arXiv subjects

J. M. Ulloa

Publications and source records attributed to J. M. Ulloa.

7 recordsLinked to original sources

arXiv2026

Thermal Control of Size Distribution and Optical Properties in Gallium Nanoparticles

Gallium nanoparticles (Ga-NPs) exhibit promising plasmonic properties spanning ultraviolet to infrared spectral regions, making them suitable for diverse nanophotonic applications. However, the synthesis of uniform and ordered Ga-NP arrays remains challenging due to size heterogeneity arising from coarsening during physical deposition. Here, we systematically investigate the influence of substrate temperature on the nucleation, growth, and homogenization dynamics of Ga-NPs formed via Joule-effect thermal evaporation on GaAs substrates. Atomic force microscopy and scanning electron microscopy reveal temperature-dependent transitions from broad, bimodal size distributions to narrow dispersed arrays within an optimal substrate temperature range of 300-350°C. Above this window, increased diffusion and desorption induce size increase, decreased density, and morphological relaxation manifested as NP shape flattening. Optical reflectance measurements identify distinct localized surface plasmon resonance (LSPR) modes whose energies are closely correlated with NP dimensions and aspect ratios. A figure of merit combining NP density and size uniformity quantifies optimal conditions at intermediate substrate temperatures, consistent with enhanced plasmonic quality factors derived from spectral LSPRs. In-situ post-deposition annealing experiments confirm Ostwald ripening as the dominant coarsening mechanism at elevated temperatures and highlight the necessity of rapid cooling and oxide shell formation to stabilize homogeneous arrays. Cross-sectional transmission electron microscopy with electron energy-loss spectroscopy validates the core-shell structure of individual Ga-NPs, quantifies temperature-dependent oxide shell thickening, and confirms the liquid metal character of the Ga within the core...

cond-mat.mtrl-sci↗

arXiv2025

Structural and optical changes induced by incorporation of antimony into InAs/GaAs(001) quantum dots

We present experimental evidence of Sb incorporation inside InAs/GaA(001) quantum dots exposed to an antimony flux immediately before capping with GaAs. The Sb composition profile inside the nanostructures as measured by cross-sectional scanning tunneling and electron transmission microscopies show two differentiated regions within the quantum dots, with an Sb rich alloy at the tip of the quantum dots. Atomic force microscopy and transmission electron microscopy micrographs show increased quantum-dot height with Sb flux exposure. The evolution of the reflection high-energy electron-diffraction pattern suggests that the increased height is due to changes in the quantum-dot capping process related to the presence of segregated Sb atoms. These structural and compositional changes result in a shift of the room-temperature photoluminescence emission from 1.26 to 1.36 microns accompanied by an order of magnitude increase in the room-temperature quantum-dot luminescence intensity.

cond-mat.mtrl-sci↗

arXiv2024

Growth interruption strategies for interface optimization in GaAsSb/GaAsN type-II superlattices

Recently, GaAsSb/GaAsN type II short-period superlattices (SLs) have been proposed as suitable structures to be implemented in the optimal design of monolithic multi-junction solar cells. However, due to strong surface Sb segregation, experimental Sb composition profiles differ greatly from the nominal square-wave design. In this work, the improvement of the interface quality of these SLs in terms of compositional abruptness and surface roughness has been evaluated by implementing different growth interruption times under Sb4/As4 (soaking) and As4 (desorption) overpressure conditions before and after the growth of GaAsSb layers, respectively. The combined effects of both processes enhance Sb distribution, achieving squarer compositional profiles with reduced surface roughness interfaces. It has been found that the improvement in compositional abruptness is quantitatively much higher at the lower interface, during soaking, than at the upper interface during desorption. Conversely, a larger decrease in surface roughness is achieved at the upper interface than at the lower interface. Fitting of the Sb segregation profiles using the 3-layer kinetic fluid model has shown that the increase in Sb incorporation rate is due to the decrease in segregation energy, presumably to changes in the surface reconstruction of the floating layer at the surface.

