Resistive Switching Properties of p-type Cobalt Oxide Thin Films and Devices Jian Yang A Thesis in Fulfilment of the Requirements for the Degree of Doctor of Philosophy School of Materials Science and Engineering Faculty of Science February 2018 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Yang First name: Jian Other name/s: Abbreviation for degree as given in the University calendar: PhD School: School of Materials Science and Engineering Faculty: Faculty of Science Title: Investigation of resistive switching properties of p-type cobalt oxide thin films and related p-n heterojunctions Abstract 350 words maximum: (PLEASE TYPE) One of the emerging semiconductor memories, the resistive random-access memory (RRAM), has attracted extensive attention for its application as the next-generation non-volatile memories, due to the excellent characteristics of high-density, high-speed, low-power­ consumption and good reliability. Cobalt oxide (CoO), a p-type binary transition metal oxide, has shown good potential as a resistive switching material but has not been extensively studied yet. In this dissertation, we investigated the RS behaviours of the oxide molecular beam epitaxy (OMBE) fabricated CoO thin films and CoO-TI02 p-n heterostructures, trying to reveal the RS mechanism in the CoO material system and to find new material structures as good RRAM candidates. RS performance and mechanism in the OMBE fabricated Coo thin film were studied. It shows bipolar RS characteristic with good endurance and retention performance, showing good potential for RRAM application. A conductive atomic force microscopy (CAFM) writing/reading study reveals the conductive filamentaryswitching nature In the CoO/Pt memory device. Growth conditions play a key role in determining the properties of thin films. The Impact of oxygen partial pressure during the deposition on the RS behaviour of CoO thin film was studied. A combination of CAFM and Kelvin probe force microscopy study shows that higher oxygen pressure leads to higher conducting filament density in the CoO film, which is attributed to higher defects concentration in this p-type oxide. The RS mechanism in this tip/CoO/Pt memory device can be explained by a novel charge­ injection/conductive-filamentary model. RRAMs with multilayered structure may exhibit different RS behaviour from those with single-layered structure. A CoOITI02 p-n heterostructure memory cell was fabricated and unique nonpolar RS behaviour has been found. Effect of growth temperatures as well as film thickness on the stability of switching parameters, such as cycle-to-cycle ret and reset voltages and resistance at HRS and LRS, have been explored. Growth conditions that result in better RS performance have been suggested. In summary, the results In this dissertation reveal cobaltoxide's great potential In RRAMsappllcatlon and provide more evince in understanding the RS mechanisms in this material. In addition, a p-n junction structured RRAM device with unique RS propertiesis developed. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in ail forms of media, now or here afterknown, subject to the provisions of the Copyright Act 1968. I retain ail property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in DissertationAbstracts International (this is applicable to doctoral theses only). 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I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.' Signed ........... .. ....... ....... ...... Date .......... .............................. COPYRIGHT STATEMENT 'I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in DissertationAbstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' Signed Date AUTHENTICITY STATEMENT 'I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.' Signed Date ACKNOWLEDGEMENT First and foremost, I would like to express deep gratitude to my supervisor, Professor Sean Li, for giving me this precious opportunity to work in his group, in UNSW and in Australia. His profound knowledge and insight has led me into an amazing world of materials science. His guidance and support in both work and life make me have a rewarding and unforgettable PhD career. I would also express my sincere appreciation to my co-supervisor, Dr Thiam Teck Tan, for his invaluable suggestion and assistance in my experiments. He is also a very good friend who gave me much help and guidance in daily life. I have learnt a lot from his attitude towards people and work, which will benefit me in my future life. In addition, I like to thank the technical staffs in the School of Materials Science, Solid State & Elemental Analysis Unit, Electron Microscope Unit at UNSW and Australian National Fabrication Facility - NSW node, for their training me on the instruments and the technical supports in my experiments. No PhD experience can be complete without the fellow schoolmates and friends. I would thank my group fellows as well as friends from other groups. They taught me on using the experimental instruments and spent valuable time discussing my results. We also had great fun and enjoyable time together when traveling and eating outside. Their company took away my loneliness and brought happiness in my life. The stress of financing overseas PhD studies is in large part shouldered by the Chinese government through the China Scholarship Council. I am extremely grateful for their support of my study in Australia. Last, but by no means least, is the selfless support I received from my parents. I would like to thank my great parents for raising me up and supporting me on my study along the past almost thirty years. I cannot grow up or make progress without their wholehearted and selfless support. Their kindness and love will always encourage me. i ABSTRACT Conventional semiconductor memories have encountered their limitations for sustained advance in data processing and storage technologies. One emerging memory, the resistive random access memory (RRAM), has attracted extensive attention for its application in the next-generation non-volatile memories, due to the excellent characteristics of high-density, high-speed, low-power-consumption and good reliability. Cobalt oxide (CoO), a p-type binary transition metal oxide, has shown good potential as a resistive switching (RS) material but has not been extensively studied yet. In this dissertation, the RS behaviours in the oxide molecular beam epitaxy (OMBE) fabricated CoO thin films and CoO-TiO2 p-n heterostructures were investigated by cyclic voltammetry measurement, conductive atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM) methods, trying to reveal the RS mechanism in the CoO material system and to find new material structures as good RRAM candidates. RS performance and mechanism in the OMBE fabricated CoO thin film were studied. It shows bipolar RS characteristic with good endurance and retention performance, showing good potential for RRAM application. A CAFM writing/reading study reveals the conductive filamentary switching nature in the CoO/Pt memory device. Growth conditions play a key role in determining the properties of thin films. The impact of oxygen partial pressure during the deposition on the RS behaviour of CoO thin film was studied. A combination of CAFM and KPFM study shows that higher growth oxygen pressure leads to higher conducting filament density in the CoO film, which is attributed to higher intrinsic defects concentration in this p-type oxide. The RS mechanism in this tip/CoO/Pt memory device can be explained by a novel charge- injection/conductive-filamentary model.