EVIDENCE FOR MIXED-PHASE NANOCRYSTALLINE BORON-NITRIDE FILMS

Boron nitride films of nearly stoichiometric composition were prepared by reactive magnetron r.f. sputtering from a BN target at various negative bias voltages between 0 and 100 V and at deposition temperatures of 50 and 500-degrees-C. The films were investigated by glancing-angle X-ray diffractometry and display a pattern consisting of two broad main peaks, which can be interpreted as due to a mixture of the hexagonal and the cubic modifications of BN. From a comparison with computer-generated diffraction spectra the approximate crystalline sizes and relative concentrations of cubic and hexagonal BN can be determined. The concentrations are bias voltage dependent. Dark field transmission electron microscopy observations confirm the mixed-phase model and different grain sizes for the hexagonal and the cubic modifications. Further support of this model is given by measurements with an ultralow load depth-sensing nanoindenter from which hardness and Young's modulus were derived. These values correlate with the cubic BN concentration in the film.