Republic of Iraq
Ministry of Higher Education
and Scientific Research
University of Al-Mustansiriya
College of Education
Effect of Annealing on the Structural and
Optical Properties of Nanostructured TiO2
Films Prepared by PLD
A thesis
Submitted to the Council of Education College of
Al-mustansiriyah University in Partial Fulfillment of the
Requirements for theDegree of M.Sc. in Physics
By
Sarmad Sabih Kaduory Al-Obaidi
B. Sc. 2010
Supervised By
Dr.Ali Ahmed Yousif Al-Shammari
(Assistant Professor)
2012 A.C.
1433 A.H.
بسم اّ الرْ الرحي
))اّ و الس اوا وا َأ ض مثل و ه َشَ فها مصباح ال صباح ِ
جاجة الزجاجة َََا كوكب د ٌي يوقد م َر مبا كة يتوة ا َقية
واغربية يَد يُا يِء ولو لم ت سسه َ و عَ و َدي اّ لنو ه
م يشاء ويْ اّ ا َأمثال للنا واّ بك َء علي((
صدق اّ العظي
النو
ii
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 University
Date:
/
/2013
Date:
(Member)
/
/2013
(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
iii
iv
Dedication
To my family, friends and
all the close people in my
life
Sarmad
v
Acknowledgment
First of all, praise be to ALLAH for helping and supporting me in every thing
I would like to express my profound sense of gratitude & appreciation to
my Supervisor’s
Dr.Ali
Ahmed Yousif Al-Shammari
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 Dr. Khaled Z. Yahya and Prof.Dr.Adawiya J. Haider
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 Mr. Kameran Yasseen Qader,
my dearest friend’s Abdulaziz Mahmood Ahmed… and I can’t forget to thank
my family whom supported me with their kind, patience and encouragement.
Allah bless you all
Sarmad
vi
Abstract
In this work, Nanostructured TiO2 thin films are grown by pulsed laser
deposition (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
vii
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 films
increases with annealing temperature. The real dielectric constant and the
imaginary part increases when the annealing temperature increasing.
viii
Table of Contents
Dedication
Acknowledgment
Abstract………………………………………………………………………i
δist of Symbols………………………………...……………………….….vii
δist of Abbreviations……………...……………………………………..…ix
δist of Tables…………………………………...……………………….…..x
Chapter One (Introduction)
3.3. Introduction……………………………………………………………..3
3.2. Fundamentals of Pulsed δaser Deposition)PδD(.…………………...…2
1.3. Chemical and Physical Properties of TiO2......…..………………..…...1
1.4. The Crystal Structure of TiO2………………...………………..………3
1.5. Applications of Nanostructured TiO2…...….………………….....……6
3.6. δiterature Survey………………………………….………........………7
3.7. Aim of the Work………………………………………………………38
Chapter Two (Theoretical Part)
2.3. Introduction………………………..…….…………………….………3λ
2.2. Pulsed δaser Deposition )PδD(….…………………...…..…….……..3λ
2.1. εechanism of Pulsed δaser Deposition ………………….…………..22
2.1.3. The Interaction of the δaser Beam and Target………….……...22
2.1.2. Plasma Plume Formation…………………..…………………...25
2.1.2. ζucleation and Growth of Thin Films………..………….……..26
ix
2.3. δimitations and Advantages of PδD……….……….…………….…..27
2.5. Pulsed Laser Deposition of Nano-Structure Semiconductor….…..…..28
2.6. Structural Properties……..………………………………………...….28
2.6.1. X-ray Diffraction ) XRD (……………...…………………...….28
2.6.2. Effect of Annealing on the X-ray Diffraction..……………...….2λ
2.6.1. Parameters Calculation………………….……………...…...….11
2.6.3.1. Full Width at Half Maximum (FWHM) (Δ(…..……….30
2.6.3.2. Average Grain Size )g(…...………….....……..……….30
3.6.3.3. Texture Coefficient (Tc)..…………….....……..……….31
3.6.3.4. Steess (Ss(………………...………….....……..……….31
3.6.3.5. εicro Strains ) (……….…………….....……..……….31
2.6.4 Atomic Force εicroscopy )AFε(….………....…….……...…...32
2.7. Optical Properties of Crystalline Semiconductors .……..………...…..11
2.7.3. The Fundamental Absorption Edge ……………..……………..13
2.7.2. Absorption Regions ……………………………..…….……….13
2.7.2.1. High Absorption Region…......………………..……….14
2.7.2.2. Exponential Region..………...………………..……….14
2.7.2.3. Low Absorption Region...…...………………..……….15
2.7.1. The Electronic Transitions …………..…………..……………..15
2.7.1.3. Direct Transitions …………...………………..……….15
2.7.1.2. Indirect Transitions ……………………….…..……….16
2.7.3. Optical Constants…………………………….…..……………..18
2.7.5. Some Optical Properties of TiO2 Thin Film…..………………..1λ
Chapter Three (Experimental Work)
1.3. Introduction….……………………..………………………………….33
x
1.2. Deposition Equipment………………………...….…...………………32
1.2.3. ζdμ YAG δaser Source.…………………….……….…….……32
1.2.2. Pulsed δaser Deposition )PδD( Technique……….….….….….31
1.2.1. Substrate Heater………………………………….….…….……35
1.2.3. Vacuum System…………..……………………….….….……..45
1.1. Target Preparation……………………………………....……….........35
1.3. Substrate Preparation…………………………………...……………..36
1.5. Characterization εeasurements……………………………………….36
1.5.3. Thickness εeasurement…...............…………………….……..36
1.5.2. Structural and εorphological εeasurements….…….....………37
3.5.2.1. X-ray Diffraction )XRD(…………….....……..……….37
1.5.2.2. Atomic Force εicroscopy )AFε(...…….….....……….37
1.5.1. Optical εeasurements………...………………….…………….38
Chapter Four (Results and Discussion)
3.3. Introduction…………………………………..………………………..50
3.2. Structural Properties……………………………….…...………….….50
4.2.1. X-ray Diffraction……....…………………….……...…….……50
4.2.2. Atomic Force Microscopy )AFε(………….……...……...……56
4.3. Optical Properties………………...…………….…....…………....…..58
3.1.3. Optical Transmission )T(…………………….……......…..……58
3.1.2. Optical Absorption )A(……….…………….……......…....……59
4.3.3. Optical Absorption Coefficient )α(…………………….….……62
4.3.4. Optical Energy Gap (Eg(...………………….……...…..….……62
3.1.5. Refractive Index )n(…...…………………….……....….………66
4.3.6. Extinction Coefficient (Ko(..………………….……......…….…67
4.3.7. The Dielectric Constants )Ԑr, Ԑi(.…….………….……..….……67
xi
3.1.8. Optical Conductivity ) (.…………………….……...…….……69
Chapter Five (Conclusion and Future work)
5.3. Conclusion ………………...……………………………………..70
5.2. Future Work ……..………………………...……………………..72
5.1. Publications………………...……………………………………..73
References…….………………...……………………………………..74
xii
List of Symbols
Symbol
Description
a
Lattice constant (Å)
Absorptance
Anatase
Absorption coefficient (cm-1)
Back flux (W/cm2)
Velocity of light in vacuum (m/s)
Thickness (nm)
laser pulse width duration (s)
Inter planer spacing (Å)
Electron charge (C)
Binding energy of vaporization per atom
Ablation energy of the pulse laser (eV)
Energy gap(eV)
Energy of phonon (eV)
Laser fluence (J/cm2)
Approximate the fluence threshold for laser pulse
Average grain size (nm)
Plank constant (J. s)
Photon energy (eV)
Laser intensity (W/cm2)
Measured intensity
JCPDS standard intensity
Wave vector (cm-1)
Boltzmann constant (J/K)
Extinction coefficient
Refractive index
Number density of atoms
A
A
α
b
c
t
tp
d
e
Eb
Eab
Eg
Eph
F
Fth
g
h
hυ
I
I
Io
∆k
KB
Kₒ
n
na
xiii
Nr
p
R
R
T
Tₒ
Tc
Ts
u
x
∆x
Ss
c
θ
r
i
γF
γS
γI
υ
υo
Reflection number
Pressure of the gas (mbar)
Reflectance
Rutile
Transmittance
Temperature (ºC)
Texture coefficient
Substrate temperature (K)
Thermal diffusion coefficient (m2/s)
Fringe width (cm)
Distance between two fringes (cm)
Stress
Micro Strains
Wavelength (nm)
Wavelength cut off ) m(
Optical conductivity
Diffraction angle (deg.)
