prr.hec.gov.pkprr.hec.gov.pk/jspui/bitstream/123456789/11818/1... · DECLARATION I Bushra Parveen, PhD Scholar in the department of Physics, University of the Punjab Lahore, hereby

Growth and Characterization of Sn-Based Transition Metal

(TM) Doped IV-VI Diluted Magnetic Semiconductors

BUSHRA PARVEEN

Department of Physics, University of the Punjab,

Quaid-e-Azam Campus, Lahore-54590, Pakistan

January 2019

Growth and Characterization of Sn-Based Transition Metal

(TM) Doped IV-VI Diluted Magnetic Semiconductors

A thesis submitted for the degree of

Doctor of Philosophy (PhD)

in

Physics

By

BUSHRA PARVEEN

Roll No. PhD-1403

(Session 2014-19)

Supervisor

Dr. Mahmood-Ul-Hassan

(Assistant Professor)

Department of Physics, University of the Punjab,

Quaid-e-Azam Campus, Lahore-54590, Pakistan

January 2019

In the name of Allah, the Most Gracious, the Most Merciful.

Dedication

To

My Parents, husband, daughters, and all family members

DECLARATION

I Bushra Parveen, PhD Scholar in the department of Physics, University of the

Punjab Lahore, hereby declare and certify that the material printed in this thesis

entitled “Growth and Characterization of Sn-Based Transition Metal (TM)

Doped IV-VI Diluted Magnetic Semiconductors” is my own original research

work, no part of thesis falls under plagiarism and has not been submitted as a whole

or in part for any degree or diploma at this or any other university. If I found guilty, at

any stage, I will be responsible for the consequences.

Signature: ________________

BUSHRA PARVEEN

CERTIFICATE

It is certified that the contents and form of the thesis entitled “Growth and

Characterization of Sn-Based Transition Metal (TM) Doped IV-VI Diluted

Magnetic Semiconductors” submitted by Bushra Parveen (Roll no. PhD-1403,

Session 2014-2019) have been found accurate for the requirement of degree of

Doctor of Philosophy in Physics.

Supervisor: ____ ___________________


Assistant Professor

Department of Physics

University of the Punjab, Lahore, Pakistan

Chairman: ______________________


Associate Professor

Department of Physics,

University of the Punjab, Lahore, Pakistan

i

ABSTRACT

The multifunctional and tunable properties of semiconductors can be achieved

by intentional doping of the host lattice with appropriate impurities. As the magnetic

dopants can induce magnetic order and result remarkable changes in the physical

properties, therefore, diluted magnetic semiconductors are being extensively studied.

SnS and SnO2 are promising IV-VI binary semiconductors, which have been widely

explored for optical applications, however, existing literature about their applications

as diluted magnetic semiconductors and dielectric properties are in scarce. In this

study, the structure, ferromagnetism and dielectric response of (Fe, Ni, Co, Mn

doped) SnS and (Zr, Mn doped) SnO2 nano-crystallites, fabricated using co-

precipitation technique, have been investigated. X-ray diffraction studies confirm the

successful fabrication of phase pure transition metal doped SnS without post-growth

heat treatment, while post-growth heating of the doped SnO2 is required for the phase

pure fabrication. The measured surface morphology shows non-uniform and denser

grain distribution. X-ray absorption spectroscopy is applied to elucidate the stabilized

oxidation state of the transition metals Co, Ni, Fe and Mn in the respective host

lattice. The dielectric and optical studies show high potential for optical device

applications. The impedance analysis shows that grains and grain boundaries

conduction exhibit different relaxation times. Furthermore, above room temperature

ferromagnetism is observed, which is quite attractive for magnetic device

applications. Hence, the studied diluted magnetic semiconductors are appealing not

only for fundamental physics but also for technical applications in the frequency

related devices and in data processing and storage devices.

ii

ACKNOWLEDGEMENTS

All the praises to Almighty Allah, Who is kind and Merciful. He is the Honor

of the Universe. Nothing is possible without His Mercy. From my deep heart and

soul, the unlimited thank to Almighty Allah, Who make me able to perform this

work. I also offers my humblest Drood-o-Salam to Holy Prophet Hazart

Muhammad (PBUH), Who led the universe as a torch of guidance and knowledge

and Whose life is an ideal example for human being.

I would like to extend my heartfelt gratitude to my supervisor Assistant

Professor Dr. Mahmood-Ul-Hassan (Department of Physics, University of the

Punjab Lahore, Pakistan), for his guidance, encouragement and excellent advice

throughout this research. He showed me different paths to approach a research

problem and the need to be persistent to accomplish any goal.

I am also thankful to the Chairman Dr. Mahmood-Ul-Hassan (Associate

Professor, Department of Physics, University of the Punjab Lahore), who allowed me

to work in my desired field. I have deep gratitude for his valuable advice.

Thanks to HEC Pakistan to provide me IRSIP grant for Nanjing University,

Nanjing, China to complete my PhD. I am also thankful to Professor Liang He

(Department of Electronic Science and Engineering, Nanjing University, Nanjing

China) to facilitate to complete my project.

On a more personal note, the support that I have received from my family over

the years is of immense value to me. I thank my parents, in-laws, sisters and brothers

for always standing by me. I am also grateful to Ms. Anjum Iqbal, Ms. Samina

Shahid, Ms. Farzana Zafar, M. Yaseen Bazmi and Hira Sarfraz, for all the help and

companionship. Aspecial thanks must be said to my husband Ali Abbas, my elder

sister Tasleem and my daughters, for always believing in me.

iii

LIST OF PUBLICATIONS

Ph.D Work

1. Parveen, B., Hassan, M., Atiq, S., Riaz, S., Naseem, S., & Zaman, S. 2017.

Structural, Dielectric and ferromagnetic properties of Nano crystalline Co

doped SnS. J Mater Sci, 52, 7369-7381. Impact factor 2.993, online

2. Parveen, B., Hassan, M., Atiq, S., Riaz, S., Naseem, S., & Toseef, M. A.

2017. Structural and dielectric study of Nano-crystalline single phase Sn1-

xNixS (xNi = 0-10%) showing room temperature ferromagnetism. Progress in

Natural Science: Mater Inter, 27(3), 303-310. Impact factor 2.572, online

3. Parveen, B., Hassan, M., Atiq, S., Riaz, S., Naseem, S., Irfan, M., & Iqbal, M.

F. 2018. Investigation of Physical properties of SnS:Fe diluted magnetic

semiconductor nanoparticles for Spintronic applications. J Mag Magnetic

Mater, 460, 111-119. Impact factor 3.046, online

4. Parveen, B., Hassan, Wattoo, A. G., Song, Z., M., Atiq, S., Riaz, S., Naseem,

S. Dielectric and Impedance spectroscopic analysis of Sn1-xZrxO2

ferromagnetic Semiconductors, under review.

5. Parveen, B., Hassan, Wattoo, A. G., Song, Z., M., Atiq, S., Riaz, S., Naseem,

S., Effect of post growth heating temperature on the stabilized phase and

physical properties of Sn1-xMnxO2, submitted.

6. Parveen, B., Hassan, M., Atiq, S., Riaz, S., Naseem, S., Dielectric and

Ferromagnetic Characterization of Sn1-xMnxS nanomaterials, submitted.

Other than Ph.D work

7. Parveen, B., Hassan, M., Khalid, Z., Riaz, S., Naseem, S. 2017. Room

temperature ferromagnetism in Ni doped TiO2 diluted magnetic semiconductor

thin films. J Appl Res Technol, 15(2), 132-139. Online

iv

8. Iqbal, M. F., Hassan, M., Ashiq, M. N., Iqbal, S., Bibi, N., Parveen, B. 2017.

High Specific Capacitance and Energy density of Synthesized Graphene

Oxide based Hierarchical Al2S3 Nanorambutan for Supercapacitor

Applications. Electrochimica Acta, 246, 1097-1103. Impact factor 5.116,

Online

9. Iqbal, M. F., Ashiq, M. N., Razzaq, A., Murtaza, G., Parveen, B., Hassan,

M.2018. Excellent Electrochemical Performance of Graphene Oxide based

Strontium Sulfide Nanorods for Supercapacitor Applications. Electrochimica

Acta, 273, 136-144. Impact factor 5.116, Online

10. Parveen, B., He, L., Wei, W., Hassan, M., Optical analysis of SnS nanowires

grown by chemical vapor deposition method, submitted

v

TABLE OF CONTENT

Page No.

