Local structures of (Ge-4/Si-4)(5) monolayer strained-layer supperlattice probed by fluorescence XAFS

Local structures of (Ge-4/Si-4)(5) monolayer strained-layer supperlattice (MSLS) have been studied by fluorescence XAFS. The distortion of local lattice around Ge atoms is evidenced by the compressed Ge-Ge and Ge-Si bond lengths. RGe-Ge (0.243 nm) and RGe-Si (0.238 nm) in (Ge-4/Si-4)(5) MSLS are 0.002 nm shorter than those of c-Ge (RGe-Ge=0.245 nm) and the sum of covalent radii (RGe-Si=0.240 nm), respectively. The determined Si coordination number (2.2) is slightly larger than the Ge coordination number (1.8). A simple mechanism of site-exchange between particular sites and the adjacent several layers is proposed to interpret the interface structure for the (Ge-4/Si-4)(5) MSLS.