Ibn Arabi and Spinoza on the Nature of GOD By Naeem Ahmad.pdf
AlAhmadi Ahmad.pdf
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FABRICATION AND CHAR ACTERIZATION OF ZnO FILM BY SPRAY
PYROLYSIS AND ZnO POLYCRYSTALLINE SINTERED PELLETS
DOPED WITH REAR EARTH IONS
A Thesis Presented to
The Faculty of the
Fritz J. and Dolores H. Russ
College of Engineering and Technology
Ohio U niversity
In Partial Fulfillment
of the Requirement for the Degree
Master of Science
by
Ahmad Al-Ahrnadi
November, 2003
OHIO UNlVERSlTY
LIBRARY
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TABLE OF CONTENTS
........................................................................................................................hapter One 1
Introduction ...................................................................................................................... 11.1 Review of Study on Zinc Oxide ............................................................................ 21.2 Characteristics of Zinc Oxide ..................................................................................
.................................................................................................................3 Rare Earth 61.4 4f-4f Luminescence of Rare Earth ......................................................................... 111.5 Characteristics of Europium ................................................................................ 131.6 Characteristics of Thulium ..................................................................................... 14
......................................................................................................................hapter Two 15...........................................................................................................ample Preparation 15
..........................................................................................1 Deposition of Thin Film 15................................................................................................1.1 Spray Pyrolysis 15...............................................................................................1.2 Film Preparation 16
2.2 Polycrystalline Sintered Pellets .............................................................................. 8.............................................................................................2.1 Pellet preparation 19
Chapter Three .................................................................................................................... 20
Experimental Setup ........................................................................................................... 0.......................................................................................1 X-Ray Diffraction (XRD) 20
3.1.1 Experimental Details ....................................................................................... 23.2 Photoluminescence (PL) ...................................................................................... 25
3.2.1 Photoluminescence Experiment Setup ............................................................ 26....................................................................................3 Cathodoluminescence (CL) 28
.......................................................3.1 Cathodoluminescence Experiment Setup 2 9
.....................................................................................................................hapter Four 31.....................................................................................................esults and Discussion 31
............................................................................................1 ZnO: RE^+Thin Films 31..............................................................................................1.1 Crystal Structure 31
4.1.2 Photoluminescent ............................................................................................ 3.........................................................................................1.2 Cathodoluminescent 37
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4.2 Polycrystalline Sintered Pellets ZnO: REC13 ......................................................... 39.............................2.1 Crystal Structure of ZnO Doped with E U ~ + ZnO: Eu. C13) 39
........2.2 Photoluminescence of Zinc Oxide Doped with Europium (ZnO: Eu.Cl) 414.2.3 Cathodoluminescent of ZnO Doped with E U ~ + ZnO: Eu. C13) ..................... 44
.............2.4 Crystal Structure of Zinc Oxide Doped with Thulium (ZnO: Tm.Cl) 454.2.5 Photo Luminescent of Zinc Oxide Doped with Thulium (ZnO: Tm,Cl) ........ 474.2.6 Cathodoluminescent of Zinc Oxide Doped with Thulium (ZnO: Tm. C13) .... 49
Chapter Five ..................................................................................................................... 0
Excitation Mechanisms and Conclusion ........................................................................ 0...........................................................................................1 Excitation Mechanisms 50
.................................................................................2 Energy Transfer Mechanisms 525.3 Conclusion ......................................................................................................... 6
............................................................................................................4 Future Work 57.........................................................................................................................eferences 58
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LIST OF TABLE
.............................................able 1.1 Electronic Configurations of Trivalent Rare Earth 8
Table 1.2 Number of Available Electron States in Some of the Electron Shells and SubShells ............................................................................................................................... 0
.............................................................................able 1.3 Characteristics of Europium 13. .
Table 1.4 Charactenstics of Thulium ............................................................................... 4.......................able 2.1 Polycrystalline Sintered Pellets: ZnO: EuC13 and ZnO:TmC13 18
Table 4.1 The Peak Assignment for the PL of ZnO: Eu.C1 .............................................. 41Table 4.2 The Peak Assignment for the PL of ZnO: Tm.Cl ......................................... 47
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LIST OF FIGURES
Figure 1.1 Periodic Table of the Elements [37] ................................................................. 7
..............igure 1.2 Dieke diagram of the energy level of trivalent lanthanide ions [27] 12
Figure 2.1 Setup of the spray pyrolysis system ............................................................... 17
Figure 3.1 Bragg X-ray diffraction condition (2d sin (0 ) = n 1) ..................................... 21
Figure 3.2 Setup of the x-ray diffraction ........................................................................ 24
Figure 3.3 Photoluminescence experiment setup ............................................................ 7
Figure 3.4 Schematic represent of bombardment (modifified after Potts; 1995) not thatthe emissions come from different depths; e.g., CL and X-ray are emitted from deepersection levels than secondary electrons ............................................................................ 28
Figure 3.5 Cathodolurninescence experiment setup ........................................................ 30
Figure 4.1 XRD spectral of ZnO ...................................................................................... 2
Figure 4.2 PL spectrum of ZnO: (Eu. C1) un-annealed ................................................... 4
Figure 4.3 PL spectrum of ZnO: (Eu. C1) annealed at 550 "C ......................................... 35
Figure 4.4 PL spectrum of ZnO: (Eu. C1) annealed at 600 "C ......................................... 36
Figure 4.5 CL spectrum of ZnO: (Eu. C1) annealed at 600 and 700 "C (a). (b) measured atlow temperature (15 K) and (c) at room temperature (300 K) .......................................... 38
Figure 4.6 (a) ZnO powder 2 hr at 1000 OC in air . (b) Zn0:Eu powder 2 hr at 1000 OC inair. EuC13 with 0.07 in concentration . (c) ZnO: Eu powder 3 hr at 1000 OC in vacuum.EuCl3 with 0.07 in concentration ..................................................................................... 0
..........................igure 4.7 PL emission spectra for ZnO : Eu sintered at 1000 "C in air 42
.................igure 4.8 PL emission spectra for ZnO : Eu sintered at 1000 "C in vacuum 43
Figure 4.9 CL emission spectra for ZnO : Eu sintered at 1000 OC in N2 ........................ 44
Figure 4.10 (a) ZnO powder sintered in air for 2 hr at 1000 OC in air (b) ZnO: Tmpowder sintered in vacuum for 3 hr at 1000 OC in vacuum. TmC13 with 0.07inconcentration .................................................................................................................. 6
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Figure 4.1 1 PL emission spectra for ZnO : Tm sintered at 1000 "C in vacuum ..............48
.....................igure 4.12 CL emission spectra for ZnO
:
Tm sintered at 1000 "C in N2 49................igure 5.1 A model of the excitation processes for ZnO doped with RE ions 5 1
Figure 5.2 Schematic diagram of trapping electron on rare earth related state. Therecombination of energy of trapped electron and the free hole excites the rear earth ions.......................................................................................................................................... 52
Figure 5.3 Schematic diagram of trapping hole on rare earth related state. Therecombination of energy of fiee electron and the trapped hole excites the rear earth ions.
