Direct observation of free-exciton thermalization in quantum-well structures

Umlauff, M.; Hoffmann, J.; Kalt, H.; Langbein, Wolfgang Werner; Hvam, Jørn Märcher; Scholl, M.; Söllner, J.; Heuken, M.; Jobst, B.; Hommel, D.

doi:10.1103/physrevb.57.1390

Cited by 127 publications

(110 citation statements)

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“…Emission of LO phonons leads to an efficient exciton formation and allows the preparation of spectrally narrow hot-exciton distributions [2,3]. Also, new quantum-kinetics phenomena like the coherent coupling of electron states by multiple phonons are observed [6].…”

Section: Introductionmentioning

confidence: 99%

“…It is thus possible to study exciton-exciton interactions in ZnSe QWs solely of 1s hh excitons [1]. The thermalization dynamics of 1s hh excitons within its dispersion parabola can be investigated starting from large center-of-mass momenta, i.e., high initial excess kinetic energies [2,3]. Also the properties of biexcitons are accessible in more detail than in III-V quantum wells [4,5].…”

Section: Introductionmentioning

confidence: 99%

“…Again, the interactions of the excitons among themselves and with phonons lead to a thermalization and the latter process also to a cooling of the exciton distribution. The distribution function of the (non-)thermal exciton population can be deduced from time-resolved phonon-sideband spectroscopy [2,3]. The LO phonon-assisted recombination of the excitons is here recorded by a synchroscan streak camera after generation of hot excitons using a picosecond pulse.…”

Section: Introductionmentioning

confidence: 99%

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Ultrafast Phenomena in II-VI Semiconductors

et al. 1998

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We review studies on the coherent and incoherent exciton dynamics in ZnSe-based quantum wells. First, the exciton-exciton scattering parameter is determined from femtosecond four-wave mixing as a function of background exciton density generated by a prepulse. A linear increase in the excitonic homogeneous linewidth is found as a function of the background exciton density with significant different scattering parameters for the cases of interaction with coherent or incoherent excitons. Second, the thermalization dynamics of spectrally narrow hot-exciton distributions is investigated by time-resolved phonon sideband spectroscopy. Thermalization assisted by acoustic phonons occurs on a 100 ps timescale, which is in good agreement with model calculations in time dependent perturbation theory.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Ultrafast Phenomena in II-VI Semiconductors

et al. 1998

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“…Therefore, this new feature should be considered for understanding the structure and dynamics of the optical response of II-VI QWs. 15 Broadening and splitting of the exciton peak at the lower band edge in III-V bulk systems have been reported, 16 and they have been explained as a consequence of the increasing of the phonon density of states. However, the exciton splitting in this last system is vaguely visible for two reasons: (i) the Fröhlich coupling is roughly one order of magnitude lower than in II-VI heteroestructures; (ii) the resonance between exciton binding energy and LO-phonon energy can not be achieved.…”

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confidence: 99%

Theory of optical spectra of polar quantum wells: Temperature effects

Rodríguez

2001

Phys. Rev. B

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Theoretical and numerical calculations of the optical absorption spectra of excitons interacting with longitudinal-optical phonons in quasi-2D polar semiconductors are presented. In II-VI semiconductor quantum wells, exciton binding energy can be tuned on-and off-resonance with the longitudinal-optical phonon energy by varying the quantum well width. A comprehensive picture of this tuning effect on the temperature-dependent exciton absorption spectrum is derived, using the exciton Green's function formalism at finite temperature. The effective exciton-phonon interaction is included in the Bethe-Salpeter equation. Numerical results are illustrated for ZnSe-based quantum wells. At low temperatures, both a single exciton peak as well as a continuum resonance state are found in the optical absorption spectra. By constrast, at high enough temperatures, a splitting of the exciton line due to real phonon absorption processes is predicted. Possible previous experimental observations of this splitting are discussed.

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“…1, 22 Initially, the ground ((n ¼ 1)) and the excited (e.g., (n ¼ 2)) excitonic states are populated. The effective radiative recombination is usually slow in bulk materials due to the well-known polariton propagation effects.…”

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Intra-excitonic relaxation dynamics in ZnO

Chernikov

Koch

Laumer

et al. 2011

Applied Physics Letters

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The temperature and carrier-density dependent excitonic relaxation in bulk ZnO is studied by means of time-resolved photoluminescence. A rate-equation model is used to analyze the population dynamics and the transitions between different exciton states. Intra-excitonic (n ¼ 1) to (n ¼ 2) relaxation is clearly identified at low excitation densities and lattice temperatures with a characteristic time constant of 6 6 0.5 ps. V C 2011 American Institute of Physics.[doi:10.1063/1.3668102] Over the last decades, ZnO has attracted much interest in the scientific community mainly due to its potential application for optoelectronic devices in the ultraviolet (UV) spectral regime. In addition, properties associated with the large exciton binding energy of 60 meV and the strong electron-phonon coupling are of fundamental interest. Recently, high quality ZnO samples have become available due to improved growth methods.2-6 These advances allow for accurate studies of the electro-optical properties of the material and are important steps towards the realization of ZnO-based technology. Significant research effort has been directed towards the growth and applications of quantum well structures, 7,8 ternary ZnO-based compounds, [9][10][11][12] electron-phonon interaction, 13 and exciton-polaritons.14 Even steady-state investigations of the excited excitonic states have become possible. 15,16 In this letter, we study the dynamics of the intraexcitonic processes by means of ultrafast emission spectroscopy with sub-picosecond time resolution. We apply a rateequation model to analyze the various relaxation channels and to investigate the influence of temperature and excitation density.All time-resolved photoluminescence (PL) measurements are performed using a streak-camera setup 12 yielding spectral and time resolutions of 0.15 nm and 500 fs, respectively; a pulsed 100 fs-Ti:sapphire laser is used as an excitation source. The investigated sample is a 300 nm thick c-plane ZnO layer grown by plasma-assisted molecular beam epitaxy.11 The emission signal is collected normally to the sample surface in reflection geometry. Structural analysis by high resolution x-ray diffraction was carried out and the lattice parameters were extracted from reciprocal space maps recorded at the asymmetric (20.5) reflex. The c-lattice parameter was determined to (5.2046 6 0.001) and the a-lattice parameter to (3.2515 6 0.001). Comparison to data reported in literature reveals that the present ZnO layer can be considered as unstrained. transition is attributed to the luminescence of the free B-excitons. Also, an additional, distinct emission peak is found at 3.423 eV. In a simplified picture, the excitonic level scheme resembles the energy structure of a hydrogen atom.1 In this case, the energy of an excited states is proportional to E b n 2 , where E b is the binding energy and n is the quantum number of the corresponding state. Thus, the energy of the first excited state with (n ¼ 2) is 1/4 E b and the separation between the (n ¼ 1) and (n ¼ 2) states eq...

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Direct observation of free-exciton thermalization in quantum-well structures

Cited by 127 publications

References 18 publications

Ultrafast Phenomena in II-VI Semiconductors

Ultrafast Phenomena in II-VI Semiconductors

Theory of optical spectra of polar quantum wells: Temperature effects

Intra-excitonic relaxation dynamics in ZnO

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