2ND-HARMONIC GENERATION FROM SI(M)GE(N) SUPERLATTICES

We report the observation of optical second harmonic generation (SHG) from Si(m)Ge(n) short-period superlattices grown on different substrat es. For a 775 nm fundamental beam, the SHG signals from odd-odd, even- even, and even-odd specimens are comparable, apart from substrate-depe ndent strain enhancement. It is concluded that the lack of a large bul k dipole-allowed signal in odd-odd superlattices predicted by Ghahrama ni, Moss, and Sipe [Phys. Rev. Lett. 64, 2815 (1990)] is due to the in evitable presence of monatomic steps on the substrate which lead to an tiphase domains; a possible circumvention of this limitation is outlin ed based on the use of a single domain vicinal Si(001) substrate.