Abstract ................................................................................................................... i

Acknowledgments.................................................................................................... ii

List of Publications .................................................................................................. iii

List of Tables ........................................................................................................... viii

List of Figures .......................................................................................................... x

Chapter 1 .................................................................................................................. 1

Magnetism in Semiconductors ..................................................................... 2

1.1 Spintronics ............................................................................................. 2

1.2 Diluted magnetic semiconductors .......................................................... 3

1.3 Origin of magnetism .............................................................................. 4

1.3.1 Types of magnetic materials ........................................................ 5

1.4 Magnetic interactions in DMS .............................................................. 7

1.4.1 Exchange interactions: Heisenberg model ................................... 7

1.4.2 Direct exchange ........................................................................... 8

1.4.3 Zener model: Indirect exchange .................................................. 9

1.4.4 RKKY interaction ........................................................................ 11

1.4.5 Bound magnetic polaron .............................................................. 12

1.5 Dielectric properties ............................................................................... 13

1.6 Material properties of SnS and SnO2 ..................................................... 14

1.7 Literature Review................................................................................... 16

1.7.1 II-VI group DMSs materials ............................................................... 16

1.7.2 III-V group DMSs materials ............................................................... 17

1.7.3 IV-VI group DMSs materials.............................................................. 19

1.7.3.1 Literature review of SnS ........................................................... 20

1.7.3.2 Literature of SnO2 .................................................................... 23

1.8 Significance of the study ....................................................................... 26

1.9 Overview of the thesis .......................................................................... 26

Chapter 2 .................................................................................................................. 28

Growth Method and Characterization Techniques ...................................... 29

2.1 Co-precipitation .................................................................................... 29

2.2 Characterization techniques ................................................................... 31

2.2.1 X-ray Diffraction (XRD) ............................................................. 31

2.2.2 X-ray absorption spectroscopy (XAS) ......................................... 32

2.2.3 Scanning Electron Microscopy .................................................... 36

2.2.4 Ultraviolet Visible Near-Infra red (UV-Vis-NIR)

spectroscopy ......................................................................................... 38

vi

2.2.4.1 Absorption spectroscopy ................................................ 38

2.2.4.2 Diffused reflectance spectroscopy ................................. 38

2.2.5 Impedance spectroscopy or dielectric spectroscopy .................... 40

2.2.6 Vibrating Sample Magnetometer ................................................. 42

Chapter 3 .................................................................................................................. 43

Study of Co and Ni Doped SnS Diluted Magnetic Semiconductors ........... 44

3.1 Synthesis of Co and Ni doped SnS ........................................................ 44

3.2 Structural study ...................................................................................... 46

3.2.1 X-ray diffraction analysis ............................................................ 46

3.2.2 X-ray absorption spectroscopic analysis ..................................... 52

3.3 Surface morphology ............................................................................... 54

3.4 Diffused reflectance spectroscopy ......................................................... 57

3.5 Dielectric study ...................................................................................... 61

3.5.1 Conduction mechanism ............................................................... 64

3.5.2 Cole-Cole or Nyquist plots .......................................................... 66

3.6 Determination of temperature dependent resistivity .............................. 71

3.7 Ferromagnetic characterization .............................................................. 72

3.8 Summary ................................................................................................ 77

Chapter 4 .................................................................................................................. 79

Growth and Characterization of Fe and Mn Doped SnS ............................. 80

4.1 Synthesis of Fe and Mn doped SnS ....................................................... 80

4.2 Structural properties ............................................................................... 82

4.2.1 X-ray diffraction studies .............................................................. 82

4.2.2 X-ray absorption spectroscopic analysis ..................................... 86

4.3 Surface morphology ............................................................................... 87

4.4 Diffused reflectance spectroscopic analysis .......................................... 90

4.5 Dielectric study ...................................................................................... 92

4.5.1 Conduction mechanism ............................................................... 94

4.5.2 Nyquist complex plane plot analysis ........................................... 96

4.6 Temperature dependent resistivity ........................................................ 100

4.7 Ferromagnetic study............................................................................... 101

4.8 Summary ................................................................................................ 105

Chapter 5 .................................................................................................................. 107

Synthesis and Characterization of Mn and Zr Doped SnO2 ........................ 108

5.1 Synthesis of Sn1-xZrxO2 and Sn1-xMnxO2 ............................................... 108

5.2 Characterization of Zr doped SnO2 ........................................................ 109

5.2.1 Structural study: XRD analysis ................................................... 109

5.2.2 Diffused reflectance spectroscopic analysis ................................ 112

5.2.3 Surface morphological study ....................................................... 114

vii

5.2.4 Dielectric study ............................................................................ 115

5.2.4.1 Conduction mechanism ................................................... 116

5.2.4.2 Complex impedance and Modulus analysis .................... 117

5.2.5 Ferromagnetic properties: VSM Study ........................................ 120

5.3 Characterization of Sn1-xMnxO2: effect of post-growth heat

treatment ..................................................................................................... 122

5.3.1 X-ray diffraction analysis ............................................................ 122

5.3.2 Diffused reflectance spectroscopic analysis ................................ 125

5.3.3 Surface morphology ..................................................................... 126

5.3.4 Dielectric study ............................................................................ 128

5.3.4.1 Conduction mechanism ................................................... 129

5.3.4.2 Complex impedance and Modulus analysis .................... 131

5.3.5 Magnetic Study ............................................................................ 134

5.4 Summary ......................................................................................... 137

Chapter 6 .................................................................................................................. 138

Comparison of the Physical Properties of Transition Metal Doped SnS

and SnO2 ...................................................................................................... 139

6.1 Comparison of the Structural properties ................................................ 139

6.2 Comparison of TM dopant effects on the Surface topography .............. 141

6.3 Shift in band gap due to TM dopants ..................................................... 142

6.4 Shift in L3 and L2 TM edges .................................................................. 144

6.5 Comparison of TM induced dielectric and Impedance responses ......... 145

6.6 Comparison of the TM induced Ferromagnetism .................................. 147

Chapter 7 .................................................................................................................. 151

Conclusions .................................................................................................. 152

Refrences.................................................................................................................. 154

viii

LIST OF TABLES

Table No. Titles Page No.

Table 3.1: The stoichiometric amounts of the precursors used to prepare

one molar solutions in 50ml distilled water to fabricate Co

and Ni doped SnS.

45

Table 3.2: Various structural parameters extracted from the measured

XRD line-scans for Co and Ni doped SnS.

52

Table 3.3: The grain resistance Rg, relaxation frequency f, grain

capacitance Cg and relaxation time tg, calculated from cole-

cole plots of Co and Ni doped SnS.

70

Table 4.1: Doping contents and stoichiometric amounts of SnCl2, Na2S,

FeCl2 and MnCl2 for synthesis of Fe and Mn doped SnS.

81

Table 4.2: Various structural parameters for pure, Fe and Mn doped

SnS.

85

Table 4.3: The calculated band gap values for pure and Fe and Mn

doped SnS.

92

Table 4.4 The grain resistance, relaxation frequency, grain capacitance

and relaxation time.

99

Table 4.5 The magnetization at 1 Tesla (M1T), remnant magnetization

(MR) and coercivity (HC) for Fe and Mn doped SnS.

103

Table 5.1: Molar masses of the precursors used to prepare Sn1-xZrxO2. 109

Table 5.2: Molar masses of the precursors used to prepare Sn1-xMnxO2. 109

Table 5.3: Various structural parameters calculated for Zr doped SnO2. 112

Table 5.4: Various parameters calculated using Nyquist plots for Zr

doped SnO2

120

Table 5.5: Various structural parameters extracted from the measured

XRD line-scans.

125

Table 5.6: The values of n, f, Rg, Cg and tg calculated from cole-cole

plots for Mn doped SnO2 heated at two different

temperatures.

134

Table 6.1: Crystallite size (D) and strain (ε) extracted from the XRD

measurements for TM (Mn, Fe, Co, Ni) doped SnS.

140

Table 6.2: Crystallite size (D) and strain (ε) extracted from the XRD

measurements for TM (Zr, Mn) doped SnO2.

140

ix

Table 6.3: The band gap (Eg) values for pure and TM (Mn, Fe, Co and

Ni) doped SnS.

143

Table 6.4: Grains resistance (Rg) and grains capacitance (Cg) for Sn1-

xTMxS.

146

Table 6.5: Grains resistance (Rg) and grains capacitance (Cg) for Sn1-

xTMxO2.

146

Table 6.6: The magnetization at 1T (M1T) for Sn1-xTMxS (TM = Mn, Fe,

Co, Ni)

147

Table 6.7: The magnetization at 1T (M1T), remnant magnetization (MR),

coercivity (HC) for Sn1-xTMxO2.

149

x

LIST OF FIGURES

Figure No. Titles Page No.

Figure 1.1: Schematic diagram showing (a) magnetic semiconductor,

(b) nonmagnetic semiconductor and (c) diluted magnetic

semiconductor.

4

Figure 1.2: Various types of magnetic materials and their

magnetization and susceptibility plotted versus applied

magnetic field and temperature, respectively (1)

Diamagnets (2) ferrimagnets (3) ferromagnets (4)

paramagnets (5) antiferromagnets (Jiles, 1990).

6

Figure 1.3: The Bethe–Slater curve showing magnetic natures. 9

Figure 1.4: Formation and interaction of bound magnetic polarons. 13

Figure 1.5: (a) Layered tetragonal SnS structure. (b) Tetragonal unit

cell of SnO2.

15

Figure 1.6: Curie temperatures of various semiconductors. 19

Figure 2.1: Flow chart showing various steps used in the co-

precipitation method.

30

Figure 2.2: Schematic diagram of X-ray diffractometer. 32

Figure 2.3: The electronic transitions from 2S, 2p (j = 1/2) and 2p (j =

3/2) to the continuum state.

34

Figure 2.4: (a) Basic design of a modern synchrotron and (b) various

components of a typical modern XAS beamline.

