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Scattering of phonons at boundaries of a crystal (grains, surfaces, orsolid/solid interfaces) is characterized by the phonon wavelength, the angle ofincidence, and the interface roughness, as historically evaluated using aspecularity parameter p formulated by Ziman [J. M. Ziman, Electrons and Phonons(Clarendon Press, Oxford, 1960)]. This parameter was initially defined todetermine the probability of a phonon specularly reflecting or diffuselyscattering from the rough surface of a material. The validity of Ziman's theoryas extended to solid/solid interfaces has not been previously validated. Tobetter understand the interfacial scattering of phonons and to test thevalidity of Ziman's theory, we precisely measured the in-plane thermalconductivity of a series of Si films in silicon-on-insulator (SOI) wafers bytime-domain thermoreflectance (TDTR) for a Si film thickness range of 1 - 10{\mu}m and a temperature range of 100 - 300 K. The Si/SiO2 interface roughnesswas determined to be 0.11+/-0.04 nm using transmission electron microscopy(TEM). Furthermore, we compared our in-plane thermal conductivity measurementsto theoretical calculations that combine first-principles phonon transport withZiman's theory. Calculations using Ziman's specularity parameter significantlyoverestimate values from the TDTR measurements. We attribute this discrepancyto phonon transmission through the solid/solid interface into the substrate,which is not accounted for by Ziman's theory for surfaces. We derive a simpleexpression for the specularity parameter at solid/amorphous interfaces andachieve good agreement between calculations and measurement values.

Interfacial Phonon Scattering and Transmission Loss in >1 um Thick Silicon-on-insulator Thin Films