M. S. Andrade

Coherent-to-incoherent transition in surfactant mediated growth of InAs quantum dots

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.

On three dimensional self-organization and optical properties of InAs quantum-dot multilayers

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...

Climbing the steps of viral atomic force microscopy: visualization of Dengue virus particles

In recent years, the application of atomic force microscopy (AFM) to biological systems has highlighted the potential of this technology. AFM provides insights into studies of biological structures and interactions and can also identify and characterize a large panel of pathogens, including viruses. The Flaviviridae family contains a number of viruses that are important human and animal pathogens. Among them, Dengue virus causes epidemics with fatal outcomes mainly in the tropics. In this study, Dengue virus is visualized for the first time using the in air AFM technique. Images were obtained from a potassium‐tartrate gradient‐purified virus. This study enhances the application of AFM as a novel tool for the visualization and characterization of virus particles. Because flavivirus members are closely related, studies of the morphologic structure of the Dengue virus can reveal strategies that may be useful to identify and study other important viruses in the family, including the West Nile virus.

Magneto-optical properties of stacked self-assembled InAs quantum dots

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.

Quasiperiodic microfacets on the surface of AlGaAs/GaAs quantum well structures grown by molecular beam epitaxy on (311)A high‐index substrates

Standard molecular beam epitaxy is used to demonstrate the growth feasibility and reproducibility on the formation of microfacet array on the top surface of higher‐index (311)A quantum well structures. The quasiperiodic microfacet array was characterized by atomic force microscopy and was observed to be along the [233] direction, whereas for the (100) reference sample the microscopic surface morphology presented the terraces formation. We have used photoluminescence to characterize the optical properties of the samples. The observed redshift of the luminescence from the (311)A sample, in relation to the (100) reference sample, was correlated with the existence of a lateral confinement potential induced by the period of faceting in this structure. The comparative analysis based on photoluminescence measurements have also shown the higher quality and the lower impurity incorporation for the (311)A oriented samples.

Influence of Doping Concentration on the Etching Rate of GaAs Studied by Atomic Force Microscopy

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.

Observation of Patterns by Magnetic Force Microcopy in Fe-alloys with Shape Memory Effect

Abstract : In this study, we investigated the magnetic domains of a FeMnSiNiCr stainless steel sample using Magnetic Force Microscope (MFM). We compared the magnetic patterns obtained by scanning the sample with three coated probes with different magnetic properties: Medium magnetic moment (MM), low magnetic moment (LM), and low coercivity (LC). The probe-surface separation was varied between 25 to 300 nm in order to quantify the magnetic microstructure of the sample. A simple model for the probe-sample interaction was used to interpret the contrast change as a function of the probe-surface separation. The experiment showed that the average maximum frequency decreases with the probe-surface separation and the intensity of the frequency is the strongest for the MM probe. X-ray diffraction experiments were used to identify the different phases present in the sample. The X-ray diffraction experiments together with the MFM showed that alpha-phase islands surrounded by a gamma-phase matrix are responsible for the magnetic properties of the sample.

Role of Te on the morphology of InAs self-assembled islands

The effect of Te presence on the morphology and distribution of InAs islands grown by molecular beam epitaxy on GaAs is investigated. Atomic force microscopy was used to follow the dependence of height, radius, and surface density on Te and InAs coverages. They ranged from zero to 0.45 monolayers (ML) of Te, and from 1.8 to 3.5 ML for InAs. We obtained a higher density of islands for samples covered with 0.3 ML of Te. The number of islands is essentially the same for samples covered with 0 and 0.45 ML of Te. A delay on the onset of island growth is observed for samples with θTe=0.45 ML. The surface morphology is also different for samples with Te when compared with the Te free sample. We suggest that for θTe=0.45 ML the coherence-incoherence transition is either delayed or absent for the InAs coverage range studied.