We report on the electronic and optical properties of boron-doped nanocrystalline diamond (NCD) thin films grown on quartz substrates by ${\text{CH}}_4/{\text{H}}_2$ plasma chemical vapor deposition. Diamond thin films with a thickness below 350 nm and with boron concentration ranging from ${10}^{17}$ to ${10}^{21}{\text{cm}}^{-3}$ have been investigated. UV Raman spectroscopy and atomic force microscopy have been used to assess the quality and morphology of the diamond films. Hall-effect measurements confirmed the expected $p$-type conductivity. At room temperature, the conductivity varies from $1.5\times {10}^{-8}{\Omega }^{-1}{\text{cm}}^{-1}$ for a nonintentionally doped film up to $76{\Omega }^{-1}{\text{cm}}^{-1}$ for a heavily $B$-doped film. Increasing the doping level results in a higher carrier concentration while the mobility decreases from 1.8 down to $0.2{\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$. For NCD films with low boron concentration, the conductivity strongly depends on temperature. However, the conductivity and the carrier concentration are no longer temperature dependent for films with the highest boron doping and the NCD films exhibit metallic properties. Highly doped films show superconducting properties with critical temperatures up to 2 K. The critical boron concentration for the metal-insulator transition is in the range from $2\times {10}^{20}$ up to $3\times {10}^{20}{\text{cm}}^{-3}$. We discuss different transport mechanisms to explain the influence of the grain boundaries and boron doping on the electronic properties of NCD films. Valence-band transport dominates at low boron concentration and high temperatures, whereas hopping between boron acceptors is the dominant transport mechanism for boron-doping concentration close to the Mott transition. Grain boundaries strongly reduce the mobility for low and very high doping levels. However, at intermediate doping levels where hopping transport is important, grain boundaries have a less pronounced effect on the mobility. The influence of boron and the effect of grain boundaries on the optoelectronic properties of the NCD films are examined using spectrally resolved photocurrent measurements and photothermal deflection spectroscopy. Major differences occur in the low energy range, between 0.5 and 1.0 eV, where both boron impurities and the $s{p}^2$ carbon phase in the grain boundaries govern the optical absorption.

The demonstration that Nanocrystalline Diamond (NCD) can retain the superior Young’s modulus (1100GPa) of single crystal diamond twinned with its ability to be grown at low temperatures (<450°C) has driven a revival into the growth and applications of NCD thin films. However, owing to the competitive growth of crystals the resulting film has a roughness that evolves with film thickness, preventing NCD films from reaching their full potential in devices where a smooth film is required. To reduce this roughness, films have been polished using Chemical Mechanical Polishing (CMP). A Logitech Tribo CMP tool equipped with a polyurethane/polyester polishing cloth and an alkaline colloidal silica polishing fluid has been used to polish NCD films. The resulting films have been characterised with Atomic Force Microscopy, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy. Root mean square roughness values have been reduced from 18.3nm to 1.7nm over 25μm2, with roughness values as low as 0.42nm over ∼0.25μm2. A polishing mechanism of wet oxidation of the surface, attachment of silica particles and subsequent shearing away of carbon has also been proposed