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We have used photoluminescence (PL) spectroscopy to study the population properties of an asymmetric GaAs/(Al,Ga)As double quantum well superlattice, which represents a simplified version of the active region of a quantum cascade laser. The investigation focuses on the anti-Stokes PL signal of the narrower well, which is observed for excitation between the excitonic states of the two quantum wells and sufficiently high electric field strengths. In this case, the photocarriers are only excited in one of the two wells, and the signal gives direct evidence for transport of electrons and holes through the (Al,Ga)As barriers. An analysis of the electric-field dependence of conventional PL as well as anti-Stokes PL demonstrates that population inhomogeneities for the electronic states can be determined.

Metal–semiconductor contacts on highly Be-doped GaAs layers grown by molecular beam epitaxy are used to examine deep electronic states by deep-level transient Fourier spectroscopy. Two hole traps at 0.40 and 0.70 eV are found, which are associated with the GaAs (Ga atom on As site) defect. The concentration versus depth profile demonstrates that the intrinsic defect is formed in a 100 nm thick layer near the as-grown surface. Our experimental results confirm thermodynamical calculations for nearly Ga-rich conditions during growth. The GaAs formation energy strongly decreases, when the Fermi level is shifted from the valence band edge to midgap position. The concentration versus depth profile therefore reflects the change of the GaAs formation energy near the GaAs surface during growth. The energy to form GaAs defects at the surface of p-type GaAs is determined to be 1.1 eV, in full agreement with theoretical results.

A new spectroscopic technique is demonstrated on three characteristically different semiconductor quantum-well samples. Coherent excitation spectroscopy reveals coherent coupling and effectively suppresses the signatures of inhomogeneous broadening and incoherent transfer between various excitonic states in quantum wells. Coherently coupled homogeneous subsystems are analyzed in an inhomogeneously broadened excitonic ensemble.

The influence of carrier–carrier scattering on intraband coherence is studied by analyzing the dephasing of the THz emission from optically excited Bloch oscillations in a GaAs/AlGaAs superlattice. Intercarrier collision-induced dephasing is investigated by varying both the density of coherent scattering partners as well as the density of an incoherent carrier background. Both situations yield homogeneous linearly density-dependent contributions. Dephasing due to an incoherent background density is shown to be twice as efficient as scattering by coherent carriers. The efficiency of incoherent scattering is shown to depend moderately on the time delay to the pre-pulse generating the background density.

The interaction between a surface acoustic wave (SAW) and photogenerated carriers in GaAs/ Al 0.3 Ga 0.7 As quantum well (QW) structures is investigated using micro-photoluminescence (μ-PL). The excitonic luminescence becomes strongly suppressed in the presence of a SAW due to the ionization of the excitons and to the transport of the photogenerated carriers by the piezoelectric field. We demonstrate that the degree of μ-PL quenching and the carrier transport efficiency depends not only on the intensity of the piezoclectric field but also on the carrier mobility in the QW.

The structural perfection of (In,Ga)As- and GaAs quantum wells and distributed Bragg reflectors for cavity structures on (110)GaAs grown by molecular-beam epitaxy is investigated. The relaxation mechanism of accumulated elastic strain is studied by transmission electron microscopy. Differences to structures with (001) orientation are addressed. The use of short-period superlattices for Bragg mirrors stabilizes the structures grown on (110)GaAs against relaxation-based defect formation as well as degradation of bare sidewalls when exposed to ambient air. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface acoustic waves (SAW) are employed to transport optically generated spin ensembles over distances exceeding 60 μ m in (1 1 0) GaAs quantum wells (QW). The dependence of the spin–orbit (SO) coupling on carrier momentum is investigated by using SAWs to transport spins with well-defined velocity along different directions in the QW plane. For transport along the [0 0 1] direction, the high relaxation rates for the in-plane spin component lead to fast spin decoherence under a magnetic field. For the [ 1 ¯ 1 0 ] direction, in contrast, a non-zero average value of the SO internal magnetic field retards the longitudinal spin relaxation.