Figure 5.4 Schematic diagram of trapping electron and hole on impurity related state. ...he recombination of energy of trapped electron and hole excites the rear earth ions. 54
Figure 5.5 Schematic diagram of excitation electron and hole pair. The recombination of..............................................nergy of free electron and hole excites the rear earth ions 55
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Chapter One
Introduction
In modern optical technology, rare earth ions in solids play an important role as
an active constituent of materials. This is because they show an affluence of sharp
fluorescent transitions representing almost every region of the visible and near infrared
portions of the electromagnetic spectrum. The sharpness of many lines is a result of the
shielding effect of the outer electrons. Most of the sharp lines are duo to 4f electrons as
long as the 4f shell is not completely filled with 14 electrons. A number of 4f levels are
unoccupied and electrons already present in the 4f shell can be raised by light absorption
into these empty level [I]. Semiconductors, such as ZnO, ZnSe, or ZnS, doped with rare
earth ions [2 , 31 may show evidence of electroluminescence; these materials are
candidates for traditional semiconductor light emitting diodes and many enable new
technologies for highly distinguishable emissive flat panel displays. It may also be used
to improve many electro optical applications that rely on the direct generation of either
narrow or broad spectra.
In this study, we have prepared and investigated the effect of chlorine ions co-
doping on luminescence sensitization of ZnO samples doped with Eu and Tm ions by two
methods. The first one is deposition of thin film ZnO: EuC13 grown by spray pyrolysis
technique, and the second one is polycrystalline sintered pellets ZnO:REC13 phosphors.
Characteristic 4f luminescence of europium (EU~') and thulium ( ~ m ~ ' ) oped into ZnO
was observed under photon excitation photoluminescence (PL) and electron excitation
cathodoluminescence (CL). Furthermore, on the basis of luminescence results of the PL,
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CL, and their kinetics, we will discuss the optical properties of ZnO: EuCl3, ZnO: TmCl3
related emissions under different excitation conditions. In addition, obtained results may
indicate that codoping of Zn0:RE with chlorine leads to efficient sensitization of RE
complexes in ZnO host. Finally, we presented some of this work at the Second
International Workshop on Zinc Oxide held in Dayton, OH, 23-25 October, 2002.
1.1 Review of Study on Zinc Oxide
In the past century, much research has been conducted on luminescence of rare
earth ions doped II-VI semiconductors compounds. Especially in the fifties and sixties
rare earth ions doped II-VI semiconductors compounds have been studied widely by
several research groups for possible applications in light emitting devices and for their
unique optical properties. In the middle of seventies, a new impetus came from the
activities aimed at multicolored electroluminescence displays. Lozykowski and Szczurek
[4, 51 were the first to investigate the electroluminescence of ZnSe thin films activated
with rare earth fluorides. Their study led to the conclusion that a wide variety of
electroluminescence centers occur and that the direct impact excitation mechanism
dominates. In the late seventies and early eighties, only a broad band from ZnO was
observed [ 6 , 7 , 81 in the photoluminescence spectra. In addition, the electroluminescence
of these materials is somewhat similar from one doped sample to the other and consists of
three different bands in the 390-640 nrn range [9]. Y. Hayashi and co-workers [lo]
observed the red band luminescence from E U ~ ' doped ZnO. The intensity and fine
structures of the E U ~ + luminescence and their temperature dependence are strongly
influenced by the doping conditions. In particular, for the increase of the E U ~ +
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luminescence intensity, Li co-doping is not affective but stoichiometric control of Zn and
0is essential. Moreover, the observed red band luminescence can be attributed to excess
oxygen. Y. Park and co-workers found from PL measurements that with increasing
doping concentration, the broad-band emission centered around 530nm gradually
disappears while the sharp red emission peaks around 620nm exhibit a pronounced
increase [ l 11. J.C. Ronfard-Haret and co-workers conducted extensive investigation of
the rare earth center in ZnO and they observed at room temperature the
triboluminescence (emission of light caused by the application of mechanical energy to
solid) of ~ r ~ + ,U ~ + , O~', sm3' and ~m~~ ions. In the 400-850 nrn range, the
triboluminescence spectra were compared with the electro and photo luminescence
spectra of the pellets of the same composition and sintered under the same conditions.
The triboluminescence and electroluminescence spectra showed only the sharp lines
characteristic of transition between the 4f level of the RE^+ ions, whereas the
photoluminescence spectra showed only the broad ZnO emission. It is concluded that the
triboluminescence of the RE3+ ions is consecutive to a d irect electrical excitation
consequence of a breakdow n in the rich inter granular material [12].
In addition to Polycrystalline sintered pellets, ZnO thin films can be prepared on
several substrates by many techniques, such as vacuum evaporation 1131, photochemical
deposition reactive evaporation[14], r.f sputtering [15], chemical vapor depositionCVD
[16], sol-gel [17], pulsed laser deposition [18] and spray pyrolysis [19].F. Paraguay and
co-workers obtained uniform high quality ZnO thin films by spray pyrolysis [20]. The
spray pyrolysis attracted several research groups because of its simplicity, efficient,
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inexpensive technique, and it produced good quality films. In this process, the spray
droplets strike the substrate directly where the pyrolytic reaction takes place leading to
the formation of a thin film. M.O.Abou H elal and W.T.Seeber doped ZnO w ith rare earth
element (Pr, Ce, Nd, Tb, Sm) by spray pyrolysis and they obtained films with optical
transmission T > 85 % and structural uniformity in terms o f average roughness< 10 nm
[21]. Recent reports of prepared p-type ZnO [22, 231 open up a novel possibilities for
optoelectronic light emitting devices. In the present time, successes in producing large
area single crystals have opened up the possibility of producing blue and UV light
emitters, high temperature, and high pow er transistors [24].
1.2 Characteristics of Zinc Oxide
Zinc oxide is a 11-VI semiconductor with properties similar to GaN(3.5 eV) and
6H -S ic (3 eV at 2K). It grows like GaN with a hexagonal crystal structure. The strangest
point of ZnO is that it has a large exciton binding energy (60 meV ), which is larger than
other 11-VI compound semiconductors and much higher than that of GaN (21-25 meV).
