O. D. D. Couto

Ferromagnetic nanoclusters formed by Mn implantation in GaAs

Ferromagnetic clusters were incorporated into GaAs samples by Mn implantation and subsequent annealing. The composition and structural properties of the Mn-based nanoclusters formed at the surface and buried into the GaAs sample were analyzed by x-ray and microscopic techniques. Our measurements indicate the presence of buried MnAs nanoclusters with a structural phase transition around 40 °C, in accord with the first-order magneto-structural phase transition of bulk MnAs. We discuss the structural behavior of these nanoclusters during their formation and phase transition, which is an important point for technological applications.

Effect of a GaAsP Shell on the Optical Properties of Self-Catalyzed GaAs Nanowires Grown on Silicon

We realize the growth of self-catalyzed core-shell GaAs/GaAsP nanowires (NWs) on Si substrates using molecular-beam epitaxy. Transmission electron microscopy of single GaAs/GaAsP NWs demonstrates their high crystal quality and shows domination of the GaAs zinc-blende phase. Using continuous-wave and time-resolved photoluminescence (PL), we make a detailed comparison with uncapped GaAs NWs to emphasize the effect of the GaAsP capping in suppressing the nonradiative surface states. Significant PL enhancement in the core-shell structures exceeding 3 orders of magnitude at 10 K is observed; in uncapped NWs PL is quenched at 60 K, whereas single core-shell GaAs/GaAsP structures exhibit bright emission even at room temperature. From analysis of the PL temperature dependence in both types of NW we are able to determine the main carrier escape mechanisms leading to the PL quench.

Strain redistribution at the phase transition of MnAs∕GaAs(001) films

We investigated the thermal evolution of the lattice parameters of a MnAs film epitaxially grown on GaAs(001) around its magnetostructural first-order phase transition using x-ray diffraction. Despite the substrate constraint, large variation of one of the in-plane lattice parameters is preserved, typical of bulk MnAs phase transition, during a large temperature range where two phases coexist. We demonstrated that the condition of the constant film length along this direction, in accord to the substrate length, is always fulfilled during the process. The effect is attributed to the gliding of misfit dislocations present on the film.

Effect of MnAs/GaAs(001) film accommodations on the phase-transition temperature

The phase-transition temperature of MnAs epitaxial films grown by molecular-beam epitaxy on GaAs(001) with different crystalline accommodations was studied by specular and grazing incidence x-ray diffraction. The transition temperature of MnAs films with tilted hexagonal c-axis orientations with respect to the GaAs substrate is higher than the most investigated nontilted films and reaches a value above room temperature, which is more suitable for device applications.

Electronic channels for acoustic transport in semiconductor heterostructures

We demonstrate electronic waveguides for the ambipolar transport of electrons and holes by surface acoustic waves (SAWs) in buried (Al,Ga)As quantum well (QW) structures, which do not require deep-etching for their fabrication. They are defined by tailoring the SAW-induced piezoelectric potential in the QW plane using thin metal stripes deposited onto the piezoelectric cap layer of the QW sample. Acoustic transport experiments show the enhanced capture and acoustic transport of photoexcited carriers underneath the metallic surface stripes.

Acoustically‐driven Single Photon Sources on (311)A GaAs

We employ surface acoustic waves to control the transfer of photo‐generated carriers between interconnected quantum wells and wires grown on pre‐patterned (311)A GaAs substrates. The wires are embedded at photo‐lithographically defined positions within (Al,Ga)As/GaAs quantum well. Optical studies on these structures have shown sharp PL lines and antibunched photons with tunable emission energy, revealing the presence of several recombination centers within the wire. The spatial separation of these recombination centers emitting single photons is determined from time‐resolved measurements.

Long‐range spin transport in (110) GaAs quantum wells

We report on long‐range (>60 μm) coherent spin transport via surface acoustic waves in (110) GaAs quantum wells. The long transport distances, attributed to the quenching of the exciton exchange interaction and to the inhibition of the D’yakonov Perel’ spin relaxation mechanism, allow the manipulation of the electron spin during the acoustic transport via external magnetic fields. Potential applications include for quantum information processing.