SPIE OPTO, 2018

Indium-doped ZnO bulk crystals grown by the hydrothermal method are highly-conductive, with resistivity at 0.01 Ωcm at room temperature as revealed by Hall-effect measurement. In this paper we report on structural and optical properties of these crystals. The grown In:ZnO crystals have been studied by high resolution X-ray diffraction, micro-Raman scattering and low-temperature photoluminescence and cathodoluminescence. It was found that the c lattice parameter of the grown In:ZnO crystal expanded 0.06% with respect to the lithium-doped ZnO crystal seed, and the In-doped ZnO overgrew the seed crystal pseudomorphically but with high quality crystallinity; the X-ray rocking curves show the FWHM of the Zn face and O faces are only 0.05o and 0.1o; and the indium concentration in the crystal reaches the solubility limit. Raman spectra show strain relaxation gradually from the regrowth interface as well as a weak spectral feature at 723 cm-1. The peak at 312 cm-1 noticed in hydrothermally grown In:ZnO nanostructures does not appear in our In-doped crystals, indicating that this peak may be associated with specific defects (e.g. surface related) of the nanostructures. Photoluminescence measurements show that an indium donor bound exciton peak I9 (In0X) is the dominant peak in the PL spectrum, located at 3.3586 eV on the zinc face and 3.3577 eV on the oxygen face. Both of them deviated from the consensus literature value of 3.3567 eV, probably due to strain in the crystal induced by impuritie

About the interaction between a laser beam and group IV nanowires: a study of the electromagnetic field enhancement in homogeneous and heterostructured nanowires

The optical properties of semiconductor nanowires (NWs) are object of study because they are the building blocks of the future nanophotonic devices. The high refractive index and its reduced dimension, make them suitable for photon engineering. The study of the interaction between NWs and visible light has revealed resonances of the light absorption/scattering by the NWs. Micro-Raman spectroscopy is used as a characterization method of semiconductor NWs. The relation between the Raman intensity and the incident electromagnetic (EM) field permits to study the light/NW interaction through the micro-Raman spectra of individual NWs. As compared to either metallic or dielectric NWs, the semiconductor NWs add additional tools to modify its interaction with light, for example, the composition, the presence of heterostructures, both axial and radial, doping, and the surface morphology. One presents herein a study of the optical response of group IV semiconductor NWs to visible photons. The study is experimentally carried out through the micro-Raman spectroscopy of different group IV NWs, both homogeneous and heterostructured (SiGe/Si), and the results are analyzed in terms of the EM modeling of the light/NW interaction using finite element methods (FEMs). The heterostructures are seen to produce additional resonances allowing new photonic capacities to the semiconductor NWs.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref VA293U13

Electromagnetic interaction between a laser beam and semiconductor nanowires deposited on different substrates: Raman enhancement in Si Nanowires

Producción CientíficaRaman scattering of Si nanowires (NWs) presents antenna effects. The electromagnetic resonance depends on the electromagnetic coupling of the system laser/NW/substrate. The antenna effect of the Raman signal was measured in individual NWs deposited on different substrates, and also free standing NWs in air. The one phonon Raman band in NWs can reach high intensities depending on the system configuration; values of Raman intensity per unit volume more than a few hundred times with respect to bulk substrate can be obtained.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13

Thermal and mechanical issues of high-power laser diode degradation

A computational model for the evaluation of the thermomechanical effects that give rise to the catastrophic optical damage of laser diodes has been devised. The model traces the progressive deterioration of the device running in continuous wave conditions. The local heating of the active layer locally leads to the onset of the plastic regime. As a result, dislocations and threads of dislocations grow across the active layers and lead to rapidly growing temperatures in the quantum well. The poor power dissipation under these conditions has been identified as the key factor driving the final degradation of the laserSpanish Government (ENE2014-56069-C4-4-R) and Junta de Castilla y León (VA29U13 and VA081U16). J.L. Pura wishes to acknowledge a grant by the FPU program of the Spanish Government (FPU14/00916

Nanoscale effects on the thermal and mechanical properties of AlGaAs/GaAs quantum well laser diodes: influence on the catastrophic optical damage

Producción CientíficaIn this work we study the catastrophic optical damage (COD) of graded-index separate confinement heterostructure quantum well (QW) laser diodes based on AlGaAs/GaAs. The emphasis is placed on the impact that the nanoscale physical properties have on the operation and degradation of the active layers of these devices. When these laser diodes run in continuous-wave mode with high internal optical power densities, the QW and guide layers can experiment very intense local heating phenomena that lead to device failure. A thermomechanical model has been set up to study the mechanism of degradation. This model has been solved by applying finite element methods. A variety of physical factors related to the materials properties, which play a paramount role in the laser degradation process, have been considered. Among these, the reduced thicknesses of the QW and the guides lead to thermal conductivities smaller than the bulk figures, which are further reduced as extended defects develop in these layers. This results in a progressively deteriorating thermal management in the device. To the best of our knowledge, this model for laser diodes is the first one to have taken into account low scale mechanical effects that result in enhanced strengths in the structural layers. Moreover, the consequences of these conflicting size-dependent properties on the thermo-mechanical behaviour on the route to COD are examined. Subsequently, this approach opens the possibility of taking advantage of these properties in order to design robust diode lasers (or other types of power devices) in a controlled manner.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. Project VA293U13 and VA081U16 (003)Ministerio de Economía, Industria y Competitividad (Proyect ENE2014-56069-C4-4-R

