Defect mediated novel structural, optical, electrical and magnetic properties in ti1 xtaxo2 thin films

DEFECT MEDIATED NOVEL STRUCTURAL, OPTICAL,
ELECTRICAL AND MAGNETIC PROPERTIES IN
Ti
1-x
Ta
x
O
2
THIN FILMS




ARKAJIT ROY BARMAN
(M.S., COLORADO STATE UNIVERSITY, U. S. A.
M.Sc., INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY,
INDIA
B.Sc., PRESIDENCY COLLEGE, CALCUTTA, INDIA)




A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN
SCIENCE

DEPARTMENT OF PHYSICS

NATIONAL UNIVERSITY OF SINGAPORE
2011













In memory of
the unconditional love bestowed on me by
Late Smt. Naba Durga Debi (Thamma)
and
Late Sri Kisor Kanti Barman (Jethu)







i

ACKNOWLEDGEMENTS

The last four years which led to this thesis have been the most defining years of my life. I am
grateful to a lot of people who have been instrumental in making them so. It humbles me to
acknowledge them.
If I have to name ONE man for whom I am writing this Thesis and this acknowledgement, he
has to be my advisor, Prof. T. Venkatesan. Venky, as he is called by one and all has been one
of the biggest influences in my life. I consider myself to be extremely fortunate to have
known, worked together with and been supervised by Venky. He has encouraged me in all
my efforts and endeavors. He has managed to keep me motivated in my research. I cannot
thank him more for not giving up on me, even though, at times I was giving up on my
research. Venky has been extremely patient with me. He has always been available to answer
my doubts, even if that meant, long international calls at the wee hours or meetings extending
till the middle of the night. Venky has had a tremendous contribution in my developing as an
individual. Apart from all these, Venky has imparted me with profound knowledge and deep
insights about Oxides and Defect Induced Magnetism and has provided me with every
possible opportunity to develop myself as an experimental scientist. I will be ever indebted to
Prof. T. Venkatesan.
I also want to take this opportunity to acknowledge my co-supervisor, Prof. Chua Soo Jin.
Prof. Chua has been extremely encouraging and had taken keen interest in my research
activities. He has always helped me out with his invaluable inputs about my work.
ii

Dr. Sankar Dhar, my mentor and colleague has worked most closely with me throughout my
graduate student life. I have learnt the basics of most of the experimental techniques that I
have used for my research from Sankar da. Whenever I had felt totally lost with my research,
I had blindly turned to Sankar da for help. He has been of phenomenal help in managing my
research work and giving direction to it. His critical inputs have definitely helped me in
taking my work to the next level. He has shown great confidence in me throughout. I fondly
remember the initial days in the lab when we toiled hard together to set up the PLD. I feel

happy to thank him for all his help.
I thank Prof. Ariando and Prof. A. Rusydi for the invaluable support. There is no doubt
whatsoever, that my work would not have been possible without them. They have been of
tremendous help with experiments as well as theoretical understandings of my subject. I will
miss our paper writing sessions together and the occasional pizza parties.
I also thank Prof. H. Yang and Prof. O. Barbaros for the many fruitful discussions and the
opportunities to work together.
I would like to extend my special gratitude to Dr. Daniel Lubrich. Dan has been a constant
source of encouragement to me. We have had many interesting discussions on varied topics. I
particularly enjoyed working with him on NanoSpark projects. It helped me a lot with
building a different outlook which is hard to develop in a strictly academic environment.
I would want to thank Dr. K. Gopinadhan. Gopi has been the epitome of sincerity whom all
graduate students in our lab have tried to idolize. Gopi has helped me a lot with transport
measurements and helped me understand the intricate physics related.
iii

I would also want to thank Dr. S. Saha. We have been good friends in the few days that we
have known each other. Surajit is a focused individual with very sharp instincts of a
researcher. He has helped me with Raman measurements and with understanding the data.
I thank Dr. C.B. Tay. Chuan Beng is a very helpful individual and is always ready to do PL
measurements, even if it is on weekend nights.
I definitely want to thank Dr. W. Lú. Weiming has helped me a lot with SQUID
measurements and also with depositions.
I have been fortunate enough to have some of the most wonderful, talented and helpful lab-
mates. I want to thank Xiao Wang , Young Jun Shin, Mahdi Jamali, Mallikarjunarao
Motapothula, Jae Sung Son, Jae Hyun Kwon, Anil Annadi, Liu Zhiqi, Yong Liang Zhao,
Teguh Citra Asmara, Zhihua Yong, Amar Srivastava, Tarapada Sarkar, Naomi
Nandakumar, Masoumeh Fazlali and last but not the least Michal Dykas. Over the years we
have been more of good friends and less of colleagues. I guess we will always remember the
night outs in the lab. I also warmly remember all the Summer Internship students who have

worked with me during my stay at NUSNNI-NanoCore. It has been an honor to know and
work with you all.
I definitely want to thank all the people who has supported with running the lab smoothly
throughout the period of my research. I want to thank Jason Lim, Syed Nizar, Malathi,
Catherine Tai Guat Hoon and all the other staffs at the NUSNNI NanoCore office.
I would like to take this opportunity to mention my friends in Singapore. Most importantly, I
want to thank my room-mate, Pankaj for putting up with the weird schedules and habits of a
graduate student. I feel great pleasure to mention Arun, Shruti, Kingshuk, Nibedita, Sahoo,
Pradipto, Adeeb, Arpan, Deepal, Sandeep, Bhavesh, Trond, Hallgeir, Cecilia, Solveig, Marit,
iv

