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Diamond Coatings on Graphite for Plasma Facing Materials

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Diamond Coatings on Graphite for Plasma Facing Materials Isaela Villalpando de la Torre, MSc Thesis Submitted for the Degree of Doctor of Philosophy Heriot-Watt University School of Engineering and Physical Sciences May 2010 The copyright in this thesis is owned by the author. Any quotation from the thesis or use of any of the information contained in it must acknowledge this thesis as the source of the quotation or information Abstract Nanocrystalline and microcrystalline diamond films have been successfully deposited on graphite substrates for the first time. The morphology of the films depended on the experimental parameters used during deposition such as: gas mixture, excitation power, pressure and deposition time, along with nucleation treatments. Experiments are reported for removing non-diamond carbon material from commercial detonation nanodiamond used for seeding nucleation. Scanning Electron Microscopy (SEM), Raman Spectroscopy and X-ray Photoelectron Spectroscopy (XPS) techniques were used to characterise the samples. Optical Emi- ssion Spectroscopy (OES) and Mass Spectroscopy were used to analyse the species formed in the gas phase during diamond growth. We observed that the excitation power used during deposition affects mainly the diamond crystallite size. Microcrystalline films were obtained when the excitation power was 3.0 and 3.6 kW and nanodiamond films were observed when 1.5 kW was used. The use of argon is essential for growing diamond on graphite and the methane content affects the morphology, the sp3/sp2 content and the crystallite size of the films. When using less than 5% of methane in the gas mixture, f100g faces are predominant even after long periods of deposition. Using 5% of methane results in a film with cauliflower-like structure. Change in the morphology caused by secondary nucleation was observed after long deposition periods of time. To study the behaviour of our prepared samples under erosion conditions, diamond films were exposed to hydrogen plasma etching and analysed in terms of film quality (sp3/sp2 content) and growth/etching mechanisms. Finally, there is also included a study about the production of carbon fibres on dia- mond films during hydrogen plasma exposure in the presence of silicon. To my husband V´ıctorand to my parents Acknowledgements I am heartily thankful to my supervisor, Professor John Wilson, whose encourage- ment, guidance and support from start to end enabled me to finish my research project. My most sincere thanks to Professor Phillip John for his supervision and his support throughout this work. I would like to thank Professor Duncan Hand for his help at the beginning of my project. My special thanks to Mr. Iain Drummond who trained me in the use of vacuum systems. Many thanks to Mr. Alan Barton and Mr. Paul Allan for their valuable help in the lab and with glassware. I am grateful to Mr. Peter Heron, Mr. Mark Stewart and Mr. Neil Ross for their technical support and useful advice. Mrs. Marian Millar for her help with the analytical equipment. Dr. Graham Beamson, NCESS facility at Daresbury Laboratory, for his help with XPS analysis. I owe my deepest gratitude to my family, friends and colleagues, whose support and goodwill kept me going through this research project. I would particularly like to thank Miss Helena Lind, Dr. Lucia Romero, Dr. Amalia I. Poblador, Dr. Iwona Szkoda, Dr. Patricia Leyva, Dr. Erinc Engin, and Dr. Rachel J.E. Lees. I want to express my gratitude to: programme Alβan, the European Union Pro- gramme of High Level Scholarships for Latin America, scholarship No. E05D056416MX; Overseas Research Students Awards Scheme (ORSAS), CONACYT and Heriot-Watt University for financial support. Contents 1 Review of diamond synthesis and characterisation techniques 1 1.1 Introduction . .1 1.2 Diamond and other carbon structures . .2 1.2.1 Diamond . .2 1.2.2 Graphite . .2 1.2.3 Carbon nanotubes . .2 1.2.4 Diamond nanorods . .3 1.2.5 Carbon nanofibres . .3 1.3 Low pressure diamond synthesis techniques . .3 1.3.1 Microwave plasma CVD . .4 1.3.2 DC Plasma CVD . .5 1.3.3 RF Plasma CVD . .5 1.3.4 Hot Filament CVD . .5 1.4 Chemical mechanisms of CVD diamond . .6 1.5 Nucleation . .7 1.5.1 Nucleation on Unscratched Substrates . .8 1.5.2 Scratching and Seeding . .8 1.6 Diamond Growth . .9 1.6.1 Atomic hydrogen . .9 1.6.2 Hydrocarbon chemistry . 