Optical properties of nanocrystalline diamond thin films

The optical properties of nanocrystalline diamond films grown from a hydrogen-rich $CH4∕H2$ gas phase by hot filament chemical vapor deposition, as well as from an argon-rich $Ar∕CH4$ gas phase by microwave plasma enhanced chemical vapor deposition, are reported. The influence of nitrogen incorporation on the optical absorption is investigated. The diamond films are characterized by photothermal deflection spectroscopy and temperature dependent spectrally resolved photoconductivity. An onset of absorption at about $0.8eV$ in undoped films is attributed to transitions from $π$ to $π$ states introduced into the band gap by the high amount of $sp2$ bonded carbon at the grain boundaries. Incorporation of nitrogen leads to a strong absorption in the whole energy spectrum, as a result of the increasing number of $sp2$ carbon atoms. The effect of surface states has been observed in the high energy region of the spectrum. Transitions to the conduction band tail and photothermal ionization processes account for the observed onset at $4.4eV$⁠. Photocurrent quenching at about $3.3eV$ is observed in the case of samples grown from a hydrogen-rich $CH4∕H2$ gas phase.