Chacterization on solution rocessed p type cuo thin films for electronic devices application

LIST OF ABBREVIATIONS ATO Antimony doped tin oxide LSDA Local spin density approximation PEDOT Poly3,4-ethylenedioxythiophene PLD Pulsed laser deposition SEM Scanning electrode microscop

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY -

NGUYEN VAN DUNG

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY -

NGUYEN VAN DUNG

CHARACTERIZATION ON PROCESSED P-TYPE CuO THIN FILMS

SOLUTION-FOR ELECTRONIC DEVICES

ACKNOWLEDGEMENT

First of all, I would like to give special thanks to my supervisor, Lecturer Dr Bui Nguyen Quoc Trinh, for supporting a greatly researching environment, and for giving helpful instructions, guidance, advices, and motivations, which inspire me a lot for my current and future researches

Secondly, I would like to thank Prof Akihiko Fujiwara at Kwansei Gakuin University, Japan, for his enthusiastic supports during internship time, and his valuable discussion Also, I am very thankful to MSc Nguyen Quang Hoa who is a researcher at VNU Hanoi University of Science, for his encouragement and valuable discussion on data analysis

Thirdly, I would like to send my sincere thanks to teachers, experts, and staffs working at Nanotechnology program of Vietnam Japan University, those who have accompanied and supported to me Without such promotion, I could not complete

my master thesis as it should be

Last but not least, I would express my thanks to friends and family members who always encourage me through past 2-year master course, to overcome any difficulty

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2012.81 Hanoi, 2018

Master‟s student

Nguyen Van Dung

TABLE OF CONTENTS

ACKNOWLEDGEMENT i

TABLE OF CONTENTS ii

LIST OF FIGURES v

LIST OF TABLES vii

LIST OF ABBREVIATIONS viii

INTRODUCTION 1

CHAPTER 1 LITERATURE REVIEW 3

1.1 CuO crystal structure 3

1.2 Properties of cupric oxide 4

1.2.1 Electrical property 4

1.2.2 Optical property 6

1.3 Techniques of thin films preparation 7

1.3.1 Vacuum processes 7

Sputtering method 7

1.3.1.1 1.3.1.2 Pulse laser deposition 8

1.3.2 Non-vacuum processes 10

Metal-organic decomposition (MOD) 10

1.3.2.1 Atmospheric pressure plasma enhanced chemical vapor deposition 1.3.2.2 (AP- PECVD) 10

Sol-gel 10

1.3.2.3 1.4 Potential applications 13

1.4.1 Thin film transistors 13

1.4.2 Solar cells 14

1.5 Thesis target 15

CHAPTER 2 EXPERIMENTAL PROCEDURES 16

2.1 Precursor solutions 16

2.1.1 Starting materials and instrument tools 16

2.1.2 Precursor processing 17

2.2 Thin films deposition 18

2.2.1 Preparation 18

2.2.2 Substrates treatment 18

2.2.3 Spin coating process 19

2.3 Thin films characterization 20

2.3.1 X-Ray Diffractometer 21

2.3.2 Scanning Electron Microscope 23

2.3.3 Four-probe measurement systems 25

2.3.4 UV-Vis Spectroscopy 26

2.3.5 Transistor-operation measurement systems 28

CHAPTER 3 RESULTS AND DISCUSSION 33

3.1 The formation of CuO thin films 33

3.2 Analysis on structural property 34

3.2.1 Effect of copper salt: MEA ratio 34

3.2.2 Effect of precursor concentration 36

3.2.3 Effect of annealing temperature 39

3.3 Analysis on morphological micrographs 41

3.3.1 Effect of Salt: MEA ratio 41

3.3.2 Effect of precursor concentration 43

3.3.3 Effect of annealing temperature 44

3.4 Physical characterization 45

3.4.1 Electrical property 45

The effect of Cu2+: MEA ratio 45

3.4.1.1 The effect of Cu2+ ions concentration 46

3.4.1.2 The effect of annealing temperature 47

3.4.1.3 3.4.2 Optical property 48

The effect of Cu2+: MEA ratio 48

3.4.2.1 The effect of Cu2+ ions concentration 50

3.4.2.2 The effect of annealing temperature 52

3.4.2.3 3.5 Operation of thin film transistor 54

3.5.1 Transfer characteristic 54

3.5.2 Output characteristic 56

CONCLUSION 58

REFERENCES 59

LIST OF PUBLICATIONS 65

LIST OF FIGURES

Page

Figure 1.1 Monoclinic structure of CuO 3

Figure 1.2 The dependence of formation energies of native point defects in CuO on Fermi level EF [18] 5