Heidi, Bård, Isaac, Cari, Sarah, and most importantly Heekyoung. I thank you all from the
bottom of my heart for the much necessary distractions. It has been a pleasure knowing all of
you.
Just because I do not want to get killed, I will mention Aritra and Anshuk. It makes no sense
for me to thank you. I should rather thank my stars that I managed to finish my thesis and am
writing the acknowledgement even with two guyz like you in my life for the past twenty-five
years.
It is always difficult to express love and gratitude to family members. It appears so futile. My
parents and my sister – you are the source of my sustenance. I could not have asked for
anything more from you. It is all because of you.

v

TABLE OF CONTENTS

ACKNOWLEDGEMENTS i
TABLE OF CONTENTS v
ABSTRACT viii
LIST OF PUBLICATIONS x
LIST OF TABLES xiii
LIST OF FIGURES xiv
LIST OF SYMBOLS xx

Introduction 1
1.1 Fundamental Physical Properties of TiO
2
1
1.1.1 Crystalline Structure of TiO
2
1
1.1.2 Electronic Band Structure of TiO
2
4
1.2 Defects and Dopings in Semiconductors 5
1.2.1 Intrinsic and Extrinsic Defects in Semiconductors 5

1.2.2 Thermodynamics of Defect Formation and Compensation 8
1.3 Applications of TiO
2
11

Structural Analysis of Pulsed Laser Deposition grown Ti
1-x
Ta
x
O
2
Thin Films 12
2.1 Pulsed Laser Deposition Technique 12
2.2 Ti
1-x
Ta
x
O
2
Thin Film Preparation 14
2.3 Structural Analysis of Ti
1-x
Ta
x
O
2
Thin Films 15
2.3.1 X-Ray Diffraction Studies 15
2.3.2 Raman Spectroscopy Studies 21
2.3.3 Rutherford Backscattering – Ion Channeling Studies 27

2.3.4 Atomic Force Microscopy Studies 31
2.4 Conclusion 33

Ti
1-x
Ta
x
O
2
: A New Alloy with Transparent Conducting Properties 34
3.1 Transparent Conducting Oxides 34
3.1.1 Electrical Conductivity 35
3.1.2 Optical Properties 38
3.2 Alloying Effect of Ti
1-x
Ta
x
O
2
Thin Films 40
3.2.1 Ultra Violet-Visible Spectroscopy and Electrical Transport of Ti
1-x
Ta
x
O
2
41
3.2.2 High Energy Optical Reflectivity of Ti
1-x
Ta

x
O
2
46
3.3 Conclusion 49

Universal Kondo Effect in Ti
1-x
M
x
O
2
(M=Nb, Ta) Thin Films 50
4.1 Brief History to Kondo Effect 50
4.2 Low Temperature Transport Data on Ti
0.94
M
0.06
O
2
(M = Nb, Ta) Thin Films 53
4.3 Conclusion 67
vi


Role of Ta versus Magnetic Contaminants in Defect Mediated Ferromagnetism in Ti
1-x
Ta
x
O

2
films 70
5.1 Dilute Magnetic Semiconductors (DMS) 70
5.1.1 Spintronic Devices 70
5.1.2 Origin of Ferromagnetism in DMS 74
5.1.3 Dilute Magnetic Semiconducting Oxides 77
5.1.4 Defect Mediated Ferromagnetism 78
5.2 Magnetic Impurity Analysis in Ti
1-x
Ta
x
O
2
Thin Films 80
5.2.1 Brief History and Motivation 80
5.2.2 RBS and PIXE Results 81
5.2.3 XAS Results 82
5.2.4 TOF-SIMS Results 85
5.2.5 APT Results 85
5.3 Conclusion 89

Cationic Vacancy Induced Room Temperature Ferromagnetism in Transparent Conducting
Anatase Ti
1-x
Ta
x
O
2
Thin Films 90
6.1 Structural, chemical, electrical and optical properties 91

6.2 Magnetic Properties 91
6.3 Theoretical Calculation 101
6.4 Origin of Ferromagnetism 102
6.5 Conclusion 108

Interplay Between Carrier and Cationic Defect Concentration in Ferromagnetism of Anatase Ti
1-
x
Ta
x
O
2
Thin Films 110
7.1 PLD Deposition of Ti
1-x
Ta
x
O
2
Thin Films 110
7.2 Experimental Results and Discussions 111
7.2.1 Dependence of Magnetization on PLD Deposition Conditions 112
7.2.2 Role of Ta in the Magnetism of Ti
1-x
Ta
x
O
2
Thin Films 115
7.3 Conclusion 120

Summary and Future Work 122
8. 1 Summary 122
8.1.1 Ti
1-x
Ta
x
O
2
: A New Alloy System 122
8.1.2 Electrical Properties in Ti
1-x
Ta
x
O
2
Alloy 123
8.1.3 Magnetic Properties in Ti
1-x
Ta
x
O
2
Alloy 124
8. 2 Future Work 125



Appendix A.1
Pulsed Laser Deposition (PLD) 127

Appendix A.2
A New Route to Graphene Layers by Selective Laser Ablation 129
vii

Appendix A.3
X-Ray Diffraction (XRD) 142
Appendix A.4
Raman Spectroscopy 144
Appendix A.5
Rutherford backscattering-Ion Channeling 146
Appendix A.6
Atomic Force Microscopy (AFM) 149
Appendix A.7
Ultra Violet- Visible (UV-Vis) Spectroscopy 151

