We report on a study of the spin coherence and on the formation dynamics of charged excitons in a p-doped CdTe/Cd 1ϪxϪy Mg x Zn y Te quantum well by time-resolved photoluminescence under strictly resonant excitation of either the neutral or the charged exciton transition. The analysis of the decay of the charged exciton photoluminescence polarization and of the oscillation of this polarization when a transverse magnetic field is applied allows us to conclude that the formation of a charged exciton via an exciton state does not affect either the electron spin orientation or its coherence. The radiative lifetime of the charged exciton is directly measured and is found to be ϳ60 ps. Its formation time is determined from the experiments via a detailed model and is found to be ϳ65 ps. We also obtain information on the excitonic spin-flip time ͑12 ps͒ and the single-carrier spin-flip times within the exciton ͑electron, 60 ps and hole, 20 ps͒.

To investigate the properties of a two-dimensional carrier gas at intermediate densities, we perform picosecond transient four-wave mixing experiments on trions ͑charged excitons͒ and neutral excitons in modulation p-doped CdTe/Cd 1ϪxϪy Mg x Zn y Te quantum wells. The determination of trion and exciton dephasing rates reveals a localization of both trions and holes in potential fluctuations induced by the ionized remote acceptors. We demonstrate that trions can be efficiently used as a charged optical probe sensitive to electrostatic potential fluctuations which are imperceptible for neutral excitons. ͓S0163-1829͑99͒15731-X͔ Modifications of optical spectra due to the introduction of carriers into a semiconductor quantum well ͑QW͒ recently have been of considerable interest, in particular as they provide information on the electronic properties in the vicinity of the metal-insulator transition, and on the role of disorder. 1 It has been known for a long time that excitons characterize the optical spectra of undoped semiconductors while bandto-band transitions are observed on strongly doped samples. It has been recognized more recently that spectra of moderately doped QW's feature also charged excitons, either positively or negatively according to the type of doping-the so-called trions. 1-5 Extensive investigations have been reported on the binding energy of trions, 2-5 on their polarization in a magnetic field, 2,3 and on their behavior in an electric field 6 and at different carrier densities. 1,3,4,7 Up to now, however, only very few papers have been devoted to the dynamics of trions, including transport and relaxation. [7][8][9] Actually, there has been some controversy in the literature about the possible localization of trions in modulation doped QW's ͑MDQW͒, in which carrier gases are formed in the QW's by introducing doping layers in the barriers. It was argued that free carriers would screen the Coulomb interaction thus forbidding the formation of excitons and trions. 1,2 Hence the charge carriers should be localized by electrostatic potential fluctuations caused by the ionized remote donors or acceptors. Recent near-field photoluminescence experiments 10 directly evidence the localization of trions in n-doped GaAs QW's and conclude indirectly to the localization of the carriers themselves. Other authors 3,11 have reported the observation of free trions in high-quality MDQW's with relatively thick spacer layers, which decreases the fluctuation depth.In this paper we demonstrate that the study of the coherence properties of trions and excitons by degenerate fourwave mixing ͑FWM͒ is a very efficient method to investigate a possible localization of both trions and carriers in a MDQW. The idea is as follows: First, localized quasiparticles have a strongly reduced scattering efficiency as compared to free ones. Second, this scattering plays an important role in the dephasing of the quasiparticles. Consequently, the determination of exciton and trion dephasing rates under different conditions ͑exciton, trion a...

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