Bristol has long standing interests in the growth, characterisation and application of diamond formed by chemical vapour deposition (CVD) methods. Developing and applying new and improved methods of probing the activated gas mixtures used in diamond CVD is an important strand of this activity. Our recent and current work centres on industrially relevant microwave (MW) activated methane/hydrogen plasmas, and includes:
- Spatially resolved optical diagnosis of selected target species within these plasmas using cavity ring down spectroscopy (CRDS) and optical emission spectroscopy (OES), as functions of process conditions like the C/H ratio in the input gas feed, the gas flow rate and/or pressure, and the applied microwave power.
- Analogous spectroscopic studies exploring the effects of adding trace dopants (diborane, nitrogen, ammonia, etc), or changing the input gas mixture (e.g. to a CH4/CO2 mixture).
- Tensioning absolute column densities (returned by CRDS measurements) and relative species number densities (from the OES studies) with the results of companion 2-D plasma-chemical modelling conducted by Mankelevich and colleagues at Moscow State University.
- QM and QM/MM calculations of elementary reaction pathways by which the more abundant free radicals species (as identified by the above plasma diagnosis and modelling) might add to, migrate on, and be incorporated into, the growing diamond surface.
Careful analyses of spectra of radicals such as d3Πg–a3Πu band of C2 shown above yield measures of the local gas temperature and the column density of the monitored radical species.
Notable recent studies include
- Measurements of C2(a), CH(X) and H(n=2) column densities in MW activated CH4/H2 and C2H2/H2 gas mixtures, as functions of height above the substrate, which show the plasma composition to be very sensitive to the C/H ratio in the input gas mixture but, for a given C/H ratio, to be independent of the choice of hydrocarbon precursor.
- Detailed inter-comparisons between species concentration profiles established by CRDS measurements, by OES (with and without added Ar acting as an actinometer) and by 2-D plasma modelling. Such studies have served to test the validity of OES measurements (which are much easier to implement), to reveal variations in the electron temperature and/or concentration with changes in process conditions, and to reveal the possible contributions of chemiluminescent reactions to the measured optical emissions.
- Identification of factors affecting B and BH radical densities in MW activated B2H6/H2 and B2H6/CH4/H2 gas mixtures.
- Explorations of the ways in which the plasma chemistry and composition in MW activated CH4/CO2 gas mixtures depends on the C/O ratio in the input source gas.
- QM/MM calculations exploring (i) radical insertion into C–H bonds on a H-terminated 2´1 reconstructed (100) diamond surface, (ii) CH2 radical migration (post-incorporation as a CH3 radical and subsequent activation by an H abstraction reaction) on this same surface, and (iii) B atom and BH radical accommodation and migration on this same diamond surface.
Much our current effort is focussed on detailed diagnosis and modelling of N containing C/H plasmas and (joint with Prof. Gans and colleagues at the University of York) the development and application of pulse induced OES (PiOES) methods to provide new insights into the electron characteristics in MW activated plasmas.
Funding for this research is provided by Element Six Ltd and EPSRC.
Representative publications:
Optical emission from microwave activated C/H/O gas mixtures for diamond chemical vapour deposition, J.C. Richley, M.W. Kelly, M.N.R. Ashfold and Yu.A. Mankelevich, J. Phys. Chem. A 116, 9447-58 (2012).
Exploring the plasma chemistry in microwave chemical vapour deposition of diamond from C/H/O gas mixtures, M.W. Kelly, J.C. Richley, C.M. Western, M.N.R. Ashfold and Yu.A. Mankelevich, J. Phys. Chem. A 116, 9431-46 (2012).
Boron incorporation at a diamond surface: A QM/MM study of insertion and migration pathways during chemical vapour deposition, J.C. Richley, J.N. Harvey and M.N.R. Ashfold, J. Phys. Chem. C 116, 18300-7, (2012).
CH2 group migration between H-terminated 2×1 reconstructed {100} and {111} surfaces of diamond, J.C. Richley, J.N. Harvey and M.N.R. Ashfold, J. Phys. Chem. C 116, 7810-6 (2012).
Spectroscopic and modeling investigations of the gas phase chemistry and composition in microwave plasma activated B2H6/CH4/Ar/H2 gas mixtures, Jie Ma, J.C. Richley, D.R.W. Davies, M.N.R. Ashfold and Y.A. Mankelevich, J. Phys. Chem. A. 114, 10076-89 (2010).
Understanding the chemical vapour deposition of diamond: recent progress,J.E. Butler, Y.A. Mankelevich, A. Cheesman, Jie Ma and M.N.R. Ashfold, J. Phys.: Condens. Matter, 21, 364201 (2009).
