Experiment and modelling of the growth of CVD diamond: towards a detailed understanding of growth chemistry and mechanisms

Diamond films have many attractive properties: they are hard, wear resistant, offer low friction, have high thermal conductivity, are electrically resistant and optically transparent over an unusually wide frequency range (mid-IR up to approaching the onset for the vacuum UV region). Electrons are readily emitted from diamond surfaces suggesting possible uses in cold cathode emission devices, e.g. ultra-fast switches or displays, or in solar cell devices. Coloured defects in diamond act as single-photon sources, thereby making diamond a realistic candidate for quantum computing devices. Recent advances in growth technology have allowed single crystal diamond with areas around 8mm x 8mm to be commercially available (Element Six), as well as single crystal diamond 'gemstones' approaching 1cm in size (Carnegie Institute, Washington). Consequently, diamond films (in their many variants) are increasingly finding application in electronics, optics and in engineering, with multi-million pound markets predicted.Somewhat surprisingly, more than 20 years after the first demonstrations of diamond CVD, details of the growth mechanism remain controversial. The 'standard' growth mechanism developed in the early 1990s provides a robust description of the general CVD process from hydrocarbon/H2 gas mixtures. However this mechanism was unable to predict growth rates reliably, or to explain why certain growth conditions gave single crystal diamond whereas others gave nanodiamond. The synergy between modelling (Moscow) and experiment (Bristol) has recently allowed us to improve the model for the standard diamond growth mechanism. This enhanced growth model requires as inputs only values describing the CVD process conditions and chamber geometry to enable accurate predictions of growth rates and approximate crystallite sizes (mm, um or nm) and thus film morphologies from single crystal to nanocrystalline diamond.Unfortunately, the race for applications has overtaken development of fundamental understanding of the chemical and physical processes occurring at the gas-surface interface during CVD. Many aspects of diamond CVD thus remain largely empirical. The technical challenges of making diamond with tailored properties on the nano-scale for new applications means that we now need to take our understanding of the basic processes further. The primary aim of this proposal is an improved understanding of the fundamental gas phase and gas-surface chemistry underpinning the deposition of various types of diamond film, enabling growth of films with characteristics optimised for the particular application. We plan to study these fundamental processes via 2 complementary work packages performed by 2 PhD students: (i) An experimental package involving measurements of the gas phase chemistry during diamond growth using molecular beam mass spectrometric (MBMS) and laser spectroscopy methods developed at Bristol; (ii) A theoretical package, involving ab initio computational techniques to identify and study potential gas-surface reactions and then to build these into a more generalised Monte Carlo model for diamond growth.The two packages will be synergistic, in that measured values from the experimental package will be fed into and used to tension the growth model in the theoretical package, whilst the greater understanding of the surface processes gleaned from the modelling will ensure that we tune the experiments to measure the important species, and to enable us to ask the right questions. A better understanding of these fundamental processes will allow us to optimise the growth conditions. This should ultimately lead to the routine production of large area high quality single crystal diamond grown at high rates. This should help to bring the cost of diamond substrates down to economically viable levels, enabling scientists and engineers, at last, to use diamond as a true 21st century engineering material.

CVD diamond substrates and coatings find a wide variety of industrial applications, as hard wear-resistant coatings on mechanical components, and medical implants, as window materials and as a protective coating on other optics and IR transparent window materials, as heat spreaders, as cutting tool inserts, as high frequency audio speaker cones, etc. Depending upon their composition, doping level and structure, diamond films can exhibit a range of electrical conductivity, band gap and surface work function values. Thus they are candidates for use in electronic, micro-electromechanical systems (MEMS), and opto-electronic devices. One of the most widely investigated of these is the possibility of using these films as the cold cathode in electron field emission devices and displays - a potential market of enormous value. The thermionic emission properties of diamond have only just begun to be investigated. Early results from our group and others suggest that diamond may find uses in efficient solar cells for cheap, green energy production. The NV and NE8 colour centres in synthetic diamond have been found to act as single-photon sources, which makes doped diamond a realistic candidate for quantum computing devices. Despite this plethora of advanced applications, it is somewhat surprising that many of the fundamental chemical reactions which underpin CVD diamond growth are still unknown. It can be said that the development of applications has overtaken the science and understanding. If this situation continues, further progress in utilising CVD diamond in high tech products will be slowed due to this lack of fundamental knowledge. The proposed project seeks to redress this balance somewhat, and 'put the science back into the engineering'. The project is designed to be a synthesis between theoretical modelling of fundamental gas-surface reactions and careful experimental measurements. The anticipated results of this particular programme of work are expected to be largely fundamental in nature, and thus will be published in the scientific literature, presented at relevant national and international conferences and posted on the group web page. Existing links with Element Six Ltd will help ensure that we are asking the important questions, and that we are well placed to put the findings in context. A better understanding of the fundamental processes involved in diamond growth will allow us to optimise the growth conditions. This should ultimately lead to the routine production of large area single crystal diamond having low defect densities and impurity levels, grown at high rates with more efficient use of MW power; and therefore bring the cost of diamond substrates down to the levels which engineers will find economically viable.