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Republic of IraqMinistry of Higher Education  and Scientific ResearchUniversity of Al-Mustansiriya   College of EducationE...
‫ْ ِ ّ ِ َّ ْ ِ َّ ِ ِ‬                                   ‫بِسم اّلل الرْحن الرحي‬    ‫ْ ٌ ْ ُ‬                          ‫...
Examination Committee Certification         We certify that we have read this thesis entitled " Effect of Annealing on the...
iv
DedicationTo my family, friends andall 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...
Abstract     In this work, Nanostructured TiO2 thin films are grown by pulsed laserdeposition (PLD) technique on glass sub...
in the range from 3.49 to 3.1 eV, while the allowed direct band gap is found todecrease from 3.74 to 3.55 eV with the incr...
Table of ContentsDedicationAcknowledgmentAbstract………………………………………………………………………iList of Symbols………………………………...……………………….….vii...
2.4. Limitations and Advantages of PLD……….……….…………….…..272.5. Pulsed Laser Deposition of Nano-Structure Semiconductor….….....
3.2. Deposition Equipment………………………...….…...………………42   3.2.1. Nd: YAG Laser Source.…………………….……….…….……42   3.2.2. Pulsed Las...
4.3.8. Optical Conductivity (ζ).…………………….……...…….……69 Chapter Five (Conclusion and Future work)5.1. Conclusion ………………...……...
List of SymbolsSymbol                       Description  a                      Lattice constant (Å)  A                   ...
Nr                  Reflection number p              Pressure of the gas (mbar)R                       ReflectanceR       ...
List of AbbreviationsSymbol                     Description AFM                   Atomic Force Microscope CVD             ...
List of TablesTable                                                                  Page                                 ...
10/23/2005Introduction
Chapter One                                                               Introduction1.1 Introduction     Thin films are ...
Chapter One                                                              Introduction     There are many methods to prepar...
Chapter One                                                              Introduction(multielement), composite materials [...
Chapter One                                                                 Introduction1.4 The Crystal Structure of TiO 2...
Chapter One                                                               Introduction                    Fig. (1.2): Ruti...
Chapter One                                                               Introductionmore stable, but both will revert to...
Chapter One                                                               Introduction1.6 Literature Survey     Lofton, et...
Chapter One                                                              Introduction     Amor, et al., (1997) [30]: They ...
Chapter One                                                             Introduction     Dzibrou, et al., (2002) [34]: The...
Chapter One                                                              Introductionannealing time needed to transform an...
Chapter One                                                                Introductionfilms, with thermal annealing param...
Chapter One                                                               Introductionanatase/rutile phase transformation ...
Chapter One                                                               Introductionobtained by UV-VIS-IR spectrophotome...
Chapter One                                                             Introduction     X-ray diffraction studies showed ...
Chapter One                                                              Introductionbetween 0.95 and 1%. The band gaps ob...
Chapter One                                                                Introduction     Pomoni, et al., (2011) [52]: T...
Chapter One                                                            Introduction    Annealing was thus demonstrated to ...
Chapter One                                                            Introduction1.7 Aim of the Work    The main objecti...
10/23/2005Theoretical Part
Chapter Two                                                            Theoretical Part2.1 Introduction     This chapter i...
Chapter Two                                                            Theoretical Partare commercially available, and the...
Chapter Two                                                                      Theoretical Part        Table (2.1): Perf...
Chapter Two                                                            Theoretical Partgeometry have to be optimized for a...
Chapter Two                                                           Theoretical Part     To achieve congruent evaporatio...
Chapter Two                                                            Theoretical Partparallel to the target surface at a...
Chapter Two                                                              Theoretical Part     Where (u) is the thermal dif...
Chapter Two                                                            Theoretical Part2.3.3 Nucleation and Growth of Thin...
Chapter Two                                                           Theoretical Part    The following thin film growth m...
Chapter Two                                                             Theoretical Part2.5 Pulsed Laser Deposition of Nan...
Chapter Two                                                              Theoretical Part              n  2d sin    ………...
Chapter Two                                                           Theoretical PartSuch as the effect of substrate temp...
Chapter Two                                                              Theoretical Part     Where (λ) is the X-ray wavel...
Chapter Two                                                            Theoretical Part2.6.4 Atomic Force Microscopy (AFM)...
Chapter Two                                                          Theoretical Part   AFM images show slow growth of cry...
Chapter Two                                                            Theoretical Part   Fig. (2.6): Shows the fundamenta...
Chapter Two                                                             Theoretical Partcoefficient (α) is equal about (1 ...
Chapter Two                                                             Theoretical Part(a) Direct Allowed Transition     ...
Chapter Two                                                                 Theoretical Part(d) Forbidden Indirect Transit...
Chapter Two                                                           Theoretical Part2.7.4 Optical Constants     The extr...
Chapter Two                                                               Theoretical Part     The optical conductivity (ζ...
Chapter Two                                                               Theoretical Part     The curves of refractive in...
10/23/2005Experimental Work
Chapter Three                                                     Experimental Work3.1 Introduction     This chapter inclu...
Chapter Three                                                     Experimental Work3.2 Deposition Equipment     The basic ...
Chapter Three                                                         Experimental Work3.2.2 Pulsed Laser Deposition (PLD)...
Chapter Three                                                   Experimental Work                Fig. (3.3): Pulsed laser ...
Chapter Three                                                        Experimental Work3.2.3 Substrate Heater     The subst...
Chapter Three                                                       Experimental Work3.4 Substrate Preparation     We use ...
Chapter Three                                                       Experimental Workthin film surface and substrate botto...
Chapter Three                                                       Experimental Work     The device has been operated at ...
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld
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Effect of annealing on the structural and optical properties of nanostructured ti o2 films prepared by pld

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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

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  1. 1. Republic of IraqMinistry of Higher Education and Scientific ResearchUniversity of Al-Mustansiriya College of EducationEffect of Annealing on the Structural andOptical 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.
