Papers by Mathieu Stoffel
Journal of Luminescence
Cerium doped SiO x (0 ox o 2) thin films have emerged as promising materials for future Si-based ... more Cerium doped SiO x (0 ox o 2) thin films have emerged as promising materials for future Si-based blue light emitting devices. The optical properties of these films are strongly dependent on the nanoscale structure such as the spatial distribution of the Ce atoms. These issues have remained difficult to observe in practice by conventional techniques. In this work, we propose to use atom probe tomography which has emerged as a unique technique that is able to provide information about the chemical composition of the films together with a 3D map indicating the position of each atom from a specimen. Ce-doped SiO 1.5 thin films fabricated by evaporation were investigated. The effect of Ce-content has been systematically studied in order to correlate the structure at the nanoscale with the optical properties measured by photoluminescence spectroscopy.
Applied Physics Letters, Jan 20, 2003
We report on the photoluminescence (PL) properties of Ge hut cluster islands on Si(001) that were... more We report on the photoluminescence (PL) properties of Ge hut cluster islands on Si(001) that were overgrown at temperatures as low as 250 °C. We find that the island-related photoluminescence systematically redshifts as the overgrowth temperature is reduced from 500 to 360 °C, which is attributed to a reduced Ge segregation. For even lower overgrowth temperatures, the emission energy saturates at 0.63 eV or 1.96 μm, more than 110 meV smaller than the band gap of unstrained bulk Ge. We report a PL peak centered at 2.01 μm at low excitation power, in good agreement with the estimated transition energy for a spatially indirect transition between holes confined in the strained Ge island and electrons confined in the surrounding Si matrix. PL is observed up to a temperature of 185 K and an activation energy of 40 meV is deduced from fitting the temperature-dependent peak intensity. Annealing experiments reveal a systematic blueshift of the hut cluster-related PL, thus verifying unambiguously, that the PL signal originates from the hut clusters and not from defects.
Surface Science, Jul 15, 2007
ABSTRACT
Applied Physics Letters, Jun 22, 2009
The authors used x-ray diffraction to investigate strain and composition in SiGe nanorings formed... more The authors used x-ray diffraction to investigate strain and composition in SiGe nanorings formed during partial Si capping of self-assembled SiGe/Si(001) islands. The obtained results are corroborated with selective wet chemical etching experiments. Clear evidence is provided that rings are composed of a Ge rich core surrounded by Si richer ridges indicating that a substantial material redistribution occurs during the shape transformation from SiGe islands to rings. The results suggest that SiGe ring formation is driven by strain relief.
Journal of Physics Condensed Matter, 2008
We report on the experimental determination of composition profiles of strained three-dimensional... more We report on the experimental determination of composition profiles of strained three-dimensional (3D) SiGe islands on Si(001) substrates by means of a combination of selective wet chemical etching and atomic force microscopy (AFM). Isocompositional profiles at 65% Ge content were obtained by etching samples in a H2O2 solution. Quantitative 3D composition profiles of individual islands were extracted by using selective
Nano Letters, Jun 1, 2008
Scanning probe microscopy combined with selective wet chemical etching is employed to quantitativ... more 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.
Physical Review B, 2010
ABSTRACT Interdiffusion in self-assembled Ge:Si(001) islands has been explained by models based o... more ABSTRACT Interdiffusion in self-assembled Ge:Si(001) islands has been explained by models based on either thermodynamic and/or surface kinetic considerations. In order to analyze the relevance of bulk and surface diffusion on the final composition state, we performed a set of controlled x-ray diffraction experiments to study both composition and atomic ordering in Ge/Si(001) islands grown by different methods. Surface diffusion strongly enhances the overall interdiffusion during island growth by solid source molecular beam epitaxy while chemical-vapor-deposited islands are closer to thermodynamic model systems. The growth conditions play a crucial role on the appearance of atomic ordering. In particular, a remarkable correlation between atomic ordering and surface diffusion kinetics is found.
Physical review letters, Jan 9, 2006
We report on the observation and study of tree-ring structures below dislocated SiGe islands (sup... more We report on the observation and study of tree-ring structures below dislocated SiGe islands (superdomes) grown on Si(001) substrates. Analogous to the study of tree rings (dendrochronology), these footprints enable us to gain unambiguous information on the growth and evolution of superdomes and their neighboring islands. The temperature dependence of the critical volume for dislocation introduction is measured and related to the composition of the islands. We show clearly that island coalescence is the dominant pathway towards dislocation nucleation at low temperatures, while at higher temperatures anomalous coarsening is effective and leads to the formation of a depletion region around superdomes.
Thin Solid Films, 2007
A combination of X-ray diffraction with anomalous X-ray scattering at the Ge K edge and specular ... more A combination of X-ray diffraction with anomalous X-ray scattering at the Ge K edge and specular reflectivity measurements is used to reveal both composition and atomic ordering in Ge:Si wetting layers. By comparing the intensity distribution close to the and surface reflections we show that the Ge wetting layer is composed of a SiGe alloy which exhibits atomic ordering. Due to the Si interdiffusion the wetting layer thickness is larger than the nominal 3 ML Ge deposition. The chemical depth distribution is obtained from X-ray reflectivity measurements and confirms the enhanced Ge interdiffusion. These phenomena evidence the crucial interplay between surface kinetics and intermixing in SiGe thin films and nanostructures on Si(001) substrates.
