Structural and chemical analysis of c-BN/diamond heterostructures

Cross-sectional transmission electron microscopy has been used to characterize the morphological features of thin boron nitride films grown on single-crystal boron-doped diamond substrates (lattice mismatch of 1.36%) using electron cyclotron resonance-plasma enhanced chemical vapor deposition. The effect of gas precursor concentration, growth temperature, and substrate cleaning method on determining the BN phase (either cubic or turbostratic), the defect density, and the orientation relationship between c-BN domains and the diamond substrate were investigated. A nucleation step involving a hydrogen-limited gas mixture promoted etching of sp2-bonded BN phases and increased the fraction of cubic phase present in the films. A growth temperature of 820 °C resulted in larger BN grains and reduced defect densities particularly in regions away from the BN/diamond interface. Substrate cleaning with hydrogen plasma was found to be associated with twin-related BN growth rather than a simple epitaxial relationship. High-resolution electron micrographs showed complex contrast features caused by the presence of a high density of twin domains and stacking faults near the BN/diamond heterointerface for samples with predominant cubic phase. Electron-energy-loss spectroscopy was used to differentiate between regions of sp2 and sp3 bonding, and showed distinct transitions to the latter for the growth of cubic materials. Growth experiments at even higher temperatures and optimal substrate cleaning methods are needed for improved defect mitigation in BN films.