35

Figure 2.5: The construction of scanning electron microscopy. 36

Figure 2.6: Representation of electron beam-matter interaction and

emission of various types of electrons.

37

Figure 2.7: Schematic diagram showing diffused reflectance

spectroscopy.

39

Figure 2.8: (a) is representing a dielectric capacitor (b) Nyquist plots

and their equivalent circuit.

41

Figure 2.9: A schematic diagram showing construction of VSM. 42

Figure 3.1: XRD line-scans measured for (a) Co doped and (b) Ni

doped SnS showing diffraction peaks from various

crystallographic planes. A single phase formation with a

polycrystalline nature is evident.

48

xi

Figure 3.2: Crystallite size and strain plotted against (a) Co and (b) Ni

content in SnS.

51

Figure 3.3: NEXAFS spectra of (a) Co L-edges and (b) Ni L-edges. 54

Figure 3.4: SEM images measured for Co doped SnS (a-f, x = 0-10%). 56

Figure 3.5: SEM images measured for Ni doped SnS (a-f, y = 0-10%). 56

Figure 3.6: Diffused reflectance spectra measured for (a) Co and (b) Ni

doped SnS. The plots showing direct absorption edges for

(c) Co and (d) Ni doped SnS.

58

Figure 3.7: Band gap energy plotted against Co and Ni doping contents

in SnS.

59

Figure 3.8: Decay of band gap with the doping contents. 59

Figure 3.9: The schematic of Moss-Burstein shift in band gap energy. 60

Figure 3.10: Dielectric constant for (a) Co and (b) Ni doped SnS.

Tangent loss for (c) Co and (d) Ni doped SnS.

63

Figure 3.11: The ac conductivity of (a) Co and (b) Ni doped SnS.

Jonsher’s power law for (c) Co and (d) Ni doped SnS.

66

Figure 3.12: Real part of impedance for (a) Co and (b) Ni doped SnS,

Imaginary part of impedance for (c) Co and (d) Ni doped

SnS plotted against frequency.

67

Figure 3.13: Nyquist plot for (a) Co and (b) Ni doped SnS determined

using real and imaginary impedance. Nyquist plot for (c)

Co and (d) Ni doped SnS determined using real and

imaginary modulus.

69

Figure 3.14: Resistance and resistivity versus temperature curves for (a)

4% Co and (b) 4% Ni doped SnS.

72

Figure 3.15: VSM loops measured for (a) Co and (b) Ni doped SnS. 74

Figure 3.16: Magnetization at 1T for (a) Co and (b) Ni doped SnS.

Retentivity (MR) for (c) Co and (d) Ni doped SnS.

Coercivity (HC) for (e) Co and (f) Ni doped SnS.

76

Figure 4.1: XRD-line scans measured for (a) Sn1-xFexS (x = 0-10%)

and (b) Sn1-yMnyS (y = 0-10%).

83

Figure 4.2: Crystallite size and strain for (a) Fe and (b) Mn doped SnS

plotted against dopant content.

84

Figure 4.3: NEXAFS spectra measured for (a) Fe L-edges and (b) Mn

L-edges.

87

xii

Figure 4.4: SEM images measured for Fe doped SnS (a-f, x = 0-10%). 88

Figure 4.5: SEM images measured for Mn doped SnS (a-f, y = 0-10%). 89

Figure 4.6: Diffused reflectance spectra for (a) Fe (x = 0-10%) and (b)

Mn (y = 0-10%) doped SnS. Direct band gap spectra for (c)

Fe (x = 0-10%) and (d) Mn (y = 0-10%) doped SnS.

91

Figure 4.7: Real dielectric constant plotted for (a) Fe (x = 0-10%) and

(b) Mn (y = 0-10%) doped SnS. Tangent loss plotted for

(c) Fe (x = 0-10%) and (d) Mn (y = 0-10%) doped SnS.

94

Figure 4.8: Conductivity versus applied frequency for (a) Fe and (b)

Mn doped SnS. Jonsher’s power law for (c) Fe and (d) Mn

doped SnS.

95

Figure 4.9: Real part of impedance for (a) Fe and (b) Mn doped SnS.

Imaginary part of impedance for (c) Fe and (d) Mn doped

SnS plotted versus applied frequency.

97

Figure 4.10: Nyquist impedance plots for (a) Fe and (b) Mn doped SnS.

Nyquist electric modulus plots for (c) Fe and (d) Mn doped

SnS.

98

Figure 4.11: Resistance and resistivity versus temperature for (a) 4% Fe

and (b) 4% Mn doped SnS.

100

Figure 4.12: VSM loops of (a) Fe and (b) Mn doped SnS. 102

Figure 4.13: Crystallite size and coercivity plotted against (a) Fe and (b)

Mn content.

104

Figure 5.1: XRD line-scans measured for Zr doped SnO2. 110

Figure 5.2: Crystallite size and strain plotted against Zr content (0-

10%).

111

Figure 5.3: The direct band gaps calculated using Kubelka-Munk

approach.

113

Figure 5.4: SEM images (a-f) measured for Zr doped (0-10%) SnO2. 114

Figure 5.5: The (a) real dielectric constant and (b) dielectric tangent

loss plotted versus log of frequency.

115

Figure 5.6: The (a) ac conductivity and (b) Jonsher’s power law for Zr

doped SnO2.

117

Figure 5.7: The (a) real and (b) imaginary impedance plotted against

frequency.

118

Figure 5.8: The Nyquist plots resulted by plotting (a) 𝐙΄΄ versus 𝐙΄ and (b) 𝐌΄΄versus 𝐌΄.

119

xiii

Figure 5.9: VSM spectra for Zr doped SnO2. 121

Figure 5.10: Ms, Mr and HC plotted versus Zr. 122

Figure 5.11: XRD lines scans for Mn doped SnO2 heated for 6 hr at (a)

500 °C and (b) 650 °C.

123

Figure 5.12: The lattice constants for Mn doped SnO2 after heating at (a)

500 °C and (b) 650 °C. The crystallite size and strain after

heating at (c) 500 °C and (d) 650 °C.

124

Figure 5.13: Direct band gap of Mn doped SnO2 after heating at (a-d)

500°C and (e-h) 650°C.

126

Figure 5.14: SEM images of Mn doped SnO2 heated at (a-d) 500°C and

(e-h) 650°C.

127

Figure 5.15: Dielectric constant of Mn doped SnO2 heated at (a) 500 °C

and (b) 650 °C. Tangent loss when heated at (c) 500 °C

and (d) 650 °C.

128

Figure 5.16: (a, b) ac-conductivity and (c, d) Jonsher’s power law for

Mn doped SnO2 heated at 500 °C and 650 °C, respectively.

131

Figure 5.17: (a, b) real and (c, d) imaginary parts of the impedance

plotted against frequency for Mn doped SnO2 heated at 500

°C and 650 °C, respectively.

132

Figure 5.18: Nyquist plots determined by plotting real and imaginary

parts of (a, b) impedance and (c, d) modulus for Mn doped

SnO2 after heating at 500 °C and 650 °C, respectively.

133

Figure 5.19: Hysteresis loops measured for pure and Mn doped SnO2 after heating at (a-d) 500 °C and (e-h) 650 °C.

135

Figure 5.20: The parameters Ms, Mr and HC plotted against Mn content

in SnO2 after heating at (a) 500 °C and (b) 650 °C. A

comparison of crystallite size and coercivity after heating

at (c) 500 °C and (d) 650 °C.

136

Figure 6.1: Surface morphology of Mn, Fe, Co and Ni doped SnS. 141

Figure 6.2: Surface morphology of Zr and Mn doped SnO2. 142

Figure 6.3: The band gap values for Zr and Mn doped SnO2. 144

Figure 6.4: L3 edges of Mn, Fe, Co and Ni doped SnS. 145

Figure 6.5: The coercivity values for Zr and Mn doped SnO2. 150

Chapter 1 Magnetism in Semiconductors

1

Chapter 1

Magnetism in Semiconductors


2

Magnetism in Semiconductors

As the inducing of magnetism in nonmagnetic semiconductors is of prime

scientific interests, therefore, in this chapter, origin of magnetism and various

magnetic interactions are described. The worth of the dielectric properties and the

material properties of SnS and SnO2 for studied dielectric and magnetic applications

are given. A complete understanding of the magnetic properties and a brief

description of dielectric properties are presented.

1.1 Spintronics

The introduction of computers has revolutionized the everyday life. The

computer processing is entirely dependent upon the silicon based semiconductor

technology. The spin transport phenomena in metals and semiconductors have

revealed the novel potential applications in the emerging electronic technologies

(Wolf et al., 2001). The fundamental electronic charge is the main factor involved for

the information transfer in silicon based integrated circuit technology. The extensive

demands to enhance the device efficiency and reduce the dimension suggest involving

the spin degree of freedom of electron. The material resistance sensitive to the applied

field was reported by W. Thomson in 1850s, which is called magneto-resistance (MR)

(Thomson, 1856). A large magneto-resistance effect (2%) was observed in

ferromagnetic metals and their alloys. This change in resistance with the direction of

applied magnetic field is called as anisotropic magneto-resistance (AMR), which

arises due to the spin-orbit coupling (Kondo, 1962). A very large change in such a

resistance is called as giant magneto-resistance (GMR), which was discovered by

Albert Fert and Peter Grünberg (Nobel Prize in 2007) in Fe/Cr multilayers (Baibich et

al., 1988). The magnetic layers couple ferromagnetically or antiferromagnetically


3

depending upon the thickness of the sandwiched nonmagnetic layer (Binasch et al.,

1989). Such a device structure exhibiting GMR is called as spin valve.

