Diagnostics of Microwave Activated Novel Gas Mixtures for diamond Chemical Vapour Deposition
By
James R Petherbridge
A thesis submitted to the
University of Bristol in accordance with the requirements for the degree of
Doctor of Philosophy in the Faculty of Science, Department of Chemistry.
July 2002
Abstract
Microwave plasma enhanced Chemical Vapour Deposition (CVD) has been used to grow diamond films at substrate temperatures as low as 435°C using CO2/CH4 gas mixtures. Molecular beam mass spectrometry (MBMS) has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for a wide range of plasma gas mixtures (0 – 80% CH4, balance CO2). The CHEMKIN computer package has also been used to simulate the experimental results in order to gain insight into the major reactions occurring within the microwave plasma. The calculated trends for all species agree well with the experimental observations. Using these data, the model for the gas phase chemistry can be reduced to just four overall reactions. Our findings suggest that CH3 radicals are likely to be the key growth species when using CO2/CH4 plasmas and provide a qualitative explanation for the observation that diamond growth occurs within a very narrow concentration window.
Diamond films have also been deposited; using both 1% CH4/H2 and 51%CH4/49%CO2 gas mixtures with various levels of H2S addition (100-5000 ppm). The former gas mixture was investigated using both microwave (MW) and hot filament (HF) activation and it was found that these two deposition techniques yield very different results. For both 1% CH4/H2 and 51%CH4/49%CO2 MW activated gas mixtures, scanning electron microscopy (SEM) observations show that the crystal quality of these films reduces with increasing H2S addition. Laser Raman and four point probe measurements showed a corresponding fall in the quality and resistivity of deposited diamond films, respectively. MBMS measurements for these S containing gas mixtures revealed significant concentrations of CS2 and CS in all of the MW plasmas that yield S-doped diamond films, whereas CS was not detected in the gas phase during HF growth. This suggests that CS may be an important intermediary facilitating S incorporation into diamond.
All of these results are rationalised by further CHEMKIN simulations using a mechanism that is based on that used to model CH4/CO2 gas phase chemistry, supplemented by additional reactions to allow for H/C/O/S chemistry. This reveals a pivotal role for the overall reversible reaction: CH4 + 2H2S ¾ CS2 + H2. This model has also allowed S‑incorporation to be investigated in terms of the product of predicted mole fractions of CH3 and CS, [CH3]×[CS].
Acknowledgements
I must take this opportunity to thank those people whose assistance I have received throughout the last three years. Firstly, great thanks go to my supervisors Dr Paul May and Prof. Mike Ashfold. Paul has been a constant source of help and encouragement, especially when it has been my round in Bennies. Mike has been tireless in his helpful discussions and thorough proof reading of reams of paper. Keith Rosser must also be thanked for his help, basically whenever anything broke or needed building. A special thanks also goes to Dr Stuart Leeds without whom this Ph.D. would not be possible.
Special thanks must go to those who have had the misfortune to have me as their supervisor during their final year projects. I am of course referring to Sean, Fuge and Ed. It is comforting to see that even the horrors of CHEMKIN were not enough to persuade these three that staying on to do a Ph.D was not advisable!
I must also mention those who have shared my days in the dungeon over the past years, these include Kevin Kuo, Matt “vodka bucket and a plateful of squid” Latto, Jamesie Smith, Fred “the Belgian” Claeyssens, Simon “reformed physicist” Henley, John “ironman” Wills, Beth Fawcett, Alistair “sarky” Parkes, and Dr In‑Deok Jeon. Thanks also to the electrochemists Gus, Liz, Steve and Ray, for their assistance and good humour and also Fitzy P for his pool skills and blue language.
I must also thank Drs Jason Riley (for his advice on all things electronic) Dudley Shallcross (for the C-S linking mechanism), Jeremy Harvey (for Guassian work) and Colin Western (for great assistance with CHEMKIN). Thanks also go to Jon Hayes for Raman data, and Dr Sean Davis, Pippa Hawes, Annella Seddon, David Jones and Les Corbin for varied assistance with SEM data. A special thank you goes to all in the mechanical workshop for the fabrication of many diverse objects throughout my time in the lab.
For financial support I thank the EPSRC and De Beers Industrial Diamonds.
Finally I must say thanks to all
within Molecular Science, BUDGies past and present, my family, and most of all
to Katie, for all her love and support throughout my Ph.D!
“Diamonds
are things that fancy or agreement have put the value on, more than real use in
the necessary support of life.”
John Locke, Philospher, in ‘Treatises on Governmnet’, 1690.
“[diamond is] an unctuous body coagulated”
Sir Isaac Newton, taken from ‘On the Nature of Diamond’ by Smithson Tennant, 1796.