Real part of dielectric constant (F/m)
Imaginary part of dielectric constant (F/m)
Free energies of the film surface (eV)
Free energies of the substrate surface (eV)
Free energies of the film-substrate interface (eV)
Frequency (Hz)
Critical frequency (Hz)
xiv
List of Abbreviations
Symbol
Description
AFM
CVD
CSP
C.B.
DSSC
FTIR
FWHM
MBE
GAXRD
JCPDS
PEC
PLD
PL
RF
RMS
RTA
SEM
SHG
SHI
TCOs
TiO2
TPD
TEVD
V.B.
XRD
XPS
Atomic Force Microscope
Chemical Vapor Deposition
Chemical Spray pyrolysis
Conduction Band
Dye-Sensitized Solar Cells
Fourier Transform- Infrared Spectroscopy
Full Width at Half Maximums (deg.)
Molecular Beam Epitaxial
Glancing Angle X-ray Diffraction
Joint Committee for Powder Diffraction Standards
Photoelectrochemical Cells
Pulsed Laser Deposition
Photoluminescence
Radio Frequency
Root Mean Square
Rapid Thermal Annealing
Scanning Electron Microscope
Second Harmonic Generation
Swift Heavy Ion Irradiation
Transparent Conducting Oxide Semiconductors
Titanium Dioxide
Thermal Pyrolysis Deposition
Thermal Evaporation in Vacuum Deposition
Valence Band
X-Ray Diffraction
X-Ray Photoelectron Spectroscopy
xv
List of Tables
Table
No.
(2.1)
(4.1)
(4.2)
(4.3)
(4.4)
Title
Performance features of Excimer and Nd: YAG lasers.
Lattice constants and interpllanar spacing of TiO2 films.
The obtained result of the structural properties from XRD
for TiO2 thin films.
Morphological characteristics from AFM images for TiO2 thin
film.
Shows allowed direct band gap and allowed indirect band gap
for different annealing temperatures of TiO2 thin films.
xvi
Page
No.
21
53
54
56
63
10/23/2005
Introduction
Chapter One
Introduction
1.1 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 by
means 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].
1
Chapter One
Introduction
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 films
deposited 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 different
substrates. This supposes to result in the different structural and micro structural
properties, different surface morphology of the nanostructures to be obtained.
1.2 Fundamentals of Pulsed Laser Deposition (PLD)
The discovery of the ruby laser prompted an evolution of theoretical
investigations into laser-target interaction. Numerous experiments were carried out
to 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 methods
and provide special advantages for the growth of chemically complex
2
Chapter One
Introduction
(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 qualities
comparable 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 highly
energetic plume, and the growth of the film on the substrate.
1.3 Chemical and Physical Properties of TiO 2
The following points show some chemical and physical properties of TiO 2:
1-TiO2 is found naturally as a white material in three forms of crystalline: Rutile,
Anatase and Brookite [10].
2-The pure of TiO2 is white solid structure solvents in H2SO4, but it is not solvent
in water or alcohol or HCl [10].
3-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.
4-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].
5-The molecular weight of TiO2 is (79.90) in which Oxygen represents (40.05%)
and Titanium (59.95%), and melting point is (1850 ºC) and boiling point is
(3000 ºC) [10].
6-The thin films of TiO2 have high band energy gap about (3.2 - 3.29) eV, (3.693.78) eV for allowed and forbidden direct transition respectively [12]
3
Chapter One
Introduction
1.4 The Crystal Structure of TiO2
There are three forms of crystalline structure of TiO2 material they are:
1-Anatase: The anatase polymorph of TiO2 is one of its two metastable phases
together with brookite phase. For calcination processes above 700 ºC all anatase
structure becomes rutile, some authors also found that 500 ºC 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 Å and
c = 9.430 Å [13], as shown in fig. (1.1).
Fig. (1.1): Anatase phase for crystalline TiO2 [14].
2-Rutile: 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 Å and c = 2.9592 Å [13].
4
Chapter One
Introduction
Fig. (1.2): Rutile phase for crystalline TiO2 [14].
3-Brookite: This form has orthorhombic surface. Its density is (4.13 gm/cm3),
refractive index is (2.5831) [10] and Lattice parameters are:a = 9.18 Å, b = 5.447 Å
and c = 5.145 Å [13], as shown in fig. (1.3).
Fig. (1.3): Brookite phase for crystalline TiO2 [14].
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
5
Chapter One
Introduction
more stable, but both will revert to the rutile phase when subjected to high
temperatures.
1. 5 Applications of Nanostructured TiO 2
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).
Fig. (1.4): some applications of TiO2
6
Chapter One
Introduction
1.6 Literature Survey
Lofton, et al., (1978) [27]: They studied titanium thin films which were a
mixture of titanium and TiO2. Auger electron spectroscopy and X-ray
photoelectron spectroscopy in combination with sputter profiling techniques were
employed to study (100-500Å) 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 (SiO 2) or sapphire
(Al2O3). The depositions were carried out at either R.T. or 450 °C 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 doping
did 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).
