Mathieu Stoffel

Three-Dimensional Composition Profiles of Single Quantum Dots Determined by Scanning-Probe-Microscopy-Based Nanotomography

Scanning probe microscopy combined with selective wet chemical etching is employed to quantitatively determine the full three-dimensional (3D) composition profiles of single strained SiGe/Si(001) islands. The technique allows us to simultaneously obtain 3D profiles for both coherent and dislocated islands and to collect data with large statistics. Lateral and vertical composition gradients are observed, and their origin is discussed. X-ray scattering measurements performed on a large sample area are used to validate the results.

Control of tensile strain and interdiffusion in Ge/Si(001) epilayers grown by molecular-beam epitaxy

Tensile-strained and n-doped Ge has emerged as a potential candidate for the realization of optoelectronic devices that are compatible with the mainstream silicon technology. Tensile-strained Ge/Si epilayers can be obtained by using the difference of thermal expansion coefficients between Ge and Si. We have combined various surface, structural, and compositional characterizations to investigate the growth mode and the strain state in Ge/Si epilayers grown by molecular-beam epitaxy. The Ge growth was carried out using a two-step approach: a low-temperature growth to produce relaxed and smooth buffer layers, which is followed by a high-temperature growth to get high quality Ge layers. The existence of a substrate temperature window from 260 to 300 °C is evidenced, which allows to completely suppress the Ge/Si Stranski-Krastanov growth. As a consequence of the high temperature growth, a tensile strain lying in the range of 0.22%–0.24% is obtained. Concerning the effect of thermal annealing, it is shown that cyclic annealing may allow increasing the tensile strain up to 0.30%. Finally, we propose an approach to use carbon adsorption to suppress Si/Ge interdiffusion, which represents one of the main obstacles to overcome in order to realize pure Ge-based optoelectronic devices.

Optical properties of tensile-strained and relaxed Ge films grown on InGaAs buffer

GeO2/Ge/InxGa1−xAs heterostructures grown on (100) GaAs substrates were studied using Raman spectroscopy and photoluminescence (PL) spectroscopy. Both nearly pseudomorphic tensile-strained and nearly completely relaxed Ge films were grown and studied. The maximum tensile strain for Ge films with a thickness of ≈7 nm reaches 2.25%. PL data confirm the conclusions that the band gap offset of Ge/InxGa1−xAs is sensitive to the polarity of the bonds at the interface, and also to a parameter of x and the relaxation of strain. Depending on these parameters, the Ge/InxGa1−xAs may be type-I or type-II heterostructures.

Optical phonons as a probe to determine both composition and strain in InxAl(1−x) As quantum dots embedded in an AlAs matrix

InxAl(1−x)As quantum dots (QDs) embedded in an AlAs matrix were studied using Raman scattering and photoluminescence spectroscopy techniques. The longitudinal optical (LO) and transverse optical (TO) In-As bond vibrations frequencies are found to depend on both composition and strain. Using different scattering geometries (allowed for LO or for TO modes) we were able to obtain the experimental values of these frequencies. By comparing these values with the calculated frequencies, one can determine both the mean composition (stoichiometry parameter x) and the biaxial strain in InxAl(1−x)As QDs embedded in an AlAs matrix. Our approach is simple, non-destructive and fast.

Structure and infrared photoluminescence of GeSi nanocrystals formed by high temperature annealing of GeOx/SiO2 multilayers

Germanium and GeSi nanocrystals were synthesized in SiGeO2 glass by high temperature annealings of GeOx(5 nm)/SiO2(5 nm) multilayers. According to electron microscopy data, the size distribution and stoichiometry of the nanocrystals depend on the annealing temperature (700, 800, or 900 °C). Spatial redistribution of Ge with the formation of large faceted nanocrystals located near the Si substrate and GeSi intermixing at the substrate/film interface were observed. In the case of the 900 °C annealed sample, we note that some nanocrystals have a pyramid-like shape. Infrared absorption spectroscopy demonstrates that intermixing takes place between the GeOx and SiO2 layers leading to the formation of SiGeO2 glass. Raman spectroscopy confirms the formation of Ge nanocrystals after annealing at 700 °C and GeSi nanocrystals after annealing at 800 and 900 °C. For all annealed samples, we report the observation of infrared photoluminescence (PL) at low temperatures in the spectral range 1300–2100 nm. The observation of PL at wavelengths close to 2000 nm may be due to defect-induced radiative transitions in the nanocrystals.

