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The metastable wurtzite crystal phase in gallium phosphide (WZ GaP) is a relatively new structure with little available information about its emission properties compared to the most stable zinc-blend phase. Here, the effect of growth conditions of WZ GaP nano- and microstructures obtained via chemical beam epitaxy on the optical properties was studied using power- and temperature-dependent photoluminescence (PL). We showed that the PL spectra are dominated by two strong broad emission bands at 1.68 and 1.88 eV and two relatively narrow peaks at 2.04 and 2.09 eV. The broad emissions are associated with the presence of carbon and a small number of extended crystal defects, respectively. For the sharp emissions, two main radiative recombination channels were observed with ionization energies estimated in the range of 50-80 meV and lower than 10 meV. No variation of the low-temperature PL spectra was observed for samples grown at different P precursor flows, while increasing Ga content enhanced the dominant broad emission at around 1.68 eV, suggesting that the group III organometallic precursor is the main source of impurities. Finally, Be-doped samples were grown, and their characteristic optical emission at 2.03 eV was identified. These results contribute to the understanding of impurity-related luminescence in hexagonal GaP, being useful for further crystal growth optimization required for the fabrication of optoelectronic devices.

Strain-based band structure engineering is a powerful tool to tune the optical and electronic properties of semiconductor nanostructures. We show that we can tune the band structure of InGaAs semic...

The results of studies of the time-resolved photoluminescence in semiconductor heterostructures containing two noninteracting InGaAs quantum wells in a GaAs matrix are reported. One of the quantum wells was undoped, and the other was uniformly doped with chromium atoms (InGaAs:Cr). It was shown that the introduction of Cr had a profound effect on the recombination lifetime of charge carriers in quantum wells. The change in the photoluminescence intensity after excitation cannot be described by a monoexponential decay function, which is attributed to a change in the built-in electric field of the surface barrier in the quantum wells because of screening by photoexcited charge carriers.

The engineering of core@multi-shell nanoparticles containing heterogeneous crystalline phases in different layers constitutes an important strategy for obtaining optical probes. The possibility of obtaining an opto-magnetic core@multi-shell nanoparticle capable of emitting in the visible and near-infrared ranges by upconversion and downshifting processes is highly desirable, especially when its optical responses are dependent on temperature and magnetic field variations. This work proposes the synthesis of hierarchically structured core@multi-shell nanoparticles of heterogeneous crystalline phases: a cubic core containing DyIII ions responsible for magnetic properties and optically active hexagonal shells, where ErIII, YbIII, and NdIII ions were distributed. This system shows at least three excitation energies located at different biological windows, and its emission intensities are sensitive to temperature and external magnetic field variations. The selected crystalline phases of the core@multi-shell nanoparticles obtained in this work is fundamental to the development of multifunctional materials with potential applications as temperature and magnetic field optical probes.

This article shows results on photoluminescence measurements of Si nanocrystals embedded in a SiO2 matrix. Photoluminescence spectra were obtained as functions of both temperature (10–510 K) and excitation power (0.01–100 mW). Optical emission bands with interesting behavior as temperature was raised were observed, and a model to explain these bands considering contributions from interface states associated with SiO x and Si–O–Si units, whose emissions are located around 1.32 eV, 1.41 eV and 1.57 eV, and from quantum confinement states associated with the Si nanocrystals, was proposed.

The Stark effect is one of the most efficient mechanisms to manipulate many-body states in nanostructured systems. In mono- and few-layer transition metal dichalcogenides, it has been successfully induced by optical and electric field means. Here, we tune the optical emission energies and dissociate excitonic states in MoSe2 monolayers employing the 220 MHz in-plane piezoelectric field carried by surface acoustic waves. We transfer the monolayers to high dielectric constant piezoelectric substrates, where the neutral exciton binding energy is reduced, allowing us to efficiently quench (above 90%) and red-shift the excitonic optical emissions. A model for the acoustically induced Stark effect yields neutral exciton and trion in-plane polarizabilities of 530 and 630 x 10(-5) meV/(kV/cm)(2), respectively, which are considerably larger than those reported for monolayers encapsulated in hexagonal boron nitride. Large in-plane polarizabilities are an attractive ingredient to manipulate and modulate multiexciton interactions in two-dimensional semiconductor nanostructures for optoelectronic applications.