We studied the Rayleigh light scattering and the optical emission in GaAs-(Al,Ga)As superlattices with ordered and intentionally disordered potential profiles, correlated and uncorrelated, in external magnetic fields. We find that the intentional disorder along z affects the resonant Rayleigh scattering and the optical emission. The external magnetic field in the same direction allows to modify the exciton localization and to study the relative modification of the light scattering and of the emission spectra.

Time-resolved degenerate four-wave mixing experiments on a multiple-quantum-well Bragg structure reveal a complex real-time pattern of the nonlinear optical response from the superradiant and the subradiant coupled exciton-photon modes. We find a free-polarization decay for the superradiant mode and a photon-echo from the subradiant modes. Rotation of the Bragg structure with respect to the incident laser beams implying a reduction of the effective quantum well separation results in a distinct change of the temporal behaviour. We estimate a value of less than 40 μeV for the nonradiative contribution to the homogeneous linewidth of free excitons in our GaAs samples.

The coherent optical response of bulk GaAs is studied after ultrafast excitation close to the band edge. Pump-probe experiments with 20-fs pulses demonstrate that coherent polarizations between heavy- and light-hole continuum states make a strong contribution to the nonlinear optical response. The beat frequency is determined by the heavy-hole--light-hole energy splitting in the spectral probe window. In addition, excitonic heavy-hole--light-hole quantum beats are observed in the slightly strained GaAs crystal. The different contributions are separated via their polarization selection rules.

Linear time-resolved reflection on the heavy-hole exciton transition of high-quality multiple GaAs quantum-well Bragg samples reveals enhanced radiative emission and accelerated decay of the coherent optical polarization due to radiative interwell coupling. It is shown that this superradiant mode gradually vanishes with increasing excitation intensity. Microscopic calculations attribute this decoupling to the carrier-carrier Coulomb interaction in the individual quantum wells leading to excitation-induced dephasing. The intricate density dependence is discussed comparing computed results for the excitation-dependent decay in single quantum wells and multiple-quantum-well Bragg structures.

We report on the bend resistance in junctions of crossed wires in the quantum Hall regime. The vanished resistance associated with the ν = 1 quantum Hall state becomes negative for the filling factor ν just below unity when the channel width is reduced. The negative resistance dip is independent of temperature in the measured range of 0.08–0.9 K. Although these features indicate the interboundary scattering between the ν = 1 edge states to be the origin, simple single-particle models fail to reproduce the negative resistance because of the cancellation of the change in the transmission probabilities caused by the scattering.

We propose a vertical transport of the photo-excited carriers under the influence of the two dimensional carrier gas (2DCG) in GaAs Ga 1−x Al x As heterojunctions before excitonic recombination. The vertical transport is responsible for a long photoluminescence (PL) rese time of the excitonic transitions as well as for the PL intensity oscillations with magnetic field. The vertical transport is proved for the first time by resonant PL intensity variation at the cyclotron resonance condition of the 2DCG, by which the effective masses of two dimensional electrons as well as two dimensional holes in the n- and p-type heterojunctions are determined.

Time resolved reflectivity measurements of multiple-quantum-well Bragg structures show a strong reduction of the reflectivity at increased excitation levels. At low intensities, a pump-induced contrast of more than 20% in the reflectivity of a 10 quantum well Bragg structure is observed and attributed to the superradiant coupling between the quantum-well excitons. This superradiant state is gradually suppressed for increasing excitation densities due to rapid loss of phase coherence via excitation induced dephasing.

The intraband phase coherence of electronic Bloch wavepackets in semiconductor superlattices is partially conserved when electrons undergo energy relaxation by LO-phonon emission. The decay time of the intraband coherence as probed by terahertz-emission spectroscopy is found to be unaffected by excitation below or above the threshold for LO-phonon emission. On the other hand, the population relaxation probed by femtosecond optical transmission spectroscope is drastically accelerated when the threshold is reached. Electrons which have undergone LO-phonon emission form a more localized distribution in k-space giving rise to a stronger THz signal.

