Conservation of bond lengths in strained Ge-Si layers

Abstract

The combined techniques of x-ray-absorption fine structure and x-ray diffraction have been used to study the strain and bond distortions in epitaxial Ge-Si on Si(001). In a 31% Ge, 340-\AA{} pseudomorphic Ge-Si film, the Ge-Ge and Ge-Si first-neighbor bond lengths have been found to be 2.44\ifmmode\pm\else\textpm\fi{}0.02 and 2.38\ifmmode\pm\else\textpm\fi{}0.02 \AA{}, respectively. The lattice parameter perpendicular to the Ge-Si/Si(001) interface has been found to be ${\mathit{a}}_{\mathrm{\ensuremath{\perp}}}$=5.552\ifmmode\pm\else\textpm\fi{}0.002 \AA{}, in agreement with the predictions of macroscopic elastic theory. These results show that the bond-length strain in the epitaxial layer appears in the second and higher coordination shells, rather than in the nearest-neighbor bond lengths, which remain the same as in unstrained Ge-Si. A microscopic model is presented that accounts for these findings.