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The aim of this work is to determine the varying dielectric constant of a biological nanostructured system via electrostatic force microscopy (EFM) and to show how this method is useful to study natural photonic crystals. We mapped the dielectric constant of the cross section of the posterior wing of the damselfly Chalcopteryx rutilans with nanometric resolution. We obtained structural information on its constitutive nanolayers and the absolute values of their dielectric constant. By relating the measured profile of the static dielectric constant to the profile of the refractive index in the visible range, combined with optical reflectance measurements and simulation, we were able to describe the origin of the strongly iridescent wing colors of this Amazonian rainforest damselfly. The method we demonstrate here should be useful for the study of other biological nanostructured systems.

Self-organized nanowires were formed on topographically and orderly modified surfaces by a focused ion beam. Silver catalyst nanoparticles were deposited on the regular shallow hole arrangements on the InP (001), which were prepared with three different gallium ion doses, and acted as preferential nucleation sites for InAs grown using Metal-Organic Vapor Phase Epitaxy. Ion doses ranging from 0.8 × 1014 to 4.2 × 1014 Ga+/cm2 were used, and InAs was grown with two sub-monolayer coverages. Two-dimensional ordered InAs nanowire arrays were grown in the templates, despite the randomly deposited silver nanoparticles and the cavities full of defects generated by ion beam irradiation. For the 1 s growth time sample using templates produced with a high ion dose, most of the InAs nanostructures were found to fill either entirely or a large fraction of the cavities. Nanowires were grown in the 2 s growth time sample, using low and medium ion dose. Although the nanowires' growth mechanism is described by the most widely employed vapor-liquid-solid model, the site-selective nanowires growth with randomly deposited catalyst nanoparticle occurs due to net adatom flow to the cavities assisted by modified surface potential. The competition between the coarsening and the vapor-liquid-solid mechanisms plays a critical role in determining the growth of nanowires or islands by providing a simple model able to properly describe the type of the nanostructure (islands or nanowires) dependence with the growth time and the ion dose.

This work describes morphological and crystalline properties of the InAs islands grown on templates created by focused ion beam (FIB) on indium phosphide (InP) substrates. Regular arrangements of shallow holes are created on the InP (001) surfaces, acting as preferential nucleation sites for InAs islands grown by Metal-Organic Vapor Phase Epitaxy. Ion doses ranging from 1015 to 1016 Ga+/cm2 were used and islands were grown for two sub-monolayer coverages. We observe the formation of clusters in the inner surfaces of the FIB produced cavities and show that for low doses templates the nanostructures are mainly coherent while templates created with large ion doses lead to the growth of incoherent islands with larger island density. The modified island growth is described by a simple model based on the surface potential and the net adatom flow to the cavities. We observe that obtained morphologies result from a competition between coarsening and coalescence mechanisms.

The MeX™ software is a useful tool for tridimensional data collection for surface evaluation and could be relevant to evaluate the same specimen in different phases of the study, assuming repeated measures of dental enamel roughness. The aim of this study was to evaluate the influence of sample metallization for dental enamel roughness analysis with 3D images reconstructed using MeX™ software from Scanning Electron Microscopy (SEM) images. The influence of 74.98% (%mol/mol) argon-oxygen plasma for carbon layer removal on surface roughness of the metallized specimen was also evaluated. Dental enamel specimens were prepared for SEM analysis with and without carbon metallization using conventional or environmental modes. Argon-oxygen plasma for carbon layer removal was used and surface roughness was re-evaluated. Roughness obtained by SEM and MeX™ reconstructed images, with or without metallization, did not differ. No significant alteration on surface roughness after carbon layer removal using plasma was found. SEM baseline evaluation using conventional mode without sample preparation and in environmental mode were not comparable. Roughness of enamel 3D images reconstructed with MeX™ software from SEM images, with or without metallization was similar. The 74.98% (%mol/mol) argon-oxygen plasma removed the carbon layer with no effect on enamel roughness.

