Current research problems and opportunities in the vapor phase synthesis of diamond and cubic boron nitride

Abstract

Current research problems in the chemical-vapor deposition (CVD) of diamond and cubic-boron nitride (c-BN) include efforts to rationalize the formation of well-crystallized diamond as part of efforts to achieve several important goals. Among these is well-crystallized c-BN by CVD, improved diamond crystal perfection important to many applications, the large-area heteroepitaxial growth of diamond (or c-BN), and possibly, new composites and superlattice materials not heretofore thought possible. It is argued that the synthesis of diamond is not inconsistent with thermodynamic analysis, provided the influence of relevant surface chemistries and the nonequilibrium nature of materials synthesis are taken into account, and that the phase composition of the solid is determined in large measure by the lowest free-energy structure of its surface during growth. From linear irreversible thermodynamics it follows that crystal perfection and the nature of the growth surface during deposition will be determined by how far from equilibrium the system is held and whether or not a locally stable stationary state is maintained during deposition. The greater the departure from equilibrium, the higher the defect levels expected, implying that the growth of relatively defect-free diamond will require a close approach to equilibrium. Much of the experimental data available are shown to be consistent with this approach, including difficulties with using methods successful for diamond to grow c-BN. Although the diamond growth surface can be stabilized by the chemisorption of hydrogen, a similar stabilization of the surfaces of c-BN is at best problematic, and new chemistries may be needed to achieve its synthesis. Similarly, the heteroepitaxial growth of diamond may require not only the provision of a suitable surface template or lattice match but, also, a compatible interfacial chemistry for the stabilization of sp3 hybridized carbon.