242 research outputs found
Tuning the ferromagnetic properties of hydrogenated GaMnAs
Hydrogenation and posthydrogenation annealings have been used as a very
efficient tool to tune the hole density over a wide range, at fixed magnetic
moment concentration, in thin GaMnAs layers. Reduction of the hole density
resulted in strong modifications of their ferromagnetic properties. In
particular, we observed in magnetotransport experiments the decrease of the
Curie temperature, along with modifications of the magnetic anisotropy, a
behavior consistent with the mean-field theory
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
Strain-Control of the magnetic anisotropy in (Ga,Mn)(As,P) ferromagnetic semiconductor layers
A small fraction of phosphorus (up to 10 %) was incorporated in ferromagnetic
(Ga,Mn)As epilayers grown on a GaAs substrate. P incorporation allows reducing
the epitaxial strain or even change its sign, resulting in strong modifications
of the magnetic anisotropy. In particular a reorientation of the easy axis
toward the growth direction is observed for high P concentration. It offers an
interesting alternative to the metamorphic approach, in particular for
magnetization reversal experiments where epitaxial defects stongly affect the
domain wall propagation
Discretized disorder in planar semiconductor microcavities: Mosaicity effect on resonant Rayleigh scattering and optical parametric oscillation
International audienceThe features of resonant secondary emission by two-dimensional multiple semiconductor microcavities are experimentally investigated. We show that, under normal laser incidence, static disorder determines the final states of the resonant Rayleigh scattering in the high symmetry axes of the GaAs matrix. Scanning transmission electron microscopy reveals a small dislocation density at the layers interfaces and step formation due to strain accumulation and relaxation ruled by the symmetry of the underlying GaAs matrix: this mosaicity effects, a common feature in thick and strained crystals, determines the scattering channels by selecting the crystallographic discretized directions. Moreover, interband optical parametric oscillation of intensity balanced signal and idler beams takes place in the directions selected by the photonic disorder in the distributed Bragg reflector
GaN/Ga2O3 Core/Shell Nanowires Growth: Towards High Response Gas Sensors
International audienceThe development of sensors working in a large range of temperature is of crucial importance in areas such as monitoring of industrial processes or personal tracking using smart objects. Devices integrating GaN/Ga2O3 core/shell nanowires (NWs) are a promising solution for monitoring carbon monoxide (CO). Because the performances of sensors primarily depend on the material properties composing the active layer of the device, it is essential to control them and achieve material synthesis in the first time. In this work, we investigate the synthesis of GaN/Ga2O3 core-shell NWs with a special focus on the formation of the shell. The GaN NWs grown by plasma-assisted molecular beam epitaxy, are post-treated following thermal oxidation to form a Ga2O3-shell surrounding the GaN-core. We establish that the shell thickness can be modulated from 1 to 14 nm by changing the oxidation conditions and follows classical oxidation process: A first rapid oxide-shell growth, followed by a reduced but continuous oxide growth. We also discuss the impact of the atmosphere on the oxidation growth rate. By combining XRD-STEM and EDX analyses, we demonstrate that the oxide-shell is crystalline, presents the β-Ga2O3 phase, and is synthesized in an epitaxial relationship with the GaN-core
Parametric generation of twin photons in vertical triple microcavities
We report the realization of a monolithic vertical-cavity, surface emitting micro-optical parametric conversion nanostructure, triply resonant with the parametric frequencies, allowing parametric oscillation with ultra-low pump power threshold. The photonic phase-space naturally provides triple resonance for the parametric frequencies, together with built-in cavity phase-matching for the pump wave at normal incidence. Parametric oscillation is observed in both the strong and weak exciton–photon coupling regime, allowing a high operating temperature. Signal and idler beams can be collected at 0° or at finite angles. The OPO threshold is low enough to envisage the realization of an all-semiconductor electrically-pumped micro-parametric oscillator
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