Effect of Annealing on the Structural and Optical Properties of Nanostructured TiO2 Films Prepared by PLD

7/29/2019 Effect of Annealing on the Structural and Optical Properties of Nanostructured TiO2 Films Prepared by PLD

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Effect of Annealing on the Structural and

Optical Properties of Nanostructured TiO2Films Prepared by PLD

(Assistant Professor)

2012 A.C. 1433 A.H.


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Examination Committee Certification

We certify that we have read this thesis entitled " Effect of Annealing on the

Structural and Optical Properties of Nanostructured TiO2 Films Prepared by PLD" as an

examine committee, examined the student ( Sarmad Sabih Kaduory Al-Obaidi ) in its

contents and that, in our opinion meets the standard of thesis for the degree of Master of

Science in physics.

Signature:

Name: Dr. Adawiya J. Haidar

Title: Professor

Address: University of Technology

Date: / /2013

(Chairman)

Signature: Signature:

Name: Dr. Alwan M. Alwan Name: Dr. Abdul-Kareem Dagher

Title: Assistant Professor Title: Assistant Professor

Address: University of Technology Address: Al-Mustansiriyah UniversityDate: / /2013 Date: / /2013

(Member) (Member)

Signature:

Name: Dr. Ali Ahmed Yousif Al-Shammari

Title: Assistant Professor

Address: Al-Mustansiriyah University

Date: / /2013(Supervisor)

Approved by the Council of the College of Education:

Signature:

Name: Dr. Ahmed Shayal Gudib

Title: Assistant Professor

Address: Dean of College of Education, Al-Mustansiriyah University


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Sarmad


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First of all, praise be toALLAHfor helping and supporting me in every thing

I would like to express my profound sense of gratitude & appreciation to

my Supervisors whom guided and

supported me in every possible way with them experience, motivation, and he

positive attitude.

Also I am very thankful to all people who are working in the Physic

Department of the Education collage of AL-mustansiriyah University.

I feel responsible to express my thanks and gratitude to all the people

working in the Laser Physics branch in the (University of Technology).

I am very thankful to and for their support, helpful and assistance.

I am very grateful to staff of XRD, AFM labs, and material sciences

directorate of ministry of Science and Technology.

I would like to express my heartfull thanks to ,

my dearest friends and I cant forget to thank

my family whom supported me with their kind, patience and encouragement.Allah bless you all


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In this work, Nanostructured TiO2 thin films are grown by pulsed laserdeposition (PLD) technique on glass substrates. TiO2 thin films are then annealed

at 400-600 C in air for a period of 2 hours. Effect of annealing on the structural,

morphological and optical properties are studied. Many growth parameters have

been considered to specify the optimum condition, namely substrate temperature

(300 C), oxygen pressure (10-2 mbar) and laser fluence energy density (0.4 J/cm2),

using Q-switching Nd:YAG laser beam (wavelength 532nm), repetition rate (1 - 6)Hz and the pulse duration of (10 ns).

The results of the X-ray testing show that all nanostructures tetragonal are

polycrystalline and orientations identical with literatures, also these results show

that increasing in grain size with increasing of annealing temperature. The XRD

results also reveal that the deposited thin film and annealed at 400 C of TiO 2 have

anatase phase. Thin films annealed at 500 C and 600 C have mixed anatase and

rutile phase. The Full Width at Half Maximum (FWHM) of the (101) peaks of

these films decreases from 0.450 to 0.301 with increasing of annealing

temperature.

The surface morphology of the thin films have been studied by using atomic

force microscopes (AFM). AFM measurements confirmed that the films grown by

this technique have good crystalline and homogeneous surface. The Root Mean

Square (RMS) value of thin films surface roughness increased with increasing

annealing temperature.

The optical properties of the films are studied by UV-VIS spectrophotometer,

in the wavelength range (350- 900) nm. The optical transmission results show that

the transmission over than ~65% decreases with the increasing of annealing

temperatures. The allowed indirect optical band gap of the films is estimated to be


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in the range from 3.49 to 3.1 eV, while the allowed direct band gap is found to

decrease from 3.74 to 3.55 eV with the increase of annealing temperature. The

refractive index of the films is found from 2.1-2.8 in the range from 350nm to

900nm. The extinction coefficient and the optical conductivity of the filmsincreases with annealing temperature. The real dielectric constant and the

imaginary part increases whenthe annealing temperature increasing.


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Dedication

Acknowledgment

Abstracti

List of Symbols.....vii

List of Abbreviations.....ix

List of Tables......x

1.1. Introduction..1

1.2. Fundamentals of Pulsed Laser Deposition(PLD)....2

1.3. Chemical and Physical Properties of TiO2.............3

1.4.The Crystal Structure of TiO2.....4

1.5. Applications of Nanostructured TiO2.........61.6. Literature Survey.........7

1.7. Aim of the Work18

2.1. Introduction....19

2.2. Pulsed Laser Deposition (PLD).........19

2.3. Mechanism of Pulsed Laser Deposition ...22

2.3.1. The Interaction of the Laser Beam and Target....22

2.3.2. Plasma Plume Formation.....25

2.3.2. Nucleation and Growth of Thin Films.....26


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2.4. Limitations and Advantages of PLD.....27

2.5. Pulsed Laser Deposition of Nano-Structure Semiconductor.....28

2.6. Structural Properties......28

2.6.1. X-ray Diffraction ( XRD ).......282.6.2. Effect of Annealing on the X-ray Diffraction......29

2.6.3. Parameters Calculation........30

2.6.3.1. Full Width at Half Maximum (FWHM) ()...30

2.6.3.2. Average Grain Size (g)...........30

3.6.3.3. Texture Coefficient (Tc)..........31

3.6.3.4. Steess (Ss)...........313.6.3.5. Micro Strains ().........31

2.6.4Atomic Force Microscopy (AFM)............32

2.7.Optical Properties of Crystalline Semiconductors ........33

2.7.1. The Fundamental Absorption Edge ....34

2.7.2. Absorption Regions ....34

2.7.2.1. High Absorption Region.........342.7.2.2. Exponential Region........34

2.7.2.3. Low Absorption Region.........35

2.7.3. The Electronic Transitions ......35

2.7.3.1. Direct Transitions ......35

2.7.3.2. Indirect Transitions ....36

2.7.4. Optical Constants.....382.7.5. Some Optical Properties of TiO2Thin Film....39



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3.2. Deposition Equipment.......42

3.2.1. Nd: YAG Laser Source....42

3.2.2. Pulsed Laser Deposition (PLD) Technique.....43

3.2.3. Substrate Heater...453.2.4. Vacuum System.......45

3.3. Target Preparation.............45

3.4. Substrate Preparation.....46

3.5. Characterization Measurements.46

3.5.1. Thickness Measurement..................46

3.5.2. Structural and Morphological Measurements......473.5.2.1. X-ray Diffraction (XRD)........47

3.5.2.2. Atomic Force Microscopy (AFM)..........47

3.5.3. Optical Measurements.....48


4.2. Structural Properties......50

4.2.1. X-ray Diffraction.........50

4.2.2. Atomic Force Microscopy (AFM).......56

4.3. Optical Properties..............58

4.3.1. Optical Transmission (T).........584.3.2. Optical Absorption (A)............59

4.3.3. Optical Absorption Coefficient ()..62

4.3.4. Optical Energy Gap (Eg)..........62

4.3.5. Refractive Index (n).........66

4.3.6. Extinction Coefficient (Ko)..........67

4.3.7. The Dielectric Constants (r,i)......67


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4.3.8. Optical Conductivity ()......69

5.1. Conclusion .....70

5.2. Future Work .......72

5.3. Publications.....73

References......7 4


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Description

Symbol

Lattice constant ( )a

AbsorptanceA

AnataseAAbsorption coefficient (cm-1)

Back flux (W/cm2)bVelocity of light in vacuum (m/s)c

Thickness (nm)tlaser pulse width duration (s)tp

Inter planer spacing ( )dElectron charge (C)e

Binding energy of vaporization per atomEbAblation energy of the pulse laser (eV)Eab

Energy gap(eV)EgEnergy of phonon (eV)EphLaser fluence (J/cm2)F

Approximate the fluence threshold for laser pulseFthAverage grain size (nm)g

Plank constant (J. s)hPhoton energy (eV)h

Laser intensity (W/cm

2

)I Measured intensityI

JCPDS standard intensityIoWave vector (cm- )k

Boltzmann constant (J/K)KBExtinction coefficientK

Refractive indexnNumber density of atomsna


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Reflection numberNrPressure of the gas (mbar)p

ReflectanceR

RutileR TransmittanceTTemperature (C)T

Texture coefficientTcSubstrate temperature (K)Ts

Thermal diffusion coefficient (m /s)uFringe width (cm)x

Distance between two fringes (cm)x

StressSsMicro Strains

Wavelength (nm)Wavelength cut off (m)c

Optical conductivityDiffraction angle (deg.)

