Solution processed semiconducting and magnetic ni doped cuo thin films preparation and characterization

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY LA THI NGOC MAI SOLUTION-PROCESSED SEMICONDUCTING AND MAGNETIC Ni-DOPED CuO THIN FILMS: PREPARATION AND CHARACTERIZATION MASTER’S THESIS VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY LA THI NGOC MAI SOLUTION-PROCESSED SEMICONDUCTING AND MAGNETIC Ni-DOPED CuO THIN FILMS: PREPARATION AND CHARACTERIZATION MAJOR: NANOTECHNOLOGY CODE: 8440140.11 QTD RESEARCH SUPERVISORS: Assoc Prof Dr BUI NGUYEN QUOC TRINH Prof Dr Sc NGUYEN HOANG LUONG Hanoi, 2021 Acknowledgement First of all, I would like to sincerely thanks my two supervisors Assoc Prof Dr Bui Nguyen Quoc Trinh and Prof Dr Sc Nguyen Hoang Luong for their unending support, encouragement, and inspiration throughout my master course Under their guidance, I have had extensive knowledge and insight into materials science and nanotechnology, including modern thin-film development, which helped me to gain a wholesome idea of the field I would always cherish the time I spent under their tutelage I would like to especially thank Assoc Prof Dr Dang Van Thanh, who guided me when I carried out experiments during my internship I learned a lot from him in the field of electrochemistry His support and insightful comments aided me in completing this research Besides, I would like to thank Dr Nguyen Quang Hoa, University of Science, Vietnam National University, Hanoi, for allowing me to perform data analysis and measurements in this study Moreover, I would like to give thanks to all the lecturers and researchers in Nanotechnology Program, Vietnam Japan University, who have imparted useful knowledge and supported me to accomplish this thesis I would express my thanks to Vietnam Japan University for providing me with the facilities I am required to implement my project Last but not least, I especially wish to thank family members and friends who always encourage and support me through the past 2-year master's course, to overcome any difficulty This work is fully supported by the project with the code number of VJU.JICA.21.03, from Vietnam Japan University, under Research Grant Program of Japan International Cooperation Agency Hanoi, 2021 Master’s student La Thi Ngoc Mai TABLE OF CONTENTS LIST OF TABLES i LIST OF FIGURES ii LIST OF ABBREVIATIONS iv INTRODUCTION CHAPTER 1: OVERVIEW 1.1 Dilute Magnetic Semiconductors (DMS) .3 1.2 Thin film semiconductors .6 1.3 Cupric oxide thin films 1.4 Overview of deposition techniques 1.4.1 Physical deposition techniques 10 1.4.2 Chemistry deposition techniques 11 1.5 Spin-coating techniques 16 1.6 Motivation and the objectives of the studies 18 CHAPTER 2: FILM DEPOSITION AND CHARACTERIZATION 19 2.1 Synthesis of precursors 19 2.1.1 Raw material 19 2.1.2 Precursor preparation .19 2.2 Thin film deposition .20 2.2.1 Substrate preparation .20 2.2.2 Deposition of Ni-doped CuO thin films 21 2.3 Film characterization 22 2.3.1 X-ray diffraction (XRD) 22 2.3.2 Scanning electron microscopy (SEM) 25 2.3.3 UV-Vis spectroscopy .26 2.3.4 Four-probe measurement system 27 2.3.5 Electrochemical Impedance Spectroscopy (EIS) 28 2.3.6 Vibrating Sample Magnetometer (VSM) 30 CHAPTER 3: RESULTS AND DISCUSSION 33 3.1 Reaction mechanism of precursor solution 33 3.2 Analysis of Ni-doped CuO thin films on glass substrates 36 3.2.1 Crystal-structure properties 36 3.2.2 Surface-morphology properties 38 3.2.3 Optical properties 40 3.2.4 Electrical properties .41 3.2.5 Magnetic properties .42 3.3 Analysis of Ni-doped CuO thin films on the ITO substrates .44 3.3.1 Crystal-structure properties 44 3.3.2 Surface-morphology properties 45 3.3.3 Optical properties 46 3.3.4 Electrical properties .48 CONCLUSION .51 REFERENCES 52 LIST OF TABLES Table 1.1 High-temperature oxide-based DMS (adapted from ref [15]) Table 1.2 Crystallographic parameters (from ref [32]) Table 1.3 Thin film deposition methods (adapted from ref [47]) .9 Table 2.1 List of chemical compounds 19 Table 2.2 Mass of chemicals used to manufacture precursors with difference Ni doping concentrations 20 Table 2.3 Example of Nyquist graph and the usual equivalent circuit of a metal oxide electrode [66] 30 Table 3.1 Measured electrical characteristics of Ni-doped CuO thin films with 0, 0.2, 0.6, 1, 2, 3, and wt.