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

金刚石薄膜具有许多吸引人的特性：它们坚硬、耐磨、摩擦力低、导热性高、耐电并且在非常宽的频率范围内（中红外直到接近真空紫外区的开始）光学透明。电子容易从金刚石表面发射，这表明可能用于冷阴极发射器件，例如超快开关或显示器，或用于太阳能电池器件。钻石中的有色缺陷充当单光子源，从而使钻石成为量子计算设备的现实候选者。生长技术的最新进展已经允许商业上可获得面积约为8 mm x 8 mm的单晶金刚石（Element Six），以及尺寸接近1 cm的单晶金刚石“宝石”（卡内基研究所，华盛顿）。因此，金刚石薄膜（在其许多变体中）越来越多地在电子、光学和工程领域中得到应用，预计将有数百万磅市场。有些令人惊讶的是，在金刚石CVD首次演示20多年后，生长机制的细节仍然存在争议。20世纪90年代早期开发的“标准”生长机制提供了对烃/H2气体混合物的一般CVD工艺的稳健描述。然而，这种机制无法可靠地预测生长速率，也无法解释为什么某些生长条件会产生单晶金刚石，而其他生长条件会产生纳米金刚石。建模（莫斯科）和实验（布里斯托）之间的协同作用，最近使我们能够改善标准金刚石生长机制的模型。该增强的生长模型仅需要描述CVD工艺条件和腔室几何形状的值作为输入，以实现生长速率和近似微晶尺寸的准确预测不幸的是，应用的竞争已经超过了对CVD期间在气体-表面界面处发生的化学和物理过程的基本理解的发展。因此，金刚石CVD的许多方面在很大程度上仍然是经验性的。为新的应用在纳米尺度上制造具有定制特性的金刚石的技术挑战意味着我们现在需要进一步了解基本过程。该提案的主要目的是提高对支撑各种类型金刚石膜沉积的基本气相和气体表面化学的理解，从而能够生长具有针对特定应用优化的特性的膜。我们计划通过2个互补的工作包，由2名博士生进行研究这些基本过程：（i）实验包，包括测量气相化学在金刚石生长过程中使用分子束质谱（MBMS）和激光光谱学方法在布里斯托开发; ㈡一套理论材料，包括从头计算技术来识别和研究潜在的气体-表面反应，然后将其构建为金刚石生长的更通用的蒙特卡罗模型。这两个包将具有协同作用，因为来自实验包的测量值将被馈送到理论包中并用于拉伸理论包中的生长模型，虽然从模型中收集到的对表面过程的更多理解将确保我们调整实验以测量重要的物种，让我们能够提出正确的问题。更好地了解这些基本过程将使我们能够优化生长条件。这将最终导致以高速率生长的大面积高质量单晶金刚石的常规生产。这将有助于将金刚石基材的成本降低到经济上可行的水平，使科学家和工程师最终能够将金刚石用作真正的世纪工程材料。

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.