Effect of Terminating Species on the Initial Growth of BN on Diamond Substrates

Abstract

The details in the layer-by-layer formation of H (or F)-terminated boron nitride onto diamond (100) have been theoretically studied using ab initio density functional theory under periodic conditions. Cubic boron nitride, c-BN, is a very interesting and promising material due to its extreme properties. However, there are severe problems during the vapor phase synthesis of c-BN because of the formation of noncubic phases in the initial grow steps, why a gentle large area chemical vapor deposition (CVD) deposition is needed. The substrate material has been experimentally shown to be very important for an ideal growth of c-BN in the initial grow process. Diamond is a material that has been found to be a good substrate material for this purpose. By alternating H (or F)-terminated B and N layers, and calculating the resulting interfacial binding strengths and geometrical structures, the initial growth has been studied and compared to earlier theoretical results that have been made without terminating species. Two different structural alignments, with respect to the underlying diamond substrate, were initially constructed. One model was heterostructurally positioned on top of the diamond (100) substrate, while the other model had the x-axis of the c-BN lattice aligned with the y-axis of the diamond lattice. For the situation with a terminated monatomic B layer on top of diamond, the heteroepitaxially built structure was the most energetically favored. This result is just the opposite from the nonterminated situation, where the nonheteroepitaxial structures were favored. The binding energy for the heteroepitaxial terminated monolayer of N was calculated even stronger than the terminated B monolayer. When applying a second atomic layer of c-BN on top of the monatomic B layer, the heteroepitaxially build structure is energetically preferred. For the nonheteroepitaxial growth the adlayer actually bonds to the diamond substrate when H-terminated. Without terminating species the nonheteroepitaxial adlayer did not bind at all. Also for the situation with two atomic adlayer with N closest to the diamond substrate, the heteroepitaxial structure is energetically preferred over the nonheteroepitaxial structure. The opposite is true without any terminating species. When four, six, and eight atom layers are applied, with N atoms closest to the diamond substrate, the heteroepitaxially built structures are favored. The results showed that the terminating species helped to uphold the cubic sp3-formation. When using terminating species, the heteroepitaxially built interfaces had generally higher binding strengths than the nonheteroepitaxially built interfaces. While, when not using terminating species, the nonheteroepitaxially built interfaces were preferred instead. The c-BN structure seems to be more easily formed when several BN layers already are formed, and the interfacial binding strength is stabilized.