Electric Field Modified Growth of Spray Pyrolysed Nano-crystalline Transparent Conducting Tin
Oxide Thin Films
by
ARCHANA GUPTA
Department of Physics
Submitted
in fulfillment of the requirements of the degree of Doctor of Philosophy to the
Indian Institute of Technology Delhi
New Delhi - 110016, INDIA
1. T. DELH.
LISP AIRY
Dedicated
My Beloved Parents
CERTIFICATE
We are satisfied that the thesis entitled Electric Field Modified Growth of Spray Pyrolysed Nano-crystalline Transparent Conducting Tin Oxide Thin Films presented by Archana Gupta is worthy of consideration for the award of the degree of Doctor of Philosophy and is a record of the original and bonafide research work carried out by her under our guidance and supervision and that the results contained in it have not been submitted in part or full to any other university or Institute for the award of any degree/diploma.
Dated : 31 December, 2004
Dr. Dinesh K. Pandya Professor of Physics Thin Film Laboratory, Department of Physics
Indian Institute of Technology Delhi Hauz Khas, New Delhi-110 016 INDIA
Dr. Subhash C. Kashyap Professor of Physics Thin Film Laboratory, Department of Physics
Indian Institute of Technology Delhi Hauz Khas, New Delhi-110 016 INDIA
AKNOWLEDGMENT
I wish to thank my thesis supervisors, Prof. Dinesh K. Pandya and Prof. Subhash.
C. Kashyap, for forcing gears into motion, making me realise what I was missing, by staging frequent gladiatorial encounters with that near-mythical and largely invisible beast called 'supervision', and constantly setting unrealistic deadlines with warnings of impending doom to make me achieve the seemingly impossible. They were responsible for providing constant advice, regular motivation, my ongoing paranoia, and frequent adrenaline highs. The freedom of learning and the inspiration they given throughout my thesis work are highly appreciable. Their critical evaluations and constructive suggestions made me to present this thesis in its final form.
I am grateful to Prof K.L Chopra for establishing and extending the state of the art facility at Thin Film Laboratory (TFL). My sincere thanks and regards to the Head of the Department, Prof. L.K.Malhotra, for his constant encouragement during my work. My sincere regards to Profs. R.D. Tarey, V.D.Vankar, V. Dutta, G.B. Reddy, and B.R. Mehta.
TFL is more than a teacher and there I learned the art of team spirit and hard work. I am thankful to my seniors Oman, Prathisha, Patnaik, Seema, and Gopal Mor for extending the helping hands in my early experiments. I am also thankful to my friends Tarsame, Somnath, Pushpendar, and Babu Dayal for their support and encouragement throughout my thesis work. Thanks are also to Suneet, Aruna, Ajay, Vidhyanad, Priyanka, Suchitra, Kanwal, Yajuvendra and Gopinadhan. Also my sincere thanks to several other faculty members and research students who directly or indirectly helped me to finish my thesis within time. Thanks are also to Mangal, Ganesh and Khatri.
The financial assistance from IIT Delhi is thankfully acknowledged.
Above all, I am indebted to the support from Mother, Father who taught me the first lesson of Physics, elder brothers Shiv Prakash and Alok Chandra also being my seniors in the Physics, and sister Pratibha. My special thanks to my in-laws and husband Pawan and daughter Sony for their moral support and kind patience.
Finally, Almighty, you have given me a life and piece of mind, and asked me to prove my worth. I am grateful to you.
t1L9\C
(Archana Chip*
ABSTRACT
The requirements of high electrical conductivity and high optical transparency for various device applications are met conventionally by incorporation of dopants in addition to oxygen deficiency. But the efforts have reached a limit and any further improvement needs a new approach.
