Roland S. Tsang






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 Physical Chemistry.


August 1997



Molecular beam mass spectrometry has proved to be capable of providing quantitative measurements of both stable free radical gas-phase species under conditions typical in the diamond CVD process.  Due care has to be taken, however, in the data reduction procedures because the overall system sensitivity is critically dependent on the local temperature, pressure and composition of the gas being sampled.  Simple empirically based correction procedures can offset such variations in the sampling efficiency.  Armed with a sensitive gas-phase analysis technique and the necessary data reduction procedures for the characterisation of a CVD environment, attention was turned towards the study of the effects that different gas additions to the standard hydrocarbon/H2 gas mixtures have on the gas-phase reaction mechanism occurring during the CVD growth process.


First, an understanding in the behaviour and applicability of chlorine-assisted CVD was accomplished in a hot filament reactor.  Quantitative measurements of the composition of the gas-phase species prevailing during diamond deposition were obtained for a variety of chlorine containing source gases.  Two forms of gas mixtures were used: (1) 1% of a chlorinated methane (CH4-xClx, x=1-4) in H2 and (2) 1% CH4 in H2, then introducing chlorine varying from 1-4%.  At filament temperatures at and above the optimum for diamond growth (~2300C), the relative concentrations of the various hydrocarbon species (CH4, C2H2, C2H4) in the gas phase are insensitive to the choice of Cl precursor used. Furthermore, the product distribution at these temperatures is remarkably similar to that measured when CH4 is the precursor.  For both forms of Cl precursor gases used, chlorine is effectively reduced to HCl at standard growth temperatures, whose concentration is proportional to the chlorine fraction input in the feed gas.  The as-grown diamond films were analysed using AES and SEM techniques.  No chlorine was detected in bulk structure of the films but ~1-2% was found on the diamond surface.  The apparent catalytic activity of Cl atoms in the CVD process is therefore likely to be due to its role in abstracting surface terminating hydrogen or H abstraction of surface C-Cl, at lower substrate temperatures.


Next, the effects of nitrogen on the CVD diamond growth mechanism was examined.  The deposition rate at optimum growth conditions depends critically on the choice of C/N precursor used, and the origin of the carbon-containing species.  The reactions occurring in the gas-phase seem to lead predominantly to the formation of HCN, (except for CH4/N2 gas mixtures).  The stability of this species precludes most of the cycling of carbon during the CVD process, resulting in low rates of diamond deposition.  Thermodynamic equilibrium calculations confirm that HCN production is highly favoured in H/C/N gas mixtures at high gas processing temperatures.  For a 1:1 C:N ratio in the feed gas, continuous films were produced after 6 hour deposition only by CH4/H2/N2 gas mixtures.  Incorporation of nitrogen in the grown diamond films was very low, consistent a theoretical calculations.  At lower temperatures N2 simply acts as a spectator to the CVD process, as evidenced by the significant increase in the C2H2 concentration and reduction in the HCN concentration in the gas-phase compared to other N source gas additions.  At optimum filament temperatures (~2400°C), addition of N2 to a CH4/H2 gas mixture leads to higher deposition rates of poor quality diamond films (determined by LRS).  We believe that this can be explained if N2 is acting as a catalyst for the destruction of H atoms, thereby reducing the etching rate of non-diamond phases on the film surface.  Thus addition of a tiny amount of N2 to the hot filament CVD process will affect not only the gas-phase chemistry, but the growth rate, the morphology and the quality of the resulting diamond films.


