Marta Luengo-Kovac

Height stabilization of GaSb/GaAs quantum dots by Al-rich capping

GaSb quantum dots (QDs) in a GaAs matrix are investigated with cross-sectional scanning tunneling microscopy (X-STM) and photoluminescence (PL). We observe that Al-rich capping materials prevent destabilization of the nanostructures during the capping stage of the molecular beam epitaxy (MBE) growth process and thus preserves the QD height. However, the strain induced by the absence of destabilization causes many structural defects to appear around the preserved QDs. These defects originate from misfit dislocations near the GaSb/GaAs interface and extend into the capping layer as stacking faults. The lack of a red shift in the QD PL suggests that the preserved dots do not contribute to the emission spectra. We suggest that a better control over the emission wavelength and an increase of the PL intensity is attainable by growing smaller QDs with an Al-rich overgrowth.

Structural differences between capped GaSb nanostructures grown by Stranski-Krastanov and droplet epitaxy growth modes

Droplet epitaxy (DE) has emerged as an alternative to Stranski-Krastanov (SK) as a method for epitaxial nanostructure formation. We find significant structural differences of similar sized nanostructures embedded in GaAs between the two methods. Atomic force microscopy and atom probe tomography measurements reveal that uncapped and capped SK structures resemble each other. However, the DE nanostructures appear as rings topographically but are quantum dots compositionally. A GaSb wetting layer is present regardless of the growth method and shares a nearly identical Sb concentration profile. DE nanostructures are shown to have a lower Sb concentration, and transmission electron microscopy measurements reveal that they produce less strain on the capping layer. Despite significant structural differences, SK and DE nanostructures exhibit the same photoluminescence response, suggesting that the emission is from a shared feature such as the wetting layer, rather than the nanostructures.

Current-induced spin polarization in InGaAs and GaAs epilayers with varying doping densities

The current-induced spin polarization and momentum-dependent spin-orbit field were measured in In

Gate control of the spin mobility through the modification of the spin-orbit interaction in two-dimensional systems

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Analyzing pattern retention for multilayer focused ion beam induced quantum dot structures

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G -factor modification in a bulk InGaAs epilayer by an in-plane electric field

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Current-induced spin polarization in InGaAs and GaAs epilayers as a function of doping density

As epilayers with varying indium concentrations and silicon doping densities. Samples with higher indium concentrations and carrier concentrations and lower mobilities were found to have larger electrical spin generation efficiencies. Furthermore, current-induced spin polarization was detected in GaAs epilayers despite the absence of measurable spin-orbit fields, indicating that the extrinsic contributions to the spin polarization mechanism must be considered. Theoretical calculations based on a model that includes extrinsic contributions to the spin dephasing and the spin Hall effect, in addition to the intrinsic Rashba and Dresselhaus spin-orbit coupling, are found to qualitatively agree with the experimental results.

The effect of doping on low temperature growth of high quality GaAs nanowires on polycrystalline films

Spin drag measurements were performed in a two-dimensional electron system set close to the crossed spin helix regime and coupled by strong intersubband scattering. In a sample with uncommon combination of long spin lifetime and high charge mobility, the drift transport allows us to determine the spin-orbit field and the spin mobility anisotropies. We used a random walk model to describe the system dynamics and found excellent agreement for the Rashba and Dresselhaus couplings. The proposed two-subband system displays a large tuning lever arm for the Rashba constant with gate voltage, which provides a new path towards a spin transistor. Furthermore, the data shows large spin mobility controlled by the spin-orbit constants setting the field along the direction perpendicular to the drift velocity. This work directly reveals the resistance experienced in the transport of a spin-polarized packet as a function of the strength of anisotropic spin-orbit fields.

Current-induced spin polarization in InGaAs epilayers with varying doping densities

Atomic force microscopy was used to investigate the effects of templating parameters on focused ion beam patterned single-, two-, and three-layer InAs/GaAs(001) quantum dot structures. The number of layers, focused ion beam dwell time, and pattern spacing affected the fidelity of the quantum dots. The highest single dot fidelities were found in regions with 1 and 3 ms dwell times and 1 and 2 μm pattern spacings. A two-layer region patterned with 1 ms dwell time and 1 μm spacing was found to have 100% single quantum dot fidelity with no off-site dot nucleation in a 20 × 20 μm2 scan. Holes that were milled with 6 and 9 ms dwell times and 0.25 μm spacing became faceted, that is, deep, tightly packed, and rhombic, by the third layer. Autocorrelation of the images was used to analyze the periodicity and size of the features.

g-factor modification by an in-plane electric field in a bulk In

We report on the modification of the g-factor by an in-plane electric field in an In