Recently, there has been considerable interest in the negatively charged exciton (X^-), which can be observed in photo-luminescence spectra of low-density, quasi-2D electron gases (2DEGs). Extensive experimental and theoretical work has been directed at understanding this electron-hole-complex since its discovery in 1993 by Kheng et al.[1]. The bulk of the experimental work to date concerns inter-band transitions. Internal (i.e., intra-band) X^- transitions, which lie in the far-infrared (FIR) region of the spectrum, can provide additional important insight into the properties of the ground and excited states of this complex. The X^- complex, consisting of a hole binding two electrons, is the semiconductor analog of the hydrogen ion, H^-, and is superficially quite similar to its close relative, the negatively charged donor, D^-, which has been studied extensively [2]. A crucial difference between the X^- complex and the D^- complex is the free motion of the center of mass (CM) of the former. A hidden symmetry associated with this free motion leads to a selection rule that prohibits families of bound-to-bound internal X^- transitions. In particular, the singlet and triplet transitions from the respective ground states associated with the lowest Landau level (LL) to excited localized states associated with the next highest LL are forbidden. In contrast these transitions dominate the FIR spectra of D^-. This selection rule applies to all charged and mobile complexes in a magnetic field, as long as the field is perpendicular to the plane of motion and the system has translational invariance [3]. Allowed electric-dipole transitions from the ground state are instead bound-to-continuum transitions, which exhibit a rich structure resulting from transitions to two magneto-exciton (MX) bands arising from the CM motion of a 1s (2p+) exciton with a scattered electron in the N_e = 1 (N_e = 0) electron LL, respectively (see Fig. 1). For the triplet state the resulting FIR absorption spectrum is dominated by an onset at each MX band, indicated by arrows (1) and (2) in the figure, and thus exhibits a characteristic double-peak structure. The X^- singlet transitions, which occur at finite fields and confinement, are qualitatively the same, as has been shown in a numerical calculation for a 200 Å GaAs/AlGaAs multiple quantum well (MQW) system [3]. We have used optically detected resonance (ODR) spectroscopy to study two undoped and one barrier-doped 200 Å wide GaAs/AlGaAs MQW structures which show neutral exciton (X) as well as X^- recombination-lines in photoluminescence measurements. This work represents an extension of earlier studies of internal transitions of neutral magneto-excitons [4]. In ODR spectroscopy the changes (intensity and shape) of a particular photoluminescence feature induced by resonant absorption of FIR radiation are monitored. The measurements, which were performed at low temperatures (4.2 K) in the Faraday geometry in magnetic fields of up to 15 T, show, in addition to cyclotron resonance, both singlet and triplet transitions of X^-, which become the dominant features in modulation doped structures. The experimental results are in good agreement with numerical calculations at 9 T in terms of both position and qualitative nature of the bands. No evidence was found for any features corresponding to the localized-to-localized state transitions that are dominant in magneto-spectroscopy of D^-.
 

[1] K. Kheng et al., Phys. Rev. Lett. 71, 1752, 1993.
[2] Z.X. Jiang et al., Phys. Rev. B 56, R1692, 1997, and references therein.
[3] A.B. Dzyubenko et al., submitted Phys. Rev. Lett., 1999.
[4] H.A. Nickel et al., Physica B, 249-251, 598-602, 1998.