The introduction of impurities(N, B, S or P) into the diamond lattice is of a major interest due to large effects on the properties of diamond (e.g. transformation to a semiconductor material). In choosing a suitable dopant for diamond, one has not only to consider its effects on the diamonds properties, but also its solubility and mode of incorporation.i,iiIt is possible to introduce the impurity during chemical vapor deposition (CVD) of diamond, and it has been observed that the presence of the dopant will largely affect the growth process (increase of growth rate, preferential surface plane formation, etc).iii
Only a small number of theoretical publications has focused on this type of phenomenon,iv,v,vi,viiImportant changes in the reactions responsible for the growth process are frequently observed in the presence of an impurity. However, this effect still not fully understood. 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 the CH2and CH3 growth species on H-terminated
(111) and (100) diamond surfaces, respectively. The effect of various impurities AHX(A = N, B, S or P; X = 0 or 1 for S; X= 1 or 2 for N, B and P), co-adsorbed on a neighboring surface C site, were then especially focused. For the (100) surface, 3 different neighbouringpositions were investigated as dopant surface positions (compared to only one for the (111) surface). The adsorption of growth precursors to diamond surface sites with neighbouringdopant species, have all been compared with corresponding adsorption processes where no dopants are present on the surface.
As a result, it has been found that the adsorption of CH2to diamond
(111) surfaces is significantly favouredin the presence of NH, BH, S and PH (i.e. the radical forms of the impurities). The much stronger adsorption energies are induced by new bond formations between the growth species and co-adsorbed dopant one. On the contrary, the corresponding adsorption of CH3is less favourable in the presence of S, BH and PH. It was observed that these dopants tend to strongly stabilize the radical carbon surface site prior to any precursor adsorption.
Consequently, this stabilization will reduce the exothermicityof the following adsorption reaction. Similar dopant-surface site interactions were also observed for the (100) diamond surface, but the stabilization intensity varied with the position of the dopant.
The dopant effects on the adsorption reaction (being an initial step to diamond growth) are very important for the CVD process. But one has to consider also other elementary surface reactions (like migration and gas phase abstraction) before any more decisive conclusion can be drawn.
Those theoretical studies will be included in a next-coming series of investigations, out of which this presentation is the result from the first one.
i Larsson K. ; Comp. Mat. Sc. (2003) 27, 23-29 ii Kajihara S. A., Antonelli A., Bernholc J. ; Physica B (1993) 185,
iii Haubner R., Lux B. ; Diamond properties, growth and application
iv Frauenheim Th. et al. ; Diam. Relat. Mater. (1998) 7, 348-355 v Zhou H. et al. ; Jpn. J . Appl. Phys. (2001) 40, 2830-2832 vi Tamura H. et al. ; J. Chem. Phys. (2001) 115, 5284-5291 vii Cheesman A. et al. ; Phys. Chem. Chem. Phys. (2005) 7, 1121-1126