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The resistivity, magnetization and magnetoresistance of (La, Ca) and yttrium-substituted manganites La0.67−xYxCa0.33MnQ3 have been studied as a function of annealing conditions under air or oxygen. Doping by Y decreases the Curie temperature and drastically increases the zero field resistivity peak value. For yttrium doped manganite, annealing at 1400°C for 1 h leads to an increase of the maximum value of magnetoresistance up to 94.6% in applied field of 5 T at 150 K and drastically improves the field sensitivity in small fields (70% in 1 T). Conversely, the MR at room temperature is highest for the undoped sample annealed at 1200°C under oxygen.

The possibility of locally varying the potential energy of the electrons and holes localized in a quantum well by a quantum-dot array deposited in the immediate vicinity of the quantum well is demonstrated. These changes in the potential energy are induced when a strain arises in the quantum-dot region.

We report on GaAs-based broad area (100 µm) 1.3 µm quantum dot (QD) lasers with high CW output power (5 W) and wall-plug efficiency (56%). The reliability of the devices has been demonstrated beyond 3000 h of CW operation at 0.9 W and 40 °C heat sink temperature with 2% degradation in performance. P-doped QD lasers with a temperature-insensitive threshold current (T0 > 650 K) and differential efficiency (T1 = infinity) up to 80 °C have been realized.

We studied magneto-optical properties of ultra-thin InAs GaAs heterostructures deposited on GaAs (311)A surfaces. Sharp heavy- and light-hole exciton resonances, as well as the features due to exciton excited states are observed in photoluminescence (PL) and photoluminescence excitation (PLE) spectra. No Stokes shift between heavy-hole exciton peaks in PL and PLE spectra is found. From magneto-PLE spectra, exciton binding energy is measured to be 12 ± 1 meV, i.e. three times higher than the surrounding GaAs matrix and ten times higher than the bulk InAs. We suggest the model of lateral confinement in monolayer InAs which enhances the exciton binding energy and suppresses the in-plane exciton transport.

We report on optical studies of relaxation processes in self-organized InAs/GaAs quantum dots (QDs). Photoluminescence excitation spectra reveal a series of almost equidistant lines. Their energy with respect to the detection energy does not depend on QD size. The energy separations are close to the LO phonon energy of 32.1 meV estimated numerically for strained pyramid-like InAs QDs. Carrier relaxation is found to proceed by multiphonon processes involving QD and wetting layer LO phonons, and an interface mode.

Electric noise characteristics of a triple-heterojunction AlGaAs/GaAs/AlAs/GaAs structure are measured at 10 GHz frequency for the electric field (≤ 1.3 kV/cm) applied in the interface plane. The maximum in the dependence of the longitudinal diffusion coefficient (deduced from the spectral density of current fluctuations) is observed at 1 kV/cm and interpreted as being brought about by back-and-forth transverse tunnelling of hot electrons between the quantum wells separated by a thin AlAs barrier, the wells being characterized by different electron mobility. The tunnelling time constant is found from the height of the maximum and, independently, from the quenching of the maximum in short samples.

We report resonant luminescence studies in ultrathin InAs quantum‐well structures. Under double resonant conditions, sharp lines induced by longitudinal optical phonons were observed in the emission spectrum, in addition to the broad band luminescence. These sharp lines exhibited moderate circular polarizations whereas the linear polarization was negligible, indicating a resonant luminescence process. The properties of these sharp lines observed in InAs monolayer quantum wells are quite different from those observed previously in GaAs‐AlxGa1−xAs quantum wells under similar conditions. InAs phonons were found to participate in the exciton resonant scattering process at high magnetic fields where double resonant conditions are fulfilled. InAs‐like phonon energy is determined to be close to 29.9 meV, in good agreement with the energy expected for a uniform ultrathin elastically strained InAs layer. We performed various circular and linear polarizations to investigate the resonant luminescence process in thin...

We report on efficient resonant spin-flip Raman scattering due to localized heavy-hole excitons in single submonolayer InAs insertions in a GaAs matrix. Exciton, heavy-hole and electron g factors are directly measured for samples with different average thicknesses of InAs. The large size uniformity of the InAs islands manifests itself in very narrow heavy- and light-hole exciton photoluminescence peaks which also allow us to measure exciton g factors by magneto-luminescence spectroscopy.

We demonstrate stable mode-locking in a Yb : KYW laser by using a quantum-dot (QD) saturable absorber. A fast component in the absorption decay in this QD structure has been measured by pump-probe technique to be /spl sim/1 ps.