In the present work we have carried out a systematic and detailed study of the growth of ultra thin tin oxide films using chemical spray pyrolytic deposition technique under the presence of electric field. This consequently leads to significant improvement in electrical and optical properties without any need of dopants. This is the first study of the parallel electric field effect on the growth of thin films using a chemical deposition technique. Applied field modifies the nucleation phenomena by the modification in the size of the critical nucleus and the nucleation barrier height, which accelerates continuous film formation at much early stage of the film growth and films are formed with high conductivity as well as with high transparency at much lower thickness of the film and much lower temperature of the substrate. The present work contains the deposition of undoped and fluorine doped tin oxide thin films using spray pyrolysis deposition technique with and without the influence of small electric field (dc) applied on the film surface during the early stage of film growth as well as annealing of the films in the presence of the applied electric field. The post deposition electric field annealing has also been studied on films grown conventionally without any electric field.
The fluorine doped tin oxide (PTO) thin films of thickness in the range of (25 to 100nm) were deposited on the glass substrates by using spray pyrolysis deposition technique. These films are prepared at a low deposition temperature 300-350°C. The sheet resistance and visible transmittance of theses films spray deposited at 300°C are found to be in the range of 3 to 50Kohm and 65-88% respectively, thicker films have exhibited
i i i
lower sheet resistance and lower transmittance. The as-deposited films are amorphous in nature. Polycrystalline films with the comparatively smaller sheet resistance as well as with the high transmittance deposited at the 350°C. From these experimental results it is concluded that for the deposition of polycrystalline film with high transparency, the deposition temperature and thickness should be more than the 350°C and 100nm respectively. In general thickness required to obtain a reasonably low sheet resistance is higher than 500 run.
Thin (60-70nm) transparent and conducting fluorine doped tin oxide films were deposited at the low deposition temperature on glass substrate by the application of different dc electric field (2.5, 7.5, 12.5 and 75V/cm) on the substrate surface between the electrodes separated by the distance 4mm during growth in spray pyrolysis deposition technique. The film with sheet resistance 4752/0, the electrical resistivity as low as 3.2x10-
40cm with a simultaneously high transparency of 88% at 625nm were prepared in the presence of an electric field of 75V/cm even at the low deposition temperature275-300°C.
However the film grown is highly resistive when prepared without applied electric field, the sheet resistance measured as high as 31(C2/0 and with quite poor transparency as low as 59% in the visible spectral region are obtained. A dramatic decrease, by a factor of 60, is brought about in sheet resistance (resistivity) by the influence of the electric field on the growth process. This observation understood on the basis of the onset of an early coalescence establishing electrical continuity at the lower film thickness.
The XRD patterns show that film prepared in the presence of the field is polycrystalline whereas the as-deposited films are amorphous in nature Growth rate of these films are as high as 210 run/min.
The influence of small dc electric field (5- 30V/cm) also studied on growth of undoped tin oxide thin films onto glass substrates using a spray pyrolysis deposition
technique. The applied electric field improves a number of the properties of the film;
critical thickness of film, resistance, and other electrical transport properties of the films.
A low sheet resistance of 530 S2/E1 obtained for films deposited at a low temperature of 300°C in the presence of field and it is about 20 times lower than for the film prepared in absence of field. Thickness of the films was measured in the range 110nm to 120nm. In addition, these films show exceptionally high visible transparency reaching up•to 97%. The high carrier concentration as high as 1020 cm-3 and mobility 12 cm2 V-1 s-I have been obtained in comparison to highly resistive films normally obtained without field. The X-ray diffraction studies have shown that the application of electric field transforms the amorphous growth to the crystalline and oriented one. The atomic force microscopy of the film reveals an improved grain structure with the compact grain boundary region and with the reduced surface, roughness when film is prepared in the presence of electric field.
The influence of application of post—deposition dc electric pulse, where an electric field is applied on the film surface for a short time of 5 sec, at high substrate temperature of 350-400°C has been studied on the properties of the different thicknesses (20, 40,.60, 80, 100 and 150nm) of fluorine doped tin oxide thin films. The annealing resulted in the reduction of the sheet resistance by a factor of 3 to 19.2 depending on the thickness of the film. Maximum reduction occurs for minimum thickness. This agrees with the conclusion that presence of field assists in bringing about coalescence in the ultra thin discontinuous film. The improved transmittance of the field treated film compared to the film annealed without electric pulse effect indicates a better crystallinity and reduced optical scattering centers. The best opto-electric properties were found in the 40run thick electric field treated film, an ultra thin continuous film with very high transmittance 92-94% as well as useful electrical resistivity of the order of 10-35-/cm.
