Inhomogeneities across boron-doped nanocrystalline diamond films

Abstract

For large-scale device fabrication, information about film inhomogeneities is crucial for high fabrication yield. In this work, inhomogeneities in two-inch diameter heavily boron-doped nanocrystalline diamond (BNCD) films have been studied. Two BNCD films were grown using chemical vapour deposition (CVD) with different boron-to-carbon (B/C) ratios. Their superconducting properties were measured as a function of distance from the centre of the film. The critical temperature ($T_c$) and critical magnetic field ($H_{c2}$) decreased radially outwards from the centre for both films. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were done on the samples to pinpoint the underlying explanation for the observed behaviour. Raman spectroscopy suggested a reduction in boron concentration and diamond purity over both films while moving radially outwards from the centre. XPS data from both films, however, did not show similar behaviours to that observed from the Raman data for the B/C ratios or diamond content. The AFM scans and SEM analysis showed a decreasing grain size further away from the film centre irrespective of the B/C ratio. This is due to the film being thinner at the edges when compared with the centre of the film. Raman analysis showed that the film with the higher B/C ratio had a higher diamond purity across the film. As expected, the film with a higher B/C ratio showed a more robust superconducting behaviour. The observed reductions in boron concentration, diamond purity, film thickness and decreased grain sizes are responsible for the diminishing superconductivity at the edge of the films.