We have studied the effect of high-temperature annealing on the properties of a laser heterostructure with InAs quantum dots in AlAs/GaAs superlattice. By increasing the time of annealing at 700°C, it is possible to provide for a smooth variation of the lasing wavelength from 1290 to 916 nm at a constant threshold current density (250 A/cm2 at T0=110 K). By annealing the structure at 750°C, the lasing wavelength can be reduced to 845 nm.

The design of the active region in InGaAsN quantum well (QW) injection lasers is investigated. Long-wavelength (1.27–1.3 mm), low-threshold ( 50%) lasing is obtained at room temperature from structures based on single InGaAsN QWs in GaAs or InGaAsN barrier layers. The principal parameters (threshold, temperature, power) of these lasers have been studied in the wide-stripe configuration. The characteristics of injection lasers with different designs of the active region are compared.

The optical properties of quantum dot arrays in the MBE-grown InAs/GaAs(100) epitaxial system with an effective InAs layer thickness of 1.9 monolayers were studied in samples exposed to the beam of As4 for various times after switching off the In beam. The results of photoluminescence measurements showed that the emission wavelength increased with the exposure time within certain limits. This behavior agrees with predictions of the kinetic model of the initial stage of quantum dot formation.

The carrier dynamics and the collective properties of a photogenerated quasi-two-dimensional electron gas in an asymmetric undoped GaAs/AlGaAs quantum well (QW) structure are investigated using an all-optical interband pump?intersubband probe absorption experiment. At a carrier density of 1 ? 1012 cm?2 per QW, an e1?e2 absorption band at 126 meV is found. Transient resonance frequency blueshifts with respect to the single-particle resonance as well as a transient spectral band narrowing are observed. At the carrier density 1 ? 1013 cm?2 per QW, the e2?e3 absorption band at 99 meV with a spectral width of 5 meV is measured. This band is blueshifted by 30 meV through collective effects and broadened at an excitation density of 3 ? 1013 cm?2 per QW.

The dynamics of interband transitions in InGaAs/AlGaAs heterostructures with funnel-shaped quantum wells, when a narrow and deep well is positioned asymmetrically in wide and shallow ones, has been studied experimentally. These structures have been proposed as a dual-colour laser in mid- and near-IR range simultaneously. The lifetime of electrons in the ground and excited states in the quantum wells has been determined by the time of the photoluminescence intensity decline. The obtained lifetime values allow precise identification of peaks in the photoluminescence spectra and a deep insight into the process of high electron states populating. The experimentally found electron lifetime for the interband transitions corresponds on the order of magnitude with the interband transitions lifetime in GaAs. Analysis of the photoluminescence spectra as well as the time dependence of the photoluminescence intensity decline indicates a possibility of population inversion achievement in these structures under high excitation.

Self-oscillations of the current with frequencies of 0.7–3.6 MHz depending on the dc bias have been detected in weakly coupled GaAs/AlGaAs superlattices at 4.2 K. A study of the static and dynamic characteristics of the structure shows that self-oscillations arise in the local region of the superlattice, restricted by the size of the domain boundary expansion, due to the periodic coupling and decoupling of subbands in adjacent quantum wells forming the expanded domain boundary.

Wavelength selective charging of a QD sub-ensemble is demonstrated, presenting a basis of future wavelength dependent parallel optical memory. High-resolution saturation spectroscopy in two-colour photocurrent experiments is used to investigate the binding energy of the positively charged trion complex as well as the temperature dependence of the coupling to acoustic phonons.

Single-mode lasers operating in the 1.3 μ m wavelength range have been obtained with the active region based on InAs/AlGaAs/GaAs quantum dot heterostructures. A minimum threshold current of about 1.4 mA is reached, which is a record value for ridge waveguide lasers. The maximum efficiency and maximum output power in the cw lasing mode are 0.73 W/A and 120 mW, respectively.

Similar effects are responsible for self-organization of periodically corrugated surface structures and ordered dot arrays on crystal surfaces. Strain relaxation on facet edges may result in the appearance of periodically corrugated surfaces for lattice-matched growth. Strain relaxation on facet edges and island interaction via the strained substrate act as driving forces for the formation of ordered arrays of uniform, strained lattice-mismatched islands on a crystal surface. A pseudoperiodic square lattice is manifested for the InAs-GaAs(100) system. Less ordered dots are formed on the GaAs(100) surface with a 4 monolayer GaSb deposition. New experimental methods are applied for the characterization of faceted nanoscale structures. For GaAs-AlAs multilayer structures grown on (311)A substrates, interface corrugation results in optical anisotropy of the same sign as expected from the low symmetry growth direction, making the main origin of the anisotropy unclear. Our quantitative optical reflectance and reflectance anisotropy studies show that the interface corrugation plays an important role for thin (less than 4 nm) GaAs layers. Mesa arrays from samples with InAs quantum dots grown on (100) surface are fabricated. The photoluminescence intensity is found to depend only weakly on the mesa size (1000 nm to 250 nm). The estimated electron-hole pair capture time into the InAs dot at room temperature is less than 1 ps. We also found a weak dependence of the threshold current density on the deep mesa stripe width (down to 3 μm) in the case of room temperature operated quantum dot injection lasers.

