bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mo-W-O thin films for CO sensing
Authors: Comini Elisabetta , Ferroni Matteo , Guidi Vincenzo , Martinelli Giuliano , Sacerdoti Michele , Mea Gianantonio Della , Vomiero Alberto , Francia Girolamo Di , Sberveglieri Giorgio 
Published: April 2011

AbstractThe Mo-W-O thin films were deposited by RF reactive sputtering from composite target of W and Mo (20:80 weight ratio). Structural characterisation was carried out by X-ray diffraction spectroscopy and the composition of the film was obtained by Rutherford backscattering analysis. The layers were investigated by volt-amperometric technique for electrical and gas-sensing properties. The films were capable of sensing CO. No effect of poisoning of the surface was recorded and recovery of the resistance was complete. A concentration of CO as low as 15 ppm produced a relative variation in the conductance of 390% with response and recovery times of about 2 minutes at a working temperature of 200°C.
Analysis of Pt-Nanoparticles Embedded on Crystalline TiO2 Studied by Transmission Electron Microscopy
Authors: Arbiol Jordi , Ruiz Ana , Cirera Albert , Peiró Francisca , Cornet Albert , Morante Joan R. , Alimoussa Abdelhadi , Casanove Marie-José 
Published: April 2011

AbstractIn this work, we studied the Pt particles embedded on crystalline TiO2 nanopowders. As it is well known, metal nanoparticles distributed on TiO2 surface can improve dramatically the reaction rate and selectivity of reaction in gas sensing. We prepared a wide set of TiO2 samples with different nominal Pt concentration. The aim of this study was to analyse the Pt distribution on TiO2 surface (density of Pt, particle size distribution, etc.), as well as the Pt phases existing in our samples. We used High Resolution Transmission Electron Microscopy (HRTEM) as a powerful tool to study Pt particles morphology. The HRTEM images were compared with computer simulations to confirm the results obtained. The chemical composition of the material has been also investigated by Inductively Coupled Plasma (ICP).
Crystallization in the limit of ultra thin layers- A new crystallization model
Authors: Zacharias Margit , Streitenberger Peter 
Published: April 2011

AbstractAnnealing of amorphous Si/SiO2 multilayer produces nanocrystals embedded between oxide interfaces with the size of the nanocrystals depending on the Si layer thickness. It is found that the crystallization temperature is strongly enhanced by the presence of the oxide interface and follows an exponential law. A model is presented, which derives the exponential dependence taking into account the interface energies, the thickness of the layers, the melting point of the system, and the bulk amorphous crystallization temperature. The critical crystallization radius and the critical thickness of the Si layer are discussed.
Single electron charging effect in individual Si nanocrystals.
Authors: Gentile P. , Baron T. , Magnea N. , Mur P. , Martin F. , Séméria M.N. 
Published: April 2011

AbstractWe present a detailed study of the electronics properties of individual silicon nano- crystals (nc-Si) elaborated by Low Pressure Chemical Vapor Deposition on 1.2 nm thick SiO2 grown on Si (100). The combination of ultra thin oxide layers and highly doped substrates allows imaging the hemispherical dots by Scanning Tunneling Microscopy. By analyzing the STM images, we deduce a size distribution, which ranges between 3 and 6 nm with a surface density around 1012 cm-2. Spectroscopic studies of single dots are made by recording the I(V) curves on the Si nanocrystal accurately selected with the metallic tip. These I(V) curves obtain on a single dot, exhibit Coulomb blockade and resonant tunneling effects. Coulomb pseudo gaps, Ec, between 0.15 and 0.2 V are measured for different dots. From the width and height of the staircases observed at bias greater than Ec, 60 meV and 40 pA respectively, capacitance of 0.5 to 1 aF and tunnel resistance of 3.5×108 and 5.7×109 Ohms are measured within the orthodox approximation for asymmetric junctions. We have determined, from experimental measurements, the energy of the first level confined in nc-Si.
Recombination Dynamics in Nitride Quantum Boxes and Quantum Wells for Colors Ranging from the UV to the Red
Authors: Lefebvre P. , Morel A. , Gallart M. , Taliercio T. , Gil B. , Allègre J. , Mathieu H. , Grandjean N. , Damilano B. , Massies J. 
Published: April 2011