BIBLIOGRAPHY 153

viii

ABSTRACT

The main objective of this thesis is to explore the defect mediated structural, optical, electrical
and magnetic properties of titanium oxide (TiO
2
) based alloy thin films grown by pulsed laser
deposition (PLD). Such properties can be harnessed for suitable applications in the field of
optoelectronics and spintronics as transparent conducting oxides (TCO) and defect induced
diluted magnetic semiconductors (DMS) respectively.
Single crystal thin films of pure TiO
2
and tantalum (Ta) incorporated TiO
2
(Ti
1-x
Ta

x
O
2
) were
grown epitaxially on lattice matched substrates such as LaAlO
3
and SrTiO
3
. By varying the
deposition temperatures and the oxygen partial pressures in the PLD process, both anatase and
rutile polymorphs of TiO
2
were grown. By investigating the growth dependence of the different
phases of TiO
2
on the deposition parameters, an elaborate phase diagram was developed.
Rutherford backscattering-Ion Channeling (RBS) spectroscopy was used to study the crystal
structure of all the films deposited. RBS-Ion Channeling studies showed that the crystallinity of
the thin films improved with increasing deposition temperature and increasing oxygen partial
pressure. Films with higher Ta incorporation also showed higher crystallinity. X-Ray Diffraction
studies showed a lattice expansion in TiO
2
with Ta incorporation in the out-of-plane direction.
This was further supported by the Raman spectroscopy data which showed the softening of the
out-of-plane vibrational modes and the hardening of the in-plane vibrational mode.
Ultra Violet-Visible (UV-Vis) Spectroscopy was done on both anatase and rutile samples to
study the effect of Ta incorporation in TiO
2
. The band gap of both anatase and rutile samples
showed a blue shift with increasing Ta concentration. Using electrical transport data, it was

argued that the band structure of TiO
2
undergoes a drastic change with Ta incorporation resulting
in the formation of a new alloy system. High energy optical reflectivity measurements were done
ix

to directly detect the huge spectral weight shift in the spectra for pure TiO
2
and Ti
1-x
Ta
x
O
2
. This
further confirmed the role of Ta in varying the band structure of TiO
2
.
Ta incorporation in TiO
2
is believed to enhance the formation of cationic vacancies such as
titanium vacancies (V
Ti
) and suppress the formation of anionic vacancies such as oxygen
vacancies (V
O
) in a crystal.
The cationic defects in a crystal lattice have been predicted to form magnetic centers. Such
magnetic centers scatters electrons resulting in an up-turn of the resistivity curve as function of
temperature. This phenomenon, known as Kondo effect has been found in Ti

1-x
Ta
x
O
2
thin films.
Thorough and systematic Hall measurements have also been done on Ti
1-x
Ta
x
O
2
thin films to
study the variation of carrier density and electron mobility with deposition conditions.
Defect mediated magnetism is an alternate and a more trustworthy route to DMS oxides. Ti
1-
x
Ta
x
O
2
thin films with cationic defects and enough free carriers showed ferromagnetism (FM) at
room temperature (RT). Extensive impurity analysis has been done by RBS, Proton Induced X-
Ray Emission Spectroscopy (PIXES), X-Ray Absorption Spectroscopy (XAS) and Secondary
Ion Mass Spectrometry (SIMS) to rule out the presence of any magnetic impurities in the thin
films. Soft X-Ray Magnetic Circular Dichroism (XMCD) and Optical Magnetic Circular
Dichroism (OMCD) measurements were done to confirm the origin of the magnetism to be
predominantly cationic defects. Detailed Superconducting Quantum Interference Device
(SQUID) Magnetometry measurements were done to study the variation of magnetism with
deposition conditions. Based on these measurements, a plausible model is devised for the defect

mediated magnetism in Ti
1-x
Ta
x
O
2
thin films.


x

LIST OF PUBLICATIONS

 AIP Advances 2, 012148 (2012), A. Roy Barman, A. Annadi, K. Gopinadhan, W.M. Lu,
Ariando, S.Dhar, T. Venkatesan; Interplay between carrier and cationic defect concentration
in ferromagnetism of anatase Ti
1-x
Ta
x
O
2
thin films.


 Appl. Phys. Lett. 99, 172103 (2011), W.M. Lu, X. Wang, Z.Q. Liu, S.Dhar, A. Annadi,
K.Gopinadhan, A. Roy Barman, H.B. Su, T. Venkatesan and Ariando; Metal-insulator
transition at a depleted LaAlO
3
/SrTiO
3

Interface: Evidence for charge transfer induced by
SrTiO
3
phase transitions.


 J. Appl. Phys. 110, 084309 (2011), S. Mathew, T.K. Chan, D. Zhan, K. Gopinadhan, A. Roy
Barman, M.B.H. Breese, S, Dhar, Z.X. Shen, T. Venkatesan and J.T.L. Thong; Mega-
electron-volt proton irradiation on supported and suspended grapheme: A Raman
spectroscopic layer dependent study.


 Phys. Rev. B 84, 165106 (2011), Z.Q. Liu, D. P. Leusink, W.M. Lu, X. Wang, X. P. Yang,
K. Gopinadhan, Y. T. Lin, A. Annadi, Y. L. Zhao, A. Roy Barman, S. Dhar, Y. P. Feng, H.
B. Su, G. Xiong, T. Venkatesan and Ariando; Reversible Metal Insulator Transition in
LaAlO
3
Thin Films Mediated by Intragap Defects: An Alternative Mechanism for Resistive
Switching.


