M.J. da Silva

Expression of correctly processed human growth hormone in seeds of transgenic tobacco plants

Human growth hormone was expressed in transgenic tobacco seeds using the monocot tissue-specific promoter from sorghum γ-kafirin seed storage protein gene. During tobacco seed ripening, the expressed hormone was directed to the endoplasmic reticulum by a signal peptide from a Coix prolamin and was secreted into the apoplastic space, where it accounted for 0.16% of total soluble seed protein. The expressed hormone has the same amino acid sequence and receptor-binding properties as the native mature hormone.

InAs/GaAs quantum dots optically active at 1.5 μm

InAs quantum dots grown by molecular-beam epitaxy on GaAs substrates are demonstrated to be suitable structures to achieve an optical emission in the 1.3–1.5-μm range. Their tuning towards such long wavelengths was made possible by combining an extreme reduction of the InAs growth rate and a fast growth of the GaAs cap layer at low temperature. Our results create perspectives for the fabrication of GaAs-based devices operating in the most important telecommunications window.

Influence of the temperature on the carrier capture into self-assembled InAs/GaAs quantum dots

Photoluminescence (PL) spectroscopy and atomic-force microscopy (AFM) were used to investigate the size evolution of InAs quantum dots on GaAs(001) as a function of the amount of InAs material. Different families of islands were observed in the AFM images and unambiguously identified in the PL spectra, together with the signal of the wetting layer. PL measurements carried out at low and intermediate temperatures showed a thermal carrier redistribution among dots belonging to different families. The physical origin of this behavior is explained in terms of the different temperature dependence of the carrier-capture rate into the quantum dots. At high temperatures, an enhancement of the total PL-integrated intensity of the largest-sized quantum dots was attributed to the increase of diffusivity of the photogenerated carriers inside the wetting layer.

Influence of indium segregation on the RHEED oscillations during the growth of InGaAs layers on a GaAs(001) surface

The surface segregation of indium (In) atoms was investigated during the growth of InGaAs layers by reflection high-energy electron diffraction (RHEED). We showed that the strong damping of the RHEED oscillations usually observed during the deposition of InGaAs on GaAs was directly related to the presence of a population of In atoms at the surface of the sample originating from the segregation phenomenon in the InGaAs layers. We proposed a simple model to estimate the segregation coefficient R in situ and in real time from the RHEED oscillations. Our results were quantitatively confirmed by several RHEED measurements carried out under very different growth conditions and were in excellent agreement with data from the literature.

Optical response at 1.3 μm and 1.5 μm with InAs quantum dots embedded in a pure GaAs matrix

We demonstrated that the growth of InAs on GaAs at rates as low as 0.003 ML/s can be used to form quantum dots (QDs) that are optically active at 1.3 and 1.5 μm. The emission at 1.5 μm (at 300 K) originating from individual InAs QDs embedded in a pure GaAs matrix is close to the important telecom window at 1.55 μm. Such emission is related to a narrow distribution of islands with a height peaked around 15 nm as shown by atomic-force microscopy.

Ex-situ investigation of indium segregation in InGaAs/GaAs quantum wells using high-resolution x-ray diffraction

Calculations using the dynamical theory of diffraction together with a sample model which considers the segregation of indium atoms were employed to fit the high-resolution x-ray spectra of strained InGaAs/GaAs quantum wells grown by molecular-beam epitaxy. The segregation coefficients obtained from the best fits to the experimental data of samples grown at different temperatures are in excellent agreement with the expected values and confirm that x-ray diffraction is a valuable tool for the investigation of the segregation phenomenon.

Correlation between structural and optical properties of InAs quantum dots along their evolution

Abstract The growth of highly inhomogeneous samples is presented as a suitable way to investigate the continuous evolution of self-assembled InAs quantum dots as a function of the thickness of deposited material. A single specific sample allowed a direct comparison of the structural and optical properties of the strained islands using atomic-force microscopy and low-temperature photoluminescence.

Carrier kinetics in quantum dots through continuous wave photoluminescence modeling: A systematic study on a sample with surface dot density gradient

A systematic study is presented of continuous wave (cw) photoluminescence (PL) of self-assembled quantum dots (QDs) grown on GaAs (001) by molecular-beam epitaxy as a function of excitation intensity and QD density. The sample used in this work was grown under nonisotropic indium flux that resulted in a QD density gradient across the sample surface ranging from 0 to 1.8×1011 cm−2. The carrier kinetics in the sample is described by a set of coupled rate equations through which the cw PL data from the GaAs barrier, wetting layer (WL), and QDs were simulated as a function of the excitation intensity and QD density. By comparing the PL data with our simulations we infer that carrier capture into the QD occurs directly from GaAs barrier. Auger and phonon-assisted carrier capture from the WL were found to give negligible contribution. With an increase of the QD density we observe an increase of the nonradiative recombination rates of the barrier and at the WL, which we tentatively correlate with the increase of...

Influence of illumination on the quantum mobility of a two-dimensional electron gas in si δ-doped GaAs/In0.15Ga0.85As quantum wells

A series of GaAs/InGaAs quantum wells with a silicon δ-doped layer in the top barrier was investigated by Shubnikov–de Haas measurements as a function of the illumination time of the samples. During the illumination process strong modifications of the electronic density and the quantum mobility of each occupied subband were observed. Based on self-consistent calculations, the dominant mechanism which caused the changes in the subband quantum mobilities with illumination was elucidated.

Anomalous blueshift in vertically coupled InAs/GaAs quantum dots using InGaAs strain-reducing layers

The vertical coupling of InAs quantum dots and their capping with In0. 1Ga0. 9As layers were investigated in order to shift the optical emission of the structures toward longer wavelengths. We observed that both ways can not be used simultaneously by just replacing part of the usual GaAs spacer layer by In0. 1Ga0. 9As because the intermixing of Ga and In atoms is enhanced in stacked layers and causes a blueshift of the emission instead of the expected redshift.