Diamond Growth at Low Pressures
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ent from 250 nm in the ultraviolet to the infrared. When doped, it is a high band-gap semiconductor, structurally analogous to silicon. This unusual combination of extrême properties, and the ability to grow diamond as films at relatively moderate conditions, has made diamond a candidate for many applications. Potential uses for vapor-grown diamond include abrasives, bearihg and wear résistant surfaces, tool coatings, heat sinks, optical coatings, optical Windows, and as active electronic device éléments. Presently however, mostof thèse applications hâve not yet been realized and await further improvements in crystalline quality, surface smoothness, and in the économies of production. Current applications, actually in the market place, are relatively few and include instrument Windows and highfrequency speaker diaphragms. Vapor-
grown diamond abrasives and tool coatings are being introduced to the market and vapor-grown diamond heat sinks may appear shortly. There are several comprehensive reviews of the field with extensive bibliographies for the interested reader.3"7 The history of metastable diamond growth has been described in détail elsewhere8 and will not be repeated hère. Structure and Stability Diamond is the prototypical example of the diamond-cubic crystal structure, formed from two interpenetrating face centered cubic lattices of atoms displaced one quarter of the cube diagonal from one another. For understanding the chemical properties of diamond, other descriptions of the lattice are more useful. In diamond each carbon atom is tetrahedrally coordinated to four other carbon atoms through cr bonds arising from sp3 hybrid atomic orbitals. The lattice may be visualized as stacked layers of six-membered rings, with each ring in the "chair" conformation. Two principal types of layer stacking are possible. If the layers of chairs are joined by staggered carbon-carbon bonds, the sixmembered rings between the layers are also chairs. This structure, in which ail rings are chairs, is the diamond cubic lattice. The four directions in diamond are in the bond directions. The stacking séquence of layers in any direction is ABC/ABC/... The
Table I. Some Properties of Natural10 and Vapor-Grown Diamond Type HA Hardness, GPa Mass density, g/cm3 Molar density, g atom/cm3 Spécifie heat at 300 K, J/g Debye température, 273-1100 K Thermal conductivity at 298 K, w/cm K Bulk modulus, N/m2 Compressibility, cm2/kg Thermal expansion coefficient at 293 K, K-1 Refractive index at 589.29 nm Dielecfric constant at 300 K
Vapor-Grown
=90* 3.515 0.293* 6.195 1860 ±10 K
80-90 2.8-3.5 0.23-0.29
=20* 4.4-5.9 x 10"** 1.7 x 10"7**
10-20
0.8 x 10-6*** 2.41726 5.7 ±0.05
=2.4 =5.7
*higher than any other known material "lower than any other known material ***lower than Invar
MRS BULLETIN/OCTOBER1989
Diamond Growth at Low Pressures
Figure la. Diamond cubic lattice: layers of six-memberedringsin the chair conformation are separated by staggered bonds; the stacking séquence in each direction is ABC/ABC/... ; in cubic diamond ail bonds a
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