Miniband structures in semiconductor superlattices investigated by modulated-reflectance spectroscopy

1.用<10>光反射谱（PR）研究了（GaAs）_ /（AlAs）_m超晶格（m= 1，2，4和6）（1个单层=-0.28nm）的微带结构.在单层AlAs的超晶格中，观察到了Gamma和π（微带边）临界点的光学跃迁。此外，成功地检测到在miniband临界点的Franz-Keldysh（FK）ascillations。从FK振荡曲线的分析中发现，在Gamma点处的能带质量几乎等于体GaAs的质量，π点质量约为Gamma点质量的三分之一。2.In（GaAs）_<10>/（Al_<0.3>Ga_<0.7>As）_<10>SL和（InAs）_1/（GaAs）_mSL（m=10和30单层）用PR谱观察到了势垒以下和势垒以上的光跃迁，用电反射（ER）谱观察到了（GaAs）_<25>/（Al_<0.1>Ga_<0.9>As）_<12>SL在p-i-n二极管中的光跃迁随电场的变化.研究发现，FK振荡曲线随电场的变化反映了从能带到Wannier-Stark（WS）局域态的转变过程。进一步证实了电场使势垒以上的能带转变为WS局域态，用有效质量近似计算的能带结构解释了上述实验结果.

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.