AVS 50th International Symposium
    Thin Films Thursday Sessions
       Session TF-ThA

Paper TF-ThA1
Thin-film Cavity Ringdown Spectroscopy (tf-CRDS) for Ultra-sensitive and Direct Detection of Defect-related Absorptions in a-Si:H Thin Films

Thursday, November 6, 2003, 2:00 pm, Room 329

Session: In-Situ / Ex-Situ & Real-Time Monitoring
Presenter: I.M.P. Aarts, Eindhoven University of Technology, The Netherlands
Authors: I.M.P. Aarts, Eindhoven University of Technology, The Netherlands
B. Hoex, Eindhoven University of Technology, The Netherlands
A.H.M. Smets, Eindhoven University of Technology, The Netherlands
R. Engeln, Eindhoven University of Technology, The Netherlands
M. Nesládek, Limburgs Universitair Centrum, Belgium
W.M.M. Kessels, Eindhoven University of Technology, The Netherlands
M.C.M. van de Sanden, Eindhoven University of Technology, The Netherlands
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We have explored the applicability of cavity ringdown spectroscopy with respect to the measurement of small, defect-related absorptions in thin films. First of all, the validity of the method has been confirmed by rigorous studies on the issues that arise when placing an optical transparent substrate inside the high-finesse optical cavity: it has been found that the stability of the cavity, the build-up time of the electromagnetic field inside the cavity as well as the changes in output signal are not significantly affected by the substrate. Furthermore, the losses caused by surface scattering induced by the sample's surface roughness have been estimated employing surface morphology data obtained from atomic force microscopy. It is shown that surface scattering will eventually limit the absorption sensitivity of the technique, which is as good as 10@super -7@ per pass. Subsequently, as a proof-of-principle, measurements have been performed on several samples of hydrogenated amorphous silicon (a-Si:H) thin films in a broad spectral range (0.7 - 1.7 eV) using an optical parametric oscillator laser. The absorption values of the a-Si:H films of various thickness (5 - 1000 nm) have been corrected for interference effects that have been modeled in a straightforward but complete manner. The optical absorption spectra obtained revealed good agreement with conventional transmission-reflection and photothermal deflection spectroscopy (PDS) measurements and the spatial distribution as well as the bulk and surface defect densities in the a-Si:H thin film have been determined. From the results, it can be concluded that tf-CRDS is an ultra-sensitive absorption technique that does not require any calibration. The technique is therefore very promising for a broad range of thin film research fields and currently experiments are carried out to detect ultra-low doping levels of rare earth metals in silicon and to apply the technique real time during film growth of a-Si:H.