ZnO is a wide and direct band gap semiconductor,E, = 3.2 eV at room temperature and
E, = 3.437 at 2K. W ide band gap sem iconductor materials have come to the forefront in
the past decade because of an increasing need for short-wave length photonic devices,
high power, and high frequency electronic devices. Also, ZnO, like indium ox ide and tin
oxide, is transparent in the visible region and electrically conductive with appropriate
dopants. This property has been w idely studied for its practical application, as transparent
conducting (TCs) electrodes, which have a wide variety of uses. Their ability to reflect
thermal infrared heat is exploited to make energy conserving windows. These low
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ernissivity windows are the largest area of current use for TCs. Oven windows employ
TCs to conserve energy and to maintain an outside temperature that makes them safe to
touch. The electrical conductivity of TCs is exploited in front-surface electrodes for solar
cells and flat-panel displays (FPDs). TCs can also be fonned into transparent
electromagnetic shields, invisible security circuits on windows, and transparent radio
antennas built into automobile windows. Indeed, polycrystalline ZnO has found
numerous applications in such diverse areas as facial powders, piezoelectric transducers,
varistors, phosphors, and transparent conducting films [36].
Zinc oxide is n-type semiconductor. It has molecular weight of 81.37 and
enthalpy of formation (298.15K) of 350.5 KJ/mol. It crystallizes in a hexagonal wurtzite
lattice, consisting of two interpenetrating hexagonal close packed lattices, one containing
the cations (Zn ++), and the other the anions ( 0 '). The lattice constants parameters a =
0.32495 * 0.00005 nrn and c = 0.52069 0.00005 nm at 298 * 5 K, which slightlychanges with stoichiometry of the composition. The melting point of ZnO is 2248 K.
ZnO has a large exciton-binding energy of 60 meV that has attracted much recent
attention. Zinc oxide thin films have valuable properties, such as chemical stability in
hydrogen plasma, high optical transparency in the visible and near infrared region of the
electromagnetic spectrum, and high refractive index [26].
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1.3 Rare Earth
As mentioned earlier, rare-earth ions play an important role in much of modern
optical technology as the active constituents of materials. The most well-known
application is in rare-earth solid-state laser materials. Rare-earth ions also play a critical
role in energy-efficient luminescent materials, such as phosphors for fluorescent lamps,
cathode ray tubes (CRT's), and plasma displays. In the periodic table, various groups of
elements can be distinguished, like the s and p-block elements, the 3d, 4d, and5d
transition elements, and the lanthanides and actinides Figure1.1. The lanthanides form a
special group of elements, usually shown at the bottom of the periodic table. They are
characterized by an incom pletely filled 4f shell.
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-- - -- - - - -Figure 1 . 1 Periodic Table of th e Elements [37]
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The electronic configuration of trivalent rare earth ions in the ground states are shown in
Table (1.1).
Table 1.1 Electronic Configurations of Trivalent Rare EarthElement Electron Covalent Ground Electronegativity
Cerium
Praseodymium
Neodymium
Promethium
Samarium
Europium
Gadolinium
Terbium
Dysprosium
Holmium
Erbium
Thulium
Ytterbium
Configuration
of RE^' Ions
4f'5s25p6
4f5s25p6
4P5s25p6
4f45s25p6
4f55s25p6
4f65s25p6
7 2 64f 5s 5p
4$5s25p6
4P5s25p6
4f'05s25p6
4f"5s25p6
4e25s25p6
4t35s25p6
Radius State Ion
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As we see the lanthanides have one to fourteen 4f electrons added to their inner
shell configuration, which is equivalent to Xe. Ions with no 4f electrons ( ~ a ~ +nd L U ~ + )have no electronic energy levels that can induce excitation and lumines cence processes in
or near the visible region. In contrast, the ions from ce3+ o yb3+which have partially
filled 4f orbital have energy levels characteristic of each ion and show a variety of
luminescence properties aro und the visible region. For exam ple, at cerium there is one 4f
electron and it begins increasing through the element until it is filled at lutetium. The
ground state is characterized by the H und rules:
1. The max imum values of the total spin s allowed by the exclusion principle.
2. The m aximum value of the orbital angular mom entum L consistent with this value
of s.
3. The value of the total angular momentumJ is equal to (L-S) when the sh ell is less
than half full and to (L+S) when the shell is more than half full. When the shell is half
full, the ap plication of the first rule gives L= 0, so that J = 0.
The azimutal quantum number( I ) of 4f orbital is 3 giving rise to 7 ( = 21 + 1 )
orbital, each of which can accommodate two electrons. In the ground state, electrons are
distributed to provide the m aximum com bined spin angular momentum (s).
The spin angular momentum s is further combined with the orbital angular
momentum (L) to give the total angular momentum(J). An electronic state is indicated
by notation 2 S + ' ~ J where L representsS, P, D, F, G, H, I, K, L, M .. ., corresponding to L=
0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ....., respectively. Table 1.2 show the number of available
electron states in som e of the electron shells and sub sh ells.
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Table 1.2 Num ber of Available Electron States in Som e of the Electron Shells and SubShells
Principal Shell Sub Shells Number of Number of Number of
Quan tum Designation States Electrons Per Electrons
Number n Sub Shell Per Shell
1 K s 1 2 2
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1.4 4f-4f Luminescence of Rare Earth
The 4f electrons are shielded from ex ternal electric fields by the o uter 5s2 and
5p6electrons so that the levels are not affected much by the environment, unlike 3d
electrons located in an outer orbit which are heavily affected by the environmental or
crystal electric field. Dieke and co-workers [27] have accurately investigated the
characteristic energy level of 4f electrons of trivalent lanthanide ions and the results are
shown in Figure (1.2), which is known as a Dieke diagram. In Figure (1.2), each level
designated by the number J is split into a number of sublevels by the stark effect due to
the crystal field. The number of split sublevels is (2J+1) or (J+1/2) for J integer or J of
half integer, respectively. The number of levels is determined by the symmetry of the
crystal field surrounding the rear earth ion. The width of each level indicates the range of
splitting within each component. In the basis of the crystal field model[28], the
Ham iltonian for the 4f electrons is w ritten as
H = Ho+ Hee +Hso+Hcf (I .4.1)
(H, is Hartree-Fock part of the Ham iltonian, He, coulomb interaction between the
electrons @art not con tained in H,), H,, spin-orbit coupling , Hcf crysta l-field poten tial).
The free ion levels result from the splitting of the 4f con figuration due to &,+H,,. In
addition, under the action of Hcf he free ion are generally split into several compon ents.
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C e P r N d P m S m Err
-1
0 2 % *--city 4 5 %
.A$ *-(I 'a,
Figure 1.2 Dieke diagram of the energy levelof trivalent lanthanide ions [27]
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1.5 Characteristics of Europium
Europium was discovered by E. Demarcay, a French chemist, in 1896. He was
able to produce reasonably pure europium in 1901. Today, europium is primarily
obtained through an ion exchange process from monazite sand, a material rich in rare
earth elements. Europium is the most reactive of the rare earth elements; it quickly
oxidizes in air. There are no commercial applications for Europium metal. Due to its
ability to absorb neutrons, it is also being studied for use in nuclear reactors. Europium
Oxide (E u20 3) s widely used as a red phosphor in television (CRT ). Table 1.3 show s the
characteristics of Europium [33].
