Conversion of strain state from biaxial to uniaxial in strained silicon

The Raman shift of Δω3 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.