Growth and characterization of nickel oxide thin films and nanostructures for novel device applications

GROWTH AND CHARACTERIZATION OF NICKEL OXIDE THIN FILMS AND NANOSTRUCTURES FOR NOVEL DEVICE APPLICATIONS REN YI (B Eng, NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY NUS Graduate School for Integrative Sciences and Engineering NATIONAL UNIVERSITY OF SINGAPORE 2012 Declaration I hereby declare that the thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously Ren Yi 26 November 2012 Abstract In this dissertation, the growth and characterization of nickel oxide (NiO) for various novel device applications are investigated In the aspect of growth, many methods of both solution-based and physical deposition were employed to optimize and improve both thin film and nanostructure growth Importantly, for the thin film growth, chemical bath deposition (CBD) with various post-deposition annealing temperatures was used to grow porous thin films with different composition and degree of crystallinity; while sputtering with substrate heating was used to deposit thin films with excellent quality and uniformity For the nanostructure growth, the Kirkendall effect was examined in detail and this was shown to be especially critical in growing NiO nanostructures The roughening effect on oxidation to form NiO nanowires was explained together, with the interplay of vacancy diffusion rate and the dimensional sizes In understanding the limitations, the growth of NiO nanotubes with suitably uniform tube walls by oxidation of nickel nanowires was demonstrated for the first time The applications of NiO thin films and nanostructures were then investigated for resistive switching memory and electrochromic devices For the NiO resistive switching memory, a filamentary switching mechanism was demonstrated It was found that the electrode material and the polarity of the voltage bias played important roles in altering the filament formation process, thus affecting the resistive switching behavior of the memory device It was believed that an electrochemically inert metal anode was essential for repeatable resistive switching of NiO, because it limited the formation of a strong metal filament which could result in permanent breakdown of the memory device For applications in the electrochromic i devices, the detailed coloration and degradation mechanisms of NiO were elucidated for the first time It was found that the initial hydration of the NiO films towards nickel hydroxide proceeded gradually through a combination of coloration from hydroxyl ions and bleaching through protons, and this process increased the optical modulation of the deposited film However, enhanced hydroxyl ion incorporation during coloration will lead to water intercalation Degradation occurs when the extensive intercalated networks of water molecules isolated colored nickel oxy-hydroxide grains which resulted in irreversible coloration of the device It was also demonstrated that the degradation can be easily reversed by thermal annealing ii Acknowledgements First and foremost, I would like to extend my sincere and greatest gratitude to my supervisor A/Prof Chim Wai Kin and co-supervisor Dr Chiam Sing Yang for their patient guidance and help throughout the candidature Their advices and suggestions are invaluable for me to overcome the problems I encountered, without which the completion of this work would not be possible I would also like to express my appreciation to my Thesis Advisory Committee consisting of A/Prof Zhu Chunxiang and A/Prof Lee Chengkuo Vincent for their time in reviewing my work and giving valuable suggestions for improvement My heartfelt gratitude also goes to my senior PhD students Dr Pi Can and Dr Huang Jinquan for their patient mentorship and guidance during my earlier candidature I treasure our friendships and all the time we spent together for both work and fun Special thanks go to Mrs Ho Chiow Mooi and Mr Koo Chee Keong for providing training and assistance on the experimental equipment and logistics required in the Centre for Integrated Circuit Failure Analysis and Reliability (CICFAR) I also wish to thank the NUS Graduate School (NGS) for providing the scholarship and various education and conference allowances during my candidature Last but not least, I wish to express my love and gratitude to my beloved parents and wife for their endless love and understanding through my entire life iii Table of Contents Abstract i Acknowledgements iii Table of Contents iv List of Figures viii List of Tables xiv Chapter Introduction .1 1.1 Background and Motivation 1.2 Objectives 1.3 Organization