Table 1.3 C h a~Symbol
Atomic Number
Atomic Mass
Melting Point
Boiling Point
Number of Protons/Electrons
Number of Neutrons
Crystal Structure
Density @ 293 K
Color
,istics of EuropiumEu
151.964 amu
822.0 OC (1095.15 OK, 1511.6 OF)
Cubic
silver
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1.6 Characteristics of Thulium
Thulium was discovered in 1879 by Cleve. Thulium occurs in small quantitiesalong with other rare earths in a number of minerals. It is obtained commercially from
monazite, which contains about 0.007% of the element. Thulium is the least abundant of
the rare earth elements, yet with new sources recently discovered, it is now considered to
be as rare as silver, gold, or cadmium. Ion-exchange and solvent extraction techniques
have recently permitted much easier separations of the rare earths w ith much lower costs.
Table 1.4 show s the characteristics of Thulium [33].
Atomic Mass
Table 1.4 Characteristics of Thulium
Melting Point
Symbol
168.9342 amu
1545.0 OC (1818.1 5 OK, 2813.0 OF)
Tm
Number of Protons/Electrons I69Boiling Point
1Number of N eutrons 110 0
1727.0 OC (2000.15 OK, 3140.6 OF)
Crystal Structure
Density @ 293 K
Color
Hexagonal
9.321 &m3
silverfish
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Chapter Two
Sample Preparation
In this study, we have prepared and investigated the effect of chlorine ions co-
doping on luminescence sensitization of ZnO samples doped with Eu and Tm ions by two
methods. The first one is deposition of thin film ZnO: EuC13 grown by spray pyrolysis
technique and the se cond one is polycrystalline sintered pellets ZnO:REC13 phosphors.
2.1 Deposition of Thin Film
As mention earlier there are several different techniques used for depositing ZnO
thin film. For example, films have been deposited by vacuum evaporation, photochemical
deposition reactive evaporation, r.f sputtering, chemical vapor deposition CVD, sol-gel,
pulsed laser deposition, and spray pyrolysis. We will discuss spray pyrolysis deposition
techniques, which w e usedin this work.
2.1.1 Spray Pyrolysis
The spray pyrolysis technique is a method that has been widely used for more
than two decades, due to its simple, inexpensive technique and possibility to produce
large area films. Spray pyrolysis offers several advantages over conventional routes for
particle synthesis. Particles produced by this route are often more uniform in size and
composition than powders produced by other methods. This technique dissolves elements
of the compound material in solution and the solution is sprayed onto a heated substrate
in the form of small droplets generated by an ultrasonic spray generator or aerosol
generator. The spray nozzle can be driven by gravity or forced by gas pressure. Gas
pressure can be controlled by the flow and used as part of the compou nds, such as oxygen
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or dry air, to deposit oxides and nitrogen o r inert gas to avoid chemical reactions between
the compound materials. Another important technique is preheating the solution. This
technique is especially useful to accelerate the reaction to get good quality films. On the
other hand, it is also important to heat the substrate enough to make sure that the
compound solvents are completely evaporated. The film's thickness depends on the
concentration of the solution and deposition time.
2.1.2 Film Preparation
Thin films of un-doped and rare earth doped ZnO were deposited on a glass substrate
cleaned by using de-ionized water and acetone to achieve successful deposition. For un-
doped thin films, the spraying solution was prepared by dissolved zinc acetate dehydrate
(ZnC4H604.2H20) with a molar concentration of (0.2M) ina mixture of de-ionized water
and methanol (3:l). Using zinc acetate as a precursor makes it possible to obtain layers
without chlorine contamination. The additional advantage of zinc acetate is its high vapor
pressure. For the rear earth doped Z nO films, a stock solution of (0.2M) zinc acetate was
mixe d in a ppropriate ratios with EuC13 to provide spray solution containing certain (0.5,
2,4) percentage of rare earth dopants comparative to zinc. The solution reaches the
substrate which is inside the furnace at a temperature of (500-550"C) in the form of small
droplets where they are decomposed. The elements react endothermic ally and they are
deposited as a thin film, whereas the other unstable species are evaporated. The spraying
time is 5-35 minutes. After deposition, the samples were annealed in furnace at
tempe rature (550-700 "C) for 1-3 hour in air or N2 at atm ospheric pressure. Figure 2.1
show the setup of the spray pyrolysis system.
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IVentilation I
-TT- SampleFurnace I
Temperature Control
-uartz Tube
CarrierGas N 2
--
Figure 2.1 Setup of the spray pyrolysis system
0 0 O 0 0 o O o o0 .P$Oo; ;oo 4- 4- 4- 4-
H
HNebulizer
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2.2 Polycrystalline Sintered Pellets
Powders of ZnO and rear earth compounds were carefully weighed and m ixed, asshown in the table below (2.1).
Table 2.1 Polycrystalline Sintered Pellets: ZnO : EuC13 and ZnO:TmC13RE Compound RE ions Concen tration Firing Conditions
[atomic (at).%]
EuC13 0.01-
0.07 2hr
at 1000 OC in air
2 hr at 1000 OC inN2 +HC1
3 hr at 1050 OC in vacuum
2 hr at 1000 OC in air
2 hr at 1000 "C in N2 + HC1
3 hr at 1050 OC in vacuum
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Chapter Three
Experimental Setup
Lum inescence is the emission of light from a solid when it is supplied with some
form of energy. We may distinguish between the various type of luminescence by the
method of excitation. For exam ple in photoluminescence(PL) the excitation arises from
the absorption of photons, in cathodoluminescence the excitation is by bom bardment w ith
a beam of electrons, and in electroluminescence the excitation results from the
application of an electric field (which ma y be either a.c or d.c).
W hatever the form of energy input to the luminescing material, the final stage in
the process is an electronic transition between two energy levels which either emit or
absorb a photon. In this chapter, we discuss X-ray diffraction, photoluminescence and
cathodoluminescence.
3.1 X-Ray Diffraction (XRD)
The most basic properties to be characterized are the c rystal structure. W.L. Bragg
derived a simple equation treating diffraction as reflection from planes in the lattice.
