Raman characterization of composition and strain in Si{sub 1{minus}x}Ge{sub x}/Si heterostructures

High quality epitaxial Si{sub 1{minus}x}Ge{sub x}/Si (x = 0.09 to 0.27) samples were studied with polarized Raman scattering technique. The LO (longitudinal optical) {minus}TO (transversal optical) phonon mode splitting due to the tetragonal strain imposed by the lattice mismatch between Si{sub 1{minus}x}Ge{sub x} and Si was observed for the first time in such structures. In contrast to the case of uniaxial stress, the biaxial stress induces a larger strain frequency shift for the LO mode than for the TO mode. The phonon strain shift coefficient for the LO mode was found to be 1,010 cm{sup {minus}1}, which is larger than most of the values reported in the literatures, and almost composition independent throughout the range of x {le} 0.27. The authors argue that the smaller LO strain shift coefficients indicate that the Si{sub 1{minus}x}Ge{sub x} films used in previous work were partially relaxed.