The applications of spin valves as read heads in hard disks has revolutionized

the data storage technology. Hence, GMR effect tunes electrical resistance due to the

spin of electrons (Chappert et al., 2007), while in the conventional electronics

transport properties are resulted due to the charge of electrons. Researchers are

continuously applying different approaches to design spintronic devices to achieve

highly efficient spin polarized transport. The GMR technology can be improved by

discovering the novel materials and optimizing the existing materials by applying

various methodologies to generate and utilize spin-polarized currents in innovative

and efficient ways. The spin based transport of electrons leads to spintronics (Bland et

al., 2008). The control over electronic spin facilitates to perform much faster

calculations as compared to the conventional electronics.

The spintronics has applications in hard disk drives, where GMR effect is employed.

The control over the electronic charge in semiconductors and spin in ferromagnets is

an attractive field to realize both characteristics in a single material (Awschalom and

Flatte, 2007).

1.2 Diluted magnetic semiconductors

To develop a spintronics based semiconductor device, ferromagnetic

semiconductors compatible with the existing microelectronic technology are needed

(Wolf et al., 2001). Therefore, traditional nonmagnetic semiconductors from III-V, II-

VI and IV-VI groups are slightly doped with a fraction of transition metal (TM) or

rare-earth elements to realize diluted magnetic semiconductors (DMS). Such DMSs

could be integrable with the existing semiconductor industry. A schematic diagram

representing magnetic semiconductors, nonmagnetic semiconductors and diluted


4

magnetic semiconductors, is shown in Figure 1.1. Magnetic semiconductor is shown

in figure 1.1a, in which one of the two elements has local magnetic moment. Figure

1.1b composed of two nonmagnetic elements, represents a nonmagnetic

semiconductor. Figure 1.1c shows DMS in which few atoms of a non-magnetic

semiconductor are substituted by magnetic elements having local magnetic moments.

Figure 1.1: Schematic diagram showing (a) magnetic semiconductor, (b)

nonmagnetic semiconductor and (c) diluted magnetic semiconductor.

DMS provide a phenomenological playground for spintronics because we can

control both charge and spin of electrons (Zutic et al., 2004, Dietl, 2010). Hence,

spintronic based semiconducting device applications demand very careful handling

and manipulation of spin (Chappert et al., 2007). The free charge carrier

concentrations in the host lattice caused by the magnetic dopants can also tune the

ferromagnetic order (Dietl, 2010).

1.3 Origin of magnetism

The origin of magnetism and spin-orbit interactions needs to be explained in

order to understand various features of spintronics. Magnetism is the response of any

material to the external magnetic fields. The electronic spin and orbital motion results

in a net magnetic moment. The magnetic moments arising due to partially occupied d

or f subshells cause a net magnetic field that interacts to the external magnetic field.


5

The magnetic moments associated with spin and orbital motions of electron interact,

giving rise to splitting of the energy levels which decides the collective order of

magnetism in any material.

1.3.1 Types of magnetic materials

The magnetic materials are classified according to the magnetic

susceptibility (χ) (Jiles, 1990), which is mathematically defined as, χ = M/H, where H

is applied magnetic field. The magnetization (M) is defined as the magnetic moment

(µ) per unit volume (V). Various types of magnetic materials, according to the net

magnetization (M) and susceptibility (χ) are shown in Figure 1.2.

1. Diamagnetic materials oppose the external magnetic field H, by producing a

negative magnetization, therefore, exhibit small and negative susceptibility.

Various examples of diamagnets are Cu, Ag, Au, Bi and Be. Diamgnetic

materials demonstrate linear behavior of the opposing magnetization to the

applied magnetic field and this trend remains temperature independent.

2. Ferrimagnetic materials are formed by two sublattices having antiparallel

magnetic moments. The unequal magnetic moments result in a net

spontaneous magnetization with positive nonlinear response to the applied

field. These materials have positive and large susceptibility values. The well-

known examples of ferrimagnets are spinel ferrites such as Fe3O4.

3. Ferromagnetic materials contain many domains magnetized in different

directions, which can be strongly aligned due to applied magnetic field to

attain saturation in one direction. Ferromagnetic materials exhibit positive and

large magnetic susceptibility. The famous transition metals Fe, Co and Ni are

ferromagnetic in nature and show nonlinear hysteresis response.


6

Ferromagnetic materials show transition to the paramagnetic phase at Curie

temperature TC.

4. Paramagnetic materials are categorized as having randomly aligned magnetic

moments giving zero net magnetization. However, M increases linearly due to

the externally applied magnetic field. Paramagnets exhibit very weak

magnetization even under the influence of the strong magnetic field, therefore,

susceptibility is positive and small. The Al and Pt are famous paramagnets.

5. Antiferromagnetic materials also consist of two sublattices but with

antiparallel magnetic moments, therefore, net magnetic moment is zero. The

increased temperature above Neel temperature (TN) converts it into

paramagnet. A finite value of M appears when temperature increases under the

presence of a magnetic field, however, above TN it becomes similar to a

typical paramagnet. The NiO, FeO and CoO are examples of

antiferromagnetic materials. Their χ values are similar to those for a typical

paramagnet.

Figure 1.2: Various types of magnetic materials and their magnetization and

susceptibility plotted versus applied magnetic field and temperature,

respectively (1) Diamagnets (2) ferrimagnets (3) ferromagnets (4)

paramagnets (5) antiferromagnets (Jiles, 1990).


7

1.4 Magnetic interactions in DMS

Various magnetic interactions may operate within a magnetic material to

exhibit a net magnitude and type of magnetism (Stoner and Wohlfarth, 1948). As the

ferromagnetic materials exhibit d-orbital splitting at the Fermi level, therefore,

exchange interactions of relatively complex nature exist in diluted magnetic

semiconductors (DMSs). The s and p orbitals of the host lattice interact with d-

orbitals of transition metals to result sp-d hybridization. The free carrier mediated

ferromagnetism and the role of sp-d exchange interaction is highly influenced by the

magnetic doping contents in DMS. The random distribution of magnetic impurities in

the host lattice also affects the magnetic order. The magnetic order can be tuned by

the density of free carriers, which result indirect exchange mechanism,that was

suggested by Zener and Ruderman-Kittel-Kasuya-Yosida (RKKY). Although various

models may not completely elucidate the real underlying mechanism but still these are

very helpful to evaluate a material for practical device implementations (Furdyna,

1988), because these models can give relatively an appropriate description of the

exhibited magnetism. Various models applied to justify the magnetic nature are

described next.

1.4.1 Exchange interactions: Heisenberg model

Heisenberg model theoretically models two electronic spins by showing two

energy states appearing due to the relative orientation and arrangement of both spins,

which can be expressed for two interacting electronic spins (S1 and S2) according to

the equation:

𝐸𝐻 = −2 𝐽𝑒 𝑆1. 𝑆2 (1.1)

Where, 𝐸𝐻 and and Je show Heisenberg exchange energy and exchange

integral, respectively. This model can also express two electronic spins of neighboring


8

atoms in the lattice. The exchange interaction for such two electrons of neighboring A

and B atoms can be written in terms of the spatial wavefunctions (Jiles, 1998):

𝐽𝑒 = ∫ 𝑑3𝑟1 𝑑

3𝑟2𝛹𝐴∗(𝑟1)𝛹𝐵

∗(𝑟2) {1

𝑅𝐴𝐵−

1

𝑟𝐴2−

1

𝑟𝐵1+

1

𝑟12} 𝛹𝐵(𝑟1)𝛹𝐴(𝑟2) (1.2)

The first term expresses mutual repulsions between both spins. The second

and third terms show attraction between the electrons of either A or B atom due to the

B or A ion, respectively. Fourth term shows mutual repulsions between the electronic

charge densities. Generally, Heisenberg model describes isolated two magnetic ions

with partially filled outer shells; however, it can also be generalized for real systems

having many spins (Cohen, 1991).

1.4.2 Direct exchange

Bethe–Slater curve describe the reasons for metals being ferromagnetic or not.

Basically, exchange energy in transition metals depends upon the ratio of the

interatomic distance (ra) to radius of 3d electron shell (r3d) (Sommerfeld et al., 1933,

Callen et al., 1977). The exchange integral (Je) is very small and positive at large

interatomic distances but it increases to maximum with decaying ra. However, with

further decrease in ra, Je reduces and becomes negative, as shown in Figure 1.3. The

magnetic ions Fe, Co and Ni show positive Je to reveal intrinsic ferromagnetic nature,

while, Mn and Cr with negative Je express antiferromagnetic interactions. Modern

computational methods have also verified this curve for 1D chain of transition metals

i.e. from vanadium to cobalt (Morán et al., 2003).


9

Figure 1.3: The Bethe–Slater curve showing magnetic natures (Callen et al., 1977).