The effects of addition of phosphine on the growth behaviour of diamond films have also been investigated.  Films were grown using gas mixtures of 1% CH4 with increasing amounts of PH3 (1000-5000 ppm).  Gas phase species prevalent during the growth process (e.g. CH4, CH3, C2H2, PH3 and HCP) have been monitored, quantitatively, and compared with the corresponding growth rates, quality and properties of the resulting films. Addition of up to 2000 ppm PH3 produced good quality diamond films at the highest growth rates, and changed the crystal morphology in favour of [100].  At higher PH3 concentrations (3000-5000 ppm) the growth rate decreases again, with predominantly [111] faceted crystals and a compromise in film quality.  These observations can be rationalised by the rapidly cycling between methyl radicals and HCP molecules.  At low PH3 input, an unusually high CH3 mole fraction was observed due to additional [H] produced as a result of rapid decomposition of PH3 to P and 3H, lending to fast deposition rates.  At high PH3 input, a reversal in growth rates was observed, which we suggest may be due to another, competing reaction, which instead serves to deplete [CH3].


Finally, as a means to test the validity of the MBMS data we have also employed a highly structured computer package called CHEMKIN, using the SPIN application code to aid in the incorporation of complex gas-phase chemical reaction mechanisms into numerical simulations.  For the first time we have performed a simulation on a 1% CH4 in H2 mixture as a function of filament temperature.  We find that the mole fractions predicted by SPIN for the various hydrocarbon species (CH4, C2H2, C2H4) in the gas phase depends critically on the amount of [H] introduced in the initial SPIN input file, for any particular filament/substrate temperature.  Filament poisoning effects (which reduce the effective concentration of H atoms produced at the filament) have been taken into account in choosing an appropriate starting [H].  At growth temperatures, Dandy and Coltrin predicted an H atom concentration of 5% at the filament (having corrected for filament poisoning effects) which was the value we used in the input with 1% CH4 in H2.  Subsequent simulation of this gas mixture under standard growth conditions produced calculated mole fractions of CH4, C2H2, and C2H4 that are accurate to within a factor of 1-2 compared to the measured values.  Different H atom concentrations were introduced to the input file for different filament temperatures, and the results obtained again were in good agreement with those measured by MBMS.



Firstly, I would like to thank my supervisor, Professor Mike Ashfold, for his ever-enthusiastic and valued advice during my postgraduate studies.


My very special thanks go to Dr. Paul May, for not only being a constant source of encouragement, inspiration and efficient proof-reading over the past three years, but also for his excellent humour and treasured friendship.


I would also like to thank Dr. Christopher Rego for his support and expertise during my first year PhD, and whose work has really set the foundations not only for my project work, but for our understanding of this area of chemistry.


I am grateful to Mr. Keith Rosser for his technical support and advice throughout my experimental work, to John Cole for successfully setting up the CHEMKIN computer package, and to Dr. Thomas Badgwell of the Chemical Engineering Department at Rice University for advice on the many aspects of the package. My special thanks also goes to Dr. Jim Butler of the Naval Research Laboratory not only for valued discussions on my research, but also a most delicious tub of chilli peanut butter.


Thanks go to Mr. John Dimery for the use of the scanning electron microscope (SEM), to Dr. Charles Younes for all the Auger analyses, and to Mr. David Jones for developing outrageously large quantities of high quality SEM photos presented within this thesis. A special acknowledgement must go to Mr. Tim Davis form the Physics Department. Thanks dude for all your help and advice with these seemingly endless Raman curve fittings and data collections, but mostly for your patience and good humour, especially on those ‘Friday afternoons’.


I am indebted to the legendary “BUDGIES”, namely Paul, Stuart, Tim, Dave, Annette, and Stefan, our temporary German virgin addition, for making the last three years so unforgettable, providing a most sparkling research atmosphere and for many of those ‘it’ll be rude not to’ pub sessions.


To my family and friends, a big thank you for all of your support during my years as a student. Finally an extra special mention goes to Vicks who never ceased to believe in me. Thank you for all your care and support during the so often underestimated task of ‘writing-up’.

“To win them, temples have been profaned, palaces looted, thrones torn to fragments, princes tortured, women strangled, guests poisoned by their hosts, and slaves disembowelled. Some have fallen on battlefields, to be picked up by ignorant freebooters, and sold for a few silver coins. Others have been cast into ditches by thieves or swallowed by guards, or sunk in shipwrecks, or broken into powder in moments of frenzy. No strain of fancy in an Arabian tale has outstripped the marvels of fact in the diamond's history.”


