Novel in situ and real-time optical probes to detect (surface) defect states of a-Si:H
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A14.3.1
Novel in situ and real-time optical probes to detect (surface) defect states of a-Si:H W.M.M. Kessels, I.M.P. Aarts, J.J.H. Gielis, J.P.M. Hoefnagels, and M.C.M. van de Sanden Dept. of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
ABSTRACT 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.
INTRODUCTION The performance of Si-based thin film devices is largely determined by their interface properties with defects such as Si dangling bonds and Si-Si strained bonds playing a key role. But also during the growth process of these films from plasma or hot-wire activated radicals, the role of dangling bonds and strained bonds appears to be essential as they are considered active sites for radical adsorption. Figure 1 shows a schematic representation of a-Si:H growth from SiH3 radicals. In the growth process different aspects can be distinguished that need to be investigated in order to come to a full understanding of film formation: (i) the key radical precursor(s); (ii) the surface reaction and sticking probability of the growth precursors; (iii) the surface hydride composition and surface hydrogen coverage; (iv) the surface active sites and their density; (v) the radical surface reactions such as adsorption, abstraction, and insertion; and (vi) surface diffusion mechanisms. Almost all these aspects have been investigated in relative detail. There have been numerous reports on measurements of radical gas phase densities, and there have
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