Quantum Chemistry Model of Surface Reactions and Kinetic Model of Diamond Growth: Effects of CH3 Radicals and C2H2 Molecules at Low-Temperatures CVD

TL;DR

This study develops a quantum chemistry-based kinetic model to understand and optimize low-temperature diamond growth in CVD processes, highlighting the roles of CH3 radicals and C2H2 molecules.

Contribution

It introduces a detailed chemical kinetic model incorporating ab initio reaction rates and a new sp2-phase nucleation mechanism to improve low-temperature diamond growth understanding.

Findings

CH3 is the primary precursor for diamond growth.

C2H2 inhibits diamond growth at low temperatures.

A new sp2-phase nucleation mechanism involving C2H2 was proposed.

Abstract

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.