Annealing induced interdiffusion and crystallization in sputtered amorphous Si/Ge multilayers

The intermixing and crystallization of amorphous Si/Ge multilayers (with individual layer thickness between 1.5 and 20 nm) and SiGe alloys produced by dc magnetron sputtering have been studied by cross-sectional transmission electron microscopy and x-ray diffraction. Measurement of the crystallization temperature as a function of the Si content showed that multilayers and alloys with equal composition crystallized at the same temperature. This implies that intermixing precedes crystallization in the multilayers. Close to the crystallization temperature, formation of Kirkendall voids was observed in the short-period Si/Ge multilayers. These voids were found at positions corresponding to the original Si layers, indicating that Si diffuses faster in amorphous Ge than Ge in amorphous Si. The Ge layers in short-period Si/Ge multilayers retained their amorphous state to much higher temperatures than thick amorphous Ge layers. This is shown to be due to inhibition of nucleation by the presence of the layer interfaces. A lower estimate for the Si diffusion constant in crystalline Ge is also determined.