The quantum confined Stark effect and Wannier Stark ladders in InxGa1-xAs quantum wells and superlattices.

Abstract

The effects of an applied bias in the longitudinal or growth direction on four In$\sb{\rm x}$Ga$\sb{\rm 1-x}$As-GaAs strained single quantum wells and three strained layer superlattices have been studied using photocurrent and electroreflectance spectroscopy at liquid helium temperature. Weak applied electric fields on the quantum well samples gives rise to a red quadratic shift to the lowest interband transition between the first confined electron (E1) and heavy-hole (H1) levels, the quantum confined Stark effect (QCSE). The magnitude of the QCSE increases with well width. This field dependence becomes subquadratic at high applied fields due to carrier accumulation on the low energy side of the wells. Superlattices with relatively small periods, i.e. 10 nm, exhibit interwell coupling giving rise to a miniband structure under flatband conditions. The application of an electric field removes the interwell coupling giving rise to a ladder like progression in energy for the interband transition energies, called Wannier Stark ladders. The measured exciton transition energies follow a linear field dependence given by the product of the Stark ladder index, the superlattice period, and the electric field. The low field behaviour is more complex due to the Coulomb interaction between the electrons and heavy-holes. The measured field dependent exciton transition energies for the quantum wells agree well with single particle model calculations, while for the superlattice samples the exciton Stark ladder calculations of Dignam and Sipe have yielded good agreement with the measured data.