 AIP Advances 1, 022151 (2011) Y.L. Zhao, A. Roy Barman, S. Dhar, A. Annadi, M.
Motapothula, J. Wang, H. Su, M. Breese, T. Venkatesan and Q. Wang; Scaling of Flat Band
Potential and Dielectric Constant as a Function of Ta Concentration in Ta-TiO
2
Epitaxial
Films.


 AIP Advances 1, 022109 (2011), S. Dhar, A. Roy Barman, G.X. Ni, X. Wang, X.F. Xu, Y.

Zheng, S. Tripathy, Ariando, A. Rusydi, K.P. Loh, M. Rubhausen, A.H. Castro Neto, B.
Ozyilmaz and T. Venkatesan; A New Route to Graphene Processing by Selective Laser
Ablation.


 Appl. Phys. Lett. 98, 081916 (2011), X. Wang, J. Chen, A. Roy Barman, S. Dhar, Q H.
Xu, T. Venkatesan and Ariando; Static and Ultrafast Dynamics of Defects of SrTiO
3
in
LaAlO
3
/SrTiO
3
Heterostructures.

xi

 Appl. Phys. Lett. 98, 072111 (2011), A. Roy Barman, M. Motapothula, A. Annadi, K.
Gopinadhan, Y. Zhao, Z. Yong, I. Santoso, Ariando, M. Breese, A. Rusydi, S. Dhar and T.
Venkatesan; Multifunctional Ti
1-x
Ta
x
O
2
: Ta Doping or Alloying?

 Nature Commun. 2: 188. (2011), Ariando, X. Wang, G. Baskaran, Z.Q. Liu, J. Huijben, J.B.
Yi, A. Annadi, A. Roy Barman, A. Rusydi, S. Dhar, Y.P. Feng, J. Ding, H. Hilgenkamp and
T. Venkatesan; Electronic Phase Separation at the LaAlO

3
/SrTiO
3
Interface.

 Carbon 49, 1720-1726 (2011), S. Mathew, T.K. Chan, D. Zhan, K. Gopinadhan, A. Roy
Barman, M.B.H. Breese, S. Dhar, Z.X. Shen, T. Venkatesan and John TL Thong; Stability
of Graphene under MeV proton beam Irradiation: Effect of Layer number and Substrate.


 Appl. Phys. Lett. 98, 041904 (2011), J. Q. Chen, X. Wang, Y.H. Lu, A. Roy Barman, G.J.
You, G.C. Xing, T.C. Sum, S. Dhar, Y.P. Feng, Ariando, Q H. Xu and T. Venkatesan;
Defect Dynamics and Spectral Observation of Twinning in Single Crystalline LaAlO
3
under
Sub-Bandgap Excitation.


 US Provisional Patent No. 61/421,265 (9
th
December 2010), T. Venkatesan, S. Dhar, A.
Roy Barman, X. Wang, Ariando, B. Oezyilmaz; Synthesis of Specific Number of Graphene
Layers by Thickness Selective Laser Ablation.


 Submitted (2012), A. Annadi, X.Wang, K. Gopinadhan, W.M. Lu, A. Roy Barman, Z.Q.
Liu, A. Srivastava, S.Saha, Y.L.Zhao, S.W.Zheng, S.Dhar, T.Venkatesan, Ariando;
Unexpected Two Dimensional Electron Gas at the LaAlO
3
/SrTiO

3
(110) Interface.


 Submitted AIP Advances (2012), M. Motapothula, A. Roy Barman, S. Dhar, M. Kodzuka,
T. Ohkubo, N.L. Yakovlev, A. Rusydi, Ariando, K. Hono, M.B.H. Breese, T. Venkatesan;
Room-Temperature Ferromagnetism in Ti
0.95
Ta
0.05
O
2
films: Role of Ta Versus Magnetic
Contaminants.


 Submitted Phil. Trans. Roy. Soc. A (2012), A. Rusydi*, S. Dhar*, A. Roy Barman*,
Ariando, D C. Qi, J.B. Yi, Y. P. Feng, K. Yang, Y. Dai, J. Ding, A.T.S.Wee, G. Neuber,
M. Ruebhausen, H. Hilgenkamp, T. Venkatesan; Cationic defect induced room temperature
ferromagnetism in transparent conducting anatase TiO2 thin film via non-magnetic Ta
doping.


xii

 Submitted Phys. Rev. B (2012), K. Gopinadhan*, A. Roy Barman*, A. Annadi, T.P.
Sarkar, Ariando, S. Dhar, T. Venkatesan; Universal Kondo effect in Ti
0.94
M
0.06

O
2
(M=Nb,
Ta) thin films.

 Submitted Book Chapter (2012), S.Dhar, A. Roy Barman, A. Rusydi, Ariando, Y.P.
Feng, M.B.H. Breese, H. Hilgenkamp, T. Venkatesan; Effect of Ta alloying on the optical,
electronic and magnetic properties of TiO
2
thin films

xiii

LIST OF TABLES


Table 1.1: Physical Properties of TiO
2
phases……………………………………………………4

Table 3.1: TCO semiconductors for thin film transparent electrodes………………………… 36

Table 4.1: List of the derived parameters both from Goldhaber-Gordon and Hamann formula
fittings for different dopants as a function of oxygen partial pressure PO
2
…………………… 63

Table 6.1: SXMCD peaks and corresponding transitions at the Ti L and O K edges………… 97

xiv

LIST OF FIGURES


Figure 1.1: Crystal structure of (a) Rutile, (b) Anatase and (c) Brookite TiO
2
…… ………… 2