Semiconductor Science and Technology, 2008
We investigate the effect of strain on the etching rate of two SiGe wet etchants, namely NH 4 OH:... more We investigate the effect of strain on the etching rate of two SiGe wet etchants, namely NH 4 OH:H 2 O 2 and H 2 O 2 . For both etchants, we found that there is no appreciable strain selectivity, i.e. the etching rates do not depend on the actual strain state in the SiGe films.
Physical Review Letters, 2003
We apply a selective etching procedure to probe the lateral composition profile of self-assembled... more We apply a selective etching procedure to probe the lateral composition profile of self-assembled SiGe pyramids on a Si(001) substrate surface. We find that the pyramids consist of highly Si intermixed corners, whereas the edges, the apex, and the center of the pyramids remain Ge rich. Our results cannot be explained by existing growth models that minimize strain energy. We use a model that includes surface interdiffusion during island growth, underlining the paramount importance of surface processes during the formation of self-assembled quantum dot heterostructures in many different material systems.
Physical Review Letters, 2004
We grow single and twofold stacked island layers on patterned substrates and investigate the mate... more We grow single and twofold stacked island layers on patterned substrates and investigate the material distribution in and around the patterned area. For both layers a pronounced material depletion region occurs outside the pattern. The material gradients across the planar-patterned interface are symmetric in the first, but highly asymmetric in the second layer. We can describe these phenomena by simulations that take into account the surface curvature for the first and a strain-field modulated surface for the second layer.
Physical Review B, 2005
ABSTRACT
Physical Review B, 2007
... This work was financially supported by the BMBF 03N8711 and the EC Contracts No. 012150 and N... more ... This work was financially supported by the BMBF 03N8711 and the EC Contracts No. 012150 and NMP4CT-2004-500101 "SANDiE" . ... 5 U. Denker, A. Rastelli, M. Stoffel, J. Tersoff, G. Katsaros, G. Constantini, K. Kern, NY Jin-Phillip, DE Jesson, and OG Schmidt, Phys. Rev. Lett. ...
Physical Review B, 2005
The effects of substrate temperature, growth rate, and postgrowth annealing on the composition of... more The effects of substrate temperature, growth rate, and postgrowth annealing on the composition of Ge islands grown on Si(001) were investigated with a combination of selective wet chemical etching and atomic force microscopy. A simple kinetic model comprising only surface diffusion processes can explain all the experimentally observed compositional profiles for pyramid and dome islands grown in the 560-620°C range.
physica status solidi (a), 2006
We report on the morphological evolution of strained SiGe islands epitaxially grown on Si (001). ... more We report on the morphological evolution of strained SiGe islands epitaxially grown on Si (001). After reviewing the morphological transitions undergone by the islands as their size changes, we present recent results obtained by an approach consisting of "reading the footprints" left by the islands on the Si substrate during growth. Such footprints, which we investigate by a combination of scanning probe microscopy and selective wet chemical etching, are trenches carved in the Si substrate. The study of these trenches allows us to discuss general phenomena occurring during growth or post-growth annealing, such as coarsening and morphological transitions, surface-mediated material intermixing and lateral island motion, evolution of plastically relaxed islands and effect on the islands surrounding them.
Nano Letters, 2008
Scanning probe microscopy combined with selective wet chemical etching is employed to quantitativ... more 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.
Microelectronics Journal, 2006
We report on recent advances in the understanding of surface processes occurring during growth an... more We report on recent advances in the understanding of surface processes occurring during growth and post-growth annealing of strained islands which may find application as self-assembled quantum dots. We investigate the model system SiGe/Si(0 0 1) by a new approach based on ''reading the footprints'' which islands leave on the substrate during their growth and evolution. Such footprints consist of trenches carved in the Si substrate. We distinguish between surface footprints and footprints buried below the islands. The former allow us to discriminate islands which are in the process of growing from those which are shrinking. Islands with steep morphologies grow at the expense of smaller and shallower islands, consistent with the kinetics of anomalous coarsening. While shrinking, islands change their shape according to thermodynamic predictions. Buried footprints are investigated by removing the SiGe epilayer by means of selective wet chemical etching. Their reading shows that: (i) during post-growth annealing islands move laterally because of surface-mediated Si-Ge intermixing; (ii) a tree-ring structure of trenches is created by dislocated islands during their ''cyclic'' growth. This allows us to distinguish coherent from dislocated islands and to establish whether the latter are the result of island coalescence. r
Materials Science in Semiconductor Processing, 2005
A novel operation mode-the so-called resonance phase operation-is demonstrated using a SiGe HBT (... more A novel operation mode-the so-called resonance phase operation-is demonstrated using a SiGe HBT (heterobipolar transistor). Transit time effects and coherent transport lead by proper design of the transistor to a phase shift larger than p: In this resonance phase mode, a current gain above 0 dB is achieved at frequency bands well above the transit frequency. We have designed and fabricated SiGe HBTs in order to investigate the resonance phase effect at frequencies below 50 GHz due to easier measurement technique. The transistors were fabricated using a lowtemperature process. They showed a rather high breakthrough voltage of up to 20 V. The RF measurement showed the typical HBT behaviour up to the transit frequency f T . At higher frequencies the current gain H 21 rose again above 0 dB. We have thus demonstrated the existence of the resonance phase effect in a SiGe HBT. r
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Papers by Mathieu Stoffel