  Polarized and azimuthal dependencies of optical second harmonics generation
(SHG) at the surface of noncentrosymmetric semiconductor crystals have been
measured on polished surfaces of ZnSe(100), using a fundamental wavelength of
1.06$\mu m$. The SHG intensity patterns were analyzed for all four combination
of p- and s-polarized incidence and output, considering both the bulk and
surface optical nonlinearities in the electric dipole approximation. We found
that the measurement using $S_{in}-S_{out}$ is particularly useful in
determining the symmetry of the oxdized layer interface, which would lower the
effective symmetry of the surface from $C_{4v}$ to $C_{2v}.$ We also have shown
that the [011] and [0$\bar{1}$1] directions can be distinguished through the
analysis of p-incident and p-output confugration.