Real part of dielectric constant (F/m)r

Imaginary part of dielectric constant (F/m)i Free energies of the film surface (eV)F(eV)Free energies of the substrate surfaceS

Free energies of the film-substrate interface (eV)IFrequency (Hz)

Critical frequency (Hz)o


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Description

Symbol

Atomic Force MicroscopeAFMChemical Vapor DepositionCVDChemical Spray pyrolysisCSP

Conduction BandC.B.Dye-Sensitized Solar CellsDSSC

Fourier Transform- Infrared SpectroscopyFTIRFull Width at Half Maximums (deg.)FWHM

Molecular Beam EpitaxialMBEGlancing Angle X-ray DiffractionGAXRD

Joint Committee for Powder Diffraction StandardsJCPDSPhotoelectrochemical CellsPEC

Pulsed Laser DepositionPLD

PhotoluminescencePLRadio FrequencyRF

Root Mean SquareRMSRapid Thermal AnnealingRTA

Scanning Electron MicroscopeSEMSecond Harmonic GenerationSHGSwift Heavy Ion IrradiationSHI

Transparent Conducting Oxide SemiconductorsTCOsTitanium DioxideTiO2

Thermal Pyrolysis DepositionTPDThermal Evaporation in Vacuum DepositionTEVD

Valence BandV.B.X-Ray DiffractionXRD

X-Ray Photoelectron SpectroscopyXPS


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Page

No.TitleTable

No.

21Performance features of Excimer and Nd: YAG lasers.(2.1)53Lattice constants and interpllanar spacing of TiO2 films.(4.1)54

The obtained result of the structural properties from XRDfor TiO2 thin films.(4.2)

56Morphological characteristics from AFM images for TiO2 thin

film.(4.3)63

Shows allowed direct band gap and allowed indirect band gapfor different annealing temperatures of TiO2 thin films.(4.4)


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10/23/2005

Introduction


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Chapter One Introduction

Thin films are first made by (Busen & Grove) in 1852 by using (Chemical

Reaction). In 1857, the scientist (Faraday) was able to obtain a thin metal film bymeans of (Thermal Evaporation) [1].The experimental and theoretical study of

semiconductor nanocrystallites has generated tremendous technological and

scientific interest recently due to the unique electronic and optical properties and

exhibition of new quantum phenomena. In the semiconductor technology, laser

induced crystallization is used because it presents selective optical absorption and

low processing temperature [2]. Oxides reveal an excellent chemical and

mechanical property and do not show deterioration. As one of the important wide

band gap (Eg3 eV) oxides, TiO2 has been subject to extensive academic and

technological research for decades, due to its unique properties such as[3,4]:

High electro-chemical properties. Non-toxic, inexpensive, highly photoactive, and easily synthesized and

handled. Highly photostable. With high dielectric constant, hardness, and transparency TiO2 films are

applicable for storage capacitor in integrated electronic, protective coatings,

and optical components.

Most of the studies focused on the nanosized TiO2 with the purpose of

improving the photocatalytic activity and optical absorption [4].

Titanium dioxide is a large band gap semiconductor of exceptional stability

that has diverse industrial applications. TiO2 thin films with their high refractive

index have broad applications in optical coatings and waveguides [5].Titanium

dioxide occurs in three crystalline polymorphs: rutile (tetragonal), anatase

(tetragonal), and brookite (orthorhombic)[2].


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There are many methods to prepare thin films, as follows [6, 5]:

Thermal Evaporation in Vacuum Deposition. (TEVD) Sputtering technique. Chemical Vapor Deposition.(CVD) Chemical Spray pyrolysis.(CSP) Thermal Pyrolysis Deposition.(TPD) sol-gel method Pulse Laser Deposition.( PLD )

Wide variations in the optical and physical properties of TiO2 thin filmsdeposited by different techniques have been reported. For Pulsed laser deposition

derived films, film properties such as crystallinity, particle size, degree of

homogeneity, etc. depend largely on annealing temperature, substrate topography

[5], laser wavelength and pulse duration.

Pulsed laser deposition (PLD) is proved to be a favorable technique for the

deposition of titanium dioxide at different technological conditions on differentsubstrates. This supposes to result in the different structural and micro structural

properties, different surface morphology of the nanostructures to be obtained.

The discovery of the ruby laser prompted an evolution of theoretical

investigations into laser-target interaction. Numerous experiments were carried outto verify the theoretical models. Ready (1963) and White (1963) studied the

interactions of intense laser beams with solid surfaces [7]. By 1965, Smith and

Turner demonstrated that an intense ruby laser could be used to deposit thin films

[7]. The main advantage of PLD is its versatility. Using high-power lasers almost

any material can be vaporized and, thus, depositing a thin-film onto any substrate.

PLD has several characteristics that distinguish it from other growth methodsand provide special advantages for the growth of chemically complex


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(multielement), composite materials [8], semiconductor, metallic, superconductor

and insulating nanostructures [9]. In other words, the composition of the any target

material can be preserved with in the film. This accomplishment is significant

because it proved that PLD could be used to produce thin films with qualitiescomparable to those produced by Molecular Beam Epitaxy (MBE) [7]. The laser is

completely separated from the actual deposition chamber. During an experiment,

the laser beam is pointed onto a target inside the chamber through a viewport in

alignment with the target. Under these unique conditions the deposition chamber

can contain any working atmosphere. The pulsed laser deposition technique

involves three main steps: ablation of the target material, formation of a highlyenergetic plume, and the growth of the film on the substrate.

The following points show some chemical and physical properties of TiO2:

TiO2 is found naturally as a white material in three forms of crystalline: Rutile,

Anatase and Brookite [10].

The pure of TiO2 is white solid structure solvents in H2SO4, but it is not solvent

in water or alcohol or HCl [10].

Because the TiO2 is not solvent and has no reaction with water; therefore, it is

used in industry like paintings, in the making of gum and some kinds of shampoo.

The material of TiO2 is semiconductors; it is one of the group Transparent

Conducting Oxide Semiconductors (TCOs) and high transparent in visible region

and absorption in ultraviolet region, and low conductivity [11].

The molecular weight of TiO2 is (79.90) in which Oxygen represents (40.05%)

and Titanium (59.95%), and melting point is (1850 C) and boiling point is

(3000 C) [10].

The thin films of TiO2 have high band energy gap about (3.2 - 3.29) eV, (3.69-

3.78) eV for allowed and forbidden direct transition respectively [12]


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There are three forms of crystalline structure of TiO2 material they are:

The anatase polymorph of TiO2 is one of its two metastable phasestogether with brookite phase. For calcination processes above 700 C all anatase

structure becomes rutile, some authors also found that 500 C would be enough for

phase transition from anatase to rutile when thermal treatment takes place. This

form is tetragonal its density is (3.9 gm/cm3), energy band gap is (3.29 eV),

refractive index is (2.5612) [10] and Lattice parameters are: a = b = 3.7710 and

c = 9.430

[13], as shown in fig. (1.1).

This form is the reddish crystal because it has obtained the impurity

influence. This form is tetragonal its density is (4.23 gm/cm3) as in fig. (1.2). It has

energy gap (3.05 eV), refractive index (2.605) [10] and Lattice parameters are: a =

b = 4.5933 and c = 2.9592 [13].


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This form has orthorhombic surface. Its density is (4.13 gm/cm 3),

refractive index is (2.5831) [10] and Lattice parameters are:a = 9.18 , b = 5.447

and c = 5.145 [13], as shown in fig. (1.3).

