Carrier-induced transition from excitonic to free-carrier-like radiative recombination in a semiconductor quantum well studied by magnetoluminescence

We have investigated the magnetic-field dependence of the photoluminescence of an $n$-type modulation-doped ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}/\mathrm{GaAs}$ single-quantum-well structure while changing the electron density in the well from zero to 7 × ${10}^{11}$ ${\mathrm{cm}}^{-2\mathrm{}}$ via an applied gate voltage. A clear transition from excitonic to free-carrier recombination was observed. At intermediate electron density, the existence of magnetoexcitons is suggested.