It has been observed that the presence of a dopant (N, B, S or P) largely affect the diamond chemical vapor deposition process (CVD) (increase of growth rate, preferential surface plane etc). A small number of theoretical publications have focused on this type of phenomenon but these studies are always centered on a specific reaction pathway and for one specific surface orientation (100). Important changes in the mechanism have been observed, but many questions are still not answered.
The purpose of the present study is to take a step further by theoretically analyzing the effect of various dopants on the CVD growth process, and their influence on the surface orientation. The investigation is based on an ultrasoft pseudopotential density functional theory (DFT) method, using generalized gradient approximation
(GGA) under periodic boundary conditions.
Within the present project, the first step has been to study the adsorption processes of CH3on H-terminated (111) and (100) diamond surfaces. The effect of various impurities AHX(A = N, B, S, P or C; X = 0 or 1 for S; X= 1 or 2 for N, B and P and X= 2 or 3 for C), co-adsorbed on a neighboring surface C site, were then especially focused. In order to highlight the different structural and electronic effects induced by the dopants, similar calculations have been performed with H or CH3(including the radical form CH2) instead of the dopant. This study is completed by the analysis of the dopant effect on another important reaction in the CVD diamond growth: the H abstraction.
As a result for both type of surfaces, it has been found that the presence of a dopant tends to disfavor CH3adsorption. In the presence of NH2, PH2, SH and CH3as dopants, this effect is small since only sterical repulsions affect the adsorption process. BH2, on the (100) surface, presents a particular interaction with the surface radical which disfavor the adsorption reaction. When PH, S, BH or CH2is present on the neighboring surface site, the formation of a strong covalent bond with the carbon radical (prior to adsorption) disfavors strongly the reaction. This phenomenon is not observed for NH
Calculations have shown that the H abstraction processes is only favored when the surface H can recombine itself with another gaseous H radical and that the energy gain during the reaction depends on the type of broken bond (P-H, N-H, S-H or B-H). Of course, the formation of new interactions (or bonds) after the abstraction process have important effects on the reaction energy.
This rather broad analysis over the effects of different dopants on some crucial part of the CVD diamond growth process, gives a first important approach to the dopant effects. This study covers the thermodynamic aspects of the problem, and it gives some clues in how to proceed to get also kinetic information (transition states determination, kinetic analysis, migration process ...), which will be included in next-coming investigations.