Phonon scattering at silicon crystal surfaces

Abstract

We have studied phonon scattering by films of metals, nonmetals, and condensed gases, 2 Å to ${10}^4$ Å thick, deposited onto polished faces of Si crystals, through measurements of the thermal conductivity of these crystals in the boundary scattering regime, between 0.05 and 2.0 K. No evidence has been found for scattering by interface states or individual atoms or molecules adsorbed at the surface. Phonons are, however, strongly scattered by thin, discontinuous films and also by thick continuous ones, independent of the type of bonding to the substrate. Scattering by the islands of discontinuous thin films, or by microscopic disorder like grain boundaries, voids, or surface roughness in the thicker films can explain the observations. The model we develop to explain our findings is based on that for scattering by rough surfaces. Our experiments indicate that the high degree of disorder on the length scale of several hundred Å is not restricted to evaporated films, but also occurs in thermally grown oxide layers, Ni silicide films grown by surface reaction and even in layers of boron-ion-implanted silicon. It is concluded that thermal phonons in the frequency regime of 10–100 GHz (quantum energies ${10}^{\mathrm{-}5}$ to ${10}^{\mathrm{-}4}$ eV, corresponding to measuring temperatures 0.1 to 1.0 K) are very sensitive probes of surface film perfection on the submicrometer length scale.