Realistic heterointerfaces model for excitonic states in growth-interrupted quantum wells

We present a model for the disorder of the heterointerfaces in GaAs quantum wells including long-range components like monolayer island formation induced by the surface diffusion during the epitaxial growth process. Taking into account both interfaces, a disorder potential for the exciton motion in the quantum well plane is derived. The excitonic optical properties are calculated using either a time-propagation of the excitonic polarization with a phenomenological dephasing, or a full exciton eigenstate model including microscopic radiative decay and phonon scattering rates. While the results of the two methods are generally similar, the eigenstate model does predict a distribution of dephasing rates and a somewhat modified spectral response. Comparing the results with measured absorption and resonant Rayleigh scattering in GaAs/AlAs quantum wells subjected to growth interrupts, their specific disorder parameters like correlation lengths and interface flatness are determined. We find that the long-range disorder in the two heterointerfaces is highly correlated, having rather similar average in-plane correlation lengths of about 60 and 90 nm. The distribution of dephasing rates observed in the experiment is in agreement with the results of the eigenstate model. Finally, we simulate highly spatially resolved optical experiments resolving individual exciton states in the deduced interface structure.Comment: To appear in Physical Review

Level-statistics in the resonant Rayleigh scattering dynamics of monolayer-split excitons

The dynamics of resonant Rayleigh scattering (RRS) from excitons localized in a disorder potential with large monolayer islands is investigated theoretically and experimentally. Experimentally, the coherent RRS dynamics from GaAs single quantum wells (SQWs) is deduced by means of passively stabilized spectral interferometry. The new technique is discussed in detail. The results retrieved by spectral interferometry are complemented by data obtained by time-resolved and spectral speckle analysis. A strong dependence of the monolayer thickness of the SQW on the RRS spectra and the RRS dynamics is found. Changes in the well width corresponding to a fraction of one monolayer lead to drastic changes in the spectral and time-resolved RRS intensity. A comparison between spectra and dynamics contradicts the assumption of uncorrelated exciton levels. The RRS intensity is derived theoretically by solving the Schrodinger equation for the center-of-mass motion of the 1s-exciton in a disorder potential taking into account both interfaces with monolayer islands, the short range correlated disorder by segregation, and a correlation between the interfaces. The simulations reproduce qualitatively all observed features, determining in this way the characteristic disorder parameters like island sizes and interface roughness. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Determining the structure of semiconductor heterointerfaces by excitonic optical spectra

A combined theoretical and experimental analysis of the resonant Rayleigh scattering by excitons in a disordered GaAs quantum well with monolayer islands is reported. Experimentally, the coherent resonant Rayleigh scattering is deduced by means of both statistical speckle analysis and passively stabilized spectral interferometry. Theoretically, the effect of disorder is modelled by solving the time-dependent Schrodinger equation for the exciton center-of-mass motion. The disorder potential used in the model includes the detailed monolayer structure of both heterointerfaces, the short-range disorder due to segregation, and a correlation between the upper and lower interfaces. The simulations reproduce all features observed both in frequency- and time-resolved measurements. The comparison between theory and experiment bears strong evidence that the monolayer fluctuations in the two heterointerfaces are highly correlated and have similar length scales

Spectral speckle analysis of resonant secondary emission from solids

A linear optical method to measure coherence and dephasing of excitations in solids is presented. The spectrally resolved degree of coherence of resonantly scattered light is deduced from the intensity fluctuations over its emission directions (speckles). The spectral intensity correlation gives a direct measure of the dephasing rate within the inhomogeneously broadened ensemble. For localized excitons in semiconductor quantum wells, the combination of static disorder, phonon scattering, and radiative decay leads to a spectral dependence of the emission coherence and the dephasing rate, which is well described by model calculations

Level-statistics in the resonant Rayleigh scattering dynamics of monolayer-split excitons

The dynamics of resonant Rayleigh scattering (RRS) from excitons localized in a disorder potential with large monolayer islands is investigated theoretically and experimentally. Experimentally, the coherent RRS dynamics from GaAs single quantum wells (SQWs) is deduced by means of passively stabilized spectral interferometry. The new technique is discussed in detail. The results retrieved by spectral interferometry are complemented by data obtained by time-resolved and spectral speckle analysis. A strong dependence of the monolayer thickness of the SQW on the RRS spectra and the RRS dynamics is found. Changes in the well width corresponding to a fraction of one monolayer lead to drastic changes in the spectral and time-resolved RRS intensity. A comparison between spectra and dynamics contradicts the assumption of uncorrelated exciton levels. The RRS intensity is derived theoretically by solving the Schrödinger equation for the center-of-mass motion of the 1s-exciton in a disorder potential taking into account both interfaces with monolayer islands, the short range correlated disorder by segregation, and a correlation between the interfaces. The simulations reproduce qualitatively all observed features, determining in this way the characteristic disorder parameters like island sizes and interface roughness

Density-matrix description of frequency-resolved secondary emission from quantum wells

The frequency-resolved secondary emission from excitons in a single 8 nm-wide quantum well is investigated using speckle analysis. We model these experiments starting with a Hamiltonian in the basis of disorder eigenstates of excitons, interacting both with light and acoustical phonons. The distinction between incoherent and coherent secondary emission is intimately related with the decomposition of a two-time exciton density-matrix into a diagonal, incoherent part and a product of polarizations. The latter gives rise to speckling (intensity variations over observation angle), and is called resonant Rayleigh scattering. The results of our simulation agree pretty well with the experimental data and allow a determination of the coherent and incoherent exciton distributions

Spectral speckle analysis: a new method to measure coherence and dephasing in semiconductor nanostructures

A new method to measure the coherence of inhomogeneously broadened optical excitations in semiconductor nanostructures is presented. The secondary emission of excitons in semiconductor quantum wells is investigated. The spectrally-resolved coherence degree of resonantly-excited light emission is deduced from the intensity fluctuations over the emission directions (speckles). The spectral correlations of the speckles give direct access to the homogeneous line width as function of spectral position within the inhomogeneously broadened ensemble. The combination of static disorder and phonon scattering leads to a partially coherent emission. The temperature dependence of the homogeneous line width is well explained by phonon scattering

Resonant Rayleigh scattering dynamics of excitons in single quantum wells

The resonant Rayleigh scattering dynamics of excitons in single GaAs quantum wells is investigated. The deviation of the measured intensity from the ideal ensemble average is analyzed as a function of the speckle ensemble size. The influence of the amplitude and correlation length of the exciton disorder potential is traced using a series of samples with varying inhomogeneous broadening and interface island sizes. The experimental data are compared with theoretical predictions using exciton states of spatially uncorrelated energies, as well as full calculations of the exciton polarization dynamics using a more realistic disorder potential including the formation of monolayer islands. Deviations from the dynamics of uncorrelated states are found at early times after excitation. They are reproduced by the calculations possessing a state correlation due to quantum mechanical level repulsion and a finite correlation length for the disorder potential. Additionally, the presence of a long-range disorder potential in the micrometer range is suggested. Changing the disorder potential by varying the island size influences the observed dynamics systematically according to the different disorder correlation lengths