A. Ruiz

Atomic layer molecular beam epitaxy (Almbe) of III–V compounds: Growth modes and applications

A new development of molecular beam epitaxy (MBE) for III–V compounds is described, based on cyclic perturbation of the growth front at atomic layer level by periodic pulsing, alternating or interrupting the molecular beams. The modification of the growth mechanism caused by this perturbations is discussed and related to periodic changes of surface stoichiometry which induce 2D growth mechanism by enhanced layer nucleation.Under appropriate modulation conditions, an atomic layer by layer growth mode can be achieved. A practical implementention of this mode, that we denote atomic layer MBE (ALMBE), is considered in which only group V beams are pulsed in a specially designed effusion cell. A number of growth applications of ALMBE are presented, including growth of highly mismatched heterostructures and short period superlattices containing two different group V elements such as arsenic and phosphorous.

Magnetic nanoparticles coated with dimercaptosuccinic acid: development, characterization, and application in biomedicine

This review intends to summarize some of the results achieved in the development of magnetic nanoparticles coated with anionic ligands, specifically dimercaptosuccinic acid applied in the biomedical area. We describe synthetic routes used to produce iron oxide-based magnetic nanoparticles, subsequently coated with DMSA as well as functionalization strategies for specific purposes with polymers, antibodies, and cytokines. Finally, we have collected data on biological interactions of DMSA-coated nanoparticles in vitro and in vivo, in particular cell interaction process, pharmacokinetics, and biodistribution in different animal models and their promising applications in drug delivery, NMR imaging, hyperthermia, nanothermometry, magnetic separation, and bioremediation.

Atomic layer molecular beam epitaxy growth of InAs on GaAs substrates

InAs layers with thickness ranging from 0.1 to 2.5 μm have been grown directly on highly mismatched (7.4%) (001) GaAs substrates by atomic layer molecular beam epitaxy (ALMBE). This growth method, based on the modulated deposition of one or both component species, provides InAs layers with excellent flat morphology, independently of the total thickness. A detailed study of the evolution of the electron diffraction (RHEED) pattern indicates that a complete decoupling between the InAs epitaxial layer and the GaAs substrate is reached in less that 10 monolayers. Evidence is obtained that layer-by-layer nucleation takes place from the beginning of the growth.

CuO nanowires for inhibiting secondary electron emission

Copper oxide nanowires (NWs) grown on copper to avoid the secondary electron emission were investigated. Optimal temperatures for NW growth were found to be in the range 700–800 K. NW surface coverage of 102 µm−2 is required to strongly reduce the secondary electron yield. A total secondary electron emission coefficient below 1 was obtained for NW aspect ratio higher than 103.

Atomic layer MBE growth and characterization of AlAsInAs strained layer superlattices on GaAs

A recent development of Molecular Beam Epitaxy (MBE) — the Atomic Layer Molecular Beam Epitaxy — has been used to grow AlAsInAs strained layer superlattices (SLS) on (001)GaAs at low growth temperatures (Ts < 400°C). The growth process basically consists on alternating group V and/or group III beams following an optimum timing established by RHEED oscillations observation during the monolayer formation sequence. This method allows to grow at low substrate temperature with excellent morphology, even for those systems which have extremely different optimum MBE growth conditions and a severe lattice mismatch of 7% like AlAsInAs. X-ray diffraction and optical characterization results for superlattices of different periodicities are presented. In particular, Raman spectra of these samples showing folded acoustic phonons demonstrate their quality.

Strain distribution and structural characterization of short period GaAsGaP strained superlattices by raman and X-Ray scattering

Abstract Raman scattering is used to study structural properties of strained-layer GaAsGaP short period superlattices grown on GaAs substrates. Different thicknesses of the constituent layers and also the effect of different types of buffer layers are studied. From the energy and width of the confined optical phonons observed, information about strain accomodation in the layers, strain relaxation and, in general, about structural quality is achieved.

Folded acoustic phonons in InAs-AlAs strained-layer superlattices

Raman spectroscopy is used to characterize highly mismatched (7%) InAs‐AlAs superlattices grown by atomic layer molecular beam epitaxy. In particular, folded acoustic modes are presented and compared with two different theoretical models (Rytov and linear chain). We find good agreement between theory and experiments. We estimate, with a simple model, the magnitude of the effect of the strain on the phonon frequency shifts.

Growth and characterization of ultrathin GaP layer in a GaAs matrix by X-ray interference effect

An ultrathin two monolayers thick layer of GaP sandwiched within a GaAs matrix was grown by atomic layer molecular beam epitaxy (ALMBE). The X-ray interference effect (Pendellösung) was used to determine the structural parameters such as thickness, lattice parameter, chemical composition, and strain. Excellent agreement between the experimental rocking curve and the simulation using the dynamical theory of X-ray diffraction was found indicating the high quality of the sample. Analysis of the scans in symmetrical (004) and asymmetrical (224) reflections, sensitive to both perpendicular and parallel strain, shows that the GaP layer is coherent with the substrate, i.e., it is below the critical thickness in agreement with critical thickness theories. Despite the competition for incorporation between arsenic and phosphorus the experimental GaP thickness is found to be identical to the nominal growth value, demonstrating full incorporation of phosphorus when growing by ALMBE. No significant out-diffusion or segregation of P is observed.

Hematotoxicity of magnetite nanoparticles coated with polyethylene glycol: in vitro and in vivo studies

Hematotoxicity of magnetite nanoparticles coated with dimercaptosuccinic acid (DMSA) and polyethylene glycol (PEG) has been evaluated by determining their safety in vitro and in vivo in a rat model up to 30 days after administration of a single dose. The in vitro analysis consists of global plasma coagulation (PT, aPTT, and fibrinogen) and platelet aggregation tests while the hematotoxicity studies in vivo include a complete blood count and the possible genotoxic effect analysis in the bone marrow hematopoietic function. Prolonged aPTT values indicate a higher anticoagulant effect for NP-DMSA compared with PEG-coated nanoparticles as a consequence of the higher surface charge of the former. The in vivo tests showed that these bioferrofluids do not cause genotoxic effects, affect erythropoiesis or increase the number of immature erythrocytes in the bone marrow at the analyzed dose. However, nanoparticle administration showed a significant effect on the leukocyte counts in animals treated with DMSA coated nanoparticles 24 h after injection. This response is not observed in animals treated with PEG modified nanoparticles which justifies the use of this polymer in biomasking strategies.

Structural characterization of gaas/gap superlattices

Powder and double-crystal X-ray diffraction were used to study the structural properties of highly strained (GaAs)N/(GaP)M short-period superlattices grown on GaAs (001) substrates. In spite of the large lattice mismatch (f=3.6%) between GaAs and GaP and the competition for incorporation between As and P, high-quality short-period superlattices of GaAs/GaP have been grown by a development of conventional molecular beam epitaxy named atomic layer molecular beam epitaxy. The in-plane lattice parameter (a/sub ///) of the different superlattices was measured and studied as a function of the GaP content. The authors found that, for a given total superlattice thickness of 4000 AA, the critical lattice mismatch is fc approximately=0.5% (corresponding to an average GaP content of 13.6% in the superlattice). This means that for an average misfit or lattice mismatch above fc the superlattice starts to relax. This experimental result is compared with predictions of critical thickness theories based on energy criteria. A clear relation of the degree of relaxation with peak width of the superlattice zeroth-order diffraction peak is found. High-resolution transmission electron microscopy has been performed to analyse the type of dislocations that relax the mismatched layers.