Publications (35)47.36 Total impact
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Article: Plasma diagnostics and device properties of AlGaN/GaN HEMT passivated with SiN deposited by plasma-enhanced chemical vapour deposition
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ABSTRACT: Key words: High Electron Mobility Transistors (HEMT), AlGaN/GaN, silicon nitride (SiN) thin films, SiH 4 -NH 3 plasmas, plasma diagnostics, plasma enhanced chemical vapour deposition (PECVD). Abstract In this work, silicon nitride thin films have been deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) on both silicon samples and AlGaN/GaN High Electron Mobility Transistors (HEMT) grown on sapphire substrates. Commercial parallel-plate RF plasma equipment has been used. During depositions, the dissociation rates of SiH 4 and NH 3 precursors and the formation of H 2 and N 2 have been analysed by mass spectrometry as a function of the NH 3 /SiH 4 flow ratio and the RF power applied to the plasma reactor. Afterwards, the properties of the films and the HEMT electrical characteristics have been studied. Plasma composition has been correlated with the SiN deposition rate, refractive index, H content and the final electric characteristics of the passivated transistors.Journal of Physics D: Applied Physics. 01/2010; 43(49). -
Article: Ion energy distributions for the identification of active species and processes in low pressure hollow cathode discharges
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ABSTRACT: Energy distributions of ions generated in hollow cathode low pressures dc discharges of different gases and gas mixtures containing Ar, H2, N2, O2 or CH4 are studied by quadrupole mass spectrometry. The ions are sampled through a small diaphragm in the grounded cathode. The measured distributions are mostly determined by the acceleration of ions in the sheath region between the negative glow and the cathode, displaying in general a narrow peak centred at energies close to the anode potential, but with specific features for the distinct ions. It is shown that information about ion production and sheath collision processes can be derived from the shapes of the different energy distributions. In some cases these distributions are used for the estimation of the relative abundance of ions with the same mass/charge ratio but different compositions in complex gas mixtures.Plasma Sources Science and Technology 07/2009; 18(3):034007. · 2.52 Impact Factor -
Article: Removal of carbon films by oxidation in narrow gaps: Thermo-oxidation and plasma-assisted studies
Journal of Nuclear Materials 01/2009; 390–391:696 - 700. · 2.05 Impact Factor -
Article: Emission Imaging of Pulsed Plasma Plumes Emerging Through a Transversal Hole in the Hollow Cathode of a DC Reactor
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ABSTRACT: A sequence of images shows the emission of brief pulsed plasma plumes emerging laterally from a small hole in the wall of a DC hollow-cathode reactor. The orifice diameter is comparable to the plasma sheath thickness. The images highlight the transient behavior under particular conditions of these plasma events. They also extend current predictions for RF discharges on the distributions of ions flowing through small apertures to the case of hollow-cathode DC discharges.IEEE Transactions on Plasma Science 09/2008; · 1.17 Impact Factor -
Article: Cleaning efficiency of carbon films by oxygen plasmas in the presence of metallic getters
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ABSTRACT: In the present work, the effect that strong metallic getters has in the carbon removal rate by He/O 2 glow discharges is addressed. Due to the stringent conditions required for the use of Be in laboratory experiments, Li and Mg have been tested as O getters, the former showing also high H-getter properties. Samples of C/Metal mixtures are produced by introducing the metallic evaporator into the glow discharge deposition chamber, which is kept at room temperature. Two schemes were used: layered deposition and full mix-up. Hydrogen methane mixtures are used as precursors of hard a-C: H films in a DC glow discharge apparatus. The film growth and removal rate was monitored in situ through laser interferometry and particle balance from the mass spectrometer data. Surface analysis techniques have been applied for the mixed film characterization prior and after the exposure to the oxidizing plasma. Removal rates up to 12 nm/min are obtained in the absence of metals. It was found that full removal of carbon from the metal/C layers was possible for the plasma conditions used. Rates of similar value to those of pure C films were achieved except for the case of layered deposition, where a decrease in the film etching rate was observed corresponding to the location of the metal layer.Journal of Physics Conference Series 01/2008; 100(6):062025. -
Article: Ion chemistry in cold plasmas of H2 with CH4 and N2.
