GeLATERAL SEGREGATION AS A DOMINANT ALLOYING MECHANISM DURING LOW KINETICSiCAPPING OF STRAINEDSi1-xGexHUT ISLANDS

Several cross-sectional transmission electron microscopy images have shed light on the intermixing and island-smoothing mechanisms which occur during Si capping of roughenedSi0.5Ge0.5hut cluster morphologies, strained on Si(001). By modifying either Si cap layer growth rates or temperatures upon identically corrugated alloys, more or less Si and Ge surface migrations are allowed, accordingly affecting the capped final morphology. In these images a Ge marker technique is used to take snapshots of the interfacial region in the first growing stages. They clearly demonstrate that, at 500°C and using low Si growth rates (a few monolayers/min), the segregation does not mainly operate by vertical site exchange but by Ge depletion from the island crests. Ge is able to diffuse laterally onto Si over distances comparable to the island rippling period (a few hundreds of Å) during Si heteregeneous growth in the morphology troughs. The initial island structure, subjected to strain gradients, thus acts as a Ge reservoir for noninterrupted Si surface energy reduction by lateral Ge segregation. The latter operates as long as Si is strain driven to accommodate heterogeneously in the island troughs where it minimizes its elastic energy. This finally results in a vanishing undulated morphology and in a smoothed Si/SiGe interface, at the cost of an alloyed region enlarged by the lateral segregation. As a consequence, the thickness of the smeared region is dominantly determined in this case by the initial hut cluster roughness and not by the usual vertical segregation length.