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The optical study of self-assembled ln(sub x)Ga (sub 1-x)As/GaAs quantum dots layer, which is grown on a (100) GaAs substrate in the vicinity of a two-dimensional electron gas is presented. The results showed an increase of the spatial density of quantum dots along the In gradient axis.

We investigate optical transitions and carrier dynamics in hybrid structures containing type-I GaAs/AlGaAs quantum wells (QWs) and type-II GaSb/AlGaAs quantum dots (QDs). We show that the optical recombination of photocreated electrons confined in the QWs with holes in the QDs and wetting layer can be modified according to the QW/QD spatial separation. In particular, for low spacer thicknesses, the QW optical emission can be suppressed due to the transference of holes from the QW to the GaSb layer, favoring the optical recombination of spatially separated carriers, which can be useful for optical memory and solar cell applications. Time-resolved photoluminescence (PL) measurements reveal non-exponential recombination dynamics. We demonstrate that the PL transients can only be quantitatively described by considering both linear and quadratic terms of the carrier density in the bimolecular recombination approximation for type-II semiconductor nanostructures. We extract long exciton lifetimes from 700 ns to 5 μs for QDs depending on the spacer layer thickness. [ABSTRACT FROM AUTHOR]

We investigated the effects of nearby Mn ions on the confined states of a InGaAs/GaAs quantum well through circularly polarized and magneto-optical measurements. The addition of a Mn delta-doping layer at the barrier close to the well gives rise to surprisingly narrow absorption peaks in the photoluminescence excitation spectra. The peaks become increasingly stronger for decreasing spacer-layer thicknesses between the quantum well and the Mn layer. Most of the peaks were identified based on self-consistent calculations; however, we observed additional peaks that cannot be identified with quantum well transitions, which origin we attribute to an enhanced exciton-phonon coupling. Finally, we discuss possible effects related to the exciton magneto-polaron complex in the reinforcement of the photoluminescence excitation peaks. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the effect of Be acceptors on the electroluminescence and the spin polarization in GaAs/AlAs p-i-n resonant tunneling diodes. The quantum well emission comprise two main lines separated by ~20meV attributed to excitonic and Be-related transitions, which intensities show remarkably abrupt variations at critical voltages, particularly at the electron resonant peak where it shows a high-frequency bistability. The circular-polarization degree of the quantum-well electroluminescence also shows strong and abrupt variations at the critical bias voltages and it attains relatively large values (of ~–75% at 15 T). These effects may be explored to design novel devices for spintronic applications such as a high-frequency spin-oscillators. [ABSTRACT FROM AUTHOR]

We present a low-temperature (73 K) study of the optical properties of multi-layer type II InP/GaAs self-assembled quantum dots by means of surface photovoltage (SPV) spectroscopy, taking advantage of its high sensitivity and contactless nature. The samples contain 10 periods of InP quantum dot planes separated by 5 nm GaAs spacers. The SPV amplitude spectra reveal two major broad peaks, situated at low and high energies, respectively. These features are analyzed taking into account the type II character of the structure, the quantum coupling effects, the spectral behavior of the SPV phase, and the photoluminescence spectra. As a result they have been attributed to optical transitions in the quantum dots and the wetting layers, respectively. The main mechanism for carrier separation in the SPV generation process is clarified via the analysis of the SPV phase spectra. The influence of the substrate absorption on the SPV spectra is discussed in details. [ABSTRACT FROM AUTHOR]

Bulk Zn1-xCoxO samples were synthesized via standard solid-state reaction route with different Co molar concentrations up to 21%. A detailed microstructural analysis was carried out to investigate alternative sources of ferromagnetism, such as secondary phases and nanocrystals embedded in the bulk material. Conjugating different techniques we confirmed the Zn replacement by Co ions in the wurtzite ZnO structure, which retains, however, a high crystalline quality. No segregated secondary phases neither Co-rich nanocrystals were detected. Superconducting quantum interference device magnetometry demonstrates a paramagnetic Curie–Weiss behavior with antiferromagnetic interactions. We discuss the observed room temperature paramagnetism of our samples considering the current models for the magnetic properties of diluted magnetic semiconductors. [ABSTRACT FROM AUTHOR]