  2. 2. ‫ْ ِ ّ ِ َّ ْ ِ َّ ِ ِ‬ ‫بِسم اّلل الرْحن الرحي‬ ‫ْ ٌ ْ ُ‬ ‫َِ ْ َ‬‫))اّلل ه ُور الس َماوات َواأل ْرض َمثَل ه ُو ِر ِه َكشَك ٍة ِفهيَا ِمص َباح الْ ِمص َباح ِِف‬ ‫َّ ُ ُ َّ َ ِ َ ِ ُ‬ ‫ٍََ‬ ‫ْ َ‬ ‫ُزجاجة الزجاج ُة َََكَّنَّ َا ك ْوكب دُري يُوقَدُ ِمن َش ََر ٍة ُم َباركة َزيْ ُتوه ٍَة ال َشِقيَّة‬ ‫َْ ٍ‬ ‫َ َ ٌ ّ ِ ٌّ‬ ‫َ َ ٍ ُّ َ َ‬ ‫ٌ َ ٍ ِ َّ ُ‬ ‫َ ْ ُ‬ ‫ِ ُ‬ ‫ٍ َُ‬‫َوالغَ ْرِبيَّة يََكد َزْي ُُتَا يُِضء َولَ ْو لَ ْم تَ ْمسسه َنَ ٌر ه ُور عََل ه ُور َيَ ْدي اّلل ِل ُنو ِر ِه‬ ‫َمن يَشَ ا ُء َويَْضب اّلل األ ْمث َال ِللنَّاس َواّلل ِبك َش ٍء عَ ِلي((‬ ‫ْ ِ ُ َّ ُ َ َ ِ َّ ُ ُ ِّ َ ْ ٌ‬ ‫ْ‬ ‫صدَ ق اّلل الْ َعظي‬ ‫َ َ َّ ُ ِ ُ‬‫النور‪‬‬‫‪‬‬ ‫‪ii‬‬
  3. 3. Examination Committee Certification We certify that we have read this thesis entitled " Effect of Annealing on theStructural and Optical Properties of Nanostructured TiO2 Films Prepared by PLD" as anexamine committee, examined the student ( Sarmad Sabih Kaduory Al-Obaidi ) in itscontents and that, in our opinion meets the standard of thesis for the degree of Master ofScience 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 DagherTitle: Assistant Professor Title: Assistant ProfessorAddress: University of Technology Address: Al-Mustansiriyah UniversityDate: / /2013 Date: / /2013 (Member) (Member) Signature: Name: Dr. Ali Ahmed Yousif Al-Shammari Title: Assistant Professor Address: Al-Mustansiriyah University Date: / /2013 (Supervisor)Approved by the Council of the College of Education:Signature:Name: Dr. Ahmed Shayal GudibTitle: Assistant ProfessorAddress: Dean of College of Education, Al-Mustansiriyah University iii
  4. 4. iv
  5. 5. DedicationTo my family, friends andall the close people in my life Sarmad v
  6. 6. 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 tomy Supervisor’s Dr.Ali Ahmed Yousif Al-Shammari whom guided andsupported me in every possible way with them experience, motivation, and hepositive attitude. Also I am very thankful to all people who are working in the PhysicDepartment of the Education collage of AL-mustansiriyah University. I feel responsible to express my thanks and gratitude to all the peopleworking in the Laser Physics branch in the (University of Technology). I am very thankful to Dr. Khaled Z. Yahya and Prof.Dr.Adawiya J. Haiderfor their support, helpful and assistance. I am very grateful to staff of XRD, AFM labs, and material sciencesdirectorate 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 thankmy family whom supported me with their kind, patience and encouragement. Allah bless you all Sarmad vi
  7. 7. Abstract In this work, Nanostructured TiO2 thin films are grown by pulsed laserdeposition (PLD) technique on glass substrates. TiO2 thin films are then annealedat 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 havebeen 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 arepolycrystalline and orientations identical with literatures, also these results showthat increasing in grain size with increasing of annealing temperature. The XRDresults also reveal that the deposited thin film and annealed at 400 °C of TiO 2 haveanatase phase. Thin films annealed at 500 °C and 600 °C have mixed anatase andrutile phase. The Full Width at Half Maximum (FWHM) of the (101) peaks ofthese films decreases from 0.450° to 0.301° with increasing of annealingtemperature. The surface morphology of the thin films have been studied by using atomicforce microscopes (AFM). AFM measurements confirmed that the films grown bythis technique have good crystalline and homogeneous surface. The Root MeanSquare (RMS) value of thin films surface roughness increased with increasingannealing 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 thatthe transmission over than ~65% decreases with the increasing of annealingtemperatures. The allowed indirect optical band gap of the films is estimated to be vii
  8. 8. in the range from 3.49 to 3.1 eV, while the allowed direct band gap is found todecrease from 3.74 to 3.55 eV with the increase of annealing temperature. Therefractive index of the films is found from 2.1-2.8 in the range from 350nm to900nm. The extinction coefficient and the optical conductivity of the filmsincreases with annealing temperature. The real dielectric constant and theimaginary part increases when the annealing temperature increasing. viii
  9. 9. Table of ContentsDedicationAcknowledgmentAbstract………………………………………………………………………iList of Symbols………………………………...……………………….….viiList of Abbreviations……………...……………………………………..…ixList of Tables…………………………………...……………………….…..x Chapter One (Introduction)1.1. Introduction……………………………………………………………..11.2. Fundamentals of Pulsed Laser Deposition(PLD).…………………...…21.3. Chemical and Physical Properties of TiO2......…..………………..…...31.4. The Crystal Structure of TiO2………………...………………..………41.5. Applications of Nanostructured TiO2…...….………………….....……61.6. Literature Survey………………………………….………........………71.7. Aim of the Work………………………………………………………18 Chapter Two (Theoretical Part)2.1. Introduction………………………..…….…………………….………192.2. Pulsed Laser Deposition (PLD)….…………………...…..…….……..192.3. Mechanism of Pulsed Laser Deposition ………………….…………..22 2.3.1. The Interaction of the Laser Beam and Target………….……...22 2.3.2. Plasma Plume Formation…………………..…………………...25 2.3.2. Nucleation and Growth of Thin Films………..………….……..26 ix
  10. 10. 2.4. Limitations and Advantages of PLD……….……….…………….…..272.5. Pulsed Laser Deposition of Nano-Structure Semiconductor….…..…..282.6. Structural Properties……..………………………………………...….28 2.6.1. X-ray Diffraction ( XRD )……………...…………………...….28 2.6.2. Effect of Annealing on the X-ray Diffraction..……………...….29 2.6.3. Parameters Calculation………………….……………...…...….30 2.6.3.1. Full Width at Half Maximum (FWHM) (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. Micro Strains (δ)……….…………….....……..……….31 2.6.4 Atomic Force Microscopy (AFM)….………....…….……...…...322.7. Optical Properties of Crystalline Semiconductors .……..………...…..33 2.7.1. The Fundamental Absorption Edge ……………..……………..34 2.7.2. Absorption Regions ……………………………..…….……….34 2.7.2.1. High Absorption Region…......………………..……….34 2.7.2.2. Exponential Region..………...………………..……….34 2.7.2.3. Low Absorption Region...…...………………..……….35 2.7.3. The Electronic Transitions …………..…………..……………..35 2.7.3.1. Direct Transitions …………...………………..……….35 2.7.3.2. Indirect Transitions ……………………….…..……….36 2.7.4. Optical Constants…………………………….…..……………..38 2.7.5. Some Optical Properties of TiO2 Thin Film…..………………..39 Chapter Three (Experimental Work)3.1. Introduction….……………………..………………………………….41 x
  11. 11. 3.2. Deposition Equipment………………………...….…...………………42 3.2.1. Nd: YAG Laser Source.…………………….……….…….……42 3.2.2. Pulsed Laser Deposition (PLD) Technique……….….….….….43 3.2.3. Substrate Heater………………………………….….…….……45 3.2.4. Vacuum System…………..……………………….….….……..453.3. Target Preparation……………………………………....……….........453.4. Substrate Preparation…………………………………...……………..463.5. Characterization Measurements……………………………………….46 3.5.1. Thickness Measurement…...............…………………….……..46 3.5.2. Structural and Morphological Measurements….…….....………47 3.5.2.1. X-ray Diffraction (XRD)…………….....……..……….47 3.5.2.2. Atomic Force Microscopy (AFM)...…….….....……….47 3.5.3. Optical Measurements………...………………….…………….48 Chapter Four (Results and Discussion)4.1. Introduction…………………………………..………………………..504.2. Structural Properties……………………………….…...………….….50 4.2.1. X-ray Diffraction……....…………………….……...…….……50 4.2.2. Atomic Force Microscopy (AFM)………….……...……...……564.3. Optical Properties………………...…………….…....…………....…..58 4.3.1. Optical Transmission (T)…………………….……......…..……58 4.3.2. Optical Absorption (A)……….…………….……......…....……59 4.3.3. Optical Absorption Coefficient (α)…………………….….……62 4.3.4. Optical Energy Gap (Eg)...………………….……...…..….……62 4.3.5. Refractive Index (n)…...…………………….……....….………66 4.3.6. Extinction Coefficient (Ko)..………………….……......…….…67 4.3.7. The Dielectric Constants (Ԑr, Ԑi).…….………….……..….……67 xi
  12. 12. 4.3.8. Optical Conductivity (ζ).…………………….……...…….……69 Chapter Five (Conclusion and Future work)5.1. Conclusion ………………...……………………………………..705.2. Future Work ……..………………………...……………………..725.3. Publications………………...……………………………………..73References…….………………...……………………………………..74 xii
  13. 13. List of SymbolsSymbol Description a Lattice constant (Å) A Absorptance A Anatase α Absorption coefficient (cm-1) b Back flux (W/cm2) c Velocity of light in vacuum (m/s) t Thickness (nm) tp laser pulse width duration (s) d Inter planer spacing (Å) e Electron charge (C) Eb Binding energy of vaporization per atom Eab Ablation energy of the pulse laser (eV) Eg Energy gap(eV) Eph Energy of phonon (eV) F Laser fluence (J/cm2) Fth Approximate the fluence threshold for laser pulse g Average grain size (nm) h Plank constant (J. s) hυ Photon energy (eV) I Laser intensity (W/cm2) I Measured intensity Io JCPDS standard intensity ∆k Wave vector (cm-1) KB Boltzmann constant (J/K) Kₒ Extinction coefficient n Refractive index na Number density of atoms xiii
  14. 14. Nr Reflection number p Pressure of the gas (mbar)R ReflectanceR RutileT TransmittanceTₒ Temperature (ºC)Tc Texture coefficientTs Substrate temperature (K)u Thermal diffusion coefficient (m2/s) x Fringe width (cm)∆x Distance between two fringes (cm)Ss Stressδ Micro Strainsλ Wavelength (nm)λc Wavelength cut off (μm)ζ Optical conductivityθ Diffraction angle (deg.)εr Real part of dielectric constant (F/m)εi Imaginary part of dielectric constant (F/m)γF Free energies of the film surface (eV)γS Free energies of the substrate surface (eV)γI Free energies of the film-substrate interface (eV)υ Frequency (Hz)υo Critical frequency (Hz) xiv