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ABSTRACT: The distributions of ions and neutrals in low-pressure (approximately 10(-2) mbar) DC discharges of pure hydrogen and hydrogen with small admixtures (5%) of CH(4) and N(2) have been determined by mass spectrometry. Besides the mentioned plasma precursors, appreciable amounts of NH(3) and C(2)H(x) hydrocarbons, probably mostly from wall reactions, are detected in the gas phase. Primary ions, formed by electron impact in the glow region, undergo a series of charge transfer and reactive collisions that determine the ultimate ion distribution in the various plasmas. A comparison of the ion mass spectra for the different mixtures, taking into account the mass spectra of neutrals, provides interesting information on the key reactions among ions. The prevalent ion is H3+ in all cases, and the ion chemistry is dominated by protonation reactions of this ion and some of its derivatives. Besides the purely hydrogenic ions, N(2)H+, NH(4)+, and CH(5)+ are found in significant amounts. The only mixed C/N ion clearly identified is protonated acetonitrile C(2)H(4)N+. The results suggest that very little HCN is formed in the plasmas under study.The Journal of Physical Chemistry A 10/2007; 111(37):9003-12. · 2.95 Impact Factor -
Article: Ion Chemistry in Cold Plasmas of H2 with CH4 and N2
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ABSTRACT: The distributions of ions and neutrals in low-pressure (≈10-2 mbar) DC discharges of pure hydrogen and hydrogen with small admixtures (5%) of CH4 and N2 have been determined by mass spectrometry. Besides the mentioned plasma precursors, appreciable amounts of NH3 and C2Hx hydrocarbons, probably mostly from wall reactions, are detected in the gas phase. Primary ions, formed by electron impact in the glow region, undergo a series of charge transfer and reactive collisions that determine the ultimate ion distribution in the various plasmas. A comparison of the ion mass spectra for the different mixtures, taking into account the mass spectra of neutrals, provides interesting information on the key reactions among ions. The prevalent ion is H3+ in all cases, and the ion chemistry is dominated by protonation reactions of this ion and some of its derivatives. Besides the purely hydrogenic ions, N2H+, NH4+, and CH5+ are found in significant amounts. The only mixed C/N ion clearly identified is protonated acetonitrile C2H4N+. The results suggest that very little HCN is formed in the plasmas under study.08/2007; -
Article: From Carbon Nanostructures to New Photoluminescence Sources: An Overview of New Perspectives and Emerging Applications of Low‐Pressure PECVD
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ABSTRACT: Low-pressure, plasma-enhanced (PE)CVD is a powerful and versatile technique that has been used for thin-film deposition and surface treatment since the early 1960s. However, it is only recently that it has been used in applications other than the different stages of microelectronic circuit fabrication. Now, PECVD is being used in emerging applications due to new materials and process requirements in a wide variety of areas, such as biomedical applications, solar cells, fuel cell development, fusion research, or the synthesis of silicon nanocrystals showing efficient photoluminescence, useful for future solid-state light sources. These new scenarios have stimulated further development of novel PECVD diagnostic techniques, together with fundamental experimental and theoretical studies aimed at a better understanding of some of the basic processes underlying the plasma/surface interaction. This paper gives an overview of some new research areas where PECVD is finding promising applications.Chemical Vapor Deposition 06/2007; 13(6‐7):267 - 279. · 1.80 Impact Factor -
Article: Atom and ion chemistry in low pressure hydrogen dc plasmas.