In this work, we present a sulfurization apparatus, we call Russian Doll, which provides a quasi-stationary sulfur rich environment. We prepared several pyrite films using the technique to sulfurize iron oxide films. We studied the influence of the synthesis parameters in the phase composition and microstructure of the disulfide films. We found no traces of iron oxides nor of secondary iron sulfide phases, such as pyrrhotite. Nevertheless, we observed the formation of the marcasite polymorph at two different temperature ranges, due to the similarity in the thermodynamic properties of both disulfide phases. At temperatures lower than 550 °C, however, marcasite undergoes a transformation reaction to pyrite, which is completed after 4 h at 550 °C. At temperatures higher than 550 °C, pyrite slowly decomposes producing pyrrhotite and sulfur gas, due to the high sulfur equilibrium fugacity. During cooling, the sulfur equilibrium fugacity drops shifting the reaction inversely towards the formation of iron disulfide, resulting in a core-surface pattern of pyrite-marcasite. We also observed that oxide films with larger particles generated pyrite films also with larger particles.

The iridescent wings of the Chalcopterix rutilans damselfly (Rambur) (Odonata, Polythoridae) are investigated with focused ion beam/scanning electron microscopy, transmission electron microscopy, and time-of-flight secondary ion mass spectrometry. The electron microscopy images reveal a natural photonic crystal as the source of the varying colors. The photonic crystal has a consistent number and thickness (∼195 nm) of the repeat units on the ventral side of the wing, which is consistent with the red color visible from the bottom side of the wing in all regions. The dorsal side of the wing shows strong color variations ranging from red to blue depending on the region. In the electron microscopy images, the dorsal side of the wing exhibits varied number and thicknesses of the repeat units. The repeat unit spacings for the red, yellow/green, and blue regions are approximately 195, 180, and 145 nm, respectively. Three-dimensional analysis of the natural photonic crystals by time-of-flight secondary ion mass spectrometry reveals that changes in the relative levels of Na, K, and eumelanin are responsible for the varying dielectric constant needed to generate the photonic crystal. The photonic crystal also appears to be assembled with a chemical tricomponent layer structure due to the enhancement of the CH6N3+ species at every other interface between the high/low dielectric constant layers.

Cadmium Sulfide (CdS) nanoparticles were successfully grown on Multi-Walled Carbon Nanotubes (MWNTs) via a simple chemical reaction. The CdS-MWNTs sample was afterwards characterized with Scanning and Transmission Electron Microscopies (SEM/TEM) and X-ray diffraction (XRD). The obtained images show clearly the decoration of the MWNTs by the CdS nanoparticles, and the XRD measurements indicate the CdS structure as Zinc Blend type.

Iron silicide samples were grown on Si (111) substrates by solid phase epitaxy and reactive deposition epitaxy. The different iron silicide phases and their correlations with the growth parameters were analyzed by x-ray photoelectron spectroscopy, conversion electron Mossbauer spectroscopy, x-ray diffraction, atomic force microscopy, and magnetic force microscopy. The authors investigated the potential of each technique for identifying and quantifying of the phases. In particular, the authors used a semiquantitative analysis of magnetic force microscopy images to spatially resolve the semiconductor β-FeSi2 phase.

The new Microscopy Center at the Federal University of Minas Gerais (UFMG) is presently in the final stages of construction. Here, we will describe the history as well as the technical aspects of this new facility, which will be fully available to the scientific, state and private enterprise communities in 2007. We envision that this multi-user facility will be a place where researchers from material sciences and biomedical areas will share space, microscopes and knowledge. Such cross- discipline interactions should promote a better interface between these diverse areas and enhance the growth of emerging areas such as nanotechnology and bioengineering.

A reproducible route for tuning localized surface plasmon resonance in scattering type near-field optical microscopy probes is presented. The method is based on the production of a focused-ion-beam milled single groove near the apex of electrochemically etched gold tips. Electron energy-loss spectroscopy and scanning transmission electron microscopy are employed to obtain highly spatially and spectroscopically resolved maps of the milled probes, revealing localized surface plasmon resonance at visible and near-infrared wavelengths. By changing the distance L between the groove and the probe apex, the localized surface plasmon resonance energy can be fine-tuned at a desired absorption channel. Tip-enhanced Raman spectroscopy is applied as a test platform, and the results prove the reliability of the method to produce efficient scattering type near-field optical microscopy probes.