InAs quantum dots in a GaAs matrix have been prepared by molecular beam epitaxy using a self-organizing mechanism. A narrow size distribution of single dots of pyramidal shape (typically with a base of 12 ± 1 nm and a height of 4–6 nm) is created as directly imaged with plan-view and cross-section transmission electron microscopy. The dots exhibit self-organized short range order and preferentially align in rows along 〈100〉. The photoluminescence of the dot ensemble has, due to fluctuations in dot size, shape and strain, a FWHM of typically 50–60 meV. However, using highly spatially and spectrally resolved cathodoluminescence it is possible to directly excite a tiny fraction of all dots (typically only 30 dots). Under these excitation conditions the spectrum changes drastically into a series of ultrasharp lines with a FWHM < 0.15 meV, each originating from a different single InAs quantum dot. This directly visualizes their δ function-like density of electronic states, especially since the lines remain sharp even for kBT⪢FWHM.

THz response of AlGaAs/InGaAs/GaAs HEMT structure has been investigated. The structure consists of the serpentine chain of series connected HEMTs. The source of one is the drain for the subsequent transistor. Experiments have been showed THz response peculiarities of such structures and enhanced noise equivalent power.

The possible mechanisms of the polarization control in single-mode intracavity- contacted vertical-cavity surface-emitting lasers (IC-VCSELs) with the rhomboidal selectively- oxidized current aperture were investigated. It was found that the lasing emission polarization of all single-mode VCSELs is fixed along the minor diagonal of the rhomboidal-shape aperture (the [110] direction). Numerical modelling of carrier transport did not reveal any sufficient injection anisotropy in the laser active region, while the transverse optical confinement factors calculated for the fundamental mode with two orthogonal polarizations are identical. Optical loss anisotropy and/or gain anisotropy are the most likely mechanisms of inducing the polarization fixation.

Lasing at 1488–1515 nm in the temperature range 20–83°C was obtained in structures with an active region based on multiply stacked arrays of self-organized quantum dots grown on GaAs substrates. The threshold current density of a laser with four cleaved facets was 800 A/cm2 at room temperature. The method of wavelength extension is based on the use of a metamorphic buffer layer with an In content of about 20% intended for relieving the lattice mismatch stress.

We have studied the optical properties of pseudo-alloy monolayer InAs/GaAsN superlattices with highly planar interfaces. In spite of the two-dimensional growth mode, we found that the photoluminescence (PL) reveals strong exciton localization through the whole PL band, dominating the spectrum up to high excitation densities and observation temperatures. Pump-and-probe PL experiments provide the following time constants: (a) the exciton relaxation time to the ground states of the localization regions is found to be ∼40–70 ps, depending on the photon energy, and (b) the time for depopulation of these localized states is between 2 and 4 ns.

The luminescence of GaAs/AlGaAs multiple-quantum-well structures with different well widths, containing A(+) centers, was studied to determine the dependence of the center binding energy on quantum-well width. It is shown that the binding energy of the A(+) centers increases markedly with decreasing well width, becoming ten times greater in 10-nm-wide wells than in the bulk material. The binding energy of A(+) centers was found to depend on their concentration.

Metamorphic modulation-doped InGaAs/InAlAs heterostructures have been MBE-grown on GaAs substrates. The optimization of low-temperature growth conditions for a graded-composition buffer layer made it possible to reduce the amount of structural defects in the active layers of the structure. The electron mobility in the 2D channel of metamorphic structures grown under optimum conditions (8100 cm2/V s at 300 K) noticeably exceeds the values achievable in strained InGaAs/AlGaAs heterostructures on GaAs substrates.

We demonstrate amplification (> 18 dB) of 200-fs pulses in a quantum-dot (QD) semiconductor amplifier. Our measurements have shown that such QD devices can provide amplification of femtosecond pulses over a spectral range that exceeds 100 nm.