Figure 1.2: Schematic band structures of (a) metal, (b) semiconductor and (c) insulator……….7

Figure 1.3: Effect on the band structure of a semiconductor due to (a) n-type and (b) p-type
doping…………………………………………………………………………………………… 8

Figure 2.1: Schematic diagram of a pulsed laser deposition chamber………………………… 13

Figure 2.2: XRD θ - 2 θ spectra showing different phases for Ti
1-x
Ta
x
O
2
thin films grown at (a)
constant oxygen partial pressure of 1x10
-5
Torr and varying deposition temperatures from 500 °C
to 750 °C. (b) constant deposition temperature of 600 °C and varying oxygen partial pressures

from 1x10
-1
Torr to 1x10
-5
Torr………………………………………………………………….16

Figure 2.3: Phase diagram for Ti
1-x
Ta
x
O
2
thin films grown by the PLD process as a function of
oxygen partial pressure and deposition temperature…………………………………………… 18

Figure 2.4: (a) A typical XRD rocking curve obtained for Ti
1-x
Ta
x
O
2
thin film; (b) Variation of
the Rocking curve FWHM with deposition temperature with fixed P(O
2
)=3x10
-5
Torr; (c)
Variation of the Rocking curve FWHM with oxygen partial pressure with deposition temperature
fixed at 700 °C………………………………………………………………………………… 20

Figure 2.5: (a) XRD θ - 2 θ spectra for anatase Ti
1-x
Ta
x
O
2
(x=0 – 0.08) thin films grown at an
oxygen partial pressure of 1x10
-5
Torr and at a deposition temperatures from 700 °C; (b)
Variation of the d
(004)
lattice parameter as a function of Ta concentration; (inset) shows the shift
in the (004) anatase peaks for pure and 8% Ta incorporated TiO
2
films……………………… 22

Figure 2.6: Raman active modes for TiO
2
. (a) In-plane E
g
mode at 144 cm
-1
; (b) In-plane E
g

mode at 197 cm
-1
; (c) Out-of-plane B
1g

mode at 399 cm
-1
; Degenerate modes of (d) A
1g
at 513
cm
-1
and (e) B
1g
519 cm
-1
; (f) In-plane E
g
mode at 639 cm
-1
…………………………………….24

Figure 2.7: Raman Spectra for anatase Ti
1-x
Ta
x
O
2
thin films (0 ≤ x ≤ 0.08) with the shaded peaks
corresponding to B
1g
mode at 399 cm
-1
, A
1g

/B
1g
degenerate mode at 513 cm
-1
& 519 cm
-1
and E
g

mode at 639 cm
-1
…………………………………………………………………………………25

Figure 2.8: (a) Variation of B
1g
mode with Ta concentration. (Inset) shows the expansion of the
d
(004)
lattice parameter as obtained from X-Ray diffraction studies; (b) Variation of the A
1g
/B
1g

degenerate mode with Ta concentration; (c) Variation of E
g
mode with Ta concentration…… 26

xv

Figure 2.9: (a) 2.0 MeV He+ RBS-Ion Channeling random and channeling spectra for 6% Ta
incorporated TiO
2
on LAO substrate; (b) (Left axis) Minimum channeling yield for Ti and Ta
and (Right axis) Ta substitutionality in Ti
1-x
Ta
x
O
2
thin films as a function of deposition
temperature; (c) Theoretical fitting of Ta substitutionality data to Arrhenius equation to calculate
activation energy for Ta incorporation in Ti lattice sites………………………………………29

Figure 2.10: Minimum channeling yield for Ti and Ta as a function of (a) oxygen partial
pressure and (b) Ta concentration in Ti
1-x
Ta
x
O
2
thin films………………………………………30

Figure 2.11: AFM images of Ti
0.94
Ta
0.06
O

2
thin films grown on (a) LaAlO
3
(b) SrTiO
3
substrates
……………………………………………………………………………………………………32

Figure 3.1: Schematic band structure for an extrinsic doped semiconductor with (a)
d
n
<
c
n
;
(b)
c
n
<
d
n
<
2c
n
; (c)
d
n
>
2c
n

……………………………………………………………37

Figure 3.2: (a) Transmittance spectra; (b)
1/2

vs
h

for anatase Ti
1-x
Ta
x
O
2
(x = 0 – 0.08) thin
films. (c) Transmittance spectra; (d)
1/2

vs
h

for rutile Ti
1-x
Ta
x
O
2
(x = 0 – 0.08) thin films
……………………………………………………………………………………………………42

Figure 3.3: Blueshift in the band gap for anatase and rutile Ti
1-x
Ta
x
O
2
thin films with Vegard‟s
law fitting as a function of Ta concentration…………………………………………………….43

Figure 3.4: (a) Temperature dependence of resistivity for anatase Ti
1-x
Ta
x
O
2
(x = 0 – 0.08) thin
films and rutile Ti
1-x
Ta
x
O
2
(x = 0.08) thin film; (b) Variation of carrier density and Hall mobility
for anatase Ti
1-x
Ta
x
O
2
thin films as a function of Ta concentration…………………………… 45

Figure 3.5: High energy optical reflectivity for anatase Ti
1-x
Ta
x
O
2
(x = 0, 0.02, and 0.04) thin
films…………………………………………………………………………………………… 48

Figure 4.1: van der Pauw geometry for transport measurements in Ti
0.94
M
0.06
O
2
(M = Nb, Ta)
thin films…………………………………………………………………………………………54

