Abstract
Time-resolved cavity ringdown spectroscopy (τ-CRDS) has been applied to determine the surface reaction probability β of Si and SiH3 radicals during plasma deposition of hydrogenated amorphous silicon (a-Si:H). In an innovative approach, our remote Ar-H2-SiH4 plasma is modulated by applying pulsed rf power to the substrate and the resulting time-dependent radical densities are monitored to yield the radical loss rates. It is demonstrated that the loss rates obtained with this τ-CRDS technique equal the loss rates in the undisturbed plasma and the determination of the gas phase reaction rates of Si and SiH3 as well as their surface reaction probability β is discussed in detail. It is shown that Si is mainly lost in the gas phase to SiH4 [reaction rate kr=(3.0±0.6)×10−16m3s−1], while the probability for Si to react at an a-Si:H surface is 0.95<βSi<1 for a substrate temperature of 200°C. SiH3 is only lost in reactions with the surface and measurements of β of SiH3 for substrate temperatures in the range of 50–450°C show that βSiH3=(0.30±0.03), independent of the substrate temperature. The implications for a-Si:H film growth are discussed.
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