Person: Nogales Díaz, Emilio
Universidad Complutense de Madrid
Faculty / Institute
Física de Materiales
Now showing 1 - 10 of 60
PublicationCharacterization of the blue emission of Tm/Er co-implanted GaN(Materials Research Society, 2006) Nogales Díaz, EmilioComparative studies have been carried out on the cathodoluminescence (CL) and photoluminescence (PL) properties of GaN implanted with Tin and GaN co-implanted with Tin and a low concentration of Er. Room temperature CL spectra were acquired in an electron probe microanalyser to investigate the rare earth emission. The room temperature CL intensity exhibits a strong dependence on the annealing temperature of the implanted samples. The results of CL temperature dependence are reported for blue emission (similar to 477 nm) which is due to intra 4f-shell electron transitions (¹G₄→³ H₆) associated with Tm^(3) ions. The 477 nm blue CL emission is enhanced strongly as the annealing temperature increases up to 1200 ⁰C. Blue PL emission has also been observed from the sample annealed at 1200 ⁰C . To our knowledge, this is the first observation of blue PL emission from Tin implanted GaN samples. Intra- 4f transitions from the ¹D₂ level (similar to 465 nm emission lines) of Tm(3+) ions in GaN have been observed in GaN:Tm films at temperatures between 20-200 K. We will discuss the temperature dependent Tm(3+) emission in both GaN:Tm,Er and GaN:Tm samples. PublicationFailure mechanism of AlN nanocaps used to protect rare earth-implanted GaN during high temperature annealing(American Institute of Physics, 2006-01-16) Nogales Díaz, Emilio; Martin, R. V.; O'Donnell, K. P.; Lorenz, K.; Alves, E.; Ruffenach, S.; Briot, O.The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300 degrees C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1-2 µm which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates. PublicationRaman and cathodoluminescence analysis of transition metal ion implanted Ga₂O₃ nanowires(Elsevier Science BV, 2017-11) Gonzalo, A.; Nogales Díaz, Emilio; Lorenz, K.; Víllora, E. G.; Shimamura, K:; Piqueras de Noriega, Javier; Méndez Martín, BianchiThe structural and luminescence properties of gallium oxide nanowires doped with chromium or manganese have been investigated. Undoped Ga₂O₃ nanostructures have been fabricated by a thermal evaporation method, while doping was subsequently achieved by ion implantation followed by thermal annealing. Scanning electron microscopy (SEM) analysis has shown that this doping process does not alter the morphology of the nanostructures. Ion implantation results in partial amorphization of the crystal lattice, as deduced from Raman spectroscopy studies. Thermal annealing at different temperatures was carried out in order to restore the crystallinity of the nanowires. Raman spectroscopy analysis demonstrates that recrystallization starts at about 700 °C and a complete recrystallization is achieved at. abOut 1000 °C. Cathodoluminescence (CL) analysis has been used to study the emissions in the 300-900 nm range. As-implanted nanowires virtually do not emit any light, which is related to their Poor crystal quality and the implantation induced defects. Thermal annealing results in effective CL emission. In particular, a clear correlation between crystallinity of the nanowires doped with Cr and the emission from the ²E-⁴A₂ and ⁴T₂-⁴A₂ intraionic transitions has been observed. On the other hand, emissions directly related to intraionic transitions of Mn have not been found in the nanowires implanted with this ion. The influence of the implantation process and annealing temperature on the observed changes in the donor -acceptor pairs (DAP) band of Ga₂O₃ is discussed. PublicationSTM-REBIC study of nanocrystalline and crystalline silicon(Materials Research Society, 2003) Nogales Díaz, Emilio; Méndez Martín, Bianchi; Piqueras de Noriega, Javier; Plugaru, R.Electrically active regions of nanocrystalline silicon (nc-Si) films as well as of a p-type crystalline silicon (c-Si) wafer have been investigated by using a scanning electron microscope/scanning tunneling microscope (SEM/STM) combined instrument. The nc-Si films were obtained by boron implantation of amorphous silicon layers with an average nanocrystal size of about 10 nm. STM current constant images reveal a cell structure in the nc-Si films which was also revealed in the STM remote electron beam induced current (REBIC) images with a resolution of up to 