We measured resistivity in the range of 30–390 K on four semi-insulating low-temperature grown molecular-beam epitaxy GaAs samples. The growth temperature range was from 215 °C to 315 °C. Arrhenius fittings with T−1 and hopping fitting with T−1/4 do not permit us the definition of the temperature ranges controlled by band and hopping conduction, respectively. This leads to major errors in the calculation of both activation energies and hopping parameters. We have used the differential activation energy in order to clearly identify the temperature range for the different transport mechanisms. Hopping dominates at low temperatures and band conduction at high temperatures. In-between, a mixed conduction regime is observed. We introduce a criterion to clearly define the temperature range of hopping, band and mixed conduction. The lower temperature at which mixed conduction is identified decreases for samples with increasing growth temperature. Only the sample grown at 215 °C presents both forms of hopping conduction before entering the mixed conduction regime. Hopping parameters were obtained from the fittings of the differential activation energy and the values are in good agreement with the usual method of calculating them if the correct temperature range is used.

Iron silicides were grown on Si (111) substrates by Solid Phase Epitaxy (SPE) and Reactive Deposition Epitaxy (RDE) to identify the optimum conditions to obtain the semiconduting β-FeSi2 phase. The films were produced under different growth and annealing conditions and analyzed in situ and ex situ by X-ray Photoelectron Spectroscopy, and ex situ by Conversion Electron Mossbauer Spectroscopy. The use of these techniques allowed the investigation of different depth regions of the grown layer. Films of the ɛ-FeSi and β-FeSi2 phases were obtained as well as the mixtures Fe3Si + ɛ-FeSi and ɛ-FeSi + β-FeSi2. The sequence Fe3Si→ɛ-FeSi→β-FeSi2 was found upon annealing, where the phase transformation occurred due to the migration of silicon atoms from the substrate to the surface region of the grown layer. The best conditions for the phase transformation in SPE samples were met after annealing in the range 700 – 800 °C. For the RDE samples, the transition to the beta phase occurred between 600 and 700 °C, but pure β-FeSi2 was obtained only after two hours of annealing at 700 °C.

We have analyzed the nonlinear electric field time series theoretically generated by a set of three coupled first-order differential equations in order to interpret the chaotic behavior present in measurements of low-frequency current oscillations in semiconductor samples. In the present work, we carry out a direct investigation of the theoretical time series applying techniques of experimental nonlinear time series analysis. We have reconstructed the phase space of the system and calculated some parameters as the correlation dimension of the reconstructed attractors, the Lyapunov exponent's spectrum and the Kaplan–Yorke dimension. The conclusions are consistent with the theory of chaotic systems and with the available experimental results for semiconductors.

Coherent emission efficiency in a 100A GaAs SQW microcavity was enhanced one order when pumped resonantly at 10 K, compared to the off-resonant excitation. The usual kink observed in the exciton emission linewidth as well as in the emission intensity in relation to the pump power, changes smoothly instead of the usual abrupt kink observed in the off-resonant microcavity laser. In addition, polarization measurements show a correlation relationship between the pump light polarization and the cavity emission polarization.

Low frequency oscillations (LFO) on current I ( V , t ) curves were studied in a GaAs sample grown by molecular beam epitaxy. Our measurements were carried out at a temperature of 200 K and under infrared illumination. The nonlinear I ( V ) characteristics give rise to spontaneous oscillations under constant applied bias V . The attractors were constructed by the delay vector technique with the delay value given by minimal mutual information. A bifurcation diagram was constructed from the minima sequence as a function of the applied bias. A noise reduction algorithm applied to the raw data allowed the cleaning of the bifurcation diagram. We have identified a bifurcation route of periodicity following the sequence 0–1–2–4–2–∞ having the applied bias as the control parameter. Fourier transforms is a merit figure of the noise reduction procedure and also a qualitative proof of the chaotic regime.