  15. 15. List of AbbreviationsSymbol Description AFM Atomic Force Microscope CVD Chemical Vapor Deposition CSP Chemical Spray pyrolysis C.B. Conduction Band DSSC Dye-Sensitized Solar Cells FTIR Fourier Transform- Infrared SpectroscopyFWHM Full Width at Half Maximums (deg.) MBE Molecular Beam EpitaxialGAXRD Glancing Angle X-ray DiffractionJCPDS Joint Committee for Powder Diffraction Standards PEC Photoelectrochemical Cells PLD Pulsed Laser Deposition PL Photoluminescence RF Radio FrequencyRMS Root Mean Square RTA Rapid Thermal Annealing SEM Scanning Electron Microscope SHG Second Harmonic Generation SHI Swift Heavy Ion Irradiation TCOs Transparent Conducting Oxide Semiconductors TiO2 Titanium Dioxide TPD Thermal Pyrolysis Deposition TEVD Thermal Evaporation in Vacuum Deposition V.B. Valence Band XRD X-Ray Diffraction XPS X-Ray Photoelectron Spectroscopy xv
  16. 16. List of TablesTable Page Title No. No.(2.1) Performance features of Excimer and Nd: YAG lasers. 21(4.1) Lattice constants and interpllanar spacing of TiO2 films. 53 The obtained result of the structural properties from XRD(4.2) 54 for TiO2 thin films. Morphological characteristics from AFM images for TiO2 thin(4.3) 56 film. Shows allowed direct band gap and allowed indirect band gap(4.4) 63 for different annealing temperatures of TiO2 thin films. xvi
  17. 17. 10/23/2005Introduction
  18. 18. Chapter One Introduction1.1 Introduction Thin films are first made by (Busen & Grove) in 1852 by using (ChemicalReaction). In 1857, the scientist (Faraday) was able to obtain a thin metal film bymeans of (Thermal Evaporation) [1].The experimental and theoretical study ofsemiconductor nanocrystallites has generated tremendous technological andscientific interest recently due to the unique electronic and optical properties andexhibition of new quantum phenomena. In the semiconductor technology, laserinduced crystallization is used because it presents selective optical absorption andlow processing temperature [2]. Oxides reveal an excellent chemical andmechanical property and do not show deterioration. As one of the important wideband gap (Eg3 eV) oxides, TiO2 has been subject to extensive academic andtechnological 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 ofimproving the photocatalytic activity and optical absorption [4]. Titanium dioxide is a large band gap semiconductor of exceptional stabilitythat has diverse industrial applications. TiO2 thin films with their high refractiveindex have broad applications in optical coatings and waveguides [5].Titaniumdioxide occurs in three crystalline polymorphs: rutile (tetragonal), anatase(tetragonal), and brookite (orthorhombic) [2]. 1
  19. 19. 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 filmsdeposited by different techniques have been reported. For Pulsed laser depositionderived films, film properties such as crystallinity, particle size, degree ofhomogeneity, 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 thedeposition of titanium dioxide at different technological conditions on differentsubstrates. This supposes to result in the different structural and micro structuralproperties, 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 theoreticalinvestigations into laser-target interaction. Numerous experiments were carried outto verify the theoretical models. Ready (1963) and White (1963) studied theinteractions of intense laser beams with solid surfaces [7]. By 1965, Smith andTurner 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 almostany material can be vaporized and, thus, depositing a thin-film onto any substrate. PLD has several characteristics that distinguish it from other growth methodsand provide special advantages for the growth of chemically complex 2
  20. 20. Chapter One Introduction(multielement), composite materials [8], semiconductor, metallic, superconductorand insulating nanostructures [9]. In other words, the composition of the any targetmaterial can be preserved with in the film. This accomplishment is significantbecause it proved that PLD could be used to produce thin films with qualitiescomparable to those produced by Molecular Beam Epitaxy (MBE) [7]. The laser iscompletely separated from the actual deposition chamber. During an experiment,the laser beam is pointed onto a target inside the chamber through a viewport inalignment with the target. Under these unique conditions the deposition chambercan contain any working atmosphere. The pulsed laser deposition techniqueinvolves three main steps: ablation of the target material, formation of a highlyenergetic 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 solventin water or alcohol or HCl [10].3-Because the TiO2 is not solvent and has no reaction with water; therefore, it isused 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 TransparentConducting Oxide Semiconductors (TCOs) and high transparent in visible regionand 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, (3.69-3.78) eV for allowed and forbidden direct transition respectively [12] 3
  21. 21. Chapter One Introduction1.4 The Crystal Structure of TiO 2 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 phasestogether with brookite phase. For calcination processes above 700 ºC all anatasestructure becomes rutile, some authors also found that 500 ºC would be enough forphase transition from anatase to rutile when thermal treatment takes place. Thisform 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 andc = 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 impurityinfluence. This form is tetragonal its density is (4.23 gm/cm3) as in fig. (1.2). It hasenergy 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
  22. 22. 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 Åand 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 fromanatase phase and submitted to an annealing process in order to reach the stablerutile phase but brookite phase never appeared. The difference in these three crystalstructures can be attributed to various pressures and heats applied from rockformations in the earth. At lower temperatures the anatase and brookite phases are 5
  23. 23. Chapter One Introductionmore stable, but both will revert to the rutile phase when subjected to hightemperatures.1. 5 Applications of Nanostructured TiO 2 TiO2 nanostructure one of the oxides family has attracted significant attentionin recent years due to it interesting electrical [15] optical [16] magnetic propertiesand applications for catalysis [17] energy conversion [18] biomedicalapplications [19] functionalized hybrid materials [20] and nanocomposites [21]. Because of its semiconductivity, photoelectrical and photochemical activityunder UV light. TiO2 nanostructures can be used as dye-sensitized solar cells(DSSC( [22] and photoelectrochemical cells (PEC) [23] photocatalysis, chemicalsensors [24] self-cleaning coating [25] and TiO2/polymer nanocomposites [26],thesome applications of TiO2 is shown in fig. (1.4). Fig. (1.4): some applications of TiO2 6
  24. 24. Chapter One Introduction1.6 Literature Survey Lofton, et al., (1978) [27]: They studied titanium thin films which were amixture of titanium and TiO2. Auger electron spectroscopy and X-rayphotoelectron spectroscopy in combination with sputter profiling techniques wereemployed to study (100-500Å) titanium thin films. The composition of the filmswas studied as a function of substrate. The samples were prepared by the electronbeam 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 typicalpressure (p) of 10-8 Torr (1.33x10-6 Pa). The effect of different temperatures oneach titanium device was studied, as well as its effect on rate deposition. Korotcenkov and Han (1997) [28]: They prepared (Cu, Fe, Co, Ni)-dopedtitanium dioxide films deposited by spray pyrolysis. The annealing at 850-1030 ◦Cwas carried out in the atmosphere of the air. For structural analysis of tested filmsthey have been using X-ray diffraction, Scanning Electron Microscopy (SEM), andAtomic Force Microscopy (AFM) techniques. It was established that the dopingdid not improve thermal stability of both film morphology and the grain size. It wasmade a concluded that the increased contents of the fine dispersion phase ofTitanium dioxide in the doped metal oxide films, and the coalescence of this phaseduring thermal treatment were the main factors, responsible for observed changesin the morphology of the doped TiO2 films. Hiso Yanagi, et al., (1997) [29]: They prepared TiO2 thin films by spraypyrolysis of titanium films on glass substrates. Depending upon the substratetemperature, morphology of the deposited TiO2 films changed from irregularaggregates at 200 ◦C to homogeneous particles with a diameter of (50-100) nmabove (400 ◦C). 7
