Trion and exciton dephasing measurements in modulation-doped quantum wells: A probe for trion and carrier localization

Abstract

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...

All-Optical Trion Generation in Single-Walled Carbon Nanotubes

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Biexciton, single carrier, and trion generation dynamics in single-walled carbon nanotubes

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Trions in a periodic potential

“…The observation of (negatively) charged excitons [1] has recalled a forty-year old concept of excitons "trions" or "charged excitons" introduced by Lampert in 1958 [2]. Recently, such excitons, consisting of two holes and an electron or two electrons and hole (denoted X ± , respectively), have been investigated both experimentally and theoretically [3][4][5].…”