Figure 1.3 A sputtering system [16] 8

Figure 1.4 Pulsed laser deposition system [16] 9

Figure 1.5 Sol-gel process 11

Figure 1.6 Spin coating process (http://www.ossila.com/pages/spin-coating) 13

Figure 2.1 Instrument tools: a) Analytical balance b) Magnetic stirrer 16

Figure 2.2 Ultrasonic cleaner 19

Figure 2.3 The entitle of spin coating process 20

Figure 2.4 The phenomenon of X-ray diffraction 21

Figure 2.5 X-ray diffractometer (XRD, Bruker, D5005) – Center of Materials Science, VNU University of Science 23

Figure 2.6 Diagram of scanning electron microscope [38] 24

Figure 2.7 Scanning electron microscope (SEM, Nova NANOSEM 450) 25

Figure 2.8 Electrical resistivity measurement by a four-probe method [16] 26

Figure 2.9 The (IDS – VGS) characteristic curve 30

Figure 2.10 The characteristic curve 31

Figure 3.1 XRD pattern with various molar ratios of MEA 35

Figure 3.2 XRD patterns with various solution concentration 38

Figure 3.3 XRD patterns with different annealing temperature 40

Figure 3.4 SEM graphs of films with different molar ratio of MEA a) 1, b) 2, c) 2.5 and d) 3 42

Figure 3.5 SEM graphs of films with different Cu2+ ions concentration 44

Figure 3.6 SEM graphs of films with various annealing temperature 45

Figure 3.7 The absorbance spectra of CuO with different MEA molar ratio 49

Figure 3.8 The bandgap of CuO with various Cu2+: MEA ratio 50

Figure 3.9 The absorbance spectra of CuO with different solution concentrations.

51

Figure 3.10 The bandgap of CuO with different Cu2+ ions concentrations 52

Figure 3.11 The absorbance spectra of CuO with different annealing temperature.

LIST OF TABLES

Page

Table 1.1 The lattice parameters and physical properties of CuO [16] 4

Table 2.1 The mass of starting materials following ratio of copper salt and MEA. 17

Table 2.2 The mass of starting materials following Cu2+ ions concentration in the precursor solution 18

Table 3.1 The lattice parameter of thin films with various Cu2+: MEA molar ratios 36

Table 3.2 The lattice parameter of thin films with Cu2+ ions concentration 39

Table 3.3 The lattice parameters of thin films with different annealing temperature. 41

Table 3.4 The sheet resistance with various MEA molar ratio 46

Table 3.5 The sheet resistance with various Cu2+ ions concentration 47

Table 3.6 The sheet resistance with various annealing temperature 48

Table 3.7 Electrical parameters of CuO TFTs with different channel lengths 56

LIST OF ABBREVIATIONS

ATO Antimony doped tin oxide

LSDA Local spin density approximation

PEDOT Poly(3,4-ethylenedioxythiophene)

PLD Pulsed laser deposition

SEM Scanning electrode microscope

SILAR Successive ionic layer adsorption and reaction TFTs Thin film transistors

UV-Vis Ultraviolet-Visible

INTRODUCTION

Recently, metal oxide based thin film transistors (TFTs) have attracted significant attention for application in display industry, for examples, flexible display, and flat panel display because of their excellent performances In covalent semiconductor consisting of sp3 orbital, the bond angle significantly affects to charge transport in the bottom of the conduction band, leading to low mobility and unsteady of silicon-based TFTs However, the overlap between neighbor metal ns orbital in metal oxide semiconductor could happen in an amorphous phase because the carrier transport paths include both metal ns orbitals, which lead to high charge carrier mobility in amorphous structures [1] Therefore, metal oxide based TFTs are considered as one of the potential candidates to alter the traditional amorphous Si (a-Si) and organic-based TFTs for application in display, for instances, active matrix organic light-emitting diodes (AMOLEDs) and an active matrix (AM) liquid crystal displays Hitherto, most of studies on oxide semiconductor based TFTs have concentrated on n-channel layer such as ZnO, In2O3, ZnInO, InGaO, SnO2 and InGaZnO [2] In contrast, only a few studies have been focused on p-type oxide TFTs owing to the lack of intrinsically p-type semiconductors and it is difficult to develop a high-quality p-type thin film [3, 4], which is the challenge for the fabrication of low power complementary logic circuits with metal-oxide based TFTs [5] Therefore, the development of TFTs using p-type metal oxide semiconductor as conducting channel is important to fabricate a complementary integrated circuits (ICs) using oxide semiconductor [2, 5]

