Dept of Physics
Higgins Hall 257
Chestnut Hill, MA 02167-3811
Dept of Engineering Physics
Hamilton, ON L8S 4L7 CANADA
Dept of Applied Mech & Engr Sci
Univ of California-San Diego
La Jolla, CA 92093-0411
Osram Sylvania Inc
Beverly, MA 01915
Aachen, D-52085 GERMANY
Proceedings published as Volume 560
* Invited paper
of the Materials Research Society
Symposium Proceedings Series.
SESSION E1/B1: JOINT SESSION:
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,
INORGANIC LOW VOLTAGE PHOSPHORS
Monday Morning, April 5, 1999
Metropolitan I (A)
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+, (Y1-n-mCenGdm)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 (Y1-n-mCenGdm)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
SESSION E2/B2: JOINT SESSION:
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.
THIN FILM PHOSPHORS
Monday Morning, April 5, 1999
Metropolitan I (A)
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
TFEL devices driven at 60 Hz: 809 and 592 cd/m2 in Zn2Si0.75Ge0.25O4:Mn
and CaGa2O4:Mn TFEL devices, respectively. Maximum
luminous efficiencies of 2.35, 1.7 and 1.2 lm/W were obtained in the Zn2Si0.7Ge0.3O4: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,
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
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)
. 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
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.
Monday Afternoon, April 5, 1999
Franciscan I (A)
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,
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
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
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
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+xBr1-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,
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,
The last four years has seen a marked increase in flat
panel display phosphor development. What achievements have we made? What
SESSION E4: SYNTHESIS, PROCESSING AND CHARACTERIZATION
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,
Tuesday Morning, April 6, 1999
Franciscan I (A)
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,
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
Y2O3'', 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
Pb1, and Pb0 centers. The 41.2 G weak
doublet concerned with this line has been tentatively attributed to a hydrogen
coupled Pb1, 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 Pb
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 , 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.  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,
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
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
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 . 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
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 . Moreover, different
defect status induced by different intensity of -ray
irradiation were also observed by deep level transient spectroscopy (DLTS)
 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.
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.
 S. Nakamura, T. Mukai, and M. Senoh, Appl. Phys.
Lett. , 64, 1687 (1994).
 C. Trager-Cowan, K.P. O'Donnell, S.E. Hooper, and
C.T. Foxon, Appl. Phys. Lett. , 68, 355 (1996).
 F.K. Koschnick, K. Michael, J.-M. Spaeth, B. Beaumont,
and P. Gibart, Phys. Rev. B , B54, R11042 (1996).
 T.S. Cheng, L.C. Jenkins, S.E. Hooper, C.T. Foxon,
J.W. Orton, and D.E. Lacklison, Appl. Phys. Lett. 66, 1509
 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.
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
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.
Tuesday Afternoon, April 6, 1999
Franciscan I (A)
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,
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 Tx, Ty and Tz
in the absence of the external magnetic field (zero field splitting). In
this designations the term Tx has the highest and the
term Tz has the lowest energy in zero magnetic field.
We have observed the magnetically induced crossing of the terms Ty
and Tz. 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:  C. Deka, et al. Appl. Phys.
Lett. 61 (1992) 2141.  N.V. Kuleshov, et al. Optical Materials. 4 (1995)
507.  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,
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 4I13/2
(first excited) and the 4I15/2 (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  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,
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
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.
Tuesday Evening, April 6, 1999
Metropolitan Ballroom (A)
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.
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.
SiOx 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 SiOx 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
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.
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
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.
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,
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
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.
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.
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)].
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 . 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 . 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 5DF1,2
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 .  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.
 R. Rolli, P. Camagni, G. Samoggia, A. Speghini, M. Wachtler and M.
Bettinelli, Spectrochim. Acta, in press.  M. Sbetti, E. Moser, M. Montagna,
M. Ferrari, S. Chaussedent and M. Bettinelli, J. Non-Cryst. Solids 220
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.
HIGH TEMPERATURE ANNEALING BEHAVIORS OF LUMINESCENT SIOX: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 SiOx:H films have
been investigated. The SiOx: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
(Ta) 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 Ta 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 SiOx, 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 SiOx, leading to Si nanocrystals embedded in SiO2
matrix. This segregation process occurred gradually with increasing Ta
greater than 700C, as confirmed
by the evolution of IR absorption spectra with Ta. AFM
provides clear images of the morphology of the films, also exhibiting the
existence of cluster-like structures and the expansion with Ta.
The evolution of optical transmission spectra with Ta
also shows the occurrence of the segregation. The dependence of the deduced
optical coefficients () near the
absorption edge (Eg) on the photon energy (h)
is found to follow the square root law: (h)1/2
(Eg - 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.
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) , 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 . 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.  A.N. Trukhin, M.
Goldberg, J. Jansons, H.-J. Fitting, I.A. Tale, J. Non-Cryst. Solids 223
(1998) 114  L.Skuja, J. Non-Cryst. solids 167 (1994) 229
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.
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.
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
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 Ce4.667(SiO4)3O and Ce2Si2O7
were the origin of the violet/blue luminescence.
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.
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,
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.
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/Zn1-xCdxTe (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 Zn1-xCdxTe 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
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.
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.
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,
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.
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.
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.
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
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.
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.
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
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
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.
Wednesday Morning, April 7, 1999
Franciscan I (A)
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,
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
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:
CONCURRENT WITH B11, B12 AND F5
ORGANIC LUMINESCENT MATERIALS
Wednesday Afternoon, April 7, 1999
Franciscan I (A)
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,
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
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,
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
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
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.
Thursday Morning, April 8, 1999
Franciscan I (A)
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
S. Chaussedent2 and S.H. Garofalini3,
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
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 Iref and Pref 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 4F3/2
manifold and terminate in the 4I11/2 or 4I9/2
manifolds. We studied the lines R1Y1,
in GSGG:Nd,Cr, and R2Y1,
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.