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

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY ` NGUYEN VAN DUNG CHACTERIZATION ON SOLUTIONPROCESSED P-TYPE CuO THIN FILMS FOR ELECTRONIC DEVICES APPLICATION MASTER'S THESIS Hanoi, 2018 VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY NGUYEN VAN DUNG CHARACTERIZATION ON SOLUTIONPROCESSED P-TYPE CuO THIN FILMS FOR ELECTRONIC DEVICES APPLICATION MAJOR: NANOTECHNOLOGY SUPERVISOR: Lecturer Dr Bui Nguyen Quoc Trinh Hanoi, 2018 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 i TABLE OF CONTENTS ACKNOWLEDGEMENT i TABLE OF CONTENTS ii LIST OF FIGURES v LIST OF TABLES vii LIST OF ABBREVIATIONS viii INTRODUCTION .1 CHAPTER LITERATURE REVIEW 1.1 CuO crystal structure 1.2 Properties of cupric oxide 1.2.1 Electrical property 1.2.2 Optical property 1.3 Techniques of thin films preparation 1.3.1 Vacuum processes 1.3.1.1 Sputtering method 1.3.1.2 Pulse laser deposition 1.3.2 Non-vacuum processes .10 1.3.2.1 Metal-organic decomposition (MOD) 10 1.3.2.2 Atmospheric pressure plasma enhanced chemical vapor deposition (AP- PECVD) 10 1.3.2.3 Sol-gel 10 1.4 Potential applications 13 1.4.1 Thin film transistors 13 1.4.2 Solar cells 14 1.5 Thesis target 15 CHAPTER EXPERIMENTAL PROCEDURES 16 2.1 Precursor solutions 16 2.1.1 Starting materials and instrument tools 16 2.1.2 Precursor processing .17 ii 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 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 3.4.1.1 The effect of Cu2+: MEA ratio 45 3.4.1.2 The effect of Cu2+ ions concentration 46 3.4.1.3 The effect of annealing temperature 47 3.4.2 Optical property 48 3.4.2.1 The effect of Cu2+: MEA ratio 48 3.4.2.2 The effect of Cu2+ ions concentration 50 3.4.2.3 The effect of annealing temperature 52 3.5 Operation of thin film transistor .54 iii 3.5.1 Transfer characteristic 54 3.5.2 Output characteristic .56 CONCLUSION 58 REFERENCES 59 LIST OF PUBLICATIONS 65 iv LIST OF FIGURES Page Figure 1.1 Monoclinic structure of CuO Figure 1.2 The dependence of formation energies of native point defects in CuO on Fermi level EF [18] Figure 1.3 A sputtering system [16] .8 Figure 1.4 Pulsed laser deposition system [16] 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) 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 v 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 .53 Figure 3.12 The bandgap of CuO annealed at the different annealing temperature .54 Figure 3.13 The transfer characteristic of CuO TFTs with various channel length of 50, 100 and 150 .55 Figure 3.14 The output characteristic of CuO TFTs with various channel length: a) 50 b) 100 c) 150 d) 200 57 vi LIST OF TABLES Page Table 1.1 The lattice parameters and physical properties of CuO [16] 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 vii LIST OF ABBREVIATIONS ATO Antimony doped tin oxide ITO Indium tin oxide LSDA Local spin density approximation MEA Monoethanolamine PEDOT Poly(3,4-ethylenedioxythiophene) PLD Pulsed laser deposition PSS Polystyrene sulfonate SEM Scanning electrode microscope SILAR Successive ionic layer adsorption and reaction TFTs Thin film transistors XRD X-ray Diffraction UV-Vis Ultraviolet-Visible viii attributed to the increasing of thin film thickness with increasing of solution concentration from 0.15 M to 0.3 M Figure 3.9 The absorbance spectra of CuO with different solution concentrations In addition, it is possible to determine the bandgap energy ( of ( ) versus photon energy ( ) from the plots ) by a linear extrapolation to x-axis as shown in Figure 3.10 From this figure, the bandgap energy of CuO thin film was estimated to be 2.09, 2.00, 1.98 and 1.96 eV corresponding to the initial Cu2+ ions concentration in a precursor solution of 015, 0.20, 0.25 and 0.3 M It is obvious that the band gap energy of thin film decreases with increasing of initial Cu2+ ions concentration The decrease of bandgap energy may be due to an increase of grain size as well as a decrease of the defects and disorder in thin films These results are matched with XRD and SEM results as discussed in previous section According to XRD and SEM results, the grain size of thin film increases with increasing of solution concentration This causes the reduction of grain boundaries of films, thus, the scattering of carriers at grain boundaries reduces Consequently, the defects and 51 disorder in films decrease, leading to the reduction of bandgap energy with increasing of solution concentration The values of bandgap energy in this research are matched with the other reported on CuO thin film fabricated by sol-gel technique [40, 46] In general, the decrease of bandgap shows that the thin film improved with increasing of solution concentration Figure 3.10 The bandgap of CuO with different Cu2+ ions concentrations 3.4.2.3 The effect of annealing temperature The absorbance spectra of the CuO thin films with different annealing temperature were measured by UV/VIS spectrophotometer (UV 2450-PC, Shimadzu) in the wavelength region from 300 to 800 nm as shown in Figure 3.11 The CuO thin films are to start an absorption at the wavelength of 850 nm and have a broad absorption band at the wavelengths of 339 - 345 nm Furthermore, the results that the optical property of CuO thin films are significant affected by annealing temperature In details, the absorption intensity of CuO thin films 52 decreases with decreasing of annealing temperatures from 550oC to 400oC The absorption intensity of CuO thin film annealed at 550oC is higher than that of CuO thin films