Interdiffusion and short-range order in amorphous Ta-Si multilayer structures

Sequential sputter deposition of amorphous 4-nm-thick Ta and 7-nm-thick Si layers leads to the formation of a periodic multilayer that can be regarded as a synthetic one-dimensional crystal. A simple kinematical diffraction model is presented to discuss x-ray diffraction patterns obtained from the multilayer structure during annealing. Post-deposition annealing up to temperatures above 530 °C, where the lattice structure disappears, leads to Si diffusion into the Ta layers forming a nonstoichiometric Ta-Si phase. The high diffusion rate of Si in Ta leads to a homogeneous distribution of Si in the Ta (silicide) regimes. As shown by cross-sectional transmission electron microscopy, the interfaces remain sharp during the interdiffusion process. This makes the observation of low-angle Bragg reflections up to the 12th order possible. The local atomic structure around the Ta atoms was analyzed by measuring the extended x-ray-absorption fine structure above the Ta LIII edge at different probing depth conditions. During annealing the local Ta environment was found to be TaSi2 like, even if only a small portion of the Si has diffused into the Ta layers, and the silicide formation is far from stoichiometry. The formation of a Ta suboxide was detected in the surface region of the top Ta layer.