% of Ni doping on the glass substrates 41 i LIST OF FIGURES Figure 1.1 Schematic showing a magnetic semiconductor (A), a non-magnetic semiconductor (B), and a diluted magnetic semiconductor (C) [4] Figure 1.2 Schematic diagram of sputtering [47] 10 Figure 1.3 Sol-gel process [54] 12 Figure 1.4 Schematic of a chemical bath deposition [56] 14 Figure 1.5 The SILAR deposition process [58] 15 Figure 1.6 The stages of thin film formation using spin-coating process [60] 17 Figure 2.1 Precursor solution preparation process 20 Figure 2.2 Spin-coating system (a), annealing furnace (b) 22 Figure 2.3 Electromagnetic radiation spectrum 23 Figure 2.4 Diffraction of X-rays by a crystal 24 Figure 2.5 Schematic diagram of a Scanning Electron Microscope [62] (a), SEM machine in this study (b) 25 Figure 2.6 Schematic of four-point probe configuration 27 Figure 2.7 Model of an electrochemical electrode system and its equivalent electrical circuit [65] 29 Figure 2.8 VSM system 31 Figure 2.9 Hysteresis loop (M-H curve) of the magnetic materials [67] .32 Figure 3.1 The change of concentration, solvent, additive, and Ni2+: MEA ratio to the solubility of nickel salt .33 Figure 3.2 CuO precursor solution (a), and Ni-doped CuO precursor solution in the range – wt.% Ni doping (b) 34 Figure 3.3 Schematic drawing of the chemical equilibria involved in the Ni and Cu ions sol-gel process 35 Figure 3.4 XRD patterns of the un-doped and Ni-doped CuO thin films on glass substrates 36 Figure 3.5 The XRD patterns in the range 30° - 40° of the films on the glass substrates 37 Figure 3.6 Surface SEM images of Ni-doped CuO thin films with various Ni ratios on glass substrates 39 Figure 3.7 The optical absorbance graph of Ni-doped CuO films on glass substrates 40 Figure 3.8 The optical bandgap versus Ni doping concentrations of Ni-doped CuO films on glass substrates 41 Figure 3.9 M-H curves of the Ni-doped CuO thin films on glass substrates 43 Figure 3.10 M-H curves for NiO powder and Ni-CuO film 43 Figure 3.11 XRD patterns (a), the enlarged diffraction pattern in the range 35° to 40° (b) of all films on the ITO substrates 45 ii Figure 3.12 SEM images of CuO thin film and Ni-doped CuO thin films with 1, 2, 3, wt.% deposited on the ITO substrates 46 Figure 3.13 Optical absorbance spectra of all films on the ITO substrates 47 Figure 3.14 Variation of bandgap energy for un-doped and Ni-doped CuO thin films on the ITO substrates 48 Figure 3.15 Nyquist plot (a), the equivalent circuit model (b) of the Ni-doped CuO thin films on the ITO substrates .49 Figure 3.16 Variation in the resistive part with frequency of the undoped and Ni- doped CuO thin films on the ITO substrates .49 iii LIST OF ABBREVIATIONS CBD : Chemical bath deposition CVD : Chemical vapor deposition DMS : Dilute magnetic semiconductor FE-SEM : Field emission scanning electron spectroscopy MEA : Monoethanolamine RTFM : Room temperature ferromagnetism SEM : Scanning electron spectroscopy SILAR : Successive ionic layer adsorption and reaction SQUID : Superconducting quantum interference device UV-VIS : Ultravilolet-Visible VSM : Vibrating sample magnetometer XRD : X-ray diffraction iv INTRODUCTION Conventionally, electronic devices use an electron state, which is an electrical charge for information processing One method for increasing information storage capacity and signal processing speed in electronic devices is to reduce the size of electronic components such as transistors or capacitors Both coding and decoding processes will face extreme difficulty as the size of the transistor cell continues to be shrunk due to the quantum effect The spintronic device is a candidate technology for addressing the major issues