GaAs/AlGaAs single quantum wells (QW) with different degrees of fluctuations of the exciton confinement potential are investigated by microphotoluminescence (μ-PL) as a function of the excitation energy and by conventional PL, PL excitation, as well as cathodoluminescence spectroscopy. The surface morphology has been studied by atomic force microscopy. For a small inhomogeneous broadening of the PL spectrum, the influence of resonant excitation of higher localized exciton states is only detected for energies below the center of the absorption line, which defines an effective exciton mobility edge Em. For broader PL spectra, resonant excitation of higher states can be observed far above the center of the absorption spectrum resulting in an effective blue-shift of Em. Furthermore, the position of Em can depend on the excitation location. In this case, in addition to nm-scale fluctuations, a variation of the random potential on the μm-scale was observed.

Using 20 fs laser pulses, we impulsively excite coherent inter-valence band polarizations in GaAs which lead to quantum beats in the free carrier continuum. The beat frequency which is determined by the energy splitting between heavy and light hole states, is varied over a broad range (5 to 30 THz) by changing the spectral position of the laser pulses relative to the bandgap. For carrier densities below 1016 cm—3, the decay of quantum coherence occurs on a 150 fs time scale and is influenced by both inhomogeneous broadening and scattering processes.

Physical review / B 56(16), 10469 - 10482 (1997). doi:10.1103/PhysRevB.56.10469, The features of surface and interface roughness in crystalline AlAs/GaAs superlattices grown by molecular beam epitaxy on vicinal (001) GaAs substrates are studied by grazing-incidence x-ray scattering (GIXS). The effects of different growth modes [step-flow or two-dimensional- (2D-) nucleation], different substrate preparations, and growth interruptions on the roughness are investigated. The results of GIXS are compared with atomic force microscopy (AFM) images of sample surfaces. For samples grown in the step-flow mode, both of the methods display a distinct anisotropy in the lateral size of roughness along the substrate miscut direction and perpendicular to it. The lateral correlation lengths given by GIXS correspond to the size of step bunches observed by AFM, while individual steps are resolved by AFM only. GIXS reveals also a strong interface-interface correlation or inheritance of roughness for all the samples which is not accessible by AFM. Moreover, the angle of inclination of the direction of this inheritance from the surface normal is found to be dependent on the growth conditions. Two effects in the skew inheritance have been observed by means of 2D mapping of GIXS in the reciprocal space: (i) in the direction of substrate miscut the angle of skew inheritance inverted its sign, (ii) in the direction perpendicular to the miscut a strongly skew inheritance appeared as an effect of growth interruptions. Conclusions concerning the improvement of GIXS experiments applied to the studies of multilayers are derived., Published by Inst., Woodbury, NY

We use molecular beam epitaxial growth in order to synthesize strain compensated GaAs AlAs-based distributed Bragg reflectors (DBR). The strain compensation is achieved by introducing carbon doping up to densities of about 2 × 10 20 cm −3 . The residual strain with respect to the GaAs substrate can be reduced to less than 1 × 10 −4 . X-ray topographs of the strain compensated DBRs showed no misfit dislocations, thus proving that the critical thickness for the formation of dislocations in strain compensated DBRs is much larger than in the undoped and Be-doped cases. X-ray reciprocal space mapping reveals a structural degradation of the DBR : C for thicknesses above 2.5 μm and the defect density is found to be much lower in AlAs : C than in GaAs : C. We study the effect of the strain compensation on the morphology and the optical reflectivity of the DBRs.