7
Chapter One
Introduction
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)
)Ω.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 by
optical absorption spectra.
8
Chapter One
Introduction
Dzibrou, et al., (2002) [34]: They deposited TiO2 thin films on quartz and
silicon wafers, by PLD method using Nd: YAG pulsed laser ) =155nm, 31 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 obtained
were 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 400°C 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 TiO 2
thin films prepared by aqueous sol-gel process at low temperature TiO2. 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 ºC) .The films have been annealed for 10 hours at the temperature 600 ºC, 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
9
Chapter One
Introduction
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 asdeposited 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 was
studied 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) ºC 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
11
Chapter One
Introduction
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 films
prepared by (PLD) KrF excimer pulsed laser system (wavelength = 248 nm) on
indium-doped tin oxide (ITO) substrates under different substrate temperature and
pressure conditions )Tₒ = 251, 311,511 and 611 °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 on
substrate 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 500°C in air for 2h. Structural, optical and electrical properties of TiO 2
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 TiO 2
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
11
Chapter One
Introduction
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 roughness
increased 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 ᵒC) in air for 1h. To
examine the substrates and morphology of the films, X-ray diffraction. Atomic
force 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
12
Chapter One
Introduction
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 to
3.42 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 repetition
rate 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.
13
Chapter One
Introduction
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 be
3.11 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 ºC. The grain size of the film
annealed at 600 ºC was about 20 nm. The lattice parameters for the films annealed
at 600 ºC were a = 3.7862 Å and c = 9.5172 Å, 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 ºC 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 under
various temperatures, 100, 150, 200, 300 and 399 ºC 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
14
Chapter One
Introduction
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 µm.
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 ºC)
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 the
RMS 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 ºC, 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.
15
Chapter One
Introduction
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 ºC). 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 ºC, the transmittance values appear
significantly reduced in comparison to those for the film treated at 400 ºC. Further
increase of the treatment temperature up to 1000 ºC 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 ºC with an important increase of their size at 1000 ºC (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 ºC- 400 ºC. 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 W–TiO2 bi-layer thin films was
increased from 200 to 400 ºC, the band gap energy shifted from 3.158 to 3.098 eV.
16
Chapter One
Introduction
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 Thermal
Annealing (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 ºC 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 of
nanocrystalline 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.
17
Chapter One
Introduction
1.7 Aim of the Work
The main objectives of this work are:
1- Initially, the series of samples has been prepared by PLD technique at
different technological conditions on glass substrates.
2- We study the preparation condition such as, substrate temperature, oxygen
pressure and energy laser influence during deposition.
3- As well as the concentration into the target on the structure, morphology
(Atomic Force Microscopy (AFM)), and XRD. Also the optical properties
for deposited films.
4- Then, we study the effect of annealing temperature on structural and optical
properties of TiO2 films.
18
10/23/2005
Theoretical Part
Chapter Two
Theoretical Part
2.1 Introduction
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 as
process 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.
2.2 Pulsed Laser Deposition (PLD)
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 elementary
targets 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
19
Chapter Two
Theoretical Part
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 present
work, 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 heater
and 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.
Fig. (2.1): Left Schematic of the PLD process. Right: Photograph of plume during
deposition [56].
21
Chapter Two
Theoretical Part
Table (2.1): Performance features of Excimer and Nd: YAG lasers [57, 58].
Parameter
Excimer System
Nd:YAG System
248 nm
1064 and 532 nm
100 - 1200 mJ
100 - 1000 mJ
Variable, 1 - 200 Hz
Fixed, 1 - 30 Hz
0.5 - 1%, RMS
8 - 12%, RMS
Wavelength
(nanometers)
Output Energy
(millijoules)
Repetition Rate (Hertz)
Shot-to-Shot Stability
(RMS)
Advantages
Disadvantages
-High power output
-Output energy sufficient for laser
-Good stability
ablation
-Flexibility for tuning
-Simple maintenance
-Laser output parameter
-Compact system
-Short operation life time.
-Large energy drop for the 3rd
-Complicated maintenance
harmonic mode
-Expansive and high purity gasses,
constants refilling
-Space consuming.
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:
1- The substrate type, orientation and temperature.
2- The laser parameters (working wavelength, fluence, pulse duration, and
repetition rate).
3- The chamber pressure and the chemical composition of the buffer gas.
4- The structural and chemical composition of the target material.
5- And the geometry of the experiment (incident angle of the laser, incident
angle 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
21
Chapter Two
Theoretical Part
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.
2.3 Mechanism of Pulsed Laser Deposition
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.
2.3.1 The Interaction of the Laser Beam with Target
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 a
single 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 melting
and 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].
22
Chapter Two
Theoretical Part
To achieve congruent evaporation the energy from the laser must be dumped
into 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 is
most 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 the
energy 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 of
electron-phonon coupling [60, 64]. When the focused laser pulse arrives at the
target 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 temperature
reached, 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 but
also 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 an
angle 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
23
Chapter Two
Theoretical Part
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 say
ablated) into a plasma plume by a focused laser beam an angle of 45 o. The
materials ablated then flow (or fly) onto the substrate surface, on which the desired
thin films are developed. Therefore, the interaction of intense laser which matters
plays 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 is
usually rotated in order to avoid repeated ablation from the same spot on the target.
Ablation Thresholds
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]
F I tp
………………………….…………………. )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 material’s electromagnetic, and thermal properties
The following equation can approximate the fluence threshold for laser pulse
durations that are larger than 10 picoseconds: [66]
Fth
1
(ut p ) 2 Ebna
…………………………………. )2-2)
24
Chapter Two
Theoretical Part
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 )α(
the absorption coefficient (cm-1).
2.3.2 Plasma Plume Formation
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).
Fig. (2.2): Shadowgraph of plume at 1200ps Source [57].
25
Chapter Two
Theoretical Part
2.3.3 Nucleation and Growth of Thin Films
The Volmer-Weber, Frank-van der Merwe and Stranski-Krastinov nucleation
and growth modes explain the nucleation and growth of thin films close to
thermodynamic equilibrium. Each growth mode is governed by the balance
between the free energies of the film surface )γF(, substrate surface )γS), and the
film-substrate interface )γI) [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, γF + γI > γS, this results in 3-dimensional island growth. When
γF + γI < γS 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.
Fig. (2.3): Growth Modes: (a) Frank-Van der Merwe; (b) Volmer-Weber; (c) StranskiKrastanov Source [69].
26
Chapter Two
Theoretical Part
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, addition
information pertaining to kinetic type growth modes can be found in [69].
