We report the optical properties of the modulated n-doped ZnSe/BeTe/ZnSe type-Ⅱ quantum wells. The reflection spectra have shown typical negatively charged exciton features only in a doped sample. The luminescence spectra and the polarization anisotropy depend strikingly on both the n-doping into the barrier layers and an applied external electric field perpendicular to the layer. These are explained by screening of the built-in electric field with n-doping and Stark effects due to the applied electric field. The electron density as well as the mass was determined in high magnetic field (up to 160 T) cyclotron-resonance measurements. It was found that the observed indirect PL transition occurs via the charged excitons in a type-II quantum configuration.