The spin-memory effect in the GaAs / InGaAs heterostructures with $\delta$ layer in GaAs barrier have been investigated. The effect consists in spin polarization of Mn atoms due to interaction with photogenerated spin-polarized holes. The investigation of the effect was carried out by analyzing the polarization of the probe photoluminescence pulse in the pump-probe technique. It was shown that the circular polarization degree of probe pulse generated photoluminescence is strongly affected by the interaction of hole spins with spins of Mn atoms polarized by the pump pulse. The latter leads to decrease of circular polarization degree as compared with single pulse excitation ($\delta P$ effect). The amplitude of $\delta P$-effect is most strongly affected by the concentration of resident electrons in the quantum well which is believed to be due the specific compliance with selection rules for optical transition with the participation of unpolarized resident electrons. The rest of the sample's parameters including the spatial separation between $\delta$ layer and InGaAs quantum well ($d_s$) have a minor effect on the $\delta P$ value which leads to a paradoxical situation of decreasing $\delta P$-effect with the decrease of $d_s$. The proposed experimental technique consisting in creating the significant concentration of resident electrons in the QW may serve as a reliable photoluminescence method determining the strength of this effect as well as the Mn spin relaxation time in a particular nanostructure.

Definitive evidence for the direct band gap predicted for Wurtzite Gallium Phosphide (WZ GaP) nanowires has remained elusive due to the lack of strong band-to-band luminescence in these materials. In order to circumvent this problem, we successfully obtained large volume WZ GaP structures grown by nanoparticle-crawling assisted Vapor-Liquid-Solid method. With these structures, we were able to observe bound exciton recombination at 2.14 eV with FHWM of approximately 1 meV. In addition, we have measured the optical absorption edges using photoluminescence excitation spectroscopy. Our results show a 10 K band gap at 2.19 eV and indicate a weak oscillator strength for the lowest energy band-to-band absorption edge, which is a characteristic feature of a pseudo-direct band gap semiconductor. Furthermore, the valence band splitting energies are estimated as 110 meV and 30 meV for the three highest bands. Electronic band structure calculations using the HSE06 hybrid density functional agree qualitatively with the valence band splitting energies.

Definitive evidence for the direct band gap predicted for Wurtzite Gallium Phosphide (WZ GaP) nanowires has remained elusive due to the lack of strong band-to-band luminescence in these materials. In order to circumvent this problem, we successfully obtained large volume WZ GaP structures grown by nanoparticle-crawling assisted Vapor-Liquid-Solid method. With these structures, we were able to observe bound exciton recombination at 2.14 eV with FHWM of approximately 1 meV. In addition, we have measured the optical absorption edges using photoluminescence excitation spectroscopy. Our results show a 10 K band gap at 2.19 eV and indicate a weak oscillator strength for the lowest energy band-to-band absorption edge, which is a characteristic feature of a pseudo-direct band gap semiconductor. Furthermore, the valence band splitting energies are estimated as 110 meV and 30 meV for the three highest bands. Electronic band structure calculations using the HSE06 hybrid density functional agree qualitatively with the valence band splitting energies.