To solve these issues, the typical p-type semiconductors such as CuO, Cu2O, NiO, and SnO was developed for the application in the p-channel TFTs Among various investigated p-type semiconductors, cupric oxide (CuO) is considered as one of potential candidates for thin film transistors (TFTs) using p-type metal oxide

As a p-type channel material in TFTs, CuO crystallizes in monoclinic structure with

a narrow bandgap energy in range of 1.2 - 2.1 eV, the excellent electrical properties, low fabrication cost, good thermal stability and non-toxic materials [6-9]

Moreover, CuO shows p-type conductivity with the hole mobility up to 100

cm2V−1s−1 [10] In addition, CuO is applicable in other fields, for examples, chemical catalyst, solar cell, energy stored batteries, gas sensor, high efficiency thermal conducting material, and magnetic recording media Recently, a few research groups have reported the operation of p-type semiconductor TFTs using CuO channel layer CuO TFTs fabricated on p-type silicon substrates showed the typical p-type operation, the ON/OFF ratio was approximate of 1.1×104 and field-effect mobility of 0.4 cm2/V⋅s in the ref [11] K Sanal has successfully fabricated TFTs using p-type CuO channel layer on SiO2/Si substrate by a sputtering technique The on/off ratio and field-eff ect mobility of p-channel CuO TFTs were

103 and 1.43 × 10−2 cm2 V−1s−1, in turn In addition, CuO devices also showed a maximum subthreshold voltage swing of 4.8 in the ref [10]

Hitherto, the CuO thin films were fabricated by means of various techniques, for examples, sputtering [12], pulsed laser deposition, chemical vapor deposition [13], thermal evaporation, spin coating [14] Among these methods, the spin coating method is simplest, which offers great advantages for the fabrication of thin film because it consumed low-power and material, has a short time of the procedure and

it is easy to fabricate on flexible substrates at atmospheric pressure

In my research, CuO thin film was developed on glass substrates by spin coating method for application in electronic devices Here, the concentration of precursor solution and the molar ratio of monoethanolamine (MEA) to copper (II) acetate salt were modified to find out the optimum condition for high-quality thin film growth The structural properties (crystal structure, morphology properties, electrical and optical properties) of the thin film, as well as the operation of thin film transistor, have been examined

CHAPTER 1 LITERATURE REVIEW

1.1 CuO crystal structure

Cupric oxide (CuO) belongs to a monoclinic crystal structure with space symmetrical group of Tetramolecular cell has four CuO molecules per unit cell where the copper atoms take positions of ( ); ( ); ( ) and ( ), respectively and the oxygen ions are located at positions ( ); ( ); ( ̅ ) and ( ) with ( ) as shown in Figure 1.1 In a CuO cell unit, each copper (Cu) atom is coordinated to four neighbor oxygen (O) atoms in a square planar coordination Meanwhile, each O atom is linked to four Cu atoms in a disorder tetrahedral type of coordination, which means the CuO structure consists of both ionic and covalent bonds In addition, the four oxygen atoms take positions of the corners of parallelogram will combine with another two oxygen atoms to create

a strongly distorted octahedron [15, 16]

The lattice parameters of cupric oxide (CuO) are as following: and The physical properties constant of CuO were also indicated in Table 1.1

Figure 1.1 Monoclinic structure of CuO

Table 1.1 The lattice parameters and physical properties of CuO [16]

Space group

Lattice parameters

Cell volume 81.08 Cell content 4 [CuO] Distances Cu-O

Cu-Cu

O-O

Molecular mass

Density

Relative permittivity 12 Melting point

Boiling point

1.2 Properties of cupric oxide

1.2.1 Electrical property

CuO crystal lattice is composed by and ions, which have the electron configuration of and , respectively There is not the existence of defects in pure crystal lattice However, in fact, the crystal lattice is not perfect and always has the existence of defects and impurities in the crystal The defects in crystal lattice can be included: point defects (vacancy, interstitial defects), substitute defects, linear defects (dislocations), planar defects (stacking fault and grain boundary) These defects have significantly affected on the electrical properties of semiconductor material [17]