In this work several structural and chemical properties of self-assembled InAs islands grown on GaAs(001) are studied using surface x-ray scattering with synchrotron radiation. The technique of x-ray diffraction under grazing incidence condition was employed to differentiate coherent and incoherent islands. We used a model of a strained pyramidal island to interpret the x-ray results and correlate size and strain-state of these islands. The degree of GaAs interdiffusion in the islands was inferred from the variation of volume of the unit cell. The Poisson's ratio of the two materials involved establishes a limit of tetragonal distortion for this material. Any variation in this distortion is associated with the presence of Ga inside the islands.

Studying the transport properties via Hall and resistivity measurements of low-temperature molecular-beam epitaxy (LT-MBE) GaAs samples, the optimal conditions for fabricating high-resistivity material are found. We present results on three LT-MBE GaAs samples grown at 215, 265, and 315 °C. The measurements were carried out at temperatures ranging from 130 to 300 K, and the hopping conduction mechanism in this range is identified as variable range hopping. The sample grown at 315 °C presents the highest hopping parameter; this appears to be due to a reduction in the density of hopping centers. The mechanisms responsible for this are discussed.

Structural and chemical properties of self-assembled InAs islands grown on GaAs(001) were studied using x-ray scattering. Two measurements performed under grazing incidence geometry were correlated to obtain the three-dimensional strain and chemical status of InGaAs coherent islands. Grazing incidence diffraction was employed to reveal the in-plane strain-size interplay. Mapping out the reciprocal space near the GaAs(022) reflection and correlating the in-plane and out-of-plane strain information, we have been able to quantify the tetragonal distortion of the unit cells at any position inside the islands. Simple theory of elasticity of alloys enabled us to analyse the elastic deformation of the unit cells. Any variation in the expected tetragonal distortion of the unit cell was associated to the presence of Ga atoms inside the islands. Using this method, the Ga content in our islands was shown to vary linearly from 25% (island bottom) to 8% (island top).

Semi-insulating GaAs samples present N-shaped negative differential conductivity under high-electric fields. This behavior can be associated with two physical processes: Impact ionization (generation) and field-enhanced trapping (recombination), both of which involve trapped and free electrons. We have analyzed the j(E) characteristic curves of a GaAs sample rich in As antisite defects at different conditions of temperature and illumination. The fitting was carried out using an analytical expression for j(E) based on the competition between the above-mentioned processes. Our analysis permits us to identify the temperature and illumination ranges in which those processes are relevant. The best fittings were obtained for measurements between 150 and 200 K and using an infrared photon flux of the order of 1011 photons/cm2 s.

In this work we attempt to directly observe anisotropic partial relaxation of epitaxial InAs islands using transmission electron microscopy (TEM) and synchrotron x-ray diffraction on a 15 nm thick InAs:GaAs nanomembrane. We show that under such conditions TEM provides improved real-space statistics, allowing the observation of partial relaxation processes that were not previously detected by other techniques or by usual TEM cross section images. Besides the fully coherent and fully relaxed islands that are known to exist above previously established critical thickness, we prove the existence of partially relaxed islands, where incomplete 60° half-loop misfit dislocations lead to a lattice relaxation along one of the 〈110〉 directions, keeping a strained lattice in the perpendicular direction. Although individual defects cannot be directly observed, their implications to the resulting island registry are identified and discussed within the frame of half-loops propagations.

