BAND-TO-BAND TRANSITIONS IN STRAIN-SYMMETRIZED, SHORT-PERIOD SI/GE SUPERLATTICES

Optical interband transitions were studied in a series of short-period, strained-symmetrized superlattices grown by molecular beam epitaxy. The spectroscopical studies were performed using both mesa photodiodes and undoped superlattice layers. Apart from defect-related signals at lower energies, transitions which can be directly attributed to the band gap in the superlattices were observed. The interband transitions in the energy range 0.6-0.83 eV shift to higher energies with decreasing superlattice period in an (n:n) series of superlattices with identical silicon/germanium ratio, i.e. 6:6, 5:5 and 4:4 respectively, demonstrating that the signal does not originate from the relaxed Si/Ge alloy buffer layer. For comparison samples consisting of an Si0.5Ge0.5 alloy were also investigated where the band gap was detected to be about 0.94 eV.