Study by means of photothermal deflection methods of the opto-electronic properties of CVD diamon in relation to the defect population

Abstract

The main objective of this thesis was to investigate if Photothermal Deflection Spectroscopy (PDS) could be applied to study the defects present in undoped and doped polycrystalline CVD diamond films. Up till now, PDS was only used for the characterisation of semiconductors such as a-Si and a-Ge. Photothermal Deflection Spectroscopy: PDS is a very sensitive characterisation technique which allows to determine the defectinduced optical absorption coefficient α of semiconducting films. We have adapted the PDS set-up in such a way that it meets the specific requirements of polycrystalline CVD diamond films and moreover, we studied the validity of the PDS theory for diamond. This resulted in the construction of two different PDS configurations: the transverse and the collinear set-up. (i) Transverse PDS was used for the first time as a gap state spectroscopic tool to study the optical transitions due to the presence of defects in the bandgap of diamond. The spectral dependence of the optical absorption coefficient α in the energy range of 0.5 to 5.4 eV can be studied. (ii) Collinear PDS was used to map the optical absorption (in our case at the CO2 laser wavelength of 10.6 µm, with a spatial resolution of about 100 µm) in high quality CVD diamond windows. In general, it is not straightforward to get absolute optical absorption values out of PDS measurements in arbitrary units. (i) We compared different scaling procedures for transverse PDS measurements to calculate the absolute optical absorption coefficient. - low quality CVD diamond films: Ssat-scaling procedure (transverse PDS + saturation value). - intermediate to high quality CVD diamond films: T-scaling procedure (transverse PDS + transmission measurements) (ii) In the case of collinear PDS, absorption values of high quality diamond films were calibrated by means of laser calorimetry on a reference sample. (Our collinear arrangement is not suitable to measure absorption in low quality diamond films.) In particular, the influence of light scattering in diamond films on transverse PDS measurements has been studied thoroughly. In the low absorption range (αd < 1), the effect of scattering and light trapping results in an artificially increased effective optical thickness. This enhancement is spectral independent and thus, the shape of the spectrum is not distorted. Actually, it comes down to an enhancement of the sensitivity of PDS. Since we measure PDS spectra in arbitrary units this influence is not relevant. In contrast with the transmission measurements, the PDS measurements are not hindered by scattering losses. Because specular transmission measurements suffer much more from scattering than integrated sphere transmission measurements, the latter are used to scale the PDS measurements....