Near-Infrared Femtosecond Laser for Studying the Strain in $\hbox {Si}_{1\hbox {-}{\rm x}}\hbox {Ge}_{\rm x}$ Alloy Films via Second-Harmonic Generation

The second-harmonic generation (SHG) from Si_1-xGe_x alloy films has been investigated by near-infrared femtosecond laser. Recognized by s-out polarized SHG intensity versus rotational angle of sample, the crystal symmetry of the fully strained Si_0.83Ge_0.17 alloy is found changed from the O_h to the C₂ point group due to the inhomogeneity of the strain. Calibrated by double crystal X-ray diffraction, the strain-induced χ⁽²⁾ is estimated at 5.7 × 10^-7 esu. According to the analysis on p-in/s-out SHG, the strain-relaxed Si_0.10Ge_0.90 alloy film is confirmed to be not fully relaxed, and the remaining strain is quantitatively determined to be around 0.1%.