  25. 25. Chapter One Introduction Amor, et al., (1997) [30]: They studied the structural and optical properties ofTiO2 films type (brookite) prepared by sputtering method and energy gap forallowed direct transition was (3.3-3.5 eV). They also studied thermal treatment onits properties where they observed that the energy gap became (3.46-3.54 eV).XRD results observed films before thermal treatment were amorphous structure butafter thermal treatment they became polycrystalline. XU, et al., (1998) [31]: They studied the effect of calcinations temperatures onphotocatalytic activity of TiO2 films prepared by an electrophoretic deposition(EPD) method. TiO2 films fabricated on transparent electro-conductive glasssubstrates and were further characterized by X-ray diffraction (XRD), X-rayphotoelectron 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, whichconsisted of nano-sized particles. Patil (1999) [32]: studied the anatase thin films TiO2 prepared by sputteringPyrolysis technique, were obtained with good crystalline. Such films had indirectband gap energy of (3.08 eV) and direct band gap energy of (3.65 eV). Films madenear 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 ◦Cwere anatase. Sekiya, et al., (2000) [33]: They studied absorption spectra of anatase TiO2single crystals heat-treated under oxygen atmosphere. The optical properties hadbeen grown by chemical vapor transport reaction as grown crystals having bluecolor were heat-treated under oxygen atmosphere, the change in crystal color fromblue through yellow to colorless depending on oxygen annealing was detected byoptical absorption spectra. 8
  26. 26. Chapter One Introduction Dzibrou, et al., (2002) [34]: They deposited TiO2 thin films on quartz andsilicon wafers, by PLD method using Nd: YAG pulsed laser (λ=355nm, 10 Hz)with laser energy density of 1.5 J/cm2. The thin films were thermally treated attemperatures of 300 °C, 400 and 500 °C in air for 1 hour. The coatings obtainedwere uniform, smooth with very good optical properties. The sample annealed atlower temperature had the characteristic appearance of an amorphous material. Thesamples treated at 400°C and 500 °C were crystallized. TiO2 had direct and indirectband gaps. The band gap values for both transitions were different in comparison tothe well-known value of 3.03 eV for the indirect band gaps and 3.43eV for thedirect. Wang, et al., (2002) [35]: They studied the optical properties of anatase TiO 2thin films prepared by aqueous sol-gel process at low temperature TiO2. Thin filmswere spin-coated on Si (100) substrates via an aqueous sol-gel, and were annealedin air at different temperatures up to 550 °C for 1h. X-Ray diffractometry indicatedthat crystallization into anatase started at 350 °C. The 350 °C-annealed films werefurther characterized by auger electron spectroscopy, X-ray photoelectronspectroscopy, and variable angle spectroscopic ellipsometry. The results showedthat homogeneous, carbon-free TiO2 films with high refractive index (n=2.3 at550 nm) were successfully obtained under an annealing temperature as low as350 °C. The indirect and direct optical absorption band gaps of the anatase filmwere estimated as 3.23 and 3.80 eV, respectively. Shinguu, et al., (2003) [36]:They studied the structural properties andmorphologies of TiO2 thin films, in which they were deposited on Si(100) andSi(111) substrates by using ArF excimer laser (operating with wavelength 248 nmat 500 ºC) .The films have been annealed for 10 hours at the temperature 600 ºC, inoxygen and air flow. The TiO2 film deposited on (111)-oriented silicon exhibited abetter anatase crystalline than that on (100)-oriented silicon. Whereas a higher 9
  27. 27. Chapter One Introductionannealing time needed to transform anatase structure into rutile structure for filmsdeposited on Si (111) than on Si (100). The AFM images showed that the substrateorientation had no great effect on the surface morphologies for both anatase as-deposited films and rutile annealed films. Tien, et al., (2004) [37]: They deposited TiO2 thin films on sapphire by usingArF 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 evaporatedTi particles with buffer O2 gas. The dependence of the buffer O2 gas pressure wasstudied by spectroscopy of ablation plume, thickness of films, morphology of thesurface using SEM and AFM micrographs, XRD patterns and Raman spectra. Themorphology showed the formation of nanostructure by interactions of evaporatedTi particles with the buffer O2 gas. The structures of the PLD thin films showedepitaxial growths in the high substrate temperature (500 °C) and an appearance ofanatase at high buffer O2 gas pressure owing to the contributions of the TiOmolecules. Suda, et al., (2004) [38]: They prepared TiO2 films on different substrate atdifferent 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) resultsindicated that the thin films fabricated at the optimized conditions showed theintense near band PL emissions. Stamate, et al., (2005) [39]: They analyzed the optical properties of TiO2 thinfilms deposited through a d.c. magnetron sputtering method on glass made. A strong dependence between the value of TiO2 optical band gap andargon/oxygen ratios had been revealed. Changes in optical properties of TiO 2 thin 11
  28. 28. Chapter One Introductionfilms, with thermal annealing parameters. The optical band gap varies from 3eV to3.4eV as function of oxygen/argon ratios. Caricato, et al., (2005) [40]: They studied nanostructured TiO2 thin filmsprepared by (PLD) KrF excimer pulsed laser system (wavelength = 248 nm) onindium-doped tin oxide (ITO) substrates under different substrate temperature andpressure conditions (Tₒ = 250, 400,500 and 600 °C, p = 10-2 and 10-1 Torr). AFMresults showed the samples prepared at 400 °C have much more uniform surfacesand smaller particle size than that prepared at 600 °C. The XPS results indicatedthat the binding energy of the Ti core level system pressure was dependent onsubstrate temperature. However, under 10-1 Torr, only anatase phase was observedeven at the temperature higher than the commonly reported anatase-to-rutile phasetransition range (~ 600 °C). Deshmukh, et al., (2006) [41]: They studied TiO2 thin films deposited ontoglass substrates by means of spray pyrolysis method. The thin films were depositedat three different temperatures of 350,400 and 450 °C. As deposited thin films wereamorphous having (100-300 nm.) thickness, the thin films were subsequentlyannealed at 500°C in air for 2h. Structural, optical and electrical properties of TiO 2thin films had been studied as well. Polycrystalline thin films with rutile crystalstructure, as evidenced from X-ray diffraction pattern, were obtained with majorreflection along (110). Surface morphology and growth stage based on atomicforce microscopy measurements were discussed. Optical study showed that TiO 2possesses direct optical transition with band gap of (3.4 eV) Mere, et al., (2006) [42]:They studied the structural and electricalcharacterization of TiO2 films grown by spray pyrolysis onto silicon wafers atsubstrate temperature between (315 °C and 500 °C) using pulsed spray solution feedfollowed by annealing in temperature interval from (500 to 800 °C) in air.According to FTIR (Fourier Transform Infra-Red), XRD, and Raman, the 11
  29. 29. Chapter One Introductionanatase/rutile phase transformation temperature was found to depend on the filmdeposition temperature. Film thickness and refractive index were determined byEllipsometry, giving refractive index (2.1-2.3) and (2.2-2.6) for anatase and rutilerespectively. According to AFM (Atomic Force Microscopic), film roughnessincreased with annealing temperature from ( 700 to 800 °C) from ( 0.60 to1.10 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 activationenergy for TiO2 films with anatase and rutile structure was found to be (100 and 60meV), respectively. Nambara and Yoshida (2007) [43]: They studied the crystalline rutile typetitanium dioxide (TiO2) thin films which were prepared by (PLD) at substratetemperature 850 °C. The optical properties of the present rutile films were differentfrom 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 bulkvalue, they considered quantum size and strain effects of PLD-TiO2 crystalline. Hassan, et al., (2008) [44]: They studied the effects of annealing temperatureon optical properties of anatase. TiO2 thin films were grown by radio frequencymagnetron sputtering on glass substrates at high sputtering pressure and roomtemperature. The anatase films were then annealed at (300-600 ᵒC) in air for 1h. Toexamine the substrates and morphology of the films, X-ray diffraction. Atomicforce microscopy (AFM) methods were used respectively. From (XRD) patterns ofthe TiO2 films, it was found that the as-deposited film showed some differencescompared with annealed films, and the intensities of the peaks of the crystallinephase increased with the increase of annealing temperature. From (AFM) images,the distinct variations in the morphology of the films were also observed. Theoptical constants were characterized using the transmission spectra of the films 12
  30. 30. Chapter One Introductionobtained by UV-VIS-IR spectrophotometer. The refractive index of films wasfound from (2.31-2.35) in the visible range. The extinction coefficient was nearlyzero in the visible range but increased with annealing temperature. The allowedindirect optical band gap of the films was estimated to be in the range from (3.39 to3.42 eV), which showed to be a small variation. The allowed direct band gap wasfound to increase from (3.67 to 3.72 eV). Walczak, et al., (2008) [45]: They studied the effect of oxygen pressure onthe structural and morphological characterization of TiO2 thin films deposited on Si(100) by using KrF Excimer laser operated at wavelength of 248 nm and repetitionrate 5Hz . The laser energy density was about 2 J/cm2). They found that thedecreasing of oxygen pressure from (10-2 Torr to 10-1 Torr) produced highlyhomogeneous nanostructured morphology with grain size as small as 40 nm andhigh 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 byusing three different Nd: YAG laser wavelengths (266nm, 532nm and 355nm). They found that the films grown at λ=266 nm has smallest nanoparticles (withaverage diameter 25 nm) and the narrowest size distribution was obtained byablation at 266 nm under 0.05 Pa of oxygen. The effects of temperature on thestructural and optical properties of these films have been investigatedsystematically by XRD, SEM, FTIR, and PL spectra. Sankar and Gopchandran (2009) [47]: They studied the effect of annealingtemperature (973 and 1173 K) on the structural, morphological, electrical andoptical properties of nanostructured titanium dioxide thin films were preparedusing reactive pulsed laser ablation technique. The structural, electrical and opticalproperties of TiO2 films are found to be sensitive to annealing temperature and aredescribed with GIXRD, SEM, AFM, UV-VIS spectroscopy and electrical studies. 13