All the TiO2 samples analyzed in the present work are firstly synthesized from

anatase phase and submitted to an annealing process in order to reach the stable

rutile phase but brookite phase never appeared. The difference in these three crystal

structures can be attributed to various pressures and heats applied from rock

formations in the earth. At lower temperatures the anatase and brookite phases are


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more stable, but both will revert to the rutile phase when subjected to high

temperatures.

TiO2 nanostructure one of the oxides family has attracted significant attention

in recent years due to it interesting electrical [15] optical [16] magnetic properties

and applications for catalysis [17] energy conversion [18] biomedical

applications [19] functionalized hybrid materials [20] and nanocomposites [21].Because of its semiconductivity, photoelectrical and photochemical activity

under UV light. TiO2 nanostructures can be used as dye-sensitized solar cells(DSSC( [22] and photoelectrochemical cells (PEC) [23] photocatalysis, chemical

sensors [24] self-cleaning coating [25] and TiO2/polymer nanocomposites [26],the

some applications of TiO2 is shown in fig. (1.4).


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Lofton, et al., (1978) [27]: They studied titanium thin films which were a

mixture of titanium and TiO2. Auger electron spectroscopy and X-rayphotoelectron spectroscopy in combination with sputter profiling techniques were

employed to study (100-500) titanium thin films. The composition of the films

was studied as a function of substrate. The samples were prepared by the electron

beam deposition of high purity (99.9 %) titanium on quartz (SiO2) or sapphire

(Al2O3). The depositions were carried out at either R.T. or 450C at typical

pressure (p) of 10-8 Torr (1.33x10-6 Pa). The effect of different temperatures on

each titanium device was studied, as well as its effect on rate deposition.

Korotcenkov and Han (1997) [28]: They prepared (Cu, Fe, Co, Ni)-doped

titanium dioxide films deposited by spray pyrolysis. The annealing at 850-1030 C

was carried out in the atmosphere of the air. For structural analysis of tested films

they have been using X-ray diffraction, Scanning Electron Microscopy (SEM), and

Atomic Force Microscopy (AFM) techniques. It was established that the dopingdid not improve thermal stability of both film morphology and the grain size. It was

made a concluded that the increased contents of the fine dispersion phase of

Titanium dioxide in the doped metal oxide films, and the coalescence of this phase

during thermal treatment were the main factors, responsible for observed changes

in the morphology of the doped TiO2 films.

Hiso Yanagi, et al., (1997) [29]: They prepared TiO2 thin films by spray

pyrolysis of titanium films on glass substrates. Depending upon the substrate

temperature, morphology of the deposited TiO2 films changed from irregular

aggregates at 200 C to homogeneous particles with a diameter of (50-100) nm

above (400 C).


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Amor, et al.,(1997) [30]: They studied the structural and optical properties of

TiO2 films type (brookite) prepared by sputtering method and energy gap for

allowed direct transition was (3.3-3.5 eV). They also studied thermal treatment on

its properties where they observed that the energy gap became (3.46-3.54 eV).XRD results observed films before thermal treatment were amorphous structure but

after thermal treatment they became polycrystalline.

XU, et al., (1998) [31]: They studied the effect of calcinations temperatures on

photocatalytic activity of TiO2 films prepared by an electrophoretic deposition

(EPD) method. TiO2

films fabricated on transparent electro-conductive glass

substrates and were further characterized by X-ray diffraction (XRD), X-ray

photoelectron spectroscopy (XPS), field emission scanning electron microscope

(FESEM), UV-vis diffuse reflectance spectra and Photoluminescence spectra (PL).

FESEM images indicated that the TiO2 films had roughness surfaces, which

consisted of nano-sized particles.

Patil (1999) [32]: studied the anatase thin films TiO2 prepared by sputtering

Pyrolysis technique, were obtained with good crystalline. Such films had indirect

band gap energy of (3.08 eV) and direct band gap energy of (3.65 eV). Films made

near 325 C substrate temperature contained only the anatase phase with 75%

optical transmittance. The photo conductivity increased from about (10 -10 - 10-8)

(.cm)-1 when illuminated at (30 mW.cm-2) intensity. The films produced at 380 C

were anatase.

Sekiya, et al., (2000) [33]: They studied absorption spectra of anatase TiO2

single crystals heat-treated under oxygen atmosphere. The optical properties had

been grown by chemical vapor transport reaction as grown crystals having blue

color were heat-treated under oxygen atmosphere, the change in crystal color from

blue through yellow to colorless depending on oxygen annealing was detected byoptical absorption spectra.


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Dzibrou, et al., (2002) [34]: They deposited TiO2 thin films on quartz and

silicon wafers, by PLD method using Nd: YAG pulsed laser (=355nm, 10 Hz)

with laser energy density of 1.5 J/cm2. The thin films were thermally treated at

temperatures of 300 C, 400 and 500 C in air for 1 hour. The coatings obtainedwere uniform, smooth with very good optical properties. The sample annealed at

lower temperature had the characteristic appearance of an amorphous material. The

samples treated at 400C and 500 C were crystallized. TiO2 had direct and indirect

band gaps. The band gap values for both transitions were different in comparison to

the well-known value of 3.03 eV for the indirect band gaps and 3.43eV for the

direct.

Wang, et al., (2002) [35]: They studied the optical properties of anatase TiO2

thin films prepared by aqueous sol-gel process at low temperature TiO 2. Thin films

were spin-coated on Si (100) substrates via an aqueous sol-gel, and were annealed

in air at different temperatures up to 550 C for 1h. X-Ray diffractometry indicated

that crystallization into anatase started at 350 C. The 350 C-annealed films were

further characterized by auger electron spectroscopy, X-ray photoelectron

spectroscopy, and variable angle spectroscopic ellipsometry. The results showed

that homogeneous, carbon-free TiO2 films with high refractive index (n=2.3 at

550 nm) were successfully obtained under an annealing temperature as low as

350 C. The indirect and direct optical absorption band gaps of the anatase film

were estimated as 3.23 and 3.80 eV, respectively.

Shinguu, et al., (2003) [36]:They studied the structural properties and

morphologies of TiO2 thin films, in which they were deposited on Si(100) and

Si(111) substrates by using ArF excimer laser (operating with wavelength 248 nm

at 500 C) .The films have been annealed for 10 hours at the temperature 600 C, in

oxygen and air flow. The TiO2 film deposited on (111)-oriented silicon exhibited a

better anatase crystalline than that on (100)-oriented silicon. Whereas a higher


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annealing time needed to transform anatase structure into rutile structure for films

deposited on Si (111) than on Si (100). The AFM images showed that the substrate

orientation had no great effect on the surface morphologies for both anatase as-

deposited films and rutile annealed films.

Tien, et al.,(2004) [37]: They deposited TiO2 thin films on sapphire by using

ArF excimer laser (operating with wavelength 193 nm, pulse width 15 ns,

repetition frequency 10 Hz and power 100 mJ ) at a substrate temperature of 500 C.

The diagnostic of the ablation plume showed the interaction of the evaporated

Ti particles with buffer O2 gas. The dependence of the buffer O2 gas pressure wasstudied by spectroscopy of ablation plume, thickness of films, morphology of the

surface using SEM and AFM micrographs, XRD patterns and Raman spectra. The

morphology showed the formation of nanostructure by interactions of evaporated

Ti particles with the buffer O2 gas. The structures of the PLD thin films showed

epitaxial growths in the high substrate temperature (500 C) and an appearance of

anatase at high buffer O2 gas pressure owing to the contributions of the TiO

molecules.

Suda, et al.,(2004) [38]: They prepared TiO2 films on different substrate at

different temperatures (100-400) C by using KrF Excimer laser (=532nm,

=3.5ns) at about 1 J/cm2 laser density. They found that all films showed (101)

anatase phase at the optimized conditions. Photoluminescence (PL) results

indicated that the thin films fabricated at the optimized conditions showed the

intense near band PL emissions.

Stamate, et al.,(2005) [39]: They analyzed the optical properties of TiO2 thin

films deposited through a d.c. magnetron sputtering method on glass made.

A strong dependence between the value of TiO2 optical band gap and

argon/oxygen ratios had been revealed. Changes in optical properties of TiO 2 thin


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films, with thermal annealing parameters. The optical band gap varies from 3eV to

3.4eV as function of oxygen/argon ratios.