We propose a new concept for the reduction of impurity scattering in remotely doped GaAs single quantum wells by using heavy-mass X-electrons in the short-period AlAs/GaAs superlattice barriers which smooth the potential fluctuations of the ionized Si dopants. Electron mobilities as high as 120 m 2 /V . s and electron densities of 1.6 x 10 16 m -2 are obtained in 10 nm GaAs single quantum wells in the one-subband occupation mode without any parallel conductance. To demonstrate the applicability of our concept magnetotransport and photoluminescence measurements together with self-consistent calculations are presented. Limitations of the effectiveness of the reduction of impurity scattering by impurity segregation and intermixing in the superlattice are discussed.

Coherent polarizations in the band-to-band continuum of bulk GaAs are studied in pump-probe experiments with 20 fs pulses. For the first time, we observe quantum beats on a 100 fs time scale that are due to an impulsively excited quantum coherence between heavy and light hole states. The beat frequency is determined by the heavy-light hole energy splitting changing continuously with the energy separation between the laser and the band gap. Theoretical calculations of the coherent response based on the semiconductor Bloch equations for a three-band scheme account for the data.

Present semiconductor technologies allow the growth of different types of nanostructures, such as quantum wells, wires, and dots on the surface of a single semiconductor crystal. The piezoelectric field of surface acoustic waves (SAWs) propagating on the crystal surface provides an efficient mechanism for the controlled exchange of electrons and holes between these nanostructures. In this review, we explore this ability of dynamic SAW fields to demonstrate acoustically driven single-photon sources using coupled quantum wells and dots based on (Al,Ga)As (311)A material system. We address the growth of the coupled nanostructures by molecular beam epitaxy, the dynamics of the acoustic carrier transfer between them, as well as the acoustic control of recombination in quantum dots. The latter provides the basis for the operation of the acoustically driven single-photon sources, which are characterized by a low jitter and repetition frequency close to 1 GHz.

Photonic modulators are one of the most important elements of integrated photonics. We have designed, fabricated, and characterized a tunable photonic modulator consisting of two 180 degrees-dephased output waveguide channels, driven by a surface acoustic wave in the GHz frequency range built on (Al,Ga)As. Odd multiples of the fundamental driven frequency are enabled by adjusting the applied acoustic power. A good agreement between theory and experimental results is achieved. The device can be used as a building block for more complex integrated functionalities and can be implemented in several material platforms. (C) 2013 Optical Society of America, We thank J. Pedros for helping with the sample preparation. This work has been supported by the international campus of excellence VLC/CAMPUS and by the program INNCIDE from the Spanish Ministry of Economy and Competitiveness (MINECO), through the program "Valoritza i Transfereix" from the Vice-Principal of Research and Scientific Policy of the Universitat de Valencia and through the program INNOVA (grant SP20120860) from the Universitat Politecnica de Valencia. The authors also acknowledge financial support by the Spanish MINECO Projects TEC2010-21337 and MAT2012-33483. A. Crespo-Poveda and B. Gargallo acknowledge financial support through FPI grants BES-2010-036846 and BES-2011-046100, respectively.

The potential of cathodoluminescence (CL) in connection with scanning electron microscopy (SEM) for the investigation of the lateral variation of interfaces in this film heterostructures is examined. GaAs/Al x Ga 1− x As single and multiple quantum wells were prepared by molecular beam epitaxy under different conditions resulting in various surface morphologies. Spectrally resolved CL line profiles, which provide information on lateral variations of the exciton confinement energy ( E ) are compared with the surface structure imaged by SEM or atomic force microscopy. For surface undulations with periods larger than the spatial resolution ( L ) of CL, a direct correlation between surface and interface features was proven. In the case of a statistical distribution of small interface growth islands, the period of the CL intensity modulation is determined by L . However, the modulation depth of the CL intensity contains information on the underlaying disorder. For substrates, in which misfit dislocations were generated during the MBE growth of the heterostructure, we found variations of E along two sets of straight lines perpendicular to each other.

