Juan Pedro Silveira

Strain relaxation and segregation effects during self-assembled InAs quantum dots formation on GaAs(001)

In segregation effects during InAs growth on GaAs(001) and critical thickness for InAs self-assembled quantum dots are studied using a real time, in situ technique capable of measuring accumulated stress during growth. Due to a large (∼50%) surface In segregation of floating In, self-assembled dot formation takes place when less than one monolayer of InAs is pseudomorphically grown on GaAs. A picture of the growth process is discussed on the basis of the equilibrium between InAs and floating In dominated by the stress energy.

Transmittance photoplethysmography and pulse oximetry with near infrared laser diodes

Photoplethysmography and pulse oximetry are widely used techniques for noninvasive monitoring of heart rate and peripheral oxygen saturation. Over the last years we have been working onto the application of near infrared range wavelengths to replace those of the classical pulse oximeters. The development of laser diodes based sensors, processing algorithms, a calibration procedure, and the comparison with a commercial pulse oximeter, have been followed by a set of experimental studies. The results exposed here demonstrate the applicability of transmittance photoplethysmography and pulse oximetry using laser diodes emitting at specific wavelengths for a wide range of situations.

In situ observation of reconstruction related surface stress during molecular beam epitaxy (MBE) growth of III–V compounds

Anisotropic surface stress associated with the different surface reconstructions of (0 0 1)GaAs is measured in situ in an MBE system. We use an optical deflection technique on [1 T 0] and [1 1 0] oriented cantilevers fabricated on thinned wafer areas. Changes of surface stress, associated with the As and Ga dimer surface coverage for c(4 x 4), 2 x 4, 3 x 1 and 4 x 2 reconstructions, are observed as substrate temperature and impinging molecular flux are being varied. Deflection oscillations can be detected also during MBE growth relating surface stress to surface roughness or step density. Sensitivity and stability of this technique demonstrates its usefulness as new tool to study in situ a variety of surface kinetics and incorporation phenomena.

In situ measurements of As/P exchange during InAs/InP(0 0 1) quantum wires growth

Unintentional As/P exchange has a significant influence on InAs/InP nanostructures growth. In this paper, we report on the As/P exchange reactions at different temperatures studied by in situ stress measurements and reflectance difference (RD) characterization. When arsenic atoms are incorporated at the InP surface, an asymmetric stress is built up that is responsible for quantum wires (QWr) formation. We obtain that arsenic atoms do not actively displace phosphorous from the surface at the range of surface temperatures and exposure times of interest for growth of InAs nanostructures by molecular beam epitaxy (MBE). Instead, the arsenic atoms bind to available In atoms left vacant by phosphorous desorption at the corresponding temperature.

Stress evolution aspects during InAs/InP (001) quantum wires self-assembling

Abstract We report here in situ measurements of stress evolution during molecular beam epitaxy growth of InAs/InP (001) quantum wires. The obtained results provide the necessary information to understand why quantum wires instead of quantum dots are formed in this heteroepitaxial system: a strong stress anisotropy along 〈110〉 directions is responsible for it. Moreover, during quantum wires growth As/P exchange reactions take place, which determine the total amount of InAs incorporated in the nanostructures and then their optical properties. In situ stress measurements have also been used to study the exchange process and they have allowed us to calculate the quantity of InAs that enters into the lattice because of it.

Surface stress effects during MBE growth of III–V semiconductor nanostructures

Abstract Surface and interface stress evolution is measured in situ and in real time during molecular beam epitaxy of InAs/GaAs and GaSb/GaAs systems, covering initial 2D growth, formation of QDs and subsequent GaAs capping. These two systems with very similar lattice parameter mismatch but involving quite different surface chemistry show a similar critical thickness for 2D/3D transition, 0.7–1.0 monolayer ML. Limited In incorporation during InAs/GaAs growth affects the relaxation process. In segregation effects are observed during GaAs overgrowth. GaSb/GaAs system presents a strong 2D/3D relaxation associated with mass transport from the 2D layer. Sb/As exchange reactions are considered.

Optical studies of GaAs quantum wells strained to GaP

Quantum wells of GaAs lattice matched to GaP have been studied by photoluminescence and electroreflectance. The quantum well thickness was varied between 1 and 6 monolayers in steps of 1 monolayer. Electron-hole transitions have been observed involving states both in the X band and in the Γ band, which have been modeled using a k.p. model in conjunction with the envelope function approximation. Overall agreement between theory and experiment is found using an unstrained valence band offset of 0.6 eV.

In segregation effects during quantum dot and quantum ring formation on GaAs(001)

Abstract In segregation during InAs growth on GaAs(001) is studied using a real time, in situ technique capable of measuring sample accumulated stress. A 50% surface In segregation of liquid-like stress free matter is deduced. A picture of growth below critical thickness for quantum dot formation is discussed on the basis of the equilibrium between pseudomorphic InAs and liquid In dominated by the stress energy. Quantum rings are produced when large (>10 nm height) quantum dots are covered with 2 nm of GaAs cap. A formation mechanism of the rings is presented. The possibility of tailoring photoluminescence emission through control over size and shape is demonstrated.

Oxygen saturation measurements in athletes attaining maximal exertion conditions

Maximal effort tests are used to check the fitness of athletes and to assess their status of health. Measurements performed during the test are also useful to understand athlete physiology and to optimise the training protocols. One of the interesting measurements that can be performed is the oxygen saturation and desaturation in blood along with cardiac rate. The task is complicated by the fact that standard instrumentation (oximeters) are affected by movement artefact, and in the case of this kind of tests movements can be so strong and wide that the measurements is invalidated. In this paper, a novel algorithm is proposed which achieve meaningful measurements even in presence of very strong movement artefact.

NIR transmittance pulse oximetry system with laser diodes

A transmittance pulse oximetry system based on near-infrared laser diodes (LD) for monitoring arterial blood hemoglobin oxygen saturation (So2) has been previously reported. In this work we present the results obtained after improvements in the sensor configuration, signal processing algorithm and calibration procedure. The pulse oximetry system also comprises the sensor electronics, and a data acquisition board installed on a handheld personal computer. The two LD chips are mounted on a single metal heat-sink and as photo- detectors are used silicon p-i-n photodiodes with the first amplifier stage situated in their back side. The real time calculation of the parameters related to So2 is carried out through a numeric separation of the pulsatile and non- pulsatile components of the photoplethysmographic signals for both wavelengths and a non-linear filtering. Patients with respiratory failure conditions were monitored as a part of the calibration procedure in order to cover a wide range of So2-values. A calibration curve have been derived through the determination of in vitro arterial So2 with a significant quantity of experimental points ranging from 60 to almost 100%. The obtained results demonstrate that it is possible to apply the proposed system to monitoring a wide range of oxygen saturation levels.