Gardner Williams (General Manager of de Beers ca.1890's)





            “The BUDGIES are getting rowdy…”


                                    Jim Butler, NRL, Diamond Conference Meal, Tours, 1996.





            “They resist blows to such an extent that the hammer rebounds and the very anvil splits asunder, but this invincible element which defies Natures two most violent forces, iron and fire, can be broken by ram's blood. But it must be steeped in blood that is fresh and warm and even so, many blows are needed.”


Pliny the Elder ca. 1 century AD.





            “Je suis vraiment desolé…”


The BUDGIES, Diamond Conference, Tours, 1996.

















To mum, dad and sis.












The research described in this thesis was carried out by the author in the School of Chemistry at the University of Bristol under the supervision of Professor MNR Ashfold, Dr. P.W. May and Dr. C.A. Rego.


The work reported herein is original to the author, except where acknowledged by reference or special recognition. No part of this work has been submitted previously for any degree.







1.1       Diamond                                                                                                              1

1.2       Historical overview of the CVD diamond process                                                  3

1.3       The diamond CVD technique                                                                                4

1.4       The CVD diamond film                                                                                         6

1.5       The choice of substrates suitable for growing CVD diamonds                                8

1.6       Mechanism of CVD growth                                                                                  9

1.7       Role of atomic hydrogen in the CVD process                                                      11

1.8       Gas-phase chemistry involved in the CVD process                                              12

1.9       Characterisation of the CVD process                                                                  13

1.10     Summary of the work performed on hot filament CVD systems.                           14

1.11     References                                                                                                         17




2.1       Introduction                                                                                                        20

2.2       Scanning Electron Microscopy (SEM)                                                                21

2.3       Laser Raman Spectroscopy (LRS)                                                                      21

2.4       Auger Electron Spectroscopy (AES)                                                                   23




3.1       Introduction                                                                                                        25

3.2       Hot filament CVD reactor                                                                                   26

            (a) The filament                                                                                                   26

            (b) The substrate                                                                                                27

            (c) Gas phase composition                                                                                  28

            (d) Gas flow system                                                                                            30

3.3       Molecular beam mass spectrometry of diamond CVD                                         34

3.4       Molecular beam mass spectrometer design                                                          35

            (a) Choice of sampling cone orifice diameters                                                      38

            (b) Two stage differential pumping                                                                       39

            (c) Hiden HAL/3F PIC 100 quadrupole mass spectrometer                                40

3.5       Characterisation of the mass spectrometer                                                           50

            (a) Energy scale calibration                                                                                 50

            (b) MS calibration                                                                                              51

            (c) Mass discrimination                                                                                       52

            (d) Temperature dependence of MS sampling efficiency                                      54

            (e) Thermal diffusion effects                                                                                56

            (f) Dissociation patterns                                                                                      56

            (g) Ionisation cross sections and potentials                                                           57

            (h) Detection of radical species                                                                           58

            (i) Procedure for obtaining quantitative measurements of CH3 radicals                  59

3.6       Step-by-step procedure for converting MBMS raw data into species

            mole fractions                                                                                                     62

            (a) Stable gas phase species                                                                                62

            (b) Methyl radicals                                                                                              66

3.7       References                                                                                                         72




4.1       Introduction                                                                                                        74

4.2       Cracking patterns of CH4                                                                                    75

4.3       Cracking patterns of C2H2                                                                                  76

4.4       Gas phase composition as a function of filament temperature

            for 1% CH4 in H2                                                                                               77

4.5       Discussion of errors                                                                                            78

4.6       Gas composition as a function of filament temperature

            for a variety of hydrocarbon precursor gases                                                       79

4.7       Analysis of the films grown using 0.5% and 1% CH4 in H2                                   79

4.8       Appendix: Experimental Data                                                                              83




5.1       Introduction                                                                                                        87