We present a new concept to reduce impurity scattering in remotely doped GaAs single quantum wells by using heavy-mass X electrons in barriers formed by short-period AlAs/GaAs superlattices to smooth the potential fluctuations of the ionized Si dopants. Electron mobilities as high as $120{\mathrm{m}}^{2}/\mathrm{V}\mathrm{s}$ and electron densities up to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{m}}^{\ensuremath{-}2}$ are obtained in 10 nm GaAs single quantum wells in the one-subband conductivity mode without any parallel conductance. In addition to magnetotransport we present voltage dependent capacitance and photoluminescence measurements as well as self-consistent calculations to demonstrate the applicability of our concept.

The bend resistance in narrow cross junctions has been investigated in the fractional quantum Hall regime. Dips in the resistance due to the ballistic transmission of composite fermions have been observed near v = 1/2 and 1/4. The temperature dependence of the dips is strong reflecting the small Fermi energy of the composite fermions.

Carrier capture induced by electric fields in -like GaAs highly doped with Si is investigated with respect to the carrier heating necessary for such a charge transfer and the energy required to release the trapped carriers. The existence of a threshold field is established. The value of the threshold field for capture is in accordance with either trapping via the L band or a process that is direct but assisted by a large-wavevector phonon. The release of the carriers from the metastable traps due to infrared excitation, measured in terms of its dependence on the wavelength, yields a significantly higher threshold than is usually required for thermal excitation. This indicates the substantial lattice relaxation involved to form the charged trap.

The occupation of the laser levels in an undoped GaAs/(Al,Ga)As quantum-cascade structure is directly detected by interband photoluminescence spectroscopy. The upper laser level and the injector state exhibit an anticrossing behavior with a splitting of about 10 meV, which is attributed to a strong coupling between them. The energy splitting matches well the oscillation period observed in recent, ultrafast, coherent electron-transport experiments by F. Eickemeyer et al. [Phys. Rev. Lett. 89, 047402 (2002)]. A comparison with the calculated transition energies qualitatively confirms the anticrossing behavior in the vicinity of the threshold field strength. Furthermore, the measured energy difference between the upper and lower laser level agrees with the lasing energies of a set of complete quantum-cascade lasers. At the same time, the variation of the lasing energies corresponds to the energy splitting between the upper laser level and the injector state.

The transport properties of the half-filled Landau level in crossed-wire junctions are presented. A dip in the bend resistance due to ballistic transmission of composite fermions has been observed near upsilon =1/2 and 1/4. The temperature dependence of the dips is found to be strong due to the small Fermi energy of the composite fermions. The dips are found to shift to lower magnetic fields, indicating that the effective magnetic field for the composite fermions in the narrow channel is not uniform.

Experiments on the controlled generation of up to 20 current filaments in n‐type GaAs have been performed in the electric field region, where impurity impact‐ionization avalanche breakdown takes place. This strongly nonlinear system is found to be very sensitive to small deviations of the technological parameters. In particular, we present the results of time‐resolved current measurements on two filaments connected in parallel, which compete for the impressed total current due to nonlocal coupling via a common load resistor. The observed switching time constants down to 100 ns make this system very interesting as a ‘‘winner takes all’’ element in synergetic computer applications.

A continuous spectrum of misorientations of all azimuthal angles and inclinations varying from 0° to 8° occurring on spherically shaped substrates was used to study the development of terrace-step structures of homoepitaxially grown GaAs (001) by reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). The singular GaAs (001) surface shows growth instabilities with respect to the step-edge profile and the terrace width distribution ranging from 0.1 to 1 μm. The vicinal surface is composed of straight Ga-terminated and ragged As-terminated steps reflecting the interplay of the different nature of both kinds of steps. Obviously, self-similar terrace and step structures are reproduced on different length scales as revealed by a comparison of the atomic force microscopy images with images known from scanning tunnelling microscopy. This scaling leads to terrace and step dimensions which are comparable to the length scale of electronic excitations, thus, influencing the electronic properties of the material. All surfaces under investigation showed a common lateral roughness on the 0.5 to 4 μm scale caused by fluctuations of the terrace width distribution (two-dimensional step bunching). This lateral modulation length increases with increasing misorientation and is always larger, parallel to the [110] direction than perpendicular to this direction. The vertical roughness increases with the amount of misorientation and is stronger for misorientations towards (111)As.

