Laser Diagnostics of a Diamond depositing Chemical Vapour
Deposition gas-phase environment
A
thesis submitted to the University of Bristol in accordance with the
requirements of the degree of Doctor of Philosophy in the Department of
Chemistry, Faculty of Science.
Studies have been carried out to
understand the gas-phase chemistry underpinning diamond deposition in hot
filament and DC-arcjet chemical vapour deposition (CVD) systems.
Resonance enhanced Multiphoton
Ionisation (REMPI) techniques were used to measure the relative H atom and CH3
radical number densities and local gas temperatures prevalent in a hot filament
reactor, operating on CH4/H2 and C2H2/H2
gas mixtures. These results were compared
to a 3D-computer simulation, and hence provided an insight into the nature of
the gas-phase chemistry with particular reference to C2®C1 species conversion. Similar experimental and theoretical studies
were also carried out to explain the chemistry involved in NH3/CH4/H2
and N2/CH4/H2 gas mixtures. It was demonstrated that the reactive nature
of the filament surface was dependent on the addition of NH3,
influencing atomic hydrogen production, and thus the H/C/N gas-phase chemistry.
Studies of the DC-arcjet diamond CVD
reactor consisted of optical emission spectroscopic studies of the plume during
deposition from an Ar/H2/CH4/N2 gas
mixture. Spatially resolved species
emission intensity maps were obtained for C2(d®a), CN(B®X) and Hb from Abel-inverted datasets. The C2(d®a) and CN(B®X) emission intensity maps both show local maxima near the substrate
surface. SEM and Laser Raman analyses
indicate that N2 additions lead to a reduction in film quality and
growth rate. Photoluminescence and SIMS
analyses of the grown films
provide conclusive evidence of nitrogen incorporation (as chemically bonded
CN).
Absolute column densities of C2(a)
in a DC-arcjet reactor operating on an Ar/H2/CH4 gas
mixture, were measured using Cavity ring down spectroscopy. Simulations of the measured C2(v=0)
transition revealed a rotational temperature of ~3300 K. This gas temperature is similar to that
deduced from optical emission spectroscopy studies of the C2(d®a) transition.
Firstly, thanks must go to my supervisor, Professor
Mike Ashfold, whose guidance and enthusiasm over the past three years has been
above and beyond.
Secondly, many thanks to Keith Rosser, without whom
the lab would grind to a halt. Thanks
also to Dr. Paul May and Prof. H. Yagi whose help and friendship has been much
appreciated.
For much appreciated guidance and discussion I would
like to thank Dr. Andrew Orr-Ewing, Dr. Colin Western, Dr. Eckart Wrede and Dr.
Steve Langford.
For their interest and enthusiasm in the work I would
like to acknowledge Moray Cook and Ewan Cameron.
I would also like to take this opportunity to thank
Jon Wills, for allowing me to include his CRDS results in this thesis, and to
Y. Mankelevich and N. Suetin for their 3-D computer modelling results.
For their assistance in aspects of the work undertaken
within this thesis I would like to acknowledge C. Younes for carrying out the
SIMS and G. Evans for help with Laser Raman.
The work undertaken by Charlie, Nigel and Gwyn within the Mechanical
workshop also deserves many thanks.
For their friendship and help during the last three
years I would also like to thank the entire diamond and laser chemistry groups
past and present, including James FitzP., Svemir, Mikhail and Pip’s
cousin! In particular I would like to
thank James P. for having his feet on the ground and his sense of humour, and
to Freddie for having his head in the clouds and his help in keeping the lab
tidy! And of course Sean…
I would like to thank my family and friends (even
Cindy) for their constant support. In
particular, I would like to thank Mum and Dad Dowdeswell who made this Ph.D.
possible. Finally, I would like to
thank Nik for everything!
The work contained in this thesis was undertaken at the Department of Chemistry, University of Bristol between October 1998 and May 2002, and has not been submitted for any other degree. It is the work of the author, except where otherwise acknowledged.
G.A.V.
Smith and E.M. Harrison
Contents Page No.
