Mechanics of relaxing SiGe islands on a viscous glass

A process has been developed recently to fabricate a structure comprising, from top to bottom, a SiGe thin film, a glass layer, and a Si wafer. The SiGe film is a perfect crystal, and is under biaxial compression. The SiGe film is patterned into islands. On annealing, the glass flows and the islands relax. The resulting strain-free islands are used as substrates, to grow epitaxial optoelectronic devices. This article describes a series of studies on the annealing process, combining experiment and theory. A small island relaxes by expansion, starting at the edges and diffusing to the center. A large island wrinkles before the expansion reaches the center. After some time, the wrinkles either disappear, or cause the island to fracture. We model the island as an elastic plate, and the glass layer as a viscous liquid. The strains in the islands are measured by X-ray diffraction and Raman spectroscopy, and the wrinkle amplitudes by atomic force microscope. The data are compared with the theoretical predictions. We determine the conditions under which the islands relax by expansion without significant wrinkling, and demonstrate that a cap layer suppresses wrinkles, relaxing a large island crack-free.