The films prepared with and without effect of the electric pulse of thickness 40nm were analyzed by the AFM revealing a transformation of the discontinuous film to continuous film. The experimental results show that the post deposition applied electric field is much effective when applied on the ultra thin film.
In order to modulate the properties of the film prepared in the presence of field, annealing for different times (0, 10, 30, 50 and 100min) in the presence of a constant electric field (30V/cm) was done. Effect has been seen in terms of the changes in the electrical and optical properties of the film on treatment. The deposition and annealing temperatures were in the range of 250-300°C and 350-400°C respectively. When the films were annealed in the presence of electric field, the sheet resistance of the as- deposited film (zero annealing time) reduced with annealing time, the maximum reduction from 31(Q/0 to 800Q/0 was observed for an annealing time of 100min. However when films were annealed without electric field, the sheet resistance of the film was found to increase slightly, from 3 KCVO to 5 1(Q/0 after 100min annealing time.
Annealing of the films in the presence of field also resulted in enhanced values of carrier concentration and mobility, from 5.7x 1019 to 9.4 x 1019/cm3 and 4.9 to 12.3
CM2 V-I
S-1
, respectively. These properties of the film changed in the opposite manner with annealing time when annealed without applied field. The improved properties of the films is ascribed to the field induced coalescence and improved grain size of film due to the presence of electric field during annealing process. The systematic increase in the intensity of the X-ray diffraction lines as a function of the annealing time was supportive of this conclusion. All the experimental results show that the application of electric field applied during or/and the post annealing resulting in the improved properties of the ultra thin films of doped and undoped tin oxide.
LIST OF FIGURES
Reflection(R), Transmittance(T) and absorption spectra of fluorine doped tin oxide thin film
Unit cell of SnO2 Band structure of SnO2
A typical electrostatic spray assisted vapor deposition set up
Schematic drawing of the resistive touch panel; (a) digital type, (b) analogue-type
SnO2ISiO2 gate ISFET structure
Block diagram of spray pyrolysis system used in the present work
Cross sectional view of the spray nozzle
The experimental arrangement for the deposition of large area oxide coatings
van der Pauw geometry used for Hall measurements Schematic diagram of the glancing angle X-ray diffractometer
Schematic diagram of the operation of AFM
Schematic of ray diagram of the working principle of .a Scanning electron Microscope (SEM)
5 11 12
22
33 35
44 45
45 51
53 55
58 Figure 1.1 :
Figure 1.2 : Figure 1.3 : Figure 1.4:
Figure 1.5 :
Figure 1.6 :
Figure 2.1:
Figure 2.2 : Figure 2.3 :
Figure 2.4 : Figure 2.5 :
Figure 2.6 : Figure 2.7 :
Figure 3.1 :
Figure 3.2 :
Figure 3.3 :
Figure 3.4 :
Sheet resistance of the fluorine doped tin oxide thin films vs thickness of films
Transmittance of the fluorine doped tin oxide thin films of different thickness 40,60, 80, 100nm prepared at 300°C
Transmittance of the fluorine doped tin oxide thin film (100nm) prepared at 350°C
X-ray diffraction spectra of fluorine doped tin
oxide thin films prepared at the different temperature
63
64
64
65
xi
Figure 3.5(a): The current through the growing films as a function of deposition time for different applied electric fields