Made available in DSpace on 2015-10-21T20:15:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-06-17. Added 1 bitstream(s) on 2015-10-22T09:48:16Z : No. of bitstreams: 1 WOS000356518200001.pdf: 803864 bytes, checksum: e41d7e36cd2e1f67d32a7945ba81b594 (MD5) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Gold is a noble metal that, in comparison with silver and copper, has the advantage of corrosion resistance. Despite its high conductivity, chemical stability and biocompatibility, gold exhibits high plasticity, which limits its applications in some nanodevices. Here, we report an experimental and theoretical study on how to attain enhanced mechanical stability of gold nanotips. The gold tips were fabricated by chemical etching and further encapsulated with carbon nanocones via nanomanipulation. Atomic force microscopy experiments were carried out to test their mechanical stability. Molecular dynamics simulations show that the encapsulated nanocone changes the strain release mechanisms at the nanoscale by blocking gold atomic sliding, redistributing the strain along the whole nanostructure. The carbon nanocones are conducting and can induce magnetism, thus opening new avenues on the exploitation of transport, mechanical and magnetic properties of gold covered by sp(2) carbon at the nanoscale. Univ Fed Minas Gerais, ICEx, Dept Fis, BR-31270901 Belo Horizonte, MG, Brazil Penn State Univ, Mat Res Inst, University Pk, PA 16802 USA Univ Fed Minas Gerais, Ctr Microscopia, BR-30123970 Belo Horizonte, MG, Brazil Ctr Desenvolvimento Tecnol Nucl, BR-31270010 Belo Horizonte, MG, Brazil Univ Estadual Campinas, Inst Fis Gleb Wataghin, BR-13083970 Campinas, SP, Brazil Univ Estadual Paulista, UNESP, IGCE, Dept Fis, Rio Claro, SP 13506900, Brazil Univ Estadual Paulista, UNESP, IGCE, Dept Fis, Rio Claro, SP 13506900, Brazil FAPESP: 2013/08293-7 FAPESP: 2011/17253-3 FAPESP: 2013/09536-0

Combining the coherence–incoherence transition of the InAs islands with the multistacking of islands planes, a very narrow photoluminescence line width of 13.6 meV was obtained. This line width is comparable to the band-to-band recombination from InGaAs quantum wells and multiquantum wells.

We have performed magneto-photoluminescence experiments in several samples containing a layer of InAs with different thicknesses, ranging from 1.2 to 2.5 ML, inserted inside a modulation doped GaAs SQW. We applied the magnetic field both parallel and perpendicular to the growth direction and analyzed the circular polarization of the emitted light. We show that between 1.4 and 2.1 ML the observed PL can be associated with a lens shaped InAs SAQD, where the lateral confinement is not strong. We have observed no clear relation between the occurrence of the QHE in the underlying 2DEG and the optical properties of the lens shaped SADQ's, although a step like behavior of the PL peak energy occur around 6 T, close to filling factor ν =2.

InAs nanocrystals were grown by MBE on GaAs(1 0 0) with Te coverage ( θ Te ) ranging from zero to 0.45 monolayers (ML). The height, radius and surface density of these nanocrystals (nominal InAs coverage varying from 1.8 to 4.5 ML) were measured by atomic force microscopy (AFM). We observed that for θ Te =0.45 ML the transition coherence–incoherence is either delayed or absent for the InAs coverage range studied. Te deposition has the opposite effect of the pressure of As, regarding the size the islands. The size of the nanocrystals, for 0.45 monolayers of Te, varies smoothly up to 3.5 monolayers of InAs. Samples with Te resulted in `soft’ small nanocrystals, which could be a sign of a lack of stoichiometry in the small nanocrystals.

We present experimental data on the magnetic properties of atomically thin fcc (\ensuremath{\gamma}-phase) Fe films (1--6 atomic layer nominal thickness) epitaxially grown on ${\mathrm{Cu}}_{84}{\mathrm{Al}}_{16}(100)$ obtained by linear magnetic dichroism in the angular distribution of Fe $3p$ core photoelectrons excited by linearly polarized synchrotron radiation. The sign and magnitude of the Fe $3p$ photoemission magnetic asymmetry indicates the onset of in-plane ferromagnetism at 2.5(2) monolayer (ML) thickness of \ensuremath{\gamma}-Fe. The Curie temperature is 288(2) K for 4 ML thickness. The magnetic splitting of the Fe $3p$ $\mathrm{mj}$ core hole sublevels is 1.10(2) eV, i.e., the same value as measured for a bcc-Fe(100) surface where large surface and near-surface enhanced moments contribute. These results characterize the epitaxial \ensuremath{\gamma}-Fe on ${\mathrm{Cu}}_{84}{\mathrm{Al}}_{16}(100)$ as a high-spin ferromagnet for thickness up to 4 ML, with an average magnetic moment per iron atom of 2.5(1)${\ensuremath{\mu}}_{B}.$ A phase transition occurs between 4 and 5 ML thickness: the magnetic order of the pseudomorphic \ensuremath{\gamma}-Fe film decreases consistently with the breaking into two phases with the deeper layers in a low-spin and/or antiferromagnetic phase and surface restricted ferromagnetism, similar to the case of \ensuremath{\gamma}-Fe/Cu(100).