Caricato, et al., (2005) [40]: They studied nanostructured TiO2 thin filmsprepared by (PLD) KrF excimer pulsed laser system (wavelength = 248 nm) on

indium-doped tin oxide (ITO) substrates under different substrate temperature and

pressure conditions (T = 250, 400,500 and 600 C, p = 10-2 and 10-1 Torr). AFM

results showed the samples prepared at 400 C have much more uniform surfaces

and smaller particle size than that prepared at 600 C. The XPS results indicated

that the binding energy of the Ti core level system pressure was dependent onsubstrate temperature. However, under 10-1 Torr, only anatase phase was observed

even at the temperature higher than the commonly reported anatase-to-rutile phase

transition range (~ 600 C).

Deshmukh, et al., (2006) [41]: They studied TiO2 thin films deposited onto

glass substrates by means of spray pyrolysis method. The thin films were deposited

at three different temperatures of 350,400 and 450 C. As deposited thin films were

amorphous having (100-300 nm.) thickness, the thin films were subsequently

annealed at 500C in air for 2h. Structural, optical and electrical properties of TiO2

thin films had been studied as well. Polycrystalline thin films with rutile crystal

structure, as evidenced from X-ray diffraction pattern, were obtained with major

reflection along (110). Surface morphology and growth stage based on atomic

force microscopy measurements were discussed. Optical study showed that TiO2

possesses direct optical transition with band gap of (3.4 eV)

Mere, et al., (2006) [42]:They studied the structural and electrical

characterization of TiO2 films grown by spray pyrolysis onto silicon wafers at

substrate temperature between (315 C and 500 C) using pulsed spray solution feed

followed by annealing in temperature interval from (500 to 800

C) in air.According to FTIR (Fourier Transform Infra-Red), XRD, and Raman, the


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anatase/rutile phase transformation temperature was found to depend on the film

deposition temperature. Film thickness and refractive index were determined by

Ellipsometry, giving refractive index (2.1-2.3) and (2.2-2.6) for anatase and rutile

respectively. According to AFM (Atomic Force Microscopic), film roughnessincreased with annealing temperature from ( 700 to 800 C) from ( 0.60 to

1.10 nm.) and from ( 0.35 to 0.70 nm.) for films deposited at ( 375 and 800 C)

respectively. The effective dielectric constant values were in the range of (36 to 46)

for anatase (53 to 70) and for rutile at (10 KHz.). The conductivity activation

energy for TiO2 films with anatase and rutile structure was found to be (100 and 60

meV), respectively.

Nambara and Yoshida (2007) [43]: They studied the crystalline rutile type

titanium dioxide (TiO2) thin films which were prepared by (PLD) at substrate

temperature 850 C. The optical properties of the present rutile films were different

from that of single crystal TiO2. UV-VIS spectra of PLD films showed a blue shift.

The value of the gap was 3.30 eV, which was shifted from 3.02 eV as the bulk

value, they considered quantum size and strain effects of PLD-TiO2 crystalline.

Hassan, et al., (2008) [44]: They studied the effects of annealing temperature

on optical properties of anatase. TiO2 thin films were grown by radio frequency

magnetron sputtering on glass substrates at high sputtering pressure and room

temperature. The anatase films were then annealed at (300-600 C) in air for 1h. To

examine the substrates and morphology of the films, X-ray diffraction. Atomicforce microscopy (AFM) methods were used respectively. From (XRD) patterns of

the TiO2 films, it was found that the as-deposited film showed some differences

compared with annealed films, and the intensities of the peaks of the crystalline

phase increased with the increase of annealing temperature. From (AFM) images,

the distinct variations in the morphology of the films were also observed. The

optical constants were characterized using the transmission spectra of the films


30/110


3

obtained by UV-VIS-IR spectrophotometer. The refractive index of films was

found from (2.31-2.35) in the visible range. The extinction coefficient was nearly

zero in the visible range but increased with annealing temperature. The allowed

indirect optical band gap of the films was estimated to be in the range from (3.39 to3.42 eV), which showed to be a small variation. The allowed direct band gap was

found to increase from (3.67 to 3.72 eV).

Walczak, et al., (2008) [45]: They studied the effect of oxygen pressure on

the structural and morphological characterization of TiO2 thin films deposited on Si

(100) by using KrF Excimer laser operated at wavelength of 248 nm and repetitionrate 5Hz . The laser energy density was about 2 J/cm2). They found that the

decreasing of oxygen pressure from (10-2 Torr to 10-1 Torr) produced highly

homogeneous nanostructured morphology with grain size as small as 40 nm and

high quality nanostructure was observed at the 10 -1 Torr of oxygen.

Sanz, et al.,(2009) [46]: They deposited TiO2 films on Si (100) by PLD by

using three different Nd: YAG laser wavelengths (266nm, 532nm and 355nm).

They found that the films grown at =266 nm has smallest nanoparticles (with

average diameter 25 nm) and the narrowest size distribution was obtained by

ablation at 266 nm under 0.05 Pa of oxygen. The effects of temperature on the

structural and optical properties of these films have been investigated

systematically by XRD, SEM, FTIR, and PL spectra.

Sankar and Gopchandran (2009) [47]: They studied the effect of annealing

temperature (973 and 1173 K) on the structural, morphological, electrical and

optical properties of nanostructured titanium dioxide thin films were prepared

using reactive pulsed laser ablation technique. The structural, electrical and optical

properties of TiO2 films are found to be sensitive to annealing temperature and are

described with GIXRD, SEM, AFM, UV-VIS spectroscopy and electrical studies.


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4

X-ray diffraction studies showed that the as-deposited films were amorphous

and at first changed to anatase and then to rutile phase with increase of annealing

temperature. The average grain size increases with increase in annealing

temperature. For the as deposited film, the value of band gap is observed to be3.11 eV. It was shifted to 3.19 eV for the film annealed at 973 K, which is observed

to be anatase in crystal structure. Annealing at 1173 K resulted in reduction of the

band gap to 3.07 eV.

Mathews, et al., (2009) [48]: They studied nanostructured TiO2 thin films

were deposited on glass substrates by sol-gel dip coating technique. The structural,

morphological and optical characterizations of the as deposited and annealed films

were carried out using X-ray diffraction (XRD), Raman spectroscopy, atomic force

microscopy (AFM), and UV-VIS transmittance spectroscopy. As-deposited films

were amorphous, and the XRD studies showed that the formation of anatase phase

was initiated at annealing temperature close to 400 C. The grain size of the film

annealed at 600 C was about 20 nm. The lattice parameters for the films annealed

at 600 C were a = 3.7862 and c = 9.5172 , which is close to the reported values

of anatase phase. Band gap of the as deposited film was estimated as 3.42 eV and

was found to decrease with the annealing temperature. At 550 nm the refractive

index of the films annealed at 600 C was 2.11, which is low compared to a pore

free anatase TiO2.

Igwe, et al.,(2010) [49]: They studied the effect of thermal annealing undervarious temperatures, 100, 150, 200, 300 and 399 C on the optical properties of

titanium Oxide thin films prepared by chemical bath deposition technique,

deposited on glass substrates. The thermal treatment streamlined the properties of

the oxide films. The films are transparent in the entire regions of the

electromagnetic spectrum, firmly adhered to the substrate and resistant to

chemicals. The transmittance is between 20 and 95% while the reflectance is


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5

between 0.95 and 1%. The band gaps obtained under various thermal treatments

are between 2.50 and 3.0 eV. The refractive index is between 1.52 and 2.55. The

thickness achieved is in the range of 0.12-0.14 m.

Pawar, et al.,(2011) [50]: They prepared TiO2 thin films on glass substrates

using spin coating technique and the effect of annealing temperature (400 - 700 C)

on structural, microstructural, electrical and optical properties were studied. The

X-ray diffraction and Atomic force microscopy measurements confirmed that the

films grown by this technique have good crystalline tetragonal mixed anatase and

rutile phase structure and homogeneous surface. The study also reveals that theRMS value of thin film roughness increases from 7 to 19 nm. The surface

morphology (SEM) of the TiO2 film showed that the nanoparticles are fine with an

average grain size of about 50 - 60 nm. The optical band gap slightly decreases

from 3.26 - 3.24 eV.