In the last two decades, many experiments were conducted in self-organization of nanocrystals into two- and three-dimensional (3D) superlattices and the superlattices were synthesized and characterized by different techniques, revealing their unusual properties. Among all characterization techniques, X-ray diffraction (XRD) is the one that has allowed the confirmation of the 3D superlattice formation due to the presence of sharp and intense diffraction peaks. In this work, we study self-organized superlattices of quantum dots of PbS prepared by dropping a monodispersed colloidal solution on a glass substrate at different temperatures. We showed that the intensity of the low-angle XRD peaks depends strongly on the drying time (substrate temperature). We claim that the peaks are originated from the 3D superlattice. Scanning electron microscopy images show that this 3D superlattice (PbS quantum dots) is formed in flake's shape, parallel to the substrate surface and randomly oriented in the perpendicular planes.

Here we show a new nanowire growth procedure, exploring the thermally activated motion of Au droplets on III-V surfaces. We show that by setting a single growth parameter we can activate the crawling motion of Au droplets in vacuum and locally modify surface composition in order to enhance vapor-solid (VS) growth along oxide-free areas on the trail of the metal particle. Asymmetric VS growth rates are comparable in magnitude to the vapor-liquid-solid growth, producing unconventional wurtzite GaP morphologies, which shows negligible defect density as well as optical signal in the green spectral region. Finally, we demonstrate that this effect can also be explored in different substrate compositions and orientations with the final shape finely tuned by group III flow and nanoparticle size. This distinct morphology for wurtzite GaP nanomaterials can be interesting for the design of nanophotonics devices.

Aqueous colloidal silver nanoparticles have substantial potential in biological application as markers and antibacterial agents and in surface-enhanced Raman spectroscopy applications. A simple method of fabrication and encapsulation into an inert shell is of great importance today to make their use ubiquitous. Here we show that colloids of silver-core/silica-shell nanoparticles can be easily fabricated by a laser-ablation-assisted chemical reduction method and their sizes can be tuned in the range of 2.5 to 6.3 nm by simply choosing a proper water-ethanol proportion. The produced silver nanoparticles possess a porous amorphous silica shell that increases the inertness and stability of colloids, which decreases their toxicity compared with those without silica. The presence of a thin 2 to 3 nm silica shell was proved by EDX mapping. The small sizes of nanoparticles achieved by this method were analyzed using optical techniques, and they show typical photoluminescence in the UV-vis range that shifts toward higher energies with decreasing size.

Espelhos de Bragg são heteroestruturas de alta e seletiva refletividade. O objetivo desse projeto é a simulação, confecção e caracterizadação de um espelho Bragg óptico feito com camadas de Al2O/TiO2 e de outro com Al2O3/ZnO. A simulação foi feita no software Mathematica™ e a caracterização foi feita pela medida da refletividade (para ambos espelhos) e também da difração de raios-x, para o estudo da camada de ZnO.

Overcoming the critical thickness limit in pseudomorphic growth of lattice mismatched heterostructures is a fundamental challenge in heteroepitaxy. On-demand transfer of light-emitting structures to arbitrary host substrates is an important technological method for optoelectronic and photonic device implementation. The use of freestanding membranes as compliant substrates is a promising approach to address both issues. In this work, the feasibility of using released GaAs/InGaAs/GaAs membranes as virtual substrates to thin films of InGaAs alloys is investigated as a function of the indium content in the films. Growth of flat epitaxial films is demonstrated with critical thickness beyond typical values observed for growth on bulk substrates. Optically active structures are also grown on these membranes with a strong photoluminescence signal and a clear red shift for an InAlGaAs/InGaAs/InAlGaAs quantum well. The red shift is ascribed to strain reduction in the quantum well due to the use of a completely relaxed membrane as the substrate. Our results demonstrate that such membranes constitute a virtual substrate that allows further heterostructure strain engineering, which is not possible when using other post-growth methods.