“It’s scientific fact, even though there is
absolutely no proof for it”
Dr Fox, in ‘Brass Eye’, 2001.
For my family
and for
Katie
I declare that the work in this dissertation is carried out by the author in the School of Chemistry at the University of Bristol, under the supervision of Dr P.W. May and Prof. M.N.R. Ashfold, and in accordance with the regulations of the University of Bristol. This work is original except where indicated by special reference in the text, and no part of the dissertation has been submitted for any other degree. Any views expressed are those of the author and in no way represent those of the University of Bristol. This dissertation has not been presented to any other University for examination, either in the United Kingdom, or overseas.
James Petherbridge
1.1. Diamonds in Nature
1.2. Man and Diamond
1.3. Properties of Diamond
1.4. Synthetic Diamond
1.4.1. HPHT Synthesis
1.4.2. Diamond Growth at Low Pressures
1.5. Diamond CVD Methods
1.6. Hot Filament CVD
1.7. Microwave Plasma CVD
1.7.1 Plasma Generation
1.7.2. Plasma properties
1.8. The Chemistry of Diamond CVD
1.8.1. Diamond Growth Precursor Species
1.9. Gas-phase Diagnostics of HFCVD
1.9.1. The Effect of Filament Temperature
1.9.2. The Effect of Input Gas Mixture
1.9.3. Species Concentrations: Variation with Distance from Filament
1.9.4. Gas Temperature: Variation with Distance from Filament
1.9.5. Modelling Studies
1.10. Gas-phase Diagnostics of MWCVD
1.10.1. Species Concentrations
1.10.2. Temperature Measurements
1.10.3. Modelling Studies
1.11. Molecular Beam Mass Spectrometry
1.12. Nucleation and Growth of the Diamond Film
1.13. Applications
1.14. Low Temperature Growth of Diamond (LTGD)
1.14.1. H/C/O Containing Diamond CVD Gas Mixtures
1.14.2. CH4/CO2 Diamond CVD Gas Mixtures
1.15. Semiconducting CVD Diamond for Electronics
1.16. p-Type CVD Diamond
1.16.1. H-induced p-type Surface Conductivity
1.17. Investigations of n-type CVD Diamond
1.17.1. Sulfur Doping Experiments
1.17.2. Ab Initio Calculations for Sulfur Related Defects
1.17.3. Gas Phase Studies of Sulfur Doping of CVD Diamond
1.18. References
2.1. Introduction
2.2. Scanning electron microscopy (SEM)
2.3. Laser Raman Spectroscopy (LRS)
2.4. X-ray photoelectron spectroscopy (XPS)
2.5. Four point probe resistance measurements
2.6. References
3.1. Introduction
3.2. Film deposition apparatus
3.2.1. Microwave System
3.2.2. Deposition Chamber and Vacuum System
3.2.3. Cooled Substrate Holder
3.2.3.1. Cooled Substrate Holder Development
3.2.3.2. Calibration of Substrate Temperature Measurement
3.2.4. Gas supply
3.2.5. Pressure Regulation and Exhaust System
3.2.6. Substrates: Selection and Preparation
3.2.7. Safety Interlock System
3.2.8. Deposition Experiment Procedure
3.2.9. Placement of Substrates during Deposition
3.2.10. Problems Encountered during Deposition Experiments
3.2.10.1 CH4/CO2Gas Mixtures: Chamber Pressure Control
3.2.10.2 Sulfur Containing Gas Mixtures
3.3. Film analytical apparatus
3.3.1. Scanning Electron Microscopy
3.3.2. Laser Raman Spectroscopy
3.3.3. X-ray Photoelectron Spectroscopy
3.3.4. Four-point Probe Measurements
3.4. Optical Emission Spectroscopy
3.5. Molecular Beam Mass Spectroscopy (MBMS)
3.5.1. Gas Sampling From Plasma Ball
3.5.2. Gas Sampling From Vicinity of Hot Filament
3.5.3. MBMS First Stage
3.5.4. MBMS Second Stage
3.5.5. Considerations for MBMS studies
3.6. Hiden Analytical HAL/3F PIC Quadrupole Mass Spectrometer
3.6.1. Source
3.6.2. Quadrupole Mass Filter
3.6.3. Detector
3.6.4. HAL/3F PIC 100 QMS: System Control Unit
3.7. Mass Spectrometer Characterisation
3.7.1. Energy Scale Calibration
3.7.2. Threshold Ionisation Technique