10 1.6.3 Effects of oxygen addition . 11 1.6.4 Effects of noble gas addition . 11 1.6.5 Growth mechanisms . 11 1.6.6 Growth mechanism of diamond (111) . 14 1.6.7 Growth mechanism on (100) surface . 14 1.6.8 Growth mechanism on (110) surface . 15 1.7 Graphitisation of Diamond . 15 1.8 Nanodiamond and ultrananocrystalline diamond . 16 1.9 Characterisation techniques . 16 1.9.1 Optical Emission Spectroscopy . 16 i 1.9.2 Mass Spectroscopy . 17 1.9.3 Infrared Spectroscopy . 18 1.9.4 Raman Spectroscopy . 20 1.9.5 X-ray Photoelectron Spectroscopy . 22 1.9.6 Scanning Electron Microscopy . 24 1.9.7 Transmission Electron Microscopy . 26 2 Synthesis and characterisation of diamond films 29 2.1 6 kW Microwave Plasma Enhanced CVD System . 29 2.2 Experimental conditions during diamond growth . 31 2.3 Characterisation Techniques . 32 2.3.1 Optical Emission Spectroscopy . 32 2.3.2 Mass Spectroscopy . 34 2.3.3 Infrared Spectroscopy . 34 2.3.4 Raman Spectroscopy . 34 2.3.5 X-ray Photoelectron Spectroscopy . 34 2.3.6 Scanning Electron Microscopy . 34 3 Removal of non-diamond material in nanocrystalline diamond pro- duced by detonation synthesis 36 3.1 Introduction . 36 3.2 Experimental . 38 3.3 Results . 39 3.4 Discussion . 42 3.5 Conclusions . 43 4 Study of nucleation treatments and microwave excitation power for CVD diamond films on graphite 44 4.1 Experimental . 45 4.2 Results . 48 4.2.1 SEM analysis . 48 4.2.2 Raman Spectroscopy . 53 4.2.3 X-ray Photoelectron Spectroscopy . 56 4.2.4 Plasma analysis during deposition by Optical Emission Spec- troscopy . 62 4.3 Discussion . 63 4.4 Conclusions . 68 ii 5 Effect of gas composition on the resulting morphology of CVD dia- mond films 69 5.1 Introduction . 69 5.2 Experimental . 71 5.3 Results . 72 5.3.1 SEM analysis . 72 5.3.2 Raman analysis . 76 5.3.3 Mass Spectroscopy . 77 5.3.4 Optical Emission analysis . 79 5.4 Discussion . 79 5.5 Conclusions . 82 6 Hydrogen plasma etching of thick diamond films 84 6.1 Introduction . 84 6.2 Experimental . 87 6.3 Results . 88 6.3.1 SEM analysis . 88 6.3.2 Raman analysis . 91 6.3.3 Optical Emission Spectroscopy analysis . 91 6.3.4 Mass Spectroscopy analysis . 93 6.4 Discussion . 93 6.5 Conclusions . 98 7 Study of other carbon structures: Carbon Fibres 99 7.1 Introduction . 99 7.2 Experimental . 102 7.3 Results . 103 7.3.1 SEM analysis . 103 7.3.2 XPS analysis . 105 7.3.3 Raman Spectroscopy analysis . 108 7.3.4 Optical Emission analysis . 109 7.4 Discussion . 111 7.5 Conclusions . 113 8 Conclusions and Future Work 114 8.1 Future Work . 115 References 117 iii List of Tables 1.1 IR absorptions observed below 1800 cm−1................ 20 2.1 Values used in equation 2.4. 33 3.1 Experimental conditions for the nanodiamond oxidation treatment. 39 3.2 Main functional groups found in nanodiamond synthesised by detona- tion technique . 40 4.1 Experimental conditions for graphite samples prepared using different nucleation pretreatments. Deposition condions for all the samples were 100 torr, 2 hours using a mixture of 5%CH4/15%H2/Ar. 46 4.2 Experimental conditions for samples prepared using different nuclea- tion pretreatments and heated in oven. Deposition condions for all the samples were 100 torr, 2 hours using a mixture of 5%CH4/15%H2/Ar. 47 4.3 % surface covered by the film. 53 4.4 Parameters from deconvoluted Raman spectra. 56 4.5 Peaks %area of deconvoluted XPS C1s spectra from samples n1 to n9. 58 4.6 OES data from the corrected spectra. 63 5.1 Experimental conditions for samples c1 to c8 deposited using different gas mixtures. 71 5.2 Growth rates of samples deposited using mixtures of CH4/H2/Ar. 75 5.3 Parameters from deconvoluted Raman spectra for samples c5 to c8. 77 5.4 Parameters from deconvoluted Raman spectra for samples tc5 to tc8. 77 5.5 Mean values for the ratio of carbon species/m, where carbon species = CH, CH2, CH3,C2,C2H, C2H2,C2H3 and C2H4 and m = methane, for sample c5 to c8. 79 6.1 Parameters from deconvoluted Raman spectra for samples etc5 to etc8. 91 7.1 Experimental conditions of the sample synthesis, letter \s" indicates the use of silicon piece masking part of the diamond film during the hydrogen treatment. 102 7.2 XPS analysis of the sample.
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