We experimentally demonstrate that the Raman active optical phonon modes of single layer graphene can be modulated by the dynamic local strain created by surface acoustic waves (SAWs). In particular, the dynamic strain field of the SAW is shown to induce a Raman scattering intensity variation as large as 15% and a phonon frequency shift of up to 10 cm$^{-1}$ for the G band, for instance, for an effective hydrostatic strain of 0.24% generated in a single layer graphene atop a LiNbO$_{3}$ piezoelectric substrate with a SAW resonator operating at a frequency of $ \sim $ 400 MHz. Thus, we demonstrate that SAWs are powerful tools to modulate the optical and vibrational properties of supported graphene by means of the high-frequency localized deformations tailored by the acoustic transducers, which can also be extended to other 2D systems., Comment: 10 pages, 7 figures

We investigate the optical properties of strain-free mesoscopic GaAs/Al x Ga1 - x As structures (MGS) coupled to thin GaAs/Al x Ga1 - x As quantum wells (QWs) with varying Al content (x). We demonstrate that quenching the QW emission by controlling the band crossover between AlGaAs (X-point) and GaAs (Γ-point) gives rise to long carrier lifetimes and enhanced optical emission from the MGS. For x = 0.33, QW and MGS show typical type-I band alignment with strong QW photoluminescence emission and much weaker sharp recombination lines from the MGS localized exciton states. For x ≥ 0.50, the QW emission is considerably quenched due to the change from type-I to type-II structure while the MGS emission is enhanced due to carrier injection from the QW. For x ≥ 0.70, we observe PL quenching from the MGS higher energy states also due to the crossover of X and Γ bands, demonstrating spectral filtering of the MGS emission. Time-resolved measurements reveal two recombination processes in the MGS emission dynamics. The fast component depends mainly on the X - Γ mixing of the MGS states and can be increased from 0.3 to 2.5 ns by changing the Al content. The slower component, however, depends on the X - Γ mixing of the QW states and is associated to the carrier injection rate from the QW reservoir into the MGS structure. In this way, the independent tuning of X - Γ mixing in QW and MGS states allows us to manipulate recombination rates in the MGS as well as to make carrier injection and light extraction more efficient.

O objetivo do projeto é investigar as propriedades estruturais por espectroscopia Raman de filmes de diamond-like carbon (DLC) depositados por método de imersão em plasma. A técnica de espectroscopia Raman irá fornecer informações relativas a natureza química e dos tipos de ligações químicas do material. Os filmes investigados têm sido utilizados em diversas aplicações, como na proteção física das superfícies em peças mecânicase em plásticos de embalagem de alimentos. A proposta deste projeto é, portanto, utilizando a espectroscopia Raman, fazer a análise de filmes depositados a diferentes potencias e pressões.

Monolayers of transition metal dichalcogenides (TMD) are promising materials for optoelectronics devices. However, one of the challenges is to fabricate large-scale growth of high quality TMD monolayers with the desired properties in order to expand their use in potential applications. Here, we demonstrate large-scale tungsten disulfide (WS2) monolayers grown by van der Waals Epitaxy (VdWE). We show that, in addition to the large structural uniformity and homogeneity of these samples, their optical properties are very sensitive to laser irradiation. We observe a time instability in the photoluminescence (PL) emission at low temperatures in the scale of seconds to minutes. Interestingly, this change of the PL spectra with time, which is due to laser induced carrier doping, is employed to successfully distinguish the emission of two negatively charged bright excitons. Furthermore, we also detect blinking sharp bound exciton emissions which are usually attractive for single photon sources. Our findings contribute to a deeper understanding of this complex carrier dynamics induced by laser irradiation which is very important for future optoelectronic devices based on large scale TMD monolayers.

Structural and optical properties of erbium- and manganese-doped strontium aluminates (SrAl 2 O 4 ) phosphor materials synthesized by a solid state reaction were investigated. The samples presented the fundamental optical transitions due to Er 3+ and Mn 2+ which are typical features of the well-diluted doping process. A significant enhancement of the Er 3+ optical emission band at 1530 nm was observed when the matrix is co-doped with Mn. Photoluminescence intensity under ultraviolet excitation was three times larger as compared to samples without Mn content. A model of energy transfer mechanism from Mn 2+ to Er 3+ ions due to optical energy matching is proposed to explain the experimental results. This result, presented first time, can be applied as useful tool for developments in optical communications.