Figure 3.1 show the Bragg X-ray diffraction condition
2d sin(0) = nh (3.1)
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Incident Diffractedbeam
d = plane distancecigure 3.1 Bragg X-ray diffraction condition (2d sin(8)
=
n h)
X-ray Diffraction (X RD) is one o f the most useful and powerful non-destructive
technique methods for characterizing crystalline materials. It provides information on
structures, phases, preferred crystal orientations (texture), and other structural parameters,
such as average grain size, crystallinity, strain, and crystal defects. X-ray diffraction
peaks are produced by constructive interference of m onochromatic beam scattered from
each set of lattice planes at specific angles. X-ray diffraction from crystalline solids
occurs as a result of the interaction of X-rays with the electron charge distribution in the
crystal lattice. The ordered nature of the electron charge distribution, which is distributed
around atomic nuclei and is regularly arranged with translational periodicity, means that
superposition of the scattered X-ray am plitudes will give rise to regions of con structive
and destructive interference producing a diffraction pattern. The X-ray has a wavelength
range from approximately 0.1 to 1008, They occur in that portion of the electromagnetic
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spectrum between gamma-rays and the ultraviolet. Like so many other absorption
phenom ena, the coefficient absorption of X-ray by a s olid follows the equation:
1 = 10 ex p (ppx) (3.2)
where p is the mass absorption coefficientp is the density and x is the thickness of the
specimen.
3.1.1 Experimental Details
X-rays produced by bombarding a metal target with 40 keV electrons produce a
continuous spectrum as well as sharp, very intense spectral features characteristic of the
atoms of the target. For the Rigaku Geigerflex DMAX-B the target in the source tube is
Cu and the characteristic X-rays have wavelengthsK,,= 1.5405 and K , 2 = 1.5443 (taken
together as the K, = 1.542 A). There are also nearby less intense lines designatedKp at
1.392 A. In this instrumen t we have a mon ochrom ating crystal "C" which is a specially
bent piece of pyrolitic graphite. The angle between the incoming beam and the crystal is
equal to the angle between the detector and the crystal. They are fixed to select the Cu
K, by Bragg scattering from the grapheme planes. Notice that the monochromating
crystal is used after the X-ray has been scattered from the sample and before the detector.
In the theta-theta goniometer on this machine, the sample sits still and the source and
detector arms each turn in the opposite direction by an angle0, with respect to the plane
of the sample surface. There are sets of slits that help to define the solid angle of the
beam reaching the detector. The first slit, which is nearest the source, is the divergence
slit (DS) which limits the area of the sample exposed to the beam. The scatter slit (SS),
which limits the scattered X-rays incident on the analyzer and which is a mating slit to
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the DS comes next. The receiving slit (RS), which is at the image position of the source
reflected in the sample and which also limits the scattered X-rays incident on the
analyzer, is important in determining the angular resolution of the measurement. The
resolution is also affected by the narrowness of the beam itself, which is determined by
the takeoff angle. In this instrument, the takeoff angle is fixed. The monochromator
receiving slit (RSM ) is at the image position of the R S reflected in the mirror of the bent
graphite crystal and which limits the X-rays entering the counter. These slits come in
various sizes that may be selected depending on the needs of the experiment. In many
diffraction measurements above 20= 20, the slits are chosen to be: DS lo , SS lo, RS 0 .3
mm and RMS 0.45 mm. This selection gives an adequate combination of intensity and
resolution for most purposes [34]. The computer is used to determine the crystal
structures by x-ray diffraction by collecting the data a nd controlling the processing of the
experiment.
In our experiment we used theta-theta Gonoimeter model CN 2182 D61 CN
2182D7. A typical X-ray setup is illustrated in F igure 3.2.The divergence beam X-ray is
controlled by two sets of slits placed between focus and the samples and the sam ples and
scatter slit, respectively. To convert the diffracted X-ray photons into voltage pulses, a
photon detector is placed behind the receiving slit. The diffracted beams travel back
toward the x-ray tube and strike the flat film. Each diffracted beam leaves a spot on the
exposed film. The position of the spots can be converted to angular readings of the
orientation of the atomic plane causing the spot, using a special chart called a Greninger
chart. These angular readings are plotted on a stereographic projection, from which the
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angles between p lanes can be read. W ith experience, luck, and a bit of trial and error, the
crystallographic indices of the planes can be deduced, and then the orientation of the
crystal is specified.
Detector\
Slitn
I
Sample
\
Source tube C u
Figure 3.2 Set up of the x-ray diffraction
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3.2 Photoluminescence (PL)
In photoluminescence, we measure physical and chem ical properties of materials
by using photons to induce excited electronic states in the material system and analyzing
the optical emission as these states relax. Typically, light is directed onto the sample for
excitation, and the emitted luminescence is collected by a lens and passed through an
optical spectrometer onto a photo-detector. The spectral distribution and time dependence
of the emission are related to electronic transition probabilities within the sample and can
be used to provide qualitative and sometimes quantitative information about chemical
composition, structure, impurities, kinetic processes and energy transfer. In a typical PL
experiment, the sam ple is cooled to 10K in a cryostat and the excitation source is a laser.
In general, the laser light will create a non-thermal distribution of electrons and holes.
Efficient non-radiative processes will however, rapidly bring the carriers into a quasi-
equilibrium described by the quasi-Fermi levels. Finally, the electron-hole pairs emit
photons as they recombine. T he recombination takes place on a time scale determined by
the transition probability. The PL spectrum is obtained by analyzing the spectral content
of the emitted light.
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- - - - - - - - - _ _ _ _ _ CCD Controller- - Camera -
StepperMotor \ - - Monochromator
I
I -I '\
Figure 3.3 Photoluminescence experiment setup
Controller ofStepper Motor
IonizationGauge Control
I \
Filter ,' & Lens Computer PC,,
\\
I : , \: I \ \
TemperatureController
ModulatorCooling System
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3.3 Cathodoluminescence (CL)
Cathodoluminescence (CL ) is the emission of photons due to the bombard ment of
an energetic electron beam on luminescent materials. The CL technique has been
recognized as a powerful and sensitive tool for micro characterization of luminescent
materials, especially in the field of optoelectronic semiconductor materials. The
interaction of the beam with the sam ple gives rise to a numb er of effects: the emission of
secondary electrons (SE), back scattering of electrons (B SE) , electron absorption , X-ray
and CL emission Figure 3.4. Most energy of the beam is converted into heat. The
penetration depth o f electrons and accord ingly, the ex citation depth depend on the energy
of the electrons.
Thin
Incident Electron Beam
Iathodoluminescence I I
SecondaryElectrons
Section
inelasticallyScatteredElectron Elastically
Scattered UnscatteredElectron Electrons
Figure 3.4 Schematic represent of bombardment (modifified after Potts; 1995) not thatthe emissions come from different depths; e.g., CL and X-ray are emitted from deepersection levels than secondary electrons.
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The m echanisms leading to the emission of light in a solid are similar for different
forms of the excitation energy. An advantage of CL, in addition to the high spatialresolution is its ability to obtain m ore detailed depth resolved information by varying the
electron beam energy. Luminescence can be divided into intrinsic CL, which is
characteristic of the host lattice, and extrinsic CL, which results from impurities.
