however predominantly ZnO films were deposited. Finally, a mosaic ZnO-Ta target was sputtered, which resulted in stoichiometric and luminescent thin films. In this presentation, the luminescent properties of Ta2Zn3O8 will be reviewed, and the aforementioned thin film sputtering issues will be discussed. Particular attention will be given to the negative ion re-sputtering phenomenon which includes four processes: 1) negative ion formation, 2)ion acceleration through the dark space, 3) ion/neutral drift through the plasma, and 4) re-sputtering at the substrate surface.

9:30 AM E4.5
PARAMAGNETIC DEFECTS IN LUMINESCING SPARK-PROCESSED SILICON. Victor Ya.Bratus' , Eugene G. Gule, Taras T. Petrenko, Igor P. Vorona, Volodymyr A. Yukhimchuk, Institute of Semiconductor Physics NASU, Dept of Optics and Magnetic Resonance, Kyiv, UKRAINE; Rolf E. Hummel, Univ of Florida, Dept of Materials Science and Engineering, Gainesville, FL.

Spark processing of silicon produces structures with strong photoluminescence (PL) in violet/blue, green or orange spectral range. Various mechanisms have been proposed for explanation of the origin of different luminescent bands, including quantum confinement and electron-hole recombination at Si/SiO₂ interface and defect states. The results of comprehensive EPR study of such structures in X- and Q-band are presented in this report and the role of defects in PL is briefly discussed. The samples of spark-processed Si (sp-Si) were created in different conditions and annealed in a nitrogen ambient with small (about 3) admixture of oxygen at temperatures ranging from 200C to 1100C for 15 minutes. The EPR spectra of the sp-Si samples consist mainly of a narrow and a broad line. The first one completely disappeared after annealing at 600C has been identified by its g-values as a set of E' centers. Two weak lines have been observed about 37.1 G on either side of the EÕ central resonance. According to the studies of ``bulk'' SiO₂, the defect with 74 G doublet structure was reasonably assigned to hydrogen coupled EÕ center. A decomposition of the broad EPR line and its slight angular variation at Q-band allow to conclude that it is associated with P_b1, and P_b0 centers. The 41.2 G weak doublet concerned with this line has been tentatively attributed to a hydrogen coupled P_b1, center. In addition, the EPR line in the range of g=2.0090 has been assigned to a broadened signal of non-bridging oxygen hole centers. The presence of EÕ centers implies that oxygen vacancies in SiO₂ are created during spark processing. The P_b defects as well-known centers of nonradiative recombination could give rise to the inhomogeneity of PL observed in sp-Si. As in Si⁺ - implanted Si/SiO₂ structures a correlation between the number of defects and the intensity of PL band at 620 nm in as-prepared samples has been found.

9:45 AM E4.6
VISIBLE PHOTOLUMINESCENCE FROM a-Si:H FIMLS GROWN BY ULTRAHIGH VACUUM ELECTRON CYCLOTRON RESONANCE CHEMICAL VAPOR DEPOSITION. S.J. Joo and S.H. Lim, Seoul National Univ., School of Materials Science and Engineering, Seoul, KOREA; W.C. Choi and E.K. Kim, Korea Institute of Science and Technology, Semiconductor Materials Research Center, Seoul, KOREA; E. Yoon , D.C. Marra and E.S. Aydil, Univ. of California, Dept. of Chemical Engineering, Santa Barbara, CA.

Since the first report of visible light emission from porous Si [1], the visible photo-/electro-luminescence at room temperature from porous Si and nanocrystalline Si particles embedded in hydrogenated amorphous silicon (a-Si:H) or SiO₂ films has attracted much attention. In general, quantum confinement of charge carriers in nanometer-sized crystalline Si is believed to be responsible for the visible luminescence from these materials, but the exact mechanism remains controversial. In this report, results of room-temperature PL analysis of a-Si:H films grown by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition (UHV-ECRCVD) are presented. The a-Si:H films were deposited on either (100) Si or 100nm-thick SiO₂ substrates without external heating. An ECR hydrogen plasma was used to dissociate SiH₄ and H₂. The films were grown under the conditions of high H₂/SiH₄ flow rate ratios (between 50 and 100). As-grown a-Si:H films showed a strong visible luminescence band centered around 700 nm at room temperature. Unlike the broad band luminescence typically observed from a-Si:H and porous Si, the spectra consisted of a series of sharp peaks (FWHM 25 nm). The peak separation between adjacent peaks increased with wavelength (from 24 nm at 550 to 60 nm at 750 nm), strongly suggesting that the PL is related to the vibrational mode of the luminescent species. Changes in deposition parameters affected the peak intensity as well as the peak position. Raman spectra showed only an amorphous Si peak at 480 cm^-1, whereas Fourier-transformed infrared transmission spectroscopy showed (SiH₂)_n-related peaks in our films. The effects of deposition parameters and subsequent annealing on the PL spectra of a-Si:H will be reported in detail. [1] L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990).

10:15 AM E4.7
SYNTHESIS OF HOMOGENEOUS HIGH LEVEL ER-DOPED SILICA GLASS THROUGH SOL-GEL PROCESSING WITH ORGANIC COMPLEXATION. Xiuhong Han and Guozhong Cao, University of Washington, Department of Materials Science and Engineering, Seattle, WA.

Synthesis of homogeneous high level Er-doped silica glass through sol-gel processing with organic complexation is presented. 3-Aminopropyltrimethoxylsilane (APS) has been used as organic complexing agent to react with Er ions through complexation and, thus, linked the Er ions into the silica gel network. As a result, high level homogeneous doping (up to 10 wt%) of Er in silica gel network was achieved. Nitrogen adsorption analysis showed that the addition of organic complexing agent had a great influence on the microstructure of the silica gel network due to a high pH value resulted from the chemical reactions between the organic ligands and water. Optical absorption, fluorescence spectra, Fourier transform infrared (FTIR), and magic angle spin - nuclear magnetic resonance (MAS-NMR) were applied to local atomic structure characterization and optical property measurements. Both fluorescence spectra and MAS-NMR relaxation time indicate that the phase segregation or formation of Er-clusters was inhibited by the complexation with organic ligands.

10:30 AM E4.8
THE STRUCTURE PROPERTY RELATIONSHIPS IN MOLECULAR FILMS DEPOSITED BY ORGANIC MOLECULAR BEAM DEPOSITION. S.M. Bayliss , S. Heutz, R.L. Middleton, G. Rumbles, J.M. Hartmann, T.S. Jones, Centre for Electronic Materials and Devices, Imperial College of Science, Technology and Medicine, London, UNITED KINGDOM.

Organic molecular beam deposition (OMBD) is an ideal technique for depositing thin films of molecular materials and forming multilayer structures with novel optical properties. The controlled and reproducible nature of the deposition process provides an excellent opportunity to study the fundamental relationship between the structure and optical properties of the films and for optimising the growth conditions. In this paper, we present results for the deposition by OMBD of single and multilayer structures of metal free phthalocyanines (H2Pc) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). The structure of the films has been characterised by X-ray diffraction and Raman spectroscopy, the optical properties by UV/visible absorption spectroscopy, and the surface morphology has been investigated by atomic force microscopy (AFM) and Nomarski interference microscopy. For both H2Pc and PTCDA, the growth parameters have a significant impact on the structure, optical properties and surface morphology of the films, and by a suitable choice of growth conditions, it is possible to produce the two phases that are known to exist in the solid state. The very different morphologies of the two phases has a significant influence on the growth of multilayer structures and the quality of interface produced. The implication of these results for potential device structures will be discussed.

10:45 AM E4.9
MATERIAL CHARACTERIZATION OF STAIN ETCHED POROUS SILICON FOR USE IN PATTERNED LEDS. Max D. Alexander, Jr. , Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH; Thomas W. Listerman, Wright State Univ, Physics Dept, Dayton, OH.

Porous silicon has been known since the mid 1960ís. However, the room temperature luminescent properties did not bring the material to great attention until 1990. The common production process requires the silicon sample to be placed in an acid bath while electric current is passed from the wafer through the solution to an inert electrode. This process then results in preferential etching of the surface resulting in the formation of nano-size silicon columns. These columns can be produced such that their radius is smaller than the radius of a free exciton. This results in interesting quantum mechanical phenomenon, such as visible luminescence. However, another less commonly discussed technique to produce these structures is the stain etching process. This process does not require assistance by electrical current, thereby reducing the complexity of the fabrication process. Several electroluminescent and photoluminescent devices have been produced by this method and have been characterized by our group. We examine the etched surface by scanning electron microscopy and high-resolution x-ray photoelectron spectroscopy to determine the structure and chemical species present at the surface. In addition, the photo and electroluminescent properties of the devices have been characterized by I-V techniques, quantum efficiency, and fluorescence lifetime spectroscopy.

11:00 AM E4.10
LOW PHONON CONCENTRATION LASING GLASSES FOR 1.3  AMPLIFICATION. M. Mennig , F. Gross, I. Lang, U. Sohling, H. Schmidt, Institut fuer Neue Materialien gem. GmbH, Saarbruecken, GERMANY.

Modified Ge-Ga-S-glasses doped with 1.5 mole  Pr³⁺ have been developed for applications in optical amplifiers for 1.3 m. Thereby it was investigated, whether the formation of crystalline phase separations in Ge₂₅Ga₁₀S₆₅ could be suppressed by an increase of the number of glass components. The Pr³⁺ doped Ge-Ga-S glasses have been modified with Sb or Sn and Iodine. Samples have been prepared by melting in sealed silica ampoules at 900 to 1000C. Ge-Ga-Sb-S-glasses, doped with 1.5 mole  Pr³⁺, were synthezied by addition of Sb₂S₃ up to contents of 14.3 mole  to the composition Ge₂₅Ga₁₀S₆₅ (corresponding 83.3GeS₂-16.7Ga₂S´₃) or by replacing up to 50 mole  of Ga by Sb. Further 1.5  doped Ge-Ga-Sn-S-I glass compositions were examined, containing 1 at  Sn and up to 12.2 at  I. All these with 1.5  Pr³⁺ doped glasses are transparent and free from crystalline phase separations and exhibit fluorescence of 15-18 s lifetimes with fluorescence maximum position between 1340 - 1341 nm. The preparation process for both glass systems could be optimized to obtain monoliths with lengths of several centimeters, which are suited for the fabrication of planar waveguides. For the monoliths optical losses of less than 0.07 dB/cm after reflection correction could be determined, which is about 2 orders of magnitude smaller compared to Pr³⁺ doped Ge-Ga-S glasses. In addition, with Judd-Ofelt analysis on the electronic transitions of Pr³⁺ radiative lifetimes between 510 and 590 s were calculated for the ¹G₄ level, corresponding to a quantum efficiency of about 3  for the 1.3 m-transition.

11:15 AM E4.11
THE -RAY RADIATION EFFECTS ON RF MAGNETRON SPUTTERED GaN THIN FILMS. Ching-Wu Wang , Jin-Yuan Liao, Yuan-Hsiu Yang, I-Shou Univ., Dept. of Electronic Engineering, Ta-Hsu Hsiang, Kaohsiung County, TAIWAN, R.O.C.

Gallium nitride (GaN), having a 3.39 eV direct band gap at room temperature, is being used in optoelectronic devices such as blue-light emitting diodes [1]. Although the fundamental characteristics of GaN thin films have been widely discussed [2, 3], the -ray radiation effects on the sputtered GaN thin films have so far less investigated. The purpose of this work is to report the optoelectronic properties, crystallization, and deep level defect status of sputtered-GaN thin films which were irradiated by different intensity of -ray radiation.
The structure of GaN thin film under test was illustrated in Fig. 1. The GaN buffer layer was first grown on n⁺-Si (111) substrate at 500C with a thickness around 200. Subsequently, the substrate temperature was elevated to 700C for growing the GaN epilayer. After that, different intensity of -rays radiation treatment (1 Mrad 12 Mrad) were employed on the sputtered GaN thin films. The -ray diffraction results (as shown in Fig. 2) revealed that the 4 Mrad -ray irradiation possesses the more effective annealing effect which produces the best crystallinity of wurtzite-GaN structure [4]. Moreover, different defect status induced by different intensity of -ray irradiation were also observed by deep level transient spectroscopy (DLTS) [5] measurement, which were shown in Fig. 3. Finally, a new viewpoint for the -ray radiation effect on the sputtered GaN thin films will be proposed based on the transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS) measurements.
ACKNOWLEDGMENT
One of the authors (Ching-Wu Wang) wishes to thank National Science Council (NSC) in Taiwan for receipt of financial assistance (contract number: NSC88-2215-E214-002) to undertake this work.
REFERENCES
[1] S. Nakamura, T. Mukai, and M. Senoh, Appl. Phys. Lett. , 64, 1687 (1994).
[2] C. Trager-Cowan, K.P. O'Donnell, S.E. Hooper, and C.T. Foxon, Appl. Phys. Lett. , 68, 355 (1996).
[3] F.K. Koschnick, K. Michael, J.-M. Spaeth, B. Beaumont, and P. Gibart, Phys. Rev. B , B54, R11042 (1996).
[4] T.S. Cheng, L.C. Jenkins, S.E. Hooper, C.T. Foxon, J.W. Orton, and D.E. Lacklison, Appl. Phys. Lett. 66, 1509 (1995).
[5] D.V. Lang, J. App. Phys. 45, 3023 (1974).

11:30 AM E4.12
ZnGa₂O₄ AND RELATED GALLATE THIN-FILM PHOSPHORS GROWN BY PULSED LASER ABLATION. Yong Eui Lee *, David P. Norton, J.D. Budai, S.J. Pennycook, C. Rouleau and G.E. Jellison, Jr., *ORISE Postdoctoral Research Program, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN.

