COMPARATIVE ASPECTS OF C-BN AND DIAMOND CVD

Various key aspects of the low-pressure synthesis of cubic boron nitride and diamond are compared and discussed. Based on recent thermodynamic data for c-BN and h-BN (heats of formation, entropies, specific heats), c-BN should be the stable phase under standard and low-pressure conditions. The current problems in c-BN CVD synthesis can be explained by the empirical rules of Ostwald and Ostwald-Volmer, which show that the formation of the metastable and less dense phase, h-BN, is favoured. Thus, for a successful CVD synthesis of c-BN it is necessary to find a way to circumvent the natural tendencies of the system. Thermodynamic equilibrium calculations for c-BN and h-BN in different gas atmospheres suggested a phase-selective reaction by which h-BN could be preferentially etched. Thus a c-BN CVD process similar to that of diamond should be possible and impingement with energetic ions should not be required to grow c-BN. However, the atomic attachment kinetic during crystal growth caused by the anisotropy of the c-BN crystals (completely N- or B-terminated facets), different bonding energies, and differences of bond lengths of B-N, B-B and N-N bends must be taken into consideration and could cause problems.