2.4 Limitations and Advantages of PLD [71, 72]
1- PLD allows the growth of films under a highly reactive gas ambient over a
wide range of pressure.
2- Complex oxide compositions with high melting points can be easily
deposited provided the target materials absorb the laser energy.
3- Multi-targets for multi-layer or alloy films could be easily modified.
4- Operated under any ambient gas.
5- Relatively inexpensive technique because the target of PLD is relatively
small and need no special preparation.
6- Fast: high quality samples can be grown reliably in 10 or 15 minutes.
7- PLD is a clean process because the films are able to be deposited in vacuume
or with background gases.
8- In the PLD process during film growth suitable kinetic energy in the range
10–100 eV and photochemical excitation exist in comparison to other
deposition techniques.
9- 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.
27
Chapter Two
Theoretical Part
2.5 Pulsed Laser Deposition of Nanostructure
…...Semiconductor
Earlier a seemingly esoteric technique of Pulsed Laser Deposition (PLD) has
emerged 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-energy
source 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 where
PLD 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.
2.6 Structural Properties
2.6.1 X-ray Diffraction (XRD)
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]:
28
Chapter Two
n 2d sin
Theoretical Part
…………….…….…………………. )2-3)
Where (n) is integer that indicates the order of the reflection, (θ) is Bragg
angle, and ( ) is the wavelength of the X-ray beam. By measuring the Bragg angle
(θ), 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.
Fig.(2.4): X-ray diffraction patterns of TiO2 nanopowder at different annealing
temperatures: (a) 400°C (b) 500 °C, (c) 600 °C and (d) 700 °C [50].
2.6.2 Effect of Annealing on the X-ray Diffraction
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.
29
Chapter Two
Theoretical Part
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 by
increasing 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 of
FWHM with a deviation toward values )2θ( least, which confirms that the
temperature role in increasing the distance between the levels of crystalline (d)
because the relationship between )d( and )Sinθ( an inverse relationship according
to the Bragg's law [76,44].
2.6.3 Parameters Calculation
Normally XRD is used to calculate different parameters which could be used
to clarify the studies of the deposited films.
2.6.3.1 Full Width at Half Maximum (FWHM) (∆)
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 maximum
intensity.
2.6.3.2 Average Grain Size (g)
The average grain size )g(, which can be estimated using the Scherer’s
formula: [77]
g (0.94 ) /( ( 2 ) cos ) ...….….……..….…….. )2-4)
31
Chapter Two
Theoretical Part
Where ) ( is the X-ray wavelength )Å(, Δ
)2θ(
FWHε )radian( and )θ( Bragg
diffraction angle of the XRD peak (degree).
2.6.3.3 Texture Coefficient (Tc)
To describe the preferential orientation, the texture coefficient, T C (hkl) is
calculated using the expression [78]:
TC (hkl )
I (hkl ) I 0 (hkl )
……………… )2-5)
1
Nr I (hkl ) I 0 (hkl )
Where (I) is the measured intensity, (Io) is the JCPDS standard intensity, (Nr)
is the reflection number and (hkl) is Miller indices.
2.6.3.4 Stress (Ss)
The residual stress (Ss) in TiO2 films can be expressed as [79]
2c213 c33(c11 c12 ) c c
Ss
2c13
c
.………………... )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.
2.6.3.5 Micro Strains (δ)
This strain can be calculated from the formula [79]:
Strain ( )
c c
c
100% ..……………….. )2-7)
31
Chapter Two
Theoretical Part
2.6.4 Atomic Force Microscopy (AFM)
Atomic force microscopy (AFM) employs a microscopic tip on a cantilever
that deflects a laser beam depending on surface morphology and properties through
an 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 thin
films 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.
Fig. (2.5): AFM images of TiO2 films deposited at room temperature and annealed:
(a) As-deposited, (b) 300 °C and (c) 600 °C [44].
32
Chapter Two
Theoretical Part
AFM images show slow growth of crystallite sizes for the as-grown films and
annealed films.
2.7 Optical properties of Crystalline Semiconductors
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].
h Eg
……….……………………………….…. )2-8)
Where )υ( frequency in )Hz.( and )h( Plank constant )6.625*31-34 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=hυo( where )υo) 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]:
c ( m)
hc
1.24
Eg
E g (eV )
…...……….….. )2-9)
Where )c( is speed of light in vacuum and ) c) is wavelength cut off.
33
Chapter Two
Theoretical Part
Fig. (2.6): Shows the fundamental absorption edge of crystal semiconductor [84].
2.7.1 The Fundamental Absorption Edge
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 in
conduction band [85, 86].
2.7.2 Absorption Regions
Absorption regions can be classified to three regions, [86]:
2.7.2.1 High Absorption Region
This region is (A) as shown in fig. (2.7), where the magnitude of absorption
coefficient )α( larger or equal to )314 cm-1). This region can be introduced to
magnitude of forbidden optical energy gap (Eg).
2.7.2.2 Exponential Region
The region (B) as shown in fig. (2.7), the value of absorption
34
Chapter Two
Theoretical Part
coefficient )α( is equal about )3 cm-1 < α < 314 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.).
2.7.2.3 Low Absorption Region
The absorption coefficient )α( in these region (C) as shown fig (2.7) is
very small about )α < 3 cm-1) the transitions happen here between the regions
because density of state inside space motion resulted from faults structural [82].
Fig. (2.7): The fundamental absorption edge and absorption regions [82].
2.7.3 The Electronic Transitions
The electronic transitions can be classified basically into two types [87]:
2.7.3.1 Direct Transitions
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=1( in this state the absorption appeared when )hυ=E g), this
transition type is required to the Law's conservation in energy and momentum.
These direct transitions have two types, they are [86]:
35
Chapter Two
Theoretical Part
(a) Direct Allowed Transition
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).
(b) Direct Forbidden Transitions
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
h B (h Eg )r
given by [88]:
………..…………..…….… )2-10)
Where: Eg: energy gap between direct transition
B: constant depended on type of material
υμ 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.
2.7.3.2 Indirect Transitions
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 ╪ 1(, this transition type happens with helpful of a like particle is
called "Phonon", for conservation of the energy and momentum law. There are two
types of indirect transitions, they are [88]:
(c) Allowed Indirect Transitions
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)
36
Chapter Two
Theoretical Part
(d) Forbidden Indirect Transitions
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
h B (h Eg E ph )r
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
Fig. (2.8): shows the transition types [86, 90].
(a) allowed direct transition
(c) allowed indirect transition
(b) forbidden direct transition (d) forbidden indirect transition
37
Chapter Two
Theoretical Part
2.7.4 Optical Constants
The extraction of optical constants from various types of optical measurement
is a field of widespread interest [91]. A large number of methods have been
proposed 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].
