Mapping of local stress distributions in SiGe/Si optical channel waveguide

Micro-Raman scattering experiments have been carried out to study a SiGe/Si photoelastic optical channel waveguide structure. A Si₃N₄ stressor stripe was fabricated on a SiGe/Si planar waveguide to create local stress variation beneath the stripe. To characterize the local stress distributions which create the channel waveguide structure, we obtained Si-Si Raman peak shifts in the SiGe layer by utilizing spatially resolved Raman spectra both from the top surface across the width of the Si₃N₄ stripe and from an edge (i.e., a waveguiding region) beneath the stripe. Longitudinal optical phonon shifts observed from the top surface show a significant increase in compressive stress within the stripe width and in tensile stress just outside of stripe edges for a sample annealed at 600°C. To map local changes of stresses in the waveguiding region beneath the stripe, spectra from two transverse optical phonons were measured from the edge and interpreted using the Raman polarization selection rules. The Si-Si Raman peak shifts obtained were used to find strain components and, in turn, to model resultant local refractive index variations which give rise to optical confinement via the photoelastic effect. Model calculations show good agreement with the experimental data and provide mode profiles for both transverse magnetic and transverse electric propagation. These experiments contribute to a more detailed understanding of the complex stress and refractive index distributions present in photoelastic semiconductor optical channel waveguide structures, structures which are potentially useful for optical device technology.