Sankar, et al., (2011) [51]: They prepared Titanium dioxide thin films were

deposited on quartz substrates kept at different O2 pressures using pulsed laser

deposition technique. The effects of reactive atmosphere and annealing temperature

on the structural, morphological, electrical and optical properties of the films are

discussed. Growth of films with morphology consisting of spontaneously ordered

nanostructures is reported. The films growth under an oxygen partial pressure of

3x10-4 Pa consist in nanoislands with voids in between them whereas the film

growth under an oxygen partial pressure of 1x10 -4 Pa, after having being subjected

to annealing at 500 C, consists in nanosized elongated grains uniformly distributed

all over the surface. The growth of nanocrystallites with the increase in annealing

temperature is explained on the basis of the critical nuclei-size model. The

structural, morphological, optical and electrical properties of titanium oxide thin

films are found to be strongly influenced by the thermodynamics involving reactive

atmosphere during deposition and annealing temperature.


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6

Pomoni, et al., (2011) [52]: They studied the effect of thermal treatment on

structure, electrical conductivity and transient photoconductivity behavior of

thiourea modified nanocrystalline titanium dioxide (TiO2) thin films were prepared

by sol-gel route and were thermally treated at five different temperatures (400, 500,600, 800 and 1000 C). The transmittance reaches approximately the value of 20%

at a wavelength of 380nm that corresponds to the band gap of TiO 2. A gradual

increase in the transmittance is observed with increase of the wavelength and

transmittance values of 60-70% are recorded for the wavelengths 600-900 nm. For

the films heat treated at 500 and 600 C, the transmittance values appear

significantly reduced in comparison to those for the film treated at 400 C. Furtherincrease of the treatment temperature up to 1000 C does not practically influence

the transmittance of the films. Average crystallite sizes a small increase from 28.2

to 58.4 nm with temperature for anatase crystallites. The rutile crystallites appear at

800 C with an important increase of their size at 1000 C (58.4 nm).

Wu, et al., (2012) [53]: They studied the effect of thickness and annealing

temperature on The crystal structure, morphology, and transmittance of TiO2 and

W-TiO2 bi-layer thin films prepared by RF magnetron sputtering onto glass

substrates and tungsten was deposited onto these thin films (deposition time

15-60 s) to form W-TiO2 bi-layer thin films. Amorphous, rutile, and anatase TiO2

phases were observed in the TiO2 and W-TiO2 bi-layer thin films. Tungsten

thickness and annealing temperature had large effects on the transmittance of the

W-TiO2 thin films. The W-TiO2 bi-layer thin films with a tungsten deposition time

of 60 s were annealed at 200 C- 400 C. The band gap energy values decreased.

The band gap energy of deposited TiO2 thin film was 3.21 eV. For the W-TiO2

bi-layer thin films, as the tungsten deposition time was increased from 15 s to 60 s,

the band gap energy shifted from 3.210 to 3.158 eV, which is in the range of visible

light. When the annealing temperature of the WTiO2 bi-layer thin films was

increased from 200 to 400 C, the band gap energy shifted from 3.158 to 3.098 eV.


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7

Annealing was thus demonstrated to be another important method to decrease

the band gap energy of TiO2-based thin films.

Thakurdesai, et al., (2012) [54]: They studied the effect of Rapid ThermalAnnealing (RTA) on Nanocrystalline TiO2 by Swift Heavy Ion Irradiation (SHI).

TiO2 were deposited using Pulsed Laser Deposition (PLD) method on fused

silica Substrate in oxygen atmosphere. These films are annealed at 350 C for 2

minutes in oxygen atmosphere by Rapid Thermal Annealing (RTA) method.

During RTA processing, the temperature rises abruptly and this thermal instability

is expected to alter surface morphology, structural and optical properties ofnanocrystalline TiO2 film. The effect of RTA processing on the shape and size of

TiO2 nanoparticles is studied by Atomic Force Microscopy (AFM) and Scanning

Electron Microscopy (SEM). Glancing Angle X-ray Diffraction (GAXRD) studies

are carried to investigate structural changes induced by RTA processing. Optical

characterization is carried out by UV-VIS spectroscopy and Photoluminescence

(PL) spectroscopy. The changes observed in structural and optical properties of

nanocrystalline TiO2 thin films after RTA processing are attributed to the

annihilation of SHI induced defects.


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8

The main objectives of this work are:

Initially, the series of samples has been prepared by PLD technique atdifferent technological conditions on glass substrates.

We study the preparation condition such as, substrate temperature, oxygenpressure and energy laser influence during deposition.

As well as the concentration into the target on the structure, morphology(Atomic Force Microscopy (AFM)), and XRD. Also the optical propertiesfor deposited films.

Then, we study the effect of annealing temperature on structural and opticalproperties of TiO2 films.


36/110

10/23/2005

Theoretical Part


37/110

Chapter Two Theoretical Part

9

This chapter introduces the basics of the laser ablation. Topics like laser

target-interaction and formation of the plasma plume will be discussed, as well asprocess parameters and formation of the deposit. Also this chapter includes a

general description of the theoretical part of this study, physical concepts,

relationships, and laws used to interpret the study results.

The pulsed laser deposition (PLD) is one of the most used techniques for

depositing thin films. In the process of laser ablation, short and high-energetic laser

pulses are used to evaporate matter from a target surface. As a result, a supersonic

jet of particles, called also (plume), due to its form (see Fig. 2.1), is ejected from

the target surface and expands away from the target with a strong forward-directed

velocity distribution. The ablated particles condense on a substrate placed opposite

to the target. The ablation process takes place in a vacuum chamber- either in

vacuum or in the presence of some background gas. The laser pulses are guided to

the vacuum chamber to the target, optimizing the energy density of the laser pulses.

While the laser pulses are hitting on its surface, the target is usually rotated with a

constant speed to achieve a homogeneous ablation process. The possibility of a

multitarget rotating wheel in the vacuum chamber enables more efficient and

complex processes. Multilayers and alloy films can be grown from elementarytargets by moving them alternately into the laser focal point.

The high energy density used in a typical PLD process is able to ablate almost

every material, and by controlling the process parameters, high-quality films can be

grown reliably in a short period of time compared to other growth techniques

(MBE,Sputtering). Another known advantage of the PLD technique is the accurate

stoichiometric transfer from target to film. There are several kinds of lasers, which


38/110


21

are commercially available, and the choice of Excimer lasers (KrF, ArF, XeCl) are

widely used to deposit complex oxide films because of the larger absorption

coefficient and small reflectivity of materials at their operating wavelengths [55],

Nd: YAG lasers are also effective from the same point of view. For the presentwork, Nd: YAG laser is used. Table (2.1) has performance parameters for current

excimer and Nd: YAG systems at the 248 nm and 3nd harmonic 266 nm

wavelengths respectively because these wavelengths are the most popular for PLD.

The temperature could be kept constant by means of an automated

temperature controller, capable to program and control several ramps and dwells

with user-defined heating and cooling rates. The thermal coupling between heaterand substrate is achieved through appropriate amount of conductive silver in the

back side of the substrate. Moreover, several gases (O2,N2,H2, Ar) can be

introduced in the deposition chamber if the presence of any background gas is

required for the film growth. The flow and the pressure of each gas is controlled by

means of gas inlet valves and pressure flow controllers.


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2

Parameter Excimer System Nd:YAG System

Wavelength

(nanometers)248 nm 1064 and 532 nm

Output Energy

(millijoules)100 - 1200 mJ 100 - 1000 mJ

Repetition Rate (Hertz) Variable, 1 - 200 Hz Fixed, 1 - 30 Hz

Shot-to-Shot Stability

(RMS)0.5 - 1%, RMS 8 - 12%, RMS

Advantages

-High power output

-Good stability

-Flexibility for tuning

-Laser output parameter

-Output energy sufficient for laser

ablation

-Simple maintenance

-Compact system

Disadvantages

-Short operation life time.

-Complicated maintenance

-Expansive and high purity gasses,

constants refilling

-Space consuming.

-Large energy drop for the 3rd

harmonic mode

Although the pulsed laser deposition process is conceptually simple,controlling the dynamics of the film growth is not an easy issue, because of the

large number of interacting parameters that govern the growth process and hence

the film properties, such as:

The substrate type, orientation and temperature.The laser parameters (working wavelength, fluence, pulse duration, andrepetition rate).The chamber pressure and the chemical composition of the buffer gas.