Figure 4.2: Carrier concentration of thin film as function of temperature at different oxygen
partial pressures for (a) Ti
0.94
Ta
0.06
O
2,
and

(b) Ti
0.94

Nb
0.06
O
2
. Mobility of the charge carriers as
function of temperature at different oxygen partial pressures for (c) Ti
0.94
Ta
0.06
O
2
and (d)
Ti
0.94
Nb
0.06
O
2
…………………………………………………………………………………… 56

Figure 4.3: Magnetoresistance (MR) of thin film as a function of external magnetic field (H) at
different magnetic field angles indicating the weak angle dependency of the MR for (a)
Ti
0.94
Ta
0.06
O
2
(prepared at PO
2

=2e-04 Torr) (b) Ti
0.94
Nb
0.06
O
2
(prepared at PO
2
=8e-05 Torr)
……………………………………………………………………………………………………59

Figure 4.4: Resistivity as a function of temperature at different oxygen partial pressures fitted by
Goldhaber-Gordon formula for (a) Ti
0.94
Ta
0.06
O
2,
and

(b) Ti
0.94
Nb
0.06
O
2
thin films, and by
Hamann formula for (c) Ti
0.94
Ta

0.06
O
2,
and

(d) Ti
0.94
Nb
0.06
O
2
thin films……………………… 64
xvi


Figure 4.5: The variation of Kondo temperature T
K
as derived from Goldhaber-Gordon formula
and Hamann formula as a function of (a) measured carrier concentration and (b) estimated
magnetic concentration………………………………………………………………………… 66

Figure 4.6: Plot of resistivity (normalized to Kondo resistivity) versus temperature (normalized
to Kondo temperature) of both Ti
0.94
Ta
0.06
O
2,
and

Ti
0.94
Nb
0.06
O
2
thin films grown at different
oxygen partial pressures demonstrating the universal character of the
Kondo……………………………………………………………………………… ………… 68

Figure 5.1: Schematic of a (a) GMR spin-valve with two magnetic layers have the same moment
orientation (left panel) and the opposite moment orientation(right panel) (b) Magnetic tunnel
junction with two magnetic layers have the same moment orientation (left panel) and the
opposite moment orientation(right panel); Low panels show independent tunnel process of two
spin states………………………………………… 73

Figure 5.2: Schematic of a (a) spin field effect transistor and (b) spin light emitting diode
………………………………………………………………………………………… 75

Figure 5.3: Expanded view of RBS spectra for (a) pure TiO2, (b) Ti
0.94
Ta
0.06
O
2
targets, (c) PLD
deposited

TiO2 thin film and (d) Ti
0.94

Ta
0.06
O
2
thin film on a Si substrate showing the positions
of common magnetic elements responsible for producing
ferromagnetism……………………………………………………………………… 83

Figure 5.4: PIXE spectra of (a) pure TiO2, (b) Ti
0.94
Ta
0.06
O
2
targets, (c) PLD deposited

TiO
2
thin
film and (d) Ti
0.94
Ta
0.06
O
2
thin film on a Si substrate showing the positions of common magnetic
elements responsible for producing
ferromagnetism……………………………………………………………………… 84

Figure 5.5: A comparison of the wide scan XAS spectrum from the Ti

0.94
Ta
0.06
O
2
film with
theoretically calculated one, in which expected absorptions edges of Mn, Fe, Co, and Ni are
shown ……………………………………………………………………………………………86

Figure 5.6: TOF-SIMS spectrum of standards with 1% magnetic impurities of Fe, Cr, Mn, Ni
and Co in comparison with the spectrum from Ti
0.94
Ta
0.06
O
2
target and PLD deposited
Ti
0.94
Ta
0.06
O
2
film on silicon substrate ………………………………………………………….87

Figure 5.7: Atom probe tomography of the Ti
0.95
Ta
0.05
O

2
film optimally grown on (001) LaAlO
3

substrate. (a) Entire map. Green dots correspond to Ta atoms and purple dots correspond to La
atoms. (b) Atoms within a sliced volume……………………………………………………….88

Figure 6.1: Magnetic hysteresis loops for pure (black), Ti
1-x
Ta
x
O
2
(x~0.06) thin films grown at
600°C (blue) and 750°C (red) in oxygen partial pressure of 1×10
-5
Torr
…………………………………………………………………….…….……………………… 93


xvii

Figure 6.2: The absorption coefficient µ at (a) Ti L
2,3
edges and (b) O K edge of the pure TiO
2

and Ti
1-x
Ta

x
O
2
(x~0.06) films grown at 600 and 750°C where µ
+
and µ
-
are parallel and anti-
parallel alignments between the photon helicity and the sample magnetisation direction. The
corresponding SXMCD spectra for the (c) Ti L
2,3
edges and (d) the O K
edge……………………………………………………………….…….……………………… 95

Figure 6.3: (a) OMCD signals obtained from a Ti
1-x
Ta
x
O
2
(x~0.06) film grown at 600°C
showing the dichroism and spin-polarised magnetisation near the optical band gap. The vertical
dotted line represents the position of the optical band-gap. (b) Magnetic hysteresis loop from the
OMCD measurement showing ferromagnetic behaviour. In the inset, this loop is overlapped with
the 40 K SQUID data… …….…….…………………………………………………………….99

Figure 6.4: The XPS analysis of Ti
1-x
Ta
x

O
2
(x~0.06) films at the Ti 2p core levels (a) Pure TiO
2

film grown at 600°C and Ti
1-x
Ta
x
O
2
(x~0.06) films grown at (b) 600°C and (c) 750°C
……………………………………………………………….………………………………….100

