Extended x-ray-absorption fine-structure study of hydrogenated amorphous silicon-germanium alloys. II. Dependence of bond length and coordination on composition.

Abstract

The local structure of hydrogenated amorphous Si-Ge alloys has been determined using the extended x-ray-absorption fine-structure (EXAFS) technique. Numerical computations and comparison with experimental Ge K-edge EXAFS have shown that Ge-Ge and Ge-Si bond lengths are constant throughout the entire compositional range studied and equal to 2.46 and 2.41 A\r{}, respectively. The Ge-Si bond length is found to be close to the average value of the bond lengths for both Ge and Si crystals. A study of the coordination around the Ge atoms has revealed that Ge and Si atoms are randomly mixed in the Ge concentration range below 40 at. %. In films with high Ge content, the coordination of Ge atoms surrounding Ge atoms is 15--20 % higher than that expected for a random mixture of Ge and Si atoms, suggesting a structural inhomogeneity in Ge-rich alloys.