ABSTRACTTime-resolved photoluminescence experiments at varying temperature are performed on a series of InxGa1−xN/GaN quantum well and quantum box samples of similar compositions (0.15 < x < 0.20). The results are analyzed by using envelope-function calculations of transition energies and oscillator strengths, accounting for internal electric fields. The respective influences of localization and electric fields on radiative and nonradiative lifetimes and on the Stokes shift are deduced. The results indicate that the spatial extension of localization centers is much smaller than the size of the quantum boxes (∼10 × 3 nm, typically). The room-temperature radiative efficiency of both quantum well and quantum box samples is enhanced by replacing the topmost GaN barrier by an AlGaN one.
Polarization effects in AlxGa1−xN / GaN superlattices
Authors: Waldron Erik L. , Schubert E. Fred , Graff John W. , Osinsky Andrei , Murphy Michael J. , Schaff William F. 
Published: April 2011

ABSTRACTRoom temperature and low temperature photoluminescence studies of AlxGa1−xN/GaN superlattices reveal a red shift of the dominant transition band relative to the bulk GaN bandgap. The shift is attributed to the quantum-confined Stark effect resulting from polarization fields in the superlattices. A theoretical model for the band-to-band transition energies based on perturbation theory and a variational approach is developed. Comparison of the experimental data with this model yields a polarization field of 4.6 × 105 V/cm for room temperature Al0.1Ga0.9N/GaN and 4.5 × 105 V/cm for room temperature Al0.2Ga0.8N/GaN. At low temperatures the model yields 5.3 × 105 V/cm for Al0.1Ga0.9N/GaN and 6.3 × 105 V/cm for Al0.2Ga0.8N/GaN. The emission bands exhibit a blue shift at high excitation densities indicating screening of internal polarization fields by photo-generated free carriers.
Electric fields at the SiC/AlN and SiC/GaN polar interfaces
Authors: Laridjani Morad Rouhani , Masri Pierre , Majewski Jacek A. 
Published: April 2011

ABSTRACTWe present first-principles calculations of structural and electronic properties of heterovalent SiC/AlN and SiC/GaN heterostructures with wurtzite AlN and GaN films pseudomorphically grown on the 6H-SiC and 3C-SiC substrates along the c-axis. We have investigated reconstructed stoichiometric interfaces consisting of one mixed layer with various lateral arrangements. The preferred bonding configurations of the reconstructed interfaces are found to be Si-N and Ga-C. The calculated valence band discontinuities for SiC/AlN and SiC/GaN heterostructures lie in the range of 1.5 - 2.3 eV and 0.4 - 1.4 eV, respectively. The SiC/AlN heterostructures are predicted to be of type I, whereas SiC/GaN heterostructures can be of type I or II. The polarization induced interface charges are of the order of 4.8 × 1012cm−2 and 0.7 × 1012 cm−2 in SiC/AlN and SiC/GaN junctions, respectively.
Polarization induced 2D hole gas in GaN/AlGaN heterostructures
Authors: Hackenbuchner S. , Majewski J. A. , Zandler G. , Ambacher O. , Vogl P. 
Published: April 2011

ABSTRACTThe generation of high density 2D hole gases is crucial for further progress in the field of electronic and optoelectronic nitride devices. In this paper, we present results of C-V profiling measurements for N-face GaN/AlGaN heterostructures and systematic theoretical studies of Mg- doped GaN/AlGaN gated heterostructures and superlattices. Our calculations are based on a self- consistent solution of the multiband k.p Schrödinger and Poisson equation and reveal that the hole 2D sheet density is mainly determined by the polarization induced interface charges. For an Aluminium concentration of 30%, the induced hole density in the heterostructure can reach values up to 1.5×1013 cm−2. In the GaN/AlGaN superlattices, the hole sheet density increases with the superlattice period and saturates for a period of 40 nm at a value of 1.5×1013 cm−2.
Characterization of Ion Implanted GaN
Authors: Skromme B.J. , Martinez G.L. , Krasnobaev L. , Poker D.B. 
Published: April 2011

ABSTRACTLow temperature photoluminescence is used to study the levels introduced by low-dose implantation of Mg, C, and Be into GaN, together with Ne, Al, P, or Ar co-implantation species. None of the co-implants successfully creates shallow C or Be acceptors. The yellow (2.2 eV) PL band is strongly introduced by both C and Be implants, but not by Ar, Al, P, or Mg. We propose that it involves Ga vacancies stabilized by complexing with C and Be interstitial (or CGa) donors, which explains why C and Be are absent as substitutional acceptors. We observe excitons bound to isoelectronic PN in P-implanted GaN. They are absent in P+Mg implanted GaN, in which we observe new donor-to-acceptor pair peaks due to two deeper donor levels. We assign these levels to PGa antisite double donors, which are stable in p-type material
Surface Elevation and Strain in Ion Implanted GaN
Authors: Molnar B. , Qadri S. B. , Schiestel S. , Stroud R. M. , Carosella C. A. 
Published: April 2011

