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Featured researches published by D. Comedi.


Journal of Applied Physics | 2006

X-ray-diffraction study of crystalline Si nanocluster formation in annealed silicon-rich silicon oxides

D. Comedi; O. H. Y. Zalloum; E. Irving; J. Wojcik; Tyler Roschuk; M. Flynn; Peter Mascher

The formation and subsequent growth of crystalline silicon nanoclusters (Si-ncs) in annealed silicon-rich silicon oxides (SRSOs) were studied by glancing angle x-ray diffraction. SRSO samples with Si concentrations (y) of 0.40, 0.42, and 0.45 were grown by inductively coupled plasma-enhanced chemical-vapor deposition (PECVD). Samples with y=0.42 grown by electron-cyclotron-resonance PECVD were also studied. Annealing treatments were performed at temperatures (T) of 900, 1000, and 1100 °C for times (t) between 0.5 and 3 h in flowing Ar. As-grown SRSO films did not present signs of Si clusters (amorphous or crystalline); however, (111), (220), and (311) Bragg peaks corresponding to c‐Si were clearly seen after annealing at 900 °C for the y=0.45 sample, but only barely seen for the y=0.42 and undetected for the y=0.40 samples. For T=1000°C, all studied SRSO samples clearly showed the c‐Si diffraction peaks, which became narrower with increasing t and T. From the width of the Si (111) peaks, the mean size of ...


Nano Letters | 2014

High-Efficiency Second Harmonic Generation from a Single Hybrid ZnO Nanowire/Au Plasmonic Nano-Oligomer

Gustavo Grinblat; Mohsen Rahmani; Emiliano Cortés; Martín Caldarola; D. Comedi; Stefan A. Maier; Andrea V. Bragas

We introduce a plasmonic-semiconductor hybrid nanosystem, consisting of a ZnO nanowire coupled to a gold pentamer oligomer by crossing the hot-spot. It is demonstrated that the hybrid system exhibits a second harmonic (SH) conversion efficiency of ∼3 × 10(-5)%, which is among the highest values for a nanoscale object at optical frequencies reported so far. The SH intensity was found to be ∼1700 times larger than that from the same nanowire excited outside the hot-spot. Placing high nonlinear susceptibility materials precisely in plasmonic confined-field regions to enhance SH generation opens new perspectives for highly efficient light frequency up-conversion on the nanoscale.


Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 1992

Pulse height response of Si surface barrier detectors to 5–70 MeV heavy ions

D. Comedi; J.A. Davies

Abstract An extensive series of pulse height measurements have been performed in partially depleted Si surface barrier detectors, using various heavy ions (Li, B, C, O, Al and Cl), at energies between 5 and 70 MeV. After correcting for the small energy loss of the incident ions in traversing the gold surface barrier layer of the detector and for the residual nuclear stopping, the resulting pulse heights per MeV for the various heavy ions were found to be up to 2.5% larger than for the 241Am (5.486 MeV) alpha particle. This increase, although significant, is smaller than had been anticipated from an extrapolation of the earlier study of H, He and Li pulse heights by Lennard et al. [Nucl. Instr. and Meth. A248 (1986) 454, and B24/25 (1987) 1035.] A new method of analysis of pulse height data, which significantly reduces the uncertainties associated with the dead layer energy loss and nuclear stopping corrections, was used in order to determine directly the variation of the average energy for electron-hole pair creation with increasing particle stopping power in Si. The dependence of the detector energy resolution (FWHM) on the energy and atomic number of the particle has also been measured in order to provide a suitable data base for selecting the optimum ion beams for various backscattering analysis problems.


Journal of Applied Physics | 2012

Electrical transport and optical model of GaAs-AlInP core-shell nanowires

A. C. E. Chia; Mónica Tirado; Y. Li; S. Zhao; Z. Mi; D. Comedi; R. R. LaPierre

GaAs nanowires were passivated by AlInP shells grown by the Au-assisted vapor-liquid-solid method in a gas source molecular beam epitaxy system. Transmission electron microscopy confirmed a core-shell GaAs-AlInP structure. Current-voltage measurements on ensemble nanowires indicated improved carrier transport properties in the passivated nanowires as compared to their unpassivated counterpart. Similarly, individual nanowires showed improved photoluminescence intensity upon passivation. A detailed model is presented to quantify the observed improvements in nanowire conduction and luminescence in terms of a reduction in surface charge trap density and surface recombination velocity upon passivation. The model includes the effects of high-level injection, bulk recombination, and surface recombination. The model can be used as a tool for assessing various passivation methods.


Applied Physics Letters | 2005

H-sensitive radiative recombination path in Si nanoclusters embedded in SiO2

D. Comedi; O. H. Y. Zalloum; Peter Mascher

The room-temperature photoluminescence (PL) from silicon nanocrystals embedded in a SiO2 matrix fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and subsequent annealing in Ar and (Ar+5%H2) was studied. In addition to strong increases of the integrated PL intensity (factors of ∼4 to 10), the selective enhancement of contributions to the PL spectra at long wavelengths was observed for (Ar+5%H2) annealings. The selective H passivation of Si dangling bonds in disordered Si nanoclusters where radiative recombination proceeds through disorder-induced shallow states is proposed as a possible explanation for the observed effects.


