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
17 results
Search Results
Now showing 1 - 10 of 17
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 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 Cathodoluminescence microscopy and spectroscopy of superconducting Bi_2Sr_2CaCu_2O_(8+x) single crystals(Physica C, 1997) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, JavierCathodoluminescence (CL) microscopy and spectroscopy in the scanning electron microscope have been applied to the microcharacterization of superconducting Bi2Sr2CaCu2O8+x single crystals. CL spectra representative of the samples at macroscopic level, exhibit at low temperature two well resolved emission bands, centered at about 2.90 and 2.35 eV respectively, related to electronic processes in the oxygen sublattice. Panchromatic CL images reveal that regions with enhanced intensity usually correspond with growth layers or other topographic features of the crystals. CL spectra from the mentioned regions show a relative increase of the local oxygen deficiency related 2.3-2.4 eV emission band. The complex character of this band is revealed by CL spectroscopy, and by CL monochromatic images that relate certain peaks observed in the spectra to specific luminescent areas. Comparison of the present observations with previous luminescence studies of other high temperature superconductors, indicates that a possible origin of the 2.3-2.4 eV CL emission band lies in the Cu-O planes, common to these materials.Item Deep-level cathodoluminescence in deformed CdTe crystals(Physica Status Solidi A-Applied Research, 1995) Díaz-Guerra Viejo, Carlos; Pal, U.; Fernández Sánchez, Paloma; Piqueras de Noriega, JavierThe effect of indentation on the cathodoluminescence (CL) of CdTe in the scanning electron microscope is investigated. Deformation produces deep level emission changes, in particular, a relative intensity increase of the 1.4 and 1.1 eV bands, explained by vacancy generation. Deep level centers are not found to be related to the CL image contrast changes caused by annealing.Item Correlative SEM/STM study of local electronic properties in compound semiconductors(Solid State Phenomena, 1998) Piqueras de Noriega, Javier; Panin, G.N.; Díaz-Guerra Viejo, Carlos; Hidalgo Alcalde, Pedro; Méndez Martín, BianchiThe use of combined SEM/STM instruments enables to position the tunnel tip in the region of the sample to be investigated by STM or/and by scanning tunneling spectroscopy (STS). In this work a STM has been implemented in the chamber of a SEM provided with EBIC and CL detection systems. With this arrangement, correlative STM and SEM-EBIC or SEM-CL measurements of local electronic properties have been performed in different compound semiconductors as CdTe, CdxHg1-xTe, GaSb or ZnO. In particular, the electronic inhomogeneities of the sample were analyzed with nm resolution by current imaging tunneling spectroscopy (CITS) or using STM-REBIC contacts configuration, in areas with different recombination properties as imaged by the SEM beam injection techniques. CITS revealed local variations of the surface band gap in the defect or precipitate-rich areas, observed by EBIC and CL, and appears as a complementary technique to study local electronic properties at a finer scale than the SEM-based techniques.Item Cathodoluminescence microscopy of superconducting and non-superconducting Tl_2Ba_2CuO_(6+ δ) polycrystals(Physica C, 1996) Díaz-Guerra Viejo, Carlos; Piqueras de Noriega, Javier; Opagiste, C.Luminescence properties of Tl_2Ba_2CuO_(6+ δ) samples with different superconducting transition temperatures have been studied by cathodoluminescence (CL) in the scanning electron microscope. Independent of the crystal structure, tetragonal or orthorhombic, the Cl spectra show two resolved bands at about 430 nm (2.9 eV) and 540 nm (2.3 eV), respectively. The results show that the 2.3 eV emission is related to oxygen content and is more intense in low-oxygen content superconducting samples. It is proposed that this band is related to a complex center involving oxygen vacancies. A comparison of spectra from Tl_2Ba_2CuO_(6+ δ) and from precursors used in the synthesis (CuO_x, Tl_2Ba_2O_5) has been carried out.