The development of efficient thin-film oxide phosphors has received considerable attention for use in field emission displays and thin-film electroluminescent devices. Oxide phosphors potentially offer enhanced stability over sulfide-based materials, which suffer degradation due to sulfur loss. Mn-doped ZnGa₂O₄ has been shown to be one of the most promising thin-film oxide phosphor candidates due to excellent chemical stability and efficient green luminescence characteristics. It has also been reported that full-color luminescence could be achieved using ZnGa₂O₄ phosphors with various dopant activators, albeit with a low luminescent intensity. We have investigated the growth and properties of ZnGa₂O₄ and related gallate thin-film materials on single crystal and glass substrates using pulsed-laser deposition. We will describe the luminescent characteristics of as-deposited single and polycrystalline films with various dopants. This research was sponsored by the Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp., for the U.S. Department of Energy, under contract DE-AC05-96OR22464. The submitted manuscript has been authored by a contractor of the U.S. Government under contract DE-AC05-96OR22464. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

11:45 AM E4.13 HIGH RESOLUTION TIME AND FREQUENCY RESOLVED SPECTROSCOPY FOR THE STUDY OF PHOTOPHYSICAL PROCESSES IN LUMINESCENT MATERIALS. Bruce Farris, Anthony Smith, Richard Martoglio and Lee H. Spangler , Montana State Univ, Bozeman, MT.

A method for obtaining time and frequency resolved spectra with high spectral and temporal resolution over a broad spectral range will be presented. Step scan fourier transform spectroscopy permits spectral coverage from visible to mid IR with sub nanosecond resolution making it possible to track emission from multiple manifolds with full temporal resolution in a single experiment. Data for several rare-earth doped crystalline materials exhibiting multiple emission bands and including evidence of energy transfer will be shown.
 
 
 
 
 

SESSION E5: MECHANISMS AND DEFECTS
Tuesday Afternoon, April 6, 1999
Franciscan I (A)

1:30 PM *E5.1
PHOTOCONDUCTIVITY AND EFFICIENCY OF RARE EARTH ACTIVATED INSULATING MATERIALS. William M. Yen , University of Georgia, Department of Physics, Athens, GA.

The importance of charge transfer processes and the effects of the intrinsic bands of the host solid on the luminescence efficiency of rare earth impurity ions has become evident recently. Some of the properties of the donor and acceptor- like charge transfer states and the interactions of the valence and conduction bands of the host can be accessed optically. However, in order to obtain a comprehensive understanding of these high lying states and their effects on the luminescent efficiency of activator ions, complementary studies, such as photon induced conductivity and thermal luminescence, need to be carried out. We present a series of studies of rare earth ions in a sequence of crystalline hosts and demonstrate that luminescence efficiencies depend critically on the positioning of the impurity levels with respect to the intrinsic bands of the host crystal. For cerium doped materials, for example, we show that the radically different emission properties of this ion in a variety of materials originate in small but critical differences in the position of the emitting 5d levels. We also believe that we can use these properties to characterize the nature of localized versus delocalized charge transfer processes.

2:00 PM E5.2
LUMINESCENCE WITH EXCITATION ABOVE AND BELOW THE BAND GAP: MECHANISTIC IMPLICATIONS. David R. Tallant , Carleton H. Seager and Regina L. Simpson, Sandia National Laboratories, Albuquerque, NM.

Photoluminescent mechanisms associated with excitation by photons whose energy lies below the band gap of the host lattice of the luminescing material are believed to be dominated by activator, rather than lattice, absorption. With photon energies above the band gap of the host, the incoming light flux is primarily attenuated by lattice absorption, due to lattice absorption coefficients exceeding 100000 cm^-1. Above-band-gap photons are absorbed near the surface of the luminescing material, with a penetration depth of 100 nm or less. Activators then receive excitation by indirect processes, e.g. the generation of excitons in the host and their migration to and recombination at activator sites. The indirect excitation processes associated with above-band-gap photons have similarities to those associated with cathodoluminescence, or electron-beam-stimulated emission. Differences in luminescent behavior from photon excitation above versus below the band gap of the host material may provide insights into the mechanisms of cathodoluminescence. Luminescence data with excitation above and below the band gap for several phosphors, including ZnS:Ag and europium-activated yttrium oxide and oxysulfide, suggests that above-band-gap mechanisms vary significantly with the host and activator. The persistence of europium-doped yttrium oxysulfide is similar whether the exciting photons have energies above or below the host band gap. Apparently the indirect (above-band-gap) processes in this system are fast enough so as not to observably impact the persistence profile. Likewise, emission band profiles are not significantly affected. On the other hand the persistence of ZnS:Ag and europium-activated yttrium oxide is extended in time for excitation with photon energies just below the band gap, as compared to excitation well-removed from the band gap. Charge transfer states involving activator-host interactions or trapping by defects may be implicated by this behavior. We will make correlations to cathodoluminescent mechanisms and discuss implications for low-voltage phosphors.

2:15 PM E5.3
EFFECTS OF GAS ENVIRONMENTS ON POROUS SILICON PHOTOLUMINESCENCE. Girolamo Di Francia , Vera La Ferrara, Laura Lancellotti, Luigi Quercia, Tommaso Fasolino, CR ENEA Portici.

The intensity and the spectral distribution of Porous Silicon (PS) photoluminescence (PL) are highly dependent on the chemical nature of the surface. Particularly when the PS is in presence of an oxidizing ambient, is evident a quenching of PL intensity. The same behaviour is reported for the electroluminescence intensity which degrades with exposure to prolonged bias in air. The study of photoluminescence and electroluminescence quenching is important not only to control PL efficiency but also to find a passivating surface treatment to maintain the stability of electrically stimulated light emission. In this work we present the PL spectra and integrated photoluminescence of PS in various gas environments. PS samples have been prepared by electrochemical anodization starting from n-type CZ silicon wafers. We investigate what happens when PS is illuminated by HeCd laser for short and long time either in an inert ambient or in an oxidizing one. While in the first environment, the PL intensity is stable, in the second one it is evident a quenching induced simultaneously by laser light and oxygen. This quenching can be reversible under short illumination and irreversible under long illumination. Short time PL quenching is due to a physical adsorption of O₂ molecules. Moreover, using the data from long time PL quenching due to a O₂ chemisorption, we estimate the thickness of the oxide layer growth on PS surface during laser light exposure.

2:30 PM E5.4
EFFECT OF MIXING OF LEVELS ON THE OPTICAL PROPERTIES OF ERBIUM-DOPED SILICON. Marco Federighi , Anthony J. Kenyon, Constantinos E. Chryssou, Christopher W. Pitt, University College London, Dept of Electronic and Electrical Engineering, London, UNITED KINGDOM.

The realization of efficient Si-based infrared light sources is a very important research goal, chiefly because of the greater ease of integration compared with III-V and fiber-based devices. Luminescence has been observed in crystalline and amorphous Si and in nanostructures, and light-emitting devices have been demonstrated, albeit with low quantum efficiency. Erbium-doped Si has also been studied, with the aim of producing electroluminescence and high optical gain at 1.54 micron. High gain, occasionally abnormally high gain [S.Honkanen et al., Electron.Lett.28:746 (1992)], has been demonstrated in glass; electroluminescence from Er has been demonstrated in Si. Studies of Rare Earth compounds have shown [Varma, Rev. Mod. Phys. 2:219 (1976)] that pressure-induced hybridisation of the f-levels and the d/s-band can lead to significant changes in physical properties such as conductivity and colour. Although these changes have been observed in the middle and at either end of the RE series, where Hund rule and nearly closed shells respectively make mixed valence possible, similar but more limited mixing is possible in Er. This has a limited effect on the position of the Er luminescence peak, which is determined chiefly by the f-levels alone, but may have a dramatic effect on the optical cross-section, which because of quantum selection rules depends on the square of the non-f component in the wave function. This might explain the abnormally high gain mentioned above. Mixing with the conduction band in Si can also increase the efficiency of electroluminescence, allowing the direct electrical pumping of the Er levels. Achieving both aims in the same material could lead to very interesting developments in Si technology. The results of our theoretical and experimental study of level mixing in Si:Er are presented in this paper.

2:45 PM E5.5
OPTICAL GAIN AND ION-ION INTERACTIONS IN ERBIUM/YTTERBIUM CO-DOPED GLASSES. Marco Federighi , Ava Malla, Keng-Chun Yeo, University College London, Dept of Electronic and Electrical Engineering, London, UNITED KINGDOM.

Er/Yb co-doped glasses with very high Er and Yb concentrations can provide gains of a few dB/cm and are thus very promising materials for integrated optics applications. Their optical properties are critically dependent on ion-ion interactions which provide the key energy transfer mechanism from Yb to Er, but are also responsible for up-conversion and concentration quenching when the Er and Yb concentrations are very high. Different applications may have different requirements in terms of device structure, size and pumping, and optimization of the chemical composition of the material is of critical importance. The transition probabilities for ion-ion interactions are strongly dependent on the average distance between the interacting ions, and therefore on the ion concentrations. This can be taken into account easily in the rate equations for the Er and Yb levels as far as Er-Yb and Er-Er interactions are concerned; however, concentration quenching does not affect the level populations and must therefore be treated by correcting the Er-Yb transition rate appropriately. We have developed a model of highly doped Er/Yb co-doped glass which takes into account all relevant ion-ion interactions, and also the dependence of the energy transfer coefficient on the concentrations of Er and Yb. In this paper we discuss the dependence of the optical gain on Er and Yb doping, and apply our results to an integrated optical equaliser for the suppression of gain peaking in Wavelength-Division Multiplexing systems.

3:00 PM E5.6
ELECTRON PARAMAGNETIC RESONANCE OF Cr⁴⁺ IN Y₂SiO₅. Rakhim Rakhimov , Holli Horton, George Loutts, Center for Materials Research, Norfolk State University, Norfolk, VA.

Chromium and other transition metal ion doped Y₂SiO₅ are considered to be promising materials for laser applications due to laser transitions in the visible and infrared spectral region. Cr⁴⁺ optical properties have been studied in Y₂SiO₅ [1-3], while no EPR study on Cr:Y₂SiO₅ has been reported yet. In this presentation we will discuss orientation dependence of the EPR spectra of Cr:Y₂SiO₅ single crystals and relation of the spin Hamiltonian parameters with the crystal structure. Cr⁴⁺ substitutes Si⁴⁺ in its slightly distorted tetrahedral site. The ground state of Cr⁴⁺ (electron configuration 3d²) inY₂SiO₅ is a spin triplet (S=1). Rhombic symmetry of the electron dipole-dipole interaction leads to three electron spin terms T_x, T_y and T_z in the absence of the external magnetic field (zero field splitting). In this designations the term T_x has the highest and the term T_z has the lowest energy in zero magnetic field. We have observed the magnetically induced crossing of the terms T_y and T_z. We show that in the vicinity of energy level crossing the linewidths and positions of the resonance lines depend on temperature. The dynamic effects can be used to measure spin relaxation rates. The support for this work from the National Science Foundation (HRD-9805059) is gratefully acknowledged. References: [1] C. Deka, et al. Appl. Phys. Lett. 61 (1992) 2141. [2] N.V. Kuleshov, et al. Optical Materials. 4 (1995) 507. [3] V. P. Mikhailov, et al. Optic. Quantum Electronics. 27 (1995)767.

3:30 PM E5.7
LONG-LIVED LUMINESCENT DEFECT SITES IN PURE SILCA XEROGELS. A.E. Stiegman and Allison S. Soult, Department of Chemistry and the Materials Research and Technology Center (MARTECH), Florida State University, Tallahassee, FL.

Silica xerogels, made from the hydrolysis and condensation of silicon alkoxides, possess luminescenct sites that are inherent to the xerogel. At room temperature, a single emission is observed at 353 nm. The emission is very long lived with a lifetime of (0.85 sec) and is also vibrationally structured. At cryogenic temperatures (77 K), a second long-lived emission appears at 420 nm with a lifetime of 0.93 sec. Both of these species show significant quenching by oxygen. The intensity of these luminescent site emission varys with processing temperature of the xerogel and, upon calcining (900C), they dissapear altogether. Franck-Condon analysis of the vibrational progression observed in the emission are compared to vibrational modes observed in Raman spectra of the materials which suggests specific structural features, unique to the sol-gel derived silica, that give rise to the emission. Photochemical properties of these luminescent sites will also be discussed.

3:45 PM E5.8
INVESTIGATION OF COUPLING MECHANISM BETWEEN ERBIUM (Er³⁺) AND YTTERBIUM (Yb³⁺) IN ALUMINA (Al₂O₃) HOST. C.E. Chryssou , A.J. Kenyon, C.W. Pitt, University College London, Dept of Electronic & Electrical Engineering, London, UNITED KINGDOM; P.J. Chandler, D.E. Hole, University of Sussex, Dept of Physics & Astronomy, Brighton, UNITED KINGDOM.

Plasma-enhanced CVD deposited alumina (Al₂O₃) thin films and single sapphire crystals were co-doped with both erbium and ytterbium using ion implantation. Yb³⁺ and Er³⁺ concentrations ranged from () to (), respectively. The samples show strong, broad, room-temperature photoluminescence (PL) at = corresponding to the intra-4f transitions between the ⁴I_13/2 (first excited) and the ⁴I_15/2 (ground) state of Er³⁺. The FWHM of the emission spectrum is as high as 67 nm for the Al₂O₃ thin films; for the sapphire crystals it is 45 nm. The fluorescence lifetime of the samples has been measured to be as high as 4.2 ms at 50 mW laser pump power. The indirect pumping mechanism of erbium through the transfer of energy from ytterbium has been demonstrated and the PL peak intensity has been studied as a function of the Yb³⁺/Er³⁺ concentration ratio when the samples are pumped at 840 nm; the PL excitation spectrum of an Er³⁺/Yb³⁺ co-implanted sample is also presented. Both the PL peak intensity at  and the fluorescence lifetime have been studied as functions of annealing temperature. Luminescence spectra attributed to defects in the alumina matrix are presented for as-implanted samples and following thermal annealing.

4:00 PM E5.9
IR STIMULATED PHOTOLUMINESCENCE OF ALKALI NITRATES PROMOTED BY UV EXCIMER AND ELECTRON BEAM IRRADIATION. Tom Dickinson , Christos Bandis and Steven Langford, Washington State University, Department of Physics, Surface Dynamics Laboratory, Pullman, WA; David Ermer, Vanderbilt University, Dept. of Physics, Nashville, TN.