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ABSTRACT: The chemical composition of a low-pressure hydrogen dc plasma produced in a hollow cathode discharge has been measured and modeled. The concentrations of H atoms and of H(+), H(2)(+) and H(3)(+) ions were determined with a combination of optical spectroscopic and mass spectrometric techniques, over the range of pressures (p approximately 0.008-0.2 m bar) investigated. The results were rationalized with the help of a zero-order kinetic model. A comparatively high fraction ( approximately 0.1+/-0.05) of H atoms, indicative of a relatively small wall recombination, was observed. Low ionization degrees (<10(-4)) were obtained in all cases. In general, the ionic composition of the plasma was found to be dominated by H(3)(+), except at the lowest pressures, where H(2)(+) was the major ion. The key physicochemical processes determining the plasma composition were identified from the comparison of experimental and model results, and are discussed in the paper.The Journal of Physical Chemistry A 05/2006; 110(18):6060-6. · 2.95 Impact Factor -
Article: Influence of rotation and isotope effects on the dynamics of the N((2)D)+H(2) reactive system and of its deuterated variants.
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ABSTRACT: Integral cross sections and thermal rate constants have been calculated for the N((2)D)+H(2) reaction and its isotopic variants N((2)D)+D(2) and the two-channel N((2)D)+HD by means of quasiclassical trajectory and statistical quantum-mechanical model methods on the latest ab initio potential-energy surface [T.-S. Ho et al., J. Chem. Phys. 119, 3063 (2003)]. The effect of rotational excitation of the diatom on the dynamics of these reactions has been investigated and interesting discrepancies between the classical and statistical model calculations have been found. Whereas a net effect of reagent rotation on reactivity is always observed in the classical calculations, only a very slight effect is observed in the case of the asymmetric N((2)D)+HD reaction for the statistical quantum-mechanical method. The thermal rate constants calculated on this Potential-Energy Surface using quasiclassical trajectory and statistical model methods are in good agreement with the experimental determinations, although the latter are somewhat larger. A reevaluation of the collinear barrier of the potential surface used in the present study seems timely. Further theoretical and experimental studies are needed for a full understanding of the dynamics of the title reaction.The Journal of Chemical Physics 01/2006; 123(22):224301. · 3.33 Impact Factor -
Article: Low-temperature rotational relaxation of CO in self-collisions and in collisions with Ne and He.
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ABSTRACT: The low-temperature rotational relaxation of CO in self-collisions and in collisions with the rare-gas atoms Ne and He has been investigated in supersonic expansions with a combination of resonance-enhanced multiphoton ionization (REMPI) spectroscopy and time-of-flight techniques. For the REMPI detection of CO, a novel 2 + 1' scheme has been employed through the A(1)Pi state of CO. From the measured data, average cross sections for rotational relaxation have been derived as a function of temperature in the range 5-100 K. For CO-Ne and CO-He, the relaxation cross sections grow, respectively, from values of approximately 20 and 7 A(2) at 100 K to values of approximately 65-70 and approximately 20 A(2) in the 5-20 K temperature range. The cross section for the relaxation of CO-CO grows from a value close to 40 A(2) at 100 K to a maximum of 60 A(2) at 20 K and then decreases again to 40 A(2) at 5 K. These results are qualitatively similar to those obtained previously with the same technique for N(2)-N(2), N(2)-Ne, and N(2)-He collisions, although in the low-temperature range (T < 20 K) the CO relaxation cross sections are significantly larger than those for N(2). Some discrepancies have been found between the present relaxation cross sections for CO-CO and CO-He and the values derived from electron-induced fluorescence experiments.The Journal of Physical Chemistry A 11/2005; 109(42):9402-13. · 2.95 Impact Factor -
Article: Low-pressure DC air plasmas. investigation of neutral and ion chemistry.