Recombination of electron hole pairs may occur via nonradiatively which can decrease
the emission. This decrease is referred to as co ncentration quenching. It can be explained
by the transfer of a part of the excitation energy to other activator ions, which is more
effective than luminescence emission. Q uenching due to lattice defects may occur if the
crystal structure is damaged by mechanical processes, radiation, growth defects, or
impurities. These lattice defects create new energy levels between the cond uction and the
valence bands resulting in absorption of the excitation energy, non-luminescent energy
transfer, or low frequency emission. Another process which may be responsible for
lowering the luminescence intensity is thermal quenching [32].
3.3.1 Cathodoluminescence Experiment Setup
A typical CL setup is illustrated in Figure 3.5. The sample was mounted on a
sample holder situated inside the optical cryostat and excited by an electron beam (up to
5 keV and maximum current emission -75pA),which was incident upon the sample at a
45" angle from an electron gun (Electronscan EG5 VS W). The optical cryostat is
pumped to Torr and cooled by a closed cycle helium gas to9 K. The emitted light
was collected by a quartz lens on the entrance slit of the spectrograph monochromator.
The monochromator separates polychromatic light it receives into monochromatic light
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of individual wavelengths. Individual wavelengths are focused at different horizontal
positions along the exit port of the spectrograph and detected simultaneously by the CCD
system. The signal from CCD is sent to a computer through a controller as ASCII data.
StepperMotor
Controller ofStepper Motor
3 rn- - - - - - - - - - - -I/------./-.:------
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Chapter Four
Results and Discussion
4.1 ZnO: RE^+ Thin Films
Undoped and rare earth doped thin ZnO films used inthis study were prepared by
spray pyrolysis on glass substrates; as have been described in Chapter 2. The crystal
structure and phase were identified using a Rigaku Geigerflex X-ray diffraction. A He-Cd
laser was used as the excitation source for PL measurement. The cathode electron gun
was used as the excitation source for CL measurement.
4.1.1 Crystal Structure
Figure 4.1 shows the XRD spectra of ZnO films deposited on glass substrates
heated to 480 "C. The film was found to be polycrystalline with a (002) preferred
orientation and this indicated that the grains were strongly oriented along with the c axis
of the ZnO wurtzite structure, perpendicular to the substrate surface. The (002) peaks are
located at 34.76O for pure ZnO. In addition, there are peaks at 32.14O, 36.61, and 47.84",
which correspond to the ZnO (loo), (101), and (102) planes. When the Eu3+ ion is
situated at the ZnO grain boundary, the ZnO crystal lattice does not affect by Eu3+ ons
and diffractive peaks related to the Eu can be detected by the XRD measurement. But in
our experiments, only the ZnO wurtzite structure is obtained and the lattice deformation
is observed, which indicates that the Eu3+ ons are inside the ZnO grains and substituted
for the zn2+ on position in the host matrice successfully. Because all the ions mix well in
the sol, the Eu3+ ion could be doped in the ZnO crystal lattice easily during the
crystallization process [3].
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Figure 4.1 XRD spectral of ZnO
annealed at 600 OC
(101)
md (102)
1(002)
unannealed
(102)
t I I I I I I I 8 I
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4.1.2 Photoluminescent
In a semiconductor crystal that contains no defects there would be mostly the
luminescence line due to radiative recombination of the free exciton (FE) on the
luminescence spectra. But if there are defects such as donor and acceptor impurities in
the crystal, than most of the excitons will be bound to defects and form the so called
bound excitons (BE'S). The BE has a lower energy by the binding energy between the
defect and exciton compared to the FE. The BE'S can be categorized into three main
types: a neutral donor bound exciton (DOX), neutral accepter bound exciton (AOX), nd
an ionized donor bound exciton (D'x).
We have studied the PL spectra of ZnO films at room temperature (300 k) and 15
k with an excitation wavelength 325nm (UV) light from He-Cd laser. Figure 4.2 shows
PL spectra of unannealed ZnO: (Eu,C13) with 0.5 % of Eu concentration which has two
distinct peaks. The first one around 380 is from the bound exciton (BE) and the second
one at 520nrn is duo to the host (ZnO). The peak of the green band spectra exists at 500
nrn and it does not shift between 15 K and 300 K. We found that there is no emission due
to rear earth. But when we annealed the sample at 550 OC we found that there are peaks
from emissions of E U ~ + overlapping with the broad band luminescence of ZnO around
620nm. We found that at low temperature, around (15 K), the luminescence from the rare
earth ions is stronger than the luminescence at room temperature. This is also shown in
Figure 4.3. In addition, when we annealed the sample at 600 OC there is no big difference
between the emissions from the sample. Only the band edge emission decreased as shown
in Figure 4.4.
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34
D'X ZnO:Eu,Cl unannealedEU * Concentration : .05 at.%h = 325 nm
exc
--
I I I I
400 600 800 1 000
Wavevlength [nm]
Figure 4.2 PL spectrum of ZnO: (Eu, C1) un-annealed
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ZnO:Eu,Cl annealed at 550 "CE U~ ' Concentration : .05 at.%
400 500 600 700 800 900 1000
Wavevlength [nm]
Figure 4.3 PL spectrum of ZnO: (Eu, C1) annealed at 550 "C
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ZnO:Eu,Cl annealed at 600 "CEU" Concentration : 0.05 at.%h = 325 nm
400 500 600 700 800 900 1000
Wavevlength [nm]
Figure 4.4 PL spectrum of ZnO: (Eu, C1) annealed at 600 OC
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4.1.2 Cathodoluminescent
Figure 4.5 presents the CL spectrum of ZnO: Eu,Cl3 thin films measured at room
temperature (300 K) and at low temperature (15 K) using an excitation voltage of 5kV.
The peak of the green band spectra exists at 500 nm and it does slightly shift between 15
K and 300K; also the emission intensity increases with decreasing sample temperature.
The sharp red-emission peak at 615nm is characteristic of the transition between
electronic energy level of E U ~ + ons 'DO - ~~ (J = 1 to 6). The annealing process causesan increase in the CL intensity. The CL intensity was found generally to increase with
increasing annealing temperature, while emission peak position did not change. This
increase may be due to two effects. First the crystallite size increases with higher
annealing temperature, and second there is increase in the density of Oxygen Vacancies.
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615 nm ZnO:Eu,Cl on g lass substrates
Concentration : at.%
Wavelength [nm]
Figure 4.5 CL spectrum o f ZnO: (Eu, Cl) annealed at 60 0 and 70 0 "C (a),(b) measured atlow temperature(15 K) and (c) at room temperature (300 K )
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4.2 Polycrystalline Sintered Pellets ZnO : RECl3
In this study, we have investigated the effect of chlorine ions co-doping on
luminescence sensitization of europium (Eu) and thulium (Tm) ions doped ZnO samples.
