Electronic and optical properties of boron-doped nanocrystalline diamond films

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 ${10}^{21}\text{ }{\text{cm}}^{\ensuremath{-}3}$ 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 $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }{\ensuremath{\Omega}}^{\ensuremath{-}1}\text{ }{\text{cm}}^{\ensuremath{-}1}$ for a nonintentionally doped film up to $76\text{ }{\ensuremath{\Omega}}^{\ensuremath{-}1}\text{ }{\text{cm}}^{\ensuremath{-}1}$ 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 $0.2\text{ }{\text{cm}}^{2}\text{ }{\text{V}}^{\ensuremath{-}1}\text{ }{\text{s}}^{\ensuremath{-}1}$. 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 $2\ifmmode\times\else\texttimes\fi{}{10}^{20}$ up to $3\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }{\text{cm}}^{\ensuremath{-}3}$. 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.