Defect-induced optical absorption in CVD diamond films

Photothermal deflection spectroscopy (PDS) was used to study defect-induced optical absorption in CVD diamond films. PDS has shown very high sensitivity, allowing to measure absorbance values down to 10(-5) level. Moreover, PDS is significantly less sensitive to elastic light scattering than optical transmission measurements. Based on PDS data, it is shown that undoped polycrystalline and homoepitaxial diamond Films exhibit a very characteristic subgap absorption, which is ascribed to electronic transitions between pi --> pi* states, appearing in the gap and due to the presence of amorphous carbon. The incorporation of N (and H and O) atoms in diamond films was studied. The optical absorption of N-doped films is compared with the optical absorption due to single substitutional N in Ib synthetic diamond. The incorporation of Li by in-situ doping with organo-metallic compounds during Film growth is discussed and the observed transitions are tentatively explained. Finally, a mathematical model for the characteristic subgap absorption is described and the experimentally obtained optical absorption coefficients are deconvoluted to obtain parameters for the description of the density of gap states.