ABSTRACTWe report on the investigation of X-ray strains and surface elevation of ion-implanted GaN as a function of ion fluence. Si, Mg and Ar ion implantation are examined. A continuous increase in lattice constants, a and c occur for doses up to near 1 × 1015/cm2. At higher doses there emerges a volume expansion of the GaN perpendicular to the surface which is an order of magnitude larger than any observed out-of-plane change in strain. The rapid expansion correlates with the onset of GaN amorphization. Transmission electron microscopy indicates that cavities form after 5 × 1016/cm2, 100 keV Ar+ implantation. For low temperature implantation, the cavities lie near the peak of the ion range For room temperature implantation, the cavities coalesce at the surface and push out the GaN perpendicular to the surface. AFM measurements confirm the presence of large bumps on the surface. The very high dose ion implantation have important consequences to etching rates of GaN and to the ability to produce n+, but not p+ doping with ion implantation.
p-InGaN/n-GaN Heterojunction Diodes and their Application to Heterojunction Bipolar Transistors
Authors: Makimoto Toshiki , Kumakura Kazuhide , Nishida Toshio , Kobayashi Naoki 
Published: April 2011

ABSTRACTp-InGaN/n-GaN heterojunction diodes were grown by metalorganic vapor phase epitaxy and characterized using current-voltage (I-V) and capacitance-voltage (C-V) measurements. We changed the In mole fraction in p-InGaN from 0 to 25% to investigate diode characteristics. All the diodes showed rectified I-V characteristics at room temperature. The ideality factors obtained from forward I-V characteristics were around 2, meaning that the recombination current is dominant instead of the tunneling current through the defects in depletion layers of the diodes. The breakdown voltage in reverse I-V characteristics depends on the net donor concentration (ND - NA) in n-GaN instead of the In mole fraction in p-InGaN. This result also means that the defects in p-InGaN do not influence the breakdown voltage. The built-in potential from C-V measurements decreases with the In mole fraction in p-InGaN, meaning that the valence band discontinuity increases with the In mole fraction. This valence band discontinuity realizes the hole confinement in an HBT with an p-InGaN base. Using these InGaN/GaN heterojunction diodes, an InGaN/GaN double heterojunction bipolar transistor was fabricated for the first time. The maximum current gain of 1.2 was obtained at room temperature.
Localized Vibrational Modes of Carbon-Hydrogen Complexes in MOCVD Grown GaN and AlGaN thin films
Authors: Chen J. , Zhou Q. , Berhane Y. , Manasreh M. O. , Tran C. A. , Pophristic M. , Ferguson I. T. 
Published: April 2011

ABSTRACTLocalized vibrational modes of carbon-hydrogen complexes in metalorganic chemical vapor deposition grown GaN on sapphire were studied using a Fourier-transform infrared spectroscopy technique. Three distinctive localized vibrational modes were observed around 2850, 2922, and 2959 cm−1 for undoped, Si- and Mg-doped samples. These peaks are related to CH, CH2, and CH3 defect complexes, respectively. It is also observed that the frequencies and intensities of the localized vibrational modes are sample dependent.
Structural and Elasticity-based Properties of SiC-based Interfaces: their Relevance to the Heteroepitaxy of III-V Nitrides
Authors: Masri P. , Laridjani M. Rouhani , Stauden Th. , Pezoldt J. , Averous M. 
Published: April 2011

ABSTRACTIn this work we evaluate the strategy of using 3C-SiC as a substrate for III-V nitrides heteroepitaxy (AlN, GaN…). Our methodology is based on the elasticity theory of strained interfaces and involves not only geometric parameters of host materials but also parameters related to their elastic properties. The basic physics involved in the theory correlates lattice dynamics and strain gradients via effective elastic constants associated with the host materials forming the heterosystem (S factor). Within this approach, the optimization of the IIIV/ 3C-SiC interface is achieved by applying, at the interface, continuity conditions to the host material S factors and the related geometric features. An alloyed layer, i.e. AlxGa1−xN, simulates the III-V compound. We find out that the optimizing composition of this layer is x=1 corresponding to a stoechiometric AlN layer. This is consistent with the result showing that AlN presents the closest structural characteristic to SiC. Our results also predict that, when used as a buffer layer, AlN may provide a mean to optimize the GaN/SiC interface.
Observation of dislocation etch pits in epitaxial lateral overgrowth GaN by wet chemical etching
Authors: Wen T. C , Lee S.C. , Chuang H. S. , Chiou C. H. , Lee W. I 
Published: April 2011