20 nm. The contrast in the STM-REBIC images indicate the existence of space charge regions at the boundaries. The influence of the thermal treatment on the cell structure was studied. For comparison, SEM-REBIC and STM-REBIC images from c-Si wafer were obtained. PublicationProbing surface states in C_60 decorated ZnO microwires: detailed photoluminescence and cathodoluminescence investigations(Royal Society Chemistry, 2019-04-01) Rodrigues, Joana; Smazna, Daria; Ben Sedrine, Nabiha; Nogales Díaz, Emilio; Adelung, Rainer; Mishra, Yogendra K; Méndez Martín, Bianchi; Correia, María R.; Monteiro, TeresaZnO microwires synthesised by the flame transport method and decorated with C_60 clusters were studied in detail by photoluminescence (PL) and cathodoluminescence (CL) techniques. The optical investigations suggest that the enhanced near band edge recombination observed in the ZnO/ C_60 composites is attributed to the reduction of the ZnO band tail states in the presence of C_60. Well-resolved free and bound excitons recombination, as well as 3.31 eV emission, are observed with increasing amount of C_60 flooding when compared with the ZnO reference sample. Moreover, a shift of the broad visible emission to lower energies occurs with increasing C_60 content. In fact, this band was found to be composed by two optical centres peaked in the green and orange/ red spectral regions, presenting different lifetimes. The orange/ red band exhibits faster lifetime decay, in addition to a more pronounced shift to lower energies, while the peak position of the green emission only shows a slight change. The overall redshift of the broad visible band is further enhanced by the change in the relative intensity of the mentioned optical centres, depending on the excitation intensity and on the C_60 flooding. These results suggest the possibility of controlling/ tuning the visible emission outcome by increasing the C_60 amount on the ZnO surface due to the surface states present in the semiconductor. An adequate control of such phenomena may have quite beneficial implications when sensing applications are envisaged. PublicationLuminescence from erbium oxide grown on silicon(Materials Research Society, 2002) Nogales Díaz, Emilio; Méndez Martín, Bianchi; Piqueras de Noriega, Javier; Plugaru, R; García, J. A.; Tate, T. J.The luminescence properties of erbium oxide grown on crystalline and amorphous silicon substrates were studied by means of photo- and cathodoluminescence techniques. Differences in the luminescence spectra for samples grown on the two type., of substrates used are explained in terms of the different types of erbium centers formed by taking into account the substrate properties and the thermal treatments during growth. For comparison, erbium implanted and oxygen coimplanted crystalline and amorphous silicon have been also investigated by luminescence techniques. In the implanted samples, the sharp transitions from erbium ions in the visible range were quenched and the main emission corresponds to the intraionic transitions in Er3+ ions in the infrared range peaked at 1,54 mum. PublicationResonant cavity modes in gallium oxide microwires(Amer Inst Physics, 2012-06-25) Inaki, Lopez; Nogales Díaz, Emilio; Méndez Martín, BianchiFabry Perot resonant modes in the optical range 660-770 nm have been detected from single and coupled Cr doped gallium oxide microwires at room temperature. The luminescence is due to chromium ions and dominated by the broad band involving the T-4(2)-(4)A(2) transition, strongly coupled to phonons, which could be of interest in tunable lasers. The confinement of the emitted photons leads to resonant modes detected at both ends of the wires. The separation wavelength between maxima follows the Fabry-Perot dependence on the wire length and the group refractive index for the Ga2O3 microwires. PublicationShape engineering driven by selective growth of SnO_2 on doped Ga_2O_3 nanowires(Amer Chemical Soc, 2017-01-11) Alonso Orts, Manuel; Sanchez, Ana M; Hindmarsh, Steven A; López, Iñaki; Nogales Díaz, Emilio; Piqueras de Noriega, Javier; Méndez Martín, BianchiTailoring the shape of complex nanostructures requires control of the growth process. In this work, we report on the selective growth of nanostructured tin oxide on gallium oxide nanowires leading to the formation of SnO_2/Ga_2O_3 complex nanostructures. Ga_2O_3 nanowires decorated with either crossing SnO_2 nanowires or SnO_2 