Time-resolved Kerr rotation measurements were performed in InGaAs/GaAs quantum wells nearby a doped Mn delta layer. Our magneto-optical results show a typical time evolution of the optically-oriented electron spin in the quantum well. Surprisingly, this is strongly affected by the Mn spins, resulting in an increase of the spin precession frequency in time. This increase is attributed to the variation in the effective magnetic field induced by the dynamical relaxation of the Mn spins. Two processes are observed during electron spin precession: a quasi-instantaneous alignment of the Mn spins with photo-excited holes, followed by a slow alignment of Mn spins with the external transverse magnetic field. The first process leads to an equilibrium state imprinted in the initial precession frequency, which depends on pump power, while the second process promotes a linear frequency increase, with acceleration depending on temperature and external magnetic field. This observation yields new information about exchange process dynamics and on the possibility of constructing spin memories, which can rapidly respond to light while retaining information for a longer period., Comment: 7 pages, 5 figures

We report on the existence of two different regimes in one-step Ag-seeded InP nanowire growth. The vapor-liquid-solid-mechanism is present at larger In precursor flows and temperatures, ∼500 °C, yielding high aspect ratio and pure wurtzite InP nanowires with a semi-spherical metal particle at the thin apex. Periodic diameter oscillations can be achieved under extreme In supersaturations at this temperature range, showing the presence of a liquid catalyst. However, under lower temperatures and In precursor flows, large diameter InP nanowires with mixed wurtzite/zincblende segments are obtained, similarly to In-assisted growth. Chemical composition analysis suggest that In-rich droplet formation is catalyzed at the substrate surface via Ag nanoparticles; this process might be facilitated by the sulfur contamination detected in these nanoparticles. Furthermore, part of the original Ag nanoparticle remains solid and is embedded inside the actual catalyst, providing an in situ method to switch growth mechanisms upon changing In precursor flow. Nevertheless, our Ag-seeded InP nanowires exhibit overall optical emission spectra consistent with the observed structural properties and similar to Au-catalyzed InP nanowires. We thus show that Ag nanoparticles may be a suitable replacement for Au in InP nanowire growth.

InAs nanowires grown by vapor–liquid–solid (VLS) method are investigated by photoluminescence. We observe that the Fermi energy of all samples is reduced by ~20 meV when the size of the Au nanoparticle used for catalysis is increased from 5 to 20 nm. Additional capping with a thin InP shell enhances the optical emission and does not affect the Fermi energy. The unexpected behavior of the Fermi energy is attributed to the differences in the residual donor (likely carbon) incorporation in the axial (low) and lateral (high incorporation) growth in the VLS and vapor–solid (VS) methods, respectively. The different impurity incorporation rate in these two regions leads to a core/shell InAs homostructure. In this case, the minority carriers (holes) diffuse to the core due to the built-in electric field created by the radial impurity distribution. As a result, the optical emission is dominated by the core region rather than by the more heavily doped InAs shell. Thus, the photoluminescence spectra and the Fermi energy become sensitive to the core diameter. These results are corroborated by a theoretical model using a self-consistent method to calculate the radial carrier distribution and Fermi energy for distinct diameters of Au nanoparticles.

© 2019 Author(s). We investigate the effect of surface acoustic waves on the atomic-like optical emission from defect centers in hexagonal boron nitride layers deposited on the surface of a LiNbO3 substrate. The dynamic strain field of the surface acoustic waves modulates the emission lines resulting in intensity variations as large as 50% and oscillations of the emission energy with an amplitude of almost 1 meV. From a systematic study of the dependence of the modulation on the acoustic wave power, we determine a hydrostatic deformation potential of about 40 meV/% for defect centers in this two-dimensional material. Furthermore, we show that the dynamic piezoelectric field of the acoustic wave could contribute to the stabilization of the optical properties of these emission centers. Our results show that surface acoustic waves are a powerful tool to modulate and control the electronic states of two-dimensional materials.

