Trions, excitons, and scattering states in multiple quantum wells with a variable-concentration electron gas

CdTe/Cd1-xZnxTe multiple quantum wells were modulation doped with indium donors compensated by nitrogen acceptors so that the two-dimensional electron concentration in the wells (n(e)) could be varied from near 0 up to approximate to10(11) cm(-2) by optical pumping. In zero field at T=2 K, the optical absorption spectra show trion (X-) and exciton (X) resonance peaks at low n(e), with an electron-exciton scattering wing extending to high energy from the exciton resonance. At the highest n(e), the spectrum evolves towards the single asymmetric peak traditionally associated with the many-body "Fermi edge singularity" but its total integrated intensity remains almost constant, in agreement with recent few-body theories of the optical response at n(e)<1/a(B)(2). Under magnetic field B=8 T at T=2 K, sharp X- and X resonance peaks are seen as well as a broad band Z situated about homega(ce) (the electron cyclotron energy) higher in energy. Band Z is attributed to a known exciton-electron scattering process [Yakovlev , Phys. Rev. Lett. 79, 3974 (1997)] where the electrons are magnetically quantized. In sigma(+) circular polarization, the X resonance attenuates rapidly with n(e) but the X- resonance grows almost as rapidly ("intensity sharing") so that their intensity sum falls only slowly. In sigma(-) the X resonance also attenuates rapidly with n(e) and the Z band grows to compensate, with the intensity sum again falling only slowly. It is concluded that the spectrum evolution as n(e) varies from 0 to 10(11) cm(-2) in CdTe is due to intensity sharing between the X and X- resonances and between these resonances and scattering processes. This is a low n(e) (and low B) model of the excitonic properties, where screening and phase-space filling contribute only to the <10% decrease of the oscillator strength sums. As regards the samples' luminescence properties, two series of phonon peaks seen in emission spectra are attributed to recombination of two-dimensional electrons with nitrogen acceptors that have migrated close to and into the wells.