particles have been obtained in a single step treatment by thermal evaporation. The reason for this dual behavior is related to the growth direction of trunk Ga_2O_3 nanowires. Ga_2O_3 nanowires grown along the  direction favor the formation of crossing SnO_2 nanowires. Alternatively, SnO_2 forms rhombohedral particles on  Ga_2O_3 nanowires leading to skewer-like structures. These complex oxide structures were grown by a catalyst-free vapor-solid process. When pure Ga and tin oxide were used as source materials and compacted powders of Ga_2O_3 acted as substrates,  Ga_2O_3 nanowires grow preferentially. High-resolution transmission electron. microscopy analysis reveals epitaxial relationship lattice matching between the Ga_2O_3 axis and SnO_2 particles, forming skewer-like structures. The addition of chromium oxide to the source materials modifies the growth direction of the trunk Ga_2O_3 nanowires, growing along the , with crossing SnO2 wires. The SnO_2/Ga_2O_3 junctions does not meet the lattice matching condition, forming a grain boundary. The electronic and optical properties have been studied by XPS and CL with high spatial resolution, enabling us to get both local chemical and electronic information on the surface in both type of structures. The results will allow tuning optical and electronic properties of oxide complex nanostructures locally as a function of the orientation. In particular, we report a dependence of the visible CL emission of Sn_O_2 on its particular shape. Orange emission dominates in SnO_2/Ga_2O_3 crossing wires, while green-blue emission is Observed in SnO_2 particles attached to Ga_2O_3 trunks. The results show that the Ga_2O_3-SnO_2 system appears to be a benchmark for shape engineering to get architectures involving nanowires via the control of the growth direction of the nanowires. Publication3D and 2D growth of SnO₂ nanostructures on Ga₂O₃ nanowires: synthesis and structural characterization(RSC Royal Society of Chemistry, 2017-11-07) Alonso Orts, Manuel; Sánchez, A. M.; López, I.; Nogales Díaz, Emilio; Piqueras de Noriega, Javier; Méndez Martín, BianchiIn this work, a simple thermal evaporation method has been used to obtain a variety of Ga₂O₃/SnO₂ nano-assemblies with different shapes and dimensionalities, which may affect their physical properties, especially those influenced by surface properties. The obtained nanostructures have been characterized using electron microscopy-related techniques in order to understand their growth mechanisms. By using both metallic gallium and tin oxide powders as precursors, Ga₂O₃ nanowires (straight or branched) decorated with SnO₂ nanoparticles or SnO₂ quasi-two dimensional plates have been produced after dynamic thermal annealing for 2.5, 8.0 and 15.0 hours. For shorter treatments, accumulation of Sn atoms at the Ga₂O₃ nanowire surface or defect planes has been observed by high resolution TEM, which suggests that they could act as nucleation sites for the further growth of SnO₂. On the other hand, longer treatments promote the formation of Ga-doped SnO2 belts, from which SnO₂ nanowires eventually emerge. High-resolution TEM imaging and microanalysis reveal that Ga accumulation at (200) SnO₂ planes could stabilize some non-stoichiometric or intermediate tin oxide phases, such as Sn₂O₃, at local areas in the belts. The presence of non-stoichiometric tin oxide is relevant in applications, since surface states affect the physical-chemical behavior of tin oxide. PublicationNear-UV optical-cavities n Ga_2O_3 nanowires(Optical Society of America, 2021-01-15) Alonso Orts, Manuel; Chilla, Gerwin; Hötzel, Rudolfo; Nogales Díaz, Emilio; San Juan, José M.; Nó, Maria L.; Eickhoff, Martin; Méndez Martín, BianchiIn this Letter, we report optical confinement in the near-ultraviolet (near-UV) range in Ga2O3 nanowires (NWs) by distributed Bragg reflector (DBR) nanopatterned cavities. High-contrast DBRs, which act as the end mirrors of the cavities of the desired length, are designed and fabricated by focused ion beam etching. The resonant modes of the cavities are analyzed by micro-photoluminescence measurements, analytical models, and simulations, which show very good agreement between each other. Experimental reflectivities up to 50% are obtained over the 350-410 nm region for the resonances in this wavelength range. Therefore, Ga2O3 NW optical cavities are shown as good candidates for single-material-based near-UV light emitters.