cond-mat.mtrl-sci↗

arXiv2020

N-nH complexes in GaAs studied at the atomic scale by cross-sectional scanning tunneling microscopy

Hydrogenation of nitrogen (N) doped GaAs allows for reversible tuning of the bandgap and the creation of site controlled quantum dots through the manipulation of N-nH complexes, N-nH complexes, wherein a nitrogen atom is surrounded by n hydrogen (H) atoms. Here we employ cross-sectional scanning tunneling microscopy (X-STM) to study these complexes in the GaAs (110) surface at the atomic scale. In addition to that we performed density functional theory (DFT) calculations to determine the atomic properties of the N-nH complexes. We argue that at or near the (110) GaAs surface two H atoms from N-nH complexes dissociate as an H$_2$ molecule. We observe multiple features related to the hydrogenation process, of which a subset is classified as N-1H complexes. These N-1H related features show an apparent reduction of the local density of states (LDOS), characteristic to N atoms in the GaAs (110) surface with an additional apparent localized enhancement of the LDOS located in one of three crystal directions. N-nH features can be manipulated with the STM tip. Showing in one case a switching behavior between two mirror-symmetric states and in another case a removal of the localized enhancement of the LDOS. The disappearance of the bright contrast is most likely a signature of the removal of an H atom from the N-nH complex.

cond-mat.mes-hall↗

arXiv2019

Topology driven g-factor tuning in type-II quantum dots

We investigate how the voltage control of the exciton lateral dipole moment induces a transition from singly to doubly connected topology in type II InAs/GaAsSb quantum dots. The latter causes visible Aharonov-Bohm oscillations and a change of the exciton g-factor which are modulated by the applied bias. The results are explained in the frame of realistic $\mathbf{k}\cdot\mathbf{p}$ and effective Hamiltonian models and could open a venue for new spin quantum memories beyond the InAs/GaAs realm.

cond-mat.mes-hall↗

arXiv2017

Intersubband polarons in oxides

Intersubband (ISB) polarons result from the interaction of an ISB transition and the longitudinal optical (LO) phonons in a semiconductor quantum well (QW). Their observation requires a very dense two dimensional electron gas (2DEG) in the QW and a polar or highly ionic semiconductor. Here we show that in ZnO/MgZnO QWs the strength of such a coupling can be as high as 1.5 times the LO-phonon frequency due to the very dense 2DEG achieved and the large difference between the static and high-frequency dielectric constants in ZnO. The ISB polaron is observed optically in multiple QW structures with 2DEG densities ranging from $5\times 10^{12}$ to $5\times 10^{13}$ cm$^{-2}$, where an unprecedented regime is reached in which the frequency of the upper ISB polaron branch is three times larger than that of the bare ISB transition. This study opens new prospects to the exploitation of oxides in phenomena happening in the ultrastrong coupling regime.

cond-mat.mtrl-sci↗

arXiv2016

Strain-balanced type-II superlattices for efficient multi-junction solar cells

We propose type-II GaAsSb/GaAsN superlattices (SLs) lattice-matched to GaAs as a novel material for the 1 eV sub-cells present in highly efficient GaAs/Ge-based multi-junction solar cells. We demonstrate that, among other benefits, the spatial separation of Sb and N allows a better control over composition and lattice matching, avoiding the growth problems related to the concomitant presence of both elements in GaAsSbN layers. This approach not only reduces clustering and improves crystal quality and interface abruptness, but also allows for additional control of the effective bandgap in the 1.0-1.15 eV spectral region through the SL period thickness. The optimized SL structure exhibits a type-II band alignment and strong electronic coupling at 0 V. Both effects cooperate to increase the minority carrier collection and leads to a net strong enhancement of the external quantum efficiency (EQE) under photovoltaic conditions with respect to bulk layers of equivalent thickness.

cond-mat.mtrl-sci↗