Evidence for the principal defect states in CVD diamond films: Optical study

Abstract

Photothermal deflection spectroscopy (PDS) was used to study the subgap optical absorption in CVD diamond films. All films exhibited a very characteristic shape of the absorption in the IR and visible spectral regions, varying in intensity over several orders of magnitude and depending on the film quality. The study of the morphology proved that the films with the highest subgap absorption were nanocrystalline with increased surface area of grain boundaries. The optical absorption spectra of these films resembled the optical absorption in amorphous carbon films which is due to the optical transitions in the pi-pi* pseudogap. The best quality CVD diamond attained the character of the fundamental indirect absorption edge of diamond with strongly reduced characteristic subgap absorption. From secondary ion mass spectrometry analysis it is shown that the characteristic absorption cannot be due to the extrinsic impurities.