Potential and magnetic field confinement of shallow donor impurities in semiconductor quantum wells

We report the observation and identification of an intersubband donor absorption line induced by applying a magnetic field in the plane of an ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs quantum-well structure which is doped with donors in the center of the GaAs wells. The in-plane magnetic field permits the transition from the donor ground state by coupling an optically allowed donor p state associated with the ground subband to a forbidden donor p state associated with the first-excited subband. The observed transition energies are compared in detail to the results of variational energy-level calculations.