LATTICE-PARAMETERS AND LOCAL ATOMIC-STRUCTURE OF SILICON-RICH SI-GE/SI (100) FILMS

The lattice parameters and Ge-to-Si nearest-neighbor and next-nearest-neighbor distances were measured using x-ray rocking curves and extended x-ray-absorption fine-structure spectroscopy for four different Si-rich Si(1-x)Ge(x)/Si(100) epitaxial films. The Si-Ge films, two prepared by molecular-beam epitaxy and two by chemical-vapor deposition, had thicknesses of 500-900 angstrom and Ge concentrations of x(Ge) = 0.06-0.18. Lattice parameters of the films, corrected for coherency strain, agreed with values reported for bulk Si-Ge alloys. Ge-to-Si distances were 2.375 +/- 0.02 angstrom and 3.85 +/- 0.06 angstrom for nearest and next-nearest neighbors, respectively, and were independent of Ge concentration within these experimental uncertainties. A simple, random-solid-solution model using composition-independent values for nearest-neighbor distances r(Si-Si), r(Ge-Ge), and r(Ge-Si) reproduced the average nearest-neighbor distance <r>(x(Ge)) deduced from observed lattice parameters a(x(Ge)) for our range of Ge concentrations. Composition variations of interatomic distances expected from various theoretical models [Martins and Zunger, Phys. Rev. B 30, 6217 (1984); Shih et al., B 31, 1139 (1985); Thorpe and Garboczi, Bull. Am. Phys. Soc. 35, 781 (1990)] all fell within our experimental uncertainty of +/- 0.02 angstrom.