  31. 31. Chapter One Introduction X-ray diffraction studies showed that the as-deposited films were amorphousand at first changed to anatase and then to rutile phase with increase of annealingtemperature. The average grain size increases with increase in annealingtemperature. For the as deposited film, the value of band gap is observed to be3.11 eV. It was shifted to 3.19 eV for the film annealed at 973 K, which is observedto be anatase in crystal structure. Annealing at 1173 K resulted in reduction of theband gap to 3.07 eV. Mathews, et al., (2009) [48]: They studied nanostructured TiO2 thin filmswere deposited on glass substrates by sol-gel dip coating technique. The structural,morphological and optical characterizations of the as deposited and annealed filmswere carried out using X-ray diffraction (XRD), Raman spectroscopy, atomic forcemicroscopy (AFM), and UV-VIS transmittance spectroscopy. As-deposited filmswere amorphous, and the XRD studies showed that the formation of anatase phasewas initiated at annealing temperature close to 400 ºC. The grain size of the filmannealed at 600 ºC was about 20 nm. The lattice parameters for the films annealedat 600 ºC were a = 3.7862 Å and c = 9.5172 Å, which is close to the reported valuesof anatase phase. Band gap of the as deposited film was estimated as 3.42 eV andwas found to decrease with the annealing temperature. At 550 nm the refractiveindex of the films annealed at 600 ºC was 2.11, which is low compared to a porefree anatase TiO2. Igwe, et al., (2010) [49]: They studied the effect of thermal annealing undervarious temperatures, 100, 150, 200, 300 and 399 ºC on the optical properties oftitanium Oxide thin films prepared by chemical bath deposition technique,deposited on glass substrates. The thermal treatment streamlined the properties ofthe oxide films. The films are transparent in the entire regions of theelectromagnetic spectrum, firmly adhered to the substrate and resistant tochemicals. The transmittance is between 20 and 95% while the reflectance is 14
  32. 32. Chapter One Introductionbetween 0.95 and 1%. The band gaps obtained under various thermal treatmentsare between 2.50 and 3.0 eV. The refractive index is between 1.52 and 2.55. Thethickness achieved is in the range of 0.12-0.14 µm. Pawar, et al., (2011) [50]: They prepared TiO2 thin films on glass substratesusing spin coating technique and the effect of annealing temperature (400 - 700 ºC)on structural, microstructural, electrical and optical properties were studied. TheX-ray diffraction and Atomic force microscopy measurements confirmed that thefilms grown by this technique have good crystalline tetragonal mixed anatase andrutile phase structure and homogeneous surface. The study also reveals that theRMS value of thin film roughness increases from 7 to 19 nm. The surfacemorphology (SEM) of the TiO2 film showed that the nanoparticles are fine with anaverage grain size of about 50 - 60 nm. The optical band gap slightly decreasesfrom 3.26 - 3.24 eV. Sankar, et al., (2011) [51]: They prepared Titanium dioxide thin films weredeposited on quartz substrates kept at different O2 pressures using pulsed laserdeposition technique. The effects of reactive atmosphere and annealing temperatureon the structural, morphological, electrical and optical properties of the films arediscussed. Growth of films with morphology consisting of spontaneously orderednanostructures is reported. The films growth under an oxygen partial pressure of3x10-4 Pa consist in nanoislands with voids in between them whereas the filmgrowth under an oxygen partial pressure of 1x10-4 Pa, after having being subjectedto annealing at 500 ºC, consists in nanosized elongated grains uniformly distributedall over the surface. The growth of nanocrystallites with the increase in annealingtemperature is explained on the basis of the critical nuclei-size model. Thestructural, morphological, optical and electrical properties of titanium oxide thinfilms are found to be strongly influenced by the thermodynamics involving reactiveatmosphere during deposition and annealing temperature. 15
  33. 33. Chapter One Introduction Pomoni, et al., (2011) [52]: They studied the effect of thermal treatment onstructure, electrical conductivity and transient photoconductivity behavior ofthiourea modified nanocrystalline titanium dioxide (TiO2) thin films were preparedby 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 gradualincrease in the transmittance is observed with increase of the wavelength andtransmittance values of 60-70% are recorded for the wavelengths 600-900 nm. Forthe films heat treated at 500 and 600 ºC, the transmittance values appearsignificantly reduced in comparison to those for the film treated at 400 ºC. Furtherincrease of the treatment temperature up to 1000 ºC does not practically influencethe transmittance of the films. Average crystallite sizes a small increase from 28.2to 58.4 nm with temperature for anatase crystallites. The rutile crystallites appear at800 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 annealingtemperature on The crystal structure, morphology, and transmittance of TiO2 andW-TiO2 bi-layer thin films prepared by RF magnetron sputtering onto glasssubstrates and tungsten was deposited onto these thin films (deposition time15-60 s) to form W-TiO2 bi-layer thin films. Amorphous, rutile, and anatase TiO2phases were observed in the TiO2 and W-TiO2 bi-layer thin films. Tungstenthickness and annealing temperature had large effects on the transmittance of theW-TiO2 thin films. The W-TiO2 bi-layer thin films with a tungsten deposition timeof 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-TiO2bi-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 visiblelight. When the annealing temperature of the W–TiO2 bi-layer thin films wasincreased from 200 to 400 ºC, the band gap energy shifted from 3.158 to 3.098 eV. 16
  34. 34. Chapter One Introduction Annealing was thus demonstrated to be another important method to decreasethe band gap energy of TiO2-based thin films. Thakurdesai, et al., (2012) [54]: They studied the effect of Rapid ThermalAnnealing (RTA) on Nanocrystalline TiO2 by Swift Heavy Ion Irradiation (SHI). TiO2 were deposited using Pulsed Laser Deposition (PLD) method on fusedsilica Substrate in oxygen atmosphere. These films are annealed at 350 ºC for 2minutes in oxygen atmosphere by Rapid Thermal Annealing (RTA) method.During RTA processing, the temperature rises abruptly and this thermal instabilityis expected to alter surface morphology, structural and optical properties ofnanocrystalline TiO2 film. The effect of RTA processing on the shape and size ofTiO2 nanoparticles is studied by Atomic Force Microscopy (AFM) and ScanningElectron Microscopy (SEM). Glancing Angle X-ray Diffraction (GAXRD) studiesare carried to investigate structural changes induced by RTA processing. Opticalcharacterization is carried out by UV-VIS spectroscopy and Photoluminescence(PL) spectroscopy. The changes observed in structural and optical properties ofnanocrystalline TiO2 thin films after RTA processing are attributed to theannihilation of SHI induced defects. 17
  35. 35. Chapter One Introduction1.7 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
  36. 36. 10/23/2005Theoretical Part
  37. 37. Chapter Two Theoretical Part2.1 Introduction This chapter introduces the basics of the laser ablation. Topics like lasertarget-interaction and formation of the plasma plume will be discussed, as well asprocess parameters and formation of the deposit. Also this chapter includes ageneral 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 fordepositing thin films. In the process of laser ablation, short and high-energetic laserpulses are used to evaporate matter from a target surface. As a result, a supersonicjet of particles, called also (plume), due to its form (see Fig. 2.1), is ejected fromthe target surface and expands away from the target with a strong forward-directedvelocity distribution. The ablated particles condense on a substrate placed oppositeto the target. The ablation process takes place in a vacuum chamber- either invacuum or in the presence of some background gas. The laser pulses are guided tothe 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 aconstant speed to achieve a homogeneous ablation process. The possibility of amultitarget rotating wheel in the vacuum chamber enables more efficient andcomplex processes. Multilayers and alloy films can be grown from elementarytargets by moving them alternately into the laser focal point. The high energy density used in a typical PLD process is able to ablate almostevery material, and by controlling the process parameters, high-quality films can begrown reliably in a short period of time compared to other growth techniques(MBE,Sputtering). Another known advantage of the PLD technique is the accuratestoichiometric transfer from target to film. There are several kinds of lasers, which 19
