Synthetic Diamond

Everybody knows that diamonds are dazzling to look at and harder than any other mineral. But its crystalline structure endows diamond with other extreme physical properties. It is one of the best conductors of heat and one of the best electrical insulators. This unique combination makes diamond an ideal material for a range of uses from cooling supercomputers to detecting ultraviolet radiation, according to Alan Collins a physicist from King’s College London.

Chemically, diamond is far from exotic. It is nothing more than crystals of carbon, itself the main constituent of soot. Natural diamonds, formed under extremes of temperature and pressure, are found in deposits of kimberlite- a rare volcanic rock. Synthetic diamond can now be produced at low pressures by a process called chemical vapour decomposition.

In CVD a mixture of hydrogen and methane (a compound of carbon and hydrogen) is heated by microwaves and blown over a layer of silicon. The temperature is high enough to split the methane so that carbon is deposited onto the silicon substrate. Only a tiny fraction of the deposited carbon is diamond, but the non-diamond carbon reacts with the hydrogen in the hot gas and is etched away. The result is that tiny diamond crystals form and grow, coalescing to give a continuous sheet.

Varying the conditions of the CVD process changes the rate of growth and the quality of the diamond. James Adair and Rajiv Singh of the University of Florida at Gainesville recently developed a new process to cover the substrate with tiny diamond crystals and grew the largest ever synthetic diamond, 30 centimetres in diameter and 1.5 mm thick.

Diamond owes its special properties to its crystalline structure. The atoms in a crystal are bound together by sharing electrons with each other in a sort of atomic handshake. Each atom becomes stable when it shares enough extra electrons to fill its outermost shell- all its chemically reactive hands are now full and are no longer available to form new chemical links. Carbon has four electrons in its outermost shell and space for another four, so diamond crystals have a repeating tetrahedral structure- each atom of carbon holds hands with four neighbours.

Carbon is the smallest and lightest atom with a four-electron outer shell. So its “arms” are very short, very strong and vibrate at very high frequencies. This is what makes diamond so tough, and such a good conductor of heat- about five times better than copper at room temperature. Its high electrical resistance comes about because the electrons are held so tightly in “handshakes” that it requires a great deal of energy to liberate one to make it carry electric current.

The high thermal conductivity of diamond is put to use in the electronics industry. Components that produce large amounts of heat, like Gunn [correct] diodes used to generate microwaves, are bonded to a heat spreader made from a tiny diamond wafer. The spreader conducts the heat away to a more massive “heat sink” which may itself be water cooled.

The miniaturisation and increased density of electronic components, which brings the increases in speed and power demanded by the supercomputer industry, aggravates the heat dissipation problem. Speculative designs for future supercomputers contain cubic stacks of layers of chips mounted on diamond heat spreaders and surrounded by water-cooled heat sinks. Diamond is not yet cheap enough for use on this scale.

The stability of electrons also accounts for the transparency of diamond. Light is absorbed by exciting electrons so that they jump from one state into another. Photons of visible and infrared light do not contain enough energy to move an electron out of a bond, so diamond does not absorb these wavelengths.

The much higher energy photons of ultraviolet light are absorbed, making diamond an excellent material for building detectors of these wavelengths. One potential application for diamond uv-detectors, blind as they are to visible and infra-red radiation, would be machines for automatically reading sequences of genes, according to Collins.

The ultimate use of synthetic diamond, according to Trevor Evans, of Reading University, would be as a substitute for silicon to make semiconductors that would work in extreme conditions of temperature and radiation. Silicon, like carbon, is group 4, it has 4-electrons in its outer shell. The essential materials for making semiconductor devices are “n-type” silicon, where some of the silicon atoms are replaced by a group 5 atom like phosphorus with an extra electron in its outer shell, and “p-type” silicon, where the replacement is a group 3 atom, with one electron less.

Unfortunately diamond is much less receptive than silicon to such substitutions. Boron fits into the carbon lattice to make p-type semiconducting diamond, but the only group 5 element that fits in is nitrogen, which does not produce the expected semiconducting behaviour “Sometimes the elementary semiconductor physics is not adequate to account for the properties of impurity atoms” says Collins.

Although diamond transistors have been produced by Kobe Steel in the US and by Daimler Benz in Germany, they do not work very well. “The fact that you can only get P-type diamond is very limiting” says Collins. So it looks as it it will be some time before the ultimate fashion accessory, a diamond transistor radio, hits the streets.