4.2.1 Crystal Structure of ZnO Doped with E U ~ + ZnO: Eu , CIS).
Figure 4.6a show s the XRD spec tral of undoped ZnO sintered in air for 2 hours at
temperatures of 1000 "C. The diffraction peaks corresponding to the ZnO (100), 002) ,
(101) and (102) planes of the wurtzite structure can be seen. For comparison, theXR D
spectral of ZnO: EuC13 with a perce ntage conce ntration of 0.07 and sintered in air for2
hours at temperatures of 1000 "C is shown in Figure 4.6b. In addition to the wurzite
peaks of ZnO host, new diffraction peaks are clearly seen. These peaks are identified as
(1 11) and (401) corresponding to the cubic phase of E u203. For sam ple sintered in
vacuum atmosphere new peaks have appeared corresponding to tetragonal phase of
EuOCl compound as shown in Figure 4 . 6 ~ . hese results suggest that for the case of
sintering in air atmosphere, most chlorine ions escape from the pellet resulting in
formation of Eu20 3. For the case of sintering in vacuum atmosphere, how ever, it seems
that EU~' ion substitutes into zn2+site and couples with oxygen in ZnO lattice together
with the interstitial C1- as a charge compensator leading to the formation of EuOCl
complex. These results are in agreement with reported work by Ref[2].
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Figure 4.6 (a) ZnO powder 2hr
at 1000 OC in air. (b) Zn0:Eu powder 2 hr at 1000 "C inair, EuC13 with 0.07 in concentration . (c) ZnO: Eu powder 3 hr at 1000 OC in vacuum,EuCI3 with 0.07 in concentration.
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4.2.2 Photoluminescence of Zinc Oxide Doped with Eu ropium (ZnO: Eu,CI).
Photoluminescence was observed from 9 K to 300 K in direct and indirect
excitation processes. In Figure 4.7, we can see the PL emission spectra for ZnO with two
different EuC13 concentrations sintered in air. On the top of the broad emission from the
ZnO host, sharp emission lines can be seen in the 6 10-620 nrn range and at 700nm. The
structures of these sharp emission lines indicate that they are due to E U ~ + ons. Shown in
Figure 4.8 are the PL spectra for Zn0:Eu phosphors with varying EuC13 concentration
sintered in vacuum at 1000 "C. It is not clear at this point what mechanism is responsible
for the quenching of unwanted broad emissions from the ZnO host. It is clear, however,
that this effect is closely related to earlier finding from XRD measurement, where we
observe that Eu couples with the ZnO host lattice by forming an EuOCl complex in
samples sintered in vacuum. The peak assignment of the transitions is shown in Table 4.1
Table 4.1 The Peak Assignment for the PL of ZnO: Eu,Cl.Peak Position Transition of Tm
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Figure 4.7 PL emission spectra for ZnO: Eu sintered at 1000 "C in air
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I I I I I I I I
400 500 60 0 7 0 0 800 900Wa v e l e n g t h [ n m ]
Z n 0 : E u phosphor sintered in vacuumE U ~ + oncen t ra t ion :
a : 0 .07 a t%6 2 6 n m b: 0 .025 a t%
h = 3 2 5 n m , 3 0 0 Ke xc
n
Figure 4.8 PL emission spectra for ZnO : Eu sintered at 1000 "C in vacuum
3d
Y
$2o r (
aQ)
.yE
n
I4P1
7
'D - (J=1. .6)0 J
I '
1 (a)
52 0 n m
I'4 L . (b)
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4.2.3 Cathodoluminescent o f ZnO Doped with E U ~ + ZnO: Eu, Cl).
In Figure 4.9, we can see the CL emission spectra for ZnO: Eu with a percentage
concentration of 0.025 and sintered inN2 for 2 hours at temperatures of 1000"C . The
broad CL spectra for ZnO: Eu sintered inN2 is known to result for the suppression of
broad band emission centered at 520nm due to recombination between self-activated
defect levels, which a re Zn interstitial and0 vacancies. Emission line at 620nrn is due to
Zn vacancies and oxygen interstitial. Luminescence was observed from 9 K to 300K in
direct and indirect excitation processes.
ZnO:Eu,Cl annealed 2h at 1000 OC in N,
5
D,,-'F~ (5=1..6)
I
J
Wavelength [nm]
Figure 4.9 CL emission spectra for ZnO: Eu sin tered at 1000 OC inN2
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Figure 4.10 (a) ZnO powder sintered in air for2 hr at 1000 O C in air. (b) ZnO: Tmpowder sintered in vacuum for 3 hr at 1000 OC in vacuum, TmC13 with 0.07inconcentration.
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4.2.5 Photo Luminescent of Zinc Oxide Doped with Thulium (ZnO: Tm ,Cl).
We have studied the PL spectra of ZnO films at room temperatures (300 K) and
10 K with an excitation wavelength 325nm (UV) light from He-Cd (Helium Cadmium)
Laser. In Figure 4.1 1 we can see the PL emission spectra for ZnO: Tm sintered in
vacuum with (0.01) at % concentrations and annealed for 2.5 hours at 1000 "C. The first
peak is from the bound exciton (BE). In addition to the ZnO luminescence, it shows a
sharp peak at 476 nm, depending upon the relative positions of the ' ~ 4nd 3 ~ 6evels of
the ~ m ~ +on (blue emission due to thulium). Also, it shows a sharp peak at 800 nm
which depend upon the relative positions ' ~ 4nd 3 ~ 5ransition. The peak assignment of
the transitions is shown in Table 4.2. For more information on Tm in various hosts
references [2,3, and 281 are good sources.
Table 4.2 The Peak Assignment for the PL of ZnO: Tm,Cl.
Peak Position Transition of Tm
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Figure 4.11 PL emission spectra forZnO : Tm sintered at 10 00"C in vacuum
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4.2.6 Cathodoluminescent of Zinc Oxide Doped with Thulium (ZnO: Tm,CI).
In Figure 4.12 we can see the CL emission spectra for ZnO: Tm sintered in N2
with (0.025) M/M concentrations. The first peak is from the bound exciton (BE). The
peak of the green band spectra exists at 500 nrn and there is no emission due to Tm rear
earth ions.
300 400 500 600 700 800 900 1000
Wavelength [nm]
Figure 4.12 CL emission spectra for ZnO : Tm sintered at 1000 O C in N2
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Chapter Five
Excitation Mechanisms and Conclusion
5.1 Excitation Mechanisms
An electron and a hole may be bound together by their attractive coulomb
interaction. The bound electron hole pair is called an exciton. It can move through the
crystal and transport energy; it does not transport charge because it is electrically neutral.
When Zn0:RE material is excited by an electron beam, a number of physical processes
occur. Primary electrons penetrate the ZnO host and produce electron hole pairs, which
then diffuse through the ZnO host and either transfer their energy to rare earth ions that
subsequently emit light, or recombine at killer centers nonradiatively [29]. Luminescence
excitation of a material below the fundamental absorption edge, or forbidden energy gap
(E,) is called direct excitation mechanism.