found in conventional electronic devices The spintronic device combines charge-based semiconductor property with magnetic property based on spin effect to carry and store information in a single device Dilute Magnetic Semiconductor (DMS) is one of the most interesting materials for the development of spintronic devices containing the functional features mentioned above The advantages of spin-based DMS application are to improve integration density and higher data processing speed, higher efficiency, low power consumption, and better stability Therefore, researchers are always seeking DMS materials with magnetic behavior at room temperature, so that they can be manufactured in the electronics field The p-type semiconductor cupric oxide (CuO) has antiferromagnetic characteristics In addition, CuO has a good light absorption coefficient with low thermal emittance The characteristic properties of CuO thin films such as photoelectric properties, crystal structure can be controlled by doping, annealing temperature, crystallization atmosphere, and so on So far, several studies have focused on the magnetic behaviors of CuO in combination with different magnetic ions However, there are still a lot of remaining rooms to search for novel materials for the electronic device application In this thesis, CuO thin films doped with transition metal ions Ni were fabricated by the solution-processed method Using the spin-coating process, thin films with various Ni doping ratios were coated on glass and ITO substrates The structural, morphological, optoelectrical and magnetic characteristics of the thin films are all studied The achievement results pointed out that the Ni-doped CuO films obtained can be considered Figure 3.11 XRD patterns (a), the enlarged diffraction pattern in the range 35° to 40° (b) of all films on the ITO substrates 3.3.2 Surface-morphology properties Figure 3.12 shows the SEM images of the CuO-based thin films with various Ni doping ratios of 0, 1, 2, 3, and wt.% Flat film surfaces of the CuO film were revealed without voids or cracks It may be observed that the texture of the Ni-doped CuO films changed significantly with nickel nano-crystallites in CuO The film surface was covered with nanoparticles, and the presence of nanoparticles increases as Ni concentrations increase For all films, the effect of Ni-doped concentration on the appearance of surface morphology was demonstrated 45 Figure 3.12 SEM images of CuO thin film and Ni-doped CuO thin films with 1, 2, 3, wt.% deposited on the ITO substrates 3.3.3 Optical properties The optical properties of the CuO film and Ni-doped CuO films were studied in the range 350 nm to 800 nm in figure 3.13 The CuO-based films started an absorption at the wavelength 800 nm In addition, the absorption was highest for photo energy in 46 the near-ultraviolet region It was found that the absorption of the films increased with an increase in Ni concentration This could be because of an increase in the density of hole states with increasing Ni doping levels Figure 3.13 Optical absorbance spectra of all films on the ITO substrates As seen from figure 3.14, the Eg values for all of films decreased from 2.69 eV with no doping to 2.66, 2.58, 2.47, 2.38 eV with 1, 2, 3, wt.% Ni doping, respectively After Ni doping, the bandgap of CuO decreases, indicating that Ni ions can be substituted or interspersed with Cu ions in the CuO lattice Baturay et al [48] reported that a bandgap decrease from 2.03 eV to 1.96 eV with the increasing the Ni concentration from to wt.% in CuO nanostructure The decrease in optical energy band is due to the presence of impurity ions entering the structure of CuO, which increases the donor density Several factors affect the band energy on CuO nanostructures, such as grain size, impurities or lattice defects [80] It clearly indicates that as the Ni ratio grows, the band energy of the films varies slightly, which should aid in adjusting the bandgap of CuO films by Ni ions 47 Figure 3.14 Variation of bandgap energy for un-doped and Ni-doped CuO thin films on the ITO substrates 3.3.4 Electrical properties The electrical properties of Ni-doped CuO thin films on ITO substrates were investigated using electrochemical impedance spectroscopy (EIS) The electrical properties of the material are represented by the appearance of semicircular arcs in the Nyquist plot The complex impedance plot of the resistivity Z’ versus the capacitance Z″ for 0, 1, 2, 3, and wt.