91 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
Electric-field control of the magnetic anisotropy in an ultrathin (Ga,Mn)As/(Ga,Mn)(As,P) bilayer
We report on the electric control of the magnetic anisotropy in an ultrathin
ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer with competing in-plane and
out-of-plane anisotropies. The carrier distribution and therefore the strength
of the effective anisotropy is controlled by the gate voltage of a field effect
device. Anomalous Hall Effect measurements confirm that a depletion of carriers
in the upper (Ga,Mn)As layer results in the decrease of the in-plane
anisotropy. The uniaxial anisotropy field is found to decrease by a factor ~ 4
over the explored gate-voltage range, so that the transition to an out-of-plane
easy-axis configuration is almost reached
Surface acoustic wave driven ferromagnetic resonance in (Ga,Mn)(As,P) epilayers
Interdigitated transducers were used to generate and detect surface acoustic
waves on a thin layer of (Ga,Mn)(As,P). The out-of-plane uniaxial magnetic
anisotropy of this dilute magnetic semiconductor is very sensitive to the
strain of the layer, making it an ideal test material for the dynamic control
of magnetization via magneto-striction. The time-domain measurement of the
amplitude and phase of the transmitted SAW during magnetic field sweeps
indicated a clear resonant behavior at a field close to the one calculated to
give a precession frequency equal to the SAW frequency. A resonance was
observed from 5K to 85K, just below the Curie temperature of the layer. A full
analytical treatment of the coupled magnetization/acoustic dynamics showed that
the magneto-strictive coupling modifies the elastic constants of the material
and accordingly the wave-vector solution to the elastic wave equation. The
shape and position of the resonance were well reproduced by the calculations,
in particular the fact that velocity (phase) variations resonated at lower
fields than the acoustic attenuation variations
Buried dislocation networks designed to organize the growth of III-V semiconductor nanostructures
We first report a detailed transmission electron microscopy study of
dislocation networks (DNs) formed at shallowly buried interfaces obtained by
bonding two GaAs crystals between which we establish in a controlled manner a
twist and a tilt around a k110l direction. For large enough twists, the DN
consists of a twodimensional network of screw dislocations accommodating mainly
the twist and of a one-dimensional network of mixed dislocations accommodating
mainly the tilt. We show that in addition the mixed dislocations accommodate
part of the twist and we observe and explain slight unexpected disorientations
of the screw dislocations with respect to the k110l directions. By performing a
quantitative analysis of the whole DN, we propose a coherent interpretation of
these observations which also provides data inaccessible by direct experiments.
When the twist is small enough, one screw subnetwork vanishes. The surface
strain field induced by such DNs has been used to pilot the lateral ordering of
GaAs and InGaAs nanostructures during metal-organic vapor phase epitaxy. We
prove that the dimensions and orientations of the nanostructures are correlated
with those of the cells of the underlying DN and explain how the interface
dislocation structure governs the formation of the nanostructures
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
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
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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