DIFFERENTIAL OPTICAL-ABSORPTION SPECTROSCOPY IN GE-SI SUPERLATTICES

We have used a differential spectroscopy technique to measure and characterize the optical absorbance spectra of Ge-Si superlattices. Because of the fundamental nature of the band-edge electronic structure, it is argued that the conventional models for absorption versus photon energy in bulk semiconductors may not apply to these Ge-Si superlattices. Results obtained on superlattices grown on Si show localized levels near the band edge, in contrast to those grown on Ge which display a band to band absorption spectrum. The superlattices grown on (001) Ge show more than 100 times the absorbance of those grown on Si, and the absolute magnitude of the absorption corresponds to a band-edge absorption coefficient greater than 10(3) cm-1.