Novel in situ and real-time optical probes to detect (surface) defect states of a-Si:H


This paper describes two novel optical diagnostics that were recently introduced to the field of Si-based thin films, in particular for probing defect states present in the bulk and at the surface of a-Si:H films. It is expected that these diagnostics, when applied in situ or real time during film growth, can provide new insights into the a-Si:H film properties as well as into the fundamental surface processes during growth. The first method is cavity ringdown spectroscopy (CRDS). From ex situ measurements on a-Si:H thin films, it is shown that this method is very powerful for measuring absolute defect-related absorptions at subgap energies without the need for a calibration procedure, even for films as thin as 4 nm. It is also shown that the method can be used for measuring rare-earth dopants - here Er3+ in silicon-rich oxide - to the extent that issues about absorption cross-sections can be resolved by using thin samples instead of waveguides. Furthermore, the in situ application of the method for thin films is discussed by presenting the evanescent-wave cavity ringdown (EW-CRDS) technique. The second method is spectroscopic second harmonic generation (SHG). It has been found that this non-linear optical technique yields a photon energy dependent signal for as-deposited a-Si:H films and that this signal has a contribution from a-Si:H surface states. From a comparison with c-Si surface science studies, the possible origin of the signal from surface Si dangling bonds and strained Si-Si bonds is discussed. The application of SHG during real-time film growth is also presented.

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The authors acknowledge the following people for their important contributions: M. Nesládek (Limburgs Universitair Centrum) for the PDS measurements; H. Mertens and Prof. A. Polman (Center for Nanophotonics, FOM-Institute AMOLF) for the collaboration on the Er-doped Si- rich oxide experiments; Dr. A.C.R. Pipino (NIST) for the collaboration on the EW-CRDS technique; B. Hoex and dr. R. Engeln (Eindhoven Univ. of Technology) for the experiments and discussions; and M.J.F. van de Sande, J.F.C. Jansen, A.B.M. Hüsken, and H.M.M. de Jong for their skilful technical assistance. This work was supported by the Netherlands Foundation for Fundamental Research on Matter (FOM). The research of W.K. has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences (KNAW).

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Kessels, W., Aarts, I., Gielis, J. et al. Novel in situ and real-time optical probes to detect (surface) defect states of a-Si:H. MRS Online Proceedings Library 862, 143 (2004).

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