1.4.3 Zener model: Indirect exchange

The Zener model relates localized magnetic moments interacting with the

delocalized electrons through exchange interactions. Zener described Bethe-Slater

curve as an incomplete representation of the direct exchange interactions in the atoms

having partially filled d-shells (Mokrousov et al., 2007). He suggested

antiferromagnetic nature is exhibited due to direct exchange at any ra value, which is

due to localized d-shell electrons. Two atoms A and B with well separated spatial

coordinates can be shown as:

𝑟𝐴2 ≈ 𝑟𝐵1 ≈ 𝑟12 ≈ 𝑟𝐴𝐵 (1.3)

Due to this condition, 1 𝑟𝐴𝐵⁄ > 0, which makes Je positive and contradicts

Zener’s model. Zener proposed that nearest neighbor antiferromagnetic interactions

occur due to interactions between the localized magnetic moments of the d-electrons

and delocalized s electrons. In solids, such interactions are prominent and are of

higher magnitudes due to the significant role of the hybridization, in contrast to that in

the isolated atoms and ions. Ferromagnetism occurs if s-d exchange interaction


10

becomes stronger than the antiferromagnetic direct exchange (Sato et al., 2000, Sato

et al., 2001). Zener expressed these s-d interactions using the formula:

∆𝐸𝑓𝑒𝑟𝑟𝑜 = 1

2 𝛼 𝑆𝑑

2 − 𝛽 𝑆𝑑 𝑆𝑐 + 1

2 𝛾 𝑆𝑐

2 (1.4)

Where, Sd and Sc are spins of localized (d-shell) and s-shell (conduction)

electrons, respectively. The parameters 𝛼 and 𝛽 show antiferromagnetic and

ferromagnetic s-d coupling, respectively. While, 𝛾 shows kinetic energy increase due

to ferromagnetic interactions between the conduction electrons. According to the

Stoner ferromagnetic model, 𝛼, 𝛽 and 𝛾 have values only a few eV, about 1 eV and

several eV, respectively. From the minimization of equation (1.4) w.r.t. Sc, the value

of Sc is 𝑆𝑐 = 𝛽

𝛾 𝑆𝑑. Hence, the expression for ferromagnetic energy is:

∆𝐸𝑓𝑒𝑟𝑟𝑜 = 1

2 (𝛼 −

𝛽2

𝛾) 𝑆𝑑

2 (1.5)

The criterion for ferromagnetism comes out as 𝛽2

𝛾

> 𝛼 and such exchange

integral might be several eV higher than the direct exchange integral. Hence, Zener

suggested that indirect exchange between the localized magnetic moments is mediated

by the conduction electrons. Furthermore, superexchange may also be an indirect

exchange in which two localized magnetic ions coupled due to the nonmagnetic ions

(e.g. Fe- O- Fe or Mn - O - Mn) instead of the conduction electrons, and this may

result either ferromagnetic or antiferromagnetic coupling.

Dietl et al. presented Zener model, which describes the appearance of RTFM

in heavily p-type doped ZnO (Dietl et al., 2000; Pearton et al., 2003). Subsequently,

few density functional theory (Sato et al., 2000, Sato et al., 2001) and experimental

studies (Ueda et al., 2001, Prellier et al., 2003) revealed that Co doped ZnO with n-

type nature also results RTFM.


11

1.4.4 RKKY interaction

Although Zener model is suitable to explain indirect exchange interactions,

however, it is limited to describe the decaying stability of the ferromagnetic phase as

the conduction electron density (nc) increases. Normally, semiconductors have nc ~

1018 - 1025 m-3, while, doping can further increase it in the range 1027 - 1028.

Therefore, Zener model cannot accurately cover the ferromagnetic stability when

carrier density enhances, and further complications arise when donors or acceptors

impurities result vacancies in the lattice (Stoner, 1938, Nolting and Ramakanth,

2009). The total concentration of carriers can be expressed as:

𝑛𝑐 = 𝑛𝑐0 + 𝛿𝑛 (1.6)

Where, 𝑛𝑐0 and 𝛿𝑛 show the electron or hole concentrations caused by the

doping elements and external factors, respectively. This challenge was addressed by

Ruderman, Kittel, Kasuya and Yosida in 1950’s, by presenting RKKY (Ruderman,

Kittel, Kasuya and Yosida) model. This model explains the ferromagnetic interactions

in DMSs by describing the coupling of the individual magnetic moments with the

conduction electrons (Fardyna, 1988). According to RKKY model, ferromagnetic

state is stable if 0.13 < (nc/ns)


12

Zunger et al. and Bahadur et al., study the effect of various magnetic dopants

to stabilize the room temperature ferromagnetism. They explain the carrier mediated

ferromagnetism by RKKY model. According to this, the natue of magnetism is

decided by the carrier density (Zunger et al., 2010, Bahadur et al., 2012). Theoretical

investigations about various III-V and II-VI have also widely explored the DMSs

applications (Dietl, 2002, Lee et al., 2002, Konig et al., 2003, Dietl, 2003). The

influence of the TM doping on the structural and magnetic properties has been

revealed to justify TC and the mechanism of carrier induced ferromagnetism using

RKKY interactions (Sanvito et al., 2002, Sato and Katayama-Yoshida, 2002).

1.4.5 Bound magnetic polaron

The sp-d exchange interactions between the localized magnetic element and

conduction electrons can explain the valence band splitting due to the magnetic field

and the polaronic effects (Coey et al., 1999). A bound magnetic polaron (BMP) is

formed when free electrons or holes get trapped in an orbit for certain magnetic

moments. Such BMPs may couple with either parallel or anti-parallel orientations

corresponding to the different energies. However, this coupling is highly temperature

dependent. The magnetic interactions are evaluated by contrasting s-d exchange

energy and thermal energy KBT. The ferromagnetic state occurs at lower temperatures

when exchange energy surpasses KBT. Coey et al. presented a model for n-type

DMSs and explained BMP formation in oxides (Coey et al., 1999). In this model,

electrons are resulted due to the oxygen vacancies, which exhibit certain orbitals of

distinctive Bohr radii, as shown in Figure 1.4. These electrons align magnetic

moments lying in their orbits, and such an indirect exchange coupling causes

ferromagnetic ordering.

http://unix12.fzu.cz/ms/bibref.php?hid=Dietl:2002_bhttp://unix12.fzu.cz/ms/bibref.php?hid=Dietl:2002_bhttp://unix12.fzu.cz/ms/bibref.php?hid=Lee:2002_ahttp://unix12.fzu.cz/ms/bibref.php?hid=Lee:2002_ahttp://unix12.fzu.cz/ms/bibref.php?hid=Konig:2003_ahttp://unix12.fzu.cz/ms/bibref.php?hid=Konig:2003_ahttp://unix12.fzu.cz/ms/bibref.php?hid=Dietl:2003_ahttp://unix12.fzu.cz/ms/bibref.php?hid=Dietl:2003_ahttp://unix12.fzu.cz/ms/bibref.php?hid=Sanvito:2002_bhttp://unix12.fzu.cz/ms/bibref.php?hid=Sanvito:2002_bhttp://unix12.fzu.cz/ms/bibref.php?hid=Sato:2002_ahttp://unix12.fzu.cz/ms/bibref.php?hid=Sato:2002_a


13

Kuppan et al., studied the induction of magnetism in Mn and Fe doped SnO2.

The induced magnetism was mainly dependent on xygen vacancies and their

interactions with magnetic ions. This interaction leads to ferromagnetism depending

on interactions between bound magnetic polaron and magnetic moments (Kuppan et

al., 2016, Kuppan et al., 2017).

Figure 1.4: Formation and interaction of bound magnetic polarons (Coey et al.

1999).

As evident from the above text, the magnetic nature might be exhibited due to

a variety of underlying mechanisms. Therefore, complete understanding of these

mechanisms is important for device applications. Furthermore, the presence of

dielectric properties could also offer versatile frequency related device applications.

1.5 Dielectric properties

Electrical properties of the grown samples and impact of the grain boundaries

can be elucidated by the dielectric measurements. The dielectric characteristics

mainly depend on the electronic, dipolar, ionic and space charge contributions.


14

Electronic polarization plays most important role in the variation of dielectric

properties of polycrystalline materials in the bulk form, which is exhibited in the

optical frequency range. The ionic polarizations arise due to relative separation of

negative and positive ions (Suresh, 2014). The permanent electric dipole moment of

the molecules shows dipolar (orientation) or space charge polarizations. These

molecules reorient in response to the applied electric field. According to the space

charge polarization, dielectric material may have conductive grains, which are

separated by grain boundaries offering high resistance. The dielectric constant is the

basic electrical property to elucidate the potential applications of a material, as it

expresses the nature of the electrical phenomenon (Hashem and Abouelhassan, 2005).

1.6 Material properties of SnS and SnO2

SnS and SnO2 belonging to IV-VI group, exhibit relatively narrow and wide

band gaps, respectively. These are efficient and less explored materials, which can be

grown by low-cost fabrication technique, having capability to exhibit not only the

ferromagnetic behavior but also the optimum dielectric response, suggesting their

potential for future device utilizations. The selected materials properties are given

below.

