Papers by SERGIO I. MOLINA
Applied Physics Letters, 2007
The formation of a quaternary InGaAsSb alloy is shown to occur in the core of epitaxial GaSb capp... more The formation of a quaternary InGaAsSb alloy is shown to occur in the core of epitaxial GaSb capped InAs/ GaAs quantum dots emitting at 1.3 m. The existence of the four constituent elements is demonstrated by using spatially resolved low-loss electron energy loss spectroscopy and aberration-corrected high angle annular dark field scanning transmission electron microscopy. The intermixing process giving rise to the formation of this quaternary alloy takes place despite the large miscibility gap between InAs and GaSb binary compounds, and is probably driven by the existence of strain in the quantum dots.
2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC), 2015
Transmission Electron Microscopy Characterization of Nanomaterials, 2013
Physics, Simulation, and Photonic Engineering of Photovoltaic Devices III, 2014
The 3D distribution of self-assembled stacked quantum dots (QDs) is a key parameter to obtain the... more The 3D distribution of self-assembled stacked quantum dots (QDs) is a key parameter to obtain the highest performance in a variety of optoelectronic devices. In this work, we have measured this distribution in 3D using a combined procedure of needle-shaped specimen preparation and electron tomography. We show that conventional 2D measurements of the distribution of QDs are not reliable, and only 3D analysis allows an accurate correlation between the growth design and the structural characteristics.
Ultramicroscopy, 2014
In this work we examined MoS 2 sheets by aberration-corrected scanning transmission electron micr... more In this work we examined MoS 2 sheets by aberration-corrected scanning transmission electron microscopy (STEM) at three different energies: 80, 120 and 200 kV. Structural damage of the MoS 2 sheets has been controlled at 80 kV according a theoretical calculation based on the inelastic scattering of the electrons involved in the interaction electron-matter. The threshold energy for the MoS 2 material has been found and experimentally verified in the microscope. At energies higher than the energy threshold we show surface and edge defects produced by the electron beam irradiation. Quantitative analysis at atomic level in the images obtained at 80 kV has been performed using the experimental images and via STEM simulations using SICSTEM software to determine the exact number of MoS 2 layers.
Journal of Applied Physics, 2010
Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (Q... more Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (QD-IBSC) exhibit a quantum efficiency (QE) that extends to below bandgap energies. However, the production of sub-bandgap photocurrent relies often on the thermal and/or tunneling escape of carriers from the QDs, which is incompatible with preservation of the output voltage. In this work, we test the effectiveness of introducing a thick GaAs spacer in addition to an InAlGaAs strain relief layer (SRL) over the QDs to reduce carrier escape. From an analysis of the QE at different temperatures, it is concluded that escape via tunneling can be completely blocked under short-circuit conditions, and that carriers confined in QDs with an InAlGaAs SRL exhibit a thermal escape activation energy over 100 meV larger than in the case of InAs QDs capped only with GaAs.
In this paper we show how neural networks can be used as powerful tools for quantitative extracti... more In this paper we show how neural networks can be used as powerful tools for quantitative extraction of relevant information from high resolution transmission electron microscopy (HRTEM) images. Different data preprocessing and modelling strategies (including Multilayer Perceptrons and Probabilistic Neural Networks) were analyzed. The methodology has been applied to the determination of thickness and defocus from HRTEM images corresponding to GaN structures with wurtzite crystalline structure using a field emission gun JEOL 2010F equipped with an energy filter.
Journal of Nanoscience and Nanotechnology, Aug 1, 2012
This paper reports the formation and characterization of spherical GaAs quantum dots obtained by ... more This paper reports the formation and characterization of spherical GaAs quantum dots obtained by nanosecond pulsed laser ablation in a liquid (ethanol or methanol). The produced bare GaAs nanoparticles demonstrate rather narrow size distribution which depends on the applied laser power density (from 4.25 to 13.9 J/cm 2 in our experiments) and is as low as 2.5 nm for the highest power used. The absolute value of the average diameter also decreases significantly, from 13.7 to 8.7 nm, as the laser power increases in this interval. Due to the narrow nanoparticle size dispersion achieved at the highest laser powers two absorption band edges are clearly distinguishable at about 1.72 and 3.15 eV which are ascribed to E 0 and E 1 effective optical transitions, respectively. A comparison of the energies with those known for bulk GaAs allows one to conclude that an average diameter of the investigated GaAs nanoparticles is close to 10 nm, i.e., they are quantum dots. High resolution transmission electron microscopy (HRTEM) images show that the bare GaAs nanoparticles are nanocrystalline, but many of them exhibit single/multiple twin boundary defects or even polycrystallinity. The formation of the GaAs crystalline core capped with a SiO 2 shell was demonstrated by HRTEM and energy dispersive X-ray (EDX) spectroscopy. Effective band edges can be better distinguished in SiO 2 capped nanoparticles than in bare ones, In both cases the band edges are correlated with size quantum confinement effect.
This paper has been withdrawn by the authors. The 3D distribution of self-assembled stacked quant... more This paper has been withdrawn by the authors. The 3D distribution of self-assembled stacked quantum dots (QDs) is a key parameter to obtain the highest performance in a variety of optoelectronic devices. In this work, we have measured this distribution in 3D using a combined procedure of needle-shape specimen preparation and electron tomography. We show that conventional 2D measurements of the distribution of QDs are not reliable, and only a 3D analysis allows an accurate correlation between the growth design and the structural characteristics.
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Papers by SERGIO I. MOLINA