Figure 6.5: Calculated density of states of Ta incorporated anatase TiO
2
system: (a) total DOS
(b) partial DOS for O 2p states. (c) t
2g
and e
g
of Ti 3d (d) t
2g
and e
g
of Ta 5d
… …….…….……………………………………………………………………… …………103

Figure 6.6: Comparison between experimental and calculated OMCD data. The blue line
through the OMCD experimental data points is a guide to the eye only ……… … …………104

Figure 6.7: The XAS for pure TiO
2
and Ti
1-x
Ta
x
O
2
(x~0.06) samples grown at (a) 600°C and (b)
750°C. The XAS for Ti
1-x
Ta
x
O
2
(x~0.06) sample grown at 600°C shows anomalous enhancement
of the spectral weight at t
2g
states compared to the pure TiO
2
grown at same temperature
confirming the formation of significant amount of V
Ti
in Ti
1-x
Ta
x
O
2

films. In contrast, the XAS
for Ti
1-x
Ta
x
O
2
(x~0.06) sample grown at 750°C shows decrease of the spectral weight at t
2g
states
compared to the pure TiO
2
grown at 750°C showing absence of V
Ti

………………………………………………… …… … ………………………………… 107

Figure 6.8: (a) Three-dimensional spin density plot of anatase TiO
2
with two V
Ti
. The yellow
isosurface represents the spin density of V
Ti
, and dashed green circles show the range of the
delocalized magnetic orbitals of V
Ti
(b) A schematic of the maximum possible ferromagnetic
ordering of magnetic centers (gray circle) at the sites of Ti vacancies coupled by itinerant
electrons mediated (RKKY) exchange mechanism

………………………………………………… ………………………… … … …………109

Figure 7.1: Variation of magnetization as a function of oxygen partial pressure (left ordinate).
Blue solid circles represent magnetization for multiple samples while the open star represents the
average value; Variation of carrier density as a function of oxygen partial pressure (right
ordinate) ……………… ………………………… … … ……………………………… 113



xviii

Figure 7.2: Variation of magnetization as a function of deposition temperature (left ordinate).
Blue solid circles represent magnetization for multiple samples while the open star represents the
average value; Decrease of minimum channeling yield for Ti and Ta with increasing deposition
temperature (right ordinate)… ……………… … … …………………………………… 114

Figure 7.3: Variation of magnetization as a function of Ta incorporation in Ti
1-x
Ta
x
O
2
thin films.
Blue solid circles represent magnetization for multiple samples while the open star represents the
average value … ……………… … ………………………….…………………………….116

Figure 7.4: Statistics for the magnetization data measured on samples grown at 600°C and 1x10
-
5
oxygen partial pressure … ……………… … …………………………………………….118

Figure 7.5: Variation of the actual activated carrier density as a function of Ta incorporation in
Ti
1-x
Ta
x
O
2
thin films (blue solid squares). Variation of calculated carrier density as a function of
Ta incorporation in Ti
1-x
Ta
x
O
2
thin films with 100% carrier activation (red solid circles).
Variation of inactivated carrier density as a function of Ta incorporation in Ti
1-x
Ta
x
O
2
thin films
(green open triangle) … ………………….………………………………………………… 119

Figure A1.1: Schematic of a Pulsed Laser Deposition ……………………………………….128

Figure A2.1: A futuristic graphene integrated circuit (not to scale), wherein the desirable
properties of various thicknesses of graphene layers are utilized along with strategic oxides
(SiO

2
, ferroelectric, ferromagnetic, multiferroic, etc.) in response to various external stimuli,
such as electric or magnetic fields. In the present illustration, the device structure is fabricated
from a very thin single-crystal graphite sheet after subsequent patterning/selective ablation. The
remaining graphite acts as a good ohmic contact and interconnection between the top Al
metallization (which also acts as a self-aligned mask, protecting the underlying graphite) and the
variable-thickness graphene-based devices … ……………………………………………….130

Figure A2.2: The laser irradiation-induced effects on a single and multilayer graphene at RT in
Ar atmosphere: (a) pristine; (b) 0.1 J/cm
2
; (c) 0.2 J/cm
2
; (d) 0.4 J/cm
2
…….………………….133

Figure A2.3: Raman spectra of the laser irradiated graphene samples whose images are
displayed in Fig. 2 (a-d), showing G, 2D, and D (in the inset) peaks …………… ………….134

Figure A2.4: The ablation of graphene layers as a function of laser energy density and graphene
layer-number N clearly showing the existence of the differences in E
Th
between single-, bi-or
more layers …………… …………………………135

Figure A2.5: The graphene layer-number N, as a function of 

of N-layers (normalized to
 shows an approximate N

-0.38
dependence at 5 eV. The inset shows as a function of
incident photon energy …………… 136




xix

Figure A2.6: The ablation threshold energy density (E
Th
)

is plotted as a function of graphene
layer-number N. The red solid line is the N
-0.38
dependence that arises from only , the green
solid line is the N
-1.38
dependence that arises from both  and flexural mode (C
f
) specific heat
(Eq. A2.2), and the blue solid line with the product of and total specific heat (Eq. A2.3)
…………… 139

Figure A2.7: The four-terminal device resistance (blue solid line) versus gate voltage of a
graphene sheet (dotted red line is the fit to Eq. A2.4) that has been irradiated at a laser energy
density of 0.2 J/cm
2
at RT in Ar atmosphere …………… 140