We investigate, using first-principles total energy calculations, the initial steps of adsorption and dimer-exchange mechanisms in the surfactant-mediated epitaxial growth of InAs over GaAs(001) surface with Te as a surfactant. We find that on the Ga-terminated GaAs(001) surface, covered by Te atoms, the dimer exchange between the layer of Te atoms and an overlayer of As atoms, based on a first neighbour exchange process, is only partial, in agreement with recent experimental results. On the other hand, if the exchange process between As and Te atoms occurs after the deposition of In and As atoms, in a second neighbour exchange process, our results indicate a complete segregation of Te atoms, reducing the surface total energy. The Te atoms also prevent the adsorption of As 2 molecule on neighbour sites, avoiding the formation of As clusters.

We report on the observation of photon-exciton coupling in a MQW microcavity grown on a (311)A GaAs substrate by photoluminescence. This coupling or Rabi splitting was observed by tuning the heavy hole exciton line and the cavity mode by varying the temperature in the range 80–200 K.

The heteroepitaxy of InAs on a Te covered GaAs surface is investigated by photoelectron spectroscopy. Core-level spectra probed with synchrotron radiation show that Te remains at the interface between InAs and GaAs in a concentration much higher than the solubility limit of this element in III–V compounds, suggesting that a new compound is being formed. We propose that this interlayer is responsible for the observed changing in the growth mode of the InAs overlayer from islands to layer by layer.

The resistance of the As-terminated Si(001) surface to oxidation in O 2 is the subject of this study. Photoemission spectra of As3d and Si2p core levels excited with synchrotron radiation reveals that spectral changes are evident, simultaneously for As and Si, only from ∼10 12 L (Langmuir) on, in stark contrast with a previous report indicating a saturation coverage in the 400–2000 L range. Oxidation proceeds slowly, as dimerized As remain intact (∼24%) up to exposures of ∼10 13 L. In the oxidized areas, the four Si oxidation states (indicative of subsurface oxidation) and three As oxidation states plus metallic arsenic show up. This suggests a reduction of the arsenic oxide by silicon. Arsenic losses are also observed, probably via sublimation of As 4 O 6 molecules.

The strong resistance of the As-terminated Si(001) surface to oxidation in molecular oxygen at room temperature is the subject of this study. As 3d and Si 2p photoemission spectra, as well as valence band spectra, excited with synchrotron radiation (135 eV ≤ hv ≤ 165 eV), have been recorded as a function of increasing O2 exposures, over the range 2400–1.4 × 1013 L (langmuir). Spectral changes, simultaneously observed in the As 3d and Si 2p core-levels near ∼ 1012 L are indicative of the onset of oxidation. However a modification of the valence band, due to the contribution of O 2p states, is clearly evident from ∼ 1011 L, pointing to the presence of weakly bound oxygen species at that exposure. These results are confronted with a previous Auger study reporting an oxygen saturation coverage (0.5 monolayer) already in the 400–2000 L range, interpreted as the insertion of an oxygen atom between two dimerized arsenic atoms. On the contrary our data show that surface oxidation is a slow process, spatially inhomogeneous, as some dimerized As remain intact (∼ 24%) up to exposures as large as ∼ 1013 L. Oxidation is not limited to the outermost layer, as three arsenic oxidation states and the four silicon oxidation states (indicative of subsurface oxidation) are detected simultaneously. Moreover metallic arsenic shows up: this suggests a reduction of the arsenic oxide by silicon. Arsenic losses are also observed, probably via sublimation of As4O6 molecules.

We demonstrate an easy way of decorating MWCNTs with CdS and CuxSy using the solvothermal method.

The ferroelectric-to-paraelectric phase transition in poly(vinylidene fluoride-trifluoroethylene) copolymers has been investigated using calorimetric and dielectric technics. In these materials, as it is well known, the ferro- and paraelectric phases coexist at large temperature intervals, which produces a smearing of the physical anomalies within the transition region. Thus, in order to treat our data, we have used an extended Landau-Devonshire treatment, which has provided a quantitative analysis of the macroscopic behavior of the systems. In particular, the Landau expansion parameters, the sample crystallinities, the spontaneous polarizations, and the coercive electric fields have been estimated for the copolymers with 25, 30, and 40 mol% of trifluoroethylene. © 1994 John Wiley & Sons, Inc.

