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1

CHARACTERIZATION OF DEEP LEVEL DEFECTS IN

N-TYPE ZINC OXIDE LAYERS GROWN BY

HYDROTHERMAL TECHNIQUE

A dissertation submitted in partial fulfillment of the requirement for the degree

of Doctor of Philosophy

in Physics

By

HADIA NOOR

Department of Physics

The Islamia University of Bahawalpur Pakistan

2012

2

Abstract

Zinc oxide (ZnO) is a promising wide-bandgap semiconductor due to its favorable

properties for a variety of demanding device applications such as UV light

emitters/detectors, high-power and high-temperature devices. The presence of defects

in the material can considerably change the electrical properties of the

semiconductors. However, recently it has been found that the terminated face of the

material significantly alter the characteristics of such devices. The defects in ZnO

have been studied in last decades, but no clear consensus has been made. This

dissertation investigates the electrical properties of defects in ZnO grown by

hydrothermal and molecular beam epitaxy techniques using deep level transient

spectroscopy (DLTS). Among the growth techniques available to grow the thin film,

the hydrothermal is one of the most cheap and user friendly technique. DLTS

provides a sensitive method for identifying defects and for determining their

parameters. The main findings are as follow:

A. Several circular Schottky contacts (1mm diameter) with Pd metal on the Zn-

face and O-face on n-type ZnO grown by hydrothermal and Ohmic contact of

nickel-gold on the backside were deposited by e-beam technique. The as-

obtained samples were labeled as group A and B samples, respectively. The

present literature on n-type ZnO has highlighted a defect, labeled as E3

irrespective of growth technique, which is also studied thoroughly in this

research project. The respective summary of each group A and B of samples

is explained below:

DLTS has been carried out on the group A samples to study deep level

defects. Its result showed two electron trap level E1 having activation

energy Ec-0.22 ±0.02 eV and E2 with activation energy Ec-0.49 ±0.05

3

eV. E1 level has time-delayed transformation of shallow donor defects

zincinterstitial and vacancyoxygen (Zni-VO) complex. It is observed through

X-ray differaction that the preferred direction of ZnO growth is along

(1010) plane i.e. VO-Zni complex, assuming that under favourable

condition (Zni-VO) complex is transformed into a zinc antisite (ZnO).

Consequently, the trap concentration increases with decreasing free

carrier concentration. Hence, the ZnO is correlated to E1 level

demonstrating the increase in concentration.

Several renowned research groups have revealed different points

defects in bulk ZnO like naming oxygen vacancy, zinc interstitial,

and/or zinc antisite. These defects having activation energy (free

carrier concentration) in the range of 0.32–0.22 eV (1014

-1017

cm-3

)

below conduction band. The results of group A and B samples also

showed activation energy (free carrier concentration) as observed by

other renowned research groups. This result is due to activation energy

of the level while it is not conceivable by with Vincent et al.,[ J. Appl.

Phys. 50 (1979) 5484]. They believed that data should be carefully

interpreted obtaining by capacitance transient measurement of diodes

having carrier concentration greater than 1015

cm-3

. Thus the influence

of background free-carrier concentration, ND induced field on the

emission rate signatures of an electron point defect in ZnO Schottky

devices has been studied by using deep level transient spectroscopy.

Many theoretical models were tested on the experimental data to

understand the mechanism. Our findings were supported by Poole-

Frenkel model based on Coulomb potential. It is revealed by

4

investigation that Zn related charged impurities were found to be

responsible for electron trap. Results were also tested through

qualitative measurements like current-voltage and capacitance-voltage

measurements.

B. Several Schottky contacts of 1mm diameter with silver were prepared on ZnO

grown by molecular beam epitaxy. These samples were labeled as group C

samples, DLTS measurements revealed a hole trap exhibiting metastability

effect in the emission rates of trap with storage time. We determined that hole

trap transfers from one configuration to other with storage time. As a result the

activation energy of the acceptor level varied in the range of 0.31 eV to 0.49

eV above the valance band at different measurement time. Impurities cannot

be removed in the growth procedure. SIMS results showed the presence of

nitrogen. During the growth process nitrogen occupies O site and produces

Zn-N complex. But Zn-N bond is not stable because of its large bonding

energy and consequently results into metastable nature of the defect. All

experimental findings and available literature support the conclusion that the

observed hole trap arise from Zn-N complex.

C. The ZnO nanorods were grown on glass substrate coated with different metal

(Ni, Al, Ag and Au) by aqueous chemical growth. These samples were labeled

as D, E, F and G, respectively. The structural properties of ZnO nanorods were

investigated by X-Ray diffraction (XRD) and scanning electron microscopy

(SEM). The intensity of ZnO (0 0 2) diffraction peak in X-ray diffraction

pattern is maximum of sample D because of nucleation of Ni metal coated on

substrate. SEM measurements strongly support our observation that thin layer

Ni metal increases the growth of nanorods.

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Declaration

I, Ms. Hadia Noor, PhD student in the subject of Physics session (2007 – 2012)

hereby declare that the material produced in this dissertation titled “Characterization

of deep level defects in n- type zinc oxide layers grown by hydrothermal technique” is

my own work and has not been submitted in as a whole or in part for any degree at

this or any other university.

Hadia Noor

6

Certificate

It is certified that the work presented in this dissertation entitled “Characterization of

deep level defects in n-type zinc oxide layers grown by hydrothermal technique” by

Hadia Noor under my supervision at the Department of Physics, The Islamia

University of Bahawalpur, Pakistan.

Supervisor

Prof. Dr. M. Asghar Hashmi

Chairman

Prof. Dr. Sh. Aftab Ahmad

Department of Physics

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Acknowledgement

All praises and thanks to Almighty ALLAH, the most Beneficent and the

Merciful, The Creator of The Universe, Who enabled me to complete my research

work successfully. I would like to send my humble salutation to the Holy Prophet

Hazrat Muhammad (peace be upon him), Who is a source of guidance and knowledge

for humanity.

Now when I just finished this dissertation, all the memories of the past years

are flashing back in my mind. Many people who helped me slowly came into my

memory one by one, even though my gratitude is beyond words.

First of all, I am cordially thankful to the most inspiring character in this

research i.e my supervisor Prof. Dr. Muhammad Asghar Hashmi, whose

encouragement, guidance and support from the initial to the final level enabled me to

surpass all the obstacles in the completion of this research work. Without his great co-

operation I would not be able to complete my research work.

I am extremely appreciative and thankful to Prof. Magnus Willander, ITN,

Linköping University, Norrköping Campus, Norrköping Sweden, for his great co-

operation, excellent guidance and providing me full access to the research laboratories

at ITN, Linköping University during my six months stay in Sweden. My gratitude and

obligations are due to Prof. Dr. Omer Nur, Dr. Peter Kalson, and Dr. Q. Wahab for

helping me during research work. I appreciate the cooperation of Ms. Sadia Faraz in

co-authoring the related articles. In addition, I would like to thank my lab fellows Dr.

Adnan Ali and Dr. M. Imran Arshad for their cooperation during all these years. I am

very grateful to Higher Education Commission for providing me financial support for

completing my Ph.D studies.

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Last but not the least I wish to express my nice feelings towards my loving

and sweet family and friends. Words cannot describe my immense feelings of

appreciation for them. Special thanks for their prayers, encouragement and

unforgettable sacrifices with patience throughout my life and Ph.D studies.

Sincerly

Hadia Noor

10

Contents

Abstract 2

Declaration 5

Certificate 6

Acknowledgement 8

List of Publications 13

List of Figures 14

List of Tables 18

CHAPTER 1 19

INTRODUCTION 20

1.1 Semiconductor Materials 20

1.1.1 Wide Bandgap Semiconductor Materials 23

1.2 Motivation 25

1.3 Dissertation Outline 29

CHAPTER 2 31

PROPERTIES OF ZnO 32

2.1 Crystal Structure 32

2.2 Point Defects 37

2.2.1 Types of Point Defects 37

2.2.2 Point Defects in ZnO 38

2.3 Contacts to Zinc Oxide 44

2.3.1 Schottky Contact to ZnO 44

2.3.2 Ohmic Contacts to ZnO 46

CHAPTER 3 49

GROWTH TECHNIQUES 50

3.1 Hydrothermal technique 50

3.1.1 Experimental Setup of Hydrothermal Method 50

3.1.2 Hydrothermal Synthesis of Zinc Oxide 51

3.2 Molecular Beam Epitaxy 54

3.3 Aqueous Chemical Growth 56

CHAPTER 4 59

CHARACTERIZATION TECHNIQUES 60

4.1 Current-Voltage Measurements (I-V) 60

11

4.2 Capacitance-Voltage Measurements (C-V) 63

4.3 Deep Level Transient Spectroscopy (DLTS) 64

4.3.1 Carrier Kinetics in Semiconductors with Deep Level- Shockley-Read-Hall

Theory 65

4.3.2 Basic Principle of DLTS 66

4.3.3 Measurement of Defect Parameters by DLTS 71

4.4 X-Ray Diffraction (XRD) 75

4.4.1 Hexagonal System 76

CHAPTER 5 79

EXPERIMENTAL DETAILS 80

5.1 Group A and B Samples 80

5.2 Group C Samples 81

5.3 Samples D, E, F and G 82

5.3.1 Preparation of Seed Solution 82

5.3.2 Pretreatment of Substrate 82

5.3.3 Chemical Bath Deposition Growth 83

CHAPTER 6 84

RESULTS AND DISCUSSION 85

Section- I 85

6.1 Current-Voltage Measurements 85

6.2 Capacitance-Voltage Measurements 88

6.3 Deep Level Transient Spectroscopy Measurements 93

6.4 X-Ray Diffraction 96

6.5 Trap Identification 99

6.5.1 Electron Level E1 99


Section- II 104

6.6 Current-Voltage Measurements 104

6.7 Capacitance-Voltage Measurements 106





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Section- III 120

6.10 Hall Measurement 120

6.11 SIMS Measurement 122

6.12 Current-Voltage Measurement 123

6.13 Capacitance-voltage Measurement 124



Section- IV 130

6.16 XRD Measurements 130

6.17 SEM Measurements 133

CHAPTER 7 136

CONCLUSIONS AND FUTURE PLAN 137

7.1 Group A Samples 137

7.2 Group B Samples 137

7.3 Group C Samples 138

7.4 Samples D, E, F, & G 139

7.5 Future Plan 139

REFERENCES 140

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List of Publications

1. Influence of background concentration induced field on the emission

ratesignatures of an electron trap in zinc oxide Schottky devices. Hadia Noor, P.

Klasan, S. M. Faraz, O. Nur, Q.Wahab, M. Asghar, and M. Willander, J. Appl.

Phys. 107 (2010) 103717.

2. Time-delayed transformation of defects in zinc oxide layers grown along

the zinc-face using a hydrothermal technique. Hadia Noor, P. Klasan, O. Nur,

Q.Wahab, M. Asghar, and M. Willander, J.Appl. Phys.105 (2009) 123510.

3. Modeling and simulations of Pd/n-ZnO Schottky diode and its comparison with

measurements. S. Faraz, Hadia Noor, M.Asghar, M. Willander, Q. Wahab,

Advanced Materials Research. 79-82 (2009) 1317-1320.

4. Post-annealing modification in structural properties of ZnO thin films on p-type Si

substrate deposited by evaporation. M. Asghar, Hadia Noor, M.S. Awan, S.

Naseem and M.-A Hasan, Mater. Sci. in Semicond Processing, 11 (2008) 30-35.

5. Characterization of ZnO Thin Film Deposited by RF Operated Thermal

Evaporation. M. Asghar, I. Hussain, Hadia Noor, M. S. Awan and M.-A. Hasan,

Proceeding IEEE Regional Symposium on Microelectronics, Penang, Malaysia

(2007) 160.

6. Growth and Characterization of Single Crystalline Cubic SiC on porous Si using

low pressure chemical vapour deposition technique. M. Asghar, Payam Shoghi,

Hadia Noor, and M.-A. Hasan, NAM Proceedings (2007) 59.

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List of Figures

Figure 1.1 The block diagram of semiconductor by conductivity. 21

Figure 1.2 The block diagram of semiconductor by structure. 21

Figure 1.3 The block diagram of semiconductor by composition. 22

Figure 1.4 The block diagram of semiconductor by bandgap. 23

Figure 2.1 Unit cell of ZnO and neighbouring atoms, viewing direction approx

parallel to c. Small spheres: O-2

, big spheres: Zn+2

. 33

Figure 2.2 Temperature dependence of PL spectrum for Zn-polar face and O-

polar face in the temperature range of 10 K– 300K. 36

Figure 2.3 Schematic representation of defects in semiconductors. 38

Figure 2.4 Electron energy diagram in equilibrium (1) and in the presence of an

electric field (2) showing field-enhanced electron emission: (a)

Poole-Frenkel emission, (b) phonon-assisted tunneling. 43

Figure 2.5 Metal-semiconductor contacts according to the simple Schottky

model . 44

Figure 2.6 Energy band diagrams for Ohmic contact. 46

Figure 3.1 Schematic drawing of a hydrothermal growth system. 53

Figure 3.2 The diagram of MBE system [36] 56

Figure 2.3 The chemicals for growth solution of ZnO. 57

Figure 3.4 The stirring of growth solution for ZnO. 58

Figure 3.5 Growth solution container placed in oven. 58

Figure 4.1 Band diagram of the intimate contact between metal and

semiconductor (n-type) for a rectifying junction. 60

Figure 4.2 Electron energy band diagram for a semiconductor with deep a level

trap. 65

Figure 4.3 The metal-semiconductor contact and the depletion layer . 66

Figure 4.4 The schematic illustration of a majority injection pulse sequence and

energy band bending. (a) bias time, (b) capacitance-time, (c) and (d)

the energy band bending during the pulse and after pulse. 67

Figure 4.5 XRD measurement of the Zn. 77

Figure 5.1 ZnO wafer grown by hydrothermal. 80

Figure 5.2 ZnO samples grown by MBE. 81

15

Figure 6.1 I-V characteristics of the Pd-Schottky contact on the Zn face of the

ZnO. 86


ZnO at 120 K. 87


ZnO at 340 K. 87

Figure 6.4 Plot between Ideality factor and 1000/T indicates the To- effect for

the Zn face Pd/ZnO Schottky diode. 88

Figure 6.5 C-V measurements of the Pd-Schottky contact on the Zn face of the

ZnO. 89

Figure 6.6 Graph between applied bias and inverse squared capacitance. 90

Figure 6.7 Depth profile of free carrier concentration of Pd/ZnO. 90

Figure 6.8 C-V measurements of the Zn face of the ZnO at different

temperatures. 91

Figure 6.9 Depth profile of free carrier concentration of Zn-face ZnO at

different temperatures. 91

Figure 6.10 DLTS spectrum displaying two electron deep level defects below

conduction band of ZnO. 94

Figure 6.11 The DLTS spectrum of levels E1 in ZnO. 94

Figure 6.12 The Arrhenius plot of levels E1 and E2 in ZnO. 95

Figure 6.13 Trap concentrations of levels E1 in Zn-face ZnO 95

Figure 6.14 Typical XRD pattern of the Zn-face ZnO layer exhibiting the Zni-VO

complex as the preferential direction of growth. Peaks other than

ZnO are seen because the XRD measurements were performed on

Pd\ZnO–Zn\Au–Cr mounted on an alumina substrate by silver paste. 97

Figure 6.15 C-V measurements indicate decrease in amplitude. 100

Figure 6.16 DLTS measurements indicate increase in amplitude. 101

Figure 6.17 Demonstration of time-delayed transformation phenomenon of

defects in ZnO layer. 101

Figure 6.18 Representative I-V measurements of group B samples. 105

Figure 6.19 Representative C-V measurements of group B samples. 105

Figure 6.20 Schottky behavior of the sample B is demonstrated in 1/C2-V, filled

squares represent the experimental data and the line corresponds to

16

the theoretical fit of the data, extrapolated to x-axis yield built-in

potential. 107

Figure 6.21 The uniform spatial distribution of the free-carriers in the as-

deposited ZnO material. 107

Figure 6.22 C-V measurements of the O face of the ZnO at different temperature. 109

Figure 6.23 Depth profile of free carrier concentration of O-face ZnO at different

temperatures. 109

Figure 6.24 Representative DLTS scans of group A and B samples to show the

variation in peak position of E1 level even measured under same

measuring setup. 111

Figure 6.25 The DLTS spectra measured at different frequencies for Arrhenius

plot of levels E1 in sample B. 112

Figure 6.26 The Arrhenius plot of levels E1 in samples A and B. 112

Figure 6.27 Depth profile of trap concentration of levels E1 in O-face ZnO. 113

Figure 6.28 Influence of background concentration ND on activation energy of E1

level. Data 1 and 2 are ours and rest of the data is taken from Refs.