Figure 1.2 The dependence of formation energies of native point defects in CuO on

Fermi level EF [18]

In bulk CuO material, native point defects consist of three different types: vacancy ((Cu vacancy) or (O vacancy)], two antisite defects ( ) and isolated interstitials ( ) as shown in Figure 1.2 LSDA+U calculation was indicated that and is the stable charge state for each defect when the Fermi level is close to the valence band maximum (VBM) Meanwhile, the stable charge state for each defect is and when Fermi level is close to the conduction-band minimum (CBM) However, the negative-charged state ( ) always has the lowest formation energy compared with other charge states of the defects, which leads Cu vacancies

in CuO crystal are the most predominant defect As a result, acceptors in undoped CuO are dominant over donors and amphoteric, which leads the p-type conduction characteristic of CuO material Hence, CuO is p-type semiconductor owing to the existence of negatively charged copper vacancies ( ) in the structure, which introduces acceptor level at the top of the valence band [17, 18]

1.2.2 Optical property

The optical properties are an essential factor of thin film for application in optoelectronic devices or absorber layer in solar cells Bulk cupric oxide (CuO) is a direct narrow bandgap material with an optical bandgap energy of 1.2 eV However, the optical bandgap energy of CuO thin film varies in the range of 1.3 – 1.9 eV depending on fabrication conditions [16] The grain size, the temperature of substrate, the thickness of thin film, doping concentration, lattice strain, structural parameters, and disorder are some factors which can be led to the changing of bandgap energy of CuO thin film Akaltun et al [19] fabricated CuO thin film on microscopic glass substrates by SILAR method with optical bandgap energy of 2.03

- 1.79 eV corresponding to the film thickness of 120 – 310 nm The decrease of bandgap values was also reported by Gopalakrishnan et al for CuO thin films fabricated on glass substrates by spray method They have observed that the optical bandgap values raises from 1.8 to 1.2 eV with raising of temperature of substrate from 250 to 350 °C [20]

Along with bandgap energy, transmission, and absorption, the refractive index

is also one of the important optical parameters of thin film The bulk CuO has a refractive index of 2.63 Depending on the different fabrication method and conditions, the refractive index of CuO thin film varies in the range of 1.5 to 3.5 [21-23] The refractive index of CuO thin films deposited by electrodeposition method was increased from 2 and 3.5 with increasing of bath temperature from 30

to 90 °C in the ref [21] Y Akaltun have shown that the refractive index of CuO thin film fabricated by SILAR method reduces from 2.80 to 2.70 when the film thickness decrease from 250 to 190 nm in the ref [19]

1.3 Techniques of thin films preparation

high-DC sputtering, radio frequency (RF) sputtering and magnetron sputtering [16, 22] The sputtering has many advantages such as easy to fabricate high-melting point material, compatibility with reactive gases as oxygen, possible to use for ultrahigh vacuum, the composition of the fabricated thin film is close to the composition of the parent target material

The sputtering technique used for preparation of CuO thin film A A Ghamdi et al deposited CuO thin films on glass substrate by means of radio frequency (RF) magnetron sputtering technique at room temperature The obtained CuO thin film is polycrystalline and oriented along various planes, in which, the plane (111) is predominant over another plane The bandgap energy of CuO thin film decrease from 2.2 eV to 1.73 eV with increasing of thickness of thin film in the refs [12] Shinho Cho [24] also used a reactive RF magnetron sputtering method to fabricate CuO thin film on glass substrate The obtained CuO thin film has a preferred crystal orientation of (002), in addition, the (111), (202) and (-202) orientations were also present The crystallite size of CuO thin film reduces from 42

Al-to 34 nm when the temperature reduces from 100 °C Al-to 25 °C

Figure 1.3 A sputtering system [16]

1.3.1.2 Pulse laser deposition

Pulsed laser deposition (PLD) is another method of a physical vapor deposition process developed in 1986s because of the demand of high temperature superconductors [25] PLD is an improved thermal process using high-power pulsed laser beams as energy source to ablate the target material for the thin film‟s fabrication inside a vacuum chamber as indicated in Figure 1.4 The mechanism of thin film deposition by PLD technique is described as follows: the source target material is stroked by the high-power pulsed laser beam and the plume would be produced Atoms are then escaped from the source target in a highly directive plume, directed toward the substrate and deposited onto the surface of substrate to form the thin film [25, 26] It is possible to obtain high-quality thin film via controlling various parameters such as ablation energy, base vacuum level, ambient gas pressure, the distance of the target to substrate and the substrate‟s temperature PLD offers various great benefits over other physical deposition methods such as fast deposition time and compatibility with oxygen and other inert gases