We have previously shown that the alkali nitrates are susceptible to decomposition of the anion by exposure to electron beam and excimer laser beam irradiation. We have also seen evidence for the production of high lying electron traps capable of generating photoelectron emission at photon energies of only 4-6 eV. Recently, we have investigated the laser induced photoluminescence (PL), electron, and positive ion emission (Na⁺) from NaNO₃) at an incident wavelength of 1064 nm (Nd:YAG fundamental-8 ns pulse width) and found that surprisingly efficient emissions are observed. The PL is very broadband (340 nm-750 nm), but tends to decay with repeated pulses of the IR laser. The positive ion emission is very energetic (2-30 eV), also tending to decay, and the electron emission shows kinetic energies < 2 eV and is robust with repeated laser exposure. All emissions are highly non-linear in 1064 nm fluence. Calculations of multiphoton vs. multiple photon excitation processes suggest the latter is more probable. The decay seen in the PL and the Na⁺ emission is reversible (recoverable) by exposure to 248 nm excimer laser irradiation. This observation indicates that redistribution of electrons in high lying traps plays a crucial role in these ìenergeticî emissions. We also observe significant increases in these emissions due to electron beam emission and will present these results. Models of electron and sorbed ion redistributions on the sample surface coupled with known energy states in the nitrate ionic crystals will be discussed. This work is supported by the Department of Energy

4:15 PM E5.10
STRONG ACTIVATOR-HOST INTERACTION IN RARE EARTH BORATE PHOSPHORS. V.Z. Mordkovich , A.G. Umnov, International Center for Materials Research, Kawasaki, JAPAN.

Rare earth borate phosphors were first introduced by R.I.Smirnova et al. in 1969 [1] and have been of little or no interest for two decades. More recently due to rising demand for new efficient phosphors for flat panel display applications the rare earth boratesí properties such as high efficiency, easy processibility, durability and often long afterglow have attracted attention and more publications appeared especially on Eu-activated red-emitting borates. In this work we found that blue-emitting Ce-activated rare earth borates exhibit unusually strong activator ion-host lattice interaction. As a result of this interaction a substantial red shift in photoluminescence and low-voltage cathodoluminescence spectra is induced by Ce concentration increase. Also luminescence efficiency dependence on activator ion concentration is far more complex than classical one-maximum dependencies observed for example in case of conventional Ce-activated yttrium silicate phosphor. 1. R.I.Smirnova et al. Otkrytiya, Izobreteniya Bulletin, 1969, v.46, 74.

4:30 PM E5.11
COLOR ENGINEERING OF GARNET BASED PHOSPHORS FOR LUMINESCENCE CONVERSION LIGHT EMITTING DIODES (LUCOLEDs). M. Batentschuk , B. Schmitt, J. Schneider*, A. Winnacker, Institute of Material Science 6, University of Erlangen-Nüuernberg, Erlangen, GERMANY; *Freiburger Materialforschungszentrum, Freiburg, GERMANY.

In a luminescence converting light emitting diode (LUCOLED) the blue primary light can be converted to a large variety of colors and, by color mixing, even to many hues of white. For white light generation the phosphors on the basis of the Al-garnet are ideally suited. The luminescent- as well as the absorption characteristics can be tuned by varying the phosphor composition, which in turn effects the position of the two lowest d-levels responsible for the optical transitions. The variation of the phosphor composition can be performed by incorporating ions either on the dodecahedral (c) or the octahedral (a) sites. Substitution of Gd3+(c) or Ga3+ (a, d- tetra) had been investigated by cathodoluminescence giving the changes of emission spectra. In this paper we shall also report the effect on the absorption spectra which is relevant for luminescence conversion. Measurements were performed on powders as well as epitaxial films. The latter were used as model systems for pair substitution, for instance La3+(c)- Lu3+(c,a), or Lu3+(c)- Sc3+(c,a), in a mixture which kept the lattice parameter equal to the one of the YAG substrate. For the use as luminescence converters the quantum efficiency is of great importance. By varying the temperature the role of thermally induced band-impurity-level transitions was investigated. Furthermore the role of Fe as fluorescence killer was studied by investigation of its effect on different excitation channels. In this context the Fe3+ was identified via EPR.
 

SESSION E6: POSTER SESSION
Tuesday Evening, April 6, 1999
8:00 P.M.
Metropolitan Ballroom (A)

E6.1
PHOSPHORS WITH SINGLE-CRYSTALLINE COLUMNAR STRUCTURE. E.I. Givargizov , L.A. Zadorozhnaya, A.N. Stepanova, M.E. Givargizov, and V.G. Galstyan, Institute of Crystallography, RAS, Moscow, RUSSIA; N.P. Soshchin, PLATAN R&D Corp., Fryazino, RUSSIA; N.N. Chubun, ISTOK R&D Corp., Fryazino, RUSSIA.

In order to avoid light scattering and, in such a way, to improve resolution and to increase effectivity of cathodoluminescent screens for low-voltage field-emission displays, phosphors with single-crystalline columnar structures are prepared. An additional advantage of the single-crystalline phosphors is their potential long life. The approach is demonstrated for ZnO and CdS phosphors, however, it is acceptable for other phosphors, too. Some characteristics of the screens are measured and given.

E6.2
INFLUENCE OF OXYGEN CONTENT IN Er DOPED SiOx FILMS ON ROOM-TEMPERATURE LUMINESCENCE. W.D. Chen , Z.X. Ma, and J.J. Liang, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, P.R.CHINA; Center for Condensed Matter Physics and State Key Laboratory for Surface, Chinese Academy of Sciences, Beijing, P.R.CHINA.

SiO_x films with oxygen concentrations in the range 13-46 were deposited by plasma enhanced chemical vapor deposition (PECVD) technique using pure SiH₄ and N₂O mixture. Erbium was then implanted at an energy of 500 KeV with dose of 2x10¹⁵ ions/cm² The samples were subsequently annealed in N₂ for 20 sec at temperature in the range 300-950C. Room temperature PL data were collected by FITS(IFS/20HR) with an argon laser at a wavelength of 514.5 nm and an output power ranging from 5 to 2500 mw. The intense room-temperature luminescence was observed around 1.54 m. The luminescence intensity increases by 2 orders of magnitude with increasing oxygen concentration in the film and by a factor of 68 with increasing excitation power respectively. The Er luminescence depends strongly on the SiO_x microstructure. This experiment showers that silicon grain radius decreases with increasing oxygen content and finally microcrystalline or nanocrystalline silicon were formed. As a result it facilitates energy transfer to Er³⁺ and thus increased the photoluminescence intensity.

E6.3
THE RELATION BETWEEN STRUCTURE AND LUMINESCENT PROPERTIES OF NANO-SCALE Y₂O₃: Eu³⁺. Qiang Li , Lian Gao, Dongsheng Yan, State Key Lab of High Performance Ceramics & Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, P.R. CHINA.

In this work, nano-scale powders of Y₂O₃:Eu³⁺ were prepared by the chemical methods. Compared with micro-scale Y₂O₃:Eu³⁺, the structure and luminescent properties of nano-scale Y₂O₃:Eu³⁺ were studied. Being examined by XRD and calculated, the decreases of the unit cells of nano-scale Y₂O₃:Eu³⁺ was found. This lattice deformations may be caused by the large surface tension of nano-structured materials. Studying the luminescent spectra, the blue-shift phenomenon of nano-scale Y₂O₃:Eu³⁺ was found. In further research work by laser time-resolved methods, it was found the luminescent life time of nano-scale Y₂O₃:Eu³⁺ is longer than that of micro-scale Y₂O₃:Eu³⁺. According to the theory of crystal field, all of these luminescent properties are connected with the change of nano-structure. The changes of the unit cell must effect the crystal field of nano-scale Y₂O₃:Eu³⁺, and return in the blue-shift phenomena of emission spectra and delay of luminescent life time.

E6.4
SOME PECULIARITIES OF THE LUMINESCENCE OF THE LEAD TUNGSTATE CRYSTALS. Anatoliy Nosenko , Ludmyla Kostyk, Liliya Kozlovs'ka, L'viv State Univ, Dept of Physics, L'viv, UKRAINE.

The lead tungstate single crystals PbWO₄ (PWO) are the promising materials for application as detectors for Large Hadron Colliders. Many investigations are devoted to optical spectra of PWO crystals. On the other hand, influence of defects of the different origin on spectral characteristics is studied insufficiently. In this work the peculiarities of the Tb³⁺ ions luminescence and activation influence on the intrinsic luminescence of the PWO crystals were investigated. Influence of the high- temperature treatments on the spectral-luminescent and kinetic properties of the crystals was studied also. The pure and activated by Tb³⁺ ions PbWO₄ crystals were used in the studies. The crystals were grown by the Chochralski method in platinum crucibles in atmosphere close to air. The patterns' surface was polished mechanically. Subsequent annealing was applied (T=700C) to remove structural distortions, produced by mechanical treatment. The X-ray and photoluminescence, excitation and absorption spectra in the temperature range 77-300 K were studied as well as the thermal glow curves. A wide band in the 400-600 nm region was observed in the photoluminescence spectrum of the pure PWO with a maximum at 486 nm ( = 317 nm). An intensive maximum in the X-ray luminescent spectrum was observed at 440 nm (80 K). At the room temperature the band's intensity reduced. In the PbWO₄ crystals activated by Tb³⁺ ions the bands due to the lattice and rare- earth ions emission are observed. The luminescence of Tb ions can be excited in the absorption band of the lattice and in the absorption bands of Tb ions. The energy transfer from the matrix defects to the activator's ions has been found. The possible mechanism of these transfer processes is discussed. An activation of PbWO₄ crystals by the Tb³⁺ ions suppresses the lattice luminescence. Luminescence spectra of PbWO₄:Tb crystals exhibit mainly green emission ( transitions) at 300 K and the lattice blue and activator green emission at 77 K. The TSL curve in the pure PWO₄shows 6 peaks with the most intensive one at 216 K. Activation by the Tb³⁺ ions leads to the sharp almost complete disappearing on the peaks. The luminescence of Tb³⁺ ions in CdWO₄, ZnWO₄ and PbWO₄ crystals was compared. A possible nature of the luminescence centres, excitation mechanisms of the intrinsic and activation luminescence as well as the activation influence on the intensity and spectral composition of XL, PL and TSL are discussed on the bases of the analysis of the results received.

E6.5
LUMINESCENCE OF Ca₃Ga₂Ge₄O₁₄ SINGLE CRYSTALS AND THIN FILMS DOPED WITN Tb³⁺ AND Eu³⁺ IONS. Anatoliy Nosenko , Roman Leshchuk, L'viv State Univ, Dept of Physics, L'viv, UKRAINE.

Acentric crystalline compounds with the trigonal Ca-gallogermanate structure (Ca₃Ga₂Ge₄O₁₄, space group P321) form a new family of disordered materials. Disorder of Ca₃Ga₂Ge₄O₁₄ crystals is connected with the statistical occupation of 3f tetrahedra and 1a octahedra by the Ga³⁺ and Ge⁴⁺ cations. In this work the results of photo- and cathodoluminescence properties of Tb³⁺ and Eu³⁺ in Ca₃Ga₂Ge₄O₁₄ single crystals and thin films are presented. The crystals were grown by Czochralsky method from high-purity oxides according to standard technique, thin films deposited onto melted quartz substrates using the RF ion-plasma sputtering. In has been found that Tb³⁺ and Eu³⁺ ions occupy Ca²⁺sites in the Ca₃Ga₂Ge₄O₁₄ crystal structure according to the heterovalent isomorphism  (Eu³⁺) under simultaneous Ga³⁺ and Ge⁴⁺ redistribution over octahedral and 3f tetrahedral crystallographic positions. The luminescence of Ca₃Ga₂Ge₄O₁₄:Tb and Ca₃Ga₂Ge₄O₁₄:Eu single crystals under photo- and cathodoexcitation due to  and  transitions in Tb³⁺ and Eu³⁺ ions respectively. In contrast to single crystals, the luminescence properties of thin films are strongly depend on type of excitation and conditions of annealing. Freshly deposited Ca₃Ga₂Ge₄O₁₄:Eu and Ca₃Ga₂Ge₄O₁₄:Tb thin films are amorphous¹ and show no luminescence. In the course of high-temperature treatment, these films undergo crystallization resulting¹ in an emission caused by transitions in Tb³⁺ and Eu³⁺ ions. The Ca₃Ga₂Ge₄O₁₄:Tb films emission in short-wavelength range ( transitions) is of low intensity, in contrast to that of single crystals. The influence of conditions of heat treatment (atmosphere, temperature and duration of annealing) on the structure perfection and luminescence properties of system films-substrate is studied. The correlation between structure changes and luminescence properties of the thin films is observed. Statistical disordering of the cation sublattice results in a partial disordering of the crystals microstructure, causing, in its turn, some fluctuation of the crystal field on activator ions. Therefore luminescence spectrum of Tb³⁺ and Eu³⁺ ions in Ca₃Ga₂Ge₄O₁₄ thin films and single crystals is characterized by broadened lines which are superpositions of radiative transitions in activators centers of different structure having closely related characteristics of Stark energy levels splitting. Comparative analysis of luminescence properties thin films and single crystals of the Ca₃Ga₂Ge₄O₁₄ has been made.
¹ R.E. Leshchuk, A.E. Nosenko, M.Ye. Grytsiv, and S.I. Chykhrii (1996) Inorganic Materials. 32, 225-227.

E6.6
THEORETICAL DESCRIPTION OF LUMINESCENT EFFECTS IN ,-Di(4'-formylphenylethynyl)-4-ethynylstyrene. Roberto Salcedo , Patricia Guadarrama, L. Enrique Sansores, Sergei Fomine and Lioudmila Fomina, Instituto de Investigaciones en Materiales, UNAM, Mexico DF, MEXICO.

The compound ,-Di(4'-formylphenylethynyl)-4-ethynylstyrene presents a luminescent behavior when is exposed to UV radiation. We studied this phenomenon from a theoretical point of view by mean of GAUSSIAN94 method searching to reproduce the experimental electronic spectrum and looking for the source of this effect. Our results show that the luminescence arises from a transition of a singlet state that involves the frontier orbitals in which the high electronic delocalization plays a very important role. The first transition is reproduced very well with respect to the experimental result. The implications and extension of this behavior to other related compounds is discussed. The molecular geometry and the electronic structure of the compound in the ground and exited state are discussed.

