  The objective of this study is to explore conditions that facilitate a
significant reduction in substrate temperature during diamond growth. The
typical temperature for this process is around 1200K; we aim to reduce it to a
much lower level. To achieve this, we need to understand processes that limit
the diamond growth at low temperatures. Therefore, we developed a detailed
chemical kinetic model to analyze diamond growth on the (100) surface. This
model accounts for variations in substrate temperature and gas composition.
Using an ab initio quantum chemistry, we calculated the reaction rates of all
major gas phase reactants with the diamond surface, totaling 91 elemental
surface reactions. Consistent with previous studies, the model identifies that
CH3 is a major precursor of diamond growth, and the contribution from C2H2 to
the growth is significantly smaller. However, C2H2 can also contribute to
forming a sp2-phase instead of a sp3-phase, and this process becomes dominant
below a critical temperature. As a result, C2H2 flux inhibits diamond growth at
low temperatures. To quantify this deleterious process, we developed a new
mechanism for sp2-phase nucleation on the (100) surface. Similar to the
so-called HACA mechanism for soot formation it involves hydrogen abstraction
and C2H2 addition. Consequently, optimal low-temperature CVD growth could be
realized in a reactor designed to maximize the CH3 radical production, while
minimizing the generation of C2H2 and other sp and sp2 hydrocarbons.
