We present the results of a detailed time resolved luminescence study carried out on a very high quality InGaAs quantum well sample where the contributions at the energy of the exciton and at the band edge can be clearly separated. We perform this experiment with a spectral resolution and a sensitivity of the set-up allowing to keep the observation of these two separate contributions over a broad range of times and densities. This allows us to directly evidence the exciton formation time, which depends on the density as expected from theory. We also evidence the dominant contribution of a minority of excitons to the luminescence signal, and the absence of thermodynamical equilibrium at low densities.PACS numbers: 71.35.Cc,71.35.Ee,73.21.Fg,78.47.+p,78.67.De Excitons in quantum wells form quite an appealing quasiparticle showing a large range of optical properties that have proven at the same time technologically useful, and physically interesting [1]. A large part of this interest is linked with the appearance of excitonic resonances in absorption up to room temperature. It is also well known, since the seminal work of Weisbuch et al [2], that free excitons appear to dominate the luminescence response of semiconductor quantum wells at low temperatures. Part of the origin of this effect lies in the breakdown of the translational symmetry which brings a very efficient recombination channel to free excitons in quantum wells [3,4].Interestingly, in the low density regime, time resolved luminescence (TR-PL) in quantum wells is observed to be always dominated by light coming at the exciton energy, even under non resonant excitation. The observations of this dominant contribution are so numerous that only a very partial list of references may be given here [5,6,7,8,9] (in order to be specific, we only consider here the case of quantum wells grown on GaAs substrates). A double question has then been debated for more than 10 years in the literature: first how do free electron hole pairs bind into excitons and second does indeed the luminescence at very short time proceed from bound excitons. A brief survey of the literature allows to find that experimentalists have reported formation times ranging from less than 10 ps up to about 1 ns [7,8,9, 10] and theoretical values range from 100 ps [11,12,13] to over 20 ns [14]. Clearly, the origin of this spreading in the reported values lies in the poor sensitivity of the experiments used in general to probe the exciton formation process, except for the case of the recent terahertz absorption experiments [10]. On the theoretical side, binding of an electron hole pair into an exciton requires, at low temperatures, the emission of an acoustic phonon, which brings long formation time due to the small coupling of acoustic phonons to excitons.The long formation time of excitons, together with the observation of luminescence at the exciton energy at the shortest times [7,15] led Kira et al [16] to introduce the idea that a free electron hole plasma, properly including Coulomb correla...

We have studied density-dependent time-resolved photoluminescence from a 80 A InGaAs/GaAs single quantum well excited by picosecond pulses. We succeed in giving evidence for the transition from an exciton-dominated population to an unbound electron-hole pair population as the pair density increases. For pair densities below this excitonic Mott transition we observe a spectrally separate emission from free electron-hole pairs in addition to excitonic luminescence, thereby proving the coexistence of both species. Exciton binding energy and band gap remain unchanged even near the upper bound of this coexistence region. Above the Mott density we observe a purely exponential high energy tail of the photoluminescence and a redshift of the band gap with pair density. The transition occurs gradually between 1 x 10(10) and 1 x 10(11) cm(-2) at the carrier temperatures of our experiment.

We show a double path mechanism for the formation of charged excitons (trions); they are formed through bi-and trimolecular processes. This directly implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is substantiated by time-resolved photoluminescence experiments performed on a very high quality In x Ga 1Àx As quantum well sample, in which the photoluminescence contributions at the energy of the trion and exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor quantum well is explained by the same model. Positively and negatively charged excitons (X þ and X À trions) [1,2] are usually compared to their atomic counterpart ions of helium He þ and hydrogen H À , respectively. In astronomy, the dynamics of the formation of these atomic ions is of great importance [3]; indeed, H À is the primary source of the continuum opacity in most stellar photospheres and contributes to the production of hydrogen and other elements in various parts of the Universe. Additionally, the abundance of free electrons in the solar atmosphere is indirectly measured in terms of H À concentration. In semiconductor quantum wells (QWs), trions show a number of properties very similar to excitons [4], such as strong coupling in microcavities [5], absorption bleaching [6,7], transport [8] and diffusion [9] properties, and radiative recombination efficiency [10,11], and thus have attracted considerable interest. Moreover, trions promise to play a key role in future applications, notably in quantum-information science [12] and in the future development of all-spin-based scalable quantum computers [13,14]. They are correlated with excitons and the free carrier plasma and offer the possibility to test a model of formation of three-particle complexes.The formation process of neutral excitons (X) in QWs has been extensively investigated over the past two decades [15,16] and recently shown to be strongly density-and temperature-dependent [17]. This is a bimolecular process, in which an electron (e) and a hole (h) are bound by Coulomb interaction with the emission of the appropriate phonon. Conversely, the formation process of trions has been much less studied. It is largely believed that trions can be formed only if a population of excess carriers is trapped in the well, producing exclusively trions with the same charge. Consequently, existing models discriminate the formation channel yielding trions of opposite charge. Current models for trion formation [18,19] surmise that trions are exclusively formed through a bimolecular process, i.e., the coalescence of an exciton and a charged free carrier. While this is conceivable at low densities, nothing attests that genuine formation of the trion from an unbound electron-hole plasma (trimolecular formation) is negligible at higher densities.In...

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