The structural and chemical composition of the target material.And the geometry of the experiment (incident angle of the laser, incidentangle of the plume, distance between target and substrate).

Being able to control the parameters for a given system, the advantages of the PLD

technique can be profited. In practice, parameters like laser settings and experiment


40/110


22

geometry have to be optimized for a given system and be kept constant, while

another parameters like substrate temperature, chamber pressure and background

gas can be varied in order to investigate their influence on the film growth.

The mechanism of the PLD process can be expressed in three steps [59]:

The interaction of the laser beam with target. Plasma Plume Formation. Nucleation and growth of thin films.

The laser-target interaction is the driving mechanism of the PLD process.

Through the years, theoretical models and experimental studies have been

formulated in the attempt to explain the processes that govern the PLD ablation

process. These studies have shown that the ablation process is not governed by asingle mechanism but by multiple mechanisms that arise due to the laser-target

interaction [57]. Ideally the plasma plume produced should have the same

stoichiometry as the target if we hope to grow a film of the correct composition.

For example, if the target surface was heated slowly, say by absorbing the light

from a CW laser source, and then this would allow a significant amount of the

incident power to be conducted into the bulk of the target. The subsequent meltingand evaporation of the surface would essentially be thermal i.e. the difference

between the melting points and vapor pressures of the target constituents would

cause them to evaporate at different rates so that the composition of the evaporated

material would change with time and would not represent that of the target. This

incongruent evaporation leads to films with very different stoichiometry from the

target [60].


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23

To achieve congruent evaporation the energy from the laser must be dumpedinto the target surface rapidly, to prevent a significant transport of heat into the

subsurface material, so that the melting and vapor points of the target constituents

are achieved near simultaneously. The high laser power density that this implies ismost readily achieved with a pulsed or Q-switched source focused to a small spot

on the target. If the energy density is below the ablation threshold for the material

then no material will be removed at all, though some elements may segregate to the

surface [61, 62].

In order for the target material to be ablated the absorbed laser pulse energy

must be greater than the binding energy of an atom to the surface which is theenergy of vaporization per atom, Eab > Eb [63].

In general the interaction between the laser radiation and the solid material

takes place through the absorption of photons by electrons of the atomic system.

The absorbed energy causes electrons to be in excited states with high energy and

as a result the material heats up to very high temperatures in a very short time.

Then, the electron subsystem will transfer the energy to the lattice, by means ofelectron-phonon coupling [60, 64]. When the focused laser pulse arrives at thetarget surface the photons are absorbed by the surface and its temperature begins to

rise. The rate of this surface heating, and therefore the actual peak temperaturereached, depends on many factors: most importantly the actual volume of material

being heated.This will depend not only upon how tightly the laser is focused butalso on the optical penetration depth of the material.

If this depth is small then the

laser energy is absorbed within a much smaller volume. This implies that we

require a wavelength for which the target is essentially opaque and it is in general

true that the absorption depth increases with wavelength. The rate of heating is also

determined by the thermal diffusivity of the target and the laser pulse energy and

duration. In a high vacuum chamber, elementary or alloy targets are struck at anangle of 45o by pulsed and focused laser beam. The atoms and ions ablated from

the target are deposited on substrate, which is mostly attached with the surface


42/110


24

abp

th

nEut

F

21

)(

ptIF

parallel to the target surface at a target-to-substrate distance of typically

2-10 cm [31]. In PLD technique, the target materials are first sputtered (or sayablated) into a plasma plume by a focused laser beam an angle of 45o. The

materials ablated then flow (or fly) onto the substrate surface, on which the desiredthin films are developed.Therefore, the interaction of intense laser which mattersplays an important role in PLD process [65].

The incident laser pulse induces extremely rapid heating of significant

mass/volume of the target material. This may cause phase transition and introduce

high amplitude stress in the solid target. The output of pulsed laser is focused onto

a target material maintained in vacuum or with an ambient gas. The target isusually rotated in order to avoid repeated ablation from the same spot on the target.

The ablation threshold is the amount of energy needed for the ablation process

to begin. In PLD this energy is expressed as (F) the laser fluence in (J/cm2): [57]

.. (2-1)

Where (I) is the laser intensity (w/cm2) and (tp) is the laser pulse width

duration (s). The ablation threshold for dielectrics and metals vary greatly because

the fluence is dependent on laser parameters and material characteristics.

Parameters that influence ablation thresholds [57] Laser pulse width, and wavelength Target materials electromagnetic, and thermal properties

The following equation can approximate the fluence threshold for laser pulse

durations that are larger than 10 picoseconds: [66]

. (2-2)


43/110


25

Where (u) is the thermal diffusion coefficient (m2/s), (Eb) the binding energy

of vaporization per atom, (na) the number density of atoms in the material and ()

the absorption coefficient (cm-1).

Various experiments and models attempt to understand plasma plume

formation in different mediums.These models give insight to plasma plume

formation down to the picosecond time scale and with different imaging techniques

can provide visual aids [67, 68]. Usual laser flux densities required for most

materials to generate a plasma plume are greater than 105 W/cm2 [57]. When the

ablation threshold is reached, the ejection of electrons, ions, and neutral particles

form a shock wave followed directly by the plasma plume, typical temperatures of

these plasmas can be in excess of tens of thousands of kelvin [67]. The material

plasma vapor plume becomes apparent in the nanosecond time scale and has a

supersonic propagation velocity of approximately 106 cm/s [68].The emitted light

and the color of the plume are caused by fluorescence and recombination processes

in the plasma. The pressure and the laser fluence both have significant effect on the

shape, size of the plume [59].As shown in fig. (2.2).


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26

The Volmer-Weber, Frank-van der Merwe and Stranski-Krastinov nucleation

and growth modes explain the nucleation and growth of thin films close tothermodynamic equilibrium. Each growth mode is governed by the balance

between the free energies of the film surface (F), substrate surface (S), and the

film-substrate interface (I) [69]. For the Volmer-Weber mode there is no bonding

between the film and substrate because the total surface energy is greater than the

substrate energy, F + I > S, this results in 3-dimensional island growth. When

F+ I< S this is characterized as Frank-van der Merwe growth mode [69].Through nucleation and island clustering these films grow as full-monolayers

with strong bonding between the film and substrate, they are a monolayer thick and

completely combine before other island clusters develop to form the next

monolayer [70]. The Frank-van der Merwe growth mode is characteristic of

homoepitaxial thin film growth. The Stranski-Krastinov mode can occur during

heteroepitaxial growth due to the lattice mismatch between the substrate and

deposited thin film [69]. Initially the growth is monolayer but becomes

3-dimensional island growth due to a biaxial strain induced by the lattice

mismatch [70] Fig. (2.3) is a schematic depiction of each growth mode.


45/110


27

The following thin film growth modes provide us with a good understanding

of the nucleation, growth, and morphology of thin film growth when close to

thermodynamic equilibrium. When films are not grown close to thermodynamic

equilibrium, kinetic effects will lead to different growth modes, additioninformation pertaining to kinetic type growth modes can be found in [69].

PLD allows the growth of films under a highly reactive gas ambient over awide range of pressure.

Complex oxide compositions with high melting points can be easilydeposited provided the target materials absorb the laser energy.

Multi-targets for multi-layer or alloy films could be easily modified.Operated under any ambient gas.Relatively inexpensive technique because the target of PLD is relativelysmall and need no special preparation.Fast: high quality samples can be grown reliably in 10 or 15 minutes.PLD is a clean process because the films are able to be deposited in vacuumeor with background gases.

In the PLD process during film growth suitable kinetic energy in the range10100 eV and photochemical excitation exist in comparison to other

deposition techniques.The main practical limitation of PLD is its relatively low duty cycle,incorporation of particulates in the deposited films, although this is not

unique to PLD, because particulate problem exists in the case of sputtering

and MOCVD as well.


46/110


28

Earlier a seemingly esoteric technique of Pulsed Laser Deposition (PLD) hasemerged as a potential methodology for growing nanostructures of various

materials including semiconductors [73].

