Limitations to the realization of noncentrosymmetricSimGensuperlattices

We extend the work of Bottomley et al. [Appl. Phys. Lett. 63, 2324 (1993)] on second-harmonic generation from short-period (${\mathrm{Si}}_{\mathit{m}}$ ${\mathrm{Ge}}_{\mathit{n}}$ ${)}_{\mathit{p}}$ strained-layer superlattices (SLS) with n and m both odd integers. Such structures are theoretically predicted to be noncentrosymmetric and therefore should possess a large bulk, nonlinear susceptibility ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$. However, the prior work showed that the inevitable presence of single atomic-height steps on the Si(001) substrate leads to the formation of antiphased domains within the SLS and hence a macroscopically centrosymmetric material. Here we report second-harmonic generation induced by a 775-nm beam on several samples of nominally (${\mathrm{Si}}_7$ ${\mathrm{Ge}}_3$ ${)}_{20}$ SLS’s grown on several substrates vicinal to Si(001), including those with biatomic-height steps; the latter substrates should allow in-phase domains to be grown. A nonzero, but weak bulk ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ is obtained. We propose that the weak nonlinear response is influenced by nonideal superlattice periodicity which results in the formation of antiphase domains along the growth direction. Suggestions are offered for how noncentrosymmetric SLS’s may eventually be realized.