Nanotechnology | 2012

ZnO nanowire co-growth on SiO2 and C by carbothermal reduction and vapour advection

N C Vega; R Wallar; J Caram; G Grinblat; Mónica Tirado; R. R. LaPierre; D. Comedi

Vertically aligned ZnO nanowires (NWs) were grown on Au-nanocluster-seeded amorphous SiO(2) films by the advective transport and deposition of Zn vapours obtained from the carbothermal reaction of graphite and ZnO powders. Both the NW volume and visible-to-UV photoluminescence ratio were found to be strong functions of, and hence could be tailored by, the (ZnO+C) source-SiO(2) substrate distance. We observe C flakes on the ZnO NWs/SiO(2) substrates which exhibit short NWs that developed on both sides. The SiO(2) and C substrates/NW interfaces were studied in detail to determine growth mechanisms. NWs on Au-seeded SiO(2) were promoted by a rough ZnO seed layer whose formation was catalysed by the Au clusters. In contrast, NWs grew without any seed on C. A correlation comprising three orders of magnitude between the visible-to-UV photoluminescence intensity ratio and the NW volume is found, which results from a characteristic Zn partial pressure profile that fixes both O deficiency defect concentration and growth rate.


Nanotechnology | 2010

Electrical characteristics of core–shell p–n GaAs nanowire structures with Te as the n-dopant

J Caram; C Sandoval; Mónica Tirado; D. Comedi; J Czaban; D A Thompson; R. R. LaPierre

GaAs nanowire (NW)-based p-n photovoltaic devices, with two distinct p and n spatial distributions and where Te was the n-dopant, have been studied by impedance spectroscopy in the 10(3)-10(7) Hz frequency range and the - 1.5-1.5 V bias range. For a large n-core/p-shell overlap region within NWs in a coaxial geometry, the p-n junction properties (DC rectification and p-n depletion capacitance) are found to prevail. The impedance data at low bias for both NW devices show large frequency dispersions with relaxation frequencies that are compatible with carrier re-emission times from traps due to GaAs surface states. An increasing conductance with increasing frequency for low bias is observed, suggesting hopping transport through localized states. For large bias the conductance increases exponentially with bias and is frequency independent, indicating conduction through extended states in this regime.


Applied Physics Letters | 2012

Hierarchical ZnO nanostructures: Growth mechanisms and surface correlated photoluminescence

Gustavo Grinblat; M.G. Capeluto; Mónica Tirado; Andrea V. Bragas; D. Comedi

ZnO nanowires were grown by vapor-transport and deposition on Au nanocluster covered fused and thermal silica and c-Si. The nanowire size and density depended strongly on the substrate type. By decreasing the O2 to local Zn partial pressure ratio, the growth pattern changed to nanocombs and nanosheets. ZnO nanohedgehogs were found on bare c-Si. We observe a remarkable correlation between the defect to exciton photoluminescence intensity ratio and the nanostructures specific surface areas. These results indicate that changes in strain and O deficiency defects at surfaces are behind the observed morphology changes, one to two-dimensional growth transition, and corresponding luminescence.


IEEE Journal of Selected Topics in Quantum Electronics | 2006

Light Emission From Hydrogenated and Unhydrogenated Si-Nanocrystal/Si Dioxide Composites Based on PECVD-Grown Si-Rich Si Oxide Films

D. Comedi; O. H. Y. Zalloum; J. Wojcik; Peter Mascher

Hydrogenated and unhydrogenated Si-nanocrystal/Si dioxide (Si-nc/SiO2) composites were obtained from SiyO 1-y (y=0.36,0.42) thin films deposited by plasma-enhanced chemical vapor deposition. The unhydrogenated composites were fabricated by promoting the Si precipitation through the thermal annealing of the films in the flowing pure Ar at temperatures up to 1100degC. The hydrogenated composites were obtained from identical films by replacing the Ar with (Ar+5% H2) in the annealing step. The photoluminescence (PL) of the composites was studied as a function of the annealing temperature (T), annealing time, and pump laser power. The PL intensity increases with increasing annealing temperature and time; however, it increases faster and attains several hundreds percent larger values when the annealing is performed under (Ar+5% H2) as compared to the annealing under pure Ar. Fourier-transform infrared spectra show that H in these hydrogenated samples incorporates mainly as Si-H bonds. The dependence of the PL spectra on y,T, and laser power are consistent with the assumption that light emission in both the hydrogenated and unhydrogenated Si-nc/SiO2 composites originates from the bandgap transitions involving the electron quantum confinement in the Si-ncs. The PL spectra from the hydrogenated films are skewed to the red as compared to those from the unhydrogenated ones. The bulk of the data indicates that H passivates the nonradiative recombination centers, most probably Si dangling bonds in disordered Si-nc/SiO2 regions, thus increasing the number of Si-ncs that contribute to the PL and modifying the distribution of the emission wavelengths


Applied Physics Letters | 2014

Luminescence and electrical properties of single ZnO/MgO core/shell nanowires

Gustavo Grinblat; Francis Bern; J. Barzola-Quiquia; Mónica Tirado; D. Comedi; P. Esquinazi

To neutralise the influence of the surface of ZnO nanowires for photonics and optoelectronic applications, we have covered them with insulating MgO film and individually contacted them for electrical characterisation. We show that such a metal-insulator-semiconductor-type nanodevice exhibits a high diode ideality factor of 3.4 below 1 V. MgO shell passivates ZnO surface states and provides confining barriers to electrons and holes within the ZnO core, favouring excitonic ultraviolet radiative recombination, while suppressing defect-related luminescence in the visible and improving electrical conductivity. The results indicate the potential use of ZnO/MgO nanowires as a convenient building block for nano-optoelectronic devices.

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Mónica Tirado

National Scientific and Technical Research Council

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I. Chambouleyron

State University of Campinas

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Oscar Marin

National Scientific and Technical Research Council

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R.R. Koropecki

National Scientific and Technical Research Council

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Yu. A. Pusep

University of São Paulo

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M. Villafuerte

National Scientific and Technical Research Council

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N C Vega

National Scientific and Technical Research Council

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