G. Saint-Girons

Sub-500-fs soliton-like pulse in a passively mode-locked broadband surface-emitting laser with 100 mW average power

We report on femtosecond operation of a broadband diode-pumped external-cavity surface-emitting semiconductor laser, passively mode locked with a fast quantum–well Semiconductor Saturable Absorber Mirror grown at 735 °C. We obtained 477 fs pulses at 1.21 GHz. The average output power is 100 mW at 1040 nm, the pulse peak power 152 W, with ∼1 W of 830 nm pump. The rf spectrum shows a linewidth <50 kHz at the noise level (−65 dB). We believe that the group-delay dispersion is compensated by the negative self-phase modulation in the absorber structure, leading to soliton-like mode locking. This system requires no additional technological step after the growth of the structures.

Near-infrared waveguide photodetector with Ge/Si self-assembled quantum dots

We have investigated near-infrared p-i-n photodetectors with Ge/Si self-assembled quantum dots. The self-assembled quantum dots were grown by chemical vapor deposition on Si(001). A vertical stacking of 20 layers of quantum dots was inserted into a near-infrared waveguide obtained with a Si0.98Ge0.02 alloy. The samples were processed into ridge waveguides. The photoresponse of the device covers the near-infrared spectral range up to 1.5 μm. At room temperature, a responsivity of 210 mA/W is measured at 1.3 μm and 3 mA/W at 1.5 μm. The photocurrent is compared to the photoluminescence and to the absorption of the quantum dots measured in the waveguide geometry. At room temperature, the onset of the absorption is around 1.9 μm (0.65 eV). The photocurrent is blueshifted as compared to the absorption.

Molecular beam epitaxy of SrTiO3 on Si (001): Early stages of the growth and strain relaxation

The molecular beam epitaxy of SrTiO3 (STO) layers on Si (001) is studied, focusing on the early stages of the growth and on the strain relaxation process. Evidence is given that even for optimized growth conditions, STO grows initially amorphous on silicon and recrystallizes, leading to the formation of an atomically abrupt heterointerface with silicon. Just after recrystallization, STO is partially strained. Further increase in its thickness leads to the onset of a progressive plastic relaxation mechanism. STO recovers its bulk lattice parameter for thicknesses of the order of 30 ML.

Structural properties of epitaxial SrTiO3 thin films grown by molecular beam epitaxy on Si(001)

This work reports on the structural properties of an epitaxial SrTiO3 (STO) layer grown by molecular beam epitaxy on a Si(001) substrate with a two step process. The study, which includes a complete characterization of large scale plane-view images of the STO layer, is based on a careful analysis of x-ray spectra and transmission electron microscopy images. The STO layer presents a good crystalline quality and a slight texturation related to the presence of extended defects. A thin Ti-rich amorphous silicate layer (thickness ≈1.3nm) is formed at the interface between the STO and the Si substrate, evidencing the thermodynamic instability of the STO/Si interface. The difference between the thermal expansion coefficients of Si and STO is shown to be at the origin of an increased in-plane lattice parameter (3.927A) of the STO layer as compared to its bulk value (3.905A). This effect of differential thermal expansion is expected to be responsible for the formation of at least part of the extended defects of th...

Monolithic integration of InP based heterostructures on silicon using crystalline Gd2O3 buffers

A new approach of monolithic integration of InP based heterostructures on silicon is proposed, based on the plastic compliant behavior of the InP/Gd2O3(111) heterointerface. When grown on a crystalline Gd2O3/Si(111) buffer, InP takes its bulk lattice parameter as soon as the growth begins, allowing the monolithic growth of good quality InAsP/InP heterostructures on Si, as attested by room-temperature photoluminescence experiments.

Photoluminescence quenching of a low-pressure metal-organic vapor-phase-epitaxy grown quantum dots array with bimodal inhomogeneous broadening

The photoluminescence (PL) behavior of a bimodal In(Ga)As/GaAs quantum dots (QDs) array grown by low-pressure metal-organic-vapor-phase-epitaxy is studied as a function of the temperature. The PL quenching is attributed to the thermal escape of charge carriers out of the QDs for the high-energy emitting QDs population, and to the presence of nonradiative defects in the immediate vicinity of the lower-energy emitting QDs population. The PL intensity behavior of both QDs population is investigated, and the experimental results are fitted with the help of a rate equations model. The nonradiative mechanisms activation energies are found to be about 180 and 40 meV for the high- and low-energy emitting QDs population, respectively. A charge carriers transfer mechanism between the two QDs populations is also evidenced, and the results are discussed in terms of laser applications.

Spontaneous compliance of the InP∕SrTiO3 heterointerface

The lattice mismatch between a growing layer and its substrate is a major limitation for heteroepitaxy. Finding solutions to overcome this limitation has given rise to many researches that have up to now not come out any satisfying solution. Here we demonstrate the compliant behavior of the InP∕SrTiO3 (STO) heterointerface. InP islands grown on STO substrates and STO/Si crystalline layers are defect-free, oriented with respect to STO, and have their InP bulk lattice parameter. This contrasts with plastic relaxation mechanisms observed for III-V, Si, or Ge mismatched heterostructures. Compliance occurs spontaneously during the growth and does not require any substrate patterning.

Pseudomorphic molecular beam epitaxy growth of γ-Al2O3(001) on Si(001) and evidence for spontaneous lattice reorientation during epitaxy

Single crystal γ-Al2O3 thin films have been epitaxially grown by molecular beam epitaxy at 850°C on Si(001) substrates. Reflection high energy electron diffraction and transmission electron microscopy experiments evidence the good crystalline quality of the Al2O3 layer. The present study shows that the two first monolayers of γ-Al2O3 are (001) oriented and coherently strained on Si. For larger thickness, a transition from (001)- to (111)-oriented Al2O3 occurs, together with the apparition of domains in the layer. In-plane epitaxial relationship between Al2O3 and Si(001) are deduced from these observations.

Silicon–on–insulator waveguide photodetector with Ge/Si self-assembled islands

We have investigated a silicon-based near-infrared photodetector using a waveguide with strong optical confinement. The high-difference index waveguide is obtained with a silicon–on–insulator substrate. The optically active region consists of self-assembled Ge/Si islands embedded in a p-i-n junction. The Ge/Si islands grown by high-pressure chemical-vapor deposition exhibit a broad photoluminescence and electroluminescence which are resonant around 1.5 μm. The photoluminescence and electroluminescence energies are correlated to the island size and to the island composition using a six-band k⋅p calculation. The spectral responsivity of the detectors is measured in a front facet coupling geometry with a broadband source and with semiconductor laser diodes. For a 0 V applied bias, responsivities of 25 and 0.25 mA/W are measured at room temperature at 1.3 and 1.55 μm, respectively.

Bimodal distribution of Indium composition in arrays of low-pressure metalorganic-vapor-phase-epitaxy grown InGaAs/GaAs quantum dots

Low-pressure metalorganic-vapor-phase-epitaxy (LP-MOVPE) grown InGaAs/GaAs quantum dots (QDs) emitting around 1.3 μm have been studied by photoluminescence and transmission electron microscopy (TEM). We demonstrate the presence of a bimodal inhomogeneous broadening of the photoluminescence, correlated with a bimodal QDs contrast distribution in the TEM micrographs. Increasing the growth temperature of the dots induces a decrease of the ratio between the number of In-poor and In-rich QDs, illustrating the crucial influence of indium desorption on the LP-MOVPE growth of InGaAs QDs.