Ultrafast optical measurements of ultrasound attenuation in amorphous silicon at 50 and 100 GHz

We present ultrafast optical measurements of the attenuation of 50 -- 100 GHz ultrasound in hydrogenated amorphous silicon (a-Si:H) thin films. The films were grown using a modified very high frequency glow discharge method on steel substrates. The deposition conditions were similar to those used for high efficiency solar cells. The measurements were performed at 300 K using the picosecond ultrasonics technique. Films of varying thickness were measured so that the effect of intrinsic acoustic loss within the a-Si:H could be determined. We find that the ultrasonic attenuation in a-Si:H at 100 GHz is more than an order of magnitude lower than is found in other amorphous materials. Our results may impact theoretical models of thermal transport in amorphous materials, and could provide a new avenue for studying voids in a-Si:H and nanocrystalline Si films.