E6.7
HIGH FIELD LIMITATION OF POOLE-FRENKEL EMISSION CAUSED BY TUNNELING. S.D. Ganichev , E. Ziemann, W. Prettl, Institut fuer Experimentelle und Angewandte Physik, Univ Regensburg, Regensburg, GERMANY; A.A. Istratov, E.R. Weber, Dept of Material Science, Univ of California, Berkeley, CA.

Capture and emission of carriers by deep impurities is an important channel of non-radiative recombination limiting luminescence efficiency. The electric field dependence of the ionization probability of charged deep impurities in semiconductors is usually attributed to the Poole-Frenkel effect. Here we show that the characteristic electric field dependence of the Poole-Frenkel effect can only be observed at sufficiently small electric field strengths. At higher field strengths the electric field stimulated thermal emission is outweighted by phonon assisted tunneling. This frequently ignored fact is important for the analysis of deep impurity charge states. The experimental investigations have been carried out on charged impurities (Si:Au, Cu-pairs in Si, Ge:Hg, Ge:Cu) and on DX-centers in AlGaAs and AlGaSb which are proved to be neutral. High voltage DC sources and high power pulsed FIR lasers have been used to apply static as well as terahertz electric fields. Short FIR laser pulses allow contactless and homogeneous application of strong electric field strengths, avoiding problems associated with avalange breakdown, current pinching etc. [E. Ziemann et al., MRS Symp.Proc. 510, 595 (1998)]. The present terahertz frequency measurements were performed in the temperature and frequency range where alternating fields act as static fields and both the Poole-Frenkel effect and phonon assisted tunneling are independent of frequency [S.D. Ganichev et al., Phys. Rev. Lett. 80, 2409 (1998)].

E6.8
FLUORESCENCE LINE NARROWING SPECTROSCOPY OF GERMANATE GLASSES DOPED WITH Eu³⁺. Raffaella Rolli , Giorgio Samoggia, Dip Fisica ``A. Volta'', Univ Pavia and Istituto Nazionale di Fisica della Materia, Pavia, ITALY; Adolfo Speghini, Marco Bettinelli, Dip Scientifico e Tecnologico, Univ Verona, Verona, ITALY; Gianluigi Ingletto, Ist Diagnostica e Tossicologia, Univ Parma, Parma, ITALY; Maurizio Montagna, Dip Fisica, Univ Trento and Istituto Nazionale di Fisica della Materia, Povo-Trento, ITALY; Maurizio Ferrari, Consiglio Nazionale delle Ricerche, CeFSA, Centro Fisica Stati Aggregati, Povo-Trento, ITALY.

Germanate glasses have much lower maximum vibrational frequencies than the ones shown by silicate, phosphate and borate glasses. The reduced vibrational cut-off frequency increases the luminescence quantum efficiency from excited states of trivalent lanthanide ions present as dopants in these hosts, leading to interesting applications in the field of optical devices [1]. In the present communication we report on the fluorescence line narrowing (FLN) spectra of Eu³⁺ in sodium germanate glasses and extend the previous work on lead germanate glasses [2]. Non-resonant FLN spectra were obtained at low temperature by pumping at different wavelengths inside the excitation profile of the ⁷FD₀ transition, and observing the ⁵DF_1,2 emission transitions. Crystal field parameters were obtained for each excitation wavelength, and the results are compared with the Brecher-Riseberg model for the local structure of the Eu³⁺ ion in oxide glasses. In the case of the lead germanate glasses, time resolved non-resonant spectra and decay curves were measured at low temperatures. The results show clearly that energy transfer processes between different sites are not efficient at the doping level under investigation (1 mol Eu₂O₃). In addition, the time resolved resonant FLN technique was applied to the ⁷FD₀ transition and the homogeneous linewidth of the ⁵DF₀ transition was measured at room temperature as a function of the excitation energy within the inhomogeneous profile. The results are compared with the data recently obtained for a zinc borate glass [3]. [1] J. Wang, J. R. Lincoln, W. S. Brocklesby, R. S. Deol, C. J. Mackechnie, A. Pearson, A. C. Tropper, D. C. Hanna and D. N. Payne, J. Appl. Phys. 73 (1993) 8066. [2] R. Rolli, P. Camagni, G. Samoggia, A. Speghini, M. Wachtler and M. Bettinelli, Spectrochim. Acta, in press. [3] M. Sbetti, E. Moser, M. Montagna, M. Ferrari, S. Chaussedent and M. Bettinelli, J. Non-Cryst. Solids 220 (1997) 217.

E6.9
NICKEL-RELATED OPTICAL CENTERS IN HPHT TREATED SYNTHETIC DIAMONDS: PHOTOLUMINESCENCE STUDY. Igor N. Kupriyanov , Inst of Automation and Electrometry, Novosibirsk, RUSSIA; Vladimir A. Gusev, Yuri N. Pal'yanov, Yuri M. Borzdov, Inst of Mineralogy and Petrography, Novosibirsk, RUSSIA.

Diamond owing to unique thermal, optical and electronic properties and also superior radiation hardness and chemical stability has been attracted a considerable attention as a perspective material for a number of high-technology applications. Substantial achievement succeeded in resent years in growing large high-quality single crystal diamonds has stimulated numerous investigations on synthetic diamond properties. The present work is devoted to study of nickel-related photoluminescence centers in high pressure/high temperature (HPHT) treated synthetic diamonds. The samples used in the present investigation were grown by the temperature gradient method using high-pressure apparatus of a split sphere type (BARS). HPHT treatments were carried out under a stabilizing pressure at temperatures 1800-2800 K. Photoluminescence (PL) measurements have been taken in the range 400-1000 nm, with the excitation range 200-800 nm. Together with already known luminescence centers a number of new previously undocumented systems have been found. The most of PL systems studied is characterized by low (<1) Huang-Rhys factor values, with the electronic transitions interacting preferentially with quasi-local vibrations. Photoluminescence excitation (PLE) spectra for PL systems with ZPLs at 598.4, 622.5, 727.1, 746.6, 752.2 and 793.2 nm have been measured for the first time. From the analysis of PL, PLE and absorption spectra possible electronic structures for the optical centers studied are suggested. Preliminary results on annealing behavior of the PL centers have been obtained. They are formed in the temperature regime of the single nitrogen aggregation, with the relative intensities depending strongly on the annealing temperature. Of all the PL systems observed only that ones with ZPLs at 752.2 and 793.2 nm together with the S2 and S3 optical centers are present in diamonds annealed at temperatures higher than 2500 K. The observed annealing behavior is consistent with suggestions on the formation of nickel-nitrogen complexes.

E6.10
HIGH TEMPERATURE ANNEALING BEHAVIORS OF LUMINESCENT SIO_X:H FILMS. Zhixun Ma, Xianbi Xiang , Xianbo Liao, Inst of Semiconductors, Chinese Academy of Sciences, Beijing, CHINA; Chunlin Shao, Dept of Electrical and Computer Engineering, Nagoya Inst of Technology, Nagoya, JAPAN.

The effects of high temperature annealing on the microstructure and optical properties of luminescent SiO_x:H films have been investigated. The SiO_x:H films were prepared by PECVD using the gas ratio R=[SiH₄]/([SiH₄]+[N₂O]) from 0.29 to 0.53, and subject to isothermal annealing at various temperatures (T_a) at nitrogen atmosphere for 30min. Two strong photoluminescence (PL) peaks were found at room temperature upon excitation of 488nm Ar⁺ laser, one is at 670nm and red-shifts with increasing T_a and R, and the other, located at 850nm, is observed only after 1170C annealing. Micro-Raman spectra provide a definite evidence for the existence of both a-Si clusters in the as-grown SiO_x, and Si nanocrystals in the films after 1170C annealing. The formation of nano-Si is not only due to the high temperature solid-phase crystallization of a-Si clusters, but due to the segregation of Si atoms from SiO_x, leading to Si nanocrystals embedded in SiO₂ matrix. This segregation process occurred gradually with increasing T_a greater than 700C, as confirmed by the evolution of IR absorption spectra with T_a. AFM provides clear images of the morphology of the films, also exhibiting the existence of cluster-like structures and the expansion with T_a. The evolution of optical transmission spectra with T_a also shows the occurrence of the segregation. The dependence of the deduced optical coefficients () near the absorption edge (E_g) on the photon energy (h) is found to follow the square root law: (h)^1/2 (E_g - h), indicating that nano-Si embedded in SiO₂ is still an indirect material as Si crystal does. And through comparison of the deduced absorption edge with the PL spectra, one sees an obvious Stokes shift, implying there should be phonons involved in the optical transition process. The above experimental data are discussed on the basis of the quantum size effects.

E6.11
CATHODOLUMINESCENCE OF QUARTZ AND OXYGEN DEFICIENT SILICA. T. Barfels , H.J. Fitting, A. von Czarnowski, Physics Department, Rostock University, Rostock, GERMANY.

Electron beam induced luminescence of crystalline and non-crystalline modifications of silicon dioxide is investigated in view of dose and temerature behaviour. In non-crystalline samples we detect three characteristic luminescence bands: a red band at 650 nm (1.91 eV), a blue band at 460 nm (2.70 eV) and an ultraviolett band at 290 nm (4.28 eV). At low temperatures (LNT) the bands are increasing in intensity, and furthermore, a fourth band in the green area at 560 nm (2.21 eV) appears. This band is attributed to the self-trapped exciton (STE) [1], it is stable in the crystalline material, but it decays in amorphous samples under electron irradiation doses already of about 1 As/cm². Obviously, the electron beam irradiation destroys quasicrystalline short range areas in the amorphous SiO₂. The dose behaviour indicates a causal relationship of the blue and the ultraviolet luminescence bands. This complex luminescence center is associated with defect precursors like the single oxygen vacancy, which is transformed to the twofold coordinated silicon center (=Si:) under electron beam irradiation [2]. Consequently we observe an increase of the blue and ultraviolett luminescence intensities in silica samples with oxygen deficit. On the other hand, the red band shows a completely different dose behaviour. This luminescence center is associated to the non-bridging oxygen hole center (NBOHC). In quartz samples at room temerature we detect only the red luminescence, but this band is spreaded widely much more than in amorphous samples. However, at low temeratures the blue and the green bands appear too. Their intensity is about 100 times higher than in amorphous samples. Parallel to the cathodoluminescence measurments we have detected infrared absorption spectra (FTIR). We recognize a softening of the oxygen stretching mode in the SiO₂ network indicating a certain destruction due to electron irradiation. [1] A.N. Trukhin, M. Goldberg, J. Jansons, H.-J. Fitting, I.A. Tale, J. Non-Cryst. Solids 223 (1998) 114 [2] L.Skuja, J. Non-Cryst. solids 167 (1994) 229

E6.12
USING POLYMERISED NANOCONTACTS TO IMPROVE THE ELECTROLUMINSESCENCE OF POROUS SILICON. Daniel J. Blackwood, Anne S.L. Chong , National University of Singapore, Dept of Materials Science, SINGAPORE.

Porous silicon (PS) with nanocrystalline structures is generating wide interest especially in the field of optoelectronics. Visible light-emitting diodes can be made from PS with a solid-state contact, commonly either gold or indium-tin-oxide deposits. Even so, the materials inability to penetrate into the pores results in a discontinuous contact of low efficiency and poor stability. Emission efficiency has been improved with the incorporation of the PS into a p-n junction, which increases the efficiencies of both the minority carrier injection and the majority carrier transport into the PS, thereby increasing the probability of light emission. This opens the door for other contact material alternatives such as the organometallic conducting polymers. These have the ability to penetrate deep into the PS network as the polymerisation occurs in a liquid medium. The active surface area of emission is thereby increased and hence so is the injection efficiency. In line with this, the main focus of our current work is to develop the idea further by using nanosize precursors that will penetrate deeper into the pores before polymerising there. The nanosize active phase is achieved through the formulation of inverse microemulsions, in which microdroplets of the aqueous phase (5-20nm in diameter) are suspended in an organic phase. Polyaniline was chosen because of its transparency in the emitting region of PS as well as its stability. In situ polymerisation in the nanosize reacting domain results in a more homogeneous contact film that is p-type. Therefore by using n-type PS, we can produce the desired p-n junction. Electroluminescence was observed through an electrochemical injection of carriers from an aqueous electrolyte. The efficiency electroluminescence through the use of this novel technique is an improvement over that observed when using other commonly used polymerisation techniques such as electrochemical polymerisation, spin coating and casting.

E6.13
ORGANIC ELECTROLUMINESCENT DEVICE EMITTING RED LIGHT. Oleg N. Efimov, Mikhail G. Kaplunov , Igor K. Yakushchenko, Anatolii P. Pivovarov, Mikhail Yu. Belov, Sergei N. Shamaev, Chernogolobka, RUSSIA.

Electroluminescent devices using an emitting layer consisting of a host material doped with a small number of fluorescent molecules offer a simple means of producing efficient light emitters with a wide range of colors. We report a new red light-emitting device of the dopant-host sceme. A novel green light-emitting metal complex, bis-(2-oxibenzilidene-4-tretbutilaniline)zinc(II), Zn(OBBA) is used as a host material and Nile Red is used as a dopant. We have studied spectral properties of of the two films of 1) Zn(OBBA) and 2) Zn(OBBA) containing 1%(mass) of Nile Red. Films were prepared by spin casting from chloroform solutions. Zn(OBBA) film is characterized by the absorption bands at 312 and 406 nm and by photoluminescence band with 545 nm maximum and 150 nm half-width. The photoluminescence excitation spectrum of the film coinsides with its absorption spectrum. The film Zn(OBBA) + 1% Nile Red is characterized by the absorption bands at 312 and 406 nm corresponding to the Zn(OBBA) complex and a very weak band at 556 nm of Nile Red. The photoluminescence spectrum of this film containes the band with 632 nm maximum and 100 nm half-width. The photoluminescence excitation spectrum of the film is close to its absorption spectrum. The band of Zn(OBBA) luminescence at 545 nm is absent for this film even at excitation wavelengths corresponding to Zn(OBBA) absorption which is an evidence of an effective nonradiative energy transfer from Zn(OBBA) molecules to those of Nile Red. We have prepared electroluminescent devises of the ITO/HTL/EML/Mg:Ag type based on Zn(OBBA) - Nile Red system. HTL is the hole transporting layer of the oligomeric triphenylamines and EML is the emitting layer of Zn(OBBA) or of Zn(OBBA) + 1% Nile Red. Organic layers were prepared by spin casting. The dvices emitted green and red light correspondingly with brightness proportional to current and maximal efficiency observed was about 20 cd/A.