Since it is a cold-wall processing, which excites only the beam focused areas

on the target enabling a clean ambient, it is highly suited for the growth of

nanostructures with high chemical purity and controlled Stoichiometry.

The other characteristics of PLD such as its ability to create high-energysource particles, permitting high quality film growth at low substrate temperatures

[74], simple and inexpensive experimental setup, possible operation in high

ambient gas pressure, and sequential multi-target and multi-component materials'

congruent evaporation make it particularly suited for the growth of oxide thin films

and nanostructures.

In this section we shall present and discuss a few representative cases wherePLD has been successfully applied for the growth of semiconductors thin films and

nanostructures. These cases of various semiconductors also illustrate the current

trend and the future promise that PLD holds.

X-ray diffraction could be used to define the preferred orientation, and from

the diffrograms one can calculate the average grain size and determines whether

the deposited films suffer from stress or not. These constants change with structural

change caused by the different parameters such as deposition technique, doping,

substrate and annealing.

The Bragg's condition for the diffraction can be written as [75]:


47/110


29

sin2dn ... (2-3)

Where (n) is integer that indicates the order of the reflection, () is Bragg

angle, and () is the wavelength of the X-ray beam. By measuring the Bragg angle(), the interplanar distant (d) can be obtained if the wavelength of the X-ray beam

is known.

Fig. (2.4) shows the X-ray diffraction patterns of nanocrystalline TiO2 powder

prepared by sol-gel method annealed at 400 - 700 C temperatures with a fixed

annealing time of 1 h in air. The effect of annealing temperature on the crystallinity

of TiO2

can be understood from the figure. TiO2

has been crystallized in a

tetragonal mixed anatase and rutile form.

There are several factors working to change the properties of structural

materials and therefore a change observed in the spectrum of its X-ray diffraction.


48/110


31

)cos/()94.0()2(

g

Such as the effect of substrate temperatures, doping, nanoscale structure, annealing

and other factors. We interested in the effect of annealing.

The effect of annealing is an important factor in determining the crystal

structure of polycrystalline materials, and as especially nanostructures byincreasing the grain size and decrease boundaries grains in most cases, thus

increasing the crystallization of the material and decrease defects inside them and

the granting of atoms of the material enough energy to rearrange themselves inside

lattice. The crystallized material means, of course, a clear increase in the intensity

of peaks belonging to the levels, found during the software of modern used for

accounts that these increases are accompanied by a decrease in the values ofFWHM with a deviation toward values (2) least, which confirms that the

temperature role in increasing the distance between the levels of crystalline (d)

because the relationship between (d) and (Sin) an inverse relationship according

to the Bragg's law [76,44].

Normally XRD is used to calculate different parameters which could be used

to clarify the studies of the deposited films.

The FWHM of the preferred orientation (peak) could be measured, since it is

equal to the width of the line profile (in degrees) at the half of the maximumintensity.

The average grain size (g), which can be estimated using the Scherers

formula: [77]

.......... (2-4)


49/110


3

)()(

)()()(

0

1

0

hklIhklIN

hklIhklIhklT

r

C

c

cc

c

ccccSs

13

121133132

2

)(2

%100)(

c

ccStrain

Where () is the X-ray wavelength (), (2)FWHM (radian) and () Bragg

diffraction angle of the XRD peak (degree).

To describe the preferential orientation, the texture coefficient, TC (hkl) is

calculated using the expression [78]:

(2-5)

Where (I) is the measured intensity, (Io) is the JCPDS standard intensity, (Nr)is the reflection number and (hkl) is Miller indices.

The residual stress (Ss) in TiO2 films can be expressed as [79]

.... (2-6)

Where(c) and (co) are the lattice parameter of the thin film and TiO2 thin film

obtained from JCPDS respectively. The value of the elastic constant (cij) from

single crystalline TiO2 are used, c11=208.8 GPa, c33=213.8 GPa, c12=119.7 GPa and

c13=104.2 GPa.

This strain can be calculated from the formula [79]:

.... (2-7)


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32

Atomic force microscopy (AFM) employs a microscopic tip on a cantilever

that deflects a laser beam depending on surface morphology and properties throughan interaction between the tip and the surface. The signal is measured with a

photodetector, amplified and converted into an image display, AFM can be

performed in contact mode and tapping mode [80]. The investigated materials

include thin and thick coatings, semiconductors, ceramics, metals,

micromechanical properties of biological samples, nucleic acids, polymers and

biomaterials, to name a few [81]. Fig. (2.5) shows nanostructured anatase TiO2 thinfilms which are grown by radio frequency magnetron sputtering on glass substrates

at a high sputtering pressure and room temperature. This is films annealed at 300

C and 600 C in air for a period of 1 hour. All the TiO2 films exhibit a smooth

surface with uniform grains.


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33

gEh

AFM images show slow growth of crystallite sizes for the as-grown films and

annealed films.

The process of basically absorptivity in crystalline semiconductors for

incident rays happens when incident photon gives its energy which was equal or

larger than forbidden energy gap (Eg) to conduction band by absorbing that

incident photon [82].

... (2-8)

Where () frequency in (Hz.) and (h) Plank constant (6.625*10-34 j.sec.)

Spectroscopy of incident rays region which start electrons in it transporting is

called (fundamental absorption edge) which equals the difference between bottom

conduction band and top valance band as in fig. (2.6) where ( c ) is cut off

wavelength [83].

When (Eg) equal to (Eg=ho) where (o) is called critical frequency and the

wavelength that opposite to it called wavelength cut off (c), this process happens

when incident energy photon equals to width of forbidden energy gap which can be

expressed in the following equation [83]:

...... (2-9)

Where (c) is speed of light in vacuum and (c) is wavelength cut off.

)(

24.1)(

eVEE

hcm

gg

c


52/110


34

The fundamental absorption edge can be defined as the rapid increasing in

absorptivity when absorpted energy radiation is almost equal to the band energy

gap; therefore, the Fundamental Absorption Edge represented the less different in

the energy between the upper point in valance band to the lower point inconduction band [85, 86].

Absorption regions can be classified to three regions, [86]:

This region is (A) as shown in fig. (2.7), where the magnitude of absorption

coefficient () larger or equal to (104 cm-1). This region can be introduced to

magnitude of forbidden optical energy gap (Eg).

The region (B) as shown in fig. (2.7), the value of absorption


53/110


35

coefficient () is equal about (1 cm-1 < < 104 cm-1), and refers to transition

between the extended level from the (V.B.) to the local level in the (C.B.) also from

local levels in (C.B.) in top of (V.B.) to the extended levels in the bottom of (C.B.).

The absorption coefficient () in these region (C) as shown fig (2.7) is

very small about ( < 1 cm-1) the transitions happen here between the regions

because density of state inside space motion resulted from faults structural [82].

The electronic transitions can be classified basically into two types [87]:

This transition happens in semiconductors when the bottom of (C.B.) be

exactly over the top of (V.B.), which means they have the same value of wave

vector i.e. (K=0) in this state the absorption appeared when (h=Eg), this

transition type is required to the Law's conservation in energy and momentum.

These direct transitions have two types, they are [86]:


54/110


36

r

gEhBh )(

This transition happens between the top points in the (V.B.) to the bottom

point in the (C.B.), as shown in fig. (2.8.a).

This transition happens between near top points of (V.B.) and bottom points

of (C.B.) as shown in fig. (2.8.b), the absorption coefficient for this transitions type

given by [88]:

..... (2-10)

Where: Eg: energy gap between direct transition

B: constant depended on type of material

: frequency of incident photon.

r: exponential constant, its value depended on type of transition,

r =1/2 for the allowed direct transition.

r =3/2 for the forbidden direct transition.

