Ultrasonic attenuation in amorphous silicon at 50 and 100 GHz

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We have measured the attenuation of longitudinal acoustic waves in a series of amorphous and nanocrystalline silicon films using picosecond ultrasonics. The films were grown using a modified very high frequency glow discharge method on steel substrates. The deposition conditions were similar to that used in the fabrication of high-efficiency solar cells. The film thicknesses were varied so we could distinguish between interface losses and intrinsic losses within the silicon films. We determine the attenuation of amorphous Si to be 780 \ifmmode\pm\else\textpm\fi{} 160 cm${}^{\ensuremath{-}1}$ at 100 GHz and 340 \ifmmode\pm\else\textpm\fi{} 120 cm${}^{\ensuremath{-}1}$ at 50 GHz, values that are lower than those predicted by theories based on anharmonic interactions of the sound wave with localized phonons or extended resonant modes. We determine the attenuation of nanocrystalline Si at 50 GHz to be nearly an order of magnitude higher than amorphous Si (2600 \ifmmode\pm\else\textpm\fi{} 660 cm${}^{\ensuremath{-}1}$) and compare that value to a simple Rayleigh scattering prediction.