EuTe is a magnetic semiconductor with potential applications in prototype optoelectronic and spintronic devices. In this work, we investigated the structural properties of EuTe grown on BaF2(111) substrates by molecular beam epitaxy. X-ray diffraction measurements were performed on two series of samples with different growth times and temperatures. The growth occurs in the Volmer-Webber mode, with initial formation of islands that then coalesce to form thin films. The islands size, mosaic spread, and strain state are deduced as a function of growth conditions. Surprisingly, the EuTe islands exhibit in-plane tensile strain, while compressive strain is expected for structures grown over a substrate with smaller lattice parameter. The islands tensile strain relaxes with increasing deposition times and substrate temperature, and it tends to zero for thick EuTe films. We propose that the EuTe/BaF2 lattice mismatch is compensated by the formation of interfacial misfit dislocations. The growth conditions out of equilibrium favor the formation of a metastable state with a high concentration of dislocations that over-compensates the original misfit. This is in agreement with the observed reduction of tensile strain as the substrate temperature increases.

We use a combined process of Ga-assisted deoxidation and local droplet etching to fabricate unstrained mesoscopic GaAs/AlGaAs structures exhibiting a high shape anisotropy with a length up to 1.2 μm and a width of 150 nm. We demonstrate good controllability over size and morphology of the mesoscopic structures by tuning the growth parameters. Our growth method yields structures, which are coupled to a surrounding quantum well and present unique optical emission features. Microscopic and optical analysis of single structures allows us to demonstrate that single structure emission originates from two different confinement regions, which are spectrally separated and show sharp excitonic lines. Photoluminescence is detected up to room temperature making the structures the ideal candidates for strain-free light emitting/detecting devices.

In this work, we study theoretically and experimentally the influence of the surface electron accumulation on the optical properties of InN nanowires. For this purpose, the photoluminescence and photoluminescence excitation spectra have been measured for a set of self-assembled InN NWs grown under different conditions. The photoluminescence excitation experimental lineshapes have been reproduced by a self-consistent calculation of the absorption in a cylindrical InN nanowires. With the self-consistent model we can explore how the optical absorption depends on nanowires radius and doping concentration. Our model solves the Schrodinger equation for a cylindrical nanowire of infinite length, assuming a parabolic conduction band. The columnar geometry introduces effects in both the electron density and in the self-consistent conduction band profile, with no equivalence in planar layer. On the other hand, the differences in the photoluminescence excitation spectra are related to the inhomogeneous electron distribution inside the nanowires, caused by a bulk donor concentration and a two-dimensional density of ionized surface states. For nanowire radii larger than 30 nm, such concentrations modify the absorption edge and the lineshape, respectively, and can be determined from the comparison with the experimental data (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate the controlled transfer of photoexcited carriers by a surface acoustic wave (SAW) between coupled quantum wells, wires, and dots grown on a semiconductor surface. The quantum wires and dots used in the experiments are embedded at photolithographically defined positions within an (Al,Ga)As/GaAs (311)A-oriented quantum well grown by molecular beam epitaxy. We give experimental evidence for the anti-bunching of photons emitted in a quantum dot pumped by electrons and holes transported from the quantum well by a surface acoustic wave.

It is now possible to acoustically control the transfer of electrons and holes between a quantum well and a quantum dot by exploiting the moving piezoelectric potential modulation induced by an acoustic phonon. The effect has been used to demonstrate a high-frequency single-photon source with tunable emission energy, by acoustically transferring carriers to selected quantum dots.

