Effect by Doping and Surface Termination on c-BN Surface Reactivity: A Theoretical DFT Approach

Abstract

The cubic phase of boron nitride, c-BN, is a very promising material due to its very interesting and extreme properties, of which some are comparable or even superior to diamond. However, there are severe problems with the initial nucleation during vapor phase synthesis of c-BN thin films. Investigations of new possibilities for growth mechanisms are therefore of greatest importance to perform. It has experimentally been shown that the film quality will increase when a small amount of oxygen is added into the film during deposition. In the present study, the adsorption and abstraction energies of the surface terminating species, the geometrical structures, and the resulting electronic structures were studied for the situation when N (or B) was substituted by O (or C) atom in combination with different terminating species (H, F, or Cl) on the c-BN (100) surface. This theoretical study was performed by using first-principles functional theory (DFT) calculations under periodic boundary conditions. The results indicate that the reactivity of the B-rich c-BN (100)-2 × 1 surface is more pronounced when doping an H-terminated surface with O (or C). For a corresponding F-terminated surface, only the C-doped surface will result in a more exothermic adsorption process. Doping with O gives a slightly less exothermic adsorption process, when compared to the nondoped situation. The situation was completely different for the N-rich c-BN (100)-1 × 1 surface, where the H and F terminating species were found to chemisorb stronger for the nondoped situation than for the O (or C)-doped one. When Cl-terminating the surfaces, all the adsorption processes became endothermic and hence not possible to perform. The results from the DFT calculations also indicate that the extra electron density induced in the system by doping with O or C atoms will not stay in the nearest vicinity to the respective dopant, but will instead move closer to the nearest surface atoms. This overall increase in electron density was observed to either strengthen the surface-terminating bonds for the B-rich surface or weaken these bonds for the N-rich surface. The reactivity of the terminated surfaces, as obtained from the Fukui functions, revealed a much larger variation in electrophilic and nucleophilic susceptibility for the doped N-rich surfaces, as compared with the doped B-rich ones.