SnS (Herzenbergite) is a brown colored solid having layered orthorhombic

crystal structure, which can be considered as distorted NaCl structure (Yue et al.,

2009, Shallal, 2015), as shown in Figure 1.5 (a).


15

Figure 1.5: (a) Layered Orthorhombic SnS structure (Yue et al. 2009; Shallal 2015).

(b) Tetragonal unit cell of SnO2 (Das and Jayaraman, 2014).

The weak van der waals forces in SnS cause strong anisotropic vibrational

characteristics (Xio et al., 2015, Abdelrahman et al., 2012, Sun et al., 2015). The

layered structure of SnS is difficult to fabricate as compared to the cubic compounds

e.g., CdS, CdSe and PbS etc. Moreover, SnS nano-crystallites exhibit material

characteristics entirely different from the bulk. Furthermore, the stoichiometry of Sn

and S may also result different crystallographic phases e.g. SnS, Sn2S3, Sn3S4 and

Sn4S5 (Chaki et al., 2015).

SnS exhibits direct band gap in the range 1.3-1.5e V, however, it can also

exhibit indirect band gap in the range 1.0-1.1 eV (Sato et al., 2005, Ogah et al., 2011).

The orthorhombic lattice parameters of SnS also known as α-SnS are a = 3.978Å, b =

4.328Å and c = 11.193Å (Vidal et al., 2014, Nasirov and Adgezalova, 2001) and may

exhibits the space groups Pmnb (Chattopadhyay et al., 1986), Pcmn (Gomes and

Carvalho, 2015) and Pnma (Burton and Walsh, 2013). Its high absorption coefficient

(>104cm-1), and holes density in the range of 1015 – 1018 cm-3 reveal applications of

SnS as absorber layer in the solar cells (Ghosh et al., 2010). The non-toxic and low-

cost SnS exhibiting p-type nature (n-type nature can also be realized) elucidate the


16

technological significance (Cheng et al., 2006, Noguchi et al., 1994, Hegde et al.,

2011). Therefore, pure and doped SnS has numerous potential device applications.

Tin dioxide also called as stannic oxide (SnO2) is water insoluble inorganic,

colorless and orderless compound with mineral name cassiterite. It is soluble in hot

concentrated alkalis and concentrated acid. It is diamagnetic having susceptibility

−4.1×10−5 cm3/mol. SnO2 has rutile tetragonal structure, as shown in Figure 1.5 (b),

with lattice parameters a = b = 4.737Å, c = 3.185Å (Das and Jayaraman, 2014). It

exhibits a wide band gap of 3.6 eV (Batzill and Diebold, 2005).

SnO2 has applications as gas sensors, photovoltaic, supercapacitors, LEDs and

solar cells (Chen et al., 2003). SnO2 is highly transparent to the visible energy and

shows large reflectivity to the infra-red region (Batzill and Diebold, 2005).

1.7 Literature Review

The semiconductors based devices exhibiting ferromagnetism can be used for

multi-functional devices that can enhance the worth of the microelectronic industry

(Zutic et al., 2004). The diluted magnetic semiconductors are considered as the most

important in the materials science and condensed-matter physics. The multifunctional

properties of DMSs have attracted huge attention of the scientific community. The

room temperature ferromagnetism in DMS materials is of great interest and

challenging for spintronic applications. There are mainly three classes of DMS

materials, and various relevant previous studies are briefly discussed as below.

1.7.1 II-VI group DMSs materials

The first class of the DMSs belongs to II-VI group for example, Zn1-xMnxTe

and Cd1-xMnxTe. These were studied in 1980s. The spin glass or very weak

ferromagnetic natures with very low curie temperatures suggested them inadequate

https://en.wikipedia.org/wiki/Inorganic_compoundhttps://en.wikipedia.org/wiki/Inorganic_compoundhttps://en.wikipedia.org/wiki/Cassiterite


17

for room temperature device applications (Ferrand et al., 2001). The origin of

magnetism within II-VI DMS has been widely investigated, but the actual picture is

still not apparent. For example, II-VI DMS are of paramagnetic nature and are

relatively difficult to grow. Various reports illustrate above room temperature

ferromagnetism (RTFM) in transition metal doped semiconductor oxides e.g. zinc

oxide (ZnO) (Ogale, 2010) and titanium dioxide (TiO2) (Assadi and Hanaor, 2013).

TM doped ZnO is transparent to the visible light and has applications in light emitting

diodes (Ogale, 2010). Dietl et al. presented Zener model, which describes the

appearance of RTFM in heavily p-type doped ZnO (Dietl et al., 2000; Pearton et al.,

2003). Subsequently, few density functional theory (Sato et al., 2000, Sato et al.,

2001) and experimental studies (Ueda et al., 2001, Prellier et al., 2003) revealed that

Co doped ZnO with n-type nature also results RTFM. The synthesis of bulk or thin

film depends upon the dopant solubility, and such solubilities can be enhanced using

non-equilibrium synthesis mechanism (Dietl et al., 2000, Pearton et al., 2003).

1.7.2 III-V group DMSs materials

The second class of DMSs belongs to III-V group (Munekata et al., 1989) e.g.

In1-xMnxAs (Ohno et al., 1992) and Ga1-xMnxAs (Ohno et al., 1996). These DMSs

showed ferromagnetism with high Curie temperatures (TC). The Mn doped GaAs

exhibited TC of 173K (Jungwirth et al., 2006, Edmonds et al., 2002), which is still too

low for room temperature (RT) applications. The solubility of magnetic dopant into

III-V semiconductors is very low because higher doping induces surface segregation

resulting in the phase separation (Chiba et al., 2003). The RTFM arises due to

interactions of transition metal d-shell with electrons of the s- and p-states of the host

lattice showing s-d, p-d and d-d couplings, which strongly affect the structural and

other physical properties.


18

Many heavily Mn doped semiconductors showing ferromagnetism,

demonstrate Curie temperature (TC) well above 100 K, have been reported (Munekata

et al., 1989, Ohno et al., 1992, Ohnoet al., 1996, Hayashi et al., 1997, Van-Esch et

al., 1997 and Ohno, 1998). The ferromagnetism primarily occurs by aligning the

magnetic moments of the conduction band electrons or valence band holes. Hence,

understanding of the transport mechanism in such magnetic semiconductors is very

important. Therefore, direct interpretation of the spin transport phenomenon is also

possible in semiconductor devices (Furdyna, 1988). Furthermore, the material

inhomogeneity, resulted due to the magnetic dopants, can also tune the spin transport

mechanism, and hence, the quantum information processing applications can be

realized.

Theoretical investigations about various III-V and II-VI have also widely

explored the DMSs applications (Dietl, 2002, Lee et al., 2002, Konig et al., 2003,

Dietl, 2003). The influence of the TM doping on the structural and magnetic

properties has been revealed to justify TC and the mechanism of carrier induced

ferromagnetism. The TC values of various semiconductors are shown in Figure 1.6.

Many density functional theory studies about Mn doped III-V DMSs have been

conducted to investigate the stability of the ferromagnetic state (Sanvito et al., 2002,

Sato and Katayama-Yoshida, 2002).

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19

Figure 1.6: Curie temperatures of various semiconductors (Dietl, 2000).

A small fraction of the magnetic dopants in DMS results in the magnetic

moments with varying distance between them in the host semiconductor lattice,

therefore, magnetic order can be carefully controlled and monitored, and

consequently, the physical properties can be tuned to the desired magnitudes. The

content of the magnetic dopant in the semiconductor lattice decides the strength of the

coupling between the localized magnetic moments and the free carriers in the host

lattice. The physical properties are also altered due to the employed growth

parameters and intrinsic defect concentration.

1.7.3 IV-VI group DMSs materials

TM doped IV-VI semiconductors have recently attained huge research

interests for realizing the practical spintronic devices. Tin mono-sulphide (SnS) and

Tin di-oxide SnO2 are the promising IV-VI semiconductors for various optical device

applications. Therefore, magnetic doping of both compounds becomes very attractive

to investigate.


20

1.7.3.1 Literature review of SnS

Tin based IV-VI chalcogenides SnS, SnS2 and Sn2S3 have gained considerable

attraction due to their potential applications in optoelectronics and data storage

devices (Chaki et al., 2015, Calderon et al., 2014). Among these various phases, SnS

is most suitable for magnetic doping to realize the desired magnetic applications

(Akkari et al., 2012).

The nature of carriers was investigated by Noguchi et al. in SnS evaporated

thin films, and p-type to n-type shift was observed due to Sb-doping. High absorption

coefficient and the band gap energy were reported as 104cm-1 and 1.4-1.5eV,

respectively (Noguchi et al., 1994). There are number of reports about SnS thin films

to study the effect of growth parameters in tuning the physical properties. The

structural parameters and band gap were found to make this material suitable for

different optical device applications (Noguchi et al., 1994, Zainel et al., 1996,

Ichimura et al., 2000, Takeuchi et al., 2003, Sato et al., 2005, Cheng et al., 2007, Yue

et al., 2009, Guneri et al., 2010, Mariappan et al., 2011, Bashkirov et al., 2011,

Reghima et al., 2013, Geetha et al., 2015). A modified solution dispersion

methodology was used by Zhao et al. to prepare SnS nanoparticles for blue-UV

emission that is suitable for various optical devices (Zhao et al., 2004).

Ramakrishna Reddy et al. characterized SnS thin films grown using spray

pyrolysis for application in the solar cells. A solar conversion efficiency of 1.3% was

exhibited by the thin films with 0.6µm thickness (Ramakrishna Reddy et al., 2006). In

another report, SnS thin films synthesized using chemical deposition showed solar

cell efficiencies 0.11%, 0.10% and 0.2% for thicknesses of 0.25 µm, 0.35 µm and 0.5,

µm respectively (David and Avellaneda, 2009). SnS nanosheets fabricated using wet

chemical method showed agglomeration. The UV-VIS-NIR optical spectrum was


21

found exhibiting direct transitions at 1.88eV (Sohila et al., 2011). The direct and

indirect transitions were found at 1.78eV and 1.2eV, respectively. Broad Raman

modes were observed which were shifted towards lower wave-number side due to

nanoparticles formation (Sohila et al., 2011).

The substrate temperature critically influences the structural and optical

properties of SnS thin films. For example, Ahmad et al. reported that variation in the

substrate temperature (200-350 oC) increases the grain size from 54nm to 74nm and

decreases the band gap from 1.6eV to 1.54eV (Ahmad et al., 2011). Various dopants

can tune the physical properties of SnS nanomaterials and thin films. For example,

Sinsermsuksakul et al. studied the Sb doping effects on the structural and

morphological properties of SnS thin layers for suggesting the solar cell applications

(Sinsermsuksakul et al., 2012).

Another report revealed that Sb doped SnS thin films exhibit orthorhombic

crystal structure. The crystallite size increased from 97nm to 129nm and band gap

reduced from 1.60eV to 1.15eV with the doping contents from 0% to 10%, which

depicted potential optoelectronic device applications (Kumar, 2013). The grain size

has been observed increasing with increasing the film thickness and is related directly

to the refractive index (Jakhar et al., 2013). The orthorhombic SnS nanoparticles and

SnS:Se thin films synthesized using precipitation and electron beam evaporation

method, respectively, showed strong FTIR peaks at 2357cm-1 and 615cm-1. The Se

doping resulted SnS as a good absorber material for solar cell applications (Henry et

al. 2013). Details of the dielectric properties of SnS have showed an increase in the

conductivity with the film thickness. The peak in the measured dielectric constant has

been reported to occur due to dipoles reorientation and relaxation phenomenon

(Hassan, 2013).


22

The hot wall deposition has been used to elucidate the effect of plasma

sputtering on the surface quality of SnS thin films (Zimin et al., 2014). Similarly,

Saminathan synthesized TM (Co and Fe) doped SnS nanoparticles for solar cell

applications (Saminathan, 2014). The varying growth conditions altered the

characterization results. For example, the impact of Bi doping to modify the structural

and electrical characteristics has been reported. The grain size and surface roughness

were reduced with Bi concentration (Calderon et al., 2014).

In another report, the Bi doped SnS thin films, has been found improving the

grain size and surface roughness, and decreasing the band gap from 1.60eV to 1.40eV

with the Bi contents 0-6%. High carrier concentration and low band gap value can

increase the efficiency to 6%, making it suitable as absorber layer material. The effect

of film thickness on the physical properties of SnS thin films grown using thermal

evaporation has been observed. Three 100, 200 and 300nm thick films have been

prepared to study he dielectric properties within 10kHz to 100MHz. The measured

grain size increased within 11-16nm with film thickness. The dielectric constant was

found increasing at lower frequency but remained almost constant at high frequency

(Hassan and Nasir, 2015).

Chaki et al. synthesized and explored In and Sb doped p-type SnS as a direct

band gap semiconductors in the single crystals form (Chaki et al., 2015). Reghima et

al. has reported an increased conductivity with the Ga (0-10%) doping content as

compared to undoped SnS thin films (Reghima et al., 2015). On the other hand, direct

band gap of SnS has been observed to decay due to Al doping which improved the

crystallinity (Kafashan et al., 2016). Pan et al. has observed the band gap and

dielectric constant of SnS thin layers by using density functional theory, and has

found that band gap decreases with the strength of electric field. At particular critical


23

electric field, band gap vanished depicting a semi-metal nature. Such an electric field

modulation suggested its applications in designing of the nano devices (Pan et al.,

2016).

The transition metals (Pd, Pt, Cu, Ag) doping has been done to improve the

thermoelectric properties of SnS. It has been observed that silver is most suitable as

thermoelectric material, as compared to palladium and platinum, because figure of

merit could be doubled (Falkenbach et al., 2016). Furthermore, SnS also has

applications for quantum confinement in the nanowires (Yue et al. 2009), solar cell

junctions (Abdelrahman et al., 2012), heterojunction photovoltaic devices (Hassan

and Shallal, 2014) photocatalytic dye degradation (Das and Dutta, 2015) and three

dimensional network hierarchitectures (Xu et al., 2015). Georgios et al. have used first

principle calculations to study the low indexed SnS surfaces. They have applied

stoichiometric models and have correlated the surface effect with the exhibited

efficiency of the photovoltaic cells (Georgios et al., 2014).

1.7.3.2 Literature of SnO2

The properties of SnO2 nanoparticles also depend upon the composition and

growth method. It can be synthesized using a wide range of techniques, which are

chemical co-precipitation (Tian et al., 2008), magnetic field assisted (Xu et al., 2009),

sol-gel (Zhang et al., 2010, Azam et al., 2010), microwave (Salah et al., 2016,

Dhinakar et al., 2016) and flash evaporation (Kuppan et al., 2017) technique.

Aluminum, Sulphur and Magnesium doped SnO2 have been explored to

elucidate optical device applications (Muramba et al., 2015, Ali et al., 2013). Wang et

al. have theoretically investigated TM (Fe, Co, Mn, V) doping effects on the

ferromagnetic properties of SnO2 using first principle calculations. They have

observed that Fe and Co doped SnO2 exhibit ferromagnetic nature with high Curie


24

temperatures, while Mn and V doped SnO2 are stabilized in the paramagnetic state.

Moreover, magnetic properties of TM doped SnO2 are also greatly influenced by the

oxygen vacancies (Wang et al., 2007). The structural properties have been used to

justify the exhibited magnetic properties of TM (V, Cr, Mn, Fe, Co, Ni with 0-12%)

doped SnO2 nanoparticles. The saturation magnetizations of 2.5% Cr, 1% Co and 5%

Fe doped SnO2 samples have shown maximum values displaying their dependence

upon the structure (Van Komen et al., 2008). Similarly, Tian et al. have correlated the

structural and magnetic properties of Mn (0-7%) doped SnO2 nanoparticles, which

have been prepared using chemical co-precipitation technique. Structural and

ferromagnetic properties were found dependent on the sintering temperature.

Ferromagnetism observed was retained even up to 450 oC with Mn content less than

5% (Tian et al., 2008).

Co doped SnO2 nanocrystals have been found showing room temperature

ferromagnetism (RTFM). The demonstrated ferromagnetism was found exhibiting

potential to get improve using high external magnetic field (Xu et al., 2009). Zhang et

al. synthesized Ni doped SnO2 thin films using sol-gel method and also reported pure

rutile structure with RTFM. The Ni magnetic moment was found decreasing with its

contents, which was justified to occur due to the antiferromagnetic coupling between

the nearest Ni ions (Zhang et al., 2010).

Similarly, Azam et al. elucidated the effect of Ni (5, 7, 9 %) doping on the

properties of SnO2 nanoparticles grown using sol-gel method. They observed a

decrease in the crystallite size from 5 – 2 nm, however, ac conductivity has been

reported increasing with Ni content. Complex impedance analysis (Nyquist plot)

revealed the onset of grain boundary contribution (Azam et al., 2010). The spin

dependent density functional theory has been reported to investigate the oxygen


25

vacancy effects on the ferromagnetic properties of Fe and Co doped SnO2 for

suggesting the spintronic applications (Borges et al., 2012). TM doped DMOs were

suggested as suitable candidates showing both the magnetic metastability and half-

metallic behavior.

The thin films of Sn1-xNixO2 have been explored to show that doping can tune

the ferromagnetism (Kuppan et al., 2016). The Mn (2, 4, 6 %) doped SnO2

nanoparticles grown using sol-gel method have been found appropriate for DMS

applications. The average crystallite size of 6nm has been reported, while band gap

decays with Mn content. They emphasized that this material can be suitable for DMS

applications (Ungureanu et al., 2016).

In another report, Mn (0.1-5 mol %) doped SnO2 nanoparticles have been

grown using microwave technique. The undoped and Mn doped nanoparticles

exhibited the rutile tetragonal structure with average crystallite size of 10nm. They

observed that Mn is a suitable dopant to modify the optical and electrical properties

(Salah et al., 2016). RTFM has been reported in Fe doped SnO2 thin films, which

were synthesized using flash evaporation. The high growth temperature was not

favorable to stabilize the ferromagnetic state (Kuppan et al., 2017).

