Effects of an electric field on the continuum energy levels in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs quantum wells terminated with thin cap layers

Electroreflectance was used to study the influence of an electric field on the bound and the continuum states of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs quantum-well structures of various well widths, terminated by thin cap layers. Optical transitions involving only bound states were not significantly affected for the cap-layer thicknesses studied, and are Stark shifted as expected by the electric field. However, strong oscillations are observed in the spectra at energies larger than the barrier band gap, and are due to quantum interference in the continuum state wave functions which is related to the finite size of the cap layer. These oscillations, which do not follow a Franz-Keldysh relation, shift linearly with the electric field.