RHEED digital image analysis system for in-situ growth rate and alloy composition measurements of GaAs-based nanostructures

Monitoring the intensity of the reflected spot in a RHEED image is the most important method used to control the growth of semiconductors in MBE. The accurate control of both layer thickness and alloy composition is particularly important for the growth of high quality heterostructures. Indeed, under such conditions, extremely uniform and high quality epitaxial devices become possible. RHEED intensity oscillation can be used as accurate, quick and direct measure of the growth rates and alloy compositions as well. Although analog signal could be obtained by using an optical fiber associated to a photo-detector PM tube and a plotter, this method is troublesome and limited. In some application, the availability of the intensity signal as a digital voltage is useful to realize further advanced analysis, and achieve feedback between growth dynamic and the external parameters, such as the cells temperatures and the synchronization of the shutters. In this paper we describe the advantages gained with the upgrade of our analog system into a digital package using CCD camera, frame grabber and a home made software. Its main purpose is to track RHEED intensity changes and measures the rate of oscillation. A state-of-the-art RHEED digital image analysis system gives us the necessary tools to gain insight into the thin film growth process and optimize material quality

RETRATO SIN IDENTIFICAR [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte, 201

Coulomb effects on the quantum transport of a two-dimensional electron system in periodic electric and magnetic fields

The magnetoresistivity tensor of an interacting two-dimensional electron system with a lateral and unidirectional electric or magnetic modulation, in a perpendicular quantizing magnetic field, is calculated within the Kubo formalism. The influence of the spin splitting of the Landau bands and of the density of states (DOS) on the internal structure of the Shubnikov-de Haas oscillations is analyzed. The Coulomb electron - electron interaction is responsible for strong screening and exchange effects and is taken into account in a screened Hartree-Fock approximation, in which the exchange contribution is calculated self-consistently with the DOS at the Fermi level. This approximation describes both the exchange enhancement of the spin splitting and the formation of compressible edge strips, unlike the simpler Hartree and Hartree-Fock approximations, which yield either the one or the other.Comment: 20 pages, revtex, 7 ps figures, to appear in Phys. Rev.

Effect of Oscillating Landau Bandwidth on the Integer Quantum Hall Effect in a Unidirectional Lateral Superlattice

We have measured activation gaps for odd-integer quantum Hall states in a unidirectional lateral superlattice (ULSL) -- a two-dimensional electron gas (2DEG) subjected to a unidirectional periodic modulation of the electrostatic potential. By comparing the activation gaps with those simultaneously measured in the adjacent section of the same 2DEG sample without modulation, we find that the gaps are reduced in the ULSL by an amount corresponding to the width acquired by the Landau levels through the introduction of the modulation. The decrement of the activation gap varies with the magnetic field following the variation of the Landau bandwidth due to the commensurability effect. Notably, the decrement vanishes at the flat band conditions.Comment: 7 pages, 6 figures, minor revisio

Effect of Interfacial Bonds on the Morphology of InAs QDs Grown on GaAs (311) B and (100) Substrates

The morphology and transition thickness (tc) for InAs quantum dots (QDs) grown on GaAs (311) B and (100) substrates were investigated. The morphology varies with the composition of buffer layer and substrate orientation. Andtcdecreased when the thin InGaAs was used as a buffer layer instead of the GaAs layer on (311) B substrates. For InAs/(In)GaAs QDs grown on high miller index surfaces, both the morphology andtccan be influenced by the interfacial bonds configuration. This indicates that buffer layer design with appropriate interfacial bonds provides an approach to adjust the morphologies of QDs grown on high miller surfaces

The effect of tobacco, XPC, ERCC2 and ERCC5 genetic variants in bladder cancer development

<p>Abstract</p> <p>Background</p> <p>In this work, we have conducted a case-control study in order to assess the effect of tobacco and three genetic polymorphisms in <it>XPC, ERCC2 and ERCC5 </it>genes (rs2228001, rs13181 and rs17655) in bladder cancer development in Tunisia. We have also tried to evaluate whether these variants affect the bladder tumor stage and grade.</p> <p>Methods</p> <p>The patients group was constituted of 193 newly diagnosed cases of bladder tumors. The controls group was constituted of non-related healthy subjects. The rs2228001, rs13181 and rs17655 polymorphisms were genotyped using a polymerase chain reaction-restriction fragment length polymorphism technique.</p> <p>Results</p> <p>Our data have reported that non smoker and light smoker patients (1-19PY) are protected against bladder cancer development. Moreover, light smokers have less risk for developing advanced tumors stage. When we investigated the effect of genetic polymorphisms in bladder cancer development we have found that ERCC2 and ERCC5 variants were not implicated in the bladder cancer occurrence. However, the mutated homozygous genotype for XPC gene was associated with 2.09-fold increased risk of developing bladder cancer compared to the control carrying the wild genotype (p = 0.03, OR = 2.09, CI 95% 1.09-3.99). Finally, we have found that the XPC, ERCC2 and ERCC5 variants don't affect the tumors stage and grade.</p> <p>Conclusion</p> <p>These results suggest that the mutated homozygous genotype for XPC gene was associated with increased risk of developing bladder. However we have found no association between rs2228001, rs13181 and rs17655 polymorphisms and tumors stage and grade.</p

