Tight-binding calculations: Electronic structure and optical properties of (CdS)n/(Ge2)m superlattices

We have performed electronic structure calculations for (CdS)n/(Ge2)m (110) superlattices (SLs) with n=m=2-16. In the case of (CdS)16/(Ge2)16 (110) SL, the indirect band gap which is located at the M point, is around 0.86 eV. The states at the conduction and valence-band edges are confined two dimensionally in the Ge layers. We found that the fundamental energy gap increases (up to 1.43 eV at the X point for n=m=2) with decreasing SL period. The present results suggest that the Ge layer plays an important role in determining the fundamental energy gap of the SL, and this is due to the spatial quantum confinement effects. In addition, the calculated absorption spectra of the SLs are found to be quite different from those of the bulk CdS and Ge, but fairly close to their average. The optoelectronic properties of the SLs composed of indirect band gap semiconductors offer great potential for device applications.