Ensembles of InAs quantum dots formed at the GaAs(100) surface have been studied by the methods of reflection high-energy electron diffraction and photoluminescence. The amount of deposited InAs corresponds to a wetting layer thickness smaller than the critical value necessary for the transition from two-to three-dimensional growth. It is experimentally shown that, at a deposited film thickness of 1.5 and 1.6 monolayers, islands are formed after keeping the sample in a flow of As4. The influence of the substrate temperature on the kinetic characteristics of the formation of InAs/GaAs islands has been studied.

Broad area lasers based on InAs-GaAs quantum dots formed by submonolayer deposition were fabricated. High modal gain of submonolayer quantum dots permits the use of broad-waveguide and highly doped design. Continuous wave output power of 6 W limited by mirror damage and conversion efficiency of 58% were demonstrated at 20 °C. The characteristic temperature of 150 K was achieved.

The temperature dependence of morphological characteristics of Ge/Si(100) quantum dots grown by molecular beam epitaxy with constant growth rate at different substrate temperatures is investigated. The mean lateral size of quantum dots at 6.2 ML is shown to increase at rising substrate temperature. Comparison of experimental and theoretical results obtained in the frame of kinetic model is performed.

Comparative near-field and beam-quality (M2) measurements on narrow stripe quantum-dot (QD) and quantum-well (QW) lasers of identical structure, both emitting at 1100 nm, are presented. Intrinsic suppression of filamentation in the QD lasers is observed. QD lasers emitting at 1300 nm again show no filamentation. For a 6-μm-stripe, QW laser, M2 increases from 2.6 to 6.1 with output power increasing from 5 to 60 mW and with increasing stripe width (20 mW, 3→10 μm, M2=2.6→4.7). In the QD lasers, filamentation is suppressed up to 8 μm (1100 nm) and 9 μm (1300 nm) stripe width and no dependence on output power is observed.

Electroluminescence spectroscopy has been used in a wide range of temperatures (77–300 K) and driving current densities to study a laser heterostructure based on vertically coupled self-assembled InGaAs quantum dots (QD). It has been found that lasing occurs via the QD ground state in the entire temperature range. The temperature-independent position of the emission peak corresponding to the second excited state in QDs is explained.

Detailed investigation of anomalous modal behavior in fabricated bottom-emitting intra-cavity contacted 960 nm range vertical cavity surface emitting lasers (VCSELs) have been performed. At low currents the broad-aperture VCSELs show multi-mode operation at 945 nm via whispering gallery-like modes. Subsequent increase of pump current results in rapid increase of fundamental mode intensity and switching to a pure single transverse mode lasing regime at 960 nm with the higher slope efficiency. As a result record single transverse mode output power of 15 mW with a side-mode-suppressionratio (SMSR) above 30 dB was achieved. The observed phenomena cannot be explained by oxide-index guiding or changes in current pumping. 2D heat transport simulations show a strong temperature gradient inside the microcavity due to an effective lateral heat-sinking. This creates an effective waveguide and results in lower optical losses for the fundamental mode. At fixed pump current in pulsed regime (pulse width < 400 ns) high-order modes dominate, however the subsequent increase of pulse width leads to a rapid rise of optical power for the fundamental mode and SMSR increasing. Thus the self-heating phenomena play a crucial role in observed VCSEL unusual modal behavior.

International audience; The results of investigations of monolithic white InGaAlN LEDs with an active region containing several thin InGaN layers, emitting in the range from blue to yellow-green, and separated by short-period InGaN/GaN superlattices, are presented. The influence of the growth conditions and layer sequence in the active region on the optical properties of monolithic white LEDs was studied with the aim of controlling their color parameters.

We report the spectrotemporal measurements on 1.3 μm quantum-dot lasers with picosecond time resolution. The relaxation oscillations of the various mode groups monitored separately are identical and thus allow us to examine the spectrally integrated transient of the laser pulse for its characteristics. A modulation bandwidth of 2.3 GHz at room temperature is determined, demonstrating the potential of high-speed operation of these devices at wavelengths relevant for optical data transmission. The differential gain at room temperature was measured to be g′=1×10−15 cm2, the gain compression factor is e=1×10−15 cm3.

Room-temperature photoluminescence (PL) at 1.55 μm from heterostructures with InAs/InGaAsN quantum dots (QDs) grown by MBE on GaAs substrates is demonstrated for the first time. The effect of nitrogen incorporated into InAs/InGaAsN QDs on the PL wavelength and intensity was studied. The integral intensity of PL from the new structure with InAs/(In)GaAsN QDs is comparable to that from a structure with InGaAsN quantum wells emitting at 1.3 μm.