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ABSTRACT: The neutral and charged species present in a direct current (dc) hollow cathode, gas flow, air reactor are experimentally studied by quadrupole mass spectrometry. The degree of ionization of the plasma and the electron mean temperature with decreasing air pressure, for constant discharge current, are measured with a double Langmuir probe. The chemical composition of the plasma changes appreciably over the 3 x 10(-3) to 5 x 10(-2) mbar range investigated: at the lowest pressures studied, O2 dissociation is up to 60% and the concentration of NO is half that of N2; concerning ions, NO+ and N2+ are dominant for the whole pressure range. A kinetic model of the plasma including electrons, neutrals, and positive ions is developed to account for the experimental observations; it is consistent with energy balance and predicts that heterogeneous processes are the main source of NO and that the contribution of ions to the global chemistry of neutrals is of minor significance even for the lowest pressures.The Journal of Physical Chemistry A 08/2005; 109(28):6255-63. · 2.95 Impact Factor -
Article: High‐resolution stimulated Raman spectrum of ethane (12C2H6) in the region of the CH symmetric stretching
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ABSTRACT: The high-resolution (ca. 0.008 cm−1) stimulated Raman spectrum of the strongly perturbed CH symmetric stretching region of 12C2H6 at room temperature and at a rotational temperature of ca. 30 K, achieved in a free supersonic jet expansion chamber, is reported. The spectrum is tentatively analysed and assigned in terms of a simplified vibration-torsion–rotation Hamiltonian. Besides the already known strong Fermi resonance, several Coriolis torsional interactions are identified as the major perturbations in this spectral region.Journal of Raman Spectroscopy 04/2005; 25(7‐8):589 - 597. · 3.09 Impact Factor -
Article: Spectrometric and kinetic study of a modulated glow air discharge
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ABSTRACT: The transients associated with the ignition and extinction of the cold plasma produced in a low-frequency, square wave modulated, hollow cathode discharge of air are characterized by time resolved mass spectrometry and emission spectroscopy. The time evolution of the concentrations of neutral products measured in the discharge (NO, N2O, N and O) is compared with the predictions of a simple kinetic model previously developed to characterize low-pressure plasmas of N2O, NO and NO2 and a good agreement is found.Plasma Sources Science and Technology 05/2004; 13(2):343. · 2.52 Impact Factor -
Article: Time resolved diagnostics and kinetic modelling of a modulated hollow cathode discharge of NO2
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ABSTRACT: The transients associated with the ignition and the extinction of the cold plasma produced in a low frequency, square-wave modulated, hollow cathode discharge of nitrogen dioxide are characterized by time resolved emission spectroscopy, mass spectrometry and electrical probes. The temporal evolution of the concentrations of neutral species created or destroyed in the NO2 discharges are compared with the predictions of a simple kinetic model previously developed for discharges of other nitrogen oxides (N2O and NO). The physical conditions of pressure, gas flow rate, modulation frequency and electrical current in the NO2 plasma were selected in order to highlight the time-dependent behaviour of some of the stable species formed in the discharge, especially the nitrogen oxide products, whose concentrations show transient maxima. The usefulness of the analysis of the transient results is emphasized as a means to evaluate the relevance of the different elementary processes and as a key to estimate the values of some of the rate constants critical to the modelling.This work is dedicated to the memory of Professor José Campos.Plasma Sources Science and Technology 11/2003; 13(1):39. · 2.52 Impact Factor -
Article: Characterization and modelling of the steady state and transients of modulated hollow cathode discharges of nitric oxide
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ABSTRACT: This work describes a systematic experimental study and kinetic modelling of the steady state and the transients associated with the turn-on and turn-off of a NO hollow cathode discharge. The local charge density and mean electron energy have been determined with a double Langmuir probe. Time-resolved Fourier transform infrared spectroscopy and electron bombardment quadrupole mass spectrometry with ionization by electron impact have been used to measure the gas temperature and the concentrations of the stable molecules present in the discharge: the precursor, NO, the major products, N2, O2, and the minor species NO2 and N2O. Emission spectroscopy has been employed to study the time behaviour of the very reactive nitrogen and oxygen atoms. A model based on a reduced set of kinetic equations including electron dissociation, gas-phase reactions, and gas-surface processes gives a global account of the measured data. From the time-resolved results, electron impact dissociation rate constants for some of the involved species under the conditions of the experiment are estimated. This model is compared with that obtained in previous works on N2O plasmas.Plasma Sources Science and Technology 08/2002; 11(4):368. · 2.52 Impact Factor -
