COMPARATIVE ASPECTS OF C-BN AND DIAMOND CVD

Various key aspects of the low-pressure synthesis of cubic boron nitri de and diamond are compared and discussed. Based on recent thermodynam ic data for c-BN and h-BN (heats of formation, entropies, specific hea ts), c-BN should be the stable phase under standard and low-pressure c onditions. 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 favour ed. Thus, for a successful CVD synthesis of c-BN it is necessary to fi nd a way to circumvent the natural tendencies of the system. Thermodyn amic equilibrium calculations for c-BN and h-BN in different gas atmos pheres suggested a phase-selective reaction by which h-BN could be pre ferentially 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 c rystal growth caused by the anisotropy of the c-BN crystals (completel y N- or B-terminated facets), different bonding energies, and differen ces of bond lengths of B-N, B-B and N-N bends must be taken into consi deration and could cause problems.