The fabrication and properties of 18 µm wide, 200 µm long edge-emitting InGaAs/GaAs lasers with a distributed Bragg reflector (DBR) mirror at one cavity end and a cleaved facet at the other are presented. The three-period third-order DBRs with high aspect ratio (~14:1) and nearly vertical sidewalls have been fabricated by reactive ion etching. The device threshold current and maximum output power are 30 mA and 80 mW, respectively. The spectral width of the lasing line is only 2 nm. The reflectivity of the DBR, determined by comparing the differential efficiencies from the DBR and the cleaved facet, is 73%. The width of the far-field pattern from the DBR end in the parallel direction (12°) is slightly larger than that from the cleaved facet (8°). By contrast, the width of the far-field pattern from the DBR end in the perpendicular direction is only 8°, which is five times smaller than that from the cleaved facet.

Photoluminescence of GaAs/AlGaAs multiple-quantum-well structures incorporating positively charged beryllium-impurity shallow-level acceptors (the so-called A(+) centers) was investigated. A novel luminescence line, which originated from radiative recombination of free electrons with A(+) centers, was observed. It was shown that its spectral position is determined uniquely by the binding energy of A(+) centers. It was also ascertained that the binding energy of A(+) centers increases with a decrease in the quantum-well width when the latter is comparable to the radius of A(+) centers.

It is demonstrated that longwave room-temperature photoluminescence (up to 1.65 μm) can be obtained using InGaAs/GaAs heterostructures of two types grown by low-temperature molecular beam epitaxy: (i) with InAs quantum dots formed at a low growth velocity and (ii) with an In0.5Ga0.5As quantum well grown in excess of the group III elements.

The penetration of an electromagnetic field in the frequency range of 20 kHz to 30 MHz through the plates of bulk polyerystalline sintered La/sub 0.60/Pb/sub 0.4/MnO/sub 3/ and La/sub 0.70/Y/sub 0.07/Ba/sub 0.23/MnO/sub 3/ manganites was studied. A giant increase, up to 10 times, of the transmission coefficient, on application of the magnetic field, was found. Skin depth variations were caused mainly by variation of the dynamical reversible magnetic permeability. The contribution of the magnetic permeability dispersion to the frequency dependence of skin depth was clarified.

The effect of hydrogenation and annealing upon photoluminescence of MBE-grown AlGaAs: Si epitaxial layers is examined in detail. Nonradiative recombination centers passivated by hydrogen are found to be reactivated in a wider temperature range in comparison with Si donors or DX centers. This process cannot be described by first-order kinetics. [Russian Text Ignore]

This paper reports the effect of growth parameters and buffer structure on the luminescence electrical and structural characteristics of thin (

Results of a photoluminescence and electroluminescence study of vertically coupled In0.5Ga0.5As/GaAs quantum dot?(QD) structures with an extended waveguide (1.24??m thick) are presented. Spectra of spontaneous and stimulated near-infrared (?1??m) emission as well as spectra of spontaneous mid-infrared?(MIR) (?12??m) emission are obtained under optical and electrical pumping. It is shown that the observed MIR emission is connected with intraband electron optical transitions in the QD structures.

Laser diode structures on GaAs substrates with an active region employing laterally associated InAs quantum dots obtained by low-temperature MBE exhibit electroluminescence at a wavelength of 1.55–1.6 μm in a temperature range from 20 to 260 K.

A narrow peak at the leading edge of the current pulse was found in samples of p-GaAs/Al0.3Ga0.7As structures subjected to a high electric field. An analysis of the shape and height of the peak as a function of the electric field, as well as the field redistribution along the sample, allows us to conclude that domain instability exists under these conditions. It is also shown that the energy of holes heated in moderate electric fields can significantly exceed the optical phonon energy.

Study of the optical properties of submonolayer germanium clusters formed by molecular beam epitaxy in a silicon matrix showed that the spectra of samples obtained at elevated deposition temperatures (∼750°C) contain a series of new bands related to the formation of germanium nanoislands.

Stimulated emission from quantum dots arrays has been observed under optical pumping. The emission has been observed from the ground state for electrons (λ ≈ 1.31 µm) and from two higher levels (λ ≈ 1.21 µm, 1.12 µm). The measured values of the amplification coefficient are rather high, up to 15 ÷ 17 cm−1. Procedure of the amplification coefficient measurements in the active media has been discussed.

Resonance enhancement of the quantum efficiency of new polarized electron photocathodes based on a short‐period strained superlattice structures is reported. The superlattice is a part of an integrated Fabry‐Perot optical cavity. We demonstrate that the Fabry‐Perot resonator enhances the quantum efficiency by the order of magnitude in the wavelength region of the main polarization maximum. The high structural quality implied by these results points to the very promising application of these photocathodes for spin‐polarized electron sources.