Alterações nas reservas de sementes de Dalbergia nigra ((Vell.) Fr. All. ex Benth.) durante a hidratação

Seed imbibitions is the first stage of the germination process and is characterized by the hydration of tissues and cells and the activation and/or induction of the enzymes responsible for mobilizing reserves for respiration and the construction of new cell structures. The objective of this study was to investigate the alterations in reserve substances during slow hydration of Bahia Rosewood (Dalbergia nigra) seeds in water. Seeds from two different lots (Lot I and II) were placed in saturated desiccators (95-99% RH) to hydrate at 15 and 25 °C until water contents of 10, 15, 20 and 25% were reached. At each level of hydration, changes in lipid reserves, soluble carbohydrates, starch and soluble proteins were evaluated. The mobilization of reserves was similarly assessed in both lots, with no differences being observed between the two hydration temperatures. Lipid contents showed little variation during hydration, while the contents of soluble carbohydrates and starch decreased after the 15% water content level. Soluble proteins showed a gradual tendency to decrease between the control (dry seeds) up to 25% water content

A novel Al₀.₃₃Ga₀.₆₇As/In₀.₁₅Ga₀.₈₅As/GaAs quantum well Hall device grown on (111) GaAs

In this study, we look at the advantages of (111) GaAs substrate over (001) one, when used to grow Hall devices by MBE. In top of that, we explore the consequence of a modified design of modulation doping pseudomorphic AlGaAs/InGaAs/GaAs, and we suggest a new quantum well structure for a Hall device grown on (111) GaAs substrate, with the objective of improving its performances. From self-consistent calculations, we find that the electron concentration ns in the interface region is enhanced. This implies that one can have a wider spacer layer and still have the same ns with the result that the mobility is improved. This result should be valuable for many types of devices. We specifically consider Hall sensors, where it is desirable to have a low electron concentration and high mobility

Influence of high-index GaAs substrates on the 2D electron density of

In this paper, we report the theoretical predictions of a high-index GaAs substrate ((111)A and (311)A) on the subband structure and thereafter on the 2D electron density of Si δ-doped Al0.33Ga0.67As/In0.15Ga0.85As/GaAs pseudomorphic high electron mobility transistor (pHEMT) with an additional InxGa1-xAs (x > 0.15) thin layer embedded in the channel. We have seen that the electronic structures and the electron density are quite sensitive to the additional InxGa1-xAs (x > 0.15) layer thickness, indium composition and to their position in the channel. An optimal position of the additional InxGa1-xAs layer was found to be corresponding to the maximum of the first eigen envelope function for the different growth directions. We report that the optimised electron density is obtained in the structure grown on (111)A GaAs substrate. In this case the electron transfer is significantly higher than those grown on (311)A and (001) GaAs substrates respectively

Spectroscopic ellipsometry study of barrier width effect in self-organized InGaAs/GaAs QDs laser diodes

725-731Molecular beam epitaxy (MBE) is used to grow InGaAs/GaAs quantum dots (QDs) laser diodes (LDs) with different barrier widths (5, 10 and 15 nm) at 580 ºC on GaAs substrates. Optical properties of the InGaAs/GaAs QDs LDs have been investigated by using the spectroscopic ellipsometry (SE) technique. A general oscillator optical model has been utilized to fit the experimental data in order to obtain the LD layer thicknesses, refractive index and absorption coefficient. The dielectric function, the energy band gap and the surface and volume energy loss functions are computed in the energy range 1-6 eV. The optical properties of the deposited InGaAs/GaAs QDs LDs are found to be affected by the barrier width, which give more insight into carriers dynamics and optical parameters in these devices. The refractive indices, the extinction coefficients and the dielectric constants of the LDs with barrier widths 15 and 10 nm are relatively larger than those of the LD with barrier width 5 nm. These indicate that optical properties of LDs with larger barrier widths (15 and 10 nm) will be improved. The interband transition energies in the three devices have calculated and identified. Two energy gaps at 1.04 and ~1.37 eV are obtained for all the heterostructures which indicates that fabricated LDs may be operating for a wavelength of 1.23 m at room temperature