% Ni-doped CuO thin films are shown in Figure 3.15a In this situation, Z′ represents the complex impedance's resistive component, while Z′′ contains its capacitive component The plot of the impedance spectrum shows a semicircular arc, showing a contribution to resistance (R) and capacitance (C) From the impedance data, the electrical circuit model was fitted, including RS as Ohmic resistance (solution, wires); R1 is charge transfer resistance, Q1 is the capacity of double electric layer, ZW is Warburg diffusion In general, the impedance data from different Ni ratios had the same circuit as shown Figure 3.15b This model has different resistance values of RS 48 Figure 3.15 Nyquist plot (a), the equivalent circuit model (b) of the Ni-doped CuO thin films on the ITO substrates Figure 3.16 Variation in the resistive part with frequency of the undoped and Nidoped CuO thin films on the ITO substrates The variation in the resistive component Z’ as a function of frequency is observed in Figure 3.16 For all films, it has been demonstrated that Z' decreases with an increase in frequency This is owing to a frequency-dependent increase in conductivity caused by the hopping phenomenon [81] Because of the efficiency of resistive grain boundaries, 49 the low-frequency region is generally associated with high resistivity Furthermore, because of the change in barrier height with doping, Z' changes with the amount of doping variation 50 CONCLUSION In summary, Ni-doped CuO thin films with various Ni doping concentrations have been successfully deposited on glass and ITO substrates via a solution process In this work, the 0.25 M concentrated solutions of copper (II) acetate monohydrate and nickel acetate tetra hydrate were used as starting materials mixed in ethanol solvent The effect of Ni doping on the structural, morphological, optoelectrical and magnetic characteristics of the CuO-based thin films was evaluated by utilizing appropriate techniques The main findings in this research can be highlighted as follows The XRD results of films on the glass and ITO substrates indicated that CuO thin films were polycrystalline with (002) and (111) preferred orientations As the Ni concentration is more than wt.%, the Ni-doped CuO thin films form a composite, which could lead to some interesting applications for this composite material The surface morphology of the films observed from the scanning electron microscope pointed out obviously the presence of linked-structure nanoparticles The optical bandgap of these thin films deposited on the two substrates decreased with the increasing Ni concentrations Ni ions improved the electrical conductivity of the CuO film deposited on the glass substrates To further evaluate the effect of Ni doping on the electrical properties of CuO film, Electrochemical Impedance Spectroscopy (EIS) was utilized to determine the electrical properties of the thin films deposited on the ITO substrate The EIS method has provided interesting information about the doping efficiency of dispersed oxide semiconductor materials Utilizing a vibrating sample magnetometer, the magnetic characteristics of all films were studied, resulting in diamagnetic behavior This could be attributed to the film's relatively thin thickness in comparison to the substrate, as well as the substrate's significant diamagnetic contribution In future works, the magnetic characteristics of the films fabricated will be measured by a more accuracy technique like Physical Property 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