E6.14
SURVEY OF RECENT RESEARCH RESULTS FOR NEW FLUOR MATERIALS. Gary L. Glass and William A. Hollerman , Acadiana Research Laboratory, University of Southwestern Louisiana, Lafayette, LA; Steven A. Allison, Oak Ridge National Laboratory, Oak Ridge, TN.

There is worldwide interest in the use of materials called fluors that emit visible light when exposed to ionizing radiation. Typically, fluors are used as components in high performance electromagnetic calorimeters, down-hole oil well loggers, temperature sensors for equipment with high speed moving parts, and beam positioning systems for large particle accelerators. A candidate fluor should have a large fluorescence light production efficiency, small reduction in output as a function of exposure, large useful visible fluorescence spectrum, large material density, and small prompt fluorescence decay time. Over the last few years, new fluor materials have been located by organizations like Oak Ridge National Laboratory, Lawrence Berkeley National Laboratory, and the Department of Defense. These new fluors, such as YSiO₅:Ce, GdSiO₅:Ce, Y₂O₂S doped with Tb, Pr, and Eu, and Gd₂O₂S doped with Tb, Pr, and Eu, show a great deal of promise for use in the applications listed above. The presentation will present a summary of fluor research completed by the authors from 1989 to 1999. Emphasis will be placed on the discussion of fluor property measurements, such as fluorescence intensity, half brightness dose, and fluorescence decay time. The long-term goal of this research was to establish a fluor characterization program at the Acadiana Research Laboratory that will move us to the front of this developing technology.

E6.15
THE ORIGIN OF THE VIOLET/BLUE LUMINESCENCE FROM CERIUM OXIDE ON SILICON. Won Chel Choi , Ho Nyung Lee, Yong Kim, Eun Kyu Kim, Semiconductor Materials Research Laboratory, Korea Institute of Science and Technology, Cheongryang, Seoul, KOREA.

As a violet/blue luminescent material, we will introduce the thermal treated cerium oxide on silicon. It has been confirmed a violet/blue luminescence ranging from 358 nm to 450 nm at room temperature after the thermal treated cerium oxide thin films on silicon. As a results of AES and HR-TEM measurement, it was confirmed that cerium silicates composed of Ce-Si-O were generated by thermal treatment and the cerium silicates such as Ce_4.667(SiO₄)₃O and Ce₂Si₂O₇ were the origin of the violet/blue luminescence.

E6.16
THE OPTICAL AND CRYSTALLINE STRUCTURE PROPERTIES OF ZnO THIN FILMS GROWN BY RF MAGNETRON SPUTTERING. Kyoung-Kook Kim, Seong-Ju Park, Dept. of Materials Science and Engineering, K-JIST, Kwangju, KOREA; Jong-Han Song , Advanced Analysis Center, Korea Institute of Science and Technology, Cheongryang, KOREA; Jung-Cho, Jae-Hoon Song, Hyung-Jin Jung and Won-Kook Choi, Thin Film Technology Research Center, KOREA; Institute of Science and Technology, Cheongryang, KOREA.

ZnO is a wide band gap (3.3 eV at RT) semiconductor with crystalline properties similar to those GaN. ZnO has been applied as a chemical sensor, transparent electrode and SAW device etc. But we investigate ZnO thin films as an optoelectronic device material. The ZnO films were deposited on sapphire substrate using RF magnetron sputtering at 550C and 600C. The FWHM of XRD  -rocking curve increases with the increase of RF power from 80 W to 120 W at 550C. For specimen that was grown at RF power 120 W, 550C and for 1 hr, it was 0.19. Also, for specimen that was grown at RF power 80 W, 550C, 1 hr, the result of XRD  scan showed that the ZnO films grew to the direction of (002) with the 6-fold symmetry and rotated by 30 about sapphire substrate. AFM images of ZnO films showed that the surface roughness and the grain size of ZnO films increased with RF power. Moreover, the channeling minimum yield (_min) value of BS/channeling was as low as 5. The FWHM of PL spectra curve decreases as the RF power increases from 80 W to 120 W at 550C. For specimen that was grown at RF power 120 W, 550C, 1 hr, the FWHM of PL spectra curve was 89 meV at room temperature and in this case the deep level peak was not observed. But the deep level peak was found as RF power decreased to 40 W and growth temperature increased 600C. Also the position of band edge peak shifted toward shorter wavelength. Such opposite trend between PL property and the crystallinty was discovered and the correlation of optical property with a mosaic structure of the ZnO thin film is discussed.

E6.17
EFFECTS OF Si-DOSE ON DEFECT-RELATED PHOTOLUMINESCENCE IN Si-IMPLANTED SiO₂ LAYERS. H.B. Kim, T.G. Kim , K.H. Chae, C.N. Whang, Yonsei Univ, Dept of Physics and ASSRC, Seoul, KOREA; J.Y. Jeong, S. Im, Yonsei Univ, Dept of Metal Engineering, Seoul, KOREA; J.H. Song, Advanced Analysis Center, Korea Institute of Science and Technology, Seoul, KOREA.

Two visible photoluminescence (PL) bands are observed from Si-implanted SiO₂ thin films grown on Si wafer. One luminescence bands with a peak 600 nm are from implantation induced defects which are present in as-implanted samples. The other bands with a peak 750 nm are related to nanocrystals formed in SiO₂ layer by post anneal at high temperature(1100C). The PL intensity from defects are usually much smaller than that from nanocrystals. In this report, the intensity of defect-related luminescence was investigated in terms of implantation dose and subsequent annealing temperature, to possibly enhance the intensity of the defect-related luminescence. Energetic Si ions with an energy of 80 keV were bombarded into SiO₂ layers at room temperature with various ion doses ranging from 110¹³ to 110¹⁷ atoms/cm². The intensity of defect-related peak from the sample implanted with an dose of 510 ions/cm² was 5 times stronger than that from the sample implanted with an dose of 110¹⁷ ions/cm². It thus means that as decreasing the dose, the intensity is in increase with a certain cut-off dose. In ESR measurements, the peak, related with non-radiative recombination defects center, of a sample implanted with 110¹⁷ is larger than that of the other sample with 510¹⁵. It is thus concluded that the ratio of radiative recombination defects center (non-bridging oxygen hole center (NBOHC) and peroxy radical) versus non-radiative recombination defects center (E' center) decreases as the dose increases. The intensity of this PL peak increases with augmentation of post-annealing temperature below 800C.

E6.18
PRESSURE-INDUCED TYPE CONVERSION IN ZnTe/ZnCdTe STRAINED-LAYER SUPERLATTICE. W.S. Li , Z.X. Shen, H.Y. Li, D.Z. Shen, X.W. Fan.

Multiple quantum-well structures and superlattices of II-VI semiconductor compounds are the subject of intensive study because of their interesting optical properties. Since the demonstration of blue laser action and electroluminescence from ZnSe-based quantum well devices. The measurements of PL under hydrostatic pressure can be used as a powerful method for the investigation of semiconductor band structure and defect states. Recently, this technique has also been employed to study the electronic properties of superlattices and quantum wells. In this paper photoluminescence (PL) spectra of ZnTe/Zn_1-xCd_xTe (x=0.23) strained-layer superlattice (SLS) samples have been measured at hydrostatic pressure up to 5.1 GPa at room temperature. A crossover of the -like conduction band states of barrier and well near 2.6 GPa is demonstrated by the pressure dependence of the intensity and linewidth of the main emission band, which corresponds to the n=1 heavy-hole (HH) exciton transition in the Zn_1-xCd_xTe well layer. The type I to type II conversion is also supported by the different pressure dependence of the PL peak energy of the HH exciton transition below and above 2.6 GPa.

E6.19
UV CONTINUOUS WAVE LASER ACTION FROM CERIA DOPED NANOALUMINA POWDERS, SYNTHESIS, AND PROPERTIES. R.M. Laine , T. Hinklin, S. Rand, G. Williams and J. Williams, University of Michigan, Depts. of Materials Science and Engineering, Chemistry, and Applied Physics Division, Ann Arbor, MI.

Nanosized -alumina powders doped in the parts-per thousand with Ce³⁺ ( one Ce atom per particle) can be produced by flame spray pyrolysis (FSP) of very simple, low cost aluminum precursors doped with ceria. FSP produces nanopowders at > 100 g/h with surface areas of 50-120 m²/g with no apparent porosity (by micro-porosi- metry). Typical powders consist of unagglomerated single crystals, with particle sizes ranging from 10-20 nm ave dia. Details concerning the alkoxide chemistry, flame spray pyrolysis, and the cathodolumi- nescent lasing behavior of Ce³⁺-doped powders will be discussed.

E6.20
OPTICAL ACTIVITY AND ORDER IN LUMINESCENT POLY(p-PHENYLENE VINYLENE) FILMS. A. Marlleta, F.E.G. Guimarães, D. Gonccalves , P.T. Campana, M.R. Fernandes, O.N. Oliveira Jr., R.M. Faria, Inst de Física de São Carlos, Univ de São Paulo, São Carlos, SP, BRAZIL.

The optical activity of poly(p-phenylene vinylene) (PPV) films was correlated with properties such as order and luminescence. A large circular dichroism (CD) was observed in light-emitting PPV films prepared by the Langmuir-Blodgett (LB) technique or by casting. LB films were obtained using a precursor, poly(xylylidene tetrahydrothiophenium chloride) (PTHT), with a sulfonic salt as the spreading solution. Cast films were prepared using PTHT in aqueous solution. PTHT films were converted to PPV under vacuum at 200 or 230C. A strong dependence with temperature was observed in the spectra of circular dichroism, absorption and polarized luminescence for ordered LB films and cast films with different thicknesses . The CD spectra of LB films exhibited a strong, structured signal at the absorption edge (520 nm) which extended to 300 nm. Such optical properties are absent in the precursor solution. Anisotropy on the film plane was verified in the CD spectra after rotating the film and in absorption and polarized luminescence measurements. Cast films, on the other hand, showed a sharp CD structure which was coincident with the onset of the - absorption band-edge of PPV around 470 and 500 nm. Lower anisotropy was observed for cast films, although such effects depend on the preparation conditions and film thickness. The origin of the CD effect can be ascribed to highly ordered packing of the PPV chains particularly to a more ordered structure of the LB film. Thermal conversion increased the conjugation length of the polymer chain and a fully conjugated structure was observed at 230C. However, the CD spectra do not show appreciable changes for PPV films converted at 200 and 230C.

E6.21
THE EFFECT OF POST-ANODIZATION CHEMICAL ETCHING ON POROUS SILICON INVESTIGATED BY MEANS OF PHOTOLUMINESCENCE AND IR SPECTROSCOPY. Nobutomo Uehara , Tetsuya Yamazaki, Akiharu Kobayashi, Shinji Fujihara, Masato Ohmukai and Yasuo Tsutsumi, Dept of Electrical Engineering, Akashi College of Technology, Hyogo, JAPAN.

We prepared porous silicon samples with several anodization time and investigated them by means of photoluminescence and infrared absorption. Although as-formed porous silicon gave little photoluminescence, post-anodization chemical etching noticeably enhanced the intensity. The observed structure in photoluminescent spectra consisted of two peaks, which showed different behaviors. A peak at the lower wavelength side of the two still shifted to the lower and the intensity was enhanced as the sample was etched the longer. This behavior was generally observed regardless of anodization time. Together with a behavior of another peak, we will discuss the relation between photoluminescence and infrared absorption data.

E6.22
APPLICATION OF PHOTOACOUSTIC SPECTROSCOPY TO POROUS SILICON. Akiharu Kobayashi, Nobutomo Uehara, Tetsuya Yamazaki, Shinji Fujihara, Masato Ohmukai and Yasuo Tsutsumi, Dept of Electrical Engineering, Akashi College of Technology, Hyogo, JAPAN.

We investigated luminescing and non-luminescing porous silicon by means of photoacoustic spectroscopy. Differences in photoacoustic spectra suggested that non-radiative centers quenched photoluminescent efficiency. Together with the excitation energy dependence in the photoluminescent spectra, we discuss a behavior of excited carriers. In addition, the influence of the chopper frequency on the photoacoustic spectra will be reported.

E6.23
INTENSE UV-VISIBLE-IR ADJUSTABLE PHOTOLUMINESCENCE FROM SILICON-RICH LAYERS PREPARED BY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION. Zingway Pei , Y.R. Chang, J.F. Tong, H.L. Hsiao, H.L. Hwang, Tsing-Hua Univ, Dept of Electrical Engineering, Hsin-Chu, TAIWAN, R.O.C.

The intense UV-visible-IR adjustable light emission from silicon-rich oxide layers without thermal annealing were observed at room temperature under 325nm He-Cd laser excitation. The silicon-rich oxide layers were deposited by plasma enhanced chemical vapor deposition (PECVD)with the mixture of 5 argon diluted silane and nitrous oxide gas. the strong naked-eye recognizable photoluminescence (blue-white-red) could adjustable by changing the process gas flowratio G=(SiH₄)/(N₂O). the fourier transform infrared (FTIR) spectrscopy was applying to investigate the microstructure-bonding configuration. The position of Si-H stretching related bonding at 2258cm^-1 present no direct Si-Si bonding in the low G value samples although filims are silicon rich. The secondary ion mass spectrometry (SIMS) was used to determine the atomic species incorporated in the thin films. The SIMS results state that all samples are contained excess silicon . To the best of our knowledge, The intense UV-blue light emission in the as-deposited thin films are first reported . Besides properties of low temperature photoluminescence were also investigated on the samples with low G value. The two UV-blue light emission are located around 370nm and 410nm in low temperature and enhancedthe intensity about double and slightly changed the position. As a results, the luminescence spectrum changes from short wavelength (370nm)to long wave length (750nm) with varying the G value from 0.4 to 1.0 were proposed in this paper and we suppose the intense UV-blue light emission could possibly be related to the luminescence centers in the silicon-rich region.