of Thesis Chapter 2.1 Literature Review Growth of NiO Thin Film 2.1.1 Physical deposition of NiO thin film 2.1.2 Solution growth of NiO thin film 2.2 Growth of NiO Nanostructures by Thermal Oxidation iv 2.2.1 Fabrication of Ni nanowires 2.2.2 Oxidation of Ni nanowires by the Kirkendall effect 10 2.3 Application of NiO in Resistive Switching Memory 12 2.3.1 Resistive switching phenomena 14 2.3.2 Resistive switching materials and mechanism 16 2.3.2.1 2.3.2.2 Filament rupture process 20 2.3.2.3 2.4 Filament formation process 18 Electrode material dependency 21 Application of NiO in Electrochromic Smart Windows 23 2.4.1 Electrochromic materials and device structure 24 2.4.2 Electrochromic mechanism of NiO 26 Chapter 3.1 Experimental Details .29 Growth of Thin Films and Nanowires 29 3.1.1 Sputtering for thin film growth 29 3.1.2 Chemical bath deposition for porous film growth 30 3.1.3 Anodization, electrodeposition and oxidation for nanowire growth 31 3.2 Materials and Devices Characterization 33 3.2.1 Physical and chemical characterization 33 v 3.2.2 Electrical characterization 35 3.2.3 Electrochemistry 36 3.3 Other Methodologies Involved 38 Chapter NiO for Resistive Switching Memory 39 4.1 Thin Film Growth and Characterization 39 4.2 RS Device Characterization 41 4.3 Factors Affecting RS Behavior and the Filamentary Mechanism 44 4.3.1 Electrode size dependency 44 4.3.2 Compliance current dependency 45 4.4 Electrode Material and Bias Polarity Dependency Study 50 Chapter NiO for Electrochromic Smart Window .59 5.1 Porous Thin Film Growth and Characterization 59 5.2 EC Device Characterization 64 5.3 EC Mechanism Study 68 5.3.1 Characterization results and discussion 68 5.3.2 Coloration and degradation mechanism 73 5.3.3 Regeneration from degradation 77 5.3.4 Summary 78 vi Chapter 6.1 NiO for Nanostructured Device 80 Growth of NiO Nanostructures 81 6.1.1 Growth and characterization of the Ni nanowires 81 6.1.2 Oxidation of the Ni nanowire within the AAO template 84 6.1.3 Oxidation of dispersed Ni nanowires in the low temperature regime 86 6.1.4 Oxidation of dispersed Ni nanowire in the high temperature regime 92 6.1.5 Fabrication of uniform NiO nanotubes by chemical wet etching 99 6.1.6 Fabrication of uniform NiO nanotubes by varying nanowire diameter 101 6.1.7 Summary 111 6.2 Characterization of Nanostructured RS Device 112 6.3 Characterization of Nanostructured EC Device 117 Chapter Conclusion 121 7.1 Summary of Findings and Conclusion 121 7.2 Recommendations for Future Work 124 References 127 Appendix A: List of Publications .139 vii List of Figures Figure 2-1 The ion diffusion process in the growth of hollow nanostructure by the Kirkendall effect 10 Figure 2-2 (a) Typical current-voltage characteristics of (a) the unipolar resistive switching and (b) the bipolar resistive switching 15 Figure 2-3 Five-layer prototype for EC smart window devices 25 Figure 3-1 Schematic diagram showing the setup for (a) the anodization of Al foil using a voltage source and (b) the electrodeposition of Ni into AAO membrane using a galvanostat 32 Figure 4-1 XRD patterns of sputtered NiO films with (300 °C) and without (room temperature) substrate heating The various NiO peaks with different crystal orientation are indicated 40 Figure 4-2 Schematic diagram of the thin film RS device structure and the electrical characterization setup through a probe station and parameter analyzer 42 Figure 4-3 Current-voltage curves for a typical SET and RESET process with current in (a) linear scale and (b) logarithmic scale 43 Figure 4-4 (a) Plot of the average ON state resistance against electrode size (b) Plot of the average RESET voltage against electrode size Results for each electrode size are taken from 200 switching cycles as indicated by the error bars 45 Figure 4-5 Plot of average ON state resistance against compliance current Results for each compliance current value are taken from numerous switching cycles (at least 100 switching cycles) as indicated by the error bars 46 Figure 4-6 Current-voltage curves for an occasional switching cycle with step-like characteristic, demonstrating the formation of multiple conductive filaments 47 Figure 4-7 Plots of (a) average RESET current and (b) average RESET voltage against the compliance current Results for each compliance current value are taken from numerous switching cycles (at least 100 switching cycles) as indicated by the error bars 48 Figure 4-8 Plot of maximum number of switching cycles before dielectric breakdown occurs (i.e endurance) against the compliance current 49 viii CBD NiO film, sputtered NiO film as well as smart window