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ScienceWeek
MATERIALS SCIENCE: DIAMOND WAFERS
The following points are made by S.T. Lee and Y. Lifshitz (Nature 2003 424:500):
1) Diamond is the king of gemstones. Less well known is that it could also be an outstanding semiconductor material, superior in many ways to silicon, which is currently the most widely used electronic material. Diamond devices could operate at higher temperatures (more than 400 °C) and higher power than those of silicon, as well as being faster, denser and more resistant to radiation. But practical diamond electronics will need large-area, single-crystal diamond wafers to be fabricated, analogous to the 6–12-inch silicon wafers commonly used in the semiconductor industry. Recent research suggests that this may be possible if sapphire wafers are used as substrates on which to grow the diamond.
2) Diamond can be grown on diamond ("homoepitaxy") by chemical vapour deposition: a diamond substrate, at a temperature of 600–800 °C, is exposed to an ionized mixture of roughly 1% hydrocarbon and 99% hydrogen. Electronics-grade diamond has been made in this way. The newly grown diamond wafer can be cut from the diamond substrate, and the process can be repeated many times, reusing the substrate. But single-crystal diamond substrates are small and expensive, so this is not a viable way to fabricate large-diameter diamond wafers.
3) The alternative is to grow diamond on a foreign (non-diamond) single-crystal wafer -- this is "heteroepitaxy", the oriented growth of one crystal on another. Heteroepitaxy is readily achieved if the atomic spacings in the foreign substrate match those in diamond crystals. Of the various substrates tested --such as silicon, silicon carbide, nickel, cubic boron nitride and platinum -- iridium is the best found so far. The quality of epitaxial growth is measured by the average angular spread of the diamond-crystal orientation along a certain direction (called "mosaicity"). The minimum angle achieved is 3.9 degrees for silicon, less than 2 degrees for platinum, 1.5 degrees for silicon carbide and a mere 0.38 degrees for iridium. The catch is that a single-crystal iridium layer has never been grown as a free-standing wafer: it requires its own single-crystal substrate. Diamond has been grown successfully on iridium that was itself grown on substrates of MgO or SrTiO3. But once again there is a size limitation, as MgO and SrTiO3 wafers can be made only 2 to 3 inches in diameter.
Nature http://www.nature.com/nature
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ON THE NATURAL OCCURRENCES OF DIAMOND
In general, in this context, the term "metamorphism" refers to the mineralogical and structural changes of solid rock in response to environmental conditions at depth in the Earth's *crust. Diamond is a good example of the metamorphic formation of a rock crystal. The most common naturally occurring forms of pure carbon are graphite and diamond. If diamond is heated to approximately 1200 degrees centigrade, it slowly transforms to graphite, the rate of transformation increasing with temperature. At 2000 degrees centigrade the transformation is extremely rapid and the crystal shatters into a heap of graphite powder. It has been found, however, that diamond can survive these high temperatures if it is subjected to a pressure of several thousand atmospheres. Conversely, diamond can be produced by heating graphite to approximately 2500 degrees centigrade under a pressure in excess of 100,000 atmospheres, and it is this process that is responsible for the production of diamonds in nature. Before the middle of the 19th century, virtually all diamonds were found in or near surface streams, and most of these diamonds were discovered either in India or Brazil. In the year 1866, the children of a Boer farmer, Daniel Jakobs, while playing on the bank of the Orange River at Hopetown near Kimberley in South Africa, found a diamond weighing 21.25 *carats (slightly more than 4 grams). Subsequent exploration in the area demonstrated that the diamond-bearing deposit was a roughly circular region a few hundred meters in diameter, effectively a cylindrical "pipe" that had been pushed upwards from a region deep in the Earth's crust. Such a geological feature is now known as a "Kimberlite pipe". At present, the main source of diamonds is the *igneous rock "kimberlite", in which diamonds are found as scattered crystals. Kimberlite pipes are believed to be the remains of magma plugs that have been forced up to the Earth's surface, and there are known Kimberlite pipes with diameters up to a kilometer. There is evidence that in some cases the magma plugs may derive from *mantle plumes. The largest gem quality diamond ever found was the 621-gram (3106 carats) Cullinan crystal discovered in 1905. Diamonds have also been found in meteorites as microcrystalline clumps up to approximately a millimeter in size, the meteoritic diamonds evidently formed from graphite nodules as *iron meteorites were subjected to intense high pressures by the shock of impact on the surface of the Earth (shock metamorphism). Concerning the use of diamond in industry, diamond is the hardest substance available. In addition, diamond has the highest thermal conductivity of any known substance (five times the thermal conductivity of copper at room temperature), which means that a diamond cutting tool transfers heat quickly and does not become hot while in use.