E6.24
EVIDENCE OF RADIATIVE ENERGY TRANSFER MECHANISMS IN ERBIUM ACTIVATED PMMA:PAAc COPOLYMER SAMPLES. Rebeca Sosa , Mario Flores, Rogelio Rodríguez and Antonio Muñoz, Physics Dept, Unìversidad Autónoma Metropolitana-Iztapalapa, Distrito Federal, MÉXICO.

The Judd-Ofelt intensity parameters for f-f transitions of Er3+ ions in PMMA:PAAc copolymer samples were determined from optical absorption measurements and their dependence on the erbium content investigated within the range of 0.1 to 3.0 mol. The densities, refractive indices and emission spectra have been measured. For our Er-PMMA:PSAAc samples an effective radiative transfer between the copolymer host and the erbium ions is observed and discussed.

E6.25
CONFOCAL AND NEAR-FIELD FLUORESCENCE MICROSCOPY OF TRIS-8-HYDROXYQUINOLINE ALUMINUM (ALQ₃). Grace M. Credo , Steven K. Buratto, Dept. of Chemistry, UC Santa Barbara, CA.

Thin films of the luminescent organic semiconductor tris-8-hydroxyquinoline aluminum (Alq₃) have been widely studied due to their tremendous potential as the active layer in organic light-emitting devices. Despite the numerous spectroscopy techniques applied to Alq₃ films, the dependence of the optical properties on film morphology, particularly on a sub-micron level, remain poorly understood. The principal reason for this is that previous studies rely on far-field spectroscopy techniques which average over many morphological domains. In order to overcome this drawback, we use confocal and near-field scanning optical microscopy (NSOM) to probe carrier transport and diffusion length in Alq₃ vacuum-deposited and spin-cast films with 10-100 nm resolution, the length scale of many interesting structural domains. We use concurrent shear force microscopy (an analog to atomic force microscopy, AFM) to correlate morphology (crystalline vs. amorphous regions) to intensity variations in our fluorescence images as well as variations in the localized fluorescence spectra. Our results lead to a better understanding of how the nanoscale structure in Alq₃ affects its optical properties.

E6.26
LOCAL CONCENTRATION OF OXYGEN DETECTED IN CONFOCAL LUMINESCENCE MICROSCOPY. I. Mihalcescu, J.C. Vial , A. Deronzier, Y. Usson, C. Quillet, J. Verdetti, Universitat Joseph Fourier de Grenoble, St Martin d'Heres Cedex, FRANCE.

Porphirin based dyes are routinely now used to determine the partial pression of oxygen (pO₂) at tissular scale (>100m). The detection mechanism, is the quenching of the triplet phosphorescence (PL) by non radiative energy transfer to the molecular oxygen. Lifetime and PL intensities are then strongly diminished with the pO₂ increase. Nevertheless, this dye becomes limited for confocal use, due to its low quantum efficiency and excesive luminescence decay time. We propose here a new class of dyes, the Ruthenium-phenantrolines, which are prepared as solvatable in water. We show their good sensitivity to the oxygen concentration and their hability to be used in confocal microscopy. A first demonstration is done on µm-sized phospholipidic vesicles.

SESSION E7: NOVEL MATERIALS
Wednesday Morning, April 7, 1999
Franciscan I (A)

8:30 AM *E7.1
LUMINESCENT AND DIELECTRIC PROPERTIES OF SEMICONDUCTOR NANOCRYSTALS. Todd Krauss, Amy Michaels, Guanglu Ge, Zhonghua Yu, Manoj Nirmal, Louis Brus , Chemistry Dept, Columbia University, New York, NY.

Semiconductor nanocrystals made by chemical synthesis can be high quantum yield chromophores, if the surface is well passivated. The intrinsic, multiphonon radiationless transition which competes with band gap luminescnece is quite slow. Single nanocrystal luminescence spectroscopy shows the complexity and variability of nanocrystal luminescence. The single nanocrystal variability of dielectric (permanent dipole and polarizability) properties is shown by direct electric field measurements in an AFM apparatus configured as a capacitor.

9:00 AM E7.2
LUMINESCENCE OF CDS NANOPARTICLES DOPED AND ACTIVATED WITH FOREIGN IONS. Jinman Huang , Catherine J. Murphy, Univ of South Carolina, Dept of Chemistry and Biochemistry, Columbia, SC.

The luminescence of II-VI semiconductors such as CdS is greatly influenced by the presence of dopant ions. We have made CdS nanoparticles by room-temperature chemical synthetic routes and have introduced foreign ions of different valencies into the interior of these nanoparticles (doped) or on the surface of pre-formed nanoparticles (activated). The luminescent behavior of the resulting materials over long time is complex; possible mechanisms will be discussed.

9:15 AM E7.3
ENERGY TRANSFER IN ERBIUM-DOPED SILICON NANOCLUSTERS: A COMPARISON OF SILICON-RICH SILICA AND SILICON NANOPOWDERS. A.J. Kenyon , C.E. Chryssou, P.F. Trwoga, C.W. Pitt, Dept. Electronic and Electrical Engineering, University College London, UNITED KINGDOM; S. Botti, ENEA, Frascati, ITALY.

A number of groups have demonstrated an efficient excitation transfer mechanism in erbium doped crystalline and porous silicon. It has been postulated that this is a carrier-mediated transfer, which points to the possibility of fabricating efficient silicon-based 1.5µm emitters. We have previously reported the existence of a similar transfer mechanism in silicon-rich silica. In this paper we extend this study to include silicon nanopowders and demonstrate the existence of a very similar mechanism. Comparing the photoluminescence excitation spectra of erbium doped silica, silicon-rich silica and silicon nanopowders we show that the coupling mechanism enables broad-band pumping of the rare-earth ion. We have also measured carrier lifetimes in the nanoclustered silicon materials using photoinduced free carrier decay. The measured lifetimes are surprisingly long (in the 2-10 millisecond regime), and we discuss the effect of this on the transfer mechanism.

9:30 AM E7.4
HIGHLY EFFICIENT AND PHOTOSTABLE ORGANIC DYES IN INORGANIC MATRICES: A NOVEL PATH TO LUMINESCENCE CONVERSION DEVICES. Hartmut Froeb , Matthias Kurpiers, Karl Leo, Institut fur Angewandte Photophysik, Technische Universitaet Dresden, Dresden, GERMANY.

Thin films of dilute organic dyes in amorphous inorganic matrix materials are emerging as promising materials for optoelectronic devices based on luminescence effects, because the separation and immobilization of the dye molecules by the matrix reduces non-radiative relaxation processes of excited molecules in bulk dyes. High quantum efficiencies were demonstrated in such diluted systems realized by sol-gel technique or by incorporation in organic matrices. However, the room temperature photostability in these systems is not suitable for many applications based on the luminescence. We present results about an organic-inorganic system prepared by co-evaporation of the components in high vacuum which allows (i) to use very stable dye molecules independent of their solubility in the matrix, (ii) to use different substrates and (iii) which results in homogeneous dye molecule distribution. In dependence on dye concentration, absorption and luminescence spectra change dramatically due to the changes in intermolecular distance and surrounding. For the lower concentrations, both absorption and luminescence spectra can be described as monomer transitions with vibrational progression and broadened peak widths. In the system perylene-3,4;9,10-tetracarboxylic-dianhydride in silicon dioxide the maximum photoluminescence quantum efficiency is obtained at a concentration of about 0.1 vol.-% as expected from energy transfer processes between the molecules, which were analyzed by luminescence anisotropy measurements. A new photostability measurement method based on luminescence detection and taking into account molecular orientation effects will be presented. In the system described above, we found stabilities favorable for lighting applications.

9:45 AM E7.5
IMPLANTATION OF ERBIUM INTO ORDERED ARRAYS OF SILICA NANO-SPHERES (OPALS). H. Grebel , Y. Zhang, M. Sosnowski, ECE Department, New Jersey Institute of Technology, Newark, NJ; D.C. Jacobson, J.L. Benton, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.

Optical properties of ordered arrays of silica spheres (opals) implanted with Er are reported. Opals with sphere diameters ranging from 220 nm to 300 nm were grown in an aqueous solution. Specimen with a given sphere size had a distinct color, corresponding to the spacing between the crystallographic planes of the ordered array. The color was independent of the varying sample thickness, signifying that the structure was the same across the entire sample. The opals were implanted with 2 MeV Er ions to a dose of 110¹⁴cm^-2 and a peak density of 210¹⁹cm^-3, located in the third layer of the spheres. After implantation the samples were annealed at 850 deg. C for one hour in Ar. The distinct sample colors were changed dramatically after annealing. Photoluminescence measurements were made with an Ar laser for various angles of the excitating beam with respect to the sample normal. A spectrometer, covering the range 550-1800 nm, was equipped with either cooled Ge detector or a photomultiplier. The results were independent of the excitation angle, yet differed for various opal sphere sizes. The Er implantation resulted im major changes in the intensity as well as in spectral distribution.

10:00 AM E7.6
POTOLUMINESCENCE INVESTIGATION OF SILICON CARBIDE NANOCRYSTAL EMBEDDED IN SILICON DIOXIDE MATRIX. Yongping Guo, Jincheng Zheng , A.T.S. Wee, C.H.A. Huan, National Univ of Singapore, Dept of Physics, SINGAPORE; Kun Li, Institute of Materials Research and Engineering, SINGAPORE.

Recently, the luminescence properties and nonlinear optical behaviour of group-IV nano-crystals in SiO₂ have received considerable attention. Several models such as quantum confinement model and interface defect model have been suggested in attempts to clarify the origins of the luminescence properties. However, the mechanism of group-IV semiconductor nanocrystals embedded in SiO₂ matrix has not been determined conclusively. In this work, we investigate the formation and luminescence properties of SiC nano-crystals in SiO₂. New data on the mechanism of blue-green emission from nano-structure semiconductors in SiO₂ matrix is presented. SiC-SiO₂ composite films were deposited on Si(100) wafers by RF co-sputtering SiC and SiO₂ targets. The concentration of SiC in the as-deposited samples was controlled by changing the area ratio of SiC to SiO₂ targets as well as the RF power density and determined by XPS. Post-annealing treatments between 400 and 1100 degree C were carried out in vacuum for 30 min. Typically, SiC nano-crystals of about 5 nm in diameter have been observed in the 740 degree C annealed samples using high-resolution transmission electron microscopy (HRTEM). The PL spectrum of this sample showed a single peak at 2.7 eV (460nm), which is similar to those observed in Si and C nano-crystals in SiO₂. Furthermore, FTIR measurements showed SiC absorption at 810 cm^-1 and the SiO₂ absorption relative to that of the SiO₂ increased by more than a factor 2 after 800 degree C thermal annealing. the results suggest that SiC nano-crystals had been incorporated into the SiO₂ matrix and O-deficient defects were formed. By comparison with the corresponding PL spectra, it appears to suppose that the blue-green luminescence resulted from the creations of defects in silicon oxides.

10:30 AM *E7.7
A COMBINATORIAL APPROACH TO THE DISCOVERY OF PHOTOLUMINESCENT AND ELECTROLUMINESCENT MATERIALS. Earl Danielson , Martin Devenney, Daniel D. Doxsee, Daniel M. Giaquinta, Damodara M. Poojary, Casper M. Reaves, W. Henry Weinberg, Symyx Technologies, Santa Clara, CA.

The combinatorial synthesis and high throughput screening of large numbers of compounds has been widely applied in the pharmaceutical industry. Recently, combinatorial arrays (libraries) of inorganic materials with luminescent, magnetoresistive and superconducting properties have been reported. The combinatorial approach is particularly well suited to ternary and higher order inorganic materials, for which efforts to predict basic properties have been unsuccessful. As there are no reliable theories to predict the relation between composition/structure and luminous efficiency/color, the exploration of ternary and higher order phase spaces is often considered as more art than science. The discovery and optimization of new phosphors is of great importance in imaging, display and lighting technologies. Utilizing both physical vapor deposition and proprietary solution techniques, we have developed automated routes for the parallel synthesis and high throughput screening of large numbers of luminescent materials. These approaches have been extended to the preparation of libraries of electroluminescent materials in a combinatorial thin film device format. It has been demonstrated that the electro-optical and photophysical characteristics of these combinatorial libraries are well correlated to those of traditionally prepared materials. In this presentation we demonstrate the application of our combinatorial techniques to the discovery and optimization of previously unreported luminescent materials. The rapid and systematic exploration and screening of novel regions of composition space is presented with several examples.

11:00 AM E7.8
LUMINESCENCE INTERMITTENCY AND QUANTUM EFFICIENCY OF INDIVIDUAL POROUS Si NANOPARTICLES. Michael D. Mason , Grace M. Credo, Paul J. Carson, Steven K. Buratto, Univ. of California, Santa Barbara, Dept of Chemistry, Santa Barbara, CA.

We have recently observed spectrally resolved vibronic structure and luminescence intermittency from size selected porous silicon nanocrystals, which emit with near unity quantum efficiency. Our results suggest that the emission from porous Si nanoparticles originates from excitons in quantum confined Si, strongly coupled to the surface of the quantum dot. The mechanism for luminescence blinking is most likely one similar to that previously observed in blinking CdSe quantum dots and may be dramatically influenced by chemisorbed surface species. In order to examine this effect more closely we employ several spectroscopy and microscopy techniques including: 1) single-particle spectroscopy, 2) shear-force microscopy, and 3) time-resolved spectroscopy, on a series of size-selected, surface-derivatized single nanocrystals. In addition we apply statistical techniques to provide a more complete picture of the electronic structure and dynamics of these systems. We will present our most recent findings and discuss them in the context of a modified quantum confinement model.