19, 33, and 34. 115

Figure 6.29 The ND-induced field effect on the thermal energy data of the level.

Data 1 and 2 are ours and rest of the data is taken from Refs. 19, 33,

and 34. 116

Figure 6.30 Qualitative evidence of the Poole–Frenkel mechanism on the ND-

induced variation in emission rate signatures of E1 level. 117

Figure 6.31 Theoretical fitting of the ND-induced field emission rates (filled

circles) obeying Poole–Frenkel mechanism associated with Coulomb

potential (curve C), while square well potential (r = 4.8 nm) is not

consistent (curve S). 118

Figure 6.32 Representative temperature dependent Hall measurements of group

C samples. The upper part, middle part, and lower part of Figure

display mobility, carrier concentrations and resistivity, respectively. 121

Figure 6.33 SIMS depth profiles of O, Zn and N elements in group C samples. 122

Figure 6.34 Representative I-V measurements of group C samples. 123

Figure 6.35 Schottky behavior of group C sample is demonstrated in A2/C

2-V,

filled squares represent the experimental data and the line

corresponds to the theoretical fit of the data. 124

17

Figure 6.36 Depth profile of free carriers of group C sample. 125

Figure 6.37 Representative DLTS spectrum displaying one hole trap of group C

samples. 126

Figure 6.38 Typical DLTS spectra of levels H measured at different frequencies

of group C samples. 126

Figure 6.39 The Arrhenius plot of hole level in group C samples. 127

Figure 6.40 Metastability behavior of hole trap H with respect to time. 129

Figure 6.41 The Arrhenius plot of hole level in group C samples with passage of

time. 129

Figure 6.42 XRD patterns of four samples (D, E, F, & G). 131

Figure 6.43 SEM image of sample D grown by ACG. 134

Figure 6.44 SEM image of sample D grown by ACG. 134

Figure 6.45 SEM image of sample E grown by ACG. 135

Figure 6.46 SEM image of sample E grown by ACG. 135

18

List of Tables

Table 1.1 III-V compound semiconductors 20

Table 1.2 II-VI compound semiconductors 22

Table 1.3 Electrical parameters such as activation energy capture cross section

and defect concentration of different defects in ZnO. 26

Table 2.1 Physical properties of Zinc oxide 34

Table 2.2 2θ, intensity, and Miller indices of peaks of Zn-polar face and O-polar

face samples calculated from XRD measurements . 36

Table 3.1 The growth conditions optimized by Sakagami for zinc oxide crystals. 52

Table 4.1 Calculated miller indices (hkl) of hexagonal system when l = 0. 77

Table 4.2 Calculated miller indices (hkl) of hexagonal system 78

Table 6.1 Electrical parameters of Zn face ZnO calculated from C-V

measurements. 91

Table 6.2 Details of electrical parameters such as activation energy, capture

cross section measured via indirect and direct methods and trap

concentration of defects observed in the DLTS spectrum of Zn-face

ZnO. 96

Table 6.3 2θ, intensity, Miller indices, and sources of peaks measured from

XRD data in ZnO layers listed. 98

Table 6.4 Electrical parameters of O face ZnO calculated from C-V

measurements 108

Table 6.5 2θ, intensityand Miller indices from XRD data of sample D, E, F

and G. 132

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CHAPTER 1

20

1 INTRODUCTION

This chapter presents an introduction about the importance of wide bandgap

semiconductor materials in the development of electronics industry. Among the wide

bandgap materials special focus is given to zinc oxide because of its potential

applications. A brief comparison of zinc oxide attractive properties with other wide

bandgap materials is given. Motivation and dissertation outline are also presented.

1.1 Semiconductor Materials

The importance of electronic industry cannot be denied in the progress of

world. Semiconductors are one of the basic and major materials of this industry.

Semiconductors can be classified in various ways. Four significant ways are given

below:

Conductivity

Structure

Composition

Bandgap

The block diagrams of semiconductor classification by foresaid ways are shown

in Figures 1.1, 1.2, 1.3 and 1.4.

Table 1.1 III-V compound semiconductors [1]

Group III A Group V A

N P As Sb

B BN BP BAs BSb

Al AlN AlP AlAs AlSb

Ga GaN GaP GaAs GaSb

21

In InN InP InAs InSb

Figure 1.1 The block diagram of semiconductor by conductivity.

Figure 1.2 The block diagram of semiconductor by structure.

22

Figure 1.3 The block diagram of semiconductor by composition.

III-V and II-VI compound semiconductors are formed by the combination of

elements of respective group in Periodic Table. These III-V and II-VI compound

semiconductors are listed in Tables 1.1 and 1.2, respectively. SiC, SiGe and GeSn are

the examples of IV-IV compound semiconductors.

Table 1.2 II-VI compound semiconductors [1]

Group II B Group VI A

O S Se Te

Zn ZnO ZnS ZnSe ZnTe

Cd CdO CdS CdSe CdTe

Hg HgO HgS HgSe HgTe

23

Figure 1.4 The block diagram of semiconductor by bandgap.

Bandgaps are direct and in-direct for various compound semiconductors.

1.1.1 Wide Bandgap Semiconductor Materials

Wide bandgap semiconductors like GaN, ZnO and SiC got significant

attention due to their importance in optoelectronics and microelectronic devices. Wide

bandgap semiconductors are related to the emission/absorption wavelength of optical

devices. Light emitting diodes, laser diodes, photodiodes, photoconductive sensors,

electro-modulation devices are the examples of optical devices. Among the wide

bandgap semiconductors there is an increased interest in ZnO because of the

following advantages over other wide bandgap materials;

ZnO is a low cost material as compared to other wide bandgap materials such

as SiC and GaN.

Quality films of ZnO can be grown by a number of cost effective methods like

aqueous chemical growth method and hydrothermal growth technique etc.,

whereas quality GaN and SiC film growth require comparatively much

expensive techniques.

Semiconductor: Bandgap

Narrow bandgap

(0.1-1.5 eV)

Moderate bandgap

(1.5-3.0 eV)

wide bandgap

(3.0-6.0 eV)

24

ZnO is more suitable material for wet chemical etching which is most

essential in the device design and fabrication [2].

ZnO is an attractive material with high luminescence efficiency as compared

to GaN and SiC [3].

The growth of large single crystals of ZnO is possible on the other hand which

is still an issue for GaN and SiC.

It is easy to make nanostructures from ZnO which emit light and sense charge

transfer efficiently [4].

ZnO is a fascinating material due to piezoelectric properties and potential uses

in electronics [5], optoelectronics [6], energy conversion [7], and biosensors

[8].

It has great tolerance for radiation [9,10].

A brief preview of ZnO applications is presented in the following: zinc oxide

(ZnO) is a II-VI direct wide bandgap (3.37 eV) semiconductor with a large exciton

binding energy (60 meV). This material is famous for its photonic and electronic

applications such as UV light emitters/detectors and as high-power and high-

temperature devices [3,11]. ZnO nanostructures have significant device applications

for example surface acoustic wave filters [12], photodetectors [13], photonic crystals

[14], light emitting diodes [15], gas sensors [16], photodiodes [17], optical modulator

waveguides [18], varistors [19], solar cells [20, 21] and nanowire, nanolasers,

biosensors and field emission devices [22]. It has superior physical parameters for

electronic applications including a high breakdown electric field strength, high

thermal conductivity, high electron saturation velocity and high radiation tolerance. In

addition to its valuable optoelectronic properties, it is a candidate for the fabrication

of a dilute magnetic semiconductor with a Curie temperature higher than room

25

temperature [23]. Piezoelectric properties of the material are being explored for

fabrication of various pressure transducers, acoustic- and opto-acoustic devices [24].

Because of these characteristics, ZnO is now considered to be in the line of traditional

semiconductors such as Si and GaAs, and it is also compatible with wide-bandgap

semiconductors such as SiC and GaN [25].

1.2 Motivation

The electronic device applications as meantioned in previous section are

linked with the in house defects chemistry and thereafter, electronic structure of the

material. Defects have detrimental effects on the working of the devices, they are

known to degrade the lifetime and efficiency of the devices. However sometimes

defects are blessings, for example, several research groups [26-31] have suggested

that oxygen vacancies (defects) are the source of green luminescence in ZnO.

Therefore, an understanding of defects in the materials is essential for improving the

material quality and device performance. The defect chemistry and electronic

structure of the material have been the subjects of recent theoretical and experimental

studies. To understand the true nature and cause of defects in ZnO, it is important to

review the previous history of the defects in the material. A comprehensive study of

defects in ZnO grown by various techniques reported in the literature is prepared in

the form of a Table 1.3.

26

Table 1.3 Electrical parameters such as activation energy capture cross section and

defect concentration of different defects in ZnO.

Defect

identificati-

on

Defect

activation

energy

(eV)

Capture cross

section

(cm2)

Defect

concentration

(cm-3

)

Growth

technique

Research

group

Oxygen

vacancy

0.1 (1.2±0.5)×10-13

(14±2) ×1014

Seeded chemical

vapor transport

Wenckstern

et al. [11] 0.11 (1.2±0.5)×10

-13 (14±2) ×10

14

Pulsed-laser

deposition

0.12 (1.3±0.5)×10-13

(14±2) ×1014

Pressurized melt

growth

0.22 5×10-13

-

Hydrothermal

Grown

Vines et al.

[32]

0.27

1.6×10-16

for Pd-

SBD

1.2×10-16

for Au-

SBD

(4-5)×1014

Vapor-phase

grown

Fang et al.

[33]

0.27 0.3×10-14

2.1×1012

Hydrothermal

grown

Simpson et

al. [34]

0.29 1.5×10-15

1016

Pressurized melt

growth Auret et al.

[35] 0.29 - 10

14

Seeded chemical

vapor transport

0.29 2.8×10-14

5.1×1012

Hydrothermal

grown

Simpson et

al. [34]

0.3

-

-

Hydrothermal

grown

Kuriyama et

al. [36]

0.3 4×10-16

1 ×1015

Pressurized melt

growth

Ling et al.

[37]

Conted….

27

Conted….

Defect

identificati-

on

Defect

activation

energy

(eV)

Capture

cross

section

(cm2)

Defect

concentration

(cm-3

)

Growth

technique

Research

group

Oxygen

vacancy

0.31 5×10-13

-

Hydrothermal

grown

Vines et al.

[38]

0.32 2.5×10-14

8.3×1012

Hydrothermal

grown

Simpson et

al. [34]

0.53 4×10-15

1×1015

Vapor

transport

process

Hofmann

et.al [39]

0.57 1.5×10-16

-

Hydrothermal

grown

Vines et al.

[32]

Zinc

interstitial

0.1 - - Solid state

method

Chattopadh-

yay et al.

[40]

0.27

1.6×10-16

for Pd-

SBD

1.2×10-16

for Au-

SBD

(4-5)×1014

Vapor-phase

grown

Fang et al.

[33]

0.32 -

1014

to 1016

Pulsed-laser

deposition

Frenzel et.

al [41]

Zinc antisite

0.22 8.22×10

-

17

110×1014

Hydrothermal

technique

Noor et. al

[42]

28

Defect

identificati

-on

Defect

activatio

n energy

(eV)

Capture

cross

section

(cm2)

Defect

concentratio

n

(cm-3

)

Growth

technique

Research

group

Intrinsic

Defect

0.29 (6.2±0.7)

×10-16

(62±7)×10

14

Pulsed-laser

deposition

Wenckster

n

et al. [11]

0.3 (2±0.4)×10

-

16

(80±4)×1014

Pressurized

melt growth

0.3 (6.2±0.7)×10

-16

(2.2±0.4)×101

4

Seeded

chemical

vapor

transport

Zn-related

defect

0.26 11.16×10-17

-

Hydrotherma

l

technique

Noor et. al

[43]

0.31 1×10-15

1.2×10

14

Vapor-phase

Frank et al.

[44] -ve U-

centre 0.54 2×10

-13

from 1015

to

1017

Donor like

defect

0.01

-

-

Pulsed laser

injection

Seghier et

al.

[45]

0.03 - 1017

Seeded

chemical

vapor

transport Wenckster

n

et al. [11] 0.05 - 5×1016

Pressurized

melt growth

0.07 - 5.7×1016

Pulsed-laser

deposition

Surface

related

defect

0.49

3.4×10-14

for

Pd-SBD

3.5×10-15

for

Au-SBD

- Vapor-phase

grown

Fang et al.

[33]

0.49 1.18×10-14

2.01×1014

Hydrotherma

l

technique

Noor et. al

[42]

H diffusion

in ZnO

0.17 &

0.37

-

-

Dc

magnetron

sputtering

Nickel [46]

29

However, the following few of the unclear results listed in Table 1.3 are given to

justify the present work:

The fluctuating activation energy of so called oxygen vacancy ( Vo) defects

varies from 0.1eV to 0.75eV [32-39] but the defects parameters should be same like

fingerprints. Similarly, defect parameters like capture cross section and defect

concentration of oxygen vacancy also vary from one to other. The defect parameters

(activation energy and trap concentration) of zinc interstitial are not clear [33, 40, 41].

In the same way activation energy and trap concentration of donor like defect are not

the same [11,45]. Identification of an electron defect level having activation energy

0.3 eV is not clear [36-38]. In addition, a number of controversial interpretations of

defects are discussed in the recent literature [47-54].

This ambiguous knowledge about defect parameters and their origin motivated us

for this study. For getting better quality, good performance, high efficiency and long

working lifetime of ZnO-based devices, it is necessary to carry out a comprehensive

study of defects in ZnO.

1.3 Dissertation Outline

This dissertation is organized into seven chapters. Chapter 1 covers motivation

and dissertation outline about the current research. Chapter 2 highlights the details of

material (ZnO) such as crystal structure, physical properties and contacts to n-type

ZnO. Chapter 3 explains the growth techniques such as hydrothermal method,

molecular beam epitaxy and aqueous chemical growth. Chapter 4 presents various

characterization techniques such as current-voltage (I-V), capacitance-voltage (C-V),

deep level transient spectroscopy (DLTS) and x-ray diffraction (XRD) used for

characterization of defects in semiconductors. Chapter 5 explains the experimental

30

details of growth techniques such as hydrothermal method, molecular beam epitaxy,

and aqueous chemical growth. Chapter 6 explains the experimental results and

discussions of current experiments. Chapter 7 describes the conclusions and future

plan.

31

CHAPTER 2

32

PROPERTIES OF ZnO

This chapter briefly explains crystal structure of ZnO. A detail review about the

different defects in ZnO is provided. Furthermore, the influence of electric field on

deep level defects is enlightened which is helpful to identify the defects. The progress

of Schottky and Ohmic contacts to ZnO are also described because quality of ZnO

based devices depends upon better quality contacts (Ohmic and Schottky).

2.1 Crystal Structure

The crystal structure of ZnO is hexagonal (Wurtzite) as shown in Figure 2.1.

The space group of this hexagonal lattice is a P63mc. ZnO is a polar material. The

values of some physical properties of ZnO are listed in Table 2.1. The lattice

parameters of ZnO are a = 3.25 Ao and c = 3.18 A

o. There is a strong ionic bond ZnO

due to the large difference in the values of electronegativity (oxygen = 3.44 and Zinc

= 1.65) [1]. ZnO is characterized by two connecting sub lattices of Zn+2

and O.-2

Each Zn ion is surrounded by tetrahedra of O ions and each O ion is surrounded by

tetrahedra of Zn ions. This tetrahedral coordination provides the polar symmetry

along the hexagonal axis. In an ideal Wurtzite crystal, the ratio of lattice parameters

c/a and the u (it is the parameter by which each atom is displaced with respect to the

next along the c-axis) are correlated by the relationship as

8

3

a

uc (2.1)

33

ZnO Unit Cell

Figure 2.1 Unit cell of ZnO and neighbouring atoms, viewing direction approx

parallel to c. Small spheres: O-2

, big spheres: Zn+2

[2].

b a

34

Table 2.1 Physical properties of Zinc oxide [3]

Property Values

Chemical formula

ZnO

Lattice parameters at 300 K

a0

c0

3.24 Ao

5.20 Ao

Density

5.606 gcm-3

Stable phase at 300 K

Wurtzite

Melting point

1975 oC

Boiling point

2360 oC

Linear expansion coefficient

a0 = 6.5×10-6 o

C-1

c0 = 3.0×10-6 o

C-1

Energy bandgap

3.4 eV, direct

Static dielectric constant

8.5

Refractive index

2.008, 2.029

Exciton binding energy

60 meV

Electron effective mass

0.28 mo

Hole effective mass

0.59 mo

Bulk Young’s modulus

111.2 ± 4.7 GPa

Bulk hardness

5.0 ± 0.1 GPa

Electron Hall mobility at 300 K

200 cm2 (Vs)

-1

Hole Hall mobility at 300 K

5-50 cm2 (Vs)

-1

35

The conditions for an ideal crystal are

3

8

a

cand

8

3u (2.2)

ZnO crystals deviate from this ideal arrangement by variation of both of these

values. This deviation occurs in the lattice because tetrahedral distances are roughly

constant. For Wurtzite ZnO, experimentally calculated values of u and c/a were in the

range u=0.3817–0.3856 and c/a=1.593–1.6035 [4 – 6].

