Table 1

Johnson’s figure of merit for a transistor’s power and frequency performance for selected semiconductor materials.

Table 2

Keyes’ figure of merit for the speed of a transistor for selected semiconductors.

2.1 Background

The free energy difference between metastable diamond and stable graphite is only 453 cal/mol [3] at 1 atm. Thus the rarity of diamond is surprising when one considers that this is less than RT (≈600cal/mol) at room temperature. There are several reasons for the uncommon occurrence of diamond. Firstly, the kinetics of graphite formation are considerably faster than those of diamond, as would be predicted by the large activation energy barrier between these two materials [4]. Secondly, the entropy of diamond is less than the entropy of graphite by 0.78 cal/K mol [3]. Finally, a large activation energy barrier exists between graphite and diamond which prevents even small particles from forming at room temperature under equilibrium conditions. The density of diamond (3.51 g/cm3) is significantly greater than the density of graphite (2.25 g/cm3); thus, the former becomes the equilibrium form of C only at high pressure.

The breakthrough in the low-pressure synthesis of diamond was mainly a result of research by Soviet scientists in the late 1970s and early 1980s which indicated that gas activation techniques could greatly increase the growth rate of diamond while suppressing the graphite deposition. The results of these efforts clearly demonstrated that a concentration of atomic hydrogen, exceeding that of the equilibrium concentration associated with the thermal disassociation of the reactant hydrocarbon gas(es), was necessary to achieve diamond growth [5]. The flow of the hydrocarbon gas and hydrogen could now be simultaneous. In 1977, Derjaguin and Fedoseev [6] described three approaches to produce higher concentrations of atomic hydrogen than obtained previously: catalytic, electrical discharge and via heated tungsten filament. The significant results of this research were not published in English until 1981 [7] with additional details in 1984 [8]. This was followed by intensive efforts throughout the 1980s at the National Institute for Research in Inorganic Materials in Japan. This research quickly resulted in numerous publications [9–15] containing scanning electron micrographs of highly faceted, thick continuous films, confirming electron diffraction results and a host of spectroscopic data which removed all doubt that diamond films containing very small amounts of graphite could be grown at reasonable rates under highly metastable conditions on a variety of substrates, including Si, SiC, WC, Ta, W, Si3N4, Cu, Ni, Mo, BN, SiO2, Ti, Hf, Nb and Al2O3. This work also resulted in several new techniques for gas activation and diamond deposition. These have been grouped into five major categories [16]: (i) thermally activated chemical vapor deposition (CVD); (ii) high-frequency plasma enhanced CVD; (iii) direct-current (DC) discharge assisted CVD; (iv) combustion flame; and (v) hybrids of these and others. Only (i), (ii) and (iii) are considered likely to result in films suitable for electronic applications. Common to all techniques is the production of a high supersaturation of atomic H with a supersaturation of carbonic species and a substrate temperature in the range of 975–1275 K. Other similarities include process parameters of gas pressure (∼0.1 atm), the percentage of hydrogen in the gas phase (∼95–99.9%) and the resulting film morphology.

2.2 Nucleation and growth