AlAhmadi Ahmad.pdf

7/27/2019 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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[37] Molecular Biology and Biochemistry w w w. s ~ e c i a l e d ~ r e ~ . n e t l ..Cellular%2OBioloa~l

c o m ~ o u n d s l htm9/9/03

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