Lattice parameter determination of a composition controlled Si _1-xGe_x layer on a Si (001) substrate using convergent-beam electron diffraction

The six lattice parameters (a, b, c, α, β and γ) of Si_1-xGe_x, which was grown epitaxially on a Si (001) substrate with a varying Ge concentration, were determined by convergent-beam electron diffraction (CBED) without any assumption of crystal lattice symmetry. It was revealed that the lattice parameter c of Si_1-xGe_x varies linearly with the Ge concentration and that the lattice symmetry is lowered from cubic to tetragonal, excluding an artifact due to the thinning of the specimen. The effect of strain relaxation that is caused by thinning specimens is discussed. Ge concentrations of examined specimen areas are evaluated using the obtained lattice parameters.