Phonon scattering at crystal surfaces

Through measurements of the low-temperature (0.06-2.0 K) thermal conductivity of dielectric crystals in the boundary-scattering regime we are searching for causes of diffuse phonon scattering by crystal surfaces. Thin (≤ 100 Å) gold films on polished sapphire surfaces cause diffuse phonon scattering above 0.3 K. Below that temperature, specular reflection again begins to dominate. The same diffuse scattering can be caused by implantation of Xe ions which damage a layer ≃500-Å thick underneath the sapphire surface. For gold films, the effect diminishes with increasing thickness, i.e., greater perfection of the film. Because of the very small thickness of the layers in which the diffuse scattering occurs the phonon-scattering strength must be very much stronger than any phonon scattering previously observed in the bulk of dielectric crystals. Several potential explanations are discussed. However, none appear to be sufficiently general to account for both kinds of surface treatments.