3.8. MBMS Experimental Procedure
3.9. Mass spectrometric Data Collection
3.9.1. Notation
3.10. Data Analysis Procedure
3.10.1. Temperature and Background Correction
3.10.2. Correction for Cracking Products
3.10.3. Room Temperature Species Calibration
3.10.4. Calibration of Radical Species
3.11. Problems encountered during MBMS investigations
3.11.1. Species Mole Fraction Calibration for CH4/CO2 Gas Mixtures
3.11.2. Investigations of CH4:CO2 Mixing Ratios
3.11.3. Investigations of H2S Addition to 51%CH4/49%CO2 Gas Mixtures
3.12. References
4.1. Introduction
4.2 Overview of CHEMKIN II
4.3. Files
4.3.1. Setting up a directory for files
4.3.2. File Handling
4.4. CHEMKIN Interpreter (chem.exe)
4.5. Input Gas-Phase Reactions (chem.inp)
4.5.1. Gri-Mech 3.0 Reaction Mechanism
4.5.2. C-S linking Reaction Mechanism
4.5.3. Leeds’ Combined Combustion Reaction Mechanism
4.6. Database of Species Thermodynamic Properties (Therm.dat)
4.6.1. Thermodynamic Properties of S Containing Species
4.7. Running the CHEMKIN Interpreter
4.7.1. Checking the chem.out file
4.8. SENKIN (senk.exe)
4.8.1. Keyword input file (senk.inp)
4.8.2. Running the SENKIN code (senk.exe)
4.8.3. SENKIN Printed Output File (tign.out)
4.9. Limitations of SENKIN
4.10. Automatic looping of SENKIN
4.10.1. The cycle Program
4.10.2 Running cycle
4.11. References
5.1. Introduction
5.2. Low Temperature Diamond Deposition From 50%CH4/50%CO2Gas Mixtures
5.2.1. Film Crystalinity
5.2.2. Film Growth rate
5.2.3. LRS Analysis of Films
5.3. OES Studies of CH4/CO2 Microwave Plasmas
5.4. Molecular Beam Mass Spectrometry studies of CH4/CO2 Microwave Plasmas
5.5. Conditions for Computer Simulations of CH4/CO2 Gas Mixtures
5.5.1. Computer Simulation Time
5.5.2. Computer Simulation Temperature
5.6. Computer simulation results and comparison with experiment for CH4/CO2 Gas Mixtures
5.7. Discussion of Results for CH4/CO2 Gas Mixtures
5.7.1. Termination of Diamond Growth Surface
5.7.2. Diamond Growth Species
5.7.3. Gas-Phase CH4/CO2 Plasma Chemistry
5.8. Modelling of the C-H-O Diamond CVD Atomic Phase Diagram
5.9. References
6.1. Introduction
6.2. Diamond Deposition From H2S/1%CH4/H2 Gas Mixtures
6.2.1. Film Crystallinity
6.2.2. Film Growth Rate
6.2.3. LRS Analysis of Films
6.2.4. Four-point Probe Resistivity Measurements of Films
6.2.5. XPS measurement of film S/C Ratio
6.3. Diamond Deposition From a CS2/H2 Gas Mixture
6.4. OES of H/C/S containing Microwave Plasmas
6.5. Molecular Beam Mass Spectrometry Studies ofH2S/1%CH4/H2 Gas Mixtures
6.6. Molecular Beam Mass Spectrometry Studies of CS2/H2 Gas Mixtures
6.7. Computer Simulations of H/C/S Gas Mixtures
6.7.1. Mass Balance Corrections to MBMS Data
6.7.2. Simulation Temperature
6.7.3. H2S/1%CH4/H2 Gas Mixtures
6.7.4. 1%CS2/H2 Gas Mixture
6.8. Discussion
6.9. References
7.1. Introduction
7.2. Diamond Deposition From H2S/51%CH4/49%CO2 Gas Mixtures
7.2.1. Film Crystallinity
7.2.2. Film Growth Rate
7.2.3. LRS Analysis of Film Quality
7.2.4. Four-point probe Film Resistivity Measurements
7.2.5. XPS measurements of Film S/C ratio
7.3. OES of H2S/51%CH4/49%CO2 Microwave Plasmas
7.4. Computer Simulations of H2S/51%CH4/49%CO2 Gas Mixtures
7.5. Molecular Beam Mass Spectrometry Studies of H2S/51%CH4/49%CO2 Microwave Plasmas
7.6. Discussion
7.7. References
8.1. Growth
of Diamond from 50%CH4/50%CO2 gas mixtures
8.2. Sulfur
Doping of Diamond Films
8.2.1. Hot Filament Versus Microwave
Activation for H/C/S Gas Mixtures
8.2.2. Growth of Diamond films using H2S
addition to 1%CH4/H2 and 51%CH4/49%CO2
gas mixtures
8.3. References
Appendix IV: Chem.inp Reaction Mechanism
Appendix V: Temp and Time Computer Programs