KAPITZA RESISTANCE BETWEEN SILICON AND HE-4

By simultaneously measuring the phonon scattering at or near a silicon surface and the phonon transmission from it into liquid helium, we ha ve correlated quantitatively these two processes. For a clean polished silicon surface, the scattering of thermal phonons below 0.3 K become s very small, and the transmission probability in this case approaches the prediction of the acoustic mismatch theory, which is 0.27%. The t ransmission probability increases, i.e. the thermal boundary resistanc e decreases, as the diffuse scattering increases either as a result of increasing the phonon frequency or the disorder at the surface. In th e limit of completely diffuse scattering, the phonon transmission prob ability exceeds 30%, and the thermal boundary resistance approaches th e value predicted by the diffuse mismatch model to within a factor of three.