Measurements of the current density as a function of the applied bias have been performed to study the electronic properties of the Al:n‐GaAs(100) interface. The samples were grown by molecular beam epitaxy. A Si planar doping was placed in GaAs(100) at different depths underneath the metal‐semiconductor interface and the charge transport across this interface has been investigated. An increasing tunneling contribution to the net current through the Schottky barrier has been observed by decreasing the planar doping depth. For sufficiently small depths, the tunneling current dominated. Planar doping near the metal‐semiconductor interface was found to be equivalent, concerning charge transport properties, to a bulk semiconductor doped well beyond the currently achievable limit.

In this letter, grazing incidence x-ray scattering is employed as a method to identify relaxed islands in an ensemble of partially coherent self-assembled InAs quantum dots. A simple model of strained pyramidal islands enables the association of the local lattice parameter of an island to its lateral size. A comparison between the island side length and its strain state allows the identification of coherent and incoherent nanostructures, revealing the size–strain interplay during growth.

In this article, we report magneto-photoluminescence measurements on stacked self-assembled InAs quantum dots. By applying a magnetic field parallel to the growth direction, we determined the exciton reduced mass and exciton radius from the photoluminescence (PL) peak energy. We observed an asymmetric increase of the full width at half maximum of the quantum dots PL peak to the high-energy side that we associate to the size selectivity of the oscillator strength of the ground state transitions. The observed increase of the integrated intensity of the quantum dots line is explained in terms of the reabsorption of the photons emitted by the GaAs substrate and the InAs wetting layer. These effects are related to the multilayer structure of the sample.

The degree of vertical alignment of InAs quantum dots in InAs/GaAs(001) multilayers was studied using grazing incidence x-ray scattering. We show that it is necessary to access one of the weak (200) x-ray reflections to observe the modulation of the GaAs lattice periodicity produced by the stacking of the InAs dots. The degree of alignment of the dots was assessed by fitting the x-ray diffuse scattering profiles near a GaAs (200) reciprocal lattice point. By using a model of gaussian lateral displacement of the dots, we show that we can determine the average value of the mistake in stacking positions of the islands from one bilayer to the next.

We report on experiments aimed at producing three-dimensional self-organization in InAs quantum-dot multilayers embedded in GaAs. These InAs/GaAs quantum-dot multilayers have been grown by molecular beam epitaxy. Employing atomic force microscopy, we have analyzed the island density in samples with different number of periods of InAs/GaAs bilayers The results reveals a decrease and a tendency to saturation of the island density with an increase in the number of periods, as a three-dimensional self-organization characteristic of these samples. Optical properties of the samples are examined via photoluminescence spectroscopy. The evolution of the quantum-dot photoluminescence peak position indicates an increment in the mean size of the buried islands and a relative homogenization in size of the quantum dots, as the number of periods increases. The results of the optical measurements agree with the morphological data, and characterize a spatial process of self-organization, related to the increment of the nu...

In this letter, we present an atomic force microscopy study of a series of Te-mediated InAs/GaAs samples with InAs coverage ranging from 1.5 to 3 monolayers. We were able to directly identify the growth mode transition and the mechanism of relaxed island formation. At the limit of coherent growth mode, strained quantum dots aggregate, forming twin quantum dots (TQDs), which are structures of two, or more, dots virtually bonded together, separated by less than 3 nm. The onset of the incoherent mode is then unambiguously characterized by the coalescence of individual TQDs forming initially small, and then larger, relaxed islands.

