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Epitaxial films of cubic boron nitride (c-BN) have been grown on single-crystal boron-doped diamond substrates by electron cyclotron resonance plasma-enhanced chemical vapor deposition using gas mixtures of Ar–He–N2–BF3–H2. The resulting c-BN films have been characterized using in situ x-ray photoelectron spectroscopy to establish the growth surface bonding (i.e., sp3 or sp2). The interface and film crystal structure were characterized with high resolution electron microscopy and electron-energy-loss spectroscopy. This study considers three stages of the growth process: in situ surface preparation, initial nucleation and growth of c-BN, and growth of the epitaxial c-BN layer. Prior studies from our group have established that hydrogen gas phase concentration affects fluorine-induced etching and c-BN nucleation. The results of this study establish that by optimizing the surface chemistry for all three stages of the growth process, it is possible to achieve an adherent, oriented epitaxial c-BN layer, a workable growth rate (∼50 nm/hr), cubic phase BN throughout, and negligible sp2 bonding except at the interface.