Optically effective inhomogeneities in semiconductor heterostructures with very different origin are investigated by cathodoluminescence in conjunction with scanning electron microscopy (CL). In GaAsAlGaAs single quantum wells with different length scales of the interface roughness (LR), the CL intensity distribution is determined by the spatial resolution of CL (Lav), when LR is smaller than Lav. QWs grown on a substrate tilted 2° towards (111) As contain a long-range roughness (μ scale), which correlates with the surface topography. In highly strained QWs, the influence of misfit dislocations (MD) on the spatial and spectral luminescence distribution is investigated. MDs cause regular variations of the exciton confinement energy in the QW and hence lead to complementary CL images recorded at two defined detection energies. In a vertically biased GaAsAlAs superlattice, depth resolved CL measurements from the top and spectrally resolved CL imaging from the cleaved edge are performed to directly observe the spatial distribution of electric-field domains. Optisch wirksame Inhomogenitaten von Halbleiterschichtsystemen werden mittels Kathodenlumineszenz (KL) in einem Rasterelektronenmikroskop (raumliches Auflosungsvermogen Lav ≪ 1 μm) untersucht. Grenzflachenrauhigkeiten von GaAsAlGaAs Einzelpotentialtopfen fuhren fur kleine Rauhigkeitskorrelationslangen (LR < Lav) zu einem Muster in den KL-Bildern, das durch Lav bestimmt ist. Auf verkippten Substraten konnen Potentialtopfe eine langreichweitige Komponente von LR enthalten (LR ≈ Lav), die mit der Probenoberflachenstruktur korreliert. In einem verspannten Potentialtopf verursachen Fehlanpassungsversetzungen eine laterale Variation der Exzitonenubergangsenergie, so das Versetzungen bei unterschiedlichen KL-Detektionsenergien exakt komplementar abgebildet werden konnen. Die elektrische Feldverteilung eines Ubergitters in einem senkrechten elektrischen Feld kann durch Ausbildung von Felddomanen inhomogen werden. Mittels tiefenabhangiger KL sowie durch direkte KL-Abbildung der Probenbruchkante wird die Verteilung elektrischer Felddomanen in einem GaAsAlAs Ubergitter bestimmt.

We present an experimental and theoretical study of the breakdown caused by impurity impact ionization in n-GaAs. The observed difference in the electric field strengths for the breakdown threshold and the bistability regime between homogeneous and partially ordered doping with Si donors is explained and confirmed by numerical simulations, applying a weighted ensemble Monte Carlo method in the case of the inhomogeneous problem. Furthermore, conditions for the appearance of hysteresis loops are experimentally investigated as a function of load resistance and sample width. The transitions between different branches of the current-voltage characteristics can be understood as switching between homogeneous states and current filaments in good agreement with theoretical predictions on the basis of a simple generation-recombination model.

Electronic states in n‐type GaAs/AlxGa1−xAs heterojunctions are studied by deep‐level transient spectroscopy in the Al mole fraction range from 0.25 to 1.00. A series of four well‐defined deep levels is characteristic of the inverted GaAs/AlxGa1−xAs interface (GaAs grown on AlxGa1−xAs) grown by conventional molecular‐beam epitaxy under arsenic‐stable conditions without growth interruption. It is shown that the series of four levels originates from intrinsic defects which are associated with arsenic vacancies and antisites. In particular, two charge states of the isolated arsenic vacancy VAs can be identified. Except for the sheet very close to the inverted heterointerface the n‐type AlxGa1−xAs layers are found to be almost free of intrinsic deep levels in the entire composition range. For the investigated samples, the compositional dependence of the level energies shows that the arsenic vacancy is predominantly surrounded by Al atoms. From the compositional dependence of the level concentrations it follow...

