Evolution of optical phonons in epitaxial Ge_1−ySn_y structures

We report polarized Raman scattering results of Ge_1−ySn_y (0 ≤ y ≤ 0.09) epitaxial layers grown on Ge-buffered Si substrates. Polarized Raman spectra from the sample surfaces revealed strong Ge–Ge longitudinal optical (LO) phonon responses. The Ge–Ge LO(z) phonon wavenumber decreased systematically as the Sn content increased. Linear fitting results of the Ge–Ge LO(z) phonon wavenumber shifts as a function of Sn content suggested that a partial strain relaxation occurred in the Ge_1−ySn_y layers. Spatially resolved Raman mapping measurements from the cross section of a Ge_0.938Sn_0.062 samplev showed that the peak wavenumber of the Ge–Ge transverse optical phonon decreased gradually toward the top surface, providing direct evidence that the residual built-in strain initially formed at the Ge_0.938Sn_0.062/Ge interface tended to relax gradually along the growth direction. Further, a hydrogen inductively coupled plasma treatment induced a greater homogeneous strain profile in the Ge_0.938Sn_0.062 layer.