bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiparametric sensor for air pollutants based on a porous silicon optical microcavity
Authors: Gaburro Z. , Faglia G. , Baratto C. , Sberveglieri G. , Pavesi L.
Published: April 2011

We experimentally demonstrate that porous silicon optical microcavities can be effectively used as multi-parametric gas sensors. As known, the photoluminescence intensity and electrical conduction of porous silicon are strongly dependent on environmental properties, such as the dipole moment of molecules of surrounding gases. The sensitivity is large due to the large surface/volume ratio of porous silicon. While these effects can be observed in any porous silicon structure, microcavities of porous silicon allow an additional sensing parameter, i.e. the spectral position of the resonance cavity peak. The position of the peak depends on the index of refraction of the environment, and gives independent additional information. Moreover, we show that the dynamic response of the peak shift is much faster comparing the other sensing parameters. The combined effects on the peak position, luminescence intensity and electrical conduction can allow discrimination between different substances, and therefore porous silicon optical microcavities can work as multi-parametric optical/electrical sensors. We report detection of 1 ppm of NO2 and 500 ppm of ethanol at room temperature. With NO2, the electrical conduction increases and PL quenches, but the peak does not shift, whereas the peak shifts with ethanol (no significant PL quenching is observed at 500 ppm). This suggests that discrimination between different species can be achieved.
Ion Beam Synthesis Based Formation of Ge – Rich Thermally Grown Silicon Dioxide Layers: A Promising Approach for A Silicon - Based Light Emitter
Authors: Gebel T. , Rebohle L. , Zhao J. , Borchert D. , Fröb H. , Borany J.V. , Skorupa W.
Published: April 2011

The development of novel devices for optical communication and integrated sensor applications is mainly focused on their possible integration into dedicated integrated circuits. The main problem concerning integrated optical systems in silicon technology has always been the formation of an highly efficient silicon-based light emitter which is a key feature to make a real step into the world of integrated optoelectronics.

One of the most promising approaches to form such a silicon based light emitter is ion beam synthesis. In this paper we will report our recent progress in extracting blue-violet (∼400 nm) electroluminescence (EL) from an ion beam synthesized Ge-rich silicon dioxide layer. The power efficiency of the EL was as high as 0.5 % which is one of the best values ever reported for Si - based light emission. The lifetime of the EL-device can reach several hours without special precautions of stabilizing the EL-active layer against ion or other contamination. Moreover, results are reported dedicated to the investigation of the excitation mechanism of this strong EL.
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

The 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

In 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

Annealing 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

We 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

Time-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

Room 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

We 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

The 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

Low 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

We 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

p-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

Localized 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

In 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

This 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

Although 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

This 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

In 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

Defect 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

Diffusivity 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

The 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

A 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

We 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.
Magnetoresistance Measurements on Boron-Doped and Undoped Ni3Al Thin Films
Authors: Patterson Edward C. , Henry L. L.
Published: April 2011

Magnetoresistance measurements were performed in magnetic fields up to 5 T, to examine the effects of boron doping on the electron transport properties of Ni3Al thin (500 Å, nominal, thickness) films. Both a doped (~200 ppm B) and undoped film were investigated. Four-probe magnetoresistance (magnetic field = 50 G and 5 T) measurements indicate that both samples undergo a phase transition in the 200 K – 300 K temperature range.
Growth Behaviour of Engineered Porous Thin Films – Measurement and Modeling
Authors: Vick D. , Smy T. , Dick B. , Kennedy S. , Brett M. J.
Published: April 2011

Recent experimental work has demonstrated that unique high porosity thin films may be ob- tained in physical deposition systems by combining glancing angle deposition with in situ sub-strate motion control [1-7]. The microstructure of these films consists of isolated columns engineered into shapes such as helices, posts, or chevrons. Due to the isolated nature of the columns, the films present a unique opportunity to study fundamental thin film growth behaviour and, in particular, the influence of the self shadowing mechanism in three dimensions. Apart from this academic motivation, there is the need to characterize the physical constraints imposed on the engineering of these films. In particular, this study will have implications for the realization of isolated, periodically arranged nanostructures envisioned for certain applications. Preliminary results from an ongoing study of growth dynamics, morphology, porosity, and scaling behaviour, and the dependence of these features on deposition parameters are presented below.
Unique Features of the Crystallization Kinetics of FeCrBSi Thin Films Obtained by Laser Ablation Deposition
Authors: Sorescu M. , Grabias A.
Published: April 2011

Pulsed laser deposition was used to obtain a metallic glass in thin film form using a Fe77Cr2B16Si5 amorphous ribbon as a target. The as-deposited films were annealed under the same conditions as the parent ribbon samples in order to compare their crystallization behaviors. Transmission Mössbauer measurements were performed to determine the crystallization products formed due to annealing of the Fe77Cr2B16Si5 ribbon. Conversion electron Mössbauer spectroscopy was used to obtain structural and magnetic information from the thin films as well as from the surface layers of the ribbons. Our Mössbauer study showed that crystallization behavior of the FeCrBSi thin films differs from both the surface and bulk crystallization of the ribbon. Crystallization occurs at lower annealing temperature for the thin film than for the bulk ribbon. Annealed films also contain iron oxides which are not observed in the crystallized ribbons. A formation of a very thin, protective oxide film at the surface of the ribbon is taken into account to explain the differences in the crystallization behavior of the films and ribbons.
Low Temperature Tungsten, Tungsten Carbide and Tantalum Carbide Film Growth
Authors: Sun Y.-M. , Lee S.Y. , Engbrecht E. R. , Pfeifer K. , Smith S. , White J. M. , Ekerdt J. G.
Published: April 2011

