Kinetic considerations of gas-phase abstraction of H and F from the c-BN(100) surface

Abstract

The cubic phase of boron nitride (c-BN) is an extremely promising multifunctional material. To exploit all possible applications, large area chemical vapor deposition (CVD) of c-BN films is required. To maintain the cubic (sp3) structure of the surface atoms, the growing surface is covered with surface stabilizing species. However, the surface stabilizing species must also be able to undergo abstraction reactions with gaseous species and, hence, leave room for an incoming B- or N-containing growth species for a continuous c-BN growth to occur. The abstraction process is therefore a central elementary reaction step in CVD growth of c-BN. Hydrogen, H, and fluorine, F, have earlier been found to be promising as surface stabilizing species for both the B- and N-terminated c-BN(100) surfaces — the sp2 structure is maintained and both H and F bind strongly to the surface. In addition, of highest importance is the chemical capability to remove these terminating species from the surface, leaving a highly reactive surface site (i.e., gas-phase abstraction). The present study has therefore focused on, by using density functional theory (DFT), the kinetics of the H- or F-abstraction processes from the B- or N-terminated c-BN(100) surface.The energetic and structural evolution for gaseous F approaching a surface-binding F species, show that F radicals are not able to abstract chemisorbed F atoms, i.e., a gas-phase containing only F is unfavorable for growth of c-BN(100). On the other hand, H radicals are able to abstract chemisorbed H atoms. However, a minor barrier of energy was observed for the N-terminated surface (+13kJ/mol). The abstraction of F from the B-terminated surface, with gaseous H radicals, was found to be highly probable from both thermodynamic and kinetic considerations, being also the situation for the F-covered N-terminated surface (with a minor energy barrier of +8kJ/mol). In addition, the energy evolution for the approaching F to a surface H, clearly shows that any abstraction reaction will never take place. Hence, the results within the present study clearly show that at realistic deposition temperatures, it is only gaseous H that will have the capacity to remove H or F from the c-BN(100) surface.