Observation of negatively charged excitons ${\mathit{X}}^{\mathrm{-}}$ in semiconductor quantum wells

The negatively charged exciton ${\mathit{X}}^{\mathrm{-}}$ is identified by its circular polarization properties in 1.7 K magentoabsorption spectra of CdTe/${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te multiple quantum wells modulation doped with electron concentrations ${\mathit{N}}_{\mathit{s}}$≊2×${10}^{10}$ to 1.5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. The binding energy of the second electron of ${\mathit{X}}^{\mathrm{-}}$ is 0.20 3D Rydbergs at 100 Å well width. The species ${\mathit{X}}^{\mathrm{-}}$ and the neutral exciton X exist in zero field for low ${\mathit{N}}_{\mathit{s}}$. At ${\mathit{N}}_{\mathit{s}}$=1.45×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ a Fermi edge singularity is seen at B=0 but ${\mathit{X}}^{\mathrm{-}}$ appears with field, at filling factor ν=2, while X appears at ν=1.