Conversion of strain state from biaxial to uniaxial in strained silicon

The Raman shift of Δ ω₃ in (001) strained silicon is found to be independent of the azimuthal angle of the patterned structures but exhibits shape dependence in strain relaxation. The tensile strain is reduced from 0.85% in the unpatterned thin film to 0.16% in the cylindrical pillars showing 82% relaxation. It becomes more significant along the width direction of the patterned gratings due to Poisson's effect and only a tensile strain of 0.07% remains. Consequently, the strain state changes from biaxial into uniaxial and is expected to enhance the carrier mobility. Finite element analysis is conducted to elucidate the mechanism.