Figure 1.4 Pulsed laser deposition system [16]

PLD technique also used to fabricate CuO thin film Prakash Chand and worker [27] studied Li-doped CuO thin films deposited on (100) oriented Si substrates by PLD The obtained Li-doped CuO thin film is polycrystalline and has

co-a preferred crystco-al orientco-ation of (111) co-and (311) The PESEM results show thco-at the grain size reduces from 97 to 47 nm with decreasing of doping concentration from 0.09 to 0% By means of pulsed laser deposition, Faiz et al [28] also fabricated Zn doped CuO thin film on Si (100) substrate with the film thickness of The Zn-doped CuO thin film has polycrystalline structure with (002) and (200) orientations The bandgap energy of thin film reduces from 2.5 to 2 eV with reducing of doping concentration from 3 to 0 atomic wt%

by means of any techniques to form a wet film To remove any remaining solvents during the deposition process, the wet films are heated Finally, the metal-organic compounds are decomposed and condensed to form a film

Atmospheric pressure plasma enhanced chemical vapor deposition (AP-

1.3.2.2

PECVD)

Atmospheric pressure plasma enhanced chemical vapor deposition PECVD) is an improved chemical vapor deposition process which uses plasma to enhance the reaction of organic and inorganic chemical monomers for the preparation of the thin film at normal pressure

(AP-AP-PECVD offers many great potentials for the deposition of thin films over large surface areas such as simple and inexpensive method, high productivity, high cross-linking density and high rate of deposition

of discrete particles or polymers networks The sol gel process consists of five key steps [29, 30]:

 The hydrolysis and condensation reactions of metal alkoxide precursors would be formed a sol

 The formation process of gel: In this process, the metal-oxo–metal or metal–hydroxy–metal bonds would be formed via polycondensation of the hydrolyzed precursor compound

 Syneresis or the aging process: The remaining liquid (solvent) phase continues condensed within the gel network, often accompanied by shrinking and densification

 The gel drying process: In this process, the dense „xerogel‟ would be formed through the collapse of the porous network or aerogel

 The calcination at high temperature to remove the surface M-OH groups

Figure 1.5 Sol-gel process

(https://www.gelest.com/applications/sol-gel-applications.)

Spin coating, dip coating, and spray coating are the three most sol-gel techniques used for the preparation of thin film However, the method which is

suited with my research is spin coating method Spin coating is one of the most popular methods for the preparation of thin film on the surface of substrates Spin coating offers many great benefits for deposition of thin film Firstly, spin coating is

a simplest method, low power, and a fast operating system Another advantage of spin coating is easy to control the film thickness by controlling some parameters such as spin speed or spin time and easy to fabricate very uniform films In addition, this method doesn‟t require the strict conditions in pressure, power, and manufacture engineering compared with vacuum methods Spin coating has fairly basic principle based on the action of centrifugal force Firstly, the substrate is placed on a flat surface, which rotates around a perpendicular axis The precursor solution was then spread on the surface of substrate Under the action of centrifugal force, the solution will spread regularly on the surface of substrate and form a thin film

The process of spin coating consists of three main steps as described in Figure 1.6

Step 1 (deposition): The precursor solution is dropped onto the surface of substrates Herein, the amount of the using solution is usually greater than that of a necessary solution

Step 2 (spin-up, spin-off): The substrate was accelerated up to the essential spin speed During the spinning process, the precursor solution majority is flung off the side Thereafter, the substrate was rotated with constant velocity and the solution will spread regularly on the surface of substrate under the action of centrifugal force The thickess of film is determined by the viscosity of the solution Step 3 (Evaporation): The solvent is evaporated and condensed to form a thin film The volatilization of the solvent determined the thickness of films

Figure 1.6 Spin coating process (http://www.ossila.com/pages/spin-coating)

