353 research outputs found
STEM analysis of deformation and B distribution in nanosecond laser ultra-doped Si B
We report on the structural properties of highly B-doped silicon (> 2 at. %)
realised by nanosecond laser doping. We investigate the crystalline quality,
deformation and B distribution profile of the doped layer by STEM analysis
followed by HAADF contrast studies and GPA, and compare the results to SIMS
analyses and Hall measurements. When increasing the active B concentration
above 4.3 at.%, the fully strained, perfectly crystalline, Si:B layer starts
showing dislocations and stacking faults. These only disappear around 8 at.%
when the Si:B layer is well accommodated to the substrate. When increasing B
incorporation, we increasingly observe small precipitates, filaments with
higher active B concentration and stacking faults. At the highest
concentrations studied, large precipitates form, related to the decrease of
active B concentration. The structural deformation, defect type and
concentration, and active B distribution are connected to the initial increase
and subsequent gradual loss of superconductivity
High density InAlAs/GaAlAs quantum dots for non-linear optics in microcavities
Structural and optical properties of InAlAs/GaAlAs quantum dots grown by molecular beam epitaxy are studied using transmission electron microscopy, temperature- and time resolvedphotoluminescence. The control of the recombination lifetime (50 ps – 1.25 ns), and of the dot density (5.10−8 – 2.1011 cm−3) strongly suggest that these material systems can find wide applications in opto-electronic devices as focusing non linear dispersive materials as well as fast saturable absorbers
Polarization dependence of electroluminescence from closely-stacked and columnar quantum dots
Quantum dots (QDs) have a potential for application in semiconductor optical amplifiers (SOAs), due to their high saturation power related to the low differential gain, fast gain recovery and wide gain spectrum compared to quantum wells. Besides all advantages, QDs realized by Stranski-Krastanov growth mode have a flat shape which leads to a gain anisotropy and a related transverse magnetic (TM) and -electric (TE) polarization dependence as compared to bulk material. This has so far prevented their applications in SOAs. It has been suggested that control of optical polarization anisotropy of the QD can be obtained through QD shape engineering, in closely stacked or columnar QDs (CQDs). To this aim, we have fabricated and tested SOA structures based on closely-stacked and columnar QDs. Closely-stacked InAs QDs with 4, 6 and 10 nm GaAs spacer showed a minor improvement in the ratio of TM and TE integrated electroluminescence (EL) over standard QDs along with a strong reduction in efficiency. In contrast, a large improvement was obtained in CQDs, depending on the number of stacked submonolayers which can be attributed to the more symmetric shape of columnar QDs. A relatively small spectral separation (ÄE ~ 21 meV) between TE- and TM-EL peaks has been observed showing that heavy- and light hole-like states, respectively are energetically close in these QDs. These results indicate that columnar QDs have a significant potential for polarization-independent QD SOA
Catalyst faceting during graphene layer crystallization in the course of carbon nanofiber growth
International audienceThe low temperature catalytic growth of multiwall carbon nanotubes (MWCNTs) rests on the continuous nucleation and growth of graphene layers at the surface of crystalline catalystparticles. Here, we study the atomic mechanisms at work in this phenomenon, by observing the growth of such layers in situ in the transmission electron microscope, in the case of iron-based catalysts. Graphene layers, parallel to the catalyst surface, appear by a mechanism of step flow, where the atomic layers of catalyst are "replaced" by graphene planes. Quite remarkably, catalyst facets systematically develop while this mechanism is at work. We discuss the origin of faceting in terms of equilibrium particle shape and graphene layer nucleation. Step bunching due to impeded step migration, in certain growth conditions, yields characteristic catalyst nail-head shapes. Mastering themechanisms of faceting and step bunching could open up the way to tailoring the structure of low temperature-grown MWCNTs, e.g. with highly parallel carbon walls and, ultimately, with controlled structure and chirality
Strain in a silicon-on-insulator nanostructure revealed by 3D x-ray Bragg ptychography
International audienceProgresses in the design of well-defined electronic band structure and dedicated functionalities rely on the high control of complex architectural device nano-scaled structures. This includes the challenging accurate description of strain fields in crystalline structures, which requires non invasive and three-dimensional (3D) imaging methods. Here, we demonstrate in details how x-ray Bragg ptychography can be used to quantify in 3D a displacement field in a lithographically patterned silicon-on-insulator structure. The image of the crystalline properties, which results from the phase retrieval of a coherent intensity data set, is obtained from a well-controlled optimized process, for which all steps are detailed. These results confirm the promising perspectives of 3D Bragg ptychography for the investigation of complex nano-structured crystals in material science
Magnetic properties and domain structure of (Ga,Mn)As films with perpendicular anisotropy
The ferromagnetism of a thin GaMnAs layer with a perpendicular easy
anisotropy axis is investigated by means of several techniques, that yield a
consistent set of data on the magnetic properties and the domain structure of
this diluted ferromagnetic semiconductor. The magnetic layer was grown under
tensile strain on a relaxed GaInAs buffer layer using a procedure that limits
the density of threading dislocations. Magnetometry, magneto-transport and
polar magneto-optical Kerr effect (PMOKE) measurements reveal the high quality
of this layer, in particular through its high Curie temperature (130 K) and
well-defined magnetic anisotropy. We show that magnetization reversal is
initiated from a limited number of nucleation centers and develops by easy
domain wall propagation. Furthermore, MOKE microscopy allowed us to
characterize in detail the magnetic domain structure. In particular we show
that domain shape and wall motion are very sensitive to some defects, which
prevents a periodic arrangement of the domains. We ascribed these defects to
threading dislocations emerging in the magnetic layer, inherent to the growth
mode on a relaxed buffer
Molecular-beam epitaxy of GaSb on 6°-offcut (001) Si using a GaAs nucleation layer
International audienceWe studied and optimized the molecular beam epitaxy of GaSb layers on vicinal (001) Si substrates using a GaAs nucleation layer. An in-depth analysis of the different growth stages under optimized conditions revealed the formation of a high density of small GaAs islands forming a quasi-two-dimensional layer. GaSb then nucleated atop this layer as three-dimensional islands before turning to two-dimensional growth within a few nanometers. Moreover, reflexion high-energy electron diffraction revealed a fast relaxation of GaAs on Si and of GaSb on GaAs. The GaSb layer quality was better than that of similar layers grown on Si through AlSb nucleation layers
Phase separation and surface segregation in Co – Au – SrTiO3 thin films: Self-assembly of bilayered epitaxial nanocolumnar composites
International audiencePhase separation and surface segregation are powerful levers that allow to synthesize nanocompos-ites via self-assembly. In the present work, we combine these concepts with 3-dimensional vertical epitaxial growth and study Co-Au-SrTiO3 thin films as a model system. We demonstrate that SrTiO3, Co and Au undergo phase separation during sequential pulsed laser deposition, giving rise to a dense array of ultrathin bilayered Co-Au nanowires (NWs) with highly anisotropic optical and magnetic properties. A detailed analysis of the structural properties of the embedded metallic NWs reveals stabilization of a Co fcc phase and pronounced coupling to the matrix, which leads to large magnetoelastic effects. We discuss possible growth mechanisms yielding bilayer phase separation in nanocolumnar composites and show how the present results can be used to estimate a lower bound for the Co/Au interface energy
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