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.

 

James Anthony Smith, May 2002

 

Contents

 

Abstract

 

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 H_b 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.

 

Acknowledgements

 

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!

 

 

Declaration

 

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.

 

 

James Anthony Smith, May 2002

 

 

Dedicated to the memory of

 

G.A.V. Smith and E.M. Harrison

 

Contents                                                                                                       Page No.

 

Section 1			1
Chapter 1 : Introduction			1
1.1	Structure and Properties of Diamond		1
1.2	Synthesis of Diamond		4
1.3	HPHT synthesis of Diamond		5
1.4	Diamond synthesis by Chemical Vapour Deposition (CVD)		5
1.5	Gas-phase chemistry		8
	1.5.1  Atomic hydrogen production		8
	1.5.2  Atomic hydrogen loss mechanisms		10
	1.5.3  Hydrocarbon Gas-phase Chemistry		12
	1.5.4  Influence of trace non-hydrocarbon additions on gas-phase chemistry		18
1.6	Surface Growth of Diamond		19
1.7	Applications of Synthetic Diamond		23
	1.7.1  Cutting and Grinding Tools		24
	1.7.2  Optical windows		25
	1.7.3  Thermal Management		25
	1.7.4  Surface Acoustic Wave (SAW) devices		26
	1.7.5  Detector devices		27
	1.7.6  CVD diamond sensors and electronic devices		28
	1.7.7  Diamond cold cathode emission devices		29
	1.7.8  Other applications		30
1.8	CVD synthesis methods		31
	1.8.1  Hot filament CVD system		31
	1.8.2  Microwave-plasma assisted CVD system		36
	1.8.3  Oxyacetylene Torch CVD		37
	1.8.4  Plasma-jet CVD system		39
References			52
Section 2			57
Chapter 2 : Experimental and data analysis of HF-CVD studies			57
2.1		Reactor Considerations	58
2.2		Laser System	63
2.3		REMPI Spectroscopy	65
		2.3.1  REMPI study of Atomic Hydrogen	67
		2.3.2  H atom Doppler Profiles	69
2.4		REMPI Studies of Methyl Radicals	73
2.5		Filament Carburisation	73
References			75
Chapter 3 : REMPI studies of H atoms and CH3 radical species in a HF-CVD reactor			76
3.1		H atom detection	76
3.2		Methyl Radical detection	81
3.3		C2H2 gas-phase chemistry	89
3.4		Gas phase simulations	92
3.5		3-D calculations of CH4/H2 and C2H2/H2 chemistry	93
3.6		Conclusions	100
References			100
Chapter 4 : Experimental and Theoretical studies of a nitrogen containing HF-CVD gas-phase environment			102
4.1		Experimental Set-up	103
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
4.5		NH radical number densities and spatial profiles in 1%CH4/x%NH3/H2 gas mixtures.	116
4.6		Conclusions	118
References			118
Section 3			120
Chapter 5 : Experimental set-up, data analysis and growth characteristics of the DC-arcjet system			120
5.1		Reactor design	121
		5.1.1  Torch-Head Arrangement	121
		5.1.2  Reactor considerations	126
5.2		Growth Studies	131
		5.2.1  Influence of methane addition	132
		5.2.2  Influence of substrate temperature	135
		5.2.3  Chamber Pressure	138
		5.2.4  Discharge Power	139
5.3		Experimental Set-up for OES collection	139
5.4		Experimental set-up required for CRDS study	143
References			145
Chapter 6  : Nitrogen addition to a DC-arcjet reactor			146
6.1		OES Measurements	146
		6.1.1  The C2 molecule	150
		6.1.2  Spatially resolved OES measurements	151
		6.1.3  The Abel Transform	152
6.2		Film Deposition	157
		6.2.1  SEM analysis	158
		6.2.2  Laser Raman analysis	160
		6.2.3  Photoluminescence (PL) spectroscopy	163
		6.2.4  Secondary-Ion Mass Spectroscopy (SIMS)	164
6.3		Nitrogen-enhanced Growth Scheme	166
6.4		Conclusions	168
References			169
Chapter 7 : Cavity Ring Down Spectroscopy studies of C2 in a DC-arcjet reactor			170
7.1		Experimental Set-up	171
7.2		CRDS of C2	173
		7.2.1  C2 rotational temperature determination	176
		7.2.2  Determination of C2 absolute column densities	179
		7.2.3  CRDS measured C2(a) column densities as a function of process conditions	181
7.3		C2 Gas-phase chemistry	184
7.4		C2 as a growth species	185
7.5		Conclusions	186
References			187
Appendices
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