4 R 2 2 R 1
....……………….… )2-12)
K0
n
2
R 1
R 1
1
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].
K
4
………………………………….…… )2-13)
Where ) ( is the wavelength of the incident radiation and )α( is given byμ
2.303 A
t
...…………….....…………….……. )2-14)
(A) is the absorbance, and (t) is the sample thickness. And (R) is calculated
R T A 1
from the following equation:
………………………………...... (2-15)
An absorbing medium is characterized by a complex dielectric constant
r i i
r n 2 K02
i 2nK0
………………………………….. (2-16)
……...…………….…………….. )2-17)
……………………....………..……. )2-18)
38
Chapter Two
Theoretical Part
The optical conductivity ) ( depends directly on the wavelength and
nc
4
absorption coefficient [94]:
……………………..…………..…… )2-19)
2.7.5 Some Optical Properties of TiO 2 Thin Film
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].
Fig. (2.9): Transmittance spectra of TiO2 films: (a) as-deposited at RT. (b) annealed at
300 °C, (c) 400 °C, (d) 500 °C and (e) 600 °C [44].
39
Chapter Two
Theoretical Part
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 an
increase of the value of refractive index with higher annealing temperature.
Fig. (2.10): Refractive index of TiO2 films: (a) as-deposited at room temp.
(b) annealed at 300 °C, (c) 400 °C, (d) 500 °C and (e) 600 °C [44].
The extinction coefficient is found to increase as the treatment temperature is
increased.
Fig. (2.11): Extinction coefficient of TiO2 films: (a) as-grown at room
temp. (b) annealed at 300 °C, (c) 400 °C, (d) 500 °C and (e) 600 °C [44].
41
10/23/2005
Experimental Work
Chapter Three
Experimental Work
3.1 Introduction
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 PLD
Tₒ= 300 oC & E= (400) mJ
Annealing films at : 400 oC,
500 oC& 600 oC
Thickness
of thin films
Optical
interferometer
method
Structural
Properties
Optical
Properties
XRD & AFM
T, A, α, Eg, n,
kₒ, r, i &
Fig. (3.1): Schematic diagram for of experimental work
41
Chapter Three
Experimental Work
3.2 Deposition Equipment
The basic components of the PLD-system, the laser and the pulse shaping
have been introduced. In addition the following sections consider the components
inside the deposition chamber, namely, the target, the substrate and also the
vacuum system.
3.2.1 Nd: YAG Laser Source
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).
Fig. (3.2): The Nd: YAG laser DIAMOND-288 Pattern EPLS system.
42
Chapter Three
Experimental Work
3.2.2 Pulsed Laser Deposition (PLD) Technique
The pulsed laser deposition experiment is carried out inside a vacuum
chamber generally at (10-2 mbar) vacuum conditions, at low pressure of a
background 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 is
kept 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:
1-The arrangement of the components inside the chamber should not disturb the
path of the laser beam.
2-Access to the target and to the substrate should be straightforward, since these
components will be changed frequently.
3-The target-substrate distance should be adjustable.
4-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.
43
Chapter Three
Experimental Work
Fig. (3.3): Pulsed laser deposition (PLD) system.
Main technical Parameters
1- Laser model: Q-switched Nd: YAG Laser Second Harmonic Generation
(SHG).
2- Laser wavelength: (1064 and 532) nm.
3- Pulse energy: (100-1000) mJ.
4- Pulse duration: 10 ns.
5- Repetition frequency: (1 - 6) Hz.
6- Cooling method: inner circulation of water for cooling.
7- Power supply: 220V.
44
Chapter Three
Experimental Work
3.2.3 Substrate Heater
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. The
temperature is measured continuously during film deposition process using a
K-type thermocouple.
3.2.4 Vacuum System
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.
3.3 Target preparation
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).
Fig. (3.4): The target after being ablated by the laser.
45
Chapter Three
Experimental Work
3.4 Substrate Preparation
We use the glass substrates (3×2) cm2 to deposit TiO2 as shown in fig. (3.5).
The substrates are first cleaned in distilled water in order to remove the impurities
and residuals from there surface, then cleaned in alcohol ultrasonically for 10 min
subsequently dried prior to film deposition experiment.
Fig. (3.5): Glass substrates after the deposition of thin film TiO2.
3.5 Characterization Measurements
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.
3.5.1 Thickness Measurement
Film thickness measurements by optical interferometer method have been
obtained. This method is based on interference of the light beam reflection from
46
Chapter Three
Experimental Work
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]:
t
…………………...………………... (3-1)
2
Where )x( is the fringe width, )∆x( is the distance between two fringes and ) (
wavelength of laser light, as shown in fig. (3.6).
Fig. (3.6): Experimental arrangement for observing Fizeau fringes.
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.
3.5.2 Structural and Morphological Measurements
3.5.2.1 X-ray Diffraction (XRD)
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 CuKα radiation.
47
Chapter Three
Experimental Work
The device has been operated at 31 Kv and 11 mA emission current, =3.53Å.
The X-ray scans are performed between 2θ values of 11° and 60°.
3.5.2.2 Atomic Force Microscopy (AFM)
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 between them to cause deflection of the cantilever. The deflection
follows Hooke's law and the spring constant of the cantilever is known, thus the
amount of deflection and further, the topographical profile of the specimen, can be
determined. All the samples are studied using Nanoscope AFM (made in USA) in
Ministry of Science and Technological in Iraq.
Fig. (3.7): The operation principle of AFM [96].
48
Chapter Three
Experimental Work
3.5.3 Optical Measurements
The optical properties measurements for TiO2 thin films are obtained by using
spectrophotometer (Shimadzu UV- 1650 PC) made by Phillips, (Japanese
company) as shown fig. (3.8) for the wavelength range from 300 nm to 900 nm.
The optical properties are calculated from these optical measurements.
Fig. (3.8): Show the photographic of measuring spectrophotometer.
49
10/23/2005
Results and Discussion
Chapter Four
Results and Discussion
4.1 Introduction
This chapter presents the results and discuss the effect of annealing, upon the
characterization such as structural and optical properties of the films grown by
PLD.
Also the structural measurements such as, morphological features by Atomic
Force Microscope (AFM) and the most relevant aspects of these analytical
techniques are discussed briefly in the following section.
4.2 Structural Properties
4.2.1 X-ray Diffraction
Throughout studying the X-ray diffraction spectrum, we can understand the
crystalline growth nature of TiO2 thin films prepared by pulsed laser deposition on
glass substrates at 300 °C at different annealing temperatures (400, 500, and
600)°C with a fixed annealing time of 2 h in air.
Fig. (4.1) shows X-ray diffraction patterns for TiO2 films. We compare
deposited film at 300 °C with annealed film at 400 °C as shown in fig. (4.1, 1),
annealed film at 500 °C as shown in fig. (4.1,2) and annealed film 600 °C as shown
in fig. (4.1,3). While in fig. (4.1, 4) compared all films.