1.4 Potential applications

CuO is one of the potential candidates for application in various fields owing

to its excellent properties such as p-type conducting characteristic, low cost, toxic, good thermal stability and electrical properties [6-8] These properties enable CuO thin film having many applications in the catalyst, solar cell, metal oxide based TFTs, sensor, high-temperature superconductors and energy stored batteries

non-1.4.1 Thin film transistors

For a few recent years, cupric oxide has attracted significant attention as a type channel layer in TFTs due to its good properties A few research group also reported the operation of p-type CuO based TFTs The success in the preparation of CuO based p-type TFTs is opening the applicability of copper oxide as active layer

p-in TFTs as well as the application of p-type oxide semiconductor materials p-in integrated logic circuits (ICs)

K.C Sanal and co-worker fabricated the transparent p-channel CuO thin film transistors on ATO/ITO/glass substrate by a sputtering technique in the refs [31] The p-channel CuO TFTs exhibited p-type operation mode with ON/OFF ratio of

104 and the field-effect mobility of Also, S K C prepared TFTs using p-type CuO channel layer on SiO2/Si substrate by sputtering technique The CuO device has the field-eff ect mobility of 1.43 × 10−2 cm2 V−1s−1 and ON/OFF ratio of 103 In addition, the maximum sub threshold voltage swing of device were estimated about 4.8 in the ref [10]

1.4.2 Solar cells

Cupric oxide (CuO) has been intensively studied as one of the most potential candidates for solar cells application owing to excellent optical and electrical properties As absorber layer material, CuO offers various benefits such as p-type semiconductor with direct bandgap energy in the range of 1.2 - 2.1 eV, high optical absorption coefficient in the visible range, low-cost, low thermal emission and non-toxic material [6-8] Furthermore, the bandgap of CuO is close to the bandgap of GaAs [33] and Si [32], which is consistent with the solar spectrum Therefore, it is possible to obtain the solar conversion efficiency up to 33% [34] Many search groups have been focused on fabrication of solar cells using CuO material as absorber layer by various techniques such as electrodeposition, radio frequency (RF sputtering) [35], plasma evaporation and spin coating [36]

S.M Panah et al [35] fabricated p-CuO/n-Si (100) heterojunction solar cells

by radio frequency magnetron sputtering method with short-circuit current ( ) of open circuit voltage ( ) of 380 mV fill factor (FF) of 0.28 and power conversion efficiency ( ) of S.M Panah showed that the FF and of the solar cells have been significant enhanced with adding of nitrogen (N) dopant at the top layer of CuO The N-doped CuO/n-Si heterojunction solar cells have of , of FF of and of Hiroki Kidowaki et al have prepared ITO/PEDOT: PSS/CuO/C60/Al solar cells by spin coating technique The fabricated CuO thin film based solar cells has of , of

, FF of 0.25, and open-circuit voltage ( ) of 0.04 V in the refs [36]

1.5 Thesis target

Among researches and developments to seek novel materials for our life, cupric oxide (CuO) has been intensively investigated as one of potential candidates, owing to its preeminent benefits such as lower cost, wide availability, non-toxicity, high stability, high absorption coefficient in visible range In addition, CuO is applicable in various fields, for examples, chemical catalyst, solar cell, thin film transistor, energy stored batteries and gas sensor Therefore, this research aims to develop CuO thin film material with high conductivity and charge mobility, stable optical properties by solution method The succeed in the preparation of CuO thin film by solution method, it will open a new direction for the manufacture of low-cost and non-toxic materials In addition, the applications of CuO thin film material play a significant role for the use of clean and green energy instead of non-renewable energy resources

 Fabricating and analyzing on the operation of thin film transistor using CuO

as p-type channel layer material

CHAPTER 2 EXPERIMENTAL PROCEDURES

 Ethanol (C2H5OH, 99.7% purity) as a solvent

 Monoethanolamine (OHCH2CH2NH2, MEA, 99% purity) as stabilizer agent

b) Instruments:

 Analytical balance

 Magnetic stirrer

 Other equipment: measuring cylinder, stirring bar, beaker, and pipet

Figure 2.1 Instrument tools: a) Analytical balance b) Magnetic stirrer

2.1.2 Precursor processing

CuO precursor solution was prepared according to the following procedure: copper (II) acetate monohydrate was firstly dissolved in a pure ethanol and stirred at room temperature After 15 minutes of stirring at room temperature, some light blue Cu(OH)2 precipitations particles were created and MEA was then added the resulting solution under constant stirring at room temperature for 15 minutes Here,

in order to find an optimum condition for the high-quality thin film growth, the molar ratio of MEA to copper (II) acetate salt was controlled to be 1.5, 2, 2.5 and 3, respectively Besides, the concentration of precursor solution was also varied in the range of 0.15 to 0.3 M Thereafter, the precursor solution was stirred at 75oC for 60 minutes to form molecules network, which links copper ions together After an hour, the dark blue solution was observed without any suspension of particles Finally, all of precursor solutions were stored in bottom and preserved at refrigerator for 24 hours before spin coating on the surface of substrates In our experiment, when Cu2+ions concentration was greater than 0.3 M, the precipitate phenomenon occurred in precursor solution and when it is smaller than 0.15 M, the surface of CuO thin film is discontinuous