11:15 AM E7.9
UNDERSTANDING THE PHOTOLUMINESCENCE OF METAL HALIDE PHOSPHORS. Roger M. Sullivan and James D. Martin, Department of Chemistry, North Carolina State University, NC.

A new class of phosphors based on metal halide analogs of aluminophosphates is exemplified by CuAlCl₄. The - and -CuAlCl₄ phases show brilliant blue to blue-green luminescence. Both phases exhibit pseudo-closest packed anion sublattices with the copper in tetrahedral interstices. This is similar to the zincblende and wurtzite phases of the ZnS:Cu phosphors. However, unlike copper dopants in a zinc chalcogenide matrix, charge balance allows CuCl₄/2 units to be stochiometric constituents of the bulk material. Remarkably, this luminescence is turned off and back on during the reversible sorption of small gas molecules by this unique, flexible metal halide framework.
 

SESSION E8/B13/F6: JOINT SESSION:
ORGANIC LUMINESCENT MATERIALS
Wednesday Afternoon, April 7, 1999
Franciscan I (A)

CONCURRENT WITH B11, B12 AND F5

1:30 PM *E8.1/B13.1/F6.1
EXCIMER FORMATION IN COPOLYMERS OF 9,9-DIALKYL FLUORENES AND ANTHRACENE(S). R.D. Miller , G. Klaerner, F.G. Klaerner, M. Kamieth, M.H. Davey, J.C. Scott, J.P. Chen, IBM Almaden Research Center, San Jose, CA.

Poly(fluorene) derivatives comprise an interesting class of electroactive materials with potential application in polymeric light emitting diodes. The formation of excimers upon thermal annealing or the passage of current is a ubiquitous problem for these materials. The interaction of polymer chains with themselves and/or their environment depends on the chemical structure and shape of the chain and the nature of the functional groups. We have prepared a variety of novel linear, U- and S- shaped rigid rod polymers by the copolymerization of 2,7-dibromo-9,9-dihexylfluorene with 2,6-, 1,8, 1,5-, and 9,10-dihaloanthracenes. Polymer chain conformations have been studied by molecular modeling and predictions regarding excimer formation have been verified by spectroscopic studies.

2:00 PM E8.2/B13.2/F6.2
PHOTOLUMINESCENT POLYMER FILMS FOR DISPLAY APPLICATIONS. Anja R.A. Palmans , Andrea Montali, Christoph Weder, Paul Smith, ETH Zurich, Dept. of Materials, Zurich, SWITZERLAND.

In order to increase the efficiency of liquid crystal displays, our research has focused on the introduction of photoluminescent (PL) polarizers into traditional displays. These PL polarizers typically comprise rigid poly-p-phenyleneethynylene (PPE) polymers, embedded and uniaxially oriented in a polyethylene (PE) matrix. Since only a limited fraction of incident light can directly be used by these PL polarizers - because their absorption is also anisotropic - we recently proposed a new approach in which coumarin dyes have been isotropically mixed into such oriented PPE/PE films to act as sensitizer. In these ternary blends, the overall efficiency of transferring the unpolarized incident light to polarized colored light has been dramatically improved through a polarizing energy transfer from the sensitizer to the PPE. To explore the scope and limitations of this approach, a variety of sensitizers was mixed into the PPE/PE blends. However, we observed that upon mixing certain sensitizers into the PE/PPE polymer blends, problems related to phase separation and crystallization arise. As a result, we have introduced the concept of covalently attaching sensitizers to the conjugated polymer backbone. This approach allows us to introduce a well-defined distance between sensitizer and PPE, which ameliorates the control over the required proximity between the sensitizer and PPE. As such, energy transfer efficiency may be further improved. In addition, problems related to phase separation are circumvented. Finally, the polarizing nature of the energy transfer can be enhanced.

2:15 PM E8.3/B13.3/F6.3
LASING IN ORGANIC THIN FILMS USING CASCADE FORSTER ENERGY TRANSFER. Martin Meier , John A. Rogers, Ananth Dodabalapur, Zhenan Bao, Richard E. Slusher, Bell Laboratories, Lucent Technologies, Murray Hill, NJ; Attila Mekis, MIT, Dept. of Physics, Cambridge, MA.

Optically pumped lasing in thin films of an absorbing host 8-hydroxyquinolinato aluminum (Alq) or PBD doped with different laser dyes (C490, DCM II, LDS 821) is investigated. In our materials, an initial molecular excited state is generated in the host compound by absorption of light. This state is then resonantly and non-radiatively transferred through one or more steps down in energy. Suitably matched dye molecules dispersed in the host ensure that the absorption losses at the final emission wavelength are very small. This approach can lower the threshold power density required for light amplification and provide broad tunability of the emission wavelength. Furthermore the optical emission properties are decoupled from the transport properties which is important for the design of future electrically driven device structures. In order to provide the necessary feedback, we employed distributed feedback (DFB), distributed bragg reflectors (DBR) and two-dimensional photonic bandgap structures based on thermally oxidized Si wafer as resonators. The carefully chosen gain media together with the resonator structures which act as wavelength-selective mirrors allow us to fabricate optically pumped lasers in the whole visible spectral range (390nm -800nm). We also fabricated rib-waveguide based DFB and DBR lasers in which the gratings are defined by soft-lithographic procedures.

2:30 PM E8.4/B13.4/F6.4
ELECTRO-LUMINESCENCE AND PHOTOLUMINESCENCE OF POLYFLUORENE-PHK COPOLYMER LED. O. Stephan, J.C. Vial , F. Muller, Lab. de spectrometrie Physique, Univ. J. Fourier and CNRS, St. Martin d'Heres cedex, FRANCE.

Thin layers of polyfluorene-polyhexylcarbazole copolymer sandwiched between ITO and aluminum electrodes show stable photoluminescence and electro-luminescence at room temperature and ambient atmosphere. I(V) and Electro-luminescence versus voltage curves present a threshold at 7 Volts for layer thicknesses of the order of 0.1 m. The emission degrades slowly when the layer is electrically polarized or UV exposed (The lifetime of the device is typically 12 hours for 25 Volts - 150 A polarization conditions). Photoluminescence and electro-luminescence spectra show the same features and are similar to those obtained for low dilution of the polymer in chloroform. Time resolved photoluminescence spectra present a fast blue band (in the sub nanosecond range) centered at 450 nm and a slower yellow one (in the range of 5 ns) centered at 550 nm. The yellow band is absent for fresh samples but become dominant (specially for the electro-luminescence) for aged samples. As already proposed, the spectral behavior suggests that this band results from the re-crystallization of the polymer, here, we add other observations in favor of this explanation. In addition, We explain the slower decays as taking their origins in indirect type electron-hole recombination, enhanced in the crystalline phase.

2:45 PM E8.5/B13.5/F6.5
FLUORESCENT, HEXACATENAR LIQUID CRYSTALS. Ben Hoag, Douglas L. Gin , Univ of California, Dept of Chemistry, Berkeley, CA; Udo Theissl, Emil J.W. List, Egbert Zojer, Guenther Leising, Technische Univ Graz, Inst für Festköperphysik, Graz, AUSTRIA.

A novel family of phasmidic liquid crystals incorporating an oligo(-phenylenevinylene) core was produced. These compounds exhibit blue luminescence in both solution and the solid state. The emission profiles of these compounds are influenced by the degree of organization in the solid state. The synthesis and liquid-crystalline behavior of these compounds will be discussed. Their photophysical properties, as studied by c.w. photoluminescence excitation, emission spectroscopy, and site selective spectroscopy, as well as their optoelectronic properties will be presented.

3:15 PM *E8.6/B13.6/F6.6
MULTICOMPONENT DESIGNS FOR POLYMER LEDs. Frank E. Karasz and Bin Hu, Department of Polymer Science & Engineering, University of Massachusetts, Amherst, MA.

Multilayer architectures provide substantially enhanced efficiencies in polymer LEDs principally by virtue of their ability to provide a more balanced charge injection and good excitonic confinement within the chromophore layer. Further improvement may be gained by using the same charge transport materials in a multi-component design which relies on the immiscibility of the respective polymeric constituents to provide micro-domains (``quantum wells'') of the chromophore in a matrix of the other components. Using alternating hard-soft block copolymers in which the oligomeric hard block forms the chromophore while the soft block provides solubility and additional phase separation, we have designed systems which emit blue, green or red, as required, from a single layer device. Further advantages of the multicomponent design include more versatility in optimizing charge injection by adjusting the ratio of the electron and hole transporting materials, and reduced concentration quenching in the chromophore because of its dispersed state. By using a chromophores blend, one may also take advantage of energy transfer from the higher to the lower band gap material. Fabrication of a single layer device is simplified in comparison to a multi-layer architecture.
Furthermore the multi-component design has been developed to produce a white light emitting polymer LED. The chromophore domain is a blend of the blue, green and red emitters in appropriate ratio, while the charge transport components optimized by the usual criterion of balanced injection from the matrix. Because of the differing band gaps and ionization potentials of the chromophore components the emission spectrum is field dependent; a CIE chromaticity diagram analysis showed a shift in the spectrum as the bias voltage was increased, progressing from orange-red to white as the green and blue chromophores became activated.

3:45 PM E8.7/B13.7/F6.7
OPTICAL PROPERTIES AND CHARACTERIZATION OF Eu3+ ACTIVATED PMMA:PAAc COPOLYMER SAMPLES: Eu CONCENTRATION EFFECTS. Rebeca Sosa , Mario Flores, Antonio Muñoz and Rogelio Rodríguez, Dept. of Physics, Universidad Autónoma Metropolitana-Iztapalapa, Distrito Federal, MÉXICO.

Samples of Eu3+-activated PMMA:PAAc copolymers were prepared in order to investigate their spectroscopic properties. The densities, refractive indices. UV-Visible absorption and emission spectra and fluorescence decay times were obtained experimentally. These data were used in conjunction with Judd-Ofelt theory to calculate the spontaneous emission probabilities and branching ratios, as a function of the europium content. The range of the concentrations was 0.1 to 2.5 mol. Our results are combined with previous information on the optical properties of rare-earth activated polymers in order to get a better understanding of the role played by the Eu3+ ions on the polymerization process in our samples.

4:00 PM E8.8/B13.8/F6.8
PHOTOLUMINESCENCE OF POLY(P-PHENYLENE VINYLENE) ENCAPSULATED IN MESOPOROUS SILICA. Andras G. Pattantyus-Abraham, Michael O. Wolf , Dept of Chemistry, University of British Columbia, Vancouver, BC, CANADA.

We have prepared poly(p-phenylene vinylene) (PPV) encapsulated in a mesoporous silica, MCM-41. In situ polymerization of xylylene bis(tetrahydrothiophenium chloride) yields a highly luminescent, yellow powder with a fluorescence spectrum matching that of unencapsulated PPV. Nitrogen absorption isotherms show that the pore size of the PPV-loaded MCM-41 is significantly smaller than that of empty MCM-41 and a polymer loading of ca. 8% by weight is obtained by thermogravimetric analysis. The hybrid material has also been characterized by IR and solid-state NMR. Detailed photoluminescence data for the encapsulated PPV will be presented, and the use of these nanostructured materials in device applications will be discussed.

4:15 PM E8.9/B13.9/F6.9
FLUORESCENCE EMISSION FROM SINGLE STRETCH-ORIENTED CONJUGATED POLYMER MOLECULES. Paul J. Carson , Jessie A. DeAro, Steven K. Buratto, Department of Chemistry, University of California, Santa Barbara, CA; Cuiying Yang, IPOS, University of California, Santa Barbara, CA.

Fluorescence from blends of derivatized poly(p-phenylenevinylene) (PPV) in stretch-oriented polyethylene (PE) has been observed with single-molecule spatial isolation using confocal microscopy. The photoluminescence and electroluminescence, as well as mechanical properties and ease of processing, have made conjugated polymers desirable as potential active materials in device applications. Single-molecule fluorescence has the advantage over ensemble spectroscopies in that it allows the direct observation of the heterogeneity of spectral line shapes and fluorescence intermittency among individual emitters. These photophysical properties, as well as others, reveal the distribution of nanoenvironments and optically relevant conformational distributions of the fluorescent species in the active material. Results on single-molecule fluorescence intermittency and spectral line shapes will be presented for the derivatized-PPV/PE system. Polarization anisotropy in both stretch-oriented and unoriented polymer blends will be discussed. New techniques for characterizing the underlying rates of molecular state fluctuation will be presented.

4:30 PM E8.10/B13.10/F6.10
PROBING OF TRIPLET STATES IN POLYMER LEDS. V. Cleave , N. Tessler, R.H. Friend, University of Cambridge, Physics Department, Cambridge, UNITED KINGDOM; G. Yahioglu, Imperial College, Chemistry Department, London, UNITED KINGDOM; P.Le Barny, Thomson-CSF, Orsay, FRANCE.

We report a study of the triplet state in doped polymer light-emitting diodes (LEDs). Using a system consisting of a luminescent polymer doped with triplet-emitting molecules, we employ time resolved electroluminescence (EL) and photoluminescence (PL) as well as sub-gap (triplet) absorption, to give direct evidence for the transfer of triplet as well as singlet excitons from the polymer host to the molecular dopant. These transfer processes offer normally non-radiative triplet excitons, as well as singlet ones, access to a radiative decay channel, lifting the 25% efficiency limit imposed by the ratio of 3:1 triplet:singlet excitons created in conventional polymer LEDs. Device efficiency can be maximized by optimising the trapping of carriers, singlet excitons (Förster transfer), and triplet excitons (Dexter transfer) by a single guest molecule. The incorporation of triplet emitting molecules also enables us to probe the position of the triplet level in the polymer host via knowledge of the guest molecule, and build a more complete picture of the polymer energy levels. We show that the established procedure can be applied to a wide range of luminescent polymers to shed light on their basic photophysics.

4:45 PM E8.11/B13.11/F6.11
FROM A REACTION MECHANISM TO NEW PPV'S FOR HIGH PERFORMANCE POLYMER-LEDS. H. Spreitzer , H. Becker, W. Kreuder, E. Kluge, H. Schenk, Aventis Research & Technologies, Frankfurt, GERMANY.