  38. 38. Chapter Two Theoretical Partare commercially available, and the choice of Excimer lasers (KrF, ArF, XeCl) arewidely used to deposit complex oxide films because of the larger absorptioncoefficient and small reflectivity of materials at their operating wavelengths [55],Nd: YAG lasers are also effective from the same point of view. For the presentwork, Nd: YAG laser is used. Table (2.1) has performance parameters for currentexcimer and Nd: YAG systems at the 248 nm and 3nd harmonic 266 nmwavelengths respectively because these wavelengths are the most popular for PLD. The temperature could be kept constant by means of an automatedtemperature controller, capable to program and control several ramps and dwellswith user-defined heating and cooling rates. The thermal coupling between heaterand substrate is achieved through appropriate amount of conductive silver in theback side of the substrate. Moreover, several gases (O2,N2,H2, Ar) can beintroduced in the deposition chamber if the presence of any background gas isrequired for the film growth. The flow and the pressure of each gas is controlled bymeans 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
  39. 39. Chapter Two Theoretical Part Table (2.1): Performance features of Excimer and Nd: YAG lasers [57, 58]. Parameter Excimer System Nd:YAG System Wavelength 248 nm 1064 and 532 nm (nanometers) Output Energy 100 - 1200 mJ 100 - 1000 mJ (millijoules)Repetition Rate (Hertz) Variable, 1 - 200 Hz Fixed, 1 - 30 Hz Shot-to-Shot Stability 0.5 - 1%, RMS 8 - 12%, RMS (RMS) -High power output -Output energy sufficient for laser -Good stability ablation Advantages -Flexibility for tuning -Simple maintenance -Laser output parameter -Compact system -Short operation life time. -Large energy drop for the 3rd -Complicated maintenance harmonic mode Disadvantages -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 thelarge number of interacting parameters that govern the growth process and hencethe 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 PLDtechnique can be profited. In practice, parameters like laser settings and experiment 21
  40. 40. Chapter Two Theoretical Partgeometry have to be optimized for a given system and be kept constant, whileanother parameters like substrate temperature, chamber pressure and backgroundgas 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 beenformulated in the attempt to explain the processes that govern the PLD ablationprocess. These studies have shown that the ablation process is not governed by asingle mechanism but by multiple mechanisms that arise due to the laser-targetinteraction [57]. Ideally the plasma plume produced should have the samestoichiometry 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 lightfrom a CW laser source, and then this would allow a significant amount of theincident power to be conducted into the bulk of the target. The subsequent meltingand evaporation of the surface would essentially be thermal i.e. the differencebetween the melting points and vapor pressures of the target constituents wouldcause them to evaporate at different rates so that the composition of the evaporatedmaterial would change with time and would not represent that of the target. Thisincongruent evaporation leads to films with very different stoichiometry from thetarget [60]. 22
  41. 41. Chapter Two Theoretical Part To achieve congruent evaporation the energy from the laser must be dumpedinto the target surface rapidly, to prevent a significant transport of heat into thesubsurface material, so that the melting and vapor points of the target constituentsare achieved near simultaneously. The high laser power density that this implies ismost readily achieved with a pulsed or Q-switched source focused to a small spoton the target. If the energy density is below the ablation threshold for the materialthen no material will be removed at all, though some elements may segregate to thesurface [61, 62]. In order for the target material to be ablated the absorbed laser pulse energymust be greater than the binding energy of an atom to the surface which is theenergy of vaporization per atom, Eab > Eb [63]. In general the interaction between the laser radiation and the solid materialtakes 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 andas a result the material heats up to very high temperatures in a very short time.Then, the electron subsystem will transfer the energy to the lattice, by means ofelectron-phonon coupling [60, 64]. When the focused laser pulse arrives at thetarget surface the photons are absorbed by the surface and its temperature begins torise. The rate of this surface heating, and therefore the actual peak temperaturereached, depends on many factors: most importantly the actual volume of materialbeing heated. This will depend not only upon how tightly the laser is focused butalso on the optical penetration depth of the material. If this depth is small then thelaser energy is absorbed within a much smaller volume. This implies that werequire a wavelength for which the target is essentially opaque and it is in generaltrue that the absorption depth increases with wavelength. The rate of heating is alsodetermined by the thermal diffusivity of the target and the laser pulse energy andduration. In a high vacuum chamber, elementary or alloy targets are struck at anangle of 45o by pulsed and focused laser beam. The atoms and ions ablated fromthe target are deposited on substrate, which is mostly attached with the surface 23
  42. 42. Chapter Two Theoretical Partparallel to the target surface at a target-to-substrate distance of typically2-10 cm [31]. In PLD technique, the target materials are first sputtered (or sayablated) into a plasma plume by a focused laser beam an angle of 45 o. Thematerials ablated then flow (or fly) onto the substrate surface, on which the desiredthin films are developed. Therefore, the interaction of intense laser which mattersplays an important role in PLD process [65]. The incident laser pulse induces extremely rapid heating of significantmass/volume of the target material. This may cause phase transition and introducehigh amplitude stress in the solid target. The output of pulsed laser is focused ontoa target material maintained in vacuum or with an ambient gas. The target isusually rotated in order to avoid repeated ablation from the same spot on the target.Ablation Thresholds The ablation threshold is the amount of energy needed for the ablation processto 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 widthduration (s). The ablation threshold for dielectrics and metals vary greatly becausethe 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 propertiesThe following equation can approximate the fluence threshold for laser pulsedurations that are larger than 10 picoseconds: [66] 1 (ut p ) 2 Ebna Fth  …………………………………. (2-2)  24
  43. 43. Chapter Two Theoretical Part Where (u) is the thermal diffusion coefficient (m2/s), (Eb) the binding energyof 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 plumeformation in different mediums.These models give insight to plasma plumeformation down to the picosecond time scale and with different imaging techniquescan provide visual aids [67, 68]. Usual laser flux densities required for mostmaterials to generate a plasma plume are greater than 105 W/cm2 [57]. When theablation threshold is reached, the ejection of electrons, ions, and neutral particlesform a shock wave followed directly by the plasma plume, typical temperatures ofthese plasmas can be in excess of tens of thousands of kelvin [67]. The materialplasma vapor plume becomes apparent in the nanosecond time scale and has asupersonic propagation velocity of approximately 106 cm/s [68].The emitted lightand the color of the plume are caused by fluorescence and recombination processesin the plasma. The pressure and the laser fluence both have significant effect on theshape, size of the plume [59]. As shown in fig. (2.2). Fig. (2.2): Shadowgraph of plume at 1200ps Source [57]. 25
  44. 44. Chapter Two Theoretical Part2.3.3 Nucleation and Growth of Thin Films The Volmer-Weber, Frank-van der Merwe and Stranski-Krastinov nucleationand growth modes explain the nucleation and growth of thin films close tothermodynamic equilibrium. Each growth mode is governed by the balancebetween the free energies of the film surface (γF), substrate surface (γS), and thefilm-substrate interface (γI) [69]. For the Volmer-Weber mode there is no bondingbetween the film and substrate because the total surface energy is greater than thesubstrate energy, γF + γI > γS, this results in 3-dimensional island growth. WhenγF + γI < γS this is characterized as Frank-van der Merwe growth mode [69]. Through nucleation and island clustering these films grow as full-monolayerswith strong bonding between the film and substrate, they are a monolayer thick andcompletely combine before other island clusters develop to form the nextmonolayer [70]. The Frank-van der Merwe growth mode is characteristic ofhomoepitaxial thin film growth. The Stranski-Krastinov mode can occur duringheteroepitaxial growth due to the lattice mismatch between the substrate anddeposited thin film [69]. Initially the growth is monolayer but becomes3-dimensional island growth due to a biaxial strain induced by the latticemismatch [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) Stranski- Krastanov Source [69]. 26