Table 2.1 The mass of starting materials following ratio of copper salt and MEA

Cu2+: MEA

molar ratio

Cu2+ ions concentration MEA ( ) Ethanol ( ) 1: 1.5

Table 2.2 The mass of starting materials following Cu2+ ions concentration in the

precursor solution

Cu2+: MEA

molar ratio

Cu2+ ions concentration ( ) MEA ( ) Ethanol ( )

 CuO precursor solution prepared as above

 HF 2% solution prepared from HF 40% solution and distilled water ( )

 Acetone, ethanol, distilled water, spin coater, ultrasonic cleaner and thermal annealing system, pipet, etc

2.2.2 Substrates treatment

The cleaning of substrate affects significantly the properties of thin film, because the contamination surface can cause phenomenon of open resistor or localized high resistance Therefore, before coating the CuO precursor solution prepared as above, commercial glass substrates were treated in acetone, ethanol, distilled water and HF 2% according to following procedure: the glass substrates were firstly cleaned in acetone and ethanol combined by ultrasonic cleaner (as shown in Figure 2.2) for 5 minutes, respectively, to remove organic and metal dusts on substrate‟s surface Thereafter, the substrates were washed with

distilled water to remove any trace of acetone and ethanol After washing in distilled water, the substrates were dried with dryer before treatment by HF 2% Finally, glass substrates were dipped into 5 ml of HF acid (2%) for 30 seconds, again washed with ethanol and distilled water to passivate the Si dangling bonds with hydrogen Here, hydrofluoric acid is a solution of hydrogen fluoride (HF) in water HF acid has been known as solution which is able to dissolve the glass because it reacts with the main component of the glass (SiO2) The dissolving process is described as below:

( ) ( ) ( ) ( ) (2.1)

Then, Silicon tetrafluoride will react with residual-HF to create hexafluorosilicic acid

( ) ( ) ( ) (2.2)

Figure 2.2 Ultrasonic cleaner

2.2.3 Spin coating process

After cleaning the surface of substrate, CuO thin films were developed on clean glass substrate according to procedure:

Step 1: The cleaned glass substrate was placed on sample holder of the spin coater CuO solution was spread on the surface of substrate with the spin speed of

500 rpm in 10 seconds and 1500 rpm for 40 seconds

Step 2: The sample was pre-heated at 90oC for 3 minutes in an air and cooled for 3 minutes for each layer To obtain the desired thickness of CuO thin film, the procedures of spin coating and drying were repeated four times

Step 3: After spin coating of final layer, the samples were annealed in air at different temperature of 400, 450, 500 and 550oC for 30 minutes to change from the gel to the crystallization states

Figure 2.3 The entitle of spin coating process

2.3 Thin films characterization

After annealing process, the CuO thin films were analyzed on structural, surface morphology, optical and electrical properties The crystal structure of CuO thin films were identified by using X-ray diffractometer (XRD, Bruker, D5005) with Cu-K radiation The morphological property of CuO thin films were

min

Repeated 4 times

examined via scanning electron microscopy (SEM, Nova NANOSEM 450) The four-probe measurement system have been used to measure electrical resistivity of the thin films Optical properties of CuO thin films were analyzed by UV-Vis measurement system (UV 2450-PC, Shimadzu) The transfer and output characterization of CuO based TFTs was measured by transistor-operation measurement system

2.3.1 X-Ray Diffractometer

X-ray diffraction (XRD) method is a non-destructive analytical tool to study the crystal structure of a solid sample X-rays are considered as electromagnetic radiations with wavelengths regions around Considering the ordered arrangement of atoms in the crystal planes and the X-ray diffraction planes as shown in Figure 2.4 When monochrome X-ray beam is incident to crystal surface

at an angle of some portion of the beam will be reflected by the electrons of the atoms