The effect of low energy exciton sinks on the spectral and spatial luminescence intensity distribution has been investigated in a single quantum well (QW) and a multiple QW structure prepared by growth interrupted molecular beam epitaxy (MBE). Both samples contain growth-related lateral variations of the exciton confinement energy on the order of the exciton diffusion length, which cause a lateral and spectral intensity distribution in cathodoluminescence images.

We report results on selection rules and polarization conditions for quantum beating between light hole and heavy hole excitons in GaAs quantum wells. The measurements were conducted using time-integrated degenerate-four-wave-mixing in a three-pulse configuration. By choosing particular polarization configurations of the ultrashort optical pulses exciting heavy and light hole excitons simultaneously, degenerate four-wave-mixing signals show either quantum beating or no modulation at all. The experiments are analyzed using a model that takes into account exciton/exciton interaction.

Hot carrier transport processes in GaAs with δ‐like high Si doping have been investigated. In the high electric field region the current density decays with time by as much as 20%, an indication of electron trapping. The electron traps are metastable and the electrons can be released by light or thermal excitation. DX− centers can explain the observations, however, other types of localized electron states cannot be unambiguously excluded.

We report time-resolved degenerate four-wave-mixing experiments on a high-quality single GaAs quantum well. The signals for parallel and cross-polarized exciting pulses show pronounced quantum beats with different frequencies corresponding to the heavy/light-hole splitting and the biexciton binding energy, respectively. Comparison of the experimental data with solutions of the optical Bloch equations for a phenomenological model system shows that these features result from strong contributions of the biexcitonic state. The importance of biexcitons to the nonlinear coherent response of the two-dimension exciton is further confirmed by the frequency dependence of the time-integrated four-wave-mixing signal amplitudes measured for the two polarization geometries.

The effect of exciton diffusion and localization on the cathodoluminescence (CL) intensity distribution using a scanning electron microscope has been investigated in a single quantum well and a multiple quantum well structure prepared by growth interrupted molecular beam epitaxy. Although of different origin, lateral variations of the exciton confinement energy occur in both samples on a length scale much larger than the exciton Bohr radius. The spectral and spatial CL intensity distribution is substantially influenced by diffusion of excitons to quantum well regions with low exciton confinement energy. The CL micrographs from both quantum well structures exhibit a similar bright/dark pattern with a typical length scale, which is determined by the exciton diffusion length in this material.

We have investigated the electrical and optical properties of GaAs/(Al,Ga)As quantum-cascade-laser (QCL) structures. In the case of an undoped structure, distinct current maxima followed by negative differential conductance regions appear, while the doped structure exhibits a plateau with saw-tooth-like features similar to the well-known ones for electric-field domain formation in doped, weakly coupled superlattices. The investigation of an undoped quasi-QCL structure, in which all (Al,Ga)As barriers are enlarged, shows a much sharper current maximum in the dark current and a clear plateau-like feature under illumination. The structures observed in the current–voltage characteristics are closely related to the electric-field dependence of the photoluminescence intensity in these three QCL structures.

The lasing properties of GaAs/Al0.33Ga0.67As quantum-cascade lasers are investigated as a function of injector doping concentration ns between 2×1011 and 1×1012 cm−2 per period. Lasing is observed for ns⩾3.5×1011 cm−2, with optimal lasing properties (minimum of the threshold current and maximum of the modified characteristic temperature) for nopt≈6×1011 cm−2. With increasing ns up to nopt, the lasing energy of 115 meV exhibits first a blueshift to 135 meV, followed by a redshift to 120 meV for higher doping levels. This shift of the lasing energy as a function of ns is discussed in terms of changes in the field distribution, occupation of additional levels above the upper laser level, and electron–electron interactions.

