Importance of Interfacial Structures in the Catalytic Effect of Transition Metals on Diamond Growth.

Here, using ab initio calculations, we investigated the interaction between transition metals (M) and diamond C(111) surfaces. As a physical parameter describing the catalytic effect of a transition metal on diamond growth, we considered interfacial energy difference, ΔE int, between 1 × 1 and 2 × 1 models of M/C(111). The results showed that the transition-metal elements in the middle of the periodic table (groups 4-10) favor a 1 × 1 M/C(111) structure with diamond bulk-like interfaces, while the elements at the sides of the periodic table (groups 3, 11, and 12) favor a 2 × 1 M/C(111) structure with the 2 × 1 Pandey chain structure of C(111) underneath M. In addition, calculations of MC carbide formation for early transition metals (groups 3-6) showed that they have a tendency to form MC rather than M/C(111), which explains their low efficiency as catalysts for diamond growth. Further analysis suggests that ΔE int could serve as another parameter (catalytic descriptor) for describing catalytic diamond growth in addition to the conventional parameter of the melting temperature of M.

Unusual Dependence of the Diamond Growth Rate on the Methane Concentration in the Hot Filament Chemical Vapor Deposition Process.

Although the growth rate of diamond increased with increasing methane concentration at the filament temperature of 2100 °C during a hot filament chemical vapor deposition (HFCVD), it decreased with increasing methane concentration from 1% CH4 -99% H2 to 3% CH4 -97% H2 at 1900 °C. We investigated this unusual dependence of the growth rate on the methane concentration, which might give insight into the growth mechanism of a diamond. One possibility would be that the high methane concentration increases the non-diamond phase, which is then etched faster by atomic hydrogen, resulting in a decrease in the growth rate with increasing methane concentration. At 3% CH4 -97% H2, the graphite was coated on the hot filament both at 1900 °C and 2100 °C. The graphite coating on the filament decreased the number of electrons emitted from the hot filament. The electron emission at 3% CH4 -97% H2 was 13 times less than that at 1% CH4 -99% H2 at the filament temperature of 1900 °C. The lower number of electrons at 3% CH4 -97% H2 was attributed to the formation of the non-diamond phase, which etched faster than diamond, resulting in a lower growth rate.