Rafael Mata

Electronic structure, optical properties, and lattice dynamics in atomically thin indium selenide flakes

The progressive stacking of chalcogenide single layers gives rise to two-dimensional semiconducting materials with tunable properties that can be exploited for new field-effect transistors and photonic devices. Yet the properties of some members of the chalcogenide family remain unexplored. Indium selenide (InSe) is attractive for applications due to its direct bandgap in the near infrared, controllable p- and n-type doping and high chemical stability. Here, we reveal the lattice dynamics, optical and electronic properties of atomically thin InSe flakes prepared by micromechanical cleavage. Raman active modes stiffen or soften in the flakes depending on which electronic bonds are excited. A progressive blue-shift of the photoluminescence peaks is observed for decreasing flake thickness (as large as 0.2 eV for three single layers). First-principles calculations predict an even larger increase in the bandgap, 0.40 eV, for three single layers, and as much as 1.1 eV for a single layer. These results are promising from the point of view of the versatility of this material for optoelectronic applications at the nanometer scale and compatible with Si and III-V technologies.

The structural properties of GaN/AlN core–shell nanocolumn heterostructures

The growth and structural properties of GaN/AlN core-shell nanowire heterostructures have been studied using a combination of resonant x-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy experiments. For a GaN core of 20 nm diameter on average surrounded by a homogeneous AlN shell, the built-in strain in GaN is found to agree with theoretical calculations performed using a valence force field model. It is then concluded that for an AlN thickness up to at least 12 nm both core and shell are in elastic equilibrium. However, in the case of an inhomogeneous growth of the AlN shell caused by the presence of steps on the sides of the GaN core, plastic relaxation is found to occur. Consistent with the presence of dislocations at the GaN/AlN interface, it is proposed that this plastic relaxation, especially efficient for AlN shell thickness above 3 nm, is promoted by the shear strain induced by the AlN inhomogeneity.

Fine optical spectroscopy of the 3.45 eV emission line in GaN nanowires

GaN nanowires grown by plasma-assisted molecular beam epitaxy are of excellent optical quality, their optical signature being characteristic of homogeneous strain-free GaN. There are however discrepancies between the low temperature luminescence spectra of GaN thin films and nanowires, in particular, a strong emission line around 3.45 eV in nanowires is not found with such a large intensity in thin film GaN. The origin of this emission line in nanowires is still debated; in this article, we shed new light on this debate notably by polarization-resolved luminescence and magneto-luminescence experiments. Our findings demonstrate, in particular, that this line cannot be attributed to a two-electron satellite of the donor bound exciton transition.

The structural properties of GaN insertions in GaN/AlN nanocolumn heterostructures

The strain state of 1 and 2.5 nm thick GaN insertions in GaN/AlN nanocolumn heterostructures has been studied by means of a combination of high resolution transmission electron microscopy, Raman spectroscopy and theoretical modeling. It is found that 2.5 nm thick GaN insertions are partially relaxed, which has been attributed to the presence of dislocations in the external AlN capping layer, in close relationship with the morphology of GaN insertions and with the AlN capping mechanism. The observed plastic relaxation in AlN is consistent with the small critical thickness expected for GaN/AlN radial heterostructures.

Analysis of Multipactor RF Breakdown in a Waveguide Containing a Transversely Magnetized Ferrite

In this paper, the multipactor RF breakdown in a parallel-plate waveguide partially filled with a ferrite slab magnetized normal to the metallic plates is studied. An external magnetic field is applied along the vertical direction between the plates in order to magnetize the ferrite. Numerical simulations using an in-house 3-D code are carried out to obtain the multipactor RF voltage threshold in this kind of structures. The presented results show that the multipactor RF voltage threshold at certain frequencies becomes considerably lower than for the corresponding classical metallic parallel-plate waveguide with the same vacuum gap.

Secondary Electron Emission of Pt: Experimental Study and Comparison With Models in the Multipactor Energy Range

Experimental data of secondary emission yield (SEY) and electron emission spectra of Pt under electron irradiation for normal incidence and primary energies lower than 1 keV are presented. Several relevant magnitudes, as total SEY, elastic backscattering probability, secondary emission spectrum, and backscattering coefficient, are given for different primary energies. These magnitudes are compared with theoretical or semiempirical formulas commonly used in the related literature.

submitter : Study of the Secondary Electron Yield in Dielectrics Using Equivalent Circuital Models

Secondary electron emission has an important role on the triggering of the multipactor effect; therefore, its study and characterization are essential in radio-frequency waveguide applications. In this paper, we propose a theoretical model, based on equivalent circuit models, to properly understand charging and discharging processes that occur in dielectric samples under electron irradiation for secondary electron emission characterization. Experimental results obtained for Pt, Si, GaS, and Teflon samples are presented to verify the accuracy of the proposed model. Good agreement between theory and experiments has been found.

Novel multipactor studies in RF satellite payloads: Single-carrier digital modulated signals and ferrite materials

In this work it is reviewed the most novel advances in the multipactor RF breakdown risk assessment devoted to RF satellite microwave passive devices employed in space telecommunication systems. On one side, it is studied the effect of transmitting a single-carrier digital modulated signal in the multipactor RF voltage threshold in a coaxial line. On the other hand, an analysis of the multipactor phenomenon in a parallel-plate waveguide containing a magnetized ferrite slab it is presented.

Atmospheric Attenuation and Scintillation Effects on the Range of EDM Instruments

AbstractThis investigation aimed to study the range of various electronic distance measurement (EDM) instruments used in survey engineering, assuming different weather conditions and different terrain altitudes, by controlling every parameter influencing the measurements and contrasting the results with the information provided by the manufacturers. The first step consisted of determining the EDM real optical wavelength to be used for control against the manufacturers’ provided values. Consequently, a spectroscopy test of the lasers installed in every EDM instrument was carried out at the ESA-VSC [European Space Agency (ESA) and Val Space Consortium (VSC)] laboratory. The second step was to study the total measurement range of each instrument in different weather conditions and at different altitudes. Three experimental tests were carried out at three different locations in eastern Spain, separated by distances of less than 90 km, and altitudes ranging from sea level to about 2,000 m. Owing to the influen...