based on other EC material can be achieved For smart windows based on nanostructured NiO, a device structure with more ordered arrangement of nanostructures is required for smaller resistance and better performance This can be achieved by fabricating the AAO template, and thus the self-assembled NiO nanostructures, on the ITO-on-glass substrate directly In such a case, standalone NiO nanostructures can be grown vertically on the substrate and the length of each nanostructure can be reduced to a value similar to the film thickness in thin film based devices This device structure can result in efficient electronic transport to the entire EC material through the bottom contact of each nanostructure, reducing the in series resistance, while the space between each nanostructure ensures efficient ion diffusion for the electrochemical reaction Electrodeposition of Ni(OH)2 can be used instead of Ni if the standalone Ni nanowires cannot be fully oxidized For the RS memory device based on nanostructured NiO, a metal-NiO-metal heterojunction nanowire can be used to reduce the effective thickness between two metal electrodes and thus the forming voltage By sequentially changing the electrolyte solution during the electrodeposition, Pt-Ni-Pt heterojunction nanowire can be fabricated and the length of the Ni portion can be adjusted by careful control of the deposition duration Subsequent oxidation of the nanowire will only oxidize Ni and produce Pt-NiO-Pt nanowire The nanowire can then be used to form a RS device with two metal electrodes which have a spacing not limited by lithography 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Journal of PhysicsCondensed Matter, vol 21, p 415303, Oct 14 2009 [118] Y Ren, W K Chim, S Y Chiam, J Q Huang, C Pi, and J S Pan, "Formation of nickel oxide nanotubes with uniform wall thickness by low-temperature thermal oxidation through understanding the limiting effect of vacancy diffusion 137 and the Kirkendall phenomenon," Advanced Functional Materials, vol 20, pp 3336-3342, Oct 2010 [119] Y Ren, S Y Chiam, and W K Chim, "Diameter dependence of the void formation in the oxidation of nickel nanowires," Nanotechnology, vol 22, p 235606, Jun 10 2011 [120] R Waser, R Dittmann, G Staikov, and K Szot, "Redox-based resistive switching memories - nanoionic mechanisms, prospects, and challenges," Advanced Materials, vol 21, p 2632, Jul 13 2009 138 Appendix A: List of Publications A1 International Publications [1] Y Ren, W K Chim, S Y Chiam, J Q Huang, C Pi, and J S Pan, “Formation of nickel oxide nanotubes with uniform wall thickness by low-temperature thermal oxidation through understanding the limiting effect of vacancy diffusion and the Kirkendall phenomenon,” Advanced Functional Materials, vol 20, p 3336, 2010 [2] C Pi, Y Ren, and W K Chim, “Investigation of bipolar resistive switching and the time-dependent SET process in silver sulfide/silver thin films and nanowire array structures,” Nanotechnology, vol 21, p 085709, 2010 [3] Y Ren, S Y Chiam, and W K Chim, “Diameter dependence of the void formation in the oxidation of nickel nanowires,” Nanotechnology, vol 22, p 235606, 2011 [4] C Pi, Y Ren, Z Q Liu, and W K Chim, “Unipolar memristive switching in yttrium oxide and RESET current reduction using a yttrium interlayer,” Electrochemical and Solid State Letters, vol 15, p G5, 2012 [5] Y Ren, W K Chim, L Guo, H Tanoto, J S Pan, and S Y Chiam, “Understanding of the coloration and degradation mechanism of electrochromic nickel oxide,” Submitted to Solar Energy Materials and Solar Cells, 2012 A2 International Conferences [1] Y Ren, S Y Chiam, and W K Chim, “Understanding the influence of vacancies diffusion in utilizing the Kirkendall phenomenon for nanotubes formation,” 2010 8th International Vacuum Electron Sources Conference and NANOcarbon conference proceedings, Nanjing, China, 2010 [2] Y Ren, S Y Chiam, and W K Chim, “Photocatalytic conversion of carbon dioxide to hydrocarbons by copper based metal oxide nanostructures,” 2nd Nano Today Conference, Hawaii, USA, 2011 139 ... dissertation, the growth and characterization of nickel oxide (NiO) for various novel device applications are investigated In the aspect of growth, many methods of both solution-based and physical... nanowires was demonstrated for the first time The applications of NiO thin films and nanostructures were then investigated for resistive switching memory and electrochromic devices For the NiO resistive... Literature Review Growth of NiO Thin Film 2.1.1 Physical deposition of NiO thin film 2.1.2 Solution growth of NiO thin film 2.2 Growth of NiO Nanostructures by Thermal