An analysis of the hydrodynamic motion of the fluid of photoinjected carriers in polar semiconductors is presented. Experiments of time-resolved photoluminescence, which provide relevant insights into the dynamical behavior of heterostructures, are analyzed. We study the propagation and recombination of carriers in semiconductor devices with a large cap layer, where carriers are photoinjected, and a quantum well where they recombine. The movement of the photoinjected, and away from equilibrium, carriers along such cap layer consists, to a good degree of approximation, in an ambipolar diffusivelike one, which decays in time as a result of recombination and the coupling, via Coulomb interaction, with the optical and acoustic plasma waves. The density of the electrons arriving at the interface with the quantum well can be determined; these electrons are transferred through the interface to recombine in the quantum well, and the resulting intensity of the time-resolved luminescence is obtained. Comparison with experimental data shows a good agreement. [ABSTRACT FROM AUTHOR]

We investigated structural and optical properties of type-II InP/GaAs quantum dots using reflection high energy electron diffraction, transmission electron microscopy, atomic force microscopy, grazing incidence x-ray diffraction, and photoluminescence techniques. The InP dots present an efficient optical emission even when they are uncapped, which is attributed to the low surface recombination velocity in InP. We compare the difference in the optical properties between surface free dots, which are not covered by any material, with dots covered by a GaAs capping layer. We observed a bimodal dispersion of the dot size distribution, giving rise to two distinct emission bands. The results also revealed that the strain accumulated in the InP islands is slightly relieved for samples with large InP amounts. An unexpected result is the relatively large blue shift of the emission band from uncapped samples as compared to capped dots. [ABSTRACT FROM AUTHOR]

We present a structural and optical characterization of scepterlike micrometer-sized free-standing structures, composed of a long InGaP rod with a metallic sphere on its top, grown on polycrystalline InP substrates. In contrast to the conventional vapor-liquid-solid growth method, no catalyst was deposited on the substrate. Instead, metallic In liberated from the InP substrate by phosphor evaporation works as the catalyst metal. We performed Raman scattering, photoluminescence spectroscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy measurements on individual structures. The alloy composition measured by microscopic techniques is in agreement with the values obtained by the optical measurements considering that the rod is strain free. The InGaP rods present essentially constant Ga composition within a fluctuation of ∼10% and efficient optical emission. We also observed a marked increase in the Raman-scattering signal at rod positions near the metallic sphere (the “neck”), which was attributed to a surface-enhanced Raman-scattering effect. Our results demonstrate the possibility of using InGaP rods for optical device applications. [ABSTRACT FROM AUTHOR]

The thermal equilibrium of the formation of negatively charged excitons is studied in this work, by measuring the current and quantum-well photoluminescence in biased resonant tunneling double barrier diodes. We observe that the intensity ratio of negatively charged and neutral excitons depends linearly on the current for fixed temperature and illumination conditions. We propose that the results can be interpreted in terms of a mass-action law governing the concentrations of neutral and charged excitons and free electrons. Measurements at different temperatures and bias yield an electron concentration and a dwell time in the well that are in good agreement with the values previously reported in the literature. We also analyze the dependence of the luminescence on excitation intensity. [ABSTRACT FROM AUTHOR]

Presents information on a study which examined some aspects of the morphology of indium arsenide island formation on indium phosphide using atomic force microscopy, photoluminescence excitation spectroscopy and Raman scattering. Experimental details; Results and discussion; Conclusions.

In this work, we report on a technique—namely, the photoinduced magnetotunneling technique—which allows the direct experimental observation of tunneling of holes through valence-band Landau levels in n-i-n double-barrier resonant tunneling structures. Photocurrent–voltage curves exhibit several peaks associated with the complex nature of the dispersion of holes under parallel electric and magnetic fields applied to the sample. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a study that have grown ferromagnetic perpendicularly magnetized epitaxial thin films of aluminum on aluminum arsenide/gallium arsenide heterostructures by molecular beam epitaxy. Polar Kerr rotation of manganese-aluminum and aluminum; Magnetization of manganese-aluminum and aluminum; Achievement of the largest polar Kerr rotation.