Article: Suppression of hydrogenated carbon film deposition by scavenger techniques and their application to the tritium inventory control of fusion devices
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ABSTRACT: The well-known radical and ion scavenger techniques of application in amorphous hydrogenated carbon film deposition studies are investigated in relation to the mechanism of tritium and deuterium co-deposition in carbon-dominated fusion devices. A particularly successful scheme results from the injection of nitrogen into methane/hydrogen plasmas for conditions close to those prevailing in the divertor region of present fusion devices. A complete suppression of the a-C : H film deposition has been achieved for N2/CH4 ratios close to one in methane (5%)/hydrogen DC plasma. The implications of these findings in the tritium retention control in future fusion reactors are addressed.Plasma Physics and Controlled Fusion 07/2002; 44(8):L37. · 2.42 Impact Factor -
Article: Transition dipole moment of the .nu.3 band of CH3
04/2002; -
Article: Experimental and Theoretical Reaction Cross Sections for the H + HCl System†
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ABSTRACT: The dynamics of the gas-phase reaction of H atoms with HCl has been studied experimentally employing the laser photolysis/vacuum-UV laser-induced fluorescence (LP/VUV-LIF) “pump-and-probe” technique and theoretically by means of quasiclassical trajectory (QCT) calculations performed on two versions of the new potential energy surface of Bian and Werner [Bian, W.; Werner, H.-J. J. Chem. Phys. 2000, 112, 220]. In the experimental studies translationally energetic H atoms with average collision energies of Ecol = 1.4 and 1.7 eV were generated by pulsed laser photolysis of H2S and HBr at 222 nm, respectively. Ground-state Cl(2P3/2) and spin−orbit excited Cl*(2P1/2) atoms produced in the reactive collision of the H atoms with room-temperature HCl were detected under single collision conditions by VUV-LIF. The measurements of the Cl* formation spin−orbit branching ratio φCl*(1.4 eV) = [Cl*]/[Cl + Cl*] = 0.07 ± 0.01 and φCl*(1.7 eV) = 0.19 ± 0.02 revealed the increasing importance of the nonadiabatic reaction channel H + HCl → H2 + Cl* with increasing collision energy. To allow for comparison with the QCT calculations, total absolute reaction cross sections for chlorine atom formation, σR(1.4 eV) = (0.35 ± 0.16) Å2 and σR(1.7 eV) = (0.13 ± 0.06) Å2, have been measured using a photolytic calibration method. In addition, further QCT calculations have been carried out for the H + DCl isotope reaction which can be compared with the results of previous reaction dynamics experiments of Barclay et al. [Barclay, V. J.; Collings, B. A.; Polanyi, J. C.; Wang, J. H. J. Phys. Chem. 1991, 95, 2921].08/2000; -
Article: Fast Processes in a N2O-Modulated Hollow Cathode Discharge: Excitation and Diffusion
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ABSTRACT: In this work the changes in the infrared absorption of N2O observed in a hollow cathode discharge modulated by a 45 Hz square wave are studied by time-resolved infrared absorption spectroscopy, with a Fourier transform infrared (FTIR) spectrometer working in the step-scan mode. These variations are attributed to alternative population and depopulation of the ground state of N2O; on the other hand, no changes are observed in the total concentration of N2O and the major products of the discharge. The experimental results are explained by means of a simple kinetic model: vibrational excitation processes are assumed to be the main cause of the observed effects, but an inhomogeneous distribution of the stable species and the incorporation of diffusion terms between the plasma volume and the rest of the discharge cell are shown to be crucial in order to justify the quick and sharp variations in the N2O ground-state population seen along the path of the IR beam. The influence of these effects has been verified also by mass spectrometric measurements of the temporal behavior of the concentration of the major products N2O, N2 and O2 at one end of the discharge cell and at different frequencies.08/2000;
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Institutions
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2006
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Spanish National Research Council
Madrid, Madrid, Spain -
Complutense University of Madrid
- Facultad de Ciencias Químicas
Madrid, Madrid, Spain
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