Inhomogeneous deformation of silicon surface layers probed by second-harmonic generation in reflection

Abstract

Semiconductor crystals possessing inversion symmetry (Si, Ge) are known to have a rather weak second-order nonlinearity of the quadrupole type [ Phys. Rev. Lett.51, 1983 ( 1983)], since the electric-dipole contribution is forbidden by symmetry. We report the experimental observation of anomalously highly efficient second-harmonic generation (SGH) in reflection from the surface of Si under inhomogeneous deformation. This effect is believed to be due to an electric-dipole contribution to the second-order susceptibility induced in the near-surface layer by inhomogeneous mechanical stress. This fact is consistent with theoretical calculations based on the molecular sp3-orbital model. Experimentally we observed an increase in the second-harmonic intensity by more than 2 orders of magnitude and a modification of the second-harmonic intensity dependence on crystal orientation with respect to the surface normal in the case of ion-implanted, pulsed-laser-annealed Si (111) samples. A similar effect was observed with thermally oxidized Si wafers and silicide-on-cSi structures. These results demonstrate the sensitivity of SHG in reflection to the presence of inhomogeneous stress in Si surface layers, which enables one to use SHG for nondestructive monitoring of stress in semiconductor structures.