annealed at the lower temperature of 400, 450 and 500oC This may be due to the increasing of thin film thickness with increasing of annealing temperature Figure 3.11 The absorbance spectra of CuO with different annealing temperature In addition, we can estimate the bandgap energy of CuO thin films via the Tauc graph plotted as a function of annealing temperature in Figure 3.12 From this figure, the bandgap energy of CuO thin films can be extracted to be 2.06, 2.04, 2.02 and 1.96 eV as increasing of annealing temperature from 400 to 550oC The decreasing of bandgap energy as increasing of annealing temperature indicates the improvement of the CuO thin film This is consistent with XRD results as mentioned in the section 3.2.3 According to XRD results, the crystallite size increases with increasing of annealing temperature, which leads to reduce the density of grain boundaries As a result, the scattering of carrier at grain boundaries 53 reduces Therefore, the bandgap energy of thin film decreases The bandgap energy of CuO the thin films has been published in range of 1.98 – 2.05 eV [40, 46], which indicates that the values of bandgap energy in our case are consistent with others Therefore, one can conclude that the CuO thin film is able to be used for optoelectrical devices Figure 3.12 The bandgap of CuO annealed at the different annealing temperature 3.5 Operation of thin film transistor 3.5.1 Transfer characteristic Figure 3.13 shows the transfer characteristic of the CuO TFTs for which the channel lengths were varied with respect to 50, 100, 150 and 200 while the channel width of 1000 μm was unchanged In this measurement, the gate to source voltage (VGS) was scaned from 40 to -200 V with a step of 0.5V Meanwhile, the bias voltage between the drain and source (VDS) was fixed at -80V As shown in Figure 3.13, the transfer characteristic curves indicated a typical p-type operation mode of cupric oxide TFTs with ON/OFF ratio of ~ 10-1 – 102 With increasing of 54 channel length from 50 to 100 m, the ON/OFF ratio decreases from The to ratio of CuO TFTs with different channel lengths is listed in Table 3.7 Figure 3.13 The transfer characteristic of CuO TFTs with various channel length of 50, 100 and 150 Besides, it is possible to calculate the electrical parameters CuO TFTs based transfer characteristic curves The electrical parameters of CuO TFTs are calculated and indicated in Table 3.7 The saturation mobility ( ) was estimated from the following equation: ( √ ) Where, W and L denote the channel width and length, and (3.8) is the gate capacitance In addition, the threshold voltage (VTH) was determined from the graphs of √ and VGS 55 Table 3.7 Electrical parameters of CuO TFTs with different channel lengths Channel length ( ) ( ) ( ) 50 -26.26 4.26 × 10−4 100 -24.82 2.78 × 10−4 150 -40.05 6.51 × 10−4 200 -29.56 8.69 × 10−4 3.5.2 Output characteristic Figure 3.14 shows the output characteristics of CuO TFTs with different channel lengths of 50, 100, 150 and 200 In this measurement, the gate to source voltage (VGS) was scanned from to -200 V with a step of -50V Meanwhile, the drain to source voltage (VDS) is also scanned from to -200V The output characteristic curves show clear linear and saturation regions, which are the behavior of field-effect transistor Importantly, the operation is a p-type channel transistor, as obviously demonstrated 56 Figure 3.14 The output characteristic of CuO TFTs with various channel length: a) 50 b) 100 c) 150 d) 200 57 CONCLUSION In summary, CuO thin films have been successfully fabricated on glass substrates by a solution method using copper (II) acetate monohydrate, ethanol and monoethanolamine (MEA) as starting materials The effect of MEA stabilizer molar ratio, that of Cu2+ ions concentration and that of annealing temperature were examined, basing on analysis of crystal structure, surface morphology, electrical property of CuO thin films In addition, the operation of thin film transistor using CuO material as a p-type channel layer was investigated and demonstrated The main findings in this research can be highlighted as follows The XRD results indicated that CuO thin films were single-phase, polycrystalline with (110), (002), (111) and (20 ̅ ) planes, and monoclinic structure SEM micrographs verified that the surface of CuO thin films was homogenous and smooth Bandgap energy of CuO thin films was determined in range of 1.96 to 2.09 eV Electrical resistance of 96.32 Ω/ was obtained as the best for the CuO thin film prepared 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November 2016, Hanoi, Vietnam [2] Van Dung Nguyen, Quang Hoa Nguyen, Van Dung Tran, Nguyen Quoc Trinh Bui (2017), Solution-processed doping and undoping zinc-oxide and copper-oxide thin films In: International Thin Films Conference (TACT), 15-18 October 2017, Hualien, Taiwan [3] Hoa Quang Nguyen, Dung Van Nguyen, Akihiko Fujiwara, Bui Nguyen Quoc Trinh (2018) “Solution-processed CuO thin films with various Cu2+ ion concentrations”, Thin Solid Films (proof corrected, in press) DOI: https://doi.org/10.1016/j.tsf.2018.03.036 65 ...VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY NGUYEN VAN DUNG CHARACTERIZATION ON SOLUTIONPROCESSED P-TYPE CuO THIN FILMS FOR ELECTRONIC DEVICES APPLICATION MAJOR:... 34 of the concentration of Cu2+ ions in solution leads to the formation of lower complex concentration Therefore, the formation of CuO phase decreased and the crystallinity of CuO thin film decreased... 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