Several properties of ZnO such as piezoelectricity and spontaneous

polarization depend on its polarity. Based on termination scheme, the Wurtzite ZnO

has four types (i) polar type terminated at Zn-face along (0001) direction (ii) polar

type terminated at O-face along (0001) direction; (iii) non-polar type along (11 2 0)

direction (iv) non-polar along (1010) direction. The number of Zn and O atoms are

equal in both non-polar (11 2 0) and (1010) faces. Furthermore, the polar surfaces and

non-polar surface (1010) are stable, but the (11 2 0) face is less stable and generally

has a higher level of surface roughness than its counterparts. Each polar face has

unique chemical and physical properties like electronic structure of O-terminated face

is slightly different from Zn-terminated [7]. The optical properties, for example in

photoluminescence, the PL intensities of Zn-polar face are different from O-polar face

because of exciton-phonon coupling strengths, opposite band bending effects and

difference of the adsorbed molecules as shown in Figure 2.2 [8-10]. In the same

manner XRD intensities of Zn-polar face are different from O-polar face as exposed

in Table 2.2.

36

Figure 2.2 Temperature dependence of PL spectrum for Zn-polar face and O-polar

face in the temperature range of 10 K– 300K [10].

Table 2.2 2θ, intensity, and Miller indices of peaks of Zn-polar face and O-polar face

samples calculated from XRD measurements [11].

Peak# Zn-face O-face Identification

(Miller Indices)

2Θ

(degree)

Intensity 2Θ

(degree)

Intensit

y

P1 31.28 1097 31.04 1844 ZnO (1010)

P2 34.48 15744 34.48 812147 ZnO (0002)

P3 36.36 455 Missing ZnO(1011)

P4 38.08 1607 Missing Al(11 1), Ag(111)

P5 40.2 32603 40.2 13862 Zn(1010)

P6 44.76 880 44. 32 90 Ca(531)

P7 Missing 52.72 1334 Unidentify

P8 Missing 57.44 59 ZnO(11 2 0)

P9 64.4 990 64.6 246 Li(211)

P10 65.48 1362 Missing Al(220)

P11 72.64 2208 72.6 49633 ZnO (0004), Zn(11 2 0)

P12 73.72 8260 Missing Ca(331)

P13 77.56 382 Missing Mg(20 2 2)

P14 81.6 181 Missing Al(222), Ag(222)

P15 86.64 1466 86.92 473 Zn (20 2 1)

37

This difference of properties can be due to defects present in Zn and O-polar.

In the following section we describe the Physics of various defects reported in

literature.

2.2 Point Defects

Semiconductors usually have small crystallographic irregularities, also called

as point defects. Point defects are introduced during growth or in postgrowth

methods for example annealing or ion implantation. Even if semiconductor materials

are free of point defects and unexpectedly pure, defects in small concentrations do

exist which have remarkable effect on properties of semiconductors. Defects are not

bad in all cases. However, sometimes they may appreciably enhance the performance

of devices instead of reducing it. The defects such as dislocation, edge dislocation and

others are not addressed because this dissertation is focused only on point defects

2.2.1 Types of Point Defects

There are many forms of crystal point defects. Defects involving only the host

atoms are called the intrinsic defects. Several types of defects are shown in Figure 2.3.

A defect wherein a host atom is missing from one of these sites is

known as a vacancy defect.

If an atom is located on a non-lattice site within the crystal, then it

is said to be an interstitial defect.

A defect in which one host atom is located on wrong site (one host

atom occupied the site of another atom), then it is called an antisite

defect.

38

Figure 2.3 Schematic representation of defects in semiconductors [12].

2.2.2 Point Defects in ZnO

There are two types of point defects in bulk ZnO which have been studied in

literature.

2.2.2.1 Intrinsic Defects

In case of ZnO, the oxygen vacancies (VO), zinc vacancies (VZn), oxygen

interstitials (Oi), zinc interstitials (Zni), oxygen antisites (OZn) and zinc antisites (ZnO)

are the intrinsic defects [13]. Similarly, complex defects are the combination of point

defects like VO-Zni in ZnO [14,15]. They are acting as free carrier concentration and

are responsible for n or p-type in bulk ZnO. The electric field induced by intrinsic

defects affects the properties of deep level defects present in bulk ZnO to be discussed

in section 2.2.2.3.

2.2.2.2 Extrinsic Defects

The defects involving the foreign atoms (impurities) are called the extrinsic

defects for example hydrogen interstitial (Hi) and hydrogen on oxygen sites (HO) in

ZnO [16, 17].

39

2.2.2.3 Influence of Electric Field on Deep Level Defects

The activation energy of traps can be lowered in strong electric fields. The

activation energy of traps is calculated from the emission rate. The emission rate of

carriers from trap is most significant parameter of defect. Each defect has a unique

emission rate. According to Martin et. al. [18] the origin and the carrier excitation

phenomenon of the defect are determined from the emission rate. The emission

rateen,p (the subscripts n and p stand for electron and holes carriers) of defect in terms

of mathematical equation is defined as:

kT

GNVge vcthpnpnpn exp.,,, (2.3)

where G ,png ,,

pn, , thV , vcN . , k, and T are the change in Gibb’s free energy at

constant temperature and pressure, degeneracy of deep states, capture cross-section,

the carrier’s average thermal velocity, the effective density of states, the Boltzmann

constant and absolute temperature, respectively. An internal electric field is produced

in depletion region due to the free carrier concentrations.The relation between internal

electric field and free carrier concentration is explained as what follows:

The electric-field is calculated by the following relation [19]:

Electric field = Qdep /εεo (2.4)

Qdep = god EqN 2 (2.5)

where Qdep, q, εo, ε, , Nd, and Eg are charges in the depletion region, electronic charge,

permittivity of free space, relative permittivity of the host crystal, free carrier

concentration and bandgap, respectively. The emission rate of defects is affected by

40

the electric field due to free carrier concentration. The electric field dependence of the

emission rate has to be taken into account, because neglecting this effect may lead to

serious misinterpretations in the determination of deep level parameters. On the other

hand, it can yield a lot of useful information on the nature of deep traps. Interaction of

fields with emission rate of defect has been explained by various models which helps

to identify the nature of defects. Some of famous models are the following:

(i) Poole-Frenkle [20]

(ii) Phonon-assisted tunneling [21, 22]

(iii) Pure tunneling [18]

Vincet et al. and Naz et al. [23, 24] proposed a simple model to distinguish

between phonon-assisted tunneling and Poole-Frenkle mechanisms, i.e to compare the

plots of ln(e) vs F2 and ln(e) vs F0.5, respectively. In the case of phonon-assisted

tunneling the plot of ln(e) vs F2 is expected to provide a good linear fit, while for

Poole-Frenkle the electric field should follow the ln(e) vs F0.5 linear variation.

2.2.2.3.1 The Poole-Frenkel

According to the Poole–Frenkel theory, when electric field is applied, the

electron band diagram is slanted and the barrier depth is reduced. The emission rate

due to the barrier lowering of the Poole-Frenkel effect is written as [20]:

kT

EEEvNe iic

nncn

exp (2.6)

It can be written as:

41

kT

FEeFe i

non

exp (2.7)

The various models like Coulomb potential, square well potential and dipole

potential have been suggested to fit Poole–Frenkel effect on the emission rates of the

carriers from defect level.The mathematical equations for emission rates at electric

field F to Coulomb potential and square well potential are given below [18,23]:

2

111

1

)0( 2

ee

Fe

n

n (2.8)

where kTqqF or // 2

1

2

11

2

1

)0(

e

e

Fe

n

n (2.9)

where kTqFr /

where F and r are the electric field and radius of potential well, respectively. All other

constants bear usual meanings.

2.2.2.3.2 Phonon-Assisted Tunneling

Phonon-assisted tunneling theory states that if the electron has coupling with

the suitable phonon, then electron will tunnel through the barrier because the emission

energy will be reduced. The field dependence emission rate of the carrier to

characterize the phonon-assisted tunneling mechanism is given below [24]:

2

2

exp0

cF

F

e

Fe (2.10)

42

where Fe and 0e are the emission rate at field F and the emission rate at zero

field. The characteristic field cF is calculated as:

3

2

2

*3

q

hmFc (2.11)

where *m , q and 2 are the carrier effective mass, charge of electron, and the tunneling

time. The tunneling time is calculated as:

122

Tk

h

B

( 2.12)

where h and 1 are the Planck’s constant and characteristic time constant of the order

of the inverse local impurity vibration frequency.

The electric field to enhance emission for Poole-Frenkle and Phonon-assisted

tunneling is in the range of 104-106 V/cm. The enhanced emission processes are

illustrated in Figure 2.4. The electric field in pure tunneling is in the range of ≥107

V/cm [20].

43

Figure 2.4 Electron energy diagram in equilibrium (1) and in the presence of an

electric field (2) showing field-enhanced electron emission: (a) Poole-Frenkel

emission, (b) phonon-assisted tunneling [25].

44

2.3 Contacts to Zinc Oxide

For quality devices, suitable metals should be used to make contacts to ZnO

thin films. Device performance depends not only on the materials quality but also on

the type of metals and its contact. There are two types of contacts that are fabricated

on ZnO, namely:

Schottky Contacts [1]

Ohmic Contacts [26]

2.3.1 Schottky Contact to ZnO

The Schottky model of the metal and n-type semiconductor is defined as [24]:

smB

(2.13)

where φm is work function of metal, defined as the potential difference between the

the vacuum level and the Fermi level EF and φB and χs are barrier height and electron

affinity of semiconductor. The electron affinity of semiconductor is defined as the

energy difference between the bottom of the conduction band EC and the vacuum

level at the semiconductor surface.

Figure 2.5 Metal-semiconductor contacts according to the simple Schottky model

[25].

EC EF EV

X S

EF

M

45

The parameters such as barrier height, series resistance, and ideality factor of

the diode are determined after the fabrication of Schottky contacts. The fabrication of

Schottky contacts is difficult due to following factors: (i) the surface states, (ii) the

contaminants, (iii) the defects in the surface layer, and the diffusion of the metal into

the semiconductor. It is fact that high-quality Schottky contacts are serious issue for

ZnO device applications. First time Mead [27] fabricated the Schottky contacts with

different metal on vacuum-cleaved n-type ZnO surfaces in 1965 but he did not

comprehensively study the thermal stability of the Schottky diodes fabricated on ZnO.

The metals such as Au, Ag, Pd, Pt, and Ti are used for Schottky - contact to n-

type ZnO [28-45]. Al has strong reaction with anions (O). Al is supposed to make the

most dissociated cations (Zn) in ZnO. This results in low barrier height and leakage

current. Simpson et al. [46] reported that the thermal stability of Ag Schottky contacts

was better than that of Au Schottky contacts. Some other reports [30,31] also show

that Au Schottky contacts have some serious problems at temperatures more than 340

K. I-Vcharacteristics of as-deposited Au and Ag Schottky contacts on n-type ZnO are

improved by exposure to the plasma at room temperature or cleaning with organic

solvent before the metal deposition [31,47]. Ohashi et al. [33] suggested that the value

of the barrier height was lower because of high donor density. The higher value of

ideality factors may be due to relatively high carrier concentration which leads to the

increase of tunneling current through the junction. Ip et al. [43] observed that the

Schottky barrier height of Pt contacts on P-doped n-type ZnO grown by plused laser

deposition decreased and ideality factor increased with increasing temperature. It is

clear from the literature that the ideality factor for the ZnO Schottky contacts is higher

than unity in the most of the cases. The higher value of ideality factor can be

explained by different phenomenon such as the prevalence of tunneling [48] the

46

presence of an interface layer, surface states [34] increased surface conductivity [47]

and/or the influence of deep recombination centers [31].

2.3.2 Ohmic Contacts to ZnO

Any contact having a linear and symmetric current-voltage relationship for

both positive and negative voltages is called Ohmic contact. An Ohmic contact obeys

Ohm’s law and is demonstrated in Figure 2.6. It is essential for carrying electrical

current into and out of the semiconductor, ideally with no contact resistance. The

contact resistance is one of the main problems for long-lifetime operation of optical

and electrical devices. The high contact resistance between the semiconductor and

metal affects the device performance because of thermal stress and/or contact failure

[49]. It is necessary to attain Ohmic contacts that have low contact resistance and

thermally stable for high-performance ZnO-based optical and electrical devices. This

can be attained by reducing the metal-semiconductor barrier height or increasing the

effective carrier concentration of the surface [50-52].

Figure 2.6 Energy band diagrams for Ohmic contact [53].

EC

EF

EV

qF

qB

Ei

Metal Semiconductor

47

Ohmic-contact metallization is essential for obtaining good electronic device

performance [50]. In case of wide-bandgap semiconductors like ZnO, low resistance

Ohmic contact can be attained by thermal annealing. But surface roughness and

structural degradation of the interface can be produced during the thermal annealing

process [54,55], resulting in poor device performance [56]. The non-alloyed Ohmic

contacts having low specific contact resistance arechosen in Ohmic-contact

technology especially for shallow junction and low voltage devices because they give

smooth metal semiconductor interfaces [52, 57, 58].

The metals such as Al, Au, In, InGa , Pt , Ti, and Ru are often used for

Ohmic-contact to n-type ZnO [59-63]. Some reports are following; Lee et al. [64]

have successfully fabricated low-resistance and non-alloyed Ohmic contacts to

epitaxially grown n-ZnO. Marlow et al. [65] deposited Ohmic-contacts to n-ZnO by

hydrogen plasma treatment by Ti/Au metallization schemes and observed the contact

resistivity decreased by Ar plasma-treated. They suggested that this decrease in

contact resistivity is due to the formation of shallow donor by ion bombardment. Kim

et al.[54] also reported that resistivity of annealed samples decrease two order of

magnitude as compared to the as deposited samples. They proposed that this reduction

in resistivity is caused by the combined effects of the increases in the carrier

concentration near the ZnO layer surface and the contact area. Kim et al. [63]

proposed that Ohmic contact of Ru to n-type ZnO is appropriate for high-temperature

ZnO based devices. The Ohmic-contacts of Pt were characterized as linear if it

deposited to n-ZnO after surface modification [50,66]. Sheng et al. [52] reported an

extensive study of Ohmic-contact to n-ZnO. They found the following results: (i) the

electron concentrations are increased and the specific contact resistivity is decreased

48

because of tunneling and (ii) the specific contact resistivity of heavily doped ZnO is

lowered than unintentionally doped ZnO.

49

CHAPTER 3

50

2 GROWTH TECHNIQUES

Zinc Oxide can be grown by various techniques such as, sol-gel chemistry, spray

pyrolysis, metal organic chemical vapor deposition (MOCVD), molecular beam

epitaxy (MBE), pulsed laser deposition (PLD), hydrothermal, reactive thermal

evaporation, and sputtering [1-7]. ZnO samples used in this study were grown by

hydrothermal technique, MBE and aqueous chemical methods. These three techniques

are hereby described in detail.

3.1 Hydrothermal Technique

Hydrothermal technique can be defined as a method of synthesis of crystals

under elevated pressure and temperature conditions in the presence of water. The term

hydrothermal usually refers to any heterogeneous reaction in the presence of aqueous

solvents under high pressure and temperature conditions to dissolve and recrystallize

(recover) materials that are relatively insoluble under ordinary conditions. Today, the

hydrothermal technique has gained a unique place in various branches of modern

science and technology like in fabrication of nanostructures. Hydrothermal method is

preferred one and economical because of its short reaction times and lower energy

requirements. It is very important for its technological efficiency in developing

bigger, pure, and dislocation-free single crystals.

3.1.1 Experimental Setup of Hydrothermal Method

In hydrothermal technique, the growth chamber is capable of containing

corrosive solvent at high temperature and pressure required for growth. The

temperature, pressure, and corrosion resistance of solvent are the important

parameters for selecting a suitable growth chamber. The nutrient is supplied to the

http://en.wikipedia.org/wiki/Single_crystal

51

growth chamber along with solvent (i.e an aqueous solution of alkali). The

temperature gradient is applied at the opposite ends of the growth chamber. The

nutrient dissolves at the hotter end of chamber and seed crystal are introduced at

cooler end of chamber for additional growth. An ideal growth chamber for

hydrothermal should have the following characteristics

Inertness to acids, bases and oxidizing agents.

Easy to assemble and dissemble.

Sufficient length to obtain a desired temperature gradient.

Leak-proof with unlimited capabilities for the required temperature and

pressure.

Rugged enough to bear high pressure and temperature condition for long

duration, so that no machining or treatment is needed after each experimental

run.

3.1.2 Hydrothermal Synthesis of Zinc Oxide

The hydrothermal technique is the most suitable method for fabricating

isometric (having equal measurement) zinc oxide crystals of good quality [8-13]. In

this method, the aqueous solution of ZnO is heated at suitable temperature in

temperature gradient chamber where ZnO seed crystals are mounted. ZnO vapors are

condensed on the seed crystals and form ZnO wafer.These wafers can be separated

from seed crystals by stress pressure. The schematic diagram of the hydrothermal

growth chamber is shown in Figure 3.1. The hydrothermal apparatus consists of

furnaces, pressure gauge, autoclave, heaters and Pt crucibles. The furnaces and

autoclave are used to increase the temperature. The Pt crucible contains baffle, vessel

of seed crystals in upper part of crucible and vessel of solution in lower part of

52

crucible. The heaters F1 and F2 are used to control the temperature in such a way that

temperature of upper part is less than lower part of Pt crucible.

Hydrothermal growth process is described as following: initially some seed

crystals are hanged in the upper part of Pt crucible by a Pt wire. The sintered ZnO as

nutrient and aqueous solution of alkali (NaOH, KOH, LiOH) or chlorides of desired

molarity as solvent are mixed together to form the solution [14-18]. The solvent

breaks the intra molecular bonds of ZnO due to week Van de Waal forces. This

solution is put in lower part of Pt crucible. A Pt baffle is installed to separate seed

crystals from solution. The crucible is placed into an autoclave after closing it by

welding. The autoclave is then placed into a two-zone vertical furnace. When solution

temperature and pressure of autoclave are increased, water of aqueous solution

converted into steam and moved to upper part of crucible. Then steam returned backin

liquid form to lower part of crucible because of condensation of water molecules.

ZnO is attached to seed crystals in upper part of Pt crucible and seed crystals grow to

bulk. Only ZnO grow in upper part of crucible due to the presence of its mother seed.

Table 3.1 The growth conditions optimized by Sakagami for zinc oxide crystals [30].

Parameters

Values

Growth temperature

370–400 °C

Temperature difference

10–15 °C

Total pressure

700–1000 kg/cm

Partial pressure

10–30 kg/cm

Solvent KOH 3.0 M + LiOH 1.5 M

Oxidizer

H2O2 0.1 M–0.3 M

Nutrient ZnO sintered

Growth run

15–20 days/run

53

Figure 3.1 Schematic drawing of a hydrothermal growth system. F: furnaces (F1,F2),

T: thermocouples for control (T1,T2) and monitor (T3), P: pressure gauge, A:

autoclave, C: Pt crucible, S: seed crystals, N: nutrient, Baffle [19].

In the zinc oxide growth, the oxidation of the chamber material produces

hydrogen due to the presence of alkalis. The amount of hydrogen formed depends on

the temperature, the concentration of the alkali, and the duration of the experiment.

54

Several research groups have successfully grown ZnO nanostructures by

hydrothermal methods for biomaterials applications [20-28]. Sun et. al. [29] had

grown ZnO nanostructures by hydrothermal technique. They obtained the outstanding

variety of ZnO nanostructures having different and controllable morphologies by

varying growth parameters such as precursor chemicals, their concentrations and/or

the growth temperature. Sakagami [30] has grown zinc oxide crystals of high purity

by the hydrothermal method using platinum-lined chamber. The growth conditions

optimized by Sakagami are listed in Table 3.1.

3.2 Molecular Beam Epitaxy

Molecular beam Epitaxy is the most advance technique to grow atom by atom

layers of thin films. There is no substitute to the quality of materials grown by MBE,

however it does have some fundamental limits. An ultra high vacuum is required to

grow the material. Advantages of MBE are the following:

Excellent interface and surface morphology.

It is possible to control the thickness of epilayer precisely.

In-situ characterization techniques (RHEED).

High purity starting materials, easy chemistry.

Low growth temperature which reduces any undesirable thermally

activated processes such as diffusion.

The slow growth rate and high cost are disadvantages of MBE growth. Now

various scientists have successfully grown ZnO by MBE [31-34].

The substrate is placed on sample holder. It is heated to required temperature

and rotated continuously (if necessary) for better quality of growth homogeneity [35].

55

The ultra high vacuum (UHV) in the range of 10-6

to 10-4

mbar is needed for MBE

growth. The O2, CO2, H2O and N2 contamination on the growing surface can be

ignored after outgasing under UHV. The reduced rate down to nm/sec is achieved by

particular growth conditions. In this way precise growth of control thickness is

possible. Prior to growth, substrate is cleaned to avoid the effect of contamination on

the properties of substrate. The basic requirements for MBE growth are following, (i)

pure startng materials are to be used. (ii) Low background pressure in the evaporator

to decrease contamination; (iii) Uniform flux of effusion cell across the substrate. (iv)

The reaction chamber is evacuated to <10-8

mbar and the walls of the chamber cooled

with liquid nitrogen. The UHV is an important characteristic of MBE. The most

common diagnostic techniques to monitor the growth is reflected high-energy electron

diffraction (RHEED). Each monolayer growth can be seen in the intensity and pattern

of the RHEED signal. Thus growth can be controlled precisely at themonolayer

level.The diagram of MBE system is shown in Figure. 3.2.

56

(1) RHEED, (2) growth chamber, (3) baking heaters, (4) ion pump, (5) gate wall, (6)

mechanical pump, (7) preparation chamber, (8) fishing lever, (9) Viewing windows.

Figure 3.2 The diagram of MBE system [36]

3.3 Aqueous Chemical Growth

It is the easiest, cheap and time saving growth technique. A number of ZnO

nanostructures such as nanowires, nanorods and nanotubes have been grown by this

technique [37-40]. In this method, first of all, seed solution of required material is

prepared by the mixture of different chemicals of desired molarity under necessary

condition of temperature and stirring. The seed solution should be transparent. Then

few drops of the seed solution were placed on the substrate and rotated with help of

1 4 3

2

5

6 7 8

9

57

spin coater at desired speed. After this the growth solution is made by the mixture of

chemicals (HMT and Zinc nitrate hexahydrate for ZnO as shown in Figure 3.3) of

preferred molarity in deionized water at room temperature and stirred using magnetic

stirrer apparatus as illustrated in Figure 3.4. The substrate is put in the solution with

face downward towards the solution. Subsequently, solution container is placed into

the oven as shown in Fig.3.5 at temperature less than 100oC for some hours. Then

substrate is removed from the solution, cleaned with deionized water and dried at

room temperature and used for characterization.

Figure 2.3 The chemicals for growth solution of ZnO.

58

Figure 3.4 The stirring of growth solution for ZnO.

Figure 3.5 Growth solution container placed in oven.

59

CHAPTER 4

60

3 CHARACTERIZATION TECHNIQUES

Characterization techniques can generally be placed into two categories i.e. Electrical

and Optical. It is one of the most important steps to study the various properties of

semiconductor materials. This chapter describes the various characterization

techniques like current-voltage (I-V), capacitance-voltage (C-V), deep level transient

spectroscopy (DLTS), and X-ray diffraction (XRD). To perform the electrical

measurements, Schottky and Ohmic contacts are necessary, described in the previous

chapter.

4.1 Current-Voltage Measurements (I-V)

The current-voltage characteristic (I-V) provides the knowledge of

fundamental parameters about the performance of devices. Figure 4.1 shows the

intimate contact between the metal and the n-type semiconductor.

Figure 4.1 Band diagram of the intimate contact between metal and semiconductor (n-

type) for a rectifying junction [1].

61

When a voltage is applied to the metal contact, current flows through a

uniform metal-semiconductor interface. The movement of majority carriers across the

interface such as electrons in n-type material or holes in p-type material generates a

flow of current. Three main current mechanisms can occur, namely, (i) Thermionic

emission (TE), (ii) thermionic field emission (TFE), and (iii) Field emission (FE).

Thermionic emission is dominant for lightly doped semiconductors, for the

intermediate doped semiconductors, the current flows as a result of thermionic field

emission whereas field emission can dominate in heavily doped semiconductors [2].

The current-voltage technique involves a range of voltages, both positive and

negative, to the metal-semiconductor system and the measurement of the resulting

current [3]. When a reverse or negative voltage is applied, the energy levels of an n-

type semiconductor are lowered with respect to the metal Fermi level. The barrier for

electrons to traverse from the semiconductor to the metal is increased as a result of the

increased band bending, associated with the lowering of the Fermi level. In this

scenario, the electrons flow from the semiconductor to the metal is decreased. When

positive or forward voltage is applied, results in the Fermi level of the semiconductor

being raised compared to the metal. In this case, the barrier for electrons to traverse

from the metal to the semiconductor remains the same, but the barrier for electrons to

traverse from the semiconductor to the metal is reduced. Thus, equilibrium conditions

in the current flow are altered, and the flow of current from the metal to the n-type

semiconductor is greater than the opposing current. Therefore the metal-

semiconductor junction with positive voltage has a rectifying behavior. A large

current exists under forward bias than the reverse bias.

62

The ideality factor (n), barrier height (φB)I-V and saturation current (Is) are

measured by forward I-V measurements at given temperature. By plotting the

experimentally measured I-V data, as ln(I) vs V, the intercept of the linear fit will give

saturation current (Is) from which barrier height (φB)I-V could be extracted, (method is

described below) and the slope of the plot will give the value of ideality factor (n),

using following relations[1]:

1exp

nkT

qVII S (4.1)

kT

qTAAI

B

S

exp2* (4.2)

slopekT

qn

(4.3)

dV

Idslope

ln (4.4)

where A is Schottky contact area, A* is the Richardson constant, T is temperature in

Kelvin, k is the Boltzmann’s constant (k = 1.3810-23

J/K or 8.61710-5

eV/K) and q

is the electric charge. Deviation of n from the ideal range 1.01 <n < 1.1 may be

generated by a thick interface layer or substantial recombination in the depletion

region. A non-uniform interface will also produce an increase in n from 1.01- to

higher value [1]. The saturation current is calculated from the intercept of semilog

graph of I-V measurements at different temperatures. In this way we have the values

of saturation current at different temperatures. The slpoe of plot of log (Is/T2

) vs

1000/T gives the barrier height.

63

4.2 Capacitance-Voltage Measurements (C-V)

Capacitance-Voltage (C-V) method is based upon the voltage dependence of

the charge in the depletion region of the diode. The C-V method measures the

electrostatic properties of the Schottky barrier and is not affected by transport

processes such as tunneling and image force lowering.

The capacitance of Schottky diodedecreases asreverse bias increases.In

capacitances-voltage characteristics, capacitance for a Schottky diode is given by

following relation [4]:

VV

qNAC

bi

roD

2

(4.5)

where C, A, ε, q, ND, V, k and T are capacitances, area of contact, dielectric constant,

electric charge free carrier concentration, applied voltage, Boltzmann constant and

temperature, respectively. In C-V measurements, a plot of (A/C)2

vs V will give a

straight line. The slope and x-intercept of straight line are (2/ εqND) and 2Vbi/εqND

respectively.The Vbi (electrostatic potential at equilibrium in semiconductor) is called

built-in potential. The free carrier concentration ND is calculated from the slope using

the following relation:

slopeq

N D

2 (4.6)

The depth profile of free carrier concentration is also found from C-V

measurements by plotting the graph between free carrier concentration and depth. The

depth x is calculated by following relations:

64

dV

dCq

N D 2

2

(4.7)

C

Ax o

(4.8)

The barrier height, (φB) C-V can be calculated from:

VoVbivcB (4.9)

D

C

ON

NIn

q

kTV (4.10)

222

ekTmNC

(4.11)

where VO is the energy difference between the Fermi level and the bottom of the

conduction band, and NC

is the conduction band density of the states, m*

is the

effective mass of material. High densities of impurities or defects with deep energy

levels in the band gap make it difficult to measure the Schottky barrier height (SBH)

by C-V, since these defects change the space-charge region and hence the intercept

voltage. These deep level defects typically cause the SBH (C-V) to be greater than the

SBH (I-V).

4.3 Deep Level Transient Spectroscopy (DLTS)

Deep Level transient spectroscopy is one of the most versatile techniques to

detect impurities and defects in semiconductors. The level of detection, going down to

~1011

cm-3

by this method, is unmatched with other characterization techniques. The

DLTS technique also provides electrical parameters of the levels associated with

impurities or defects. By monitoring capacitance transients produced by pulsing the

65

semiconductor junction at different temperatures, a spectrum is generated which

exhibits a peak for each level. The height of the peak is proportional to trap density

and the position of the peak on the temperature axis leads to the determination of the

fundamental parameters governing thermal emission and capture (activation energy

and cross section).

4.3.1 Carrier Kinetics in Semiconductors with Deep Level- Shockley-

Read-Hall Theory

The carrier kinetics in extrinsic semiconductors can be explained by

Shockley-Read-Hall theory [2]. To support our results, we will describe this theory

qualitatively only. Consider ND, Pt, and Nt which denote the doping concentration, a

hole trap concentration and an electron trap concentration, respectively. Both the

electron and the hole traps are uniformly distributed throughout the semiconductor

with the trap activation energy of ET as shown in Figure. 4.2 Ec, Ev, cn, en,cp and ep

represent the energy of conduction band, energy of valence band, capture of electrons,

emission of electrons, capture of holes and emission of holes, respectively.

Figure 4.2 Electron energy band diagram for a semiconductor with deep a level trap

[2].

66

Figure 4.2 shows a recombination event (a) followed by (c) and a generation

event (b) followed by (d). The impurity is a G-R center and both the conduction and

valence bands take part in recombination and generation. A trap has only two charge

states, a filled state which is occupied by an electron, or an empty state which is

unoccupied. If the capture and emission of electrons are dominating, then the total

electron trap concentration can be expressed as

NT = nt + pt (4.12)

where nt is the concentration of the trapped electrons (occupied states), pt is the

concentration of unoccupied states. Similarly, if the capture and emission of holes are

dominant, then the total hole trap concentration is:

PT = nt + pt (4.13)

where nt and pt , are respectively the concentrations of trapped holes (occupied states)

and neutral traps (unoccupied traps).

4.3.2 Basic Principle of DLTS

In the following, we consider an n-type semiconductor containing only

electron traps. A depletion layer of width x will be formed as show in Figure 4.3.

Figure 4.3 The metal-semiconductor contact and the depletion layer [2].

x

Depletion region

V

67

When bias voltage V is imposed across the junction, the depletion layer

capacitance per unit area is given by:

VV

qNC

bi

roD

2

(4.14)

where Vbi is the built in potential and V is the bias voltage, positive for forward bias

and negative for reverse bias.

Figure 4.4 The schematic illustration of a majority injection pulse sequence and

energy band bending. (a) bias time, (b) capacitance-time, (c) and (d) the energy band

bending during the pulse and after pulse [2].

68

Assuming that a forward bias pulse ∆V is applied, then the capacitance prior to

the application of the forward bias pulse is:

VV

qNC

bi

roD

o

2

(4.15)

When a pulse ∆V<V is applied, electrons will move into the n-side and they will

fill the traps step by step. After a certain period of time, when the traps are filled then

the capacitance will increase.

VVV

nNqC

bi

tDo

2

(4.16)

When all traps are filled with electrons, nt= NT. At the moment when ∆V= 0 at t

= 0 as shown in Figure 4.4 (b), the capacitance C changes to:

VV

NNqtC

bi

tDo

20

(4.17)

After ∆V= 0, the trapped electrons are regularly re-emitted to the conduction

band as shown in Figure. 4.4 (d). This means that nt slowly reduces and finally nt, = 0,

then Eq. (4.17) becomes Eq. (4.15) and C returns to Co. The change of the capacitance

C with time after t = 0 is referred to as the capacitance transient, which can be written

as:

VV

tnNqtC

bi

tDo

2

(4.18)

Eq. (4.18) can be written as:

69

tnNK

VV

tC D

bi

22

1 (4.19)

where oqK 2 , is a constant, by defining S (t) = -dV/d (1/C2), we obtain

tt nnKSS 00 2 (4.20)

Suppose that nt(0) = NT and nt (∞) = 0, then from Eq. (4.20) we have

2

0

K

tStSN tt

T

(4.21)

Thus, from above Eq. (4.21) the deep-level impurity concentration can be

calculated by the DLTS measurements. The emissions of the trapped electrons from

traps change the capacitance with time. Since the capacitance transient reveals the

emission process of trapped electrons. The emission rate depends upon the following

factors: (i) trap activation energy ET; (ii) trap capture cross section σn;

(iii) temperature T. Hence these trap parameters can be calculated by capacitance

transient spectroscopy. The capacitance of a junction due to impurities with a single

level of activation energy can be written as:

eD

to

t

N

nCtC

exp

2

01 (4.22)

where nt (0) the concentration of trapped electrons at t = 0, ND is the doping

concentration which can be considered as the free electron concentration, τe is the

trapped electron emission time constant, which is given by:

70

2

exp

T

kT

EE

nn

TC

e

(4.23)

where

2

3

2

1

T

N

T

v Cthn , Vth is the electron thermal velocity and NC is the election

effective density of states in the conduction band. If the trap concentration is

uniformly distributed in the depletion region and nt (t = 0) = NT, then NT be expressed

as:

o

DTC

CNN

2 (4.24)

By using the Equations (4.22-4.24), we can find out all trap parameters such as

the trap energy level ET, the trapped electron emission time constant τe and trap

concentration NT through a series of proper C-t measurements at various temperatures

[5, 6].

The type of above foresaid technique is isothermal single shot technique (IST).

IST is time domain technique and therefore takes extremely long period to just get a

single data point. It produces data of emission rate of very high quality. But there are

some disadvantages of IST, for instance; (i) IST requires extremely good control over

temperature; (ii) Analysis of data becomes difficult when more than one defect is

present. Thus IST is converted into method commonly known as Deep Level

Transient Spectroscopy (DLTS) by introducing the filtering function. It determines the

temperature where the transient has a certain time constant instead of the time

constant for the transient at a certain temperature as is the case in the single shot

71

transient technique. The change of time domain to temperature domain can be

explained by the following relation:

dttWTtfTSt

)(,)(

(4.25)

where S(T), f(t),W(t) and T are the output DLTS signal, current or capacitance signal,

the filtering function and temperature, respectively. In this technique, the electronic

system responds only within a desired emission rate based on mechanical rate

window, which can be changed if required, while the temperature is increased from a

low value to higher values. The transients due to different deep levels are appeared

into peaks at different temperatures for a single desired rate window. There are many

methods to make rate window for example (i) Dual – Gated Integrator (Double

Boxcar) (ii) Lock in amplifier. The DLTS used in our lab is lock in amplifier. In lock-

in amplifiers DLTS, a square wave weighting function is used whose period set by the

frequency of the lock-in amplifier, the frequency in return yields carriers emission

rate (en) defined as: en = 2.17 × f. When this frequency has the proper relationship

with emission rate of defect, a peak of level is obtained at certain temperature. The

defect parameters like activation energy, the capture cross section and the

concentration of the defect level are calculated as described below.

4.3.3 Measurement of Defect Parameters by DLTS

From DLTS, activation energy and capture cross section of the defect level are

calculated as a result of several temperature scans performed. In temperature scan the

frequency is kept constant while the temperature is traversed. After performing

several measurements with different frequencies referred as (T-scan) or at different

temperatures referred as (F-scan), there are frequency-temperature data pairs

72

belonging to each peak. The obtained f-T i.e. (en,T) data pairs can be illustrated

according to the following equation, which is the logarithmic form of the expression

of the emission rate. The emission of majority carriers from deep levels is a process

which depends exponentially on the inverse temperature and the position in the band

gap. The evaluation of the detailed balance relation is performed on its logarithmic

form. This is the equation:

ktEENve TCCnthn /exp (4.26)

where en, σn are emission rate and capture cross section. The definition of the thermal

velocity according to the Boltzmann velocity distribution:

m

kTvth

3 (4.27)

Since the example refers to n-type material, the effective mass of the electrons

should be put in this equation. The effective density of states in conduction band is:

2/3

2

22

h

kTmNC

(4.28)

After substituting vth and NC the equation (4.26) becomes:

kTEETh

mke TCnn /)(exp{)3(2

2 22/1

2/3

2

2

(4.29)

The constants can be marked with K (the capture cross section and the

effective mass are supposed to be temperature independent, which is a simplification

again):

73

}/)(exp{2 kTEEKTe TCnn (4.30)

Taking the logarithmic form of both sides of the equation:

kTEEKTe TCnn /)()ln()/ln( 2 (4.31)

This relation is the so-called Arrhenius plot. To illustrate this curve, emission

rate-temperature (en-T) data pairs are needed which are characteristic of the deep

level. These data come from the fitting of the DLTS spectra.

kTEKTe actnn /)ln()/ln( 2 (4.32)

where Eact is the activation energy of the level needed to promote electron from the

level to the conduction band. Measuring the emission rate as a function of temperature

and plotting ln (e/T 2) vs 1/T (an Arrhenius plot) will give a straight line, from which

the apparent activation energy, Eact can be extracted from the slope and the apparent

carrier capture cross section, σn from the intersection of the line with the y-axis.

Trap concentration (NT) is calculated using the following formula:

21

2VV

VV

C

CNN RB

DT

(4.33)

where VB,VR, V1 V2 andC

C are the built in potential, reverse bias, and forward biases,

amplitude of peak, respectively.

The capture cross section is a characteristic parameter of the deep level. It can

be investigated by varying the width of the filling pulse and measuring the peak

amplitude. The measurements are frequency scans performed at constant temperature.

74

The list of filling pulse width and peak amplitude data pairs. The capture cross section

is supposed to be an exponential process. The capture time constant can be described

by the following equation:

thD

cvN

1

(4.34)

Supposing that the trap is empty before the filling pulse, the trap concentration at the

end of the pulse is:

)]/exp(1[)( cpTp tNtN (4.35)

This equation has a logarithmic form including the relative occupation n(t) which is

proportional with the peak amplitude at a given filling pulse width:

T

p

pN

tNtn

)()( (4.36)

c

p

p

ttn

)](1ln[ (4.37)

According to this modal, illustrating the left side of the above equation as a function

of the filling pulse the received points can be fitted by straight line. The slope of the

line gives the time constant of the capture process whose reciprocal value is the

capture velocity:

c

cV

1 (4.38)

The capture cross section is calculated according to the equation:

75

thc vDN

1

(4.39)

This series of measurements can be repeated at different temperature. In this way

(known as indirect method) the temperature dependence of the capture cross section

can be investigated.

4.4 X-Ray Diffraction (XRD)

X-ray diffraction is a technique in which the pattern produced by the

diffraction of X-rays through the closely spaced lattice of atoms in a crystal is

recorded and then analyzed to reveal the crystal structure of that lattice. This generally

leads to an understanding of the material and molecular structure of a substance. The

spacing in the crystal lattice can be determined using Bragg’s law.

Miller indices are a symbolic vector representation for the orientation of an

atomic plane in a crystal lattice and are defined as the reciprocals of the fractional

intercepts which the plane makes with the crystallographic axes. Indexing pattern of

any crystal system can be found by analytical methods. Analytical methods of

indexing involve arithmetical manipulation of the observed sin2 θ values in an attempt

to find certain relationships among them. Since each crystal system is characterized

by particular relationships between sin2θ values, recognition of these relationships

identifies the crystal system and leads to a solution of the line indices. Since we are

working with ZnO and its crystal structure is hexagonal. The Miller indices of

hexagonal system are determined as described below.

76

4.4.1 Hexagonal System

For hexagonal crystal system, sin2 θ values are given by [7]:

2222 ClkhkhASin (4.40)

where2

2

3aA

and

2

2

4cC

where λ is wavelength of x-rays, a, and c are lattice constants of material. The

possible values of (h2+hk+k

2) are S = (h

2+hk+k

2) = 1, 3, 4, 9, etc. The indexing

procedure is explained by a particular example (shown in Figure.4.5) such as the

powder pattern of zinc (its structure is hexagonal), the observed sin2θ values are

tabulated in Table 4.1. Initially we suppose l = 0 and divide the sin2θ values by the

integers (S) 1, 3, 4, etc, and put the results in Table as shown in Table 4.1. This

applies to the first line of the pattern. After this we then observe those numbers, which

are equal to one another or equal to one of the observed sin2θ values. In this case, the

two values, 0.112 and 0.111, are almost equal, so we suppose that lines 2 and 5 are

hk0 lines. Afterward we cautiously put A = 0.112 which is equivalent to saying that

line 2 is 100. As the sin2θ value of line 5 is very nearly 3 times that of line 2, line 5

should be 110. We find the value of C by using the equation:

2222 ClkhkhASin (4.41)

We now subtract AS from each sin2 θ value, the values of A = 0.112, 3A =

0.336, 4A = 0.448, etc., and look for remainders (Cl2) which are in the ratio of 1, 4, 9,

16, etc. The resultant numbers are given in Table 4.2. Now the five numbers are

important because these entries (0.024, 0.079, 0.221, and 0.390) are very close in ratio

1, 4, 9, and 16. Therefore we put 0.024 = C (1)2, 0.097 = C (2)

2 , 0.221 = C (3)

2, and

77

0.390 = C(4)2. This gives C = 0.024 and indices of line 1 as 002 and line 6 as 004.

Since line 3 has a sin2 θ value equal to the sum of A and C, its indices must be 101. In

this fashion, the indices of lines 4 and 5 are found to be 102 and 103, respectively.

Similarly, indices are found to all the lines on the pattern. It can be rechecked by a

comparison of observed and calculated sin2 θ values.

Figure 4.5 XRD measurement of the Zn.

Table 4.1 Calculated miller indices (hkl) of hexagonal system when l = 0.

Line sin2θ Sin

2θ/3 sin

2θ/4 Sin

2θ/7 hkl

1 0.097 0.032 0.024 0.014 -

2 0.112* 0.037 0.028 0.016 100

3 0.136 0.045 0.034 0.019 -

4 0.209 0.070 0.052 0.030 -

5 0.332 0.111* 0.083 0.047 110

6 0.390 0.130 0.098 0.056 -

30 40 50 60 70 80

0

50

100

150

200

Inte

nsi

ty (

cp

s)

2 (degree)

78

Table 4.2 Calculated miller indices (hkl) of hexagonal system

Line sin2θ sin

2θ - A sin

2θ – 3A hkl

1 0.097* - - 002

2 0.112 0.000 - 100

3 0.136 0.024* - 101

4 0.209 0.097* - 102

5 0.332 0.221* - 110, 103

6 0.390* 0.278 0.054 004

79

CHAPTER 5

80

4 EXPERIMENTAL DETAILS

ZnO samples used in this study were grown by hydrothermal technique, MBE and

aqueous chemical method.. The samples (“group A” and “group B”) were grown by

hydrothermal technique. The samples (“group C”) were grown by MBE. The other

samples labeled as sample “D, E, F, G” were grown by aqueous chemical method.

The experimental details of all samples are explained in this chapter.

5.1 Group A and B Samples

Hydrothermally grown, wurtzite (0001) single crystal bulk n type ZnO wafers

were purchased from ZnOrdic AB original 10×10×5 mm in size shown in Figure. 5.1,

but were cut into pieces for various characterization purposes. The rocking curve of x-

ray diffraction (XRD) peak at 17.74o was 20-60 arcsec. All the samples had both Zn

and O-face. For the purpose of electrical characterization, metal contacts are

necessary. Schottky contacts of 1mm diameter (thickness ~ 2000Å) with palladium

metal were prepared on the Zn-face and O-face and samples were grouped into

“group A” and “group B”, samples respectively.Ohmic contacts of nickel/gold

(thickness ~ 200/2000Å) were prepared on the relevant backside of group A and B

samples. These samples were provided by our collaborator Prof. Dr. Magnus

Willander.

Figure 5.1 ZnO wafer grown by hydrothermal [1].

81

5.2 Group C Samples

Thin film of ZnO on silicon (111) was grown by molecular beam epitaxity.

Prior to the growth of ZnO, cleaning was performed by the standard procedure:

substrate was cleaned by dipping it in the mixture of 100 ml of H2O2 (deionized

water) and 150 ml of H2SO4 for 20 minutes and subsequently cleaned in the HF

(hydrofluoric acid) for 15 seconds. This cleaning procedure is known as piranha

procedure. After cleaning, substrate was loaded into the chamber. To attain high

quality film, a proper vacuum is required. For this purpose rotary pump and Turbo

molecular pump were used to achieve a pressure of 0.75×10-4

Torr in 4 hrs. To

perform the growth, effusion cell was heated at temperature 286 0C to evaporate zinc

from the cell. Atomic oxygen was produced by an RF-plasma source equipped with

an electrostatic ion trap operated at 300 W. The substrate temperature and chamber

pressure were varied from 382-430o

C and (1- 4) ×10-4

×0.75 Torr, respectively during

the growth period of 24 hours. The samples grown by MBE were categorized as

“group C”. For the electrical measurements again Schottky contacts of different

diameter with silver were prepared as shown in Figure. 5.2. These samples were

grown at University of North Carolina Charlotte, North Carolina, USA.

Figure 5.2 ZnO samples grown by MBE.

82

5.3 Samples D, E, F and G

Samples D, E, F and G were grown by aqueous chemical growth. This growth

procedure consists of three steps [2]:

Preparation of seed solution

Pretreatment of substrate

Chemical bath deposition growth

5.3.1 Preparation of Seed Solution

A solution was made by mixing the 274 mg zinc acetate dehydrate in 125 ml

of methanol, the molarity of this solution was 0.01M (molarity). The solution was

heated up to 60 oC with stirring using magnetic stirrer apparatusuntil the solution

became transparent. Another solution was prepared by mixing 109 mg KOH in 65 ml

of methanol (this will give a 0.03 M concentration) at 60 0C. Similarly this solution

was stirred until it becomes transparent. Then the solution of KOH + methanol was

added to the solution of zinc acetate dehydrate + methnol at 60 oC very slowly, while

stirring was continued. After mixing these two solutions, the resulting solution was

kept at 60 oC for 2 hrs with stirring using magnetic stirrer apparatus.

5.3.2 Pretreatment of Substrate

We used the metal coated glass as substrates: Ni, Al, Au and Ag coated glass

labeled as samples “D, E, F, G, respectively were used to form nanostructures of ZnO.

Finally, few drops of the seed solution were placed on the each metal coated glass

substrate. The substrate was placed on to the spinner that spins at a spin speed of 4000

rpm for 30 seconds [3]. Then metal coated glass substrates were placed into the oven

at 100 oC for 10 minutes.

83

5.3.3 Chemical Bath Deposition Growth

Two equimolar aqueous solutions of Zinc nitrate hexahydrate (ZNH) (Zn

(NO3)2 .6H2O) and hexa-methylene-tetramine (HMT) (C6H12N4) were prepared. Both

solutions have molarity of 0.1M. Then both solutions were mixed at room temperature

with stirring using magnetic stirrer apparatus. The substrates were placed in this

solution with face downward towards the solution. After this solution containers were

put in the oven at a constant temperature 95oC for 7 hrs. After 7 hrs the substrates

were removed from the solution, cleaned with deionized water and dried at room

temperature.

84

CHAPTER 6

5

85

6 RESULTS AND DISCUSSION

This chapter is divided into four sections.

Section- I

The first section presents study of defects in Zn-face ZnO (group A samples) grown

by hydrothermal method. Results described in this section are based in part on paper

[Noor et.al. 2009[1]].

6.1 Current-Voltage Measurements

To study the deep level defects metallic contacts are required (Schottky and

Ohmic). It is necessary, before starting the deep level measurements, to determine the

quality of the contacts. I-V measurements provide a deep and detail characteristics of

the Schottky diode / Schottky contact. A number of measurements were performed on

all the samples (group-A), the representative results are presented in this section.

Figure 6.1 shows the forward and reverse biased I-V characteristics of Pd/ZnO SBD

(Schottky barrier diode) at temperature 306 K. A low leakage current is found ~ 6 μA

at higher reverse bias i.e. -3V. This low leakage current is appropriate for capacitance-

voltage (C-V) and DLTS measurements. I-V measurements were analyzed to

determine the quality parameters ideality factor, and barrier height for group “A”

samples. But results related with barrier height of the devices were not convincing

therefore are not being presented here.

The value for the ideality factor (n) is obtained by following the procedure

described in chapter 4 and by using the equations 4.3 and 4.4. The I-V measurements

were performed in the temperature range 120 – 340K. As a result, the upper and lower

limits of the n appear to be 1.7 at 340 K and 13.5 at 120 K, (Figures. 6.2 and 6.3)

86

respectively. The variation in the values of the ideality factor could be related to the

thermionic transport mechanism that is extremely affected in the low temperature

regime. This phenomenon is known as To-effect [2, 3]. A plot of n versus 103/T as

shown in Fig. 6.4 demonstrates that To-effect is present in the temperature range 335-

250 K while it is not observed in the low temperature range 250-120 K. It could be

connected to the To anomalous effect. The product of n and temperature is almost the

same for the data having the To-effect. According to the literature survey, this constant

product is a sign of a trap assisted carrier transport mechanism [4,5].

Figure 6.1 I-V characteristics of the Pd-Schottky contact on the Zn face of the ZnO.

-3 -2 -1 0 1 210

-9

10-7

10-5

10-3

10-1

C

urren

t(A

)

Voltage(V)

T = 306K

n = 4.1

87

Figure 6.2 I-V characteristics of the Pd-Schottky contact on the Zn face of the ZnO at

120 K.

Figure 6.3 I-V characteristics of the Pd-Schottky contact on the Zn face of the ZnO at

340 K.

-3 -2 -1 0 1 2

1E-7

1E-6

1E-5

1E-4

1E-3

Cu

rren

t(A

)

Voltage(V)

T = 340K

n = 1.7

88

Figure 6.4 Plot between Ideality factor and 1000/T indicates the To- effect for the Zn

face Pd/ZnO Schottky diode.

6.2 Capacitance-Voltage Measurements

Figure. 6.5 shows the experimental capacitance–voltage characteristics of the

Pd/ZnO SBD. C–V analysis is done at 1 MHz ac signal as a function of voltage at

room temperature. The depth profile of the apparent free carrier concentration (ND)

from C-V measurement is shown in Figure 6.7. The free carrier concentration (ND)

can be obtained from the plot of (A/C)2 –V (Figure. 6.6) by means of the given

equation:

1

2

22

C

A

dV

d

qN

o

D

(6.1)

The contact area (A), charge of electron (q), permittivity of free space (ε0) and

dielectric constant (ε) are 0.78×10-2

cm2, 1.6×10

-19 C, 8.85×10

-12 Fm-1 and 8.5 for

Pd/ZnO, respectively. The free carrier concentration (ND) is found to be 3.44×1017

cm-3. The depth x is calculated from the depletion capacitance C by using the

following equation:

2 4 6 80

6

12

1000/T (K -1)

n (

idea

lity

fa

cto

r)

89

Rbi

D

VVN

mx

71005.1 (6.2)

where Vbi and VR are built in potential and applied voltage respectively. The built-in

potential Vbi is determined by the linear extrapolation to the intercept on the voltage

axis of the (A/C)2–V. The value of barrier height (φB)C-V is obtained from C–V

measurement by using the equation 4.7. The barrier height (φB)C-V is found to be 0.12

eV.

As can be seen in Figure 6.5 C-V characteristics have an unusual peak in

forward bias region. This peak points out the defects in depletion region that

strengthen our explanation for the observed higher value of the ideality factor. These

features again show the presence of majority carrier traps on the surface and/or in the

bulk of the semiconductor material. The C-V measurements at different temperature

shown in Figure 6.8 and their parameters (built in potential and free carrier

concentration) at different temperatures are listed in Table 6.1. Depth profiles of free

carrier concentration of Zn-face ZnO at different temperatures shown in Figure 6.9.

Figure 6.5 C-V measurements of the Pd-Schottky contact on the Zn face of the ZnO.

-3 -2 -1 0 1150

200

250

300

Applied Bias (V)

Cap

aci

atn

ce (

pF

)

90

Figure 6.6 Graph between applied bias and inverse squared capacitance.

Figure 6.7 Depth profile of free carrier concentration of Pd/ZnO.

-3 -2 -1 0 18.0x10

14

1.0x1015

1.2x1015

1.4x1015

1.6x1015

1.8x1015

2.0x1015

A

2/C

2 (

cm

/F)2

Voltage(V)

200 150 100 50 010

16

1017

1018

ND

(cm

-3)

Depth (nm)

91

Figure 6.8 C-V measurements of the Zn face of the ZnO at different temperatures.

Figure 6.9 Depth profile of free carrier concentration of Zn-face ZnO at different

temperatures.

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0150

200

250

300

350

400

450

C

ap

aci

atn

ce (

pF

)

Voltage(V)

159 K

199 K

304 K

338 K

240 260 280 300 320 340 360

4

6

N

D (10

17cm

-3)

Depth (nm)

T = 159 K

220 240 260 280 300 320 340

4

6

8

ND

(1

01

7cm

-3)

Depth (nm)

T = 199 K

135 140 145 150 155 160 1652

3

4

5

6

7

8

9

10

N

D (1

01

7cm

-3)

Depth (nm)

T =304 K

200 220 240 260 280 300

6

8

10

12

14

16

N

D (10

17cm

-3)

Depth (nm)

T = 338 K

92

Table 6.1 Electrical parameters of Zn face ZnO calculated from C-V measurements.

Temperature

(K)

Built in

potential (V)

Free carrier

concentration

(cm-3

)×1017

122 7.92 3.13

128 7.95 3.15

140 7.82 3.14

141 8.16 3.19

151 7.94 3.2

152 8.13 3.28

159 7.14 3.28

160 7.75 3.15

169 7.89 3.19

170 7.14 3.3

178 7.11 3.29

180 7.78 3.23

189 7.15 3.32

193 6.86 3.22

200 7.78 3.23

207 6.92 3.28

209 7.87 3.3

240 7.46 3.19

293 7.62 3.01

294 6.62 3.29

295 6.58 3.08

296 6.72 3.43

296 6.34 3.28

296 6.6 3.37

302 6.86 3.55

93

6.3 Deep Level Transient Spectroscopy Measurements

Two electron defect levels labeled as E1 & E2 were observed from the DLTS

scan of group A representative sample having Pd-Schottky contacts of 0.78 mm2 or 1

mm in diameter, shown in Figure 6.10. DLTS spectrum was taken from 150 K to 350

K with digital operating systems. For DLTS measurements, following parameters

were used: UR (reverse bias) = –2 V, Vp (filling pulse) = +2 V, tp (filling time) = 20 μs

and en (emission rate) = 2170 s-1

. During the DLTS scan, the temperature ramping rate

was limited at 0.01 K/sec for the precise temperature control. Representative DLTS

spectra at different frequencies of E1 were shown in Figure 6.11.

The activation energies, i.e., energy level positions in the bandgap (Ec–ET)

and capture cross sections (σ∞) of the defects (E1 & E2) have been obtained from the

temperature dependence of the thermal emission rates by means of Arrhenius plots (

Figure 6.12), defect E1 has activation energy ( Ec-Et ~ 0.22 ± 0.02 eV) with capture

cross sections of about (8.22 ± 0.4)×10-17

cm2

and defect E2 has activation energy ( Ec-

Et ~ 0.49 ± 0.02 eV) with capture cross section of about (1.18 ± 0.5)×10-14

cm2. The

trap concentration (NT) of the defects (E1 & E2) is calculated by using the equation

[12]:

2

22

2p

rp

D

t

W

WW

N

N

C

C

(6.3)

where ND is the free carrier concentration, Wr and Wp the thickness of the depletion

layer at reverse bias and at the pulse voltage, respectively and

D

tFo

Nq

EE2

2

(6.4)

94

where ε, εo, q, EF, and Et consist of usual meanings. The trap concentration (NT) of the

defects (E1) was shown in Figure 6.13. All the parameters of defects levels are listed

in Table 6.2.

Figure 6.10 DLTS spectrum displaying two electron deep level defects below

conduction band of ZnO.

Figure 6.11 The DLTS spectrum of levels E1 in ZnO.

150 200 250 300 350

E2

E1

emission rate = 2170 s

-1 D

LT

S S

ign

al

(a.u

)

Temperature(K)

x10

2.11015

cm-3

100 150 200 250 300 350 400 450

-15

-10

-5

0

DL

S (

mV

)

Temperature(K)

10Hz

100Hz

500Hz

1000Hz

2500Hz

95

Figure 6.12 The Arrhenius plot of levels E1 and E2 in ZnO.

Figure 6.13 Trap concentrations of levels E1 in Zn-face ZnO

0.150 0.155 0.160 0.165 0.170 0.175 0.180 0.185 0.1902

4

6

8

10

12

Nt (

x1

014 c

m-3

)

Depth (m)

4 6

1E-3

0.01

E1

E2

en

T -

2(s

-1K

-2

)

1000/T (K -1)

96

Table 6.2 Details of electrical parameters such as activation energy, capture cross

section measured via indirect and direct methods and trap concentration of defects

observed in the DLTS spectrum of Zn-face ZnO.

Defect

ID

Tmeas

(K) E (eV) σ∞ (cm

2)

σT (cm2)

(× 10-19

)

NT (cm-3

)

(× 1012

)

E1 200 0.22 ± 0.02 (8.22±0.4)×10-17

6.8±0.3

110 ± 50

E2 291 0.49 ±0.02 (1.18±0.5)×10-14

1.5 ±0.5

2.01 ± 0.6

6.4 X-Ray Diffraction

Figure 6.14 displays the representative XRD spectrum of group “A” sample

exhibiting twelve peaks (P1-P12). The corresponding planes of peaks were calculated

by using standard formula explained in “The Elements of X-ray diffraction” by B. D.

Cullity [13]. 2θ, intensity, Miller indices, and sources of peaks are listed in Table. 6.3.

It is clear from the table that P1(1010), P2 (0002), P3(1011), P5 (1010), P9 (0004)

and P12 Zn (20 2 1) were originated from ZnO. P4 Al(111), Ag(111), P6 Ca(531), P8

Al(220), P10 Ca(331) and P11 Mg(20 2 2) were originated from alumina which was

used for back contacts.

97

Figure 6.14 Typical XRD pattern of the Zn-face ZnO layer exhibiting the Zni-VO

complex as the preferential direction of growth. Peaks other than ZnO are seen

because the XRD measurements were performed on Pd\ZnO–Zn\Au–Cr mounted on

an alumina substrate by silver paste.

40 60 80

0.0

2.0x104

4.0x104

p8p12

p11

p10

p9p7p6

p5

p4p3

p2

Inte

nsi

ty (

cp

s)

2 (degree)

p1

98

Table 6.3 2θ, intensity, Miller indices, and sources of peaks measured from XRD

data in ZnO layers listed.

Peak# 2θ

Intensity

Zn-Face

Identification

(Miller Indices)

Source of Peak

P1 31.28 1097 ZnO(Zni-VO) (1010) ZnO layer

P2 34.48 15744 ZnO (0002) ZnO layer

P3 36.36 455 ZnO(1011) ZnO layer

P4 38.08 1607 Al(111), Ag(111) Alumina

P5 40.2 32603 Zn (Zni-VO) (1010) ZnO layer

P6 44.76 880 Ca(531) Alumina

P7 64.4 990 Li(211) Hydrothermal growth of ZnO

layer

P8 65.48 1362 Al(220) Alumina

P9 72.64 2208 ZnO (0004), Zn(11 2 0) ZnO layer

P10 73.72 8260 Ca(331) Alumina

P11 77.56 382 Mg(20 2 2) Alumina

P12 86.64 1466 Zn (20 2 1) ZnO layer

99

6.5 Trap Identification

In the following section, detail discussion of E1 and E2 are given.

6.5.1 Electron Level E1

We focus on E1 level in this section. A deep level defect is defined as defect

level having activation energies ≥ 0.1 Eg and trap concentration ≤ 0.01ND. E1 having

activation energy 0.22 eV below the conduction band edge, may be a shallow dopant

level in ZnO (the energy bandgap Eg of ZnO is 3.37 eV). In literature Kitchill et

al.[14] have given the same justification of Mg-related level in p-GaN (Eg ~ 3.34 eV)

as a shallow acceptor level with activation energy 0.2 eV above the valence band.

Several research groups have observed same electrical properties as our E1 level in

the recent literature. Some well-known papers are as follows: Dong et al. [15]

reported a similar defect level at energy of 0.3 eV, attributed to an oxygen vacancy.

Vineset al. [16] found the trap of energy 0.31eV below the conduction band and they

linked this trap to the oxygen vacancy. An electron trap associated with Zn

interstitials having activation energy (trap concentration) of 0.32 eV (1014

–1016

cm−3

)

below the conduction, was discovered by Frenzel et al. [17]. The intrinsic donor like

defects in ZnO having activation energy in the range of 0.30–0.37 eV, were reported

by Wenckstern et al. [18]. A Zn related defect level having ionization energy of 0.31

eV, was observed by Frank et al. [19]. A similar level at an energy of 0.29 eV,

connected to an oxygen vacancy, was found by Auret et al . [20]. By comparing it

with literature we conclude that research groups were not agreed on the energy of

level (E1) and its origin. After this, we carried out further experiments in order to

resolve this issue.

100

C-V and DLTS measurements were performed after equal interval of time and

displayed in Figures 6.15 and 6.16. It is observed that the free carrier density ND

decreases with increasing trap E1 concentration NT with a passage of time as shown in

Figure 6.17. The possible justification of this phenomenon could be: (a) site

competition [21] (b) lattice relaxation [22] and/or (c) transformation of defect states

[23]. In accordance with the study of ZnO material, we can experimentally consider

the above mechanism with the transformation of defect states as described below.

ZnO has intrinsically n-type conductivity. The Zni, VO and the Zni-VO complex

are common sources of n-type conductivity in ZnO [24, 25].Our ZnO sample had

been grown along Zn face and/or under Zn rich condition. As expected, Zn atoms are

present in large numbers in our samples. Zn atoms may occupy interstitial Zni site or

make complex with oxygen vacancy Zni-VO. To support our expectations XRD

measurement was performed.

Figure 6.15 C-V measurements indicate decrease in amplitude.

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

200

240

280

320

360

400

Cap

aci

atn

ce (

pF

)

Voltage(V)

0 hours

744 hours

2208 hours

2760 hours

101

Figure 6.16 DLTS measurements indicate increase in amplitude.

Figure 6.17 Demonstration of time-delayed transformation phenomenon of defects in

ZnO layer.

0 1000 2000 3000

0.0

0.4

0.8

1.2

0.0

0.4

0.8

1.2

8

16

24

32

ND

(x1

016 c

m-3

)

NT

(x1

016 c

m-3

)

Time (hours)

120 160 200 240 280 320 360-350

-300

-250

-200

-150

-100

-50

0

D

LS

(m

V)

Temperature(K)

0 hours

744 Hours

2208 hour

2760 hours

102

X-ray diffraction measurement (as shown in Figure 6.14) of the sample clearly

reveals that the preferable growth direction is along (1010) plane because the

intensity of peak P5 (1010) is highest. It is interesting to note that this highest peak

originates from Zni-VO complex [26] as mentioned in Table 6.2. From this result of

XRD measurement, we can suppose that our sample has n-type conductivity due to

Zni-VO complex. The presence of Zni in ZnO matrix makes Zni-VO complex unstable

bond [20]. Under certain conditions, the bond (Zni–VO) could break, consequently ND

decreases and Zni may occupy VO site and make a zinc antisite and as a result, the

concentration of E1 will increase. This change in concentration of free carrier ND and

trap Nt with respect to time is shown in Figure. 6.17. In this manner, the relationships

of ND with the Zni-VO complex and ZnO with the level E1 support our explanations. As

a result, the transformation description of defects phenomenon seems to be justified.

Some theoretical work is present in literature about the presence of zinc

antisites and vacancy complexes in the ZnO lattice such as in an electron irradiated

ZnO lattice presence of VO-OZn and VZn-ZnO complexes using the Doppler broadening

annihilation-radiation measurements together with local density approximation

calculations were observed by Chen et al. [27]. Oba et al. [28] also discussed the

existence of multiple charged states of Zn antisites, +2/+1/0 in Zn-rich ZnO crystal.

Accordingly to Janotti and Vande Walle [25], formation energy of Zn antisite is lower

than the Zn-interstitial, which is 0.57 eV. The foresaid literature study (experimental

and theoretical) clearly supported that the E1 level is zinc antisite.

103


By Arrhenius plot, the activation energy and capture cross section of E2 are Ec-

Et ~ 0.49 ± 0.02 eV and (1.18 ± 0.5)×10-14

cm2, respectively. The capture cross section

of E2 calculated by indirect method is (1.5 ±0.5)×10-19

cm2. The trap concentration of

the E2 is (2.01±0.6)×1014

cm-3

. Dong et al. [8] observed an electron trap level having

similar features as observed in our sample. They attributed it to a surface defect in

ZnO.

104

Section- II

The second part presents study of defects in O-face ZnO (group B samples) grown by

hydrothermal method. Results described in this section are based in part on paper

[Noor et.al. 2010[29]].

6.6 Current-Voltage Measurements

The numbers of measurements were performed on all the samples (group-B),

the representative results are presented in this section. Figure 6.18 shows

representative current-voltage (I-V) measurements of group B samples. Schottky

barrier height φB and ideality factor n of the diodes are calculated from the data which

are based on thermionic emission theory. I-V relationship for Schottky diode is

described by the equations 4.1 and 4.2 [30]. By plotting the measured I-V data on

semilog graph, the ideality factor is calculated from the slope of linear fit for forward

bias with the help of the Eq. 4.3. The ideality factor (quality parameter) of the

Schottky device was found to be 3.4.

The ideality factor n is greater than the practical limits i.e. 1-2 (diffusion –

recombination nature of current) [31]. It means fewer amount is recorded as Schottky

current and remaining current followed some other parallel paths. Such paths were

possibly offered by thermionic field emission (TFE), interface/surface states and/or

ND-induced barrier height lowering. The high temperature is a necessary condition for

thermionic field emission as carrier may tunnel through the thinner part of the barrier

[32]. Thus TFE cannot be valid in our case as measurements were carried out at room

temperature. On the other hand, interface and/or surface states cannot be avoided.

Characteristically, these states can act as carrier trap and/or recombination center. As

105

a result, Schottky current is reduced and hence n becomes large. But results related

with barrier height of the devices were not convincing therefore are not being

presented here.

-4 -3 -2 -1 0 1 2

10-6

10-5

10-4

10-3

10-2

T = 300K

n = 3.4 Cu

rren

t(A

)

Voltage(V)

Figure 6.18 Representative I-V measurements of group B samples.

Figure 6.19 Representative C-V measurements of group B samples.

106

6.7 Capacitance-Voltage Measurements

Figure 6.19 shows the representative experimental capacitance–voltage

characteristics of the group B samples. C–V analysis is performed at 1 MHz ac signal

as a function of voltage at room temperature. The depth profile of the apparent free

carrier concentration (ND) from C-V measurement is shown in Figure 6.21. The free

carrier concentration (ND) and depth (x) are calculated from the plot of (A/C) 2

–V

(shown in Figure 6.20) by means of equations 6.1 and 6.2. The free carrier

concentration (ND) is found to be 3×1016

cm-3

.

The built-in potential Vbi is determined by the linear extrapolation to the

intercept on the voltage axis of the (A/C) 2

-V. The built-in potential Vbi is found to be

1.43 V. The value of barrier height (φB) C-V is obtained from C-V measurement by

using the equation 4.7. The barrier height (φB) C-V is found to be 1.56 eV. The plot of

1/C2 versus V was linear, as can be seen in Figur 6.20, it indicates uniform variation in

free carrier concentration as a function of depth (see Figure. 6.21). Similar results in

intrinsically (bulk) n-ZnO Schottky diodes were observed by Dong et al. [8] and Fang

et al. [33]. They suggested that these results are due to the surface defects. In this

way, our results are consistent with the literature. C-V measurements at different

temperature are shown in Figur 6.22. As the carrier freeze out at low temperature,

carrier concentration increased with increasing temperature.The associated parameters

such as built in potential and free carrier concentration measured at different

temperatures are listed in Table 6.4. Free carrier concentration depth profiles of O-

face ZnO at different temperatures are shown in Figure. 6.23.

107

Figure 6.20 Schottky behavior of the sample B is demonstrated in 1/C2-V, filled

squares represent the experimental data and the line corresponds to the theoretical fit

of the data, extrapolated to x-axis yield built-in potential.

80 100 120 140 160 1802

3

4

ND

(10

16cm

-3)

Depth (nm)

Figure 6.21 The uniform spatial distribution of the free-carriers in the as-deposited

ZnO material.

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.50.0

5.0x1014

1.0x1015

1.5x1015

2.0x1015

2.5x1015

3.0x1015

3.5x1015

A

2/C

2 (

cm

/F)2

Voltage(V)

108

Table 6.4 Electrical parameters of O face ZnO calculated from C-V measurements

Temperature

(K)

Built in

potential (V)

Free carrier

concentration

(cm-3

)×1016

133 2.34 2.81

146 2.5 3.15

155 2.53 3.22

158 2.56 3.26

175 2.11 3.38

181 2.51 3.28

187 2 3.24

196 2.06 3.34

206 1.95 3.27

216 2 3.35

225 1.91 3.28

235 1.9 3.3

245 1.84 3.3

255 1.84 3.33

263 1.76 2.99

274 1.62 2.9

284 1.29 2.65

290 1.38 2.99

291 1.96 4.08

295 1.93 3.94

298 2.06 3.94

300 1.73 3.54

302 1.49 2.95

109

Figure 6.22 C-V measurements of the O face of the ZnO at different temperature.

Figure 6.23 Depth profile of free carrier concentration of O-face ZnO at different

temperatures.

200 250 300 350 400 4500

2

4

6

8

10

ND

(10

16cm

-3)

Depth (nm)

T = 133 K

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5120

140

160

180

200

220

240

260

280

Cap

aci

atn

ce (

pF

)

Voltage(V)

133 K

206 K

298 K

110


We expected that all results of DLTS measurements to be same in both group

of samples because both are n-type ZnO grown by same technique i.e hydrothermal

having Pd-Schottky contacts of 0.78 mm2 or 1mm in diameter. But DLTS results of

group B samples are entirely different from group A samples as can be seen below.

DLTS results of both groups of samples are following: Two majority electron

defect levels labeled as E1 & E2 were observed from the DLTS scans of group A and

B representative samples are shown in Figure 6.24. DLTS spectra were taken from

150 K to 350 K with digital operating systems. For DLTS measurements, following

parameters were used: UR (reverse bias) = –3 V, Vp (filling pulse) = +3 V, tp (filling

time) = 20 μs and en (emission rate) = 2170 s-1

. During DLTS scans, temperature

ramping rate was limited at 0.01 K/sec for the precise temperature control.

Representative DLTS spectra of group B samples measured at different frequencies of

E1are shown in Figure 6.25. DLTS spectra at different frequencies are required for

Arrhenius plots to calculate activation energy and capture cross-section of level. Level

E2 exhibits the same emission rate but E1 appears at different emission rates in

samples A and B (see Figure 6.24). First level E1 has different activation energy and

capture cross-sections in both groups of samples i.e EC-0.22 eV and 8.22×10-17

cm2

for group A samples and EC - 0.26 eV and 11.16 ×10-17

cm2 for group B samples by

means of Arrhenius plots as shown in Figure 6.26. Second level E2 has same

activation energy and capture cross-section that is EC-0.49 eV and 1.18×10-14

cm2 for

both groups A and B samples. Depth profile of trap concentration (NT) of the defects

(E1) in group B samples calculated by using equations 6.3 and 6.4 is shown in Figure

6.27.

111

Level E1 has different emission rates in both groups A and B of the samples. If

we carefully consider the experimental detail of both groups A and B of samples, they

are different by two factors: (i) face difference (ii) free carrier concentration. As face

of material is assumed to cause surface contamination, therefore, we suppose that ND

may be the only factor that affects the emission rates of the level E1.

Figure 6.24 Representative DLTS scans of group A and B samples to show the

variation in peak position of E1 level even measured under same measuring setup.

100 150 200 250 300 350-350

-300

-250

-200

-150

-100

-50

0

Temperature(K)

DL

TS

Sig

na

l (m

V)

2.11015

cm-3

emission rate = 2170 s-1

x15

E2

E1

Sample B

Sample A

112

150 200 250 300

-300

-250

-200

-150

-100

-50

0

DL

TS

Sig

na

l (m

eV

)

Temperature(K)

1 Hz

1000 Hz

100 Hz

2000 Hz

Figure 6.25 The DLTS spectra measured at different frequencies for Arrhenius plot of

levels E1 in sample B.

Figure 6.26 The Arrhenius plot of levels E1 in samples A and B.

4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8

10-3

10-2

10-1

Sample B

en

T -

2(s

-1K

-2

)

1000/T (K -1)

Sample A

113

0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38

0

20

40

60

80

100

120

140

Nt (

x1

014 c

m-3

)

Depth (mm)

Figure 6.27 Depth profile of trap concentration of levels E1 in O-face ZnO.


Detail study of trap E1 and E2 are explained in following section.


As mentioned earlier, electron level E1 has different activation energy in both

groups A and B of samples. Free carrier concentration ND may be the only factor that

affects the emission rates of the foresaid level. This contention is consistent with the

following reports:

Miyajima et al. [35] discovered two defects in Ga doped ZnSe (1015 - 1018

cm-3)

and defined them as trap A and B. Their results revealed that the activation energy of

trap A did not vary with ND but it was not same in the case for trap B. The activation

energy of trap B changed from 0.4–0.56 eV as a function of ND (1018-1015 cm-3

). They

114

observed the activation energy of trap B increased as free carrier concentration

decreased. They attributed trap B to the complex of Zn vacancy and Ga or the

complex of interstitial Se and Ga. Baber et al [36] reported that activation energy (Ec -

Et) of Ti donor level in InP using DLTS. They calculated activation energy change

from 0.54 ± 0.03 eV to 0.63 ± 0.03 eV. They observed that the specimen with the

largest activation energy (0.63 ± 0.03 eV) having the lowest background free carrier

concentration of 5×1015

cm-3

whereas the specimen attained the lowest activation

energy (0.54 ± 0.03 eV) containing the highest free carrier concentration of (1-

3)×1017

cm-3

. They suggested that emission rate was changed by electric field due to

free carrier concentration. Diaconu et al. [37] reported the similar results. They doped

Co (0.02, 0.20 and 2.00 at %) in ZnO samples labeled as “a, b and c”, respectively.

The free carrier concentrations of samples (a, b and c) were 3.7×1017 cm-3 ,

0.5×1017cm-3and 31.2×1017 cm-3, respectively. They found three defects E1, E2 and E3

in all (a, b and c) samples. The activation energy of E1 was calculated to be 0.29 eV,

0.34 eV and 0.17 eV in samples a, b, and c, respectively. Similarly the activation

energies (0.37 eV, 0.39 eV and 0.30 eV) were calculated for E2 in samples a, b and c,

respectively. They also calculated activation energies (0.45 eV, 0.59 eV and 0.33 eV)

for E3 in all three samples. They found activation energy of defects (E1, E2 and E3)

shifted towards lower value with increasing of free carrier concentration in all

samples (a, b and c).

The above reports qualitatively justify our argument. Some well-known

groups have reported E1 like level in intrinsically n-type ZnO samples having

activation energy in the range of 0.32 to 0.22 eV with background free carrier

concentration (1014

-1017

) cm-3

. They linked it to oxygen vacancy, zinc interstial

and/or zinc antisite. This defect identification is probably dependent upon the

115

activation energy of the level which according to Vincent et al. [38], is not credible.

They proposed that it was necessary to become careful before interpreting the data

obtained by means of capacitance transient measurement of diodes having free carrier

concentration greater than 1015

cm-3

. That is why we plotted the graph between

activation energy of the defect E1 against the background free carrier concentration in

our samples and already reported by other research groups [19, 33,34] as shown in

Figure 6.28. It is evident from the graph that activation energy decreases as the

background free carrier concentration increase. The information from the literature

indicates that the reduction in thermal emission energy of a defect level is linked with

the electric field enhanced emission. Figure 6.29 illustrates the variation of activation

energy as a function of electric-field created in depletion region due to ND calculated

using the equation 2.4.

Figure 6.28 Influence of background concentration ND on activation energy of E1

level. Data 1 and 2 are ours and rest of the data is taken from Refs. 19, 33, and 34.

10-1

100

101

102

103

160

180

200

220

240

260

280

300

320

340

Acti

vati

on

En

eerg

y (

meV

)

Free Carrier Concentration (1015 cm-3)

116

0.1 1

180

240

300

360

Internal Field (105 V/cm)

Acti

va

tio

n E

neerg

y (

meV

)

Figure 6.29 The ND-induced field effect on the thermal energy data of the level. Data

1 and 2 are ours and rest of the data is taken from Refs. 19, 33, and 34.

It is clear from the Figure 6.29 that an electric field has a remarkable effect on

the activation energy; the value of activation energy becomes lower due to the

increase of internal electric field. It means activation energy of defect is sensitive to

the internal electric field. Several models have used to explain the origin of defect as

described in section 2.2.2.3.

Since in our case, ND-induced barrier height lowering causing thermal

emission of the trap to the lower value. The linear relationship of log (en) with

F1/2

(depicted in Figure. 6.30) proves the Poole-Frenkel mechanism [39]. Therefore we

will concentrate only on Poole–Frenkel mechanism for our data. Two models to

evaluate the emission rate for the Poole–Frenkel effect are: (i) Columbic potential (ii)

square well potential [38,40] as explained in section 2.2.2.3.1. The mathematical

117

equations for emission rates at electric field F to Coulomb potential and square well

potential are given below:

2

111

1

)0( 2

ee

Fe

n

n (6.5)

where kTqqF or // 2

1

2

11

2

1

)0(

e

e

Fe

n

n (6.6)

where kTqFr /

Figure 6.36 shows the experimental emission data (filled circles) together with

theoretical emission rates (lines) which were calculated using equations 6.5 and 6.6

for the observed trap E1.

Figure 6.30 Qualitative evidence of the Poole–Frenkel mechanism on the ND-induced

variation in emission rate signatures of E1 level.

200 400 600 800

100

101

102

E

mis

sio

n r

ate

(s-1

)

F 0.5(V/cm)

118

Figure 6.31 Theoretical fitting of the ND-induced field emission rates (filled circles)

obeying Poole–Frenkel mechanism associated with Coulomb potential (curve C),

while square well potential (r = 4.8 nm) is not consistent (curve S).

It is clear from the Figure 6.31 that experimental data is consistent with the

Poole–Frenkel model associated with Coulomb potential. Consequently, the level E1

is identified as a charged impurity. Additionally, in intrinsically n-type ZnO material,

most of the research groups have reported Zn-related electron defects (interstitials and

antisites) showing relatively shallower energy spectrum (0.22–0.32 eV)

[1,17,19,33,34]. As a result, we attribute the foresaid charged impurity to Zn. This

justification is in agreement with the theoretical results that Zn-interstitial is shallower

than O-related defects (interstitials and antisites) in ZnO.

0 10 20 30 40

0

40

80

120

C

Em

issi

on

ra

te (

s-1)

F( 106 V/m)

S

119


An electron level having activation energy Ec-0.49 eV was observed by Fang

et al. [33]. They labeled this electron trap as E4 and linked it with surface defects. By

comparing defect parameters (activation energy, capture cross section and built in

potential) of E2 with Fang et al., we attribute E2 to the surface defect.

120

Section- III

Third part describes Hall measurement, current-voltage, capacitance- voltage, deep

level transient spectroscopy, and secondary ion mass spectroscopy (SIMS) of group C

samples grown by MBE.

6.10 Hall Measurement

Figure 6.32 shows representative temperature dependent Hall measurement of

group C samples. The upper part, middle part, and lower part of Figure 6.32 display

mobility, carrier concentrations and resistivity, respectively. As a general rule,

positive and negative data values of carrier concentration correspond to p-type

conductivity and n-type conductivity of the sample under test, respectively.

Accordingly, we can see that as mixed conductivity with respect to the temperature as

shown in middle part Figure 6.32. Data of mobility and carrier concentration Vs

temperature is scattered. Conductivity is not stable with temperature. The values of

mobility become zero as temperatures vary. To explain mobility ( H ) results,

consider the Hall mobility equation given below [41]:

HH R (6.7)

where HR and are the Hall coefficient and conductivity, Hall coefficient is define as

222

np

np

Hnpe

npR

(6.8)

where all parameters bear the usual meanings. Mobility becomes zero when 2

pp =

2

nn .

121

The scatter data of mobility becomes zero at some temperatures when number

of electrons equals to number holes by equation 6.8. The mixed conductivity transits

from one type to another at zero mobility.

Figure 6.32 Representative temperature dependent Hall measurements of group C

samples. The upper part, middle part, and lower part of Figure display mobility,

carrier concentrations and resistivity, respectively.

122

6.11 SIMS Measurement

Figure 6.33 shows typical SIMS measurement of group C samples. SIMS

measurement (Figure 6.33) shows the presence of oxygen, zinc and nitrogen as a

function of film depth. The elements are arranged as oxygen, zinc and nitrogen by

decreasing the contents in the layer, discussion of the results are presented in Sec.

6.15.

Figure 6.33 SIMS depth profiles of O, Zn and N elements in group C samples.

123

6.12 Current-Voltage Measurement

Figure 6.34 shows semilog current-voltage (I-V) measurement of sample C.

According to the thermionic emission theory, ideality factor n is determined from the

slope of the linear region of the forward- bias region of semilog I-V characteristics

through the relation 4.3. Calculated value of the ideality factor was 5.1. The ideality

factor (n) which is equal to 1 for an ideal diode has usually a value greater than unity.

High values of n can be attributed to the presence thermionic field emission, interface

states, and generation – recombination centers.

Figure 6.34 Representative I-V measurements of group C samples.

124

6.13 Capacitance-Voltage Measurement

Capacitance – voltage measurement of group C samples is performed at 1

MHz ac signal as a function of voltage at room temperature. The depth profile of the

apparent free carrier concentration (ND) from C-V measurement is shown in Figure

6.36. The free carrier concentration (ND) and depth (x) are calculated from the plot of

(A/C) 2

–V (shown in Figure 6.35) by means of given equations 5.1 and 5.2. The free

carrier concentration (ND) is found to be 2.84×1015

cm-3

.The built-in potential Vbi is

determined by the linear extrapolation to the intercept on the voltage axis of the (A/C)

2 -V. The built-in potential Vbi was found to be 0.07 V. The low value of built-in

potential is due to majority carrier traps on the surface and/or in the bulk of the

semiconductor materials to be discussed later.

Figure 6.35 Schottky behavior of group C sample is demonstrated in A2/C

2-V, filled

squares represent the experimental data and the line corresponds to the theoretical fit

of the data.

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.50.0

2.0x1015

4.0x1015

6.0x1015

8.0x1015

1.0x1016

1.2x1016

1.4x1016

1.6x1016

1.8x1016

2.0x1016

A2/C

2 (

cm/F

)2

Voltage(V)

125

Figure 6.36 Depth profile of free carriers of group C sample.


A hole defect level labeled as H is observed from the DLTS scans of group C

samples having Ag-Schottky contacts of 0.78 mm2 or 1 mm in diameter is shown in

Figure 6.37. DLTS spectrum was taken from 120 K to 350 K with digital operating

systems. For DLTS measurements, the following parameters were used: UR (reverse

bias) = –1 V, Vp (filling pulse) = +1 V, tp (filling time) = 20 μs and en (emission rate)

= 5425 s-1

. During the DLTS scan, the temperature ramping rate was limited at 0.01

K/sec for the precise temperature control. Representative DLTS spectra of H level

measured at different lock in frequencies are shown in Figure 6.38.

The activation energy, i.e., energy level position in the bandgap (Ev + ET) and

capture cross section (σ∞) of the defect (H) have been obtained from thermal

emission rates by means of Arrhenius plots (shown in Figure 6.39), defect H ( Ev + Et

~ 0.45 eV) with capture cross section of about 3.78×10-17

cm2, respectively. By

comparing this result with class A samples, the defects to be found in class A samples

126

were electron traps but observed in class C samples is the hole trap. The nature of

defects was different in both groups of samples because class A samples were grown

by hydrothermal and class C samples were grown by MBE. But defect labeled as E1

in class A samples and H in class C samples had relationship with time. The trap

concentration of E1 increased with decreasing free carrier concentration with passage

of time.

Figure 6.37 Representative DLTS spectrum displaying one hole trap of group C

samples.

Figure 6.38 Typical DLTS spectra of levels H measured at different frequencies of

group C samples.

100 150 200 250 300 350 400 450

-60

-45

-30

-15

0

D

LS

(m

V)

Temperature(K)

5 Hz

10 Hz

50 Hz

1000 Hz

100 150 200 250 300 350 400 450

-60

-50

-40

-30

-20

-10

0

Temperature(K)

DL

TS

Sig

na

l (m

eV

)

127

Figure 6.39 The Arrhenius plot of hole level in group C samples.


In semiconductor, impurities and defects occur as energy states in bandgap. If

a defect is transformed from one structural configuration to another, it is called

metastable defect. The configuration of metastable defect can be explored by

changing the measuring parameters such as electric field, temperature, storage time,

and carrier injection conditions. Several groups have reported the metastable defect by

using the DLTS [42,43]. A metastable hole trap with storage time (shown in Figure.

6.40) is detected in DLTS measurements. This hole trap located at different energy

states is observed by repeating DLTS measurements, while all the measuring

parameters remain same except time. As a result, the activation energy of the acceptor

level varies from 0.31 to 0.49 eV above the valance band (as shown in Figure 6.41)

Our results are in good agreement with available reported literature [44]. Wang et.al.

[44] found the hole trap having energy 0.45 eV above the valance band by generalized

gradient approximation (GGA). They related it to Zn-N complex.

3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1

10-4

10-3

1000/T (K-1

)

Ev+ Et= 0.451 eV

= 3.78 10-17cm2

eT-2

(s-1

K-2

)

128

Impurities introduced during growth process are very hard to eliminate. They

can come from sources, substrate, atmosphere, and walls of growth chamber. Even in

the ultrahigh vacuum atmosphere used in growth. The oxygen used for the production

of oxygen plasma could be the real source of nitrogen. During the deposition of film,

N occupied at oxygen vacancy along with one less electron in the crystal. It is

preferable for N to occupy at O site rather than Zn site because its ionic radius is

comparable to the O. The SIMS measurement (Figure 6.33) shows the presence of

nitrogen which is constant as a function of film depth. This SIMS result supports our

assumption about existence of nitrogen. Zn-N bond is not stable because of excess

amount of its bonding energy and it could be changed with time. This excess amount

of its bonding energy provides the driving force for the change of local bonding.

These changes of local bonding depend upon detailed kinetic factor for example

lattice locations and the activation energy for N displacement [45, 46]. Consequently,

we observed activation energy of hole trap H varying from 0.31 to 0.49 eV above the

valance band. Hence, it indicates that hole trap H consists of more than one

metastable conureuration. Owing to the foresaid properties of Zn-N complex, the

origin of the metastable hole defect H with Zn-N complex is justified.

129

0 100 200 300 400 500 600 700 800

320

340

360

380

400

420

440

460

480

500

Time (hours)

Act

iva

tio

n E

nee

rgy

(m

eV)

Figure 6.40 Metastability behavior of hole trap H with respect to time.

3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.31E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

100

eT

-2(s

-1K

-2)

1000/T(K-1

)

Figure 6.41 The Arrhenius plot of hole level in group C samples with passage of time.

130

Section- IV

The fourth part presents the measurements of ZnO samples (D, E, F, & G) grown on

glass coated with metals (Ni, Al, Au and Ag) by aqueous chemical growth described

in section 5.3 .

6.16 XRD Measurements

The XRD measurements of four samples (D, E, F, & G) grown by aqueous

chemical growth were performed. The source of XRD machine is copper that has

wavelength 1.54 Ao. XRD patterns of four samples (D, E, F, & G) are shown in

Figure 6.42. In this patterns we observed 8 peaks (P1, P2, P3, P4, P5, P6, P7, and P8)

at angles (2θ) 31.81o, 34.47

o, 36.31

o, 38.19

o, 44.71

o, 47.59

o, 56.55

o, and 62.87

o

respectively. Miller indices were calculated by hexagonal formula which has been

explained in Sec 4.4. The 6 peaks (P1, P2, P3, P6, P7, and P8) are due to hexagonal

structure of ZnO. Miller indices of peaks (P1, P2, P3, P6, P7, and P8) are 100, 002,

101, 102, 110 and 103, respectively. The 2θ, intensity and Miller indices of four

samples (D, E, F, & G) are listed in Table 6.5.

131

Figure 6.42 XRD patterns of four samples (D, E, F, & G).

30 40 50 60

0

200

400

600

800

1000

1200

1400

P6 P8

P4

P2

P3P1

Inte

nsi

ty (

cp

s)

2 (degree)

Sample F

30 40 50 60

0

50

100

150

200

P8P6

P7P5

P4

P3

P2

P1

Sample G

2 (degree)

Inte

nsi

ty (

cp

s)

30 40 50 60

0

100

200

300

400

500

600

P8

P7

P6

P3

P2

P1

Sample D

Inte

nsi

ty (

cp

s)

2 (degree)

30 40 50 60

0

100

200

300

400

P8P6

P3

P2

P1

P1

Sample E

Inte

nsi

ty (

cp

s)

2 (degree)

132

Table 6.5 2θ, intensity and Miller indices from XRD data of sample D, E, F and G.

Peak no 2θ (degree)

Intensity

Miller

indices Sample

D (Ni)

Sample E

(Al)

Sample F

(Au)

Sample G

(Ag)

P1 31.81 76 21 45 79 ZnO (100)

P2 34.47 538 400 228 199 ZnO (002)

P3 36.31 190 102 96 108 ZnO (101)

P4 38.19 - - 1274 92 -

P5 44.71 - - - 18 -

P6 47.59 66 41 24 28 ZnO (102)

P7 56.55 17 - - 17 ZnO (110)

P8 62.87 104 59 36 24 ZnO (103)

It is clear from above results that P2 is the highest peak in all samples except

sample F. P4 is the highest peak in sample F but it is unidentified. However P2 (002)

is the highest intense peak in all samples if we neglect the unidentified peak. The

intensity of the XRD peak indicates the atomic positions within the unit cell [13]. The

strong peak P2 (002) indicates the preferable direction of growth of ZnO is c-axis.

This result is in good agreement with available data reported in literature [47]

Peaks (P1, P2, P3, P6 & P8) originated from hexagonal ZnO are observed

in all four samples (D-G) but intensities of these 5 peaks in all samples are

different.The sharp and narrow diffraction peaks point out that the material has good

crystal quality for characterization as shown in Figure 6.41. In our case, thin layer of

metals (Ni, Al, Au & Ag) coated on substrates behaves as nucleation layer for growth

of ZnO nanostructure. Ni seemed to play a significant role for the nucleation in the

ZnO growth because intensity of peak P2 in sample D is maximum i.e 538 cps than

other samples (E, F and G).To the best of our knowledge, we are the first who used Ni

metal as nucleation in the ZnO growth. Reports about other metals are available in

133

literature for example Zhang et al. [48] reported the nucleation of ZnO nanowires

grown by PLD using thin coating of Au on substrates. Yamabi et al. [49] attained

high-quality heterogeneous nucleation by applying layer of Zn(O2CCH3)2 on

substrates such as glass.

6.17 SEM Measurements

Figure 6.43 and 6.44 show SEM images of sample D with different

magnification level of the equipment. The image shown in Figure 6.43 is magnified

by 1630 times. The image shown in Figure 6.44 is magnified by 46580 times. SEM

images of sample E with different magnification level of the equipment are shown in

Figure 6.45 and 6.46. SEM images shown in Figure 6.45 and 6.46 are magnified by

2200 and 16700 times, respectively. All images have been obtained at electron beam

energy 20 KeV. The growth of nanostructures in samples D is more intense than

sample E. Figures 6.43 and 6.45 clearly exhibit the flower like structure which

consists of nanorods as exposed at higher magnification (shown in Figures 6.44 and

6.45). It is supposed that thin metal layer of Ni enhances the growth. This result is

consistent with XRD measurement.

134

Figure 6.43 SEM image of sample D grown by ACG.

Figure 6.44 SEM image of sample D grown by ACG.

135

Figure 6.45 SEM image of sample E grown by ACG.

Figure 6.46 SEM image of sample E grown by ACG.

136

CHAPTER 7

137

7 CONCLUSION AND FUTURE PLAN

The following sections contain conclusion of each group of samples and future plan.

7.1 Group A Samples

The deep level defects in Pd/n-ZnO sample of Zn-face grown by hydrothermal

technique using DLTS have been studied. DLTS performed under various conditions.

The salient features of the research activity are following:

Two electron defects named as E1 (dominant) and E2 located at 0.22 ± 0.02 eV

and 0.49 ± 0.02 eV below the conduction band minimum, respectively are found in

Zn-face (sample A) ZnO. Level E1 shows time delayed transformations as a function

of free carrier concentration ND of the device during isothermal annealing studies.

Based on the available information in the literature together with the vigilant analysis

of the level E1, the level E1 can be linked to a zinc antisite defect and that ZnO is

intrinsically n-type because of Zni-VO complex. Under certain conditions, zinc

interstitial sits on the VO site when the Zni-VO complex is broken. Consequently, the

trap concentration NT increases and the free carrier concentration ND decrease.

Qualitative measurements such as I-V, C-V, and XRD measurements also support our

conclusion.

7.2 Group B Samples

The deep level defects in Pd/n-ZnO samples of different face (Zn-face & O-

face) grown by hydrothermal technique using DLTS has been studied. DLTS

performed under various conditions. The out standing features of the research activity

are following:

138

The effect of background doping concentration induced field on an electron

defects in ZnO Schottky devices has been studied. We investigated two samples A

(Zn-face) & B (O-face) having doping concentration 3.44 × 1017

cm-3 and 3 × 1016

cm-

3, respectively. Two electron trap levels E1 and E2, having activation energies (0.22 &

0.26) eV and (0.49 & 0.49) eV below the conduction band minimum, of samples A

and B, respectively are observed. Level E2 is due to the surface states. Some famous

research groups observed an electron trap having the activation energy in the range of

0.22- 0.32 eV with free carrier concentration in the range of 1017 - 1014 cm-3 in bulk-

ZnO devices. Therefore, decrease in emission rate of level E1 is connected to the

lowering of ND-induced barrier height. After getting these interesting results, we

applied Poole–Frenkel model carrying Coulomb potentialon the emission rate data

(ours + reported) related with E1 and observed the data to be well fitted with this

model. Zn-interstitials in ZnO are residual shallower donors in ZnO according to the

Look et al. [Phys. Rev. Lett. 82 (1999) 2552] theoretical calculations, hence E1 level

has been attributed as a charged impurity that is originated from Zn.

7.3 Group C Samples

We have studied the hole trap in ZnO grown by molecular beam epitaxy using

DLTS. This hole trap shows metastable behavior which can exist in more than one

configurations, each with distinct electric properties. The configuration of hole

changed with storage time. It is hard to remove the impurities during the growth

process. N sits on the O site during the growth process to form the Zn-N complex.

This Zn-N complex is locally instable because of its excess bonding energy and might

have changed its configuration with time. SIMS measurement shows the presence of

nitrogen which support our conclusion about hole trap.

139

7.4 Samples D, E, F, & G

We have studied the effect of thin layers of different metals (Ni, Al, Ag and

Au coated on substrates) on the fabrication of nanorods grown by aqueous chemical

growth. XRD measurements reveal the presence of ZnO nanorods with c-axis

orientation. The peak (002) observed in sample D has more intense than remaining

three samples. The largest intensity of peak originated from 002 plane of sample D is

associated with nucleation due to Ni metal coated on substrate. SEM measurements

also agree with these results that thin layer Ni metal enhances the production of

nanorods.

7.5 Future Plan

With that detailed investigations of defects contained in ZnO grown by

different growth techniques, fabrication of ZnO based devices having high efficiency

and better lifetime is possible. The study of deep level defects in ZnO grown by

different growth techniques and the influence of annealing on defects is the issue for

future work. The radiation study may be helpful for strengthening the proposed

identification of the observed defects.

140

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141

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