Excitation by photon with an energy greater than the band gap results in the
creation of a hot electron hole pairs which transfer energy to the 4f electron system. We
call this process Indirect excitation mechanism. The excitation mechanism in CL and EL
involves direct impact excitation of RE3+ ions by hot electrons, as well as an energy
transfer from the generated electron-hole pairs, or by impact excitation (or ionization)
involving impurity states outside the 43shel1, with subsequent energy transfer to this shell
[301.
Figure 5.1 shows a model of the excitation processes for ZnO doped with RE
ions. It is assumed that the indirect excitation of RE ions in ZnO proceeds through the
nonradiative transfer of energy from the exciton bound to the RE complex trap, which
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contains RE ions and native defects or impurities required for charge compensation in the
ZnO host. The energy overlap between the broad-band emission andRE energy level
does not guarantee energy transfer because the local environment around theRE ion
plays a significant part in the transfer process so that any oneRE configuration may only
couple to a selected band. Also, if the collapsing energy of an exciton bound to such a
complex center is not sufficient to excite the 4f shell of theRE^' ion ,the characteristic4f
emission of the RE ion will not be present. For some ions, the lowest 4f energy levels are
excited and only infrared emission is observed. In the case ofCL, excitation occurs
through both direct impact excitations of RE ions by hot electrons or indirectly by
generation electron hole pairs and transfers energy to the 4f shell via the mechanism
discussed previously [3 I] .
Figure 5.1 A model of the excitation processes for ZnO doped withRE ions
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5.2 Energy Transfer Mechanisms
In the indirect excitation, there are four possible mechanisms for the energy
transfer in ZnO doped rare earth[31]. The first mechanisms is shown in Figure5 .2 , the
excitation electron excites the electron hole pair and the generated electron could be
trapped by some impurity state which is related to the rare earth ions and inside the
forbidden gap.
Figure 5 .2 Schematic diagram of trapping electron on rare earth related state. Therecombination of energy of trapped electron and the free hole excites the rear earth ions.
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In the second mechanisms shown in Figure5.3, the hole in the valance band is trapped by
the impurity state that is related to the rare earth ions. Then by Auger energy transfer
process, the recombination energy of the bound electron and free hole (or the
recombination energy of the free electron and the bound hole) is transferred to the rear
earth luminescence center.
Figure 5.3 Schematic diagram of trapping hole on rare earth related state. Therecombination of energy of free electron and the trapped hole excites the rear earth ions.
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The third mechanism is shown in Figure 5.4, where both the electron and the hole are
trapped by the impurity state inside the forbidden gap.
D-A Pair energy transfer mechanism
Figure 5.4 Schematic diagram of trapping electron and hole on impurity related state.The recombination of energy of trapped electron and hole excites the rear earth ions.
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For the fourth mechanism, shown in Figure5.5, it is possible for the recombination
energy of the free exciton is transferred to the localized rare earth ions 4f state, whe re the
electron is in the conduction b and and the hole is in the valance band[2,29] .
Figure 5. 5 Schematic diagram of excitation electron and hole pair. The recombination ofenergy o f free electron and hole excites the rear earth ions.
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5.3 Conclusion
In this study we have prepared and investigated the effect of chlorine ions co-
doping on luminescence sensitization of Eu and Tm ions doped ZnO samples. We have
successfully obtained polycrystalline thin films of un-doped and E U ~ + doped ZnO by
spray pyrolysis technique. We have investigated and studied the optical characterization
of the sample by X-ray diffraction, photoluminescence and cathodoluminescence. The
peak of ZnO green band spectra exists at 500 nm and it does not shift between 15 K and
300 K. We found that there is no emission due to rear earth in as grown ZnO. But when
we annealed the sample at 550 "C we found that there are peaks from emissions of E U ~ +
overlapping with the broad band luminescence of ZnO around 620nm. We found that at
low temperatures around (15 K), the luminescence from the rare earth ions is stronger
than the luminescence at room temperature. We also studied the sintered polycrystalline
ZnO: RE pellets co-doped with C1-. The analysis of XR D measurements indicate that for
ZnO:Eu,C13 and ZnO:Tm,C13 sintered in vacuum Eu and Tm exist in the host lattice as
EuOCl and TmOC1. The presence of the complexes effectively removes the ZnO broad
band host emission here was not observed quenching of RE emission due to
concentration effect in the investigated doping range. Both Zn0:Eu and Zn0:Tm show
Re 4f4f emissions overlapped with broad host emission band. Luminescence was
observed from 15 K to 300 K in direct and indirect excitation processes.
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5.4 Future Work
As m entioned earlier ZnO is usually n-type. The carrier concentrations (electrons)
in the as-grown films are usually high, which makes p-type doping of ZnO very difficult
due to a self compensation effect. Zhenguo Ji and his coworker reported that they
obtained P-type ZnO by spray pyrolysis[23]. More experimental investigation of p-type
doped with rear earth ions can be undertaken. Present of both n-type and p-type doping
would enable us m ore com plicated device structures such as p-i-n diodes and detectors.
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[l o] H ayashi Y., Narahara H., UchidaT., Noguchi T. and Ibuki S.," Photoluminescence
of Eu doped ZnO phosphors"Jpn. J. Appl.Phys Vol. 34, pp. 1878-1882,1995.
[I I ] Park Y.K., Han J. L. , Kwak M. G., Yang, Ju S. H. and Cho S., "Effect of coupling
structure of Eu on the photoluminescent for ZnO: EuC13 phosphors",Appl.Phys.Letter.
Vol. 72 Number 6, pp.668-670, 1998.
[12] Ronfard-Haret J.C., Valat P., Wintgens V., Kossanyi J.," Triboluminescence of
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91, pp.71-77,2000.
[13] Jin Ma., Feng Ji., Hong-lei Ma., Shu-ying Li., "Preparation and characterization of
ZnO films by an evaporating method",J. Vac. Sci. Technol. A,Vol. 13 (I), pp.92 ,1995.
[14] Gupta A. , Gupta P., and Srivastava V.R., "Annealing effects in indium oxide films
prepared by reactive evaporation",Thin Solid FilmsVo1.123 (4), pp.325, 1985.
[15] Nunes P., Fortunato E. and Martins R.," properties of ZnO thin films deposited by
spray pyrolysis and magnetron sputtering",Mat. Res. Soc.Vol. 685E, pp D5.8.1,2001.
[16] Mar L. G., Timbrel1 P.Y., and Lamb R.N.," An XPS study of zinc oxide thin film
growth on copper using zinc acetate as a precursor",Thin Solid FilmsVol. 223(2), pp341,
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