Undoped and doped zno based thin films by a solution process preparation and characterization

The bandgap energy and transmission of CZO thin films with various annealing temperatures, and Cu doped concentration of 0.5%.. The bandgap energy and transmission of CZO thin films wit

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

LE THI HIEN

UNDOPED AND DOPED ZNO – BASED THIN FILMS BY A

SOLUTION PROCESS: PREPARATION AND CHARACTERIZATION

MASTER’S THESIS

Ha Noi, 2019

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

LE THI HIEN

UNDOPED AND DOPED ZNO – BASED THIN FILMS BY A

SOLUTION PROCESS: PREPARATION AND CHARACTERIZATION

ACKNOWLEDGMENTS

First of all, I would like to send special thanks to my supervisor, Dr Bui Nguyen Quoc Trinh, a Senior Lecturer at University of Engineering and Technology and Vietnam Japan University, Vietnam National University in Hanoi, for supporting a great academic environment, helpful advices and strong motivations, which should be an inspiration for me, now and future He always encourages me in doing experiments, in thinking physical meanings independently, and in writing the thesis

Second, apart from my supervisor in Vietnam, I am grateful to Prof Akihiko Fujiwara at Department of Nanotechnology for Sustainable Energy, Kwansei Gakuin University in Japan, for his unforgettable supports to my internship program Also, I am thankful to MSc Nguyen Quang Hoa at VNU Hanoi University of Science for X-ray diffractormeter measurement and scanning electron microscope observation

Third, I would like to thank all faculty members of Nanotechnology Program, Vietnam Japan University, Vietnam National University for teaching and helping

me within 2-year master course

Last but not least, my profound gratitude would be expressed to my parents, sisters, brother, and friends, because of their unconditional loves when facing difficulties in completion of master degree and whole life

This thesis is supported by the research project in 2019 from Vietnam Japan University (VJU), Research Grant Program of Japan International Cooperation Agency (JICA), and the project No QG.19.02 of Vietnam National University, Hanoi