From fig. (4.1), as-deposited TiO2 film at 300 °C is found to be crystalline and
possesses anatase structure as it shows few peaks of anatase (101) and (004), while
film annealed at 400 °C having peaks of anatase (101), (004) and (200), film
annealed at 500 °C having peaks of anatase (101), (004), (200) and rutile (110) and
film annealed at 600 °C having peaks of anatase (101), (004), (200) and rutile
(110), (211).
51
Chapter Four
Results and Discussion
Fig. (4.1): XRD patterns of TiO2 thin films annealed at: (1) 400 °C, (2) 500 °C, (3) 600 °C
and (4) compared all films.
51
Chapter Four
Results and Discussion
The X-ray spectra show well-defined diffraction peaks showing good
crystallinity, it is found that all the films are polycrystalline with a tetragonal
crystal structure and no amorphous phase is detected. The diffraction peaks are in
good agreement with those given in JCPD data card (JCPDS No. 21– 1272 & 21 –
1276) [97] for TiO2 anatase and rutile as shown in fig. (4.2). It is observed that the
intensities of the peaks of few TiO2 planes increased slightly with the increase of
annealing temperature. In addition, the location of the (101) peaks is shifted to
lower 2θ angles from 2θ=25.27 ° to 2θ=25.33 °.
Fig. (4.2): JCPD data card for TiO2. a- anataes. b- rutile [97].
For a crystalline phase to develop, the depositing atoms should have sufficient
energy. High substrate temperatures can achieve the sufficient energy to generate
crystalline phases [98]. X-ray diffraction analysis reveal that TiO2 thin films are
amorphous if the temperature substrate is lower than 300 °C [99]. It is found that
anatase films are deposited and crystallized effectively for heated substrate at
300 °C and working pressure of 10-2 mbar. The reason may be that the
kinetic.energy of the particle is high enough to initiate crystallization. For the
samples annealed at 500 and 600 °C, other characteristic peaks of anatase and rutile
52
Chapter Four
Results and Discussion
phase. These results are in agreement with other reports on the mixed phase TiO2
by PLD method [100-102].The transformation from anatase to rutile occurs at
temperatures higher than 500 °C.
The increase in peak intensity indicates an improvement in the crystallinity of
the films. This leads to decrease in Full Width at Half Maximums (FWHM) of peak
and increase in grain size.
The lattice constants and lattice constants ratio (c/a), in the diffraction pattern
of TiO2 films are given in table (4.1). The lattice constants obtained are found to be
in good agreement with JCPD.
Table (4.1): Lattice constants and interpllanar spacing of TiO 2 fims.
Temperature ( oC)
As-deposited at 300
400
500
600
degree)
(hkl)
25.27
Interplanar
Lattice constant Å
spacing, d Å
a
c
A(101)
3.15
3.3439
/
37.83
A(004)
2.37
/
9.5050
25.2
A(101)
3.53
3.8
/
37.81
A(004)
2.37
/
9.5098
48.1
A(200)
1.89
3.7803
/
25.17
A(101)
3.53
3.8
/
37.79
A(004)
2.38
/
9.5147
48.12
A(200)
1.89
3.7788
/
27.47
R(110)
3.24
4.5881
/
25.11
A(101)
3.54
3.8170
/
37.72
A(004)
2.38
/
9.5317
48
A(200)
1.89
3.7877
/
27.41
R(110)
3.25
4.5979
/
54.35
R(211)
1.68
/
2.9484
c/a
2.842
3.502
3.503
2.497
The values of Full Width at Half Maximum (FWHM) of the peaks decreases
with annealing temperature, this goes in agreement with [50].
The average grain size in thin films is calculated using Scherer’s formula )2-4),
the values of average grain size listed in table (4.2) shows an increasing at
annealing temperature for TiO2 thin films. The average grain size and Full Width at
Half Maximum (FWHM) of the (101) plane as a function of annealing temperature
for TiO2 thin films are shown in fig. (4.3).
53
Chapter Four
Results and Discussion
Increases of annealing temperature result in the reduction of (101) FWHM for
TiO2 films deposited on glass substrate, and the lowest line width of 0.301° has
been achieved at 600 °C. Narrow FWHM of (101) peak means large grain size of
film.
The micro strain depends directly on the lattice constant (c) and its value
related to the shift from the JCPD standard value which could be calculated using
the relation (2-7). The film annealed at 600 ºC temperature shows the maximum
compressive strain (3.640 ×10-3), which decreased to nearly zero at 500 ºC, as
shown in table (4.2).
The calculation of the film stress is based on the strain model, which could be
calculated using the relation (2-6), as shown in table (4.2).
The values of texture coefficient (Tc) of the thin films are listed in table (4.2).
The texture coefficient is calculated using the relation (2-5) for crystal plane (101),
the values of texture coefficient decrease with increasing of annealing temperature
as shown in fig. (4.4).This is a usual result because increase of annealing
temperature causes an increase in the surface roughness.
Table (4.2): The obtained result of the structural properties from XRD
for TiO2 thin films.
o
Temperature ( C)
As-deposited at 300
400
500
600
degree)
(hkl)
25.27
A(101)
37.83
A(004)
25.2
A(101)
37.81
A(004)
48.1
Stress Ss
Strain
(GPa)
)10-3)
0.218
0.9354
Main
FWHM°
Grain
Size (nm)
0.450
19.02
0.440
19.94
0.421
20.19
0.412
21.27
A(200)
0.410
22.16
25.17
A(101)
0.352
24.16
37.79
A(004)
0.400
21.94
48.12
A(200)
0.349
25.99
27.47
R(110)
0.334
25.59
25.11
A(101)
0.301
28.25
37.72
A(004)
0.288
30.43
48
A(200)
0.338
26.88
27.41
R(110)
0.315
27.13
54.35
R(211)
0.293
31.85
0.098
-0.019
-0.435
0.849
54
0.4225
0.084
1.8709
3.640
Texture Tc
1.813
1.126
1.154
0.952
Chapter Four
Results and Discussion
Fig.(4.3): The main grain size and FWHM for TiO2 A (101) grown
on glass substrate at different annealing temperature
Fig.(4.4): Variation of texture coefficient versus Temperature
55
Chapter Four
Results and Discussion
4.2.2 Atomic Force Microscopy (AFM)
Fig. (4.5) shows the AFM images of the TiO2 thin films deposited at 300 °C
temperature and annealed at 400, 500 and 600 °C.
The surface morphology of the TiO2 thin films changes with the different
annealing temperatures, as observed from the AFM micrographs figures (4.5) left
pictures proves that the grains are semiuniformly distributed within the scanning
area )31 m × 31 µm(, with individual columnar grains extending upwards.