We investigate the modulation of optical phonons in semiconductor crystals by a surface acoustic wave (SAW) propagating on the crystal surface. The SAW fields induce changes on the order of ${10}^{\ensuremath{-}3}$ in the time-averaged Raman peak intensity by optical phonons in Si and GaN crystals. The SAW-induced modifications in the intensity of the Raman lines are dominated by the modulation of the longitudinal optical (LO) phonon energy by the SAW strain field. We show that while the strain field of the excited Rayleigh SAWs changes the LO phonon energy, it does not mix it with the transversal optical modes. In addition to the previous contribution, which is of a local character, the experiments give evidence for a weaker and nonlocal contribution attributed to the spatial variation of the SAW strain field. The latter activates optical modes with large wave vectors and, therefore, lower energies. The experimental results, which are well described by theoretical models for the two contributions, prove that optical phonons can be manipulated by SAWs with $\ensuremath{\mu}\mathrm{m}$ wavelengths.

We review processes for long-range spin transport and manipulation in GaAs quantum wells using mobile potentials created by the piezoelectric field of a surface acoustic wave. By reducing spin dephasing mechanisms associated with the spin orbit-coupling, these potentials can coherently transport spins over distances on the order of 100 µm. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigated single InP quantum dots embedded in GaAs using micro-photoluminescence as a function of the excitation intensity. InP/GaAs dots exhibit a type-II band alignment, which leads to a spatial separation of the carriers. The effect of a magnetic field on these type-II quantum dots were also investigated through micro-photoluminescence measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InGaAs/GaAs self-assembled quantum dots in the presence of a two-dimensional electron gas were studied by micro-photoluminescence. Several sharp optical emission lines attributed to individual quantum dots were observed. These lines exhibit a blue shift and a broadening as the electron gas density is increased. We discuss the origin of the blue shift considering the variation of the built-in electric field in the asymmetric geometry of the dots. The observed broadening of the quantum dot emission lines is an indication of coupling between the two-dimensional electron gas and the electrons in the quantum dot.

The point defects of single ZnO microwires grown by carbothermal reduction were studied by microphotoluminescence, photoresistance excitation spectra, and resistance as a function of the temperature. We found the deep level defect density profile along the microwire showing that the concentration of defects decreases from the base to the tip of the microwires and this effect correlates with a band gap narrowing. The results show a characteristic deep defect levels inside the gap at 0.88 eV from the top of the VB. The resistance as a function of the temperature shows defect levels next to the bottom of the CB at 110 meV and a mean defect concentration of 4 1018 cm3 . This combination of techniques allows us to study the band gap values and defects states inside the gap in single ZnO microwires and opens the possibility to be used as a defect spectroscopy method. Fil: Villafuerte, Manuel Jose. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Solido; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Ferreyra, J. M.. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Solido; Argentina Fil: Zapata, C.. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Solido; Argentina Fil: Barzola Quiquia, J.. University of Leipzig; Alemania Fil: Iikawa, F.. Instituto de Física “Gleb Wataghin"; Brasil Fil: Esquinazi, P.. University of Leipzig; Alemania Fil: Huleani, S. P.. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Solido; Argentina Fil: de Lima, M. M.. Universidad de Valencia; España Fil: Cantarero, A.. Universidad de Valencia; España

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.

Properties of structures containing an InGaAs quantum well (QW) or InAs quantum dots (QDs) as well as δ〈C〉-and δ〈Mn〉-doped layers are investigated. Most of these structures are fabricated by the MOCVD epitaxy method; δ〈Mn〉-doped layers are obtained by low-temperature laser deposition directly in the epitaxial reactor. The structures under study exhibit three conduction channels: the δ〈C〉-doped layer, δ〈Mn〉-doped layer, and QW or wetting layer in the case of QDs. The contribution of each channel into the total conduction depend on the measurement temperature. Photoluminescence of the structures as a function of thickness of δ〈Mn〉-doped layer is studied.