Figure 2.4 The phenomenon of X-ray diffraction

The constructive interference of scattering beam with crystal plane obeys Bragg‟s law as indicated below:

where

 denotes X-ray wavelength,

 is the distance between two adjacent crystal planes which is characterized for each crystal sample,

 θ is the incident angle between the X-ray radiation and the crystal plane, and

 n is the integer number

The above formula is Bragg formula described the phenomenon of X-ray diffraction on crystal planes This formula expresses the relationship between the diffraction angle (θ), the distance between the atomic planes (d) and the incident X-ray wavelength (λ) Bragg‟s law also suggests that the diffraction occurs only when λ< 2d

From the Bragg‟s law, we can identify of a set planes based on X-ray wavelength of and diffraction angle of corresponding to the obtained diffraction line By compared with the values of the standard sample, we can identify the crystalline phases, preferred orientations and crystal lattice parameters of material For a monoclinic crystal structure, the lattice parameters can be calculated by using formula below [11]:

For this study, the crystalline quality of CuO thin films fabricated on glass substrates was studied by X-ray diffraction (XRD), with Cu-kα radiation of λ = 1.5418 Å at Center of Materials Science, Department of Physics, VNU University

of Science

Figure 2.5 X-ray diffractometer (XRD, Bruker, D5005) – Center of Materials

Science, VNU University of Science

2.3.2 Scanning Electron Microscope

Scanning electron microscope (SEM) is a useful equipment which utilizes a high energy electron beam concentrated on the sample surface to obtain high-resolution images of the material surface SEM is one of the most versatile technique for examination and investigation on morphological properties of samples Similar to other types of electron microscope, SEM also consists of many different components which operate in tandem

Figure 2.6 Diagram of scanning electron microscope [38]

The operation principle of SEM: the electron beam is first created by the electron gun and accelerated in an electric field Then, the electron beam is converged into a narrow electron beam by magnetic lenses system and swept on the sample‟s surface thanks to electrostatic scan coils When high-energy electron beam interacts with surface atoms of sample, various signals used to form an image are generated Finally, the detectors collect these signals and display them on the output devices The surface morphology of specimen is displayed on a computer screen via analysis on these signals These signals involve backscattered electrons (BSE), diffracted backscattered electrons, secondary electrons (SE), X-rays, visible light, and heat In which, SE and BSE are used to create an image of sample‟s surface morphology Diffracted backscattered electrons are used to analyze the crystalline structure and orientations of minerals, while X-rays are used to analyze the chemical composition [38] The secondary electrons (SE) are electrons emitted from the surface of sample with few nanometers depth The kinetic energy of SE is lower than that of backscattered electrons The backscattered electrons (BSE) are electrons emitted from specimen by elastic or inelastic scattering

In my thesis, the morphology surface of CuO thin film was examined by scanning electron microscope (SEM, Nova NANOSEM 450) at Center of Material Science, University of Science (HUS), VNU

Figure 2.7 Scanning electron microscope (SEM, Nova NANOSEM 450)

2.3.3 Four-probe measurement systems

Four-probe measurement method is a useful method to measure the resistivity

of small size samples as shown in Figure 2.8 In this method, the current across between the two outer metal probes and the resistivity of samples can be determined via the measurement of the voltage between the inner probes When four probes are placed on sample, the electrical resistivity is defined by [11, 39]:

Where U is the voltage between two inner probes, is the distance between probes, and I is the current flowing through sample from two outer probes, then [39]:

If the distance between two adjacent probes is three times larger than thin film‟s thickness, the electrical resistivity is determined via the measurement of surface resistance, :

Herein, d denotes the thickness of sample

The conductivity of samples is determined via electrical resistivity:

Figure 2.8 Electrical resistivity measurement by a four-probe method [16]

2.3.4 UV-Vis Spectroscopy

Optical properties of the thin film are an important factor for their application

in various optical and optoelectrical devices Optical absorbance and transmission

of the thin film were analyzed by UV-Vis spectroscopy From the spectra of transmission and absorption, we can calculate the optical specific parameters of a

thin film such as absorption coefficient ( ), band gap energy ( ) and extinction coefficient (k)

Principle: when the sample are illuminated by a beam of monochromatic light with initial light intensity Some of incident light will be absorbed by material sample corresponding to light intensity a part of incident light will reflect through sample with the reflected intensity of and the rest will transmit with the intensity of According to energy conservation, the intensity of incident is the sum of and and expressed by following equation:

(2.10) The absorbance (A) and transmittance (T) is expressed by: