Origin of characteristic subgap optical absorption in CVD diamond films

Photothermal deflection spectroscopy (PDS) is used to study subgap optical absorption in chemical vapor deposited (CVD) diamond films. The high PDS sensitivity over a broad investigated spectral range enabled the investigation of the main defect structure in CVD diamond upon changing the deposition conditions. All films exhibit a very characteristic shape of the subgap optical absorption in the infrared and visible spectral regions, which scales with the content of amorphous carbon phase in grain boundaries. In best-quality films the character of the fundamental absorption edge is attained and the subgap absorption reduced. For nanocrystalline approaching films, an absorption spectrum resembling closely the one for amorphous carbon (a-C:H) films is observed. Based on these results a mathematical model for the description of the optical absorption coefficient in the subgap region is developed. A numerical deconvolution procedure is applied to fit the experimental data.