From the extensive review of the existing literature, it is evident that these

materials still needs to be further explored for multi-device applications, because there

is a scarcity of the comprehensive dielectric and magnetic studies of both SnS and

SnO2. Despite of the fact that there exist a considerable literature about

comprehensive optoelectronic applications, the dielectric and magnetic investigations

of both materials will be quite interesting, for both, the fundamental physics and

technological point of view.


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1.8 Significance of the study

As pure SnS and SnO2 reveal relatively narrow and wide band gaps,

respectively, therefore comprehensive study of the origin of magnetism in such

versatile materials looks attractive for exploitation of the DMSs in the practical

magnetic device applications. According to the extensive literature review, structural,

dielectric and ferromagnetic investigations of transition metal (TM) doped SnS and

SnO2 synthesized by co-precipitation technique are in scarcity. Although few reports

exists in the literature about TM doped SnO2 for DMS applications but a

comprehensive study about room temperature dielectric and ferromagnetic properties

is still required. Therefore, inspired by the potential multi-scale device utilization of

the transition-metal doped SnS and SnO2, the presented study has been planned.

The less explored materials grown using low-cost method, exhibiting optimum

ferromagnetic and dielectric responses closely related with structure and surface

morphology are quite demanding to study. Therefore, investigations of transition

metal (Co, Ni, Fe, Mn) doped SnS and SnO2 diluted magnetic semiconductors with

high temperature ferromagnetism and efficient dielectric response, prepared using

low-cost fabrication method could be important not only for the fundamental physics

but also for the economy of the country.

1.9 Overview of the thesis

This chapter includes details of basic introduction of origin of magnetism,

various magnetic interactions in diluted magnetic semiconductors, dielectric propertie,

introduction of materials and literature review, used in this research work.

The details of the basic principles, construction and application of various

employed techniques are presented in chapter 2. The transition metal (TM) doped IV-


27

VI SnS and SnO2 semiconductors grown by employing low cost co-precipitation

method have been characterized with a variety of techniques.

In chapter 3, Co and Ni doped SnS diluted magnetic semiconductors,

synthesized at relatively low temperature by employing a very simple co-precipitation

technique, are studied to observe the structural, surface morphological, dielectric and

ferromagnetic characteristics.

Chapter 4 deals with the synthesis and characterization of Fe and Mn doped

SnS diluted magnetic semiconductors. The growth is done using chemical co-

precipitation technique. The structural, surface morphological, optical, dielectric, and

ferromagnetic characteristics of Sn1-xFexS and Sn1-yMnyS (x/y = 0–10%) nanoparticles

have been discussed in detail.

In chapter 5, SnO2 has been doped with Mn (0-3) % and Zr (0-10) % using co-

precipitation technique. The Mn doped SnO2 has been subjected to two different post-

growth heat treatments to investigate the impact of elevated temperatures on the

exhibited physical properties. The detailed investigations of the dielectric response

with respect to the incident frequency have been observed and linked to the structural

parameters. The magnetic characters are also studied under the influence of doping

concentrations.

Chapter 6 is related to transition metal dependent variation of physical

properties of SnS and SnO2

In chapter 7, the results derived from all characterization techniques are

presented.

Chapter 2 Growth Method and Characterization Techniques

28

Chapter 2

Growth Method and Characterization Techniques


29

Growth Method and Characterization Techniques

The transition metal (TM) doped IV-VI SnS and SnO2 semiconductors, grown

by employing low cost co-precipitation method, have been characterized with a

variety of techniques. The details of the basic principles, construction and application

of various employed techniques are presented in this chapter.

2.1 Co-precipitation technique

The use of low cost and simple growth method is very important to illustrate

the potential device applications of the fabricated product. Various techniques used to

grow nanomaterial include sol-gel, solid state reaction, hydrothermal method,

chemical vapor deposition and sputter deposition. However, we have applied

chemical co-precipitation method because it is very simple, cheap and involves low

synthesis temperature.

The co-precipitation technique is used to synthesize nanomaterials and is

based on three steps such as inclusion, adsorption and occlusion. During inclusion,

doping elements (impurity) substitutionally replace the cations of the host lattice. In

the adsorption process, impurity elements are weakly coupled to the precipitate.

Finally in the occlusion, the further grown impurity becomes a physical part of the

host lattice. The solutions of the precursor materials are prepared in a common solvent

that is selected on the basis of salts being used e.g. nitrates, sulfates or chlorides. The

uniform and homogeneous anion and cation solutions are realized by magnetically

stirring the solutions, separately. The solutions are mixed and stirred again for

nucleation and growth of the desired product. The precipitates formed are filtered,

washed and then dried. The speed of the precipitate formation mainly relies on the

solubility and surface tension. The dried samples are calcinated at particular

calcination temperature to improve the crystallinity. Various growth parameters like


30

molar ratio of salts, reaction temperature, pH and molarity strongly influence the size,

shape and purity of the synthesized product (Hahn, 1936, Patnaik and Dean's, 2004).

The flow chart showing various steps involved in the precipitation technique are

shown in Figure 2.1.

The magnetic stirring avoids particle aggregation during crystallization

process and results in the formation of uniform crystals. Above the solubility limit,

aggregates merge to form larger aggregates. The tendency to aggregate depends upon

the supersaturation. The extremely large supersaturation causes the fabrication of the

amorphous materials. The amorphous precipitates in various materials have been

observed possible to be slowly transformed to the crystalline one (Hahn, 1936).

Figure 2.1: Flow chart showing various steps used in the co-precipitation method.

This technique has advantage over the other growth methods because very

large amount of the desired products can be prepared at low temperatures.


31

Furthermore, co-precipitation is unique due to its simplicity, cost effectiveness and

low fabrication temperature.

2.2 Characterization techniques

Various techniques employed to reveal the structural, optical, dielectric and

magnetic properties are briefly discussed in this section.

2.2.1 X-ray Diffraction (XRD)

X-ray diffraction (XRD) is used to confirm the appearance of the required

phase exhibiting specific structure as well as the phase purity. The residual stress,

induced during the synthesis of the required materials, can also be calculated using

XRD (Barlow, 1883). Because around 95% solid materials are crystalline, therefore,

in 1919 A. W. Hull reported that in a mixture, each material component produces its

independent diffraction pattern. Hence, the X-ray diffraction pattern of any substance

is actually a fingerprint for it. In 1912, Max von Laue, revealed that when X-rays are

incident on a crystalline material, crystallographic planes act like 3D diffraction

grating. Therefore, X-ray diffraction (XRD) can expose the stabilized crystal

structures. There are three major components of X-ray diffractometer, X-ray tube,

sample stage and X-ray detector, as shown in Figure 2.2.

The working principle of XRD is based on the constructive interference of X-

rays reflected from crystallographic planes of the crystalline sample, according to the

Bragg’s condition (n λ = 2d sin θ), which generates diffraction pattern (Cullity, 1978).

The diffracted X-rays are detected using detector for crystal structure determination.

In powder samples, planes are randomly oriented, therefore, the sample is rotated for

a certain range of 2θ to measure the maximum part of the diffraction. The diffraction

pattern measured in this way show peaks intensity versus 2θ. The measured diffracted


32

peaks are compared with the standard reference diffraction patterns to identify the

material. The schematic diagram of X-ray diffractometer is presented in Figure.2.2.

Figure 2.2: Schematic diagram of X-ray diffractometer (Bish and Post, 1989).

Cu target is most commonly used to generate X-rays for diffraction (Cu-

kα radiations with wavelength 1.5418Å). The geometry of diffractometer allows the

sample to rotate at an angle θ, while the mounted X-ray detector collects X-rays by

rotating at an angle 2θ. The diffraction signals are highly sensitive to the sample

volume. Various sections of the diffraction rings observed using detector reveal total

no of particles per unit volume. XRD have applications in phase and purity

identification, differentiation of mixed clays, evaluation of preferred orientations

(texture analysis), lattice mismatch between the interfaces, evaluation of residual

stress and dislocation densities. In XRD, powders with particle size less than ~10 μm

are preffered, because all possible crystallographic orientations are made available for

diffraction.

2.2.2 X-ray absorption spectroscopy (XAS)

X-ray absorption spectroscopy (XAS) is used to investigate the electronic

states of elements in a compound. X-ray beam striking the atom will excite the core

electrons causing them to move to the unoccupied energy state or continuum by


33

leaving a (hole) vacancy. As the electronic transitions occur from 1s or 2p shells,

therefore, X-ray energies of thousands of eV are needed. Since the X-rays have

wavelength around 1 Angstrom, which is comparable to the atomic separations,

therefore, this technique is very useful to study the local atomic structures. When the

impinging X-ray energy becomes equal to the binding energy of the core electrons, it

abruptly increases the absorption. This mechanism gives rise to different absorption

edges corresponding to binding energies of different core electrons. X-ray absorption

is divided into four sections. First region is pre-edge that appears due to electronic

transitions from the core level to higher empty or filled levels e.g. s→ p or p→ d.

Second region is called as X-ray absorption near edge structure (XANES), which is

resulted due to the electron transitions from core to the no

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prr.hec.gov.pkprr.hec.gov.pk/jspui/bitstream/123456789/11818/1... · DECLARATION I Bushra Parveen, PhD Scholar in the department of Physics, University of the Punjab Lahore, hereby