In these transitions types, the bottom of (C.B.) is not over the top of (V.B.), in

curve (E-K), the electron transits from (V.B.) to (C.B.) not perpendicularly where

the value of the wave vector of electron is not equally before and after transition of

electron. (K 0), this transition type happens with helpful of a like particle iscalled "Phonon", for conservation of the energy and momentum law. There are two

types of indirect transitions, they are [88]:

These transitions happen between the top of (V.B.) and the bottom of

(C.B.) which is found in different region of (K-space) as shown in fig. (2.8.c)


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37

r

phg EEhBh )(

These transitions happen between near points in the top of (V.B.) and near

points in the bottom of (C.B.) as shown in fig. (2.8.d), the absorption coefficient for

transition with a phonon absorption is given by [89]:

....... (2-11)

Where Eg: energy gap for indirect transitions

Eph: energy of phonon, is (+) when phonon absorption

and (-) when phonon emission

(r = 2) for the allowed indirect transition(r = 3) for the forbidden indirect transition


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38

11

1

42

1

2

02

R

RK

R

Rn

t

A303.2

1 ATR

iri

2

0

2Knr

02nK

i

4K

The extraction of optical constants from various types of optical measurement

is a field of widespread interest [91]. A large number of methods have beenproposed for the determination of the optical parameters real part of refractive

index (n), extinction coefficient (K0) and the real and imaginary part of dielectric

constant [92].

..... (2-12)

Where (R) is the reflectance.

The extinction coefficient (K0) is related to the exponential decay of the wave

as it passes through the medium and it is defined to be [93].

. (2-13)

Where () is the wavelength of the incident radiation and () is given by:

.......... (2-14)

(A) is the absorbance, and (t) is the sample thickness. And (R) is calculated

from the following equation:

...... (2-15)

An absorbing medium is characterized by a complex dielectric constant

.. (2-16)

...... (2-17)

....... (2-18)


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39

4

nc

The optical conductivity () depends directly on the wavelength and

absorption coefficient [94]:

.... (2-19)

The optical transmission spectra for the anatase TiO2 thin films are presented

in fig. (2.9). Anatase TiO2 thin films are prepared by RF magnetron sputtering

system with a titanium target of 99.99% purity on microscope glass slides as

substrates.The substrates deposited at room temperature with TiO2 are annealed at

300 C, 400 C, 500 C and 600 C using an electric furnace for 1 h in air [44].

From fig.(2.9), it is found that average transmittance of as-deposited and

annealed TiO2 films is about 85% in the visible region. Annealing shows a slight

decrease in transmittance with the increase of annealing temperature. The films

which are annealed at 600 C show a significant decrease in visible light

transmittance [44].


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41

The curves of refractive index and extinction coefficient for as-grown and

annealed TiO2 films are shown in fig. (2.10) and fig. (2.11). Here, it is found that

the refractive index at 550 nm for as deposited, annealed at 300 C, 400 C, 500 C

and 600 C are 2.31, 2.34, 2.33, 2.33 and 2.35 respectively. This trend shows anincrease of the value of refractive index with higher annealing temperature.

The extinction coefficient is found to increase as the treatment temperature is

increased.


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10/23/2005

Experimental Work


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Chapter Three Experimental Work

4

This chapter includes a description of pulsed laser deposition system which

has been used to prepare titanium dioxide TiO2 thin films and explanation for

substrate cleaning method. Also, it deals with method of measuring thickness of

thin films, structural and optical properties measurements. A schematic diagram

illustrates the experimental work as shown in fig. (3.1).

Experimental work

Thin films By PLDT= 300

oC & E= (400) mJ

Annealing films at : 400 oC,500 oC& 600 oC

Thicknessof thin films

XRD & AFMOpticalinterferometer

method

T, A, , Eg, n,k, r, i &

OpticalProperties

StructuralProperties


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42

The basic components of the PLD-system, the laser and the pulse shaping

have been introduced. In addition the following sections consider the componentsinside the deposition chamber, namely, the target, the substrate and also the

vacuum system.

Nd:YAG laser (Huafei Tongda Technology- DIAMOND- 288 pattern EPLS)

is used for the deposition of TiO2 on glass substrate . The whole system consists of

light route system, power supply system, computer controlling system, cooling

system, etc. The light route system is installed into the hand piece, but power

supply, controlling and cooling systems are installed into the machine box of power

supply, as shown in fig. (3.2).


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43

The pulsed laser deposition experiment is carried out inside a vacuum

chamber generally at (10-2

mbar) vacuum conditions, at low pressure of abackground gas for specific cases of oxides and nitrides. Photograph of the set-up

of laser deposition chamber, is given in fig. (3.3), which shows the arrangement of

the target and substrate holders inside the chamber with respect to the laser beam.

The focused Q-switching Nd: YAG laser beam coming through a window is

incident on the target surface making an angle of 45 with it. The substrate is placed

in front of the target with its surface parallel to that of the target. Sufficient gap iskept between the target and the substrate so that the substrate holder does not

obstruct the incident laser beam. The shape of the deposition chamber is

cylindrical; the geometry of the chamber can be designed quite freely. The chamber

has typically a large number of ports, e.g. for pumping system, gas inlets, pressure

monitoring, target, substrate, laser beam and view ports. When designing a

chamber, at least following aspects should be taken into account:

The arrangement of the components inside the chamber should not disturb the

path of the laser beam.

Access to the target and to the substrate should be straightforward, since these

components will be changed frequently.

The target-substrate distance should be adjustable.

The deposition of the laser window should be eliminated as well as possible.

Modification of the deposition technique is done by many investigators from time

to time with the aim of obtaining better quality films by this process. These include

rotation of the target, heating the substrate, positioning of the substrate with respect

to target.


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44

Laser model: Q-switched Nd: YAG Laser Second Harmonic Generation(SHG).

Laser wavelength: (1064 and 532) nm.Pulse energy: (100-1000) mJ.Pulse duration: 10 ns.Repetition frequency: (1 - 6) Hz.Cooling method: inner circulation of water for cooling.Power supply: 220V.


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45

The substrate heater raises the substrate temperature up to 300 C and this is

achieved by using halogen lamp, which is mounted adjacent to the substrate. Thetemperature is measured continuously during film deposition process using a

K-type thermocouple.

The deposition chamber is fixed on a stainless steel flange containing a groove

with O-ring for vacuum sealing and feed-through in the base for electrical

connections (control the stepper motor and the substrate heater) and the chamber

evacuated using rotary pump connecting directly to the chamber by stainless steel

flexible tubes to get a vacuum up to 10-2 mbar and monitoring the pressure inside

the chamber by using (Leybold- Heraeus) Pirani gauge.

Titanium dioxide powder with high purity (99.999%) pressing it under 5 Ton

to form a target with 2.5 cm diameter and 0.4 cm thickness. The target should be as

dense and homogenous as possible to ensure a good quality of the deposit. The

target after being ablated is shown in fig. (3.4).


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46

We use the glass substrates (32) cm2 to deposit TiO2 as shown in fig. (3.5).

The substrates are first cleaned in distilled water in order to remove the impuritiesand residuals from there surface, then cleaned in alcohol ultrasonically for 10 min

subsequently dried prior to film deposition experiment.

The characteristic measurements of this technique are used to investigate the

thickness, the structural features of the films are X-ray diffraction (XRD), and

atomic force microscopy (AFM). The optical features of the films are investigated

by transmission through UV-VIS absorption spectroscopy.

Film thickness measurements by optical interferometer method have been

obtained. This method is based on interference of the light beam reflection from


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47

thin film surface and substrate bottom, with error rate at 3%. He-Ne laser

(632.8nm) was used and the thickness was determined using the formula [95]:

...... (3-1)

Where (x) is the fringe width, (x) is the distance between two fringes and ()

wavelength of laser light, as shown in fig. (3.6).

The film thickness is about 200 nm for all TiO2 films at same deposition

conditions; the number of laser pulses is in the range of 100 pulses.

To define the preferred orientation also to determine the nature of the growth

and the structured characteristics of TiO2 films, X- Ray diffraction is carried and

the phase is determined by using the JCPD data for TiO2

anatase and rutile, using

Shimadzu 6000 made in Japan. The source of X-Ray radiation has CuK radiation.

2

t


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48

The device has been operated at 40 Kv and 30 mA emission current, =1.54.

The X-ray scans are performed between 2 values of 30 and 60.

To determine the size and other characteristics of the synthesized

nanoparticles, an atomic force microscope (AFM) is used. The operation principle

of an AFM is presented in fig. (3.7). The AFM consists of a cantilever and a sharp

tip at its end. The surface of the specimen is scanned with the tip. The distance

between the specimen surface and the tip is short enough, to allow the van der

Waals forces betwee

Documents

Effect of Annealing on the Structural and Optical Properties of Nanostructured TiO2 Films Prepared by PLD