Thermomechanical issues of high power laser diode catastrophic optical damage

Catastrophic optical damage (COD) of high power laser diodes is a crucial factor limiting ultra high power lasers. The understanding of the COD process is essential to improve the endurance of the high power laser diodes. COD is observed as a process in which the active part of the laser diode is destroyed, forming characteristic defects, the so called dark line defects (DLDs). The DLDs are formed by arrays of dislocations generated during the laser operation. Local heating associated with non-radiative recombination is assumed to be at the origin of the COD process. A summary of the methods used to assess the COD, both in real time and post-mortem is presented. The main approaches developed in recent years to model the heat transport in the laser structures under non homogeneous temperature distribution are overviewed. Special emphasis is paid to the impact of the low dimensionality of QWs in two physical properties playing a major role in the COD process, namely, thermal conductivity and mechanical strength. A discussion about the impact of the nanoscale in both physical properties is presented. Finally, we summarize the main issues of the thermomechanical modelling of COD. Within this model the COD is launched when the local thermal stresses generated around the heat source overcome the yield stress of the active zone of the laser. The thermal runaway is related to the sharp decrease of the thermal conductivity once the onset of plasticity has been reached in the active zone of the laser.Junta de Castilla y León (Projects VA081U16 and VA283P18)Spanish Government (ENE 2014-56069-C4-4-R, ENE 2017-89561-C4-3-R, FPU programme 14/00916)

About the physical meaning of the critical temperature for catastrophic optical damage in high power quantum well laser diodes

Producción CientíficaIt is usually assumed that the catastrophic optical damage of high power laser diodes is launched when a critical local temperature (Tc) is reached; temperatures ranging from 120ºC to 200ºC were experimentally reported. However, the physical meaning of Tc in the degradation process is still unclear. In this work we show that, in the presence of a local heat source in the active region, the temperature of the laser structure, calculated using finite element methods, is very inhomogeneously distributed among the different layers forming the device. This is due to the impact that the low dimensionality and the thermal boundary resistances have on the thermal transport across the laser structure. When these key factors are explicitly considered, the quantum well (QW) temperature can be several hundred degrees higher than the temperature of the guides and cladding layers. Due to the size of the experimental probes, the measured critical temperature is a weighted average over the QW, guides and claddings. We show the existence of a great difference between the calculated average temperature, equivalent to the experimentally measured temperature, and the peak temperature localized in the QW. A parallel study on double heterostructure lasers is also included for comparison.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13

Una evaluación de la iniciativa pública en las sociedades de garantía recíprocas andaluzas

La Pequeña y Mediana Empresa (PYME) encuentra limitaciones en su financiación: dificultad de acceso al crédito bancario, plazos de amortización cortos, costes elevados respecto a la gran empresa, etc. Para solventar estas restricciones, las Sociedades de Garantía Recíproca (SGR) conceden avales, que permiten a la PYME acceder a la financiación bancaria en unas condiciones más favorables en sus créditos que las que conseguirían por sí solas en el mercado. En esta ponencia, realizamos un análisis tanto de la actividad de las SGR como del fomento público de las mismas. Dichos resultados se enmarcan dentro de un proyecto de investigación más amplio, de título “Evaluación ex-post de la eficiencia y eficacia de las medidas públicas de fomento de sociedades intermedias en la financiación de PYMES”, financiado por la Consejería de Economía y Hacienda de la Junta de AndalucíaSmall and Medium Company finds limitations in its financing: difficulties to access the bank credit, short paying-off terms, high costs in comparison with the great company, etc. In order to avoid such restrictions, the Reciprocal Guarantee Companies (RGC) supply backing, that allow small business have access to the bank financing under some more favourable conditions in their credits that those that would get by themselves in the market. In this report, we carry out an analysis of the activity of RGC and the public support of the same ones. These results are framed inside a wider investigation project, of title “Evaluation ex-post of the efficiency and effectiveness of the measures of public support of intermediate companies in the financing of small business", financed by the Andalusian Government

Spectrally and spatially resolved cathodoluminescence of undoped/Mg-doped GaN core-shell nanowires: a local probe into activation of Mg acceptors in non-polar and semi-polar crystal faces

Producción CientíficaSpectrally and spatially resolved cathodoluminescence (CL) measurements were carried out at 80 K on undoped/Mg-doped GaN core-shell nanowires grown by selective area growth metalorganic vapor phase epitaxy in order to investigate locally the optical activity of the Mg dopants. A study of the luminescence emission distribution over the different regions of the nanowires is presented. We have investigated the CL fingerprints of the Mg incorporation into the non-polar lateral prismatic facets and the semi-polar facets of the pyramidal tips. The amount of Mg incorporation/activation was varied by using several Mg/Ga flow ratios and post-growth annealing treatments. For lower Mg/Ga flow ratios, the annealed nanowires clearly display a donor-acceptor pair band emission peaking at 3.26-3.27 eV and up to 4 LO phonon replicas, which can be considered as a reliable indicator of effective p-type Mg doping in the nanowire shell. For higher Mg/Ga flow ratios, a substantial enhancement of the yellow luminescence emission as well as several emission subbands are observed, which suggests an increase of disorder and the presence of defects as a consequence of the excess Mg doping,Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13

About the impact of the materials properties in the catastrophic degradation of high power GaAs based laser

Producción CientíficaThe catastrophic degradation of high power lasers depends on both external factors, associated with the technological processes followed to fabricate the laser, and also on intrinsic aspects related to the materials forming the laser structure, more specifically the active zone composed by the QW, guide layers and claddings. The materials properties: optical, thermal and mechanical, play a paramount role in the degradation of the laser. We analyse here how these properties have an impact on the mechanisms responsible for the catastrophic degradation.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. Project VA293U13 and VA081U16 (003))Ministerio de Economía, Industria y Competitividad (Proyect ENE2014-56069-C4-4-R