The following points are made by Stephen E. Haggerty (Science 1999 285:851):
1) Carbon is the fourth most abundant element in the Solar System after hydrogen, helium, and oxygen. As a nucleosynthetic bridge to the heavy elements, to stellar evolution, and to biosynthesis, carbon holds an interesting position in the periodic system of elements. Diamond is pure carbon and is an impervious time capsule: Some diamonds are pre-Solar and have recorded such extraordinary astrophysical events as *supernovae explosions; other diamonds bear witness to Solar System formation; and diamonds from our planet are a window to the geodynamic evolution of Earth's deep interior.
2) The study of diamond and its various crystal forms (polymorphs) (e.g., *lonsdaleite, *fullerene, and graphite) has seen a recent burst of activity in geochemistry and geophysics, in novel methods of synthesis, and in the development of useful applications that take advantage of its covalent bonding, clarity, extreme hardness, high thermal conductivity, and high electrical resistance.
3) Diamond is now recognized as an extraordinary recorder of astrophysical and geodynamic events that extend from the far reaches of space to Earth's deep interior. Many diamonds are natural antiques that formed a) in presolar supernovae by carbon vapor deposition, b) in asteroidal impacts and meteorite craters by shock metamorphism, and c) from fluids and melts in Earth's mantle 1 to 2 billion years after *planetary accretion. The carbon in diamond is primordial, but there are unexplained isotopic fractionations and uncertainties in heterogeneity.
Science http://www.sciencemag.org
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Notes:
crust: The crust is the outermost of the 3 major layers of the planet (core, mantle, crust), its depth varying from approximately 5 kilometers to as much as 60 to 80 kilometers.
carats: 1 carat = 0.2 grams
igneous rock: Igneous rocks are rocks that have congealed from a molten mass.
mantle plumes: Mantle plumes are thin vertical conduits of molten rock material from the core-mantle boundary to the crust. Seismic studies indicate the interior of the Earth consists of three parts: a metallic core, a dense rocky mantle, and a thin low-density crust. The central part of the core is solid, but the outer part of the core is evidently liquid. The mantle, the layer of dense rock and metal oxides between the molten part of the core and the surface, has plastic properties (i.e., it is a solid capable of flow under pressure).
iron meteorites: Meteorites composed mostly of iron, with less than 30 percent nickel, and with only a small proportion of silicate minerals. They form only a few percent of meteorite falls.
supernovae explosions: Supernovae are stellar explosions in which virtually an entire star is disrupted. The estimate is that in our own Galaxy approximately 1 supernova occurs every 30 years, with most of the supernovae obscured by galactic dust.
lonsdaleite: A form of carbon found in meteorites. It can also be formed synthetically by ballistically accelerating metal projectiles into carbon targets.
fullerene: Fullerenes are large molecules composed entirely of carbon, with the chemical formula C(n), where n is any even number from 32 to over 100. They apparently have the structure of a hollow spheroidal cage with a surface network of carbon atoms connected in hexagonal and pentagonal rings.
planetary accretion: Planetesimals, present in the vicinity of a young star, are bodies with dimensions of 10^(-3) to 10^(3) meters that are believed to form planets by a process of accretion. The term "accretion" refers to an aggregation, an increase in the mass of a body by the addition of smaller bodies that collide and adhere to it, provided the relative velocities are low enough for coalescence.
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