ABSTRACTThis work investigates dislocation etch pits in epitaxial lateral overgrowth (ELO) GaN by wet chemical etching. A mixture of H2SO4 and H3PO4 was used as a dislocation etchant, and SEM and AFM were employed to observe the surface topography. For the as-grown sample, SEM images present the flat, smooth surface without any pits or hillocks. After the chemical etching, hexagonal shaped etch pits were observed at the edge of ELO GaN. AFM observation of etched ELO GaN displayed high densities of etch pits clustered in the “window” region and the coalescent line of two growing fronts. In contrast, the overgrowth region was nearly free of etch pits. Moreover, we observed that different sizes of etch pits dominated in “window” region and coalescent region. This implied different types dislocations dominated in these regions.
Optimization of the GaN epilayer quality using in-situ reflectance measurements
Authors: Ruffenach-Clur Sandra , Moret Matthieu , Briot Olivier , Moreaud Nathanaël , Calas Joseph , Aulombard Roger L. 
Published: April 2011

ABSTRACTAlthough a tremendous amount of work has been done these last years on the nitride semiconductor system, a lot is still to be understood regarding the growth mechanisms of GaN. The standard GaN MOCVD growth process includes the low temperature deposition of a nucleation layer, followed by an anneal at high temperature, and the GaN layer is then deposited. The number of process parameters which can be used to tune the growth is very large (temperatures, times, thicknesses, molar flow rates and ratios …) and, due to the coupling between them, the role of each one is not clearly understood. In this paper, we present systematic series of growth experiments, where in-situ reflectance monitoring was used and correlated to ex-situ optical characterization of the samples by photoluminescence at low temperature (2K). Here, we demonstrate that the nucleation layer and its annealing have a determining effect. The nucleation layer growth temperature was not found to be a very sensitive parameter, while the amount of re-crystallization is. Surprisingly, the amount of ammonia present in the gas phase has a determining effect on the recrystallization behavior of the nucleation layer. Another interesting point is the sensitivity versus growth temperature for the main GaN layer, which was found to affect the initial stages of the growth in a drastic manner when changed by only 5°C. In-situ reflectance allowed us to tune precisely our process and to obtain GaN layers with 500 cm2/Vs electron mobility at room temperature and photoluminescence fwhm of 1.7 meV at 2K for the donor-bound exciton.
Observation of Band Gap Energy Fluctuation of Microcrystalline InGaN:Zn
Authors: Kanie Hisashi , Sugimoto Kosei , Okado Hiroaki 
Published: April 2011

ABSTRACTThis paper describes a comparison of the optical properties of InGaN:Zn with that of GaN:Zn and InGaN by measuring photoluminescence excitation (PLE) spectra at 77 K. It is well known that MOCVD grown InGaN films tend to have a fluctuation in In concentration which results in a fluctuation of the band gap energy. The PL mechanism in InGaN films has been assigned to the annihilation of an exciton at the potential minima caused by the fluctuated band gap potential. We grew InGaN(:Zn) and GaN:Zn microcrystals emitting intense blue luminescence by a reaction of GaN and In2S3 with NH3 in the range of 850 to 900 °C. The samples grown at various temperatures show two PLE peaks: one weak peak is located around 3.47 eV, which we attribute to the band gap energy, and the other peak around 3.15 eV, which we attributed to the In localized state level. We had proposed an atomic structure of the localized state based on an isoelectronic trap theory. However, it is necessary to estimate the order of potential fluctuation of the grown InGaN microcrystal is small in order to assure the isoelectronic trap theory. PLE spectra of InGaN:Zn were measured and compared with that of GaN:Zn to estimate the degree of energy gap fluctuation. As the shape of a PLE peak of InGaN:Zn at around 3.47 eV was comparable to that of GaN:Zn, we concluded that the isoelectronic trap model holds for the grown InGaN microcrystals.
X-Ray Reciprocal space mapping studies of strained GaN/AlGaN quantum wells
Authors: Briot Olivier , Ruffenach-Clur Sandra , Moret Matthieu , Aulombard Roger Louis 
Published: April 2011

ABSTRACTIn this work, we report on the systematic X-ray reciprocal space mapping of a series of GaN/AlGaN samples, with different Al content and well thickness. For coherently grown samples, we present a calculation which allow us to precisely determine the strain state and Al content in the samples in one diffraction experiment. In our samples, both GaN and AlGaN are strained, and we discuss the effect of these strains on the band structure of GaN, which is probed by low temperature reflectivity and correlates perfectly our x-ray results.
Photoluminescence Study of Defects in GaN Grown by Molecular Beam Epitaxy
Authors: Reshchikov Michael A. , Zhang Manhong H. , Cui Jie , Visconti Paolo , Yun Feng , Morkoç Hadis 
Published: April 2011

