MPCVD growth of 13C-enriched diamond single crystals with nitrogen addition

Abstract

This study describes the growth of 13C-enriched (100)-oriented diamond single crystals by the MPCVD (microwave plasma chemical vapour deposition) process. All crystals are at least 6×6mm2 in area and 0.5–1.0mm in thickness. The samples with nominal 13C percentages (R=[13C]/[13C+12C]) of 0.011 (natural abundance), 0.10, 0.21, 0.24 and 0.34 were obtained by controlling the flow of the carbon-13 and carbon-12 methane feed gases. To obtain thicker and near-colourless quality 13C-enriched diamond, 190ppm of nitrogen was added into the gas mixture. The shift towards lower frequency in the Raman peak positions and decrease in the thermal conductivities of the near-colourless crystals with increasing 13C percentages are similar to previous studies of isotopically-controlled diamond grown with no nitrogen additive. The images of the structural defects associated with 13C-enrichment obtained by spatially-resolved X-ray rocking curve measurement show distinct patterned structures that runs parallel to the <100> direction. Moreover, the broadening in the line width of the nuclear magnetic resonance (NMR) peak from sp313C correlates with increasing R. We also expand the study by injecting 500ppm of nitrogen. Higher nitrogen concentration leads to the formation of brown crystals. The brown crystals show far greater 13C NMR peak intensity than the near-colourless. This suggests that paramagnetic nitrogen impurities in the brown crystals hasten the spin-lattice relaxation time of the 13C nuclear spin that resulted in higher intensity. The isotopic splitting observed for the localized vibrational mode of the NVH0 defect in brown crystals is attributed to the co-existence of both the 13C (3114.2cm−1) and 12C (3123.5cm−1). Unlike the isotopic splitting observed for NVH0 defect, the peak position of the Ns+ defect shifts towards lower frequency as R increases. Not only have we demonstrate the growth of bigger isotopically-controlled diamond single crystals, the results shown here have provided a framework to further investigate the interplay between 13C atoms and nitrogen impurity.