Nanocrystalline diamond films heavily doped by boron: structure, optical and electrical properties

A set of nanocrystalline diamond films was grown using microwave plasma enhanced chemical vapor deposition on fused silica substrates from methane diluted by hydrogen: with and without the addition of trimethylborane. The boron to carbon ratio in the gas phase was varied from 0 to 8000 ppm. The boron doped nanocrystalline diamond films were investigated using atomic-force microscopy, Raman spectroscopy, transmittance spectroscopy and electro-physical methods. For analysis of Raman spectra of heavily doped p-type nanocrystalline diamond using Fano contour one should take into account the shift and broadening of the phonon line due to phonon confinement in grains, or phonon scattering by defects. Raman spectra were calculated using a phonon confinement model and Fano contour. Good agreement was found between the calculated and experimental spectra. Analysis of the spectra showed both the phonon confinement effect in nanocrystalline grains and Fano interference effect due to the contribution of electron Raman scattering in heavily doped p-type diamond films. An increase in boron concentration led to a decrease in the size of crystalline diamond grains and also formation of defects (supposedly inclusion of sp2 hybridized carbon) in the nanocrystalline diamond films. Raman spectroscopy data was supplemented by data from atomic-force microscopy. The conductivity of undoped films was 0.066 Ω-1cm-1, the conductivity of doped films grew with increasing boron to carbon ratio and reached 418 Ω-1cm-1 (8000 ppm). Films were semitransparent and have good conductivity, so can be used as transparent electrodes in large-scale electronics and optoelectronics.