Raman Characterization of Composition and Strain in Si1−xGex/Si Heterostructures

Abstract

ABSTRACTHigh quality epitaxial Sil−xGex/Si (x= 0.09 to 0.27) samples were studied with polarized Raman scattering technique. The LO (longitudinal optical) -TO (transversal optical) phonon mode splitting due to the tetragonal strain imposed by the lattice mismatch between Sil−xGexand 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 1010 cm−1, which is larger than most of the values reported in the literatures, and almost composition independent throughout the range ofx≤0.27. We argue that the smaller LO strain shift coefficients indicate that the Sil−xGexfilms used in previous work were partially relaxed.