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

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 MAJOR: NANOTECHNOLOGY CODE: PILOT SUPERVISOR: Senior lecturer Dr Bui Nguyen Quoc Trinh Ha Noi, 2019 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 i TABLE OF CONTENTS ACKNOWLEDGMENTS i TABLE OF CONTENTS ii LIST OF FIGURES v LIST OF TABLES vii LIST OF ABBREVIATIONS viii ABSTRACT INTRODUCTION .2 CHAPTER LITERATURE REVIEW 1.1 Overview of ZnO material 1.1.1 Crystal structure 1.1.1.1 Wurtzite structure .6 1.1.1.2 Zinc blende structure 1.1.1.3 NaCl structure (Rocksalt) 1.1.2 Energy bandgap structure of ZnO 1.1.3 Properties of Zinc Oxide 1.1.3.1 Electrical property 1.1.3.2 Optical properties 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 ii 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 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 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 iii 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 iv LIST OF FIGURES Page Figure 1.1 Three crystal structures of ZnO [6] 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 Figure 1.5 Schematic representation of a NaCl (Rock salt) Figure 1.6 Energy bandgap structure of ZnO [4] 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 v 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 vi LIST OF TABLES Page Table 1.1 Characteristics of ZnO material at room temperature [6] Table 2.1 The mass of starting materials following Cu doped ZnO with different doping concentrations 17 Table 3.1 The lattice parameter of CZO with various Cu doped concentration 31 Table 3.2 The lattice parameters of CZO with various annealing temperatures 34 Table 3.3 The lattice parameters of CZO with various annealing temperatures 35 Table 3.4 The bandgap energy and transmission of CZO thin films with various Cu 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 vii LIST OF ABBREVIATIONS CZO Copper doped zinc oxide CuO Copper oxide ZnO Zinc oxide MEA Monoethanolamine SEM Scanning electronic microscope XRD X-ray diffraction UV-Vis Ultraviolet – Visible viii Figure 3.21 The bandgap of CZO with various annealing temperature, 2% Tables 3.5 and 3.6 indicate the Eg and T% of CZO thin films with various annealing temperatures for 0.5 and 2% doping concentrations Table 3.5 The bandgap energy and transmission of CZO thin films with various annealing temperatures, and Cu doped concentration of 0.5% CZO 0.5% 400oC 450oC 500oC Eg (eV) 3.18 3.22 3.16 T (%) 79.67 96.50 86.10 Table 3.6 The bandgap energy and transmission of CZO thin films with various annealing temperatures, and Cu doped concentration of 2% CZO 2% 400oC 450oC 500oC Eg (eV) 3.29 3.22 3.27 T (%) 87.50 87.10 40.30 48 Figure 3.22 The transmission spectra with various annealing temperature, 0.5% From the transmission spectrum of CZO thin film of 0.5%, it is found that the CZO thin film has the largest transmittance with annealing temperature of 450oC All thin films are strongly absorbed in the ultraviolet region, and have high transmittance in the visible region In this region, the thin films have a transmittance of about 70% or more Here, with the annealing temperature of 450°C, the transmittance is the best CZO thin films at 400°C with low transmittance can be explained by the fact that the thin film quality is not optimum at this temperature, according to the XRD pattern, and the film coating process may contain some of impurities Lamen substrates such as Calcium and Magnesium may affect on the transparency of thin films [37] 49 Figure 3.23 The transmission spectra with various annealing temperature, 2% 3.3.2 Electrical property 3.3.2.1 The effect of Cu doped concentration Table 3.7 displays the sheet resistance of CZO thin films with various Cu doped concentrations, at the annealing temperature of 500oC It can be extracted from the results below that the sheet resistance of CZO thin films is greatly affected by the Cu doping concentration: the sheet resistance of CZO thin films declines from 169.02 Ω/ sq to 85.46 Ω/ sq when the Cu doping concentration is raised from 0% to 2% Table 3.7 shows that the highest sheet resistance is 169.02 Ω/ sq when the Cu doping concentration is set to 0%, which means lowest value of conductivity The sheet resistance continues to reduce as the Cu concentration is raised to 0.5% and 1% However, at concentration of 1.5%, the sheet resistance rises sharply This shows that the effects of Cu doping concentration on the sheet resistance is still inconsistent [38] 50 Another conclusion can be extracted from Table 3.7 that the Cu doping concentration of 2% yields the highest level of conductivity, i.e., the sheet resistance of 85.46 Ω/sq is the lowest These results is in agreement with the obtained results from XRD patterns and SEM images as shown before Table 3.7 The sheet resistance with various Cu doped concentrations Concentration Sheet resistance Rs (Ω/ sq) 0% 0.5% 1% 1.5% 2% 169.02 85.46 87.12 93.71 89.25 3.3.2.2 The effect of annealing temperature In addition to the Cu doping concentration, the annealing temperature also has a significant impact on the electrical properties of the CZO thin films Table 3.8 demonstrates the changes in the sheet resistance of CZO thin films at annealing temperatures of 400oC, 450oC and 500oC On the one hand, at 0.5% Cu doped concentration, the sheet resistance of CZO thin films reduces from 93.34 Ω/sq to 73.42 Ω/sq as the annealing temperature rises from 400oC to 500oC At the same time, at 2% Cu doped concentration, as the annealing temperature increases from 400oC to 500oC the sheet resistance of CZO thin films gradually declines from 85.46 Ω/sq to 81.07 Ω/sq Hence, it is empirical that the rising of annealing temperature reduces the sheet resistance, thus enhancing conductivity From the calculated results, it is obvious that the lowest sheet resistance of CZO thin films (73.42 Ω/sq), which means highest conductivity, corresponds to 0.5% Cu doped concentration with annealing temperature of 450oC [9, 39] 51 Table 3.8 The sheet resistance with various annealing temperatures Concentration 0.5% 2% 93.34 (Ω/sq) 83.28 (Ω/sq) 450oC 73.42 (Ω/sq) 81.07 (Ω/sq) 500oC 89.25 (Ω/sq) 85.46 (Ω/sq) Annealing temperature 400oC 52 CONCLUSIONS Cu doped ZnO (CZO) thin films were successfully fabricated by by using a solution-processed method, with various Cu doping concentration of 0%, 0.5%, 1%, 1.5% and 2% The XRD results indicated that CZO thin films were polycrystalline with (100), (002) and (101) preferred orientations, and they were determined in a single phase of Wurtzite structure The optimum crystallization is corresponded to the dopant concentration of 0.5%, and the annealing temperature of 500oC Observation of SEM micrographs showed that the grain size is relatively uniform, but some porous spaces existed One obtained that the grain size decreased with the increase of doped concentration, but increased with the increase of annealing temperature Bandgap energy of CZO thin films is in range of 3.13 - 3.24 eV Sheet resistance of CZO thin films is lower than 100 Ω/sq The achievement results bring the fabricated CZO thin film to be a promising candidate for p-type semiconducting layer in transistor, solar 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(2019), “Characterization on Cu doped ZnO thin films prepared by solution processing”, The 2019 Hanoi International Symposium on Advanced Materials and Devices (HISAMD 2019), Hanoi, Vietnam, 10-12th January, 2019 62 ...VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY LE THI HIEN UNDOPED AND DOPED ZNO – BASED THIN FILMS BY A SOLUTION PROCESS: PREPARATION AND CHARACTERIZATION MAJOR: NANOTECHNOLOGY... (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... 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