TABLE OF CONTENTS

ACKNOWLEDGMENTS i

TABLE OF CONTENTS ii

LIST OF FIGURES v

LIST OF TABLES vii

LIST OF ABBREVIATIONS viii

ABSTRACT 1

INTRODUCTION 2

CHAPTER 1 LITERATURE REVIEW 4

1.1 Overview of ZnO material 4

1.1.1 Crystal structure 4

1.1.1.1 Wurtzite structure 6

1.1.1.2 Zinc blende structure 7

1.1.1.3 NaCl structure (Rocksalt) 7

1.1.2 Energy bandgap structure of ZnO 8

1.1.3 Properties of Zinc Oxide 8

1.1.3.1 Electrical property 8

1.1.3.2 Optical properties 9

1.2 Techniques of thin films preparation 10

1.2.1 Vacuum processes 10

1.2.1.1 Sputtering method 10

1.2.1.2 Pulse laser deposition 11

1.2.2 Non-vacuum processes 12

1.2.2.1 Chemical vapor deposition (CVD) 12

1.2.2.2 Chemical bath deposition (CBD) 12

1.2.2.3 Sol-gel 12

1.3 Potential applications 13

1.4 Thesis target 13

CHAPTER 2 EXPERIMENTAL PROCEDURES 15

2.1 Precursor solutions 15

2.1.1 Preparation of precursor solutions 15

2.1.2 Precursor processing 16

2.2 Thin films deposition 18

2.2.1 Tool and equipment 18

2.2.2 Thin films fabrication 18

2.3 Thin films characterization 19

2.3.1 X-ray Diffractometer 19

2.3.2 Four- probe measurement systems 25

2.3.3 UV-Vis Spectroscopy 26

CHAPTER 3 RESULTS AND DISCUSSION 29

3.1 Analysis on structural property 29

3.1.1 Effect of Cu doping concentration 29

3.1.2 Effect of annealing temperature 32

3.2 Analysis on morphological micrographs 35

3.2.1 Effect of Cu doping concentration 35

3.2.2 Effect of annealing temperature 38

3.3 Physical characterization 40

3.3.1 Optical property 40

3.3.1.1 The effect of Cu doped concentration 40

3.3.1.2 Effect of annealing temperature 44

3.3.2 Electrical property 50

3.3.2.1 The effect of Cu doped concentration 50

3.3.2.2 The effect of annealing temperature 51

CONCLUSIONS 53

REFERENCES 54

LIST OF FIGURES

Page

Figure 1.1 Three crystal structures of ZnO [6] 5

Figure 1.2 Wurtzite structure 6

Figure 1.3 Schematic of a Wurtzitic ZnO structure 6

Figure 1.4 Schematic representation of a Zinc blende 7

Figure 1.5 Schematic representation of a NaCl (Rock salt) 7

Figure 1.6 Energy bandgap structure of ZnO [4] 8

Figure 1.7 Sputter deposition 11

Figure 1.8 Pulsed laser deposition 11

Figure 1.9 Sol- gel process 13

Figure 2.1 Zn(CH3COO)2.H2O] 15

Figure 2.2 Cu(CH3COO)2.H2O] 15

Figure 2.3 Ethanol 16

Figure 2.4 Mono Ethanol Amine 16

Figure 2.5 Hotplate 16

Figure 2.6 Analytical balance 16

Figure 2.7 Process of making precursor solution 18

Figure 2.8 Thin films fabrication 19

Figure 2.9 Bragg-Brentano XRD geometry 21

Figure 2.10 Glancing incidence geometry 21

Figure 2.11 Glancing incidence XRD and conventional XRD The sample is a thin film of metal on glass [26] 22

Figure 2.12 X-ray diffractometer (XRD, Bruker, D5005) 22

Figure 2.13 Schemantic representation of the basic SEM components 23

Figure 2.14 Scanning electron microscope (SEM, Nova NANOSEM 450) 25

Figure 2.15 Schematic of four-point probe configuration 26

Figure 2.16 Schematic of a conventional spectrophotometer 28

Figure 3.1 XRD patterns with various Cu doped concentrations: 0%, 0.5%, 1%,

1.5% and 2% 29

Figure 3.2 XRD patterns of 0.5% Cu doping concentration, and temperarure changed: 400, 450, and 500oC 32

Figure 3.3 XRD patterns of 2% Cu doping concentration, and temperarure changed: 400, 450, and 500oC 34

Figure 3.4 SEM graph of CZO thin film with 0% Cu doped concentration 35

Figure 3.5 SEM graph of CZO thin film with 0.5% Cu doped concentration 36

Figure 3.6 SEM graph of CZO thin film with 1% Cu doped concentration 36

Figure 3.7 SEM graph of CZO thin film with 1.5% Cu doped concentration 36

Figure 3.8 SEM graph of CZO thin film with 2% Cu doped concentration 37

Figure 3.9 SEM graph of CZO thin film at 400oC, 0.5% 38

Figure 3.10 SEM graph of CZO thin film at 450oC, 0.5% 38

Figure 3.11 SEM graph of CZO thin film at 500oC, 0.5% 39

Figure 3.12 SEM graph of CZO thin film at 400oC, 2% 39

Figure 3.13 SEM graph of CZO thin film at 450oC, 2% 39

Figure 3.14 SEM graph of CZO thin film at 500oC, 2% 40

Figure 3.15 The absorbance spectra of CZO with various Cu doped concentration: 0%, 0.5%, 1%, 1.5% and 2% 41

Figure 3.16 The bandgap of CZO with various Cu doped concentration 42

Figure 3.17 The transmission spectra with various Cu doped concentration: 0%, 0.5%, 1%, 1.5% and 2% 43

Figure 3.18 The absorbance spectra with various annealing temperatures, 0.5% 45

Figure 3.19 The absorbance spectra with various annealing temperatures, 2% 46

Figure 3.20 The bandgap of CZO with various annealing temperatures, 0.5% 47

Figure 3.21 The bandgap of CZO with various annealing temperature, 2% 48

Figure 3.22 The transmission spectra with various annealing temperature, 0.5% 49

Figure 3.23 The transmission spectra with various annealing temperature, 2% 50

doped concentration at 500oC 44

Table 3.5 The bandgap energy and transmission of CZO thin films with various

annealing temperatures, and Cu doped concentration of 0.5% 48

Table 3.6 The bandgap energy and transmission of CZO thin films with various

annealing temperatures, and Cu doped concentration of 2% 48

Table 3.7 The sheet resistance with various Cu doped concentrations 51 Table 3.8 The sheet resistance with various annealing temperatures 52

LIST OF ABBREVIATIONS

CZO Copper doped zinc oxide

CuO Copper oxide

ZnO Zinc oxide

ABSTRACT

In this study, CZO thin films, whose doping concentrations varied at 0%, 0.5%, 1.0%, 1.5% and 2.0%, were successfully fabricated by using sol-gel method The annealing temperatures were 400oC, 450oC, and 500oC for all concentration conditions XRD results revealed that the preferential orientations of CZO thin films are along with (100), (002) and (101) The best crystallization is corresponded

to dopant concentration of 0.5%, and annealing temperature of 500oC SEM observation indicates that the grain size is relatively uniform, but the surface morphology is porous Also, the grain size is inversely proportional to Copper (Cu) doping concentration, but directly proportional to the annealing temperature Optical analysis points out that the CZO thin films absorb most strongly at wavelengths ranged from 360 to 375 nm; and bandgap energy of CZO thin films was in range of 3.13 - 3.23 eV Further findings of CZO thin films are discussed in detail in the thesis

INTRODUCTION

Recently, semiconductor material Zinc Oxide (ZnO) has attracted many research groups for three main reasons: first, ZnO has large exciton binding energy

of 60 meV; second, it has potential in optoelectronic applications because of its

wide direct bandgap is Eg = 3.3 eV at room temperature; third, ZnO requires a

simple crystal-growth technique, which might lower cost of the mass production In addition, ZnO consists of oxygen and zinc, which are popular metal in earth resources, and lowers further price if commercialized [1]

ZnO has described that it is very resistant to high – energy radiation It is easily imprinted in all acids and alkalis, which provide an occasion for deposition of small-size devices Moreover, the crystal structure and lattice parameters of ZnO is close to those of Gallium Nitrogenium, hence it can be used as a seed for epitaxial growth of high-quality Recently, ZnO has been considered for applications such as transparent thin-film transistors, of which the defensive light exposure is reduced, because ZnO layer is impervious to visible light [4] It is interesting that, by monitoring the concentration of doping material, ZnO is able to change: from

insulating through n-type semiconductor, and to metal, but optical transparency is

still maintained for transparent electrodes in flat-panel displays and light emitting diode Furthermore, ZnO is well known as a favorable contestant for spintronic applications Based on the dominant properties of ZnO as described above, a base

of Zinc Oxide with dopping Copper (CZO) has been fabricated in a thin film and then characterized At present, CZO is attracting a lot of attentions owing to its potential to become p-type semiconductor layer

CZO thin films to become p-type semiconductor, Zinc oxide based material is usually doped with group I or V elements like: Copper, silver, nitrogen and phosphorus However, silver has stable valence but high cost, nitrogen and phosphorus are cheap but have more unstable valences While copper has only two valances and it is low-cost material Moreover, Copper and Zinc atomic sizes are

not very different Therefore, copper has been selected for doping in ZnO background to fabricate p-type semiconductor thin films

In my study, ZnO thin film was fabricated on glass substrates by solution process method to find out the optimum condition for doping concentration and annealing temperature Here, Zn2+ salt ion concentration in precursor solution is 0.5

M, and the molar ratio between Zn2+ salt and monoethanolamine is 1: 1 The structural properties have been investigating Thereby, this research aims to investigate the effects of doping concentration and annealing temperature to find out optimum condition of CZO thin films for high quality to application in the electronic devices

CHAPTER 1 LITERATURE REVIEW

1.1 Overview of ZnO material

Zinc Oxide (ZnO) is a II-VI compound semiconductor [2] The ZnO semiconductor has the bandgap energy of 3.37 eV and is an n-type semiconductor ZnO materials have a lot of properties such as good transmittance, high electron mobility, wide bandgap, and strong luminescence at 300K [3]

ZnO is an important and hopeful material in numerous fields It has notable advantages over Copper Oxide (CuO) such as non-toxicity, low material costs, simple fabricating procedure, chemical stability, high transparency in the visible

and near infrared spectral region, wide bandgap (Eg = 3.37 eV at 300 K), large

exciton binding energy (60 meV) Furthermore, its connected quantum well may have 100% inside quantum efficiency and most of its dopants are gladly obtainable These benefits are of considerable interest for practical use in transparent conductive films, solar cell, surface acoustic wave devices and gas sensing applications [4-6]

1.1.1 Crystal structure

The crystal structure of ZnO is devided into three forms, Wurtzite structure, Rocksalt structure and Zinc Blende structure “Most of the group II–VI binary compound semiconductors crystallize in either cubic zinc blende or hexagonal Wurtzite (Wz) structure where each anion is surrounded by four cations at the corners of a tetrahedron, and vice versa This tetrahedral coordination is typical of

sp3 covalent bonding nature, but these materials also have a substantial ionic character that tends to increase the bandgap beyond the one expected from the covalent bonding ZnO is a II–VI compound semiconductor whose ionicity resides

at the borderline between the covalent and ionic semiconductors” [3, 5]

(a) Rocksalt (b) Zinc blende (c) Wurtzite

Figure 1.1 Three crystal structures of ZnO [6]

The typical crystal structures of ZnO is shown in Fig 1.1, and they are signified the Strukturbericht designations for the three phases

Table 1.1 Characteristics of ZnO material at room temperature [6]

Parameter (T = 300K)

Under ambient surroundings, the Wurtzite symmetry is the most thermodynamically steady and has low crystallization temperature or activation energy, whereas the Zinc blende ZnO structure can be stabilized only by growing

on cubic substrates, and the Rocksalt or Rochelle salt (NaCl) structure is obtained at relatively high pressures Furthermore, the Wurtzite structure has good electrical

and optical properties Therefore, the Wurtzite crystal structure is the most preferred

1.1.1.1 Wurtzite structure

At room temperature, the Wurtzite structure has a hexagonal unit cell with two lattice parameters a and c in the ratio of c/a = 1.633 and belongs to the space group

Figure 1.2 Wurtzite structure

In each basic cell, the Wurtzite structure consists two molecules of ZnO Zn atoms are located at (0,0,0) and (1/3,1/3,1/3) O atoms are positioned at (0,0,u) and (1/3,1/3,1/2+u) Each Zn atom will bind to 4 oxygen atoms located at the 4 vertices

of a tetrahedral [7, 8]

Figure 1.3 Schematic of a Wurtzitic ZnO structure

1.1.1.2 Zinc blende structure

Figure 1.4 Schematic representation of a Zinc blende

The Zinc blende structure is one of the two main structures of ZnO material

In each basic cell of the Zinc, blende structure consists four molecules ZnO Zn atoms are positioned at (1/4,1/4,3/4), (1/4,3/4,3/4), (3/4,1/4,3/4), (3/4,3/4,1/4) O atoms are located at (0,0,0), (0,1/2,1/2), (1/2,0,1/2), (0,0,1/2) [9]

1.1.1.3 NaCl structure (Rocksalt)

Figure 1.5 Schematic representation of a NaCl (Rock salt)

Rocksalt is a durable imitation of ZnO crystals in high pressure conditions (10 Gpa) The lattice of rocksalt is similar to that of NaCl In each basic cell, the rocksalt structure consists of four molecules of ZnO [10]

1.1.2 Energy bandgap structure of ZnO

Figure 1.6 Energy bandgap structure of ZnO [4]

The ZnO material is a semiconductor with a direct wide bandgap, Eg = 3.37

eV, making it transparent for a large wavelength range in the solar spectrum [11]

1.1.3 Properties of Zinc Oxide

1.1.3.1 Electrical property

The electrical properties of ZnO thin films are influenced by two main factors: the annealing temperature and the Cu doping concentration The ZnO crystal lattice is composed of cation Zn2+ and anion O2- At room temperature, the zinc and oxygen atoms in the lattice are usually closely linked This explains why ZnO has poor conductivity at room temperature However, as the temperature gradually increases to 400oC, the electrons affected by the temperature will become more flexible and will be separated from the network nodes, forming the conductive electrons and permitting the ZnO material to be conductive

The electrical properties of ZnO are also affected by the doping concentration According to Ohm's law, the conductivity of the material is calculated using the formula:

σ = n.μ.q (1.1)

In this formula :

n is the charge carrier density

μ is the mobile particle loading flexibility

q is the electric charge

ZnO thin films are prepared by Sol-gel method While not yet annealed, the films have oxygen holes, which causes Zn to have two extra electrons These two electrons participate in the electrical conductivity procedure of ZnO materials However, the material in this case has no homogeneous microstructure; therefore,

the scattering process occurs strongly, which reduces the mobility μ, consequently

leading to reduce conductivity of ZnO materials After performing the annealing process in the air or oxygen-rich environment, due to the diffusion of oxygen into

the holes, the charge carrier density n decreases However, the homogeneity of the

surface structure favours the mobility of the carriers, which enhances the conductivity of ZnO material If we choose the appropriate annealing temperature,

we can obtain ZnO thin films having not only good electrical conductivity but also throughout property [11, 12]

1.1.3.2 Optical properties

Optical properties of a material depend on its bandgap energy structure and its crystal lattice The results from the spectral analysis of ZnO nano thin films pointed out many drawbacks related to the emission of donor – acceptor pairs The bandgap energy structure is extended from 1.9 eV to 2.8 eV The luminescence of ZnO thin film in the blue light is not good because the existence of various impurities [13] The optical properties of ZnO materials are expressed through the interaction of

electromagnetic waves with the surface of the material When we put ZnO materials under a light beam with a high level of energy, electrons will become more flexible and move from the valence band to the conduction band After existing in a steady state in a period of time, electrons have a tendency to shift to lower energy levels

This process releases an energy E = hf in the form of photons ZnO materials are

transparent with the transmittance of approximately 80% By studying their spectral transmittance and absorbance spectrum, it is concluded that ZnO materials absorb most strongly at wavelengths from 320 nm to 330 nm [14]

1.2 Techniques of thin films preparation

1.2.1 Vacuum processes

1.2.1.1 Sputtering method

RF-Magnetron Sputter is most effective in thin film fabrication technique Gas stream (usually argon or argon O2+, argon N2+) is injected the vacuum chamber produces plasma that forms Ar + ions These ions are directed to the target (metal to create a thin film) which is negative These ions moving at high velocity and bombard the target and dislodge atoms from the target of target out The atoms

"evaporating" and move on to substrate (glass or silicon wafer), accumulating on substrate and forming thin films when the number of atoms is large enough In the process of sputtering, secondary ions increases the rate of film forming or reduce the pressure on the target This technique is magnetron sputtering It uses a magnetic field to target This magnetic field will keep the secondary electron knives

on the magnetic field lines around the target The electrons oscillate near surface target will contribute ionized argon atoms much more [13-19] This speeds up the process of creating thin films The schematic of the sputtering system illustrating in Fig 1.7

Figure 1.7 Sputter deposition

1.2.1.2 Pulse laser deposition

“Pulse laser deposition offers various great benefits over other physical deposition methods such as fast deposition time and compatibility with oxygen and

other inert gases” [18]

Figure 1.8 Pulsed laser deposition

Pulse lase deposition method has been attracted in the past few years This method has successfully covered complex compounds [15, 16] The Pulse laser deposition technique was first used to fabricate the superconducting membrane

YBa2Cu3O7 Many materials are difficult to fabricate in normal methods but this method has been successfully fabricated [17].

1.2.2 Non-vacuum processes

1.2.2.1 Chemical vapor deposition (CVD)

In the Chemical vapor deposition (CVD) method, vapor phase generated enough reachable by chemical methods Thin film coating is achieved by the process of depositing atoms, molecules or ions through chemical reactions Chemical vapor deposition method is used to manufacture thin-film semiconductors such as Si, AIIBVI, AIIIB, transparent conductive thin films like SnO2, In2O3: Sn (ITO), dielectric thin films such as SiO2, Si3N4, BN, Al2O3 and metal thin films

1.2.2.2 Chemical bath deposition (CBD)

Chemical bath deposition (CBD) is one among method useful for the preparation of compound semiconductors from aqueous solutions This technique is extensively used to deposit buffer layers in thin film photovoltaic cells The major advantage of CBD is that it requires in its simplest form only solution containers and substrate mounting devices Chemical bath deposition yields stable, adherent, uniform and hard films with good reproducibility by a relatively simple process

[20, 21]

In recent years, more progress has been made in the development and commercialization of this method Nowadays it is one of the most pyromising manufacturing techniques in nanotechnology Spin coating is the most sol-gel technique used fo preparation of thin film because of its simplicity, low cost, easy to control chemical components, and large-fabrication scale [21-28]

Figure 1.9 Sol- gel process

1.3 Potential applications

ZnO nanoparticles have many application in solar cell, transitor, photocatalytic, photoluminescence, gas sensor, curved LED display and other application Regarding ZnO nanoparticle application in solar cells, Suliman et al reported the synthesis of ZnO nanoparticles with average diameter of 30 nm to create ZnO films on transparent conductive glass (TCO), ZnO nanoparticles were dissolved in ethanol and then applied on the TCO surface by the doctor blade technique

fabricated two materials with both benefits, that is, they have good conductivity and transparent as well Some are known as conductive polymers and conductive metal ZnO is one of special oxides among them In order to increase the electrical properties of the ZnO thin films, we apply the phase impurities However, in order not to lose the transparency of the material, the impurities must be mixed in a suitable ratio P-doped ZnO thin films are formed, when we perform acceptor elements like N, Li, Ag, As, Cu Cu doped ZnO thin films are being studied by many research groups, and applied for a lot of specific applications in our life Moreover, ZnO has other great properties such as lighting in the ultraviolet and visible light areas at room temperature, which can be used in frontiers such as: gas sensors, biological sensor, solar cell Nowadays, there are many different methods

to prepare ZnO thin films such as sputtering, electronic beam epitaxy, chemical vapor phase deposition, electrochemical deposition, sol-gel In this study, I used sol-gel method to make Cu doped ZnO thin films because of simple and low-cost prcess

My research contents include:

 Design and prepare ZnO and CuO precursor solution from Zinc (II) acetate, copper (II) acetate monohydrate, monoethanolamine (MEA) and pure ethanol

 Optimize and fabricate Cu doped ZnO thin film by spin coating method

 Investigate and evaluate structural property, surface morphology as well as electrical and optical properties of fabricated ZnO thin films

CHAPTER 2 EXPERIMENTAL PROCEDURES

2.1 Precursor solutions

2.1.1 Preparation of precursor solutions

Instruments and chemical sources:

- Copper (II) acetate monohydrate [Cu(CH3COO)2.H2O]

- Zinc (II) acetate monohydrate [Zn(CH3COO)2.H2O]

- Mono-ethanolamine (HOCH2CH2NH2, monoethanol amin)

- Ethanol (C2H5OH, 99.7% purity)

- Hotplate, analytical balance, and magnet stirrer

The figures of instruments and chemical are shown as below

Figure 2.1 Zn(CH3COO)2.H2O] Figure 2.2 Cu(CH3COO)2.H2O]

Figure 2.3 Ethanol Figure 2.4 Mono Ethanol Amine

Figure 2.5 Hotplate Figure 2.6 Analytical balance

2.1.2 Precursor processing

The formation of precursors was based on chemical raw sources, reacting at room temperature In detail, precursors for spin coating were prepared by dissolving [Zn(CH3COO)2.H2O] and [Cu(CH3COO)2.H2O] into ethanol solvent, with a molarity of 0.5 M, using a 4 ml volumetric bottle Molar ratio beetwen MEA: salt

is 1 : 1 The formula for the relationship between molar and molar concentratios is:

CM =

From the above formula, we calculated the number of molar salts to be used, then we adjusted the mass of two salts in order to correspond the various doping concentrations

m = n M (2.1) With [Zn(CH3COO)2.H2O] = 219.49 u (abbreviated from: unified atomic mass unit) and [Cu(CH3COO)2.H2O] = 199.95 u From the molar number of the salt, we can derive the molar number of MEA, which is consistent with molar ratio between MEA and salt, for example, 1 : 1 And, we calculate the mass of MEA needed, by using the volume-density relationship:

Particularly, CM is 0.5 M and molar ratio between MEA and salt is 1 : 1,

corresponding to the volume of MEA = 60 µl

Table 2.1 The mass of starting materials following Cu doped ZnO with different

Figure 2.7 Process of making precursor solution

2.2 Thin films deposition

2.2.1 Tool and equipment

- Precusor solution

- The 22 × 22 mm2 ITO/glass (lamen) substrates

- Aceton and NaOH

- Ultrasonic vibrators

- Spin coating machine

- Annealing furnace

2.2.2 Thin films fabrication

The process of making thin films consists of 4 steps as follows

Step 1: Cleaning substrates The substrates were, in turn, cleaned in acetone and water by using Untrasonic Cleaner (UC) By this rinsing process, all organic and inorganic contaminants located on the substrates surface can be removed Details will be described as follows Firstly, the substrates were cleaned in acetone for 5 minutes and keeping under distilled water to remove organic contaminants on the substrate‟s surface Next, the substrates were cleaned by ultrasonic into sodium

hydroxide (NaOH) 1M for 5 minutes to clean metal contaminants, and to change the surface to hydrophilic, because the solution poses polar property and the surface of substrate is hydrophobic Consequently, thin films can be coated well

Step 2: After substrate treatment, precursor were dropped on the substrate with rotation speed of 1500 rpm for 30 seconds

Step 3: Samples were dried at 90oC for 5 minutes, then repeated the step 2 and the step 3 several times to achieve the desired thickness

Step 4: Finally, all samples were annealed for 30 minutes, in air, at a wide range of temperature for structural characterization, by using a furnace

Figure 2.8 Thin films fabrication

2.3 Thin films characterization

After finishing the thin films depostion, I proceed to measure the properties of CZO thin films

2.3.1 X-ray Diffractometer

X-ray measurement can be used to study solid state materials like crystalline, polycrystalline or amorphous materials, and can be used to estimate the surface roughness and interface width (arising from roughness and interdiffusion) nondestructively XRD measurement is the most well- known family of techniques

single-to investigate structural properties of a material [22] The benefits of the XRD

method are non-destructive method, no requirement on special sample preparation

[23], ability to measurements in atmosphere and under a special atmosphere such as

a high-temperature or high-pressure condition, and ability to obtaining information

on the average structure in a relatively large area (mm to cm) Besides, irradiation

damages in organic materials are low, possible to control the analysis depth by the

incident angle onto the surface, and possible to characterize buried interface

structure

XRD signal is a result of an elastic scattering of monochromatic X-ray by core

electrons of atoms in a sample Since X-ray can pass through material and atoms in

crystal structure arrange periodically and cyclically as diffraction grating, it

produces the well-known XRD patterns The diffraction of X-rays by crystals is

described by Bragg‟s law for constructive interference [22, 23]:

(2.3)

Where d is the spacing among Bragg planes, θ is the incident angle or Bragg

angle, n is the diffraction order with any integer (1, 2 ), and λ is wavelength of the

beam

“The X-ray beam of a conventional Bragg-Brentano XRD is incident at an

angle of half the diffraction angle 2θ (Fig 2.9), typically between 15° and 50° The

depth of penetration into the sample is given by the absorption length times the sine

of the incidence angle Because the diffraction peaks from a typical sample material

almost lie in the region of 20° to 100° (2θ), then incidence angles are equivalent in

the range of (30 ± 20)° [24-27] Here, the diffracted beam is focused back onto a

narrow receiving slit; the beam emerging from the thin-film geometry is broad and

close to parallel.”

Figure 2.9 Bragg-Brentano XRD geometry

“In the case of thin film, most of the X-ray beam passes through the film and

is scattered by the substrate It is important to pick an X-ray diffraction geometry that allows working at tiny incident angles, increasing the path length of the X-rays

in the film and reducing the number of X-rays that penetrate through to the substrate XRD at very shallow angles of incidence is called glancing incidence XRD, shown in Fig 2.10 The collimator is made of a set of closely spaced metal plates with an acceptance angle of approximately 0.25° To define the incidence angle exactly, the incident beam is collimated by a narrow divergence slit During a measurement, the incidence angle is fixed while the detector is scanned over the

range of diffraction angles

Figure 2.10 Glancing incidence geometry

Figure 2.11 describes the typical gain in sensitivity achieved using glancing incidence XRD In this case, collimation and monochromation of the incident X-ray

beam by reflection from a parabolic X-ray mirror decreases the background,

increasing the sensitivity of the experiment.”

Figure 2.11 Glancing incidence XRD and conventional XRD The sample is a thin

film of metal on glass [26]

In my thesis, crystal structure of ZnO thin films deposition on glass substrates was investigated by X-ray diffraction (XRD), Department of Physics, VNU University of Science

Figure 2.12 X-ray diffractometer (XRD, Bruker, D5005)

Scanning electron microscopy (SEM) is a device used to study the surface morphological structure of materials From SEM images, it is possible to determine the surface morphology as well as the crystal size of the material Fig 2.13 shows

diagram of a scanning electron microscope device

Figure 2.13 Schemantic representation of the basic SEM components

The operation principle of the device is when shotgun emits an electron beam The acceleration and passing through the lens system from an electron beam with high energy, then the electron beam is scanned over the sample surface by the electrostatic scanning coils The electron beam converges on the sample and creates

an interactive area with depth of about 1 µm The occurrence of interactions appears the radiation emitted and the signal receiver detector will record the radiation, analyze and give results on the computer The radiation emitted is divided into two categories:

- The electronic beam has low energy ( less than 50 eV) It recorded by photomultiplier tube It is usually the electrons destroy the sample surface, therefore SEM system create a two-dimensional image of the material

- The electrons reflect from the sample by elastic scattering, the electrons interact with the opposite pattern should enable the electron beam has large energy Signal intensity of the backscattered electron depends on the molecular mass of matter Elements with heavy mass molecules make the electrons scatter strongly and vice versa The electrons create the image including the white area corresponding to the heavy elements and the dark areas correspond to the light element This is also the basis of the method of qualitative analysis of elements present in the sample

The resolution of the scanning electron microscope is suitable for most atomic dimensions Compare with optical microscopes, both devices can be measured surface morphological The images of scanning electron microscopes are sharper It‟s quality is better The resolution of SEM does not depend on the optical limit, lens, mirror or image resolution of the detector Instead, SEM image resolution depends on the size of electronic particles are ejected Moreover, the resolution is limited by the interaction of the electron beam on the specimen volume Scanning electron microscope method is commonly used in research field surface structure of the material Figure 2.14 illustrates the surface morphology measurement system

Figure 2.14 Scanning electron microscope (SEM, Nova NANOSEM 450)

2.3.2 Four- probe measurement systems

Four- probe measurement systems is the method of determining the sheet resistance of the material At constant temperature, the sheet resistance of the

material has a surface area S is determined by the following formula:

R = ρl / S (2.4)

Where ρ is resistivity, l is lengthiness, S is surface area

Four- probe measurement equipment is the main component four point made

of vofram tip with equidistant to contact the sample surface The distance from the

nearest probe to the outer edge of the sample must be greater than the 3l distance In

order to ensure good contact, the probe tip is tightened by four springs Principle of the method is the current 4-point probe is supplied between two external probes and the voltage between the two probes are placed inside.The electrical current is very small in the difference between the voltage at the first contact is not great The

formula for calculating the sample resistivity thin film thickness smaller than 3l:

(2.6)

Here: Io, I: Incident light intensity, transmitted light intensity

c: The concentration of the solution (mol/l)