5.2       Experimental Details                                                                                           91

            (a) Deposition experiments                                                                                  91

            (b) Film analysis                                                                                                  91

            (c) Gas phase composition measurements                                                            92

5.3       Analysis of the diamond films                                                                              94

5.4       Gas composition versus filament temperature for a variety of chlorine

            containing precursor gases in H2                                                                          99

            (a) Chloromethane (1% CH3Cl in H2)                                                                 99

            (b) Dichloromethane (1% CH2Cl2 in H2)                                                             99

            (c) Trichloromethane (1% CHCl3 in H2)                                                           100

            (d) Tetrachloromethane (1% CCl4 in H2)                                                           100

5.5       Gas composition versus filament temperature for various CH4/Cl2/H2

            containing gas mixtures                                                                                      104

5.6       Gas composition versus filament temperature using 1% CF4 in H2                      105

5.7       Discussion                                                                                                        106

5.8       References                                                                                                       112

5.9       Appendix                                                                                                         114

            (I)   Ionisation potentials (I.P.) of the various precursor gases used                     114

            (II) Experimental Data                                                                                      114




6.1       Introduction                                                                                                      122

6.2       Experimental Details                                                                                         124

            (a) Deposition experiments                                                                                124

            (b) Film analysis                                                                                                125

            (c) Gas-phase composition measurements                                                         125

6.3       Analysis of the diamond films                                                                            126

            (a) Methane and Ammonia as precursor gas mixture                                          126

            (b) Methylamine as precursor gas mixture                                                          128

            (c) Hydrogen cyanide as precursor gas mixture                                                  128

            (d) Methane and nitrogen as precursor gas mixture                                            132


6.4       Gas composition versus filament temperature for a variety

            of C-/N-containing precursor gases in H2                                                          139

            (a)  Methane and Ammonia as source gas mixture                                             139

            (b)  Methylamine as source gas mixture                                                             144

            (c) Hydrogen cyanide as source gas mixture                                                      145

            (d) Methane and nitrogen as source gas mixtures                                               145

6.5       Conclusions                                                                                                      152

6.6       References                                                                                                       154

6.7       Appendix                                                                                                         156

            (I)   Synthesis of Hydrogen Cyanide                                                                  156

            (II)  Ionisation potentials and the user selected electron energies

                   of the various gas-phase species monitored                                                157

            (III) SEM Photo Library                                                                                   158

            (IV) Experimental Data                                                                                     166




7.1       Introduction                                                                                                      175

7.2       Experimental Details                                                                                         176

            (a) Deposition experiments                                                                                176

            (b) Film analysis                                                                                                177

            (c) Gas-phase composition measurements                                                         177

            (d) Cracking patterns of PH3                                                                             177

7.3       Analysis of the diamond films                                                                            179

7.4       Gas-phase composition measurements                                                              182

7.5       Discussion                                                                                                        185

7.6       References                                                                                                       189

7.7       Appendix                                                                                                         190

            (I)   Ionisation potentials and the user selected electron energies

                   of the various gas-phase species monitored                                                190

(II)  SEM Photo Library                                                                                   191

(III) Laser Raman spectra (uv - 325 nm) of films grown using 1% CH4

        in PH3/H2, the amount of phosphine varying from 0.1%-0.5%.                   205

            (IV) Experimental Data                                                                                     211




8.1       Introduction                                                                                                      226

8.2       Structure of CHEMKIN                                                                                   227

8.3       Structure of the Transport Property Fitting Code

and SURFACE CHEMKIN                                                                             232

8.4       Application codes                                                                                             237

            (a) SENKIN                                                                                                    238

            (b) SPIN                                                                                                          241

8.5       Numerical simulation of the diamond CVD process                                           246

8.6       Numerical simulation of the gas-phase composition vs. filament temperature

for 1% CH4 in H2 at 20 Torr                                                                             251

8.7       Filament poisoning effects                                                                                 265

8.8       References                                                                                                       272