Person: Díaz-Guerra Viejo, Carlos
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First Name
Carlos
Last Name
Díaz-Guerra Viejo
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Department
Física de Materiales
Area
Física Aplicada
Identifiers
69 results
Search Results
Now showing 1 - 10 of 69
Item Anomalies in the cathodoluminescence of the antiferromagnetic oxides NiO and CoO(Solid State Communications, 1997) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, JavierCathodoluminescence (CL) of the antiferromagnetic oxides NiO and CoO has been investigated. Both compounds show luminescence bands in the visible and near infrared spectral ranges, tentatively attributed to intra-ionic transitions. Dependence of the visible CL intensity with temperature presents anomalies at about half of the Neel temperature, which are explained by the existence of magnetic ordering effects. The results are briefly compared with previous reports on the CL from YBa_2Cu-3O-(7-x).Item Electrical characterization of ZnO ceramics by scanning tunneling spectroscopy and beam-induced current in the scanning tunneling microscope(Journal of Applied Physics, 1999) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, JavierA correlative study of the electrically active grain boundary structure of ZnO polycrystals has been carried out using a scanning electron microscope/scanning tunneling microscope (SEM/STM) combined instrument. Current imaging tunneling spectroscopy (CITS) measurements reveal reduced surface band gaps, as compared with grain interiors, at the charged boundaries imaged by SEM-based remote electron beam induced current (REBIC). ZnO grain boundaries were also imaged in the STM-REBIC mode with a resolution of up to 20 nm. The contrast differences observed in the SEM-REBIC and STM-REBIC images are discussed in terms of the different experimental conditions used in both techniques.Item Defect assessment of Mg-doped GaN by beam injection techniques(Journal of Applied Physics, 2003) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, Javier; Castaldini, A.; Cavallini, A.; Polent, L.The electronic recombination properties of Mg-doped GaN have been investigated by steady state and time-resolved cathodoluminescence (TRCL) in the scanning electron microscope, photocurrent (PC) spectroscopy, and optical beam induced current (OBIC). CL and OBIC maps reveal an inhomogeneous recombination activity in the investigated material. Deep levels giving rise to level-to-band transitions were detected by PC spectroscopy. A large PC quenching observed upon illumination with light of (2.65-2.85) eV is tentatively attributed to metastable traps within the band gap. CL spectra reveal the existence of emission bands centered at 85 K at 3.29, 3.20, 3.15, and 3.01 eV, respectively. Both time-resolved and steady-state CL measurements carried out under different excitation conditions indicate that the 3.15 and 3.01 eV emissions are likely related to donor-acceptor pair transitions. TRCL measurements also reveal different recombination kinetics for these bands and suggest that deep donors are involved in the mechanism responsible for the 3.01 eV emission.Item Study of charged defects in CdTe and CdHgTe by scanning electron and tunneling microscopy techniques(Izvestiya Akademii Nauk Seriya Fizicheskaya, 1998) Panin, G. N.; Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, JavierItem Cathodoluminescence microscopy and secondary electron emission in mechanically polished and electron irradiated YBa_2Cu_3O_(7-x) ceramics(Physica Status Solidi A-Applied Research, 1996) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, Javier; Tomashpolsky, Y. Y.; Sadovskaya, N. V.Cathodoluminescence (CL) and secondary electron emission (SEE) in the scanning electron microscope have been used to study the effects of mechanical polishing and electron irradiation on YBa2Cu3O7-x ceramics. CL spectra from the mechanically polished superconductors show an enhanced 2.4 eV luminescence band, which is related to oxygen content redistribution or depletion. Electron irradiation induces strong morphological and compositional changes which are proposed to be connected with melting-resolidification processes and surface phase segregation. SEE yield measurements allow to detect an oxygen depleted area with stoichiometric cationic composition surrounding the irradiated region. CL spectra recorded from this oxygen-deficient area also show an enhanced 2.3 to 2.4 eV emission band in comparison with the spectra recorded from the non-irradiated material. Strongly luminescent non-superconducting phases appear in the irradiated regions, showing intense 3.1 and 0.78 to 0.81 eV emission bands.Item Electron beam induced structural changes in bi_2Sr_2CaCu_2O_(8+x) studied by cathodoluminescence microscopy and secondary electron emission(Applied Physics A-Materials Science&Processing, 1997) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, Javier; Tomashpolsky, Y. Y.; Sadovskaya, N. V.Bi_2Sr_2CaCu_2O_(8+x) superconducting