SUBGAP OPTICAL-ABSORPTION IN CVD DIAMOND FILMS DETERMINED FROM PHOTOTHERMAL DEFLECTION SPECTROSCOPY

Photothermal deflection spectroscopy (PDS) was applied for the first time to measure the subgap optical absorption, induced by defects in free-standing diamond films. Compared with the optical transmission measurement, PDS is far more sensitive and even a defect concentration below parts per million level can be identified. Moreover, the elastic light scattering influences the measured data significantly less. The optical absorption measurement shows an optical transition which starts at about 1 eV, independently of the sp(2) content in the film. This level agrees well with the 1 eV activation energy of the dark conductivity. Additionally, an optical transition starting at about 3 eV is observed for white colourless films.