Ultrasonic attenuation in amorphous silicon at 50 and 100 GHz

Abstract

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 that we could distinguish between interface losses and intrinsic losses within the silicon films. We determine the attenuation of amorphous Si to be 780 ± 160 cm -1 at 100 GHz and 340 ± 120 cm -1 at 50 GHz, values that are lower than 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 ± 660 cm -1 ) and compare that value to a simple Rayleigh scattering prediction.PACS numbers: 43.35.+d, 63.20.Pw, 63.50.Lm, 65.60 Under this condition, a uniform crystalline volume fraction was demonstrated by Raman and cross-sectional TEM (X-TEM) measurements with roughly 50% Raman determined crystalline volume fraction. X-ray diffraction (XRD) showed a (220) preferential orientation with grain size around 20-30 nm. Thin aluminum layers for use as transducers were deposited on the top surface of the a-Si:H layers using a thermal evaporation method.To measure the attenuation we have used the now well-established technique of picosecond ultrasonics 10,11 and have specifically followed the methods described by Morath and Maris in a 1996 article. 12 A diode laser pumped Ti:sapphire oscillator (Coherent Mira) that operates at a repetition rate of 76 MHz and emits pulses that are ~ 100 fs in duration was used to perform the optical pump and probe experiment. Pump and probe beams with average power ~ 10 mW were focused down to the same ~ 15 μm diameter spot on the Al coated silicon film samples. The absorption of pump pulses caused rapid thermal expansion of the Al transducer layer, which launched longitudinal acoustic pulses into the Si film. The acoustic pulses are roughly single cycle with the compressive strain leading the rarefacting strain, and they contain a broad range of frequencies of coherent long wavelength acoustic phonons. We used Al transducer thicknesses of 30 nm and 15 nm in order to obtain variation of the central acoustic 4 frequency generated. The 30 nm films provided measureable frequencies (well above the noise floor) for a-Si:H in the range of 30 -90 GHz while the 15 nm films provided measureable frequencies ranging from 40 -120 GHz. The strain pulses were detected by time delayed optical probe pulses that are sensitive to changes in the reflectivity, ΔR of the Al film. Figure 1 shows ΔR versus delay time for three a-Si:H samples and two nc-Si:H samples of varying thicknesses. For this graph, the initial electronic response of the Al film near 0 ps has been removed for all five sets of data, as has the thermal decay of the ref...

Origin of micrometer-scale propagation lengths of heat-carrying acoustic excitations in amorphous silicon

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Thermal conductivity accumulation in amorphous silica and amorphous silicon

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