ceramics have been irradiated in the scanning electron microscope (SEM), and the irradiation-induced effects investigated by cathodoluminescence (CL), secondary electron emission (SEE) and X-ray microanalysis. Electron beam irradiation causes a slight Bi depletion and an inhomogeneous Ca distribution. A higher CL intensity emission is found in the irradiated areas, where besides an enhancement of the oxygen content related 2.4 eV band, another CL bands probably related to new non-superconducting phases induced by irradiation can be observed. SEE yield measurements allow to detect an oxygen depleted region surrounding the irradiated areas. X-ray microanalysis shows that this intermediate region retains the cationic composition of the unaffected material. CL spectra from bright zones inside the same area also show a dominant 2.4 eV emission band, which supports its relation with oxygen deficiency or rearrangement in high-T-c superconductors.Item Electron beam induced compositional and structural changes in Tl_2Ba_2CuO_(6+σ)(Superconductor Science & Technology, 1996) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, Javier; Tomashpolsky, Y. Y.; Sadovskaya, N. V.; Opagiste, C.Tl_2Ba-2CuO_(6+σ) ceramics have been irradiated in a scanning electron microscope. The resultant changes in the cationic stoichiometry and oxygen content of the samples were investigated by cathodoluminescence (CL), secondary electron emission (SEE) and EDX microanalysis. ii depletion and Ba enrichment are observed in the irradiated areas. CL spectroscopy shows that irradiation causes the increase of a 540 nm emission band attributed to complex centres involving oxygen vacancies. Other features of the CL spectra strongly suggest the presence of radiation induced new phases. The SEE yield, Delta, has been found to depend on the oxygen content of the samples This dependence can be explained on the basis of two charge carrier subsystems and their mutual interaction, Measurements of ∆ allow us to detect an oxygen deficient region, sometimes extending up to a distance of about 1000 mu m, that surrounds the irradiated material.Item Technical developments and principal results of vertical feeding method for GaSb and GaInSb alloys(Thermophotovoltaic Generation of Electricity, Thermophotovoltaic Generation of Electricity, 2007) Vincent López, José Luis; Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, Javier; Dieguez, E.In this work, the experimental set-up of the Vertical Feeding Method (VFM) which has been used for the preparation of GaSb and GaInSb materials is presented. The different elements of the set-up are detailed as well as the preparation process. The different configurations that have been used for material production are briefly described and the principal results are summarized. In the case of Te-doped GaSb materials, the study, focused on the grain size and structure of the as-grown materials, showed that single crystals can be obtained with the VFM. In the case of GaInSb materials, the study, focused on the spatial indium distribution in the solid phase prepared with the different VFM configurations, showed that an effective band edge reduction can be achieved.Item Luminescence from indented Te-doped GaSb crystals(Semiconductor Science and Technology, 2004) Chioncel, M. F.; Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, JavierCathodoluminescence in the scanning electron microscope has been used to investigate the effect of plastic deformation, produced by indentation, in Te-doped GaSb crystals. Deformation has been found to cause a strong quenching of the luminescence emission as well as spectral changes. In particular, the decrease of the near band edge emission is accompanied by the relative increase of the defect band A and of the 730 meV band related to the VGaGaSbTesb centres. Annealing treatments up to 600 degreesC lead to partial recovery of the deformation induced defects, and reveal the existence of Te out-diffusion processes from the deformed areas.Item Cathodoluminescence microscopy and spectroscopy of n-type 4H-SiC epilayers(European Physical Journal-Applied Physics, 2004) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, JavierCathodoluminescence (CL) microscopy and spectroscopy have been used to investigate the nature and spatial distribution of defects and impurities in n-type epitaxial 4H-SiC. CL microscopy reveals the existence of 6H-SiC polytype inclusions, while CL spectra recorded at different excitation conditions show luminescence emission related to deep levels in the SiC epilayer. Deconvolution of the mentioned spectra indicates the complex character of the deep-level CL emission, that is actually composed of four bands centred near 2.72, 2.56, 2.42 and 2.00 eV at 88 K. The origin of these bands is discussed considering previous deep level transient spectroscopy measurements carried out in the same material investigated in the present work.