Lattice strain and static disorder determination in Si/Si1-xGex/Si heterostructures by convergent beam electron diffraction

Lattice strain and static disorder present in Si1-xGex alloys forming Si/Si1-xGex/Si heterostructures with a Ge atomic fraction x equal to 0.1, 0.2, and 0.3, have been studied by convergent beam electron diffraction and large-angle convergent beam electron diffraction. These techniques have been used in order to perform structural analysis of the alloys with a spatial resolution comparable with the dimensions involved in Si/SiGe applications. Strain values along directions both perpendicular and parallel to the sample surface have been determined from a single high-order Laue zone deficiency lines pattern. The resulting relaxed alloy lattice constant has been found to depend on the Ge atomic fraction following a nearest-neighbor model alloy, where die Si-Si, Ge-Ge, and Si-Ge bond lengths combine themselves with a negligible dependence on the Ge atomic fraction. From lattice strain values, both the strain-induced bond bend in the plane of the interface, and the strain-induced bond stretch have been determined. Static disorder measurements, performed by comparing the integrated intensity of high-angle diffracted beams in the silicon substrate and in the SiGe layers, allow the determination of the atomic mean-square displacements produced by the presence in the same coordination shell of Si-Si, Ge-Ge, and Si-Ge atomic pairs with different bond lengths. The measured atomic displacements are greater than the ones predicted by both. the nearest-neighbor solution model (which only accounts for the different bond lengths, and not for both band trend and strain) and a relaxed alloys structure Monte Carlo simulation (which accounts for differences in bond lengths and bond bend, but neglects the effect of strain). The component of the atomic displacement related to the macroscopic strain has been determined as the difference between the experimental values and those computed by Monte Carlo code. A linear correlation between the strain-induced atomic displacement and the strain-induced bond bend in the plane of the interface has been found. [S0163-1829(99)04343-X].