The phase-transition temperature of MnAs epitaxial films grown by molecular-beam epitaxy on GaAs(001) with different crystalline accommodations was studied by specular and grazing incidence x-ray diffraction. The transition temperature of MnAs films with tilted hexagonal c-axis orientations with respect to the GaAs substrate is higher than the most investigated nontilted films and reaches a value above room temperature, which is more suitable for device applications.

We investigate the magnetic field dependence of the magnetic and structural phase transition of an epitaxial MnAs layer grown on GaAs (1 0 0) by using SQUID magnetometry and X-ray diffraction. The phase transition temperatures obtained from magnetic and structural results revealed an apparent splitting for magnetic field less than ∼1 kOe, while for higher magnetic fields a behavior similar to that reported for bulk MnAs is observed.

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 ...

Photoluminescence and photomodulated transmission measurements on In0.2Ga0.8As/ GaAs/Al0.3Ga0.7As modulation doped pseudomorphic single quantum wells are presented. Photomodulated transmission spectra at low temperatures showed sharp lines that are separated with respect to the luminescence peaks due to the Stokes shift. From the Stokes shift we estimated the Fermi energy and the two‐dimensional electron gas density. The obtained results are in good agreement with Shubnikov–de‐Haas data. The temperature dependence of the optical spectra was also investigated. The photomodulated transmission technique is shown to be a good tool to evaluate the electronic properties of modulation doped single quantum well structures, including an estimation of the Fermi energy and the two‐dimensional electron gas density.

We have studied type-II InP/GaAs self-assembled quantum dots by microphotoluminescence spectroscopy. Sharp spectral features were observed on top of a broad emission band. They are associated to statistical fluctuations from the ensemble of dots. Photoluminescence measurements as a function of the excitation intensity revealed markedly distinct behaviors: the broadband contour shows a large blueshift while the energy positions of the sharp features remain basically constant. We show that the large blueshift of the broad emission band in type-II quantum dots is not due to the barrier interface potential variation, but to the state filling of higher-energy states.

We report a study of magnetic responses of neutral and charged excitons in quantum wells, which are very sensitive to the strong spin hybridization of holes. This effect can be used to engineer the spin character of excitonic complexes in two-dimensional systems tuned by the magnetic field strength. Conditions for spin flip for each kind of excitonic complex is detailed and the nature of the effect discussed. Differences in the effective Zeeman splitting between neutral and charged excitons are theoretically predicted and unambiguously confirmed experimentally. Circularly polarized resolved photoluminescence has been used to study these effects under applied magnetic fields. The intertwining of spin dynamics of excitons and trions is discussed.

We report polarized Raman scattering and resonant Raman scattering studies on single InAs nanowires. Polarized Raman experiments show that the highest scattering intensity is obtained when both the incident and analyzed light polarizations are perpendicular to the nanowire axis. InAs wurtzite optical modes are observed. The obtained wurtzite modes are consistent with the selection rules and also with the results of calculations using an extended rigid-ion model. Additional resonant Raman scattering experiments reveal a redshifted E1 transition for InAs nanowires compared to the bulk zinc-blende InAs transition due to the dominance of the wurtzite phase in the nanowires. Ab initio calculations of the electronic band structure for wurtzite and zinc-blende InAs phases corroborate the observed values for the E1 transitions.

Photoluminescence excitation (PLE) spectra have been measured for a set of self-assembled InN nanowires (NWs) and a high-crystalline quality InN layer grown by molecular-beam epitaxy. The PLE experimental lineshapes have been reproduced by a self-consistent calculation of the absorption in a cylindrical InN NW. The differences in the PLE spectra can be accounted for the inhomogeneous electron distribution within the NWs caused by a bulk donor concentration $({N}_{D}^{+})$ and a two-dimensional density of ionized surface states $({N}_{ss}^{+})$. For NW radii larger than 30 nm, ${N}_{D}^{+}$ and ${N}_{ss}^{+}$ modify the absorption edge and the lineshape, respectively, and can be determined from the comparison with the experimental data.