Charged exciton dynamics in GaAs quantum wells

Le Journal de Physique IV, 1993

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1994

We present a comprehensive investigation of the dynamics of resonantly excited nonthermal excitons in high-quality GaAs/Al Ga& "As multiple-quantum-well structures on picosecond time scales. The dynamics was investigated using the luminescence upconversion technique with two independently tunable, synchronized dye lasers, which allowed measurements of the time evolution of polarized resonant luminescence with 4-ps time resolution. We show that the evolution of excitons from the initial nonthermal distribution to the thermal regime is determined by three different physical processes: (1) the enhanced radiative recombination of the metastable two-dimensional exciton polaritons, (2) the spin relaxation of excitons, and (3) the momentum relaxation of excitons. We also show that these three processes have comparable rates, so that a unified model accounting for all important processes is essential for a correct analysis of the experimental results. Using such a unified model, we have determined the rates of these processes contributing to the initial relaxation of excitons as a function of quantum-well width, temperature, and applied electric field. Quantum confinement strongly influences the radiative recombination and spin relaxation of excitons, and our study provides significant insights into these processes in quantum wells. The measured radiative recombination rate is about a factor of 2 smaller than calculated theoretically. The electric field reduces the electron-hole overlap and hence reduces the spinrelaxation rate of excitons between the optically allowed~+1) states. The measured variation is in good qualitative agreement with a recent theory, but somewhat slower than predicted by the theory.

Physical Review Letters, 1991

Superlattices and Microstructures, 1989

The dynamics of exciton formation and decay are investigated in GaAs-AlxGal_xAS (x < 0.Is) multiquantum well structures under low excitation conditions by transient photoluminescence spectroscopy. The time resolution (10ps) is achieved by using the frequency up-conversion technique. At low temperature (T < 50 K), the luminescence is dominated by localized exciton formation and recombination processes: significant Stokes shifts from the exciton absorption line, long non-exponential rise time (ranging from 200 ps to 600 ps) and short radiative lifetime (from 200 ps to Q00 ps) are observed. At higher temperature (T > 80 K), the luminescence is dominated by free exciton radiative recombination with shorter formation time and much larger exciton lifetime (from 3 ns to 6 ns). We suggest that the exciton may trap efficiently at low temperature on interface defects, the areal density of which can be evaluated in a simple manner.

Extended Abstracts of the 1993 International Conference on Solid State Devices and Materials, 1993

We present the detailed analyses of temporal profiles of photoluminescence from heavy-hole excitons in AlGaAs/GaAs quantum wells. We have observed the two-component exponential decay when the excitation density is low. We have confirmed by measuring the lateral spatial motion of excitons that the fast component is attributed to radiative recombinations of free excitons while the slowly decaying component is due to localized excitons. To the best of our knowledge, this is the first direct observation of localization of 2-dimensional excitons in quantum wells.

physica status solidi (c), 2005

We present the results of a detailed time resolved luminescence study carried on a very high quality In-GaAs quantum well sample. We clearly separate the contributions to the total intensity at the energy of the exciton and at the band by performing experiments with the proper spectral resolution and sensitivity, allowing to keep the observation of these two separate contributions over a broad range of times and densities. This enables us to access to the exciton formation rate over a broad range of pair densities. We also evidence the dominant contribution of excitons to the luminescence signal even when their population is small, and the absence of thermal equilibrium between excitons and free carriers at low enough densities.

Europhysics Letters (EPL), 1990

We report on continuous-wave and time-resolved measurements of the photoluminescence of goodquality single quantum wells at low temperatures (4-30 K). Luminescence arising from both free and localized excitons is observed and the inhuence of excitation localization on the photoluminescence decay time is investigated. Resonant pumping at the light-hole exciton transition is found to greatly increase the generation of free heavy-hole excitons. In this case, the free-exciton lifetime increases linearly with temperature (10-30 K) and the dependence of the slope on the well thickness is found to be in good quantitative agreement with the theoretical model of Andreani et al. [Solid State Commun. 77, 641 (1991)].

El Instituto Colombiano de Normas Técnicas y Certificación, ICONTEC, es el organismo nacional de normalización, según el Decreto 2269 de 1993.

Epithermal Cu-Au deposits hosted within volcanic rocks (Radka, Elshitsa, Krassen) are related to Late Cretaceous andesite-dacite volcanic terrain in the Panagyurishte ore district. The Cu-Au ores are linked by a similar mineralogy and differ by the ratio of tennantite, bornite, enargite and discrete trace minerals of Ga, Ge, In and Bi (e.g., roquesite, germanite, betekhtinite, renierite, vinciennite, aikinite). Bi-Se-Te and Ga-Ge-In-Sn signature with pronounced Au-enrichment of the bornite rich ores is a characteristic feature underlying the increasing role of the fS2/fO2 control during the transition from IS to HS environment. Formation of the epithermal Cu-Au deposits appears to have occurred during a single broad event of contemporaneous formation of epithermal and porphyry systems.The close connection between the volcano-plutonic structures facilitates the multistage and polycyclic character of their hydrothermal systems, the similar character of the epithermal ores and the mineral succession in Elshitsa, Radka and Krassen deposits. The δ 34 S ratios in the sulphide minerals range from -6.7 to 4.0, suggesting comparable magmatic sources for the epithermal mineralizing fluids and close link with porphyry environment.