143 research outputs found
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
Evidence for the formation of two phases during the growth of SrTiO3 on silicon
International audienceEpitaxial SrTiO3 (STO)/Si templates open a unique opportunity for the integration of ferroelectric oxides, such as BaTiO3 on silicon and for the realization of new devices exploiting ferroelectricity. STO itself has been shown as ferroelectric at room temperature when deposited in thin layers on Si, while bulk STO is tetragonal and, thus, ferroelectric below 105 K. Here, we demonstrate the coexistence, at room temperature, of strained cubic and tetragonal phases in thin STO/Si layers. The tetragonal STO phase presents a pronounced tetragonality for thicknesses up to 24 ML. Above this thickness, the strained cubic STO phase starts relaxing while the tetragonal STO phase progressively transits to cubic STO. The origin of the simultaneous formation of these two phases is analyzed and is attributed to oxygen segregation at the early stages of the growth
Confinement of superconducting fluctuations due to emergent electronic inhomogeneities
The microscopic nature of an insulating state in the vicinity of a
superconducting state, in the presence of disorder, is a hotly debated
question. While the simplest scenario proposes that Coulomb interactions
destroy the Cooper pairs at the transition, leading to localization of single
electrons, an alternate possibility supported by experimental observations
suggests that Cooper pairs instead directly localize. The question of the
homogeneity, granularity, or possibly glassiness of the material on the verge
of this transition is intimately related to this fundamental issue. Here, by
combining macroscopic and nano-scale studies of superconducting ultrathin NbN
films, we reveal nanoscopic electronic inhomogeneities that emerge when the
film thickness is reduced. In addition, while thicker films display a purely
two-dimensional behaviour in the superconducting fluctuations, we demonstrate a
zero-dimensional regime for the thinner samples precisely on the scale of the
inhomogeneities. Such behavior is somehow intermediate between the Fermi and
Bose insulator paradigms and calls for further investigation to understand the
way Cooper pairs continuously evolve from a bound state of fermionic objects
into localized bosonic entities.Comment: 29 pages 9 figure
Determination of the local concentrations of Mn interstitials and antisite defects in GaMnAs
We present a method for the determination of the local concentrations of
interstitial and substitutional Mn atoms and As antisite defects in GaMnAs. The
method relies on the sensitivity of the structure factors of weak reflections
to the concentrations and locations of these minority constituents. High
spatial resolution is obtained by combining structure factor measurement and
X-ray analysis in a transmission electron microscope. We demonstrate the
prevalence of interstitials with As nearest neighbors in as-grown layers.Comment: 10 pages, 4 figure
An aqueous one-pot route to gold/quantum rod heterostructured nanoparticles functionalized with DNA
International audienc
Nondestructive three-dimensional imaging of crystal strain and rotations in an extended bonded semiconductor heterostructure
International audienceWe report the 3D mapping of strain and tilts of crystal planes in an extended InP nanostructured layer bonded onto silicon, measured without sample preparation. Our approach takes advantages of 3D x-ray Bragg ptychography combined to an optimized inversion process. The excellent agreement with the sample nominal structure validates the reconstruction while the evidence of spatial fluctuations hardly observable by other means, underlines the specificities of Bragg ptychography
Polarity in GaN and ZnO: Theory, measurement, growth, and devices
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919.The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade
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