Isotopic substitution of boron and carbon in superconducting diamond epilayers grown by MPCVD

Sets of heavily boron-doped {100}-oriented diamond epilayers have been grown by MPCVD from isotope-enriched gases such as 13CH4 and 10B2H6 or 13CH4 and naturally abundant 11B2H6, and the resulting concentration profiles of the various boron isotopes were measured by SIMS. In the particular case of the 13C10B set, the high absorption cross section of 10B made neutron depth profiling possible for this element.The vibrational and superconducting properties of these epilayers were compared to those of films grown from gases containing the isotopes in their natural abundance. The critical temperature TC values were found to be lower both in the 13C11B and 13C10B sets, as expected for a conventional pairing mechanism mediated by phonons. In good agreement with recent studies, the isotope-effect coefficient for TC was found to be much higher than the classical 0.5 value. Various origins for this behaviour are suggested.