2D Raman mapping of stress and strain in Si waveguides (Vol. 43 No. 5)

In 27, 085009 (2012)this work, we characterized the mechanical stress of strained silicon waveguides by micro-Raman spectroscopy. We performed accurate measurements on the waveguide facet by using a confocal Raman microscope. The silicon-on-insulator waveguide is strained by depositing thin stressing silicon nitride (SiN) overlayers. The applied stress is varied by using different deposition techniques. By investigating the waveguides facets and modeling the measured Raman shifts, the local stress and strain are extracted. Thus, two-dimensional maps of stress distribution as a function of SiN deposition parameters are drawn showing different strain distributions depending on the deposition technique. Moreover, the results show a relevant role played by the buried oxide layer, which strongly affects the waveguides final stress. Hence, the combined actions of SiN and buried oxide layers deform the whole silicon core layer and cause an inhomogeneous strain distribution in the waveguides.

The strain inhomogeneity is fundamental to enable second order nonlinear optical devices because it breaks the silicon centro-symmetry yielding a huge second order nonlinear susceptibility (χ(2)). As we demonstrate in Nature Mater. 11 148-154 (2012), a χ(2) of several tens of picometers per volt is observed and a relation between the χ(2) values and the strain inhomogeneity and magnitude is revealed. Particularly, the largest conversion efficiency is observed in the waveguides where the micro-Raman spectroscopy shows the highest deformation. Currently, no explicit theory relating the strain with the χ(2) exists. Thus, the two-dimensional micro-Raman maps could be a valuable tool for experimental confirmation of future theories.