Figure A3.1: Schematic of a X-Ray Diffraction Technique 142

Figure A4.1: Schematic of a few radiative processes…… 145

Figure A5.1: (a) Schematic of a Rutherford backscattering experiment; (b) RBS spectrum in a
random unaligned mode…………………………… …… 147

Figure A5.2: RBS ion channeling mode for (a) perfect crystalline solid; (b) a disordered
lattice 148

Figure A6.1: Schematic of an atomic force microscopy system 149

Figure A7.1: Schematic of an ultraviolet (UV)-Visible spectrophotometer…… 151















xx

LIST OF SYMBOLS

PLD pulsed laser deposition
TCO transparent conducting oxide
DMS dilute magnetic semiconductors
RBS Rutherford backscattering
XRD X-ray diffraction
AFM atomic force microscopy
UV-Vis ultra violet- visible
PIXES proton induced X-ray emission spectroscopy
XAS X-ray absorption spectroscopy
SIMS secondary ion mass spectrometry
SXMCD soft X-ray magnetic circular dichroism
OMCD optical magnetic circular dichroism
SQUID superconducting quantum interference device
PPMS physical properties measurement system
XPS X-ray photoelectron spectroscopy
FM ferromagnetism
RT room temperature
ITO Indium Tin Oxide
LDA local density functional approximation
GGA generalized gradient approximation
HF Hartree-Fock
OLCAO orthogonalized linear combination of
atomic orbitals
VBM valence band maximum
CBM conduction band minimum
DOS density of states
LAO lanthanum aluminium oxide

STO strontium titanium oxide
FWHM full width half maximum
GMR giant magnetic resonance
MTJ magnetic tunnel junction
MRAM magnetic random access memory
FET field effect transistor
RKKY Ruderman-Kittel-Kasuya-Yosida
AHE anomalous hall effect
WL weak localization
MR magnetoresistance
AA Altshuler-Aronov
BMP bound magnetic polaron
CVD chemical vapour deposition
ALD atomic layer deposition
APT atomic probe tomography
1

Chapter 1
Introduction
Metal oxides are one of the most versatile material systems in terms of exhibited properties and
potential applications. Oxides can range from insulators to conductors [1], transparent
conductors [2] and even superconductors [3, 4]. They find applications in CMOS devices [5],
memristors [6], optoelectronic devices [7, 8], solar panels [9], detectors [10], fuel cells [11] and
even as chemicals or catalysts [12, 13]. Phenomenal progress has been made in the synthesis and
growth of high quality functional metal oxide films, nanomaterials and interface systems in
recent times [14]. This promises the emergence of an even more advanced frontier in the
research and application of „oxide electronics‟. Titanium oxide (TiO
2
) being one of the most
important oxides [15], the present thesis deals with defect mediated properties in single crystal

TiO
2
thin films.

1.1 Fundamental Physical Properties of TiO
2

1.1.1 Crystalline Structure of TiO
2

TiO
2
is a wide band gap semiconductor with three distinct structural polymorphs: rutile, anatase
and brookite. Rutile is the most stable structure in the bulk form and has been the more well
studied system among the three [16, 17]. The rutile structure is very simple as shown in Fig. 1.1a
It is characterized by the tetragonal space group P4
2
/mnm . The unit cell contains two TiO
2
units
with Ti at (0,0,0), ( 1/2 , 1/2 , 1/2 ), and O at ± (u,u,0), ± (u+1/2 , 1/2-u, 1/2). The lattice
parameters are: a=b=4.587 Å, c=2.954 Å, and u=0.305 Å
2





Figure 1.1: Crystal structure of (a) Rutile, (b) Anatase and (c) Brookite TiO
2

.

3

[18, 19]. Each Ti ion is octahedrally coordinated to six O ions and this TiO
6
octahedron is
distorted. The four equatorial O ions are in the plane of (110). The equatorial Ti-O bond length is
~1.95 Å, while the apical Ti-O bond length is ~1.98 Å. The octahedra form chains that share
edges along the [001] direction and share vertices in the (001) plane.
Anatase TiO
2
has been of more interest to the research community in the recent times. In its
anatase phase, TiO
2
has been shown to be metallic in nature, making it a possible replacement to
tin doped indium oxide (ITO). Adding to that, the Kondo effect has also been observed in the
anatase phase of TiO
2
, making it more interesting a system for studying defect mediated novel
properties. The anatase structure of TiO
2
, is shown in Fig. 1.1 (B). It belongs to the tetragonal
space group I4/amd [20, 21] with the unit cell containing two TiO
2
units. The Ti ions are at (0, 0,
0) and (0, 1/2, 1/4) and the O ions are at (0, 0, u), (0, 0,-u), (0, 1/2, u+1/4) and (0, 1/2, 1/2-u). The
lattice parameters are a=b=3.782 Å, c = 9.502 Å, and u = 0.208 Å [18, 19]. As in the rutile
structure, each Ti ion is octahedrally coordinated to six O ions. The octahedrons are also
distorted, with the short Ti-O bond length of ~1.93 Å and long bond length of ~ 1.98 Å, forming

zigzag chains along the [100] and [010] directions.
The brookite phase of TiO
2
is very unstable and has a complex structure as shown in Fig. 1.1 (C)
[20, 21]. It is characterized by the orthorhombic space group Pbca. The Ti ion is octahedrally
coordinated to six O ions as well. In contrast to that of rutile and anatase, the Ti-O bond length in
the octahedron is different from each other and ranges from 1.87 to 2.04 Å. The O-Ti-O bond
angle ranges from 77° to 105°. The common material properties of all the three phases are
tabulated below.