  Optical second-harmonic generation is demonstrated to be a sensitive probe of
the buried interface between the lattice matched semiconductors gallium
phosphide and silicon with (001) orientation. Rotational anisotropy
measurements of SHG from GaP/Si show a strong isotropic component of the
response not present for pure Si(001) or GaP(001). The strength of the
overlaying anisotropic response directly correlates with the quality of the
interface as determined by atomically resolved scanning transmission electron
microscopy.Optical second-harmonic generation is demonstrated to be a sensitive
probe of the buried interface between the lattice matched semiconductors
gallium phosphide and silicon with (001) orientation. Rotational anisotropy
measurements of SHG from GaP/Si show a strong isotropic component of the
response not present for pure Si(001) or GaP(001). The strength of the
overlaying anisotropic response directly correlates with the quality of the
interface as determined by atomically resolved scanning transmission electron
microscopy. Systematic comparison of samples fabricated with different growth
modes in metal organic vapor phase epitaxy reveals that the anisotropy for
different polarization combinations can be used as a selective fingerprint for
the occurrence of anti-phase domains and twins. This all-optical technique can
be applied as an {\it in-situ} and non-invasive monitor even during growth.
Systematic comparison of samples fabricated with different growth modes in
metal organic vapor phase epitaxy reveals that the anisotropy for different
polarization combinations can be used as a selective fingerprint for the
occurrence of anti-phase domains and twins. This all-optical technique can be
applied as an {\it in-situ} and non-invasive monitor even during growth.
