Binding energy of charged excitons in ZnSe-based quantum wells

Excitons and charged excitons (trions) are investigated in ZnSe-based quantum well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of magneto-optical spectroscopy. Binding energies of negatively ${(X}^{\ensuremath{-}})$ and positively ${(X}^{+})$ charged excitons are measured as functions of quantum well width, and free carrier density and in external magnetic fields up to 47 T. The binding energy of ${X}^{\ensuremath{-}}$ shows a strong increase from 1.4 to 8.9 meV with decreasing quantum well width from 190 to $29 \AA{}.$ The binding energies of ${X}^{+}$ are about 25% smaller than the ${X}^{\ensuremath{-}}$ binding energy in the same structures. The magnetic field behavior of ${X}^{\ensuremath{-}}$ and ${X}^{+}$ binding energies differ qualitatively. With growing magnetic field strength, ${X}^{\ensuremath{-}}$ increases its binding energy by 35\char21{}150%, while for ${X}^{+}$ it decreases by 25%. Zeeman spin splittings and oscillator strengths of excitons and trions are measured and discussed.