  45. 45. Chapter Two Theoretical Part The following thin film growth modes provide us with a good understandingof the nucleation, growth, and morphology of thin film growth when close tothermodynamic equilibrium. When films are not grown close to thermodynamicequilibrium, kinetic effects will lead to different growth modes, additioninformation 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
  46. 46. Chapter Two Theoretical Part2.5 Pulsed Laser Deposition of Nanostructure…...Semiconductor Earlier a seemingly esoteric technique of Pulsed Laser Deposition (PLD) hasemerged as a potential methodology for growing nanostructures of variousmaterials including semiconductors [73]. Since it is a cold-wall processing, which excites only the beam focused areason the target enabling a clean ambient, it is highly suited for the growth ofnanostructures with high chemical purity and controlled Stoichiometry. The other characteristics of PLD such as its ability to create high-energysource particles, permitting high quality film growth at low substrate temperatures[74], simple and inexpensive experimental setup, possible operation in highambient gas pressure, and sequential multi-target and multi-component materialscongruent evaporation make it particularly suited for the growth of oxide thin filmsand nanostructures. In this section we shall present and discuss a few representative cases wherePLD has been successfully applied for the growth of semiconductors thin films andnanostructures. These cases of various semiconductors also illustrate the currenttrend and the future promise that PLD holds.2.6 Structural Properties2.6.1 X-ray Diffraction (XRD) X-ray diffraction could be used to define the preferred orientation, and fromthe diffrograms one can calculate the average grain size and determines whetherthe deposited films suffer from stress or not. These constants change with structuralchange caused by the different parameters such as deposition technique, doping,substrate and annealing. The Braggs condition for the diffraction can be written as [75]: 28
  47. 47. Chapter Two Theoretical Part n  2d sin  …………….…….…………………. (2-3) Where (n) is integer that indicates the order of the reflection, (θ) is Braggangle, and (λ) 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 beamis known. Fig. (2.4) shows the X-ray diffraction patterns of nanocrystalline TiO2 powderprepared by sol-gel method annealed at 400 - 700 °C temperatures with a fixedannealing time of 1 h in air. The effect of annealing temperature on the crystallinityof TiO2 can be understood from the figure. TiO2 has been crystallized in atetragonal 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 structuralmaterials and therefore a change observed in the spectrum of its X-ray diffraction. 29
  48. 48. Chapter Two Theoretical PartSuch as the effect of substrate temperatures, doping, nanoscale structure, annealingand other factors. We interested in the effect of annealing. The effect of annealing is an important factor in determining the crystalstructure of polycrystalline materials, and as especially nanostructures byincreasing the grain size and decrease boundaries grains in most cases, thusincreasing the crystallization of the material and decrease defects inside them andthe granting of atoms of the material enough energy to rearrange themselves insidelattice. The crystallized material means, of course, a clear increase in the intensityof peaks belonging to the levels, found during the software of modern used foraccounts that these increases are accompanied by a decrease in the values ofFWHM with a deviation toward values (2θ) least, which confirms that thetemperature role in increasing the distance between the levels of crystalline (d)because the relationship between (d) and (Sinθ) an inverse relationship accordingto the Braggs law [76,44].2.6.3 Parameters Calculation Normally XRD is used to calculate different parameters which could be usedto 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 isequal to the width of the line profile (in degrees) at the half of the maximumintensity.2.6.3.2 Average Grain Size (g) The average grain size (g), which can be estimated using the Scherer’sformula: [77] g  (0.94  ) /( ( 2 ) cos  ) ...….….……..….…….. (2-4) 31
  49. 49. Chapter Two Theoretical Part Where (λ) is the X-ray wavelength (Å), Δ (2θ) FWHM (radian) and (θ) Braggdiffraction angle of the XRD peak (degree).2.6.3.3 Texture Coefficient (Tc) To describe the preferential orientation, the texture coefficient, T C (hkl) iscalculated using the expression [78]: I (hkl ) I 0 (hkl ) TC (hkl )  ……………… (2-5) N r  I (hkl ) I 0 (hkl ) 1 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   .………………... (2-6) 2c13 c Where (c) and (co) are the lattice parameter of the thin film and TiO2 thin filmobtained from JCPDS respectively. The value of the elastic constant (cij) fromsingle crystalline TiO2 are used, c11=208.8 GPa, c33=213.8 GPa, c12=119.7 GPa andc13=104.2 GPa.2.6.3.5 Micro Strains (δ) This strain can be calculated from the formula [79]: c  c Strain ( )   100% ..……………….. (2-7) c 31
  50. 50. Chapter Two Theoretical Part2.6.4 Atomic Force Microscopy (AFM) Atomic force microscopy (AFM) employs a microscopic tip on a cantileverthat deflects a laser beam depending on surface morphology and properties throughan interaction between the tip and the surface. The signal is measured with aphotodetector, amplified and converted into an image display, AFM can beperformed in contact mode and tapping mode [80]. The investigated materialsinclude thin and thick coatings, semiconductors, ceramics, metals,micromechanical properties of biological samples, nucleic acids, polymers andbiomaterials, to name a few [81]. Fig. (2.5) shows nanostructured anatase TiO2 thinfilms which are grown by radio frequency magnetron sputtering on glass substratesat 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 smoothsurface 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
  51. 51. Chapter Two Theoretical Part AFM images show slow growth of crystallite sizes for the as-grown films andannealed films.2.7 Optical properties of Crystalline Semiconductors The process of basically absorptivity in crystalline semiconductors forincident rays happens when incident photon gives its energy which was equal orlarger than forbidden energy gap (Eg) to conduction band by absorbing thatincident photon [82]. h  Eg ……….……………………………….…. (2-8) Where (υ) frequency in (Hz.) and (h) Plank constant (6.625*10-34 j.sec.) Spectroscopy of incident rays region which start electrons in it transporting iscalled (fundamental absorption edge) which equals the difference between bottomconduction band and top valance band as in fig. (2.6) where ( λ c ) is cut offwavelength [83]. When (Eg) equal to (Eg=hυo) where (υo) is called critical frequency and thewavelength that opposite to it called wavelength cut off (λ c), this process happenswhen incident energy photon equals to width of forbidden energy gap which can beexpressed in the following equation [83]: hc 1.24 c ( m)   …...……….….. (2-9) Eg E g (eV ) Where (c) is speed of light in vacuum and (λc) is wavelength cut off. 33
  52. 52. 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 inabsorptivity when absorpted energy radiation is almost equal to the band energygap; therefore, the Fundamental Absorption Edge represented the less different inthe energy between the upper point in valance band to the lower point inconduction 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 absorptioncoefficient (α) larger or equal to (104 cm-1). This region can be introduced tomagnitude 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
  53. 53. Chapter Two Theoretical Partcoefficient (α) is equal about (1 cm-1 < α < 104 cm-1), and refers to transitionbetween the extended level from the (V.B.) to the local level in the (C.B.) also fromlocal 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) isvery small about (α < 1 cm-1) the transitions happen here between the regionsbecause 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.) beexactly over the top of (V.B.), which means they have the same value of wavevector i.e. (∆K=0) in this state the absorption appeared when (hυ=E g), thistransition type is required to the Laws conservation in energy and momentum. These direct transitions have two types, they are [86]: 35
  54. 54. Chapter Two Theoretical Part(a) Direct Allowed Transition This transition happens between the top points in the (V.B.) to the bottompoint 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 pointsof (C.B.) as shown in fig. (2.8.b), the absorption coefficient for this transitions typegiven by [88]:  h  B (h  Eg )r ………..…………..…….… (2-10) Where: Eg: energy gap between direct transition B: constant depended on type of material υ: 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.), incurve (E-K), the electron transits from (V.B.) to (C.B.) not perpendicularly wherethe value of the wave vector of electron is not equally before and after transition ofelectron. (∆K ╪ 0), this transition type happens with helpful of a like particle iscalled "Phonon", for conservation of the energy and momentum law. There are twotypes 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