The values of the root mean square (RMS) and surface roughness of TiO2
films are shown in the table (4.3), i.e. the root mean square (RMS) and surface
roughness increased with increasing the annealing temperature as shown in
fig. (4.6), this result is in agreement with the previous work [44]. The three
dimensional AFM right pictures of TiO2 films are shown in fig. (4.5).
Table (4.3): Morphological characteristics from AFM images for TiO2 thin film.
Temperature (oC)
Roughness average Root Mean Square (RMS)
(nm)
(nm)
As-deposited at 300
46.5
60.5
400
76.6
95
500
84.3
105
600
88.6
114
In general as the annealing temperature increases, the RMS and roughness of
the TiO2 films and the grain size increase.
The surface roughness of the TiO2 thin films increases with film thickness,
annealing temperature, and annealing time [103].
56
Chapter Four
Results and Discussion
a
b
c
d
Fig.(4.5): 2D and 3D AFM images of TiO2 films deposited at 300 °C temperature and
annealed: (a) As-deposited, (b) 400 °C, (c) 500 °C and (d) 600 °C.
57
Chapter Four
Results and Discussion
Fig. (4.6): The variation of surface roughness with annealing temperature in the TiO2
films deposited on glass substrate.
4.3 Optical Properties
The optical properties of the TiO2 films deposited by pulsed laser deposition
technique are measured by UV-VIS spectrophotometer on glass substrate at
300 °C temperature in the range from 300nm to 900nm. The laser fluence energy
density is 0.4 J/cm2 and the oxygen pressure is maintained at 10-2 mbar various
annealing temperatures (400, 500 and 600) °C with film average thickness 200 nm.
The absorptance and transmittance have been studied. Also the optical energy
gap and optical constants have been determined.
58
Chapter Four
Results and Discussion
4.3.1 Optical Transmission (T)
The optical transmittance measurements of the TiO2 films depend stronger on
the annealing temperature as shown in fig. (4.7). It is found that average
transmittance of as-deposited TiO2 films is about 65% in the near-infrared region.
For all the films analyzed it is observed that the optical transmittance decreases
with increasing the annealing temperature.
The films annealed at 600 °C shows a significant decrease in the range from
350nm to 900nm transmittance as shown in the fig. (4.7.3). It is clearly, that the
increasing of the annealing temperature has an obvious effect on the transmittance
decreasing, and this is resulted from roughness increasing for film surface and
increase of the surface scattering of the light from obtained by increasing the
columnar growth with needle and rod like shape [44,104].
TiO2 films annealed at a higher temperature show a lower transmittance.
Because annealing treatment causes a film surface to be more rough which scatters
light [44].
4.3.2 Optical Absorption (A)
The optical absorbance of TiO2 films depends on the annealing temperature as
shown in fig. (4.8). Further observation shows that the absorbance of the TiO 2
films increases with increasing of annealing temperature. This is probably ascribed
to the increase of particle sizes and surface roughness. Therefore, the TiO 2 film
annealed at 600 ºC has the strongest absorbance.
Furthermore, the absorption edges of the TiO2 films have a small red shift
with increasing of annealing temperature. There are two possible factors resulting
in the red shift of absorption edge. One is that the increase of crystalline size can
cause red shift of absorption edge. The other is that the part phase transforms from
anatase to rutile leads to the decrease of band gap [99].
59
Chapter Four
Results and Discussion
1
2
3
4
Fig. (4.7): The optical transmission of TiO2 thin films annealed at: (1) 400 °C, (2) 500 °C,
(3) 600 °C and (4) compared all films.
61
Chapter Four
Results and Discussion
11
2
3
4
Fig.(4.8): The optical absorption spectra of TiO2 thin films annealed at: (1) 400 °C,
(2) 500 °C, (3) 600 °C and (4) compared all films.
61
Chapter Four
Results and Discussion
4.3.3 Optical Absorption Coefficient (α)
Fig )3.λ( shows the absorption coefficient )α( of the TiO2 thin films with
different annealing temperatures determined from absorbance measurements using
equation (2-14). The absorption coefficient of TiO2 thin films increases sharply in
the UV range, and then decreased gradually in the visible region because it is
inversely proportional to the transmittance. The absorption coefficient is increasing
with the annealing temperature increasing, its value is larger than (10 4 cm-1). This
can be linked with increase in grain size and it may be attributed to the light
scattering effect for its high surface roughness [105] .
Fig. (4.9): Absorption coefficient as a function of wavelength for the TiO2 thin films at
different annealing temperatures.
4.3.4 Optical Energy Gap (Eg)
The energy gap can be calculated from equation (2-10), the relations are
drawn between )αhυ(2 , )αhυ(1/2 and photon energy )hυ(,as in fig. )3.31( illustrates
62
Chapter Four
Results and Discussion
allowed direct transition electronic and fig. (4.11) illustrates allowed indirect
transition electronic.The energy gap value depends on the films deposition
conditions and its preparation method which influences in the crystalline
structure [106].
The variation in the structural properties and other variations is a reason for
making variation in energy gap. Figures obtained for all the other thin films have a
similar type of curve.
The respective values of Eg is obtained by extrapolation to (αhυ(n = 0. The Eg
values for direct and indirect band gap for all the thin films are summarized in table
(4.4). It is found in literature that TiO2 has a direct and indirect band gaps and the
band gap values changes according to the preparation parameters and conditions.
Table (4.4): Shows allowed direct band gap and allowed indirect band gap for different
annealing temperatures of TiO2 thin films.
Temperature (oC)
Allowed direct band Allowed indirect band
gap (eV)
gap (eV)
As-deposited at 300
3.74
3.49
400
3.7
3.39
500
3.68
3.35
600
3.55
3.1
From table (4.4) and the following figures, it can be observed that (E g) is
decreasing with the increasing of annealing temperature for all films. This result is
consistent with previous researches [47, 50]. Annealing led to increased levels of
localized near valence band and conduction band and these levels ready to receive
electrons and generate tails in the optical energy gap and tails is working toward
reducing the energy gap, or can be attributed decrease energy gap to the increased
size of particles in the films [47].
63
Chapter Four
Results and Discussion
1
2
3
4
Fig. (4.10): Allowed direct electronic transitions of TiO2 thin films annealed at:
(1) 400 °C, (2) 500 °C, (3) 600 °C and (4) compared all films.
64
Chapter Four
Results and Discussion
2
1
3
4
Fig. (4.11): Allowed indirect electronic transitions of TiO2 thin films annealed at:
(1) 400 °C, (2) 500 °C, (3) 600 °C and (4) compared all films.