Section 1
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1 |
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1 |
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1.1 |
Structure and Properties of
Diamond |
1 |
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1.2 |
Synthesis of Diamond |
4 |
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1.3 |
HPHT synthesis of Diamond |
5 |
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1.4 |
Diamond synthesis by Chemical
Vapour Deposition (CVD) |
5 |
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1.5 |
Gas-phase chemistry |
8 |
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1.5.1 Atomic hydrogen production |
8 |
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1.5.2 Atomic hydrogen loss mechanisms |
10 |
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1.5.3 Hydrocarbon Gas-phase Chemistry |
12 |
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1.5.4 Influence of trace non-hydrocarbon additions on gas-phase chemistry |
18 |
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1.6 |
Surface Growth of Diamond |
19 |
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1.7 |
Applications of Synthetic
Diamond |
23 |
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1.7.1 Cutting and Grinding Tools |
24 |
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1.7.2 Optical windows |
25 |
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1.7.3 Thermal Management |
25 |
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1.7.4 Surface Acoustic Wave
(SAW) devices |
26 |
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1.7.5 Detector devices |
27 |
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1.7.6 CVD diamond sensors and electronic devices |
28 |
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1.7.7 Diamond cold cathode emission devices |
29 |
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1.7.8 Other applications |
30 |
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1.8 |
CVD synthesis methods |
31 |
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1.8.1 Hot filament CVD system |
31 |
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1.8.2 Microwave-plasma assisted CVD system |
36 |
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1.8.3 Oxyacetylene Torch CVD |
37 |
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1.8.4 Plasma-jet CVD system |
39 |
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References |
52 |
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Section 2 |
57 |
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Chapter 2 :
Experimental and data analysis of HF-CVD studies |
57 |
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2.1 |
Reactor Considerations |
58 |
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2.2 |
Laser System |
63 |
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2.3 |
REMPI Spectroscopy |
65 |
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2.3.1 REMPI study of Atomic Hydrogen |
67 |
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2.3.2 H atom Doppler Profiles |
69 |
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2.4 |
REMPI Studies of Methyl Radicals
|
73 |
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2.5 |
Filament
Carburisation |
73 |
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References
|
75 |
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Chapter 3 : REMPI studies of H atoms and
CH3 radical species in a HF-CVD reactor |
76 |
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3.1 |
H atom detection |
76 |
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3.2 |
Methyl
Radical detection |
81 |
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3.3 |
C2H2
gas-phase chemistry |
89 |
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3.4 |
Gas
phase simulations |
92 |
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3.5 |
3-D
calculations of CH4/H2 and C2H2/H2
chemistry |
93 |
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3.6 |
Conclusions |
100 |
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References
|
100 |
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102 |
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4.1 |
Experimental Set-up |
103 |
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4.2 |
Modifications
of the hot filament upon addition of nitrogen containing gases |
104 |
|
4.3 |
Gas-phase H atom and CH3 radical relative number densities
observed with the addition of NH3 and N2 |
107 |
|
4.4 |
Gas-phase H atom and CH3 radical number densities in a 1%CH4/1%NH3/H2
gas mixture as a function of Tfil |
113 |
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4.5 |
NH radical number densities and spatial profiles in 1%CH4/x%NH3/H2 gas
mixtures. |
116 |
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4.6 |
Conclusions |
118 |
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References
|
118 |
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Section 3 |
120 |
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Chapter 5 : Experimental set-up, data
analysis and growth characteristics of the DC-arcjet system |
120 |
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5.1 |
Reactor
design |
121 |
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5.1.1 Torch-Head Arrangement |
121 |
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5.1.2 Reactor considerations |
126 |
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5.2 |
Growth Studies |
131 |
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5.2.1 Influence of methane addition |
132 |
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5.2.2 Influence of substrate temperature |
135 |
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5.2.3 Chamber Pressure |
138 |
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5.2.4 Discharge Power |
139 |
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5.3 |
Experimental
Set-up for OES collection |
139 |
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5.4 |
Experimental set-up required for CRDS study |
143 |
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References
|
145 |
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Chapter 6 : Nitrogen addition to a DC-arcjet reactor
|
146 |
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6.1 |
OES
Measurements |
146 |
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6.1.1 The C2 molecule |
150 |
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6.1.2 Spatially resolved OES measurements |
151 |
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6.1.3 The Abel Transform |
152 |
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6.2 |
Film
Deposition |
157 |
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6.2.1 SEM analysis |
158 |
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6.2.2 Laser Raman analysis |
160 |
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6.2.3 Photoluminescence (PL) spectroscopy |
163 |
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6.2.4 Secondary-Ion Mass Spectroscopy (SIMS) |
164 |
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6.3 |
Nitrogen-enhanced
Growth Scheme |
166 |
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6.4 |
Conclusions |
168 |
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References
|
169 |
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Chapter 7 : Cavity Ring Down
Spectroscopy studies of C2 in a DC-arcjet reactor |
170 |
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7.1 |
Experimental
Set-up |
171 |
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7.2 |
CRDS of
C2 |
173 |
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7.2.1 C2 rotational temperature
determination |
176 |
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7.2.2 Determination of C2 absolute
column densities |
179 |
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7.2.3 CRDS measured C2(a) column
densities as a function of process conditions |
181 |
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7.3 |
C2
Gas-phase chemistry |
184 |
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7.4 |
C2
as a growth species |
185 |
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7.5 |
Conclusions |
186 |
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References
|
187 |
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Appendices |
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A1 |
Resonance-Enhanced
Multiphoton Ionisation (REMPI) Spectroscopy |
189 |
|
A2 |
Laser
Induced Fluorescence (LIF) Spectroscopy |
191 |
|
A3 |
Cavity Ringdown Spectroscopy
(CRDS) |
192 |
|
A4 |
Optical
emission spectroscopy (OES) |
193 |
|
A5 |
Saha
Equation |
195 |
|
A6 |
GRI_MECH
3.0 (C / H / N) Gas-phase chemistry |
197 |
|
A7 |
Laser Raman Spectroscopy (LRS) |
201 |
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