Excitonic effects in the miniband formation of graded-gap superlattices


The formation of an electron miniband has been investigated in two differently realized graded-gap superlattices (SL’s). The first SL is compositionally graded, while the second system consists of layers with different well and barrier thicknesses. The formation of the miniband in the conduction band is studied experimentally by photocurrent (PC), electroreflectance (ER), and photoluminescence (PL) spectroscopy. The experimental results are compared with the field dependence of calculated differential absorption spectra for both systems. In the compositionally graded SL, the formation of a miniband in the conduction band at finite electric fields can be clearly observed in the ER spectra. However, in the well width graded system, both the PC and the ER spectra are dominated by excitonic effects, which prevent the direct observation of the miniband formation.

Phys. Rev. B 55, 15720 (1997)