Acoustic phonon scattering at sapphire surfaces coated with epitaxial silicon

Abstract

As a method of studying the phonon scattering mechanisms that occur at crystal boundaries and interfaces, the thermal conductivity of water geometry sapphire substrates has been measured. A model has been developed that enables a phonon specular reflection coefficient to be determined for a particular surface when the sample surfaces are inequivalent. This model has been applied to sapphire surfaces that have undergone three types of treatment; highly polished, roughened and coated with epitaxial silicon. The authors conclude that for the most carefully prepared sapphire surface the dominant mechanism of phonon scattering is from surface asperities of near atomic dimensions while the coated surfaces show scattering from vibrating dislocations within the silicon layer. They estimate the mean dislocation pinning lengths to be of the order of 10-6 cm with a resolved shear stress factor Omega approximately 10-4.