In the recent years, organic light emitting materials made dramatic progress in comparison to classical semiconductor light-emitting diodes. Especially light emitting polymers provide an ideal combination of processability and performance to allow for an innovative low cost technology for LCD-backlighting, illumination, and display applications. Recent advances in materials and device preparation lead to high-performance polymer LEDs (PLEDs) with power efficiencies exceeding 10 lm/W.
Soluble Poly-para-phenylenes vinylenes (PPVs) have been proven to be highly efficient light emitting materials for different parts of the visible spectrum. Those materials can be prepared by dehydrohalogenation polymerization of 1,4-bischloromethyl benzenes. The mechanism of this polymerization is assumed to incorporate a p-quinodimethane species as a key intermediate. We could show, that this mechanism may lead to polymerization defects in the polymer which then will cause interruptions in conjugation. Due to the fact, that high charge carrier mobilities are required for a stable polymeric emitter, they have been closely investigated by ¹³C NMR measurements of ¹³C isotope labeled polymers. It has now been shown, that the expected breaks in the conjugation chain could indeed be identified. The amount of defects differs regarding the substitution pattern of the PPV. The detrimental effect of conjugation breaks can clearly be observed in lifetime tests of PLEDs.
By modifying the substitution pattern of the PPV, the occurrence of polymerization defects was reduced drastically. Those new materials now allow the preparation of very stable and highly efficient PLEDs. Efficiencies of up to 16 lm/W (@ 100 cd/m²) and lifetimes exceeding 25000 h (as determined by accelerated aging) are now routinely achieved. The new class of high performance polymers allows easy access to various colors.
 

SESSION E9: THEORY AND LUMINESCENCE SENSORS
Thursday Morning, April 8, 1999
Franciscan I (A)

8:30 AM *E9.1
HOST EXCITATION AND LUMINESCENCE IN LARGE BAND GAP OXIDES. P.C. Schmidt , V. Eyert, J. Sticht, TU-Darmstadt, Department of Physical Chemistry, Darmstadt, GERMANY; K.C. Mishra, Central Research, Osram Sylvania Inc., Beverly, MA.

A good understanding of the electronic states near the band gap of large gap materials is essential for designing phosphors when the energy of exciting radiation exceeds the band gap. This is particularly true when phosphors are designed for application in rare gas discharges with plasma output in the vacuum UV (VUV) region. In this presentation, we have analysed the nature of host excitations near the band gap energy for a number of complex oxides: LaPO₄, AlPO₄, BaB₂O₄, YBO₃ and LuBO₃. Borates and phosphates are in general large gap oxides. They are being investigated with increasing interest for application in VUV light sources, and other opto-electronic applications. Because of their large optical gap, they are suitable hosts for investigating transitions from higher excited states of rare earth ions, and other non-linear luminescent processes. Using density functional ab initio methods (augmented spherical waves and full potential linear muffin-tin orbitals), we will explore common trends in the optical transitions in borates and phosphates and the corresponding binary oxides La₂O₃, Lu₂O₃, Y₂O₃, BaO, B₂O₃ and P₂O₅. In the metal oxides, the optical gap is determined by electronic transitions from almost pure 2p like states of oxygen to the metallic d-like states near the conduction band minimum. Lanthanum oxide is an exception in which the d-like states are about 1 eV above the one-electron f levels. The states involved in excitations across the band gap are different in the covalent oxides B₂O₃ and P₂O₅. Both the valence and conduction band states are hybridised states of oxygen and boron or phosphorous, and the band gap transitions involve occupied bonding and unoccupied antibonding states. In the complex oxides, one observes further hybridisation between bonding and antibonding states of anionic groups and metallic states. In general, the conduction states are a covalent mixture of the metallic d states and the antibonding states of the anionic group. We find almost pure oxygen 2p like states at the top of the valence states and the hybridised valence states of the anionic groups deep inside the valence band. The lowest conduction bands are either predominantly metal d-like states or molecular antibonding states of the anionic group. The actual ordering of these levels and the band of these materials depend on band gaps of corresponding binary oxides and overlap of the metallic d-like states with hybridised orbitals of the anionic groups. The nature of impurity states in large gap oxides, particularly those of Eu³⁺ and Pr³⁺ substituting for Y³⁺ will be discussed. It will be shown how relative ordering of the one-electron states of activator ions with respect to band states of the host lattice may affect their luminescence properties under host excitation.

9:00 AM E9.2
SIMULATION OF THE EMISSION SPECTRA FOR THE Eu³⁺ ENVIRONMENTS IN LEAD SILICATE GLASS. T. Peres¹, J.A. Capobianco ¹, S. Chaussedent² and S.H. Garofalini³, ¹Department of Chemistry and Biochemistry, Concordia University, Montréal, Québec, CANADA; ²Laboratoire des Propriétés Optiques des Materiaux et Applications E.P. CNRS, Angers, FRANCE; ³Department of Ceramic Engineering, Rutgers University, Piscataway, NJ.
 

Molecular dynamics simulations (MD) employing both two and three-body potential models, were used to simulate the structure of undoped and Eu³⁺-doped lead silicate glass (PbOSiO₂). In order to study the effect of PbO on the silicate network, a compositional study was performed on the undoped glass. The structure of the doped and undoped glasses were analyzed and the results were compared to those found experimentally using X-ray diffraction, nuclear magnetic resonance (NMR), and EXAFS. The three-body simulations showed the presence of two networks, one in which silicate tetrahedra are present and the other in which lead-oxygen polyhedra form a continuous secondary network. Simulations of the Eu³⁺-doped lead silicate glass demonstrated that the Eu³⁺ ions are found in the lead network. The structural model of the Eu³⁺:PbOSiO₂ glass showed that the Eu³⁺ ions are bonded in their first coordination shell to approximately 6.5 oxygen ions. Two main structures were found for Eu³⁺; a distorted pentagonal bipyramid and a distorted octahedron. Using crystal field theory, we have calculated the electronic energy levels and transition probabilities of the simulated Eu³⁺ ions, resulting in the simulation of the emission spectrum. A comparison to the experimental room temperature fluorescence spectrum of the corresponding laboratory glass is presented.

9:15 AM E9.3
FIRST-PRINCIPLES CALCULATION OF OPTICAL SPECTRA FOR TRANSITION-METAL IMPURITIES DOPED IN ZnO AND ZnS. Kazuyoshi Ogasawara , Takugo Ishii, Fumiyasu Oba, Hirohiko Adachi, Kyoto Univ, Dept of Materials Science and Engineering, Sakyo-ku, Kyoto, JAPAN; Isao Tanaka, Kyoto Univ, Dept of Energy Science and Technology, Sakyo-ku, Kyoto, JAPAN.

Multiplet structures of transition-metal (TM) ions doped in ZnO and ZnS are calculated from first principles using the recently developed discrete variational multielectron (DV-ME) method, in which the matrix elements of electron-electron repulsion are calculated numerically using the molecular orbitals obtained by cluster calculations. In this method, many-electron wavefunctions are expressed as linear combinations of Slater determinants corresponding to various electronic configurations. In ZnO or ZnS, the impurity TM ions are surrounded by four oxygen ions and the local symmetry around them is approximately tetrahedral. Therefore, the electric-dipole transitions are allowed due to the covalency between the impurity TM ions and the host crystals. In the present work, the transition probabilities of electric-dipole transition are calculated from first principles, using the many-electron wavefunctions obtained by the DV-ME method. The optical spectra of these materials are well reproduced and the effects of covalency and configuration interaction on these spectra are evaluated quantitatively.

9:30 AM *E9.4
LASER COOLING IN THE SOLID STATE. T.R. Gosnell , Los Alamos National Laboratory, Los Alamos, NM.

It is possible for a luminescent material to radiate more light energy than it absorbs; hence the material must cool if it is otherwise adiabatically isolated. For example, consider a transparent host medium within which is embedded an electronic impurity possessing just three energy levels: a ground state 0 and two closely space excited states 1 and 2 situated in energy well above the ground state. If the medium is then exposed to narrow band light resonant with the 0-to-1 transition, the excited population so created will undergo thermal redistribution among levels 1 and 2 according to the Boltzmann prescription. When subsequent relaxation back to the ground state is exclusively radiative, that is when the photoluminescence quantum efficiency is unity, conservation of energy requires that the temperature of the host medium decrease. We have investigated this anti-Stokes cooling mechanism using the fluoride glass ZBLAN as a host medium and trivalent ytterbium ions as the active electronic impurities. With a sample in the form of an optical fiber, a temperature drop of 65 K starting from room temperature has been observed following excitation of the material with a cw Ti:sapphire laser tuned to 1015 nm.

10:15 AM *E9.5
EPITAXIAL RUBY FILMS AS SENSORS. D.R. Clarke , Materials Dept., Univ. of California at Santa Barbara, CA.

The luminescence from ruby (Cr3+ -doped sapphire) is perhaps the most-intensively studied of all luminescence phenomena. This provides the scientific basis for a number of sensors, ranging from the traditional use of ruby chips to monitor pressure in high-pressure diamond anvil cells to more recent commercial applications as a temperature sensor and still more recent exploratory sensor applications to be described in this talk. In making sensors for the measurement of local strain, temperature and radiation damage, we have developed a solid-state thin-film sensor based on the epitaxial regrowth of Cr-doped amorphous alumina on a Cr-free sapphire substrate. Being made of sapphire, the sensor is both mechanically robust and is inert to most chemical species. The materials aspects of the sensor fabrication and its application to high-spatial resolution measurements of strain and temperature will be discussed.

10:45 AM E9.6
POROUS POLYMER/CERAMIC COMPOSITE FILMS FOR LUMINESCENCE-BASED TEMPERATURE AND PRESSURE MEASUREMENT. Aaron Scroggin , Elliott Slamovich, Purdue University, School of Materials Engineering, West Lafayette, IN; Nathan Lachendro, Jim Crafton, John Sullivan, Purdue University, School of Aeronautics and Astronautics, West Lafayette, IN.

Porous polymer/ceramic composite films have been developed to house luminescent molecules for temperature and pressure measurement. TiO₂/cellulose acetate butyrate (CAB) composite films (thickness <10m) were processed by dissolving titanium diisopropoxide bis(ethylacetoacetate), (CAB), and tributyl phosphate (a plasticizer) in a mutual solvent, casting the solution on silver-coated glass substrates and reacting the dried films in water at 80 C. The film was then dipped in a solution of Pt Octaethylprophine dissolved in toluene and dried. The dependence of luminescent intensity on pressure was fit to a Langmuir isotherm:  where I_ref and P_ref are the reference intensity and pressure respectively, and A, B, and C are constants. The TiO₂/(CAB) films exhibited Stern-Volmer constants of A=-0.173, B=1.174, and C=0.266 at room temperature (dry air). The pressure sensitivity of the film decreased with temperature, yielding A=-3.52, B=4.53, and C=0.004 at 123K (100 ppm O₂). Flexible polymer/ceramic films (thickness >100m) were prepared by tape casting Al₂O₃ (coated with Pt Octaethylprophine) using an acrylic polymer binder. These films exhibited excellent oxygen sensitivity, yielding nearly linear Stern-Volmer plots with A=0.011, B=1, and C=0.780, and microsecond response times to changes in pressure. Tape cast films containing Rhodamine B were prepared for use as cryogenic temperature (100-250K) sensors. The film temperature sensitivity increased with Rhodamine B concentration.

11:00 AM E9.7
THERMAL EFFECTS ON SHARP SPECTRAL LINES IN Nd-DOPED LASER SOLIDS. Xuesheng Chen , Dept of Physics, Wheaton College, Norton, MA; Baldassare Di Bartolo, Dept of Physics, Boston College, Chestnut Hill, MA.

We investigated the thermal behaviors of sharp luminescence lines in the rare-earth-ion Nd doped laser solids. We measured the line widths, positions and shapes of many emission lines in GSGG:Nd,Cr and CYMGG:Nd,Cr from 77 to 600K. The lines examined initiate in the ⁴F_3/2 manifold and terminate in the ⁴I_11/2 or ⁴I_9/2 manifolds. We studied the lines R₁Y₁, R₁Y₂, R₂Y₁, R₂Y₃, R₁Y₆, R₂Y₆, R₁Z₅ in GSGG:Nd,Cr, and R₂Y₁, R₁Z₅ in CYMGG:Nd,Cr. We observed that all the lines shifted to longer wavelengths (red shift) with increasing temperature except for the R₁Z₅ line which shifted to shorter wavelength (blue shift) in both crystals. For the widths, all the lines became broader with increasing temperature. We also observed that the residual linewidths in CYMGG:Nd,Cr were much broader than those in GSGG:Nd,Cr. The results will be compared with those found in YAG:Nd done by other workers. We will discuss the mechanisms responsible for the observed lineshifts (redshifts and blueshifts), line broadening, and difference in residual widths.

11:15 AM E9.8 SILICA SOL-GELS DOPED WITH FLUORESCENT LIPID BILAYERS FOR METAL ION SENSING. Lauren Shea , Darryl Sasaki, and Michael Sinclair, Sandia National Laboratories, Albuquerque, NM.

Synthetic lipid membranes immobilized in silica sol-gel materials can be employed as highly selective metal ion sensors. Lipid membranes, functionalized on the surface with ion recognition sites (e.g., iminodiacetic acid, dithioamide) and in the bilayer with pyrene fluorophores, show highly selective binding for Cu(II) and Hg(II) ions. Binding of the metal ion produces an inversion of the excimer and monomer characteristic emission peaks in the fluorescence spectrum of the pyrene fluorophores. Immobilization of these lipid membranes in silica sol-gels offers protection of the fluorescent membranes from biological predators (e.g., fungi, bacteria) while allowing the transport of small molecules and ionic species. The silicate acts as a preconcentrator of metal ions from solution increasing the sensitivity of the lipid membranes 4-50 fold. In this work, the lipid membranes were immobilized in silica sol-gels and coated onto optical fibers. After removing the cladding, the fiber surface was treated with silane surface modifiers to promote coating uniformity. Thickness and homogeneity of the coatings was optimized. Coated fibers were coupled to a bifurcated optical fiber platform and characterized using a pulsed Xe lamp at an excitation wavelength of 346 nm.

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3/8/1999