Interfaces of strained layer (GenSim)p superlattices studied by second-harmonic generation

Second-harmonic (SH) generation has been applied to study the interfaces of short-period (GenSim)p strained layer superlattices. From the SH signal and its symmetry, we have concluded that (i) over an area of optical wavelength size, the superlattices were grown with nonuniform layer thickness in each period; (ii) along the growth direction, the atomic structure of Si-on-Ge and Ge-on-Si interfaces are not symmetric; and (iii) a structural change of the interfaces, possibly from the increase of misfit dislocation or other defect density, occurs when the thickness of the strained Ge layer in each period exceeds about six layers. The SH spectroscopic results showed defects contributed dominantly for (Ge7Si8)4, but substantial contribution from Si–Ge bonds for (Ge4Si4)3 and (Ge5Si5)3 still exists.