2.5V/cm, 7.5V/cm, 12.5V/cm 67
Figure 3.5(b): The current through the growing film as a function
of the deposition time for 75 V/cm applied electric field 68 Figure 3.6 : The variation of sheet resistance for different applied
electric fields 69
Figure 3.7 : X-ray diffraction pattern for the films grown in the
presence of (12.5V/cm) and without electric field 72 Figure 3.8 : SEM of the fluorine doped tin oxide thin film
deposited in the presence of field 75
Figure 3.9 : SEM of the fluorine doped tin oxide thin film
deposited without field 75
Figure 3.10: AFM photograph of FTO film deposited at 350°C
in the presence of field (75V/cm) 78
Figure 3.11 : Transmittance of the fluorine doped tin oxide thin
film grown in presence of field (12V/cm) and without field 79 Figure 4.1 : Variation of sheet resistance of undoped tin
oxide thin films vs applied electric field during
deposition. The sheet resistance of films prepared without
applied electric field was 7KS-2/0 to 10Kohms measured. 85 Figure 4.2: The variation of electrical resistivity (p), carrier
concentration (n) and mobility (u) of the undoped tin oxide thin films prepared under the different applied
electric fields. 86
Figure 4.3 : Transmittance of the undoped tin oxide thin films
deposited under influence of different electric field 89 Figure 4.4 : The variation of visible transmittance at 700run and
sheet resistance of the undoped tin oxide thin films prepared with electric field of 20 V/cm, with thickness
of the films 90
Figure 4.5 : Visible transmittance of the undoped tin oxide thin films prepared under the different applied electric fields applied
LIST OF TABLES
Table 1.1: Various processes and their corresponding defect structure 4
Table 1.2: Etchants for TCOs 7
Table 1.3: Hardness of some TCOs 8
Table 1.4: Deposition Techniques 13
Table 1.5: Properties of TCO thin films prepared by various deposition
Techniques 28
Table 1.6: Growth parameters used by various researchers for spray
pyrolysing tin oxide films 29
Table 3.1 Details of the physical parameters of different samples. 67 Table 3.2: Effect of substrate temperature on the sheet resistivity
(SR) and transmittance (T%) of the F-loped ITO Films 77 Table 3.3: Deposition parameters and properties of ITO/FTO thin films 81 Table 4:1: Electro-transport properties of undoped tin oxide thin films
deposited under different applied electric fields 88 Table 5.1: Comparison of different TCO's by their method of
preparation and its properties 107
Table 5.2: Electrical properties of the as deposited and annealed
films of undoped Sn02 with and without applied electric field 116
xv
CONTENTS
Page No.
Certificate
Acknowledgement ii
Abstract iii
Contents vii
List of Figures xi
List of Tables xv
CHAPTER 1 : INTRODUCTION
1.1 Transparent Conducting Oxide Thin Films — A Brief History 1 1.2 The Basic Properties of Transparent conducting Oxides 3
1.2.1 Electrical conductivity 3
1.2.2 The optical properties 5
1.3 n-type and p-type Transparent Conducting Oxides 5 1.4 Selection of a Suitable Transparent Conducting Oxide 6
1.4.1 Preparation methods 6
1.4.2 Physical and chemical stability 7
1.4.3 Mechanical Hardness of TCOs 7
1.4.4 Thermal stability of TCOs 8
1.4.5 Diffusion of Sodium into the TCOs 8
1.4.6 Toxicity 9
1.4.7 Thickness 9
1.4.8 Production Costs 10
1.4.9 Summary 10
1.5 Tin Oxide (Sn02) - General Properties 10
1.6 Deposition Techniques 13
1.6.1 Pulsed Laser Deposition 13
1.6.2 Filtered Vacuum Arc Deposition 14
1.6.3 Reactive Evaporation 15