Semi-insulating GaAs samples show low-frequency current oscillations (LFO) under high dc voltage. These spontaneous oscillations are caused by traveling high-electric-field domains and can be explained based on the well-accepted model of electron trapping in deep levels by multiphonon emission processes. This paper aims to explain today the most important open question about LFO, namely, the cause of the very slow propagation of the domain resulting in the low frequencies. Our claim is that hopping conduction acts as a low mobility barrier for the free carriers leading to the formation of the electric-field domains. The barrier blocks, through a Coulomb shield, the flow of the high mobility free carriers leading to a charge accumulation, which produces the current pulse in the external circuit when the domain reaches the anode where it is destroyed. LFO occurs, therefore, only in the regime of band and hopping mixed conduction mechanisms.

Electric transport properties measured by Van der Pauw resistivity experiments of Low-Temperature Molecular Beam Epitaxy (LT-MBE) GaAs samples are used to identify a method to improve the resistivity of GaAs material. We present results on five samples grown at 265, 310, 315, 325, and 345 oC. The electric measurements were carried out at temperatures ranging from 130 to 300 K. In this temperature range the dominant transport process is identified as variable range hopping. The hopping parameter plotted against the growth temperature is shown to present a maximum. The mechanisms responsible for this behavior are discussed in relation to the compensation ratio.

We report on optical experiments in self-assembled InAs quantum dots grown on (100) and (311)A GaAs surfaces which were precovered with Te. We observe a strong reduction of the luminescence intensity with increasing Te coverage in the (100)-oriented samples. The Te-induced luminescence reduction is, however, much smaller in the (311)A oriented samples.

We have observed low frequency current oscillations in a semi-insulating GaAs sample grown by low temperature molecular beam epitaxy. For this, an experimental setup proper to measure high impedance samples with small external noise was developed. Spontaneous oscillations in the current were observed for some bias conditions. Although measurements were carried out from room temperature down to liquid helium, the dynamical analysis was carried out around 200 K where the signal to noise ratio was fairly favorable. To increase the data quality we have also used a noise reduction algorithm suitably developed for nonlinear systems. We observed attractors having low embedding dimension, limit cycle bifurcations, and chaotic behavior characteristic of nonlinear dynamical processes in route to chaos. Attractor reconstruction, Poincare sections, Lyapunov exponents, and correlation dimension were also analyzed.

In this work, the effects of doping concentration on the etching rate of GaAs were investigated. The fabrication of several etching steps on each sample, forming an “etching staircase,” enabled the use of atomic force microscopy for the determination of the etching rate. We have employed a common etching agent and samples in which the only varying parameter was the bulk doping concentration. A strong correlation between etching rate and doping concentration was found, indicated by a decrease in the etching rate as the bulk doping concentration is increased. The experimental results were well accounted for by an oxidation-reduction process, which rules the etching process in the present case.

The adsorption and desorption of Te on GaAs (100) has been investigated using Soft X-ray Spectroscopy (SXS) and X-ray Photoelectron Spectroscopy (XPS). The SXS measurements have been done at the Brazilian synchrotron facility LNLS, and the XPS study was performed using laboratory sources. The samples were MBE GaAs (100) layers covered in-situ by Te from an effusion cell. Two families of samples were studied: samples analyzed in-situ, and samples that were exposed to air for at least one week. The desorption of Te of the non-oxidized samples indicates that traces of Te remains in the surfaces for temperatures as high as 680 °C. However, for the case of the oxidized samples, the Te film sublimates easily at around of 250 °C, leaving a clean GaAs surface. It seems that the oxidized Te film works as a suitable protection for epitaxial GaAs layers.

We study the influence on the photocurrent of the final state in bound-to-quasibound transitions in self-assembled quantum dot infrared photodetectors. We investigate two structures designed to explore different mechanisms of carrier extraction and therefore achieve a better insight on these processes. We observe photocurrent in opposite directions, with positive and negative sign, for different incident frequencies at the same applied external electric field. This phenomenon is attributed to the asymmetry of the potential barriers surrounding the quantum dots.

Silver aggregates find their technological applications in electricity conducting cements and as catalysts in biomedical sciences. We have developped a method for synthesis of colloidal silver aggregates, based in redox reactions. Aggregate size and structure are strongly influenced by preparation physico-chemical conditions. We have investigated the pH influence over the growth mechanism through dynamic and static light scattering techniques. We introduce here an analysis of the sedimentation rate from which the aggregation regime can also be inferred.