65
Chapter Four
Results and Discussion
4.3.5 Refractive Index (n)
The refractive indices (n) of the TiO2 thin films are determined from
equation (2-12). Fig.(4.12) shows the variation in refractive index of TiO2 films in
the wavelength range of (350-900)nm. The increase in the annealing temperature
results in an increase in the refractive index in the visible/near infrared region. And
the refractive index decreases as the wavelength increases in the visible range. This
trend shows an increase of the value of refractive index with higher annealing
temperature. The increase may be attributed to higher packing density and the
annealing temperature influence on the morphology (change in crystalline
structure) of the films and hence caused change in the refractive index. The values
of the refractive index for the films of different annealing temperatures vary in the
range from 2.1 to 2.8 [44, 98, 107, 108]. Few researchers reported that the asdeposited or annealed TiO2 films have refractive index in the range of 2.1-2.9 and
annealing treatment caused refractive index to increase due to the enhancement of
crystallization [109, 110].
Fig. (4.12): Refractive index as a function of wavelength for the TiO2 thin films at
different annealing temperatures.
66
Chapter Four
Results and Discussion
4.3.6 Extinction Coefficient (Ko)
The extinction coefficient (Ko) is directly related to the absorption of light
then related to absorption coefficient () by the equation (2-13), so (Ko) can
measured by using the previous relation. The curves of extinction coefficient for asgrown and annealed TiO2 films are shown in fig. (4.13). Excitation coefficient
behaves in the same behavior of absorption coefficient )α( because they are joined
by previous relation, extinction coefficient increasing with increasing of annealing
temperature.
Fig. (4.13): Extinction Coefficient as a function of wavelength for the TiO2 thin films at
different annealing temperatures.
4.3.7 The Dielectric Constants (Ԑr, Ԑi)
Both real ) r( and imaginary ) i) dielectric constant are measured for prepared
films by using relations (2-17) and (2-18) respectively. Figures (4.14) and (4.15)
illustrate variation of ) r( and ) i) as a function of wavelength. The figures show
that in all samples the real part behaves like the refractive index because of the
smaller value of (Ko2) compared to (n2), while ( i) depends mainly on the (Ko)
67
Chapter Four
Results and Discussion
values, which is related to the variation of the absorption coefficient. This means
that real part and the imaginary part increases when the annealing temperature
increasing.
Fig. (4.14): Real (Ԑr) parts of the dielectric function as a function of wavelength for the
TiO2 thin films at different annealing temperatures.
Fig. (4.15): Imaginary (Ԑi) parts of the dielectric function as a function of wavelength for
the TiO2 thin films at different annealing temperatures.
68
Chapter Four
Results and Discussion
4.3.8 Optical Conductivity (σ)
Fig.(4.16), shows the variation of optical conductivity as a function of photon
energy for different annealing temperatures. The optical conductivity is calculated
by using equation (2-19). From fig. (4.16), we can see that the optical conductivity
increases with increasing photon energy. This suggests that the increase in optical
conductivity is due to electron exited by photon energy, and the optical
conductivity of the films increases with increasing of annealing temperature. High
absorption of thin film behavior is due to the photon-atom interaction. This effect
increases the optical conductivity.
Fig. (4.16): Optical conductivity as a function of photon energy for the TiO2 thin films at
different annealing temperatures.
69
10/23/2005
Conclusion and Future work
Chapter Five
Conclusion and Future Work
5.1 Conclusion
Nanostructured titanium dioxide thin films are prepared by pulsed laser
deposition techniques on the glass substrate. The effect of annealing temperature
on structure, morphology and optical properties of TiO2 thin films are studied by
XRD, AFM and UV-VIS measurements. The variation of annealing temperature
has a great influence on the structural and optical properties.
The XRD results reveal that the deposited thin film and annealed at 400 °C
of TiO2 have a good nanocrystalline tetragonal anatase phase structure. Thin
films annealed at 500 °C and 600 °C have mixed anatase and rutile phase
structure. And it is observed that the TiO2 films exhibit a polycrystalline
having (101), (110), (004), (200), (211) planes of high peak intensities, also
micro strain ) ( and grain size )g( increases while the texture coefficient )T c)
decreases with increasing of annealing temperature.
The AFM results show the slow growth of crystallite sizes for the as-grown
films and annealed films from 400 to 600 °C. The root mean square (RMS)
is increased from 50.5 to 114 nm as the annealing temperature is increased
up to 600 °C.
The average transmittance (T) of deposited TiO2 films is about 65% in the
near-infrared region. The film annealed at 600 °C has the least transmittance
among the films, while optical absorptance (A) is high at short wavelength,
therefore; the film is good to be detector within ultra-violet region range.
The optical absorption coefficient )α( of TiO2 films is about )α>314cm-1),
thus absorption coefficient has higher increase at wavelength ) <311 nm(.
This converts to a large probability that direct electronic transition will
happen.
Also optical properties of TiO2 thin films show that the films have allowed
direct transition and allowed indirect transition. Increasing of the annealing
71
Chapter Five
Conclusion and Future Work
temperature for all films cause a decrease in the optical band gap value and
an increase in the optical constants (refractive index (n), extinction
coefficient (Ko), real and imaginary parts of the dielectric constant and
optical conductivity ) ((.
71
Chapter Five
Conclusion and Future Work
5.2 Future Work
According to the results of this study, the following future studies are
suggested:
1- Effect of annealing on the electrical properties of nanostructure TiO2 films
prepared by PLD.
2- Study of structural, optical and electrical properties of nanocrystalline TiO2
films prepared by Chemical Spray Pyrolysis.
3- Studying the effect of different preparation conditions on the photolumincent
efficiency and Scanning Electron Microscopy (SEM) of the prepared
samples.
4- Study of nanostructured TiO2 thin films prepared by PLD for gas sensor
applications.
72
Chapter Five
Conclusion and Future Work
5.3 Publications
“Annealing Effect on the growth nanostructured TiO2 thin films by pulsed
laser deposition (PLD),” Accepted at Journal of Eng. & Tech., University of
Technology., No: 3291, (27/11/2012).
“Synthesis of nanostructured TiO2 thin films by pulsed laser deposition
(PLD) and the effect of annealing temperature on structural and
morphological properties,” Accepted at Ibn Al-Haitham Jour. for Pure &
Appl. Sci., University of Baghdad., No: 3/1791, (9/12/2012).
“Study of Annealing Effect on the Some Physical Properties of
Nanostructure TiO2 films prepared by PLD,” Accepted at Journal of the
colleg of the education, Al-Mustansiriya University., No: 94, (27/12/2012).
73
References
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References
Address: Iraq – Baghdad.
Email: Sarmadsapeeh@gmail.com.
Date of Discussion: 27/12/2012.
Thank you
88
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