1.6.4 Sol-Gel Coating 16
1.6.5 Spin Coating 17
1.6.6 Chemical Vapor Deposition (CVD) 18
1.6.7 Sputtering 19
1.6.9 Electrostatic Spray Assisted Vapor Deposition (ESAVD) 21 1.7 Deposition of TCO Thin Films & its Control Parameters 22
1.7.1 Doping effect 24
1.7.2 Optimum Temperature 25
1.7.3 Optimum thickness 25
1.7.4 Post deposition annealing 26
1.7.5 Present status 28
1.8 Ultra thin transparent conducting oxide films 30 1.9 , Applications of Transparent Conducting Oxide Thin Films 31
1.9.1 Electrode Material For OLEDs 31
1.9.2 Defrosting Windows 32
1.9.3 Touch-Panel Controls 32
1.9.4 Invisible Security Circuits 33
1.9.5 Improving the Durability of Glass 34
1.9.6 Heat Mirror 34
1.9.7 pH-Sensitive Material for ISFETs 35
1.9.8 Solar Cell Applications 36
1.9.9 Sensor applications 37
1.10 Objective of the present work 37
1.11 Thesis Plan 39
CHAPTER 2: DEPOSITION AND CHARACTERIZATION TECHNIQUES
2.1 Introduction 41
2.2 Spray Pyrolysis 41
2.3 Advantages of the Technique 42
2.4 Spray Pyrolysis Set-Up 43
2.5 Electric—Field Modified Spray Pyrolysis 46
2.5.1 Electrodes preparation 46
2.5.2 Substrate cleaning 47
2.6 Thin Film Deposition 47
2.7 Volmer-Weber Theory for Nucleation 48
2.8 Characterization Techniques 49
2.8.1 Transmittance Measurement 49
2.8.2 Thickness Measurement 50
2.8.3 Hall Effect Measurement 51
viii
2.8.4 Glancing Angle X-ray Diffraction (GAXRD) 52
2.8.5 Atomic Force Microscopy 53
2.8.6 Scanning Electron Microscopy 56
CHAPTER 3: FLUORINE DOPED TIN OXIDE THIN FILMS GROWN IN PRESENCE OF ELECTRIC FIELD
3.1 Introduction 60
3.2 Deposition of Fluorine Doped Tin Oxide Thin Film 62
3.2.1 Sheet resistance measurement 62
3.2.2 Transmittance measurement 63
3.2.3 Structural measurement 65
3.3 Fluorine Doped Tin Oxide Thin Films by Electric Field Modified
Spray Pyrolysis 66
3.3.1 Growth of films under electric field 67 3.3.1.1 Critical thickness for continuous film 67 3.3.1.2 Sheet Resistance Measurement 69
3.3.2 A proposed model 71
3.3.3 Structural characterization 72
3.3.4 Surface morphology 74
3.3.5 Deposition of films at different temperatures 76
3.3.6 Transmittance measurement 78
3.3.7 Effect of electric field on deposition parameters 80
3.4 Conclusion 82
CHAPTER 4: ELECTRIC FIELD GROWN UNDOPED TIN OXIDE THIN FILMS
4.1 Introduction 83
4.2 Deposition of Thin Films 84
4.3 Electrical Measurements 84
4.3.1 Sheet resistance measurement 84
4.3.2 Hall measurement 86
4.4 Transmittance Measurement 89
4.5 Surface Morphology 91
4.6 Structural Characterization Using GAXRD 94
4.7 Conclusions 95
CHAPTER 5: POST DEPOSITION MODIFICATION OF THIN FILMS BY PULSED AND CONTINUOUS ELECTRIC FIELD
5.1 Introduction 97
5.2 Annealing under the Influence of Electric Field Pulse 98
5.2.1 Sheet resistance 99
5.2.2 Surface morphology 101
5.2.3 Transmittance 104
5.3 Variable Electric Pulse Annealing 108
5.3.1 Measurement of charge flow 108
5.3.2 Sheet resistance 109
5.4 Electric Field Treatment of Field-Grown Films 110
5.4.1 Experimental details 110
5.4.2 Structural characterization using GAXRD 111 5.4.3 Electrical properties of post-annealed films 112 5.4.3.1 Sheet resistance measurement 113
5.4.3.2 Electrical measurement 114
5.4.3.3 Transmittance measurement 117
5.5 Conclusions 118
CHAPTER 6: CONCLUSIONS AND FUTURE WORK
6.1 Processing Parameters 120
6.2 Electrical and Optical Properties 121
6.3 Surface Morphology and Structural Studies 123
6.4 Growth Model 124
6.5 Model for Field-Induced Reactivity 125
6.6 Future Work 126
REFERENCES 127
LIST OF PUBLICATIONS 140
BIO-DATA 141
