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-x
Ge
_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
_0.83
Ge
_0.17
alloy is found changed from the O
_h
to the C
₂
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
_0.10
Ge
_0.90
alloy film is confirmed to be not fully relaxed, and the remaining strain is quantitatively determined to be around 0.1%.