Observation of a nanoscale phase separation in blue-emitting Ce-doped SiO1.5 thin films

Both the optical and structural properties of Ce-doped SiO1.5 thin films were investigated. The Ce-related blue luminescence, which can be seen even at room temperature for as-grown films, exhibits a rather complex evolution with the annealing temperature. In particular, a strong decrease is observed when the films are annealed at 900 °C. Structural characterizations combining scanning transmission electron microscopy and atom probe tomography reveal the formation of Si- and Ce-rich clusters at this temperature, thus demonstrating that the decreasing Ce-related luminescence is due to concentration induced quenching. For annealing temperatures higher than 900 °C, the Ce-related luminescence increases. The different structural characterizations provide clear experimental evidence of a phase separation occurring at the nanoscale between pure Si nanocrystals and Ce-rich clusters having a stoichiometry close to the cerium silicates Ce2Si2O7 or Ce4.667(SiO4)3O. The latter compounds are optically active thereby explaining the increased Ce-related luminescence observed at the highest annealing temperature.

Structural and optical study of Ce segregation in Ce-doped SiO1.5 thin films

Cerium doped SiO1.5 thin films fabricated by evaporation and containing silicon nanocrystals were investigated by atom probe tomography. The effect of post-growth annealing treatment has been systematically studied to correlate the structural properties obtained by atom probe tomography to the optical properties measured by photoluminescence spectroscopy. The atom probe results demonstrated the formation of Ce-Si rich clusters upon annealing at 900 °C which leads to a drastic decrease of the Ce-related luminescence. At 1100 °C, pure Si nanocrystals and optically active cerium silicate compounds are formed. Consequently, the Ce-related luminescence is found to re-appear at this temperature while no Si-nanocrystal related luminescence is observed for films containing more than 3% Ce.

Infrared photoluminescence from GeSi nanocrystals embedded in a germanium–silicate matrix

We investigate the structural and optical properties of GeO/SiO2 multilayers obtained by evaporation of GeO2 and SiO2 powders under ultrahigh vacuum conditions on Si(001) substrates. Both Raman and infrared absorption spectroscopy measurements indicate the formation of GeSi nanocrystals after postgrowth annealing at 800°C. High-resolution transmission electron microscopy characterizations show that the average size of the nanocrystals is about 5 nm. For samples containing GeSi nanocrystals, photoluminescence is observed at 14 K in the spectral range 1500–1600 nm. The temperature dependence of the photoluminescence is studied.

Epitaxial Growth of SiGe Interband Tunneling Diodes on Si(001) and on Si 0.7 Ge 0.3 Virtual Substrates

We present room temperature current voltage characteristics from SiGe interband tunneling diodes epitaxially grown on highly resistive Si(001) substrates. In this case, a maximum peak to valley current ratio (PVCR) of 5.65 was obtained. The possible integration of a SiGe tunnel diode with a strained Si transistor lead us to investigate the growth of SiGe interband tunneling diodes on Si 0.7 Ge 0.3 virtual substrates. A careful optimization of the layer structure leads to a maximum PVCR of 1.36 at room temperature. The latter value can be further increased to 2.26 at 3.7 K. Our results demonstrate that high quality SiGe interband tunneling diodes can be realized, which is of great interest for future memory and high speed applications.

Electrical initialization of electron and nuclear spins in a single quantum dot at zero magnetic field

The emission of circularly polarized light from a single quantum dot relies on the injection of carriers with well-defined spin polarization. Here we demonstrate single dot electroluminescence (EL) with a circular polarization degree up to 35% at zero applied magnetic field. The injection of spin-polarized electrons is achieved by combining ultrathin CoFeB electrodes on top of a spin-LED device with p-type InGaAs quantum dots in the active region. We measure an Overhauser shift of several microelectronvolts at zero magnetic field for the positively charged exciton (trion X+) EL emission, which changes sign as we reverse the injected electron spin orientation. This is a signature of dynamic polarization of the nuclear spins in the quantum dot induced by the hyperfine interaction with the electrically injected electron spin. This study paves the way for electrical control of nuclear spin polarization in a single quantum dot without any external magnetic field.