Análisis económico de la implementación de SAIR

Unlike HPHT growth, which emulates natural growth conditions, CVD growth of diamond occurs outside the diamond-stable region of the carbon phase-space (figure 2-1), and relies on chemical kinetics rather than thermodynamics during growth [36].

During modern CVD growth, carbon is deposited epitaxially onto a substrate

directly from the gas phase. Atomic hydrogen at the surface promotessp3 _diamond

growth by etching sp2 _{carbon and hence preventing growth of graphitic carbon}

[36]. Activation of the reactants (primarily by dissociation of molecular hydrogen to atomic hydrogen [37]) may be achieved by several methods including direct hot- filament heating [38] and microwave plasma heating [39]. Hot-filament approaches are prone to contamination from the filament material itself (e.g. tungsten), and so the highest-purity diamond is grown by microwave plasma CVD (MPCVD) [37].

MPCVD growth operates within a very large parameter space: source gasses, tem- peratures, plasma shape and power density, dopant gasses, substrate material etc.

are all potential variables. Typically, input gasses are H2-CH4 ( H2 Q CH4)

and growth is operated at several different temperature and plasma power density

regimes, usually at a microwave frequency of 2.45 GHz [40, 41]. MPCVD growth

rates may range over 1–150µm h1 _{[42], with higher growth rates tending to pro-}

duce lower film quality [43–46]. Modern reviews of MPCVD growth are available in [37, 47].

As with HPHT-grown diamond, CVD diamond can be grown in both polycrys- talline and single crystalline forms. Where non-diamond substrates are used, the final material is usually polycrystalline due to a combination of lattice constant mismatch and simultaneous nucleation at multiple sites on the surface. Competi- tion from the initially high number of differently-oriented grains (with nucleation

densities of up to 1012_cm2 _{[48]) acts to reduce the total number of grains as the} film grows, leading to macroscopic differences between the early- and late-growth material [49].

Single crystal CVD-grown diamond can be grown on diamond substrates (ho- moepitaxial growth), or carefully prepared multilayer non-diamond structures (heteroepitaxial growth) [50]: using the latter method, single crystal wafers with diameters up to 100 mm have been produced [51].

In addition to the surface / bulk structure, polycrystalline and single crystal ma- terial possess different optical, electrical and mechanical properties, and hence are utilised in different application areas; [4] gives a modern overview of the ap- plications of CVD diamond. Only single crystal diamond is investigated in this thesis.

2.1.2.1 Characteristics of CVD-grown diamond

The material and orientation used for the substrate has significant effects on the final CVD-grown material. Any crystallographic defects present at the substrate surface pre-growth will propagate through the CVD sample as it grows, leading to high dislocation densities [52–54]. Chemical etching of the substrate to remove e.g.

polishing damage has been successful in reducing dislocation densities [55]. ˜1 0 0

substrate orientations currently allow the highest-quality single crystal growth, with low-strain HPHT substrates employed in optical applications to minimise birefringence [56, 57].

Nitrogen is often incorporated as a dominant impurity in CVD-grown material, as with HPHT-grown and natural diamond. Whilst the incorporation efficiency (probability of being incorporated onto the surface relative to that of a carbon)

is low, at approximately 105_–104 _{[58, 59], it is difficult to exclude all sources of}

nitrogen from the growth atmosphere. Small quantities of nitrogen in the chamber (up to 100 ppm) are found to modify the growth morphology of the sample [58, 60– 62], in addition to significant increases in growth rate by a factor of 2.5 [59]. High concentrations (up to 2 % number density in the gas phase) have a detrimental

effect on film quality [63]. Nitrogen concentrations as low as 0.2 ppb have been observed in CVD-grown material [59].

Boron-doped CVD diamond may exhibit semiconducting, metallic, or even super- conducting behaviour [64, 65]. Its incorporation efficiency is much higher than

nitrogen, with efficiencies as high as 102_–101 _{[66]. As with nitrogen, the incor-}

poration of boron modifies the morphology of the sample during growth [43, 61, 67, 68].

Etching of silicon containing materials inside the reactor — windows, components, potentially the substrate itself — can lead to incorporation of silicon into the

growing diamond [69]. In turn, silicon-related defects such as ˆSiV may then be

frozen-in during growth [70, 71], presumably primarily as substitutional silicon (which has yet to be identified).

Hydrogen, the dominant element in the gas phase by number density, is also incorporated into the sample during growth at levels of up to 1 atomic percent in polycrystalline material [72]. A number of hydrogen-related defects have been identified by electron paramagnetic resonance (EPR) and infrared (IR) absorption studies.

2.2

Irradiation damage in diamond

Irradiation and subsequent characterisation has long been employed in the in- vestigation of semiconductors. Fundamental study of the irradiation-introduced damage, both intrinsic and extrinsic, can provide great insight into the physical and chemical processes occurring inside the crystal.

Scientific interest in the irradiation of diamond was first reported in 1904, with radium salts as the radiation source [73]. Increasing radiation doses were observed to impart a strong green colour to the samples. The seminal work of Clark et al. in the mid-1950s was the first systematic spectroscopic investigation into irradiation damage (and subsequent annealing) in diamond [74, 75].

particles, a constituent atom may be knocked off its lattice site if the incident particle transfers kinetic energy to the atom of greater than the threshold energy

Td (see§4.5). The exact details of the remaining damage post-irradiation depends

upon the conditions under which the irradiation is performed (a discussion of

irradiation conditions is given in §5.1.1), but generally irradiation will introduce

both interstitials and vacancies into the lattice [76].

Irradiation damage defects can introduce new mid-gap levels as well as increase disorder in the crystal: it therefore has a profound effect on the optical and elec- tronic properties of a given sample [74, 75, 77–81].