Chemical Vapour Deposition Diamonds (CVD Diamonds)

Chemical Vapour Deposition Diamonds

Researchers have found a way to grow diamond at a pressure of less than 1 atmosphere by a process of CVD (Chemical Vapour Deposition). In this procedure, a carbon-containing gas, usually methane, mixed with a large amount of hydrogen, is passed into a quartz tube and a pump maintains the pressure at about 5% of atmospheric pressure. Microwave power fed into the tube creates a plasma and also heats a substrate to approximately 800°C. The carbon is subsequently deposited on the substrate, mostly as graphite, but some carbon atoms bond together in the diamond crystal structure. The atomic hydrogen in the plasma removes graphite very readily but does not react with the diamond. The net result is that a thin film of diamond remains on the substrate.

The diamond films were generally grown on substrates such as flat sheets of silicon or molybdenum and were polycrystalline. The brown colour of early films is generally attributed to non-diamond carbon at the grain boundaries. As research in this area continued, ways to speed up the growth rate and to minimize the non-diamond carbon content of the CVD layers were developed, and De Beers demonstrated the ability to produce transparent wafers of CVD diamond more than 100 mm in diameter and with thicknesses of at least 630 mm. This material was suitable for a number of industrial applications, but being polycrystalline, had a hazy transparency, because of light-scattering at the grain boundaries, and so was not suitable for the production of diamond gemstones.

The most recent research into the growth of CVD diamond has been concerned with thick layers deposited on a diamond substrate. If carefully controlled, this homo-epitaxial process can produce single-crystal specimens of CVD diamond several millimetres thick. Using high-purity gases and a well-designed growth chamber, it is possible to grow diamonds with no obvious colour and with electronic properties that exceed those of the best natural diamonds. However, the addition of nitrogen to the growth chamber results in CVD diamond that has a brown colour and, for diamonds of the same thickness, the saturation of the brown colour is correlated with the concentration of the nitrogen in the process gases. This brown colour may be due to the presence of non-diamond carbon, and in some specimens can be removed by HPHT annealing. If, instead of nitrogen, a small concentration of a boron-containing gas (diborane B2H6) is added to the gas stream, the resulting CVD material is blue in colour.

Some material is lost when the CVD layer is separated from the diamond substrate, but the thickest layers have allowed round brilliants up to 0.82 ct to be fashioned. However, most CVD diamond gemstones are prepared using rather flat cuts to make better use of the full area of the material.

At present, there is no commercial production of CVD diamond, but it is believed that with the right know-how and an adequate investment, it would be possible to produce CVD diamond gemstones on a commercial basis.