  55. 55. Chapter Two Theoretical Part(d) Forbidden Indirect Transitions These transitions happen between near points in the top of (V.B.) and nearpoints in the bottom of (C.B.) as shown in fig. (2.8.d), the absorption coefficient fortransition with a phonon absorption is given by [89]:  h  B (h  Eg  E ph )r ……….…...……..…. (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
  56. 56. Chapter Two Theoretical Part2.7.4 Optical Constants The extraction of optical constants from various types of optical measurementis a field of widespread interest [91]. A large number of methods have beenproposed for the determination of the optical parameters real part of refractiveindex (n), extinction coefficient (K0) and the real and imaginary part of dielectricconstant [92]. 1  4 R  2  2 R  1 n    R  12   K 0   R  1  ....……………….… (2-12)      Where (R) is the reflectance. The extinction coefficient (K0) is related to the exponential decay of the waveas it passes through the medium and it is defined to be [93].  K  ………………………………….…… (2-13) 4 Where (λ) is the wavelength of the incident radiation and (α) is given by: 2.303 A  ...…………….....…………….……. (2-14) t (A) is the absorbance, and (t) is the sample thickness. And (R) is calculatedfrom the following equation: R T  A 1 ………………………………...... (2-15) An absorbing medium is characterized by a complex dielectric constant    r  i i ………………………………….. (2-16)  r  n 2  K 02 ……...…………….…………….. (2-17)  i  2nK0 ……………………....………..……. (2-18) 38
  57. 57. Chapter Two Theoretical Part The optical conductivity (ζ) depends directly on the wavelength andabsorption coefficient [94]:  nc   ……………………..…………..…… (2-19) 42.7.5 Some Optical Properties of TiO 2 Thin Film The optical transmission spectra for the anatase TiO2 thin films are presentedin fig. (2.9). Anatase TiO2 thin films are prepared by RF magnetron sputteringsystem with a titanium target of 99.99% purity on microscope glass slides assubstrates. The substrates deposited at room temperature with TiO2 are annealed at300 °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 andannealed TiO2 films is about 85% in the visible region. Annealing shows a slightdecrease in transmittance with the increase of annealing temperature. The filmswhich are annealed at 600 °C show a significant decrease in visible lighttransmittance [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
  58. 58. Chapter Two Theoretical Part The curves of refractive index and extinction coefficient for as-grown andannealed TiO2 films are shown in fig. (2.10) and fig. (2.11). Here, it is found thatthe refractive index at 550 nm for as deposited, annealed at 300 °C, 400 °C, 500 °Cand 600 °C are 2.31, 2.34, 2.33, 2.33 and 2.35 respectively. This trend shows anincrease of the value of refractive index with higher annealing temperature. 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 isincreased. 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
  59. 59. 10/23/2005Experimental Work
  60. 60. Chapter Three Experimental Work3.1 Introduction This chapter includes a description of pulsed laser deposition system whichhas been used to prepare titanium dioxide TiO2 thin films and explanation forsubstrate cleaning method. Also, it deals with method of measuring thickness ofthin films, structural and optical properties measurements. A schematic diagramillustrates 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 Structural Optical of thin films Properties Properties Optical XRD & AFM T, A, α, Eg, n, interferometer kₒ, εr, εi & ζ method Fig. (3.1): Schematic diagram for of experimental work 41
  61. 61. Chapter Three Experimental Work3.2 Deposition Equipment The basic components of the PLD-system, the laser and the pulse shapinghave been introduced. In addition the following sections consider the componentsinside the deposition chamber, namely, the target, the substrate and also thevacuum 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 oflight route system, power supply system, computer controlling system, coolingsystem, etc. The light route system is installed into the hand piece, but powersupply, controlling and cooling systems are installed into the machine box of powersupply, as shown in fig. (3.2). Fig. (3.2): The Nd: YAG laser DIAMOND-288 Pattern EPLS system. 42
  62. 62. Chapter Three Experimental Work3.2.2 Pulsed Laser Deposition (PLD) Technique The pulsed laser deposition experiment is carried out inside a vacuumchamber generally at (10-2 mbar) vacuum conditions, at low pressure of abackground gas for specific cases of oxides and nitrides. Photograph of the set-upof laser deposition chamber, is given in fig. (3.3), which shows the arrangement ofthe 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 isincident on the target surface making an angle of 45° with it. The substrate is placedin front of the target with its surface parallel to that of the target. Sufficient gap iskept between the target and the substrate so that the substrate holder does notobstruct the incident laser beam. The shape of the deposition chamber iscylindrical; the geometry of the chamber can be designed quite freely. The chamberhas typically a large number of ports, e.g. for pumping system, gas inlets, pressuremonitoring, target, substrate, laser beam and view ports. When designing achamber, at least following aspects should be taken into account:1-The arrangement of the components inside the chamber should not disturb thepath of the laser beam.2-Access to the target and to the substrate should be straightforward, since thesecomponents 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 timeto time with the aim of obtaining better quality films by this process. These includerotation of the target, heating the substrate, positioning of the substrate with respectto target. 43
  63. 63. 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
  64. 64. Chapter Three Experimental Work3.2.3 Substrate Heater The substrate heater raises the substrate temperature up to 300 °C and this isachieved by using halogen lamp, which is mounted adjacent to the substrate. Thetemperature is measured continuously during film deposition process using aK-type thermocouple.3.2.4 Vacuum System The deposition chamber is fixed on a stainless steel flange containing a groovewith O-ring for vacuum sealing and feed-through in the base for electricalconnections (control the stepper motor and the substrate heater) and the chamberevacuated using rotary pump connecting directly to the chamber by stainless steelflexible tubes to get a vacuum up to 10-2 mbar and monitoring the pressure insidethe chamber by using (Leybold- Heraeus) Pirani gauge.3.3 Target preparation Titanium dioxide powder with high purity (99.999%) pressing it under 5 Tonto form a target with 2.5 cm diameter and 0.4 cm thickness. The target should be asdense and homogenous as possible to ensure a good quality of the deposit. Thetarget after being ablated is shown in fig. (3.4). Fig. (3.4): The target after being ablated by the laser. 45
  65. 65. Chapter Three Experimental Work3.4 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 impuritiesand residuals from there surface, then cleaned in alcohol ultrasonically for 10 minsubsequently 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 thethickness, the structural features of the films are X-ray diffraction (XRD), andatomic force microscopy (AFM). The optical features of the films are investigatedby transmission through UV-VIS absorption spectroscopy.3.5.1 Thickness Measurement Film thickness measurements by optical interferometer method have beenobtained. This method is based on interference of the light beam reflection from 46
  66. 66. Chapter Three Experimental Workthin 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 (λ)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 depositionconditions; the number of laser pulses is in the range of 100 pulses.3.5.2 Structural and Morphological Measurements3.5.2.1 X-ray Diffraction (XRD) To define the preferred orientation also to determine the nature of the growthand the structured characteristics of TiO2 films, X- Ray diffraction is carried andthe phase is determined by using the JCPD data for TiO2 anatase and rutile, usingShimadzu 6000 made in Japan. The source of X-Ray radiation has CuKα radiation. 47
  67. 67. Chapter Three Experimental Work The device has been operated at 40 Kv and 30 mA emission current, λ=1.54Å.The X-ray scans are performed between 2θ values of 30° and 60°.3.5.2.2 Atomic Force Microscopy (AFM) To determine the size and other characteristics of the synthesizednanoparticles, an atomic force microscope (AFM) is used. The operation principleof an AFM is presented in fig. (3.7). The AFM consists of a cantilever and a sharptip at its end. The surface of the specimen is scanned with the tip. The distancebetween the specimen surface and the tip is short enough, to allow the van derWaals forces between them to cause deflection of the cantilever. The deflectionfollows Hookes law and the spring constant of the cantilever is known, thus theamount of deflection and further, the topographical profile of the specimen, can bedetermined. All the samples are studied using Nanoscope AFM (made in USA) inMinistry of Science and Technological in Iraq. Fig. (3.7): The operation principle of AFM [96]. 48

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