Dj. Bottomley et al., LIMITATIONS TO THE REALIZATION OF NONCENTROSYMMETRIC SIMGEN SUPERLATTICES, Physical review. B, Condensed matter, 50(12), 1994, pp. 8564-8568
We extend the work of Bottomley et al. [Appl. Phys. Lett. 63, 2324 (19
93)] on second-harmonic generation from short-period (SimGen)(p) strai
ned-layer superlattices (SLS) with n and m both odd integers. Such str
uctures are theoretically predicted to be noncentrosymmetric and there
fore should possess a large bulk, nonlinear susceptibility chi((2)). H
owever, the prior work showed that the inevitable presence of single a
tomic-height steps on the Si(001) substrate leads to the formation of
antiphased domains within the SLS and hence a macroscopically centrosy
mmetric material. Here we report second-harmonic generation induced by
a 775-nm beam on several samples of nominally (Si7Ge3)(20) SLS's grow
n on several substrates vicinal to Si(001), including those with biato
mic-height steps; the latter substrates should allow in-phase domains
to be grown. A nonzero, but weak bulk chi((2)) is obtained. We propose
that the weak nonlinear response is influenced by nonideal superlatti
ce periodicity which results in the formation of antiphase domains alo
ng the growth direction. Suggestions are offered for how noncentrosymm
etric SLS's may eventually be realized.