We report on the enhancement of the nonlinear properties of exciton-polariton macroscopic coherent phases created by resonant optical excitation under the spatial modulation of a square acoustic lattice. The confinement increases the local particle density by preventing particle drift due to repulsive interactions and modifies the coupling to the exciting laser field. Both effects lead to the enhancement of the nonlinear properties of the system, which is reflected in a reduction of the pump laser intensity threshold of formation of the coherent phase, pronounced blueshift of the emission energy and screening of the acoustic potential.

Charge transport in an electron-hole plasma driven by high-field terahertz (THz) pulses is strongly influenced by electron-hole interactions, as has been shown in a recent publication [P. Bowlan et al., Phys. Rev. Lett. 107, 256602 (2011)]. We introduce a picture of high-field THz transport which accounts for the roles of both types of carriers including their interactions. While holes make a negligible contribution to the current, they are heated by absorbing energy from the driving THz field and introduce a friction force for the electrons over a period of about 500 fs. Our model uses an extended version of the loss-function concept to calculate the time-dependent friction. The local field that drives the electrons differs from the incident THz field by screening due to Coulomb correlations in the plasma. We illustrate how spatial correlations between charged particles (electrons, holes, impurities) create a significant local-field correction to the THz driving field. The dominant contribution stems from Coulomb-correlated heavy-hole wave packets, which are strongly polarizable via inter-valence-band transitions.

We report on measurements of first-passage-time distributions associated with current switching in weakly coupled $\mathrm{GaAs}/\mathrm{AlAs}$ superlattices driven by shot noise, a system that is both far from equilibrium and high dimensional. Static current-voltage ($I\mathrm{\text{\ensuremath{-}}}V$) characteristics exhibit multiple current branches and bistability; precision, high-bandwidth current switching data are collected in response to steps in the applied voltage to final voltages ${V}_{1}$ near the end of a current branch. For a range of ${V}_{1}$ values, the measured switching times vary stochastically. At short times ($\ensuremath{\lesssim}10\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$), the switching time distributions decay exponentially, while at longer times the distributions develop nonexponential tails that follow an approximate power law over several decades. The power law decay behavior is attributed to the presence of multiple switching pathways, which may arise from small spatial variations in the superlattice growth parameters.

Nonlinear terahertz (THz) spectroscopy gives insight into high-field charge transport in semiconductors. Strong THz transients with field amplitudes of up to megavolts/cm serve as a driving field for free carriers and the resulting transport behavior is directly inferred from the field radiated by the moving charges. We study the transition from a ballistic to a drift-like transport regime of electrons in bulk GaAs. While electrons in the lowest conduction band of an n-type sample display ballistic transport, a transition to a drift-like behavior is found in an optically generated electron-hole plasma. Time-resolved measurements reveal the onset of friction on a time scale of a few picoseconds, mainly due to interactions of electrons with the hole distribution heated by the intense THz driving field. Experiments in which photoexcited electrons undergo intervalley scattering from the Γ to the L valley reveal characteristic changes of the transport behavior due to the picosecond backscattering to the Γ valley. The experimental results are in agreement with theoretical calculations of the time-dependent friction including both electron-hole scattering and local-field effects.

Phonons emitted by the application of an electric field to delta -doped (001) GaAs are investigated by the time-of-flight method. Using a sensitive bolometer and an accumulation of up to 105 signals we investigate TA and LA phonons. As well as the decay products of optical phonons. Concerning the latter contributions the experimental results are in agreement with Monte Carlo simulations. The peculiar features of the LA spectrum are discussed.

Investigations of the differential conductivity and time-of-flight spectra of phonons emitted by hot electrons were performed in Si delta -doped GaAs. Together with a Green function balance equation method for numerical simulations, carrier heating in an electrical field is convincingly described. The results emphasize the importance of carrier transfer from the confined to the extended states and the role of phonon emission.