Miniband structures in semiconductor superlattices investigated by modulated-reflectance spectroscopy

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1. Miniband structure of (GaAs)_<10>/(AIAs)_m superlattices (SLs) with m=1,2,4, and 6 monolayrs (1 monolayr=-0.28nm) have been systematically investigated by using photoreflectance (PR) spectroscopy. The optical transitions at the GAMMA and pi (miniband edge) critical points were celarly observed in the SL with one-monolayr AlAs. Furthermore, Franz-Keldysh (FK) ascillations at the miniband critical points were successfully detected. From the analysis of the FK oscillation profiles, it has been found that the miniband mass at GAMMA point is almost equal to the mass of bulk GaAs and that the pi-point mass is bout one third of the GAMMA-point mass.2.In (GaAs)_<10>/(Al_<0.3>Ga_<0.7>As)_<10>SL and (InAs)_1/(GaAs)_mSLs with m=10 and 30 monolayrs, the optical transitions associated with the below-barrier and above-barrier minibands were clearly obwerved by using PR spectroscopy.In a (GaAs)_<25>/(Al_<0.1>Ga_<0.9>As)_<12>SL embedded in a p-i-n diode, the electric-field dependence of the optical transitions were observed by using electroreflectance (ER) spectroscopy. It has been found that the change of the FK-oscillation profiles with increasing the electric field reflects the transformation process from the minibands to the Wannier-Stark (WS) localization states. Furthermore, it has been confirmed that the electric field transforms the above-barrier minibands into the WS-localization states.The above experimental results are consistently explained by the miniband structures calculated by using an effective-mass approximation.