ABSTRACTDefect related photoluminescence (PL) in unintentionally doped GaN layers grown by molecular beam epitaxy (MBE) was studied. Under certain growth conditions, we observed new defect-related bands: a red band with a maximum at about 1.88 eV and a green band with a maximum at about 2.37 eV. The quenching of these bands with increasing temperature took place with an activation energy of about 120-140 meV at temperatures above 100 K. Moreover, the red band exhibited an increase of PL intensity with an activation energy of 1.2 meV in the range of 10-60 K. The observed behavior is explained by invoking a configuration coordinate model and that we speculate the defects to be partially nonradiative and related to Ga atoms.
Strain-Induced Diffusion in Strained Sige/Si Heterostructures
Authors: Lim Y.S. , Lee J.Y. , Kim H.S. , Moon D.W. 
Published: April 2011

AbstractDiffusivity of a strained heterostructure was theoretically investigated, and general diffusion equations with strain potential were deduced. There was an additional diffusivity by the strain potential gradient as well as by the concentration gradient. The strain-induced diffusivity was a function of concentration, and its temperature dependence was formulated. The activation energy of the strain-induced diffusivity was measured by high-resolution transmission electron microscopy. This result can be generally applied for the investigation of the diffusion in strained heterostructures.
Epitaxial Al and Cu films grown on CaF2/Si(111)
Authors: Shusterman Yuriy V. , Yakovlev Nikolai L. , Dovidenko Katharine , Schowalter Leo J. 
Published: April 2011

AbstractThe ability to grow single-crystalline Al and Cu films is of significance for several areas of materials research, such as the resistivity size effect in thin metal films, electromigration failure of interconnects, and magneto-resistance studies. Here, we explore the microstructure and resistivity of thin Al and Cu films grown on CaF2/Si(111). A three-step technique of CaF2 growth is described that permits deposition under imperfect vacuum conditions and promotes smoothness of subsequent thin metal films. Reflection high-energy electron diffraction shows that epitaxial Al(111) is obtained directly on CaF2, while epitaxial Cu(111) is obtained only by growing on a 1 nm thick Al seed layer pre-deposited on CaF2. Transmission electron microscopy reveals that 75 nm thick Al films have 150 nm wide sub-grains misoriented by less than 1 degree. For 75 nm thick Cu, the grains are only 30 nm wide and are misoriented by as much as 10 degrees. Room temperature resistivity measurements of the 10-300 nm thick Al films agree with the Fuchs-Sondheimer model in which conduction electrons scatter totally diffusely at the film interfaces. For 50-1000 nm thick Cu films, the resistivity size effect is substantially greater than the prediction of this model, which may be explained in terms of grain boundary scattering.
Pattern Selection of Surface-based Nanostructures
Authors: Wang E. G. , Liu B. G. , Wu J. , Zhu W. G. , Zhang Z. 
Published: April 2011

AbstractA rich variety of two-dimensional patterns can be formed in the early stages of film growth. In this paper, we will show that, when a surfactant layer is used to mediate the growth, a counter-intuitive fractal-to-compact island shape transition can be induced by increasing deposition flux or decreasing growth temperature. Specifically, we introduce a reaction limited aggregation (RLA) theory, where the physical process controlling the island shape transition is the shielding effect of adatoms stuck to stable islands on incoming adatoms. Also discussed is the origin of a transition of compact islands from triangular to hexagonal then to inverted triangular in Pt (111) homoepitaxy with the presence of CO adsorbates. We will provide a coherent and unified picture for the interpretation of these intriguing observations based on kinetic Monte Carlo simulations, with energy barriers from first-principles calculations.
The Stability of the Low Temperature Surface Reconstruction in Au(111)
Authors: Trimble Todd M. , Cammarata Robert C. 
Published: April 2011

AbstractWe have performed computer simulation studies on the 22×√3 surface reconstruction of Au(111). This reconstruction involves a uniaxial contraction of the top monolayer corresponding to a surface strain of about 4.3% and has been observed to be the stable structure for clean surfaces at low temperatures. A continuum model yields a stability criterion that depends on the knowledge of a small number of measurable physical quantities: surface stress f, surface free energy γ, lattice parameter a0 and shear modulus µ. The simulations using EAM potentials accurately reproduce many observed features of the reconstruction and tend to support the continuum model and the resulting stability criterion.