Abstract:Low temperature chemical vapor deposition of tungsten, tungsten carbide and tantalum carbide films on SiO2/Si(100) surfaces was studied by X-ray photoelectron spectroscopy (XPS) and electron microscopy. Tungsten carbide films were deposited using the W(CO)6precursor with and without ethylene over temperatures ranging from 250 to 500 °C. The films grown without ethylene contained approximately 13 % carbon and 6 % oxygen. Cross section scanning electron microscopy imaging of the films grown at various temperatures without ethylene shows a polycrystalline microstructure, and the grain size increases dramatically as the growth temperature increases. Introducing ethylene increased carbon incorporation and changed the microstructure to amorphous-like. The tungsten to carbon ratio was approximately 2 at growth below 500 °C, and reached ~ 1.2 above 500 °C. The tantalum carbide films were deposited in a plasma enhanced chemical vapor deposition (PECVD) process using methane. The PECVD tantalum carbide films were conductive with a resistivity of ~1000 µΩ cm, which is about one order of magnitude lower than thermally grown films from pentakisdimethylamino tantalum.
Molecular Dynamics Simulation of Confined Glass Forming Liquids
Authors: Varnikl Fathollah , Scheidlerl Peter , Baschnagel Jörg , Kob Walter , Binder Kurt
Published: April 2011

Two model studies are presented in order to elucidate the effect of confinement on glass forming fluids, attempting to study the effects of the interactions between the confining walls and the fluid particles. In the first model, short bead-spring chains (modelling a melt of flexible polymers) are put in between perfectly flat, structureless walls, on which repulsive potentials act. It is shown that chains near the walls move faster (in the direction parallel to the walls) than chains in the bulk. A significant decrease of the (mode-coupling) critical temperature with decreasing film thickness is found. In the second model, a binary Lennard-Jones liquid is confined in a thin film, whose surface has an amorphous structure similar to the liquid. Although, as expected, the static structural properties of the liquid are not affected by the confinement, relaxation times near the wall are much larger than in the bulk. Consequences for the interpretation of experiments are briefly discussed.
Local Environment of Surface-Polyelectrolyte-Bound DNA Oligomers
Authors: Jeon Sangmin , Bae Sung Chul , Zhao Jiang John , Granick Steve
Published: April 2011

Two-photon time-resolved fluorescence anisotropy methods were used to study the dynamical environment when fluorescent-labelled DNA oligomers (labelled with FAM, 6-fluorescein-6-carboxamido hexanoate) formed surface complexes with quaternized polyvinylpyridine (QPVP) cationic layers on a glass surface. We compared the anisotropy decay of DNA in bulk aqueous solution, DNA adsorbed onto QPVP, and QPVP-DNA-QPVP sandwich structures. When DNA was adsorbed onto QPVP, its anisotropy decay was dramatically retarded compared to the bulk, which means it had very slow rotational motion on the surface. Motions slowed down with increasing salt concentration up to a level of 0.1 M NaCl, but mobility began to increase at still higher salt concentration owing to detachment from the surface-immobilizing QPVP layers.
Electrokinetic Phenomena in Montmorillonite
Authors: Marry V. , Dufrêche J.-F. , Bernard O. , Turq P.
Published: April 2011

Clays present remarkable electrokinetic features since they exist from very dilute colloidal state to nanoporous materials, according to the ration water/clay. The case of low volume fraction Vwater/Vtot which corresponds to compact systems is examined. The ionic distributions havebeen evaluated from Poisson-Boltzmann like models and compared to discrete solvent simulations. Electroosmosis and conductance ha v ebeen calculated, in the framework of the Mean Spherical Approximation (MSA) introduced in the Fuoss-Onsager transport theory.
Structure Determination of Ba8CoRh6O21, a New Member of the 2H-Perovskite Related Oxides
Authors: zur Loye H.-C. , Smith M. D. , Stitzer K. E. , Abed A. El , Darriet J.
Published: April 2011

Single crystals of Ba8CoRh6O21 were grown out of a potassium carbonate flux. The structure was solved by a general method using the superspace group approach. The superspace group employed was R3m(00γ)0s with a = 10.0431(1) Å, c1 = 2.5946(1) Å and c2 = 4.5405(1) Å, V = 226.60(1) Å3. Ba8CoRh6O21 represents the first example of an m = 5, n = 3 member of the A3n+3mA'nB3m+nO9m+6n family of 2H hexagonal perovskite related oxides and contains chains consisting of six consecutive RhO6 octahedra followed by one distorted CoO6 trigonal prism. These chains in turn are separated from each other by [Ba]∞ chains.
Study of Fluoride Ion Motions in PbSnF4 and BaSnF4 Compounds With Molecular Dynamics Simulation and Solid State NMR Techniques
Authors: Chaudhuri Santanu , Castiglione Michael , Wang Francis , Wilson Mark , Madden Paul A. , Grey Clare P.
Published: April 2011

A combined approach, using solid state NMR and Molecular Dynamics (MD) simulations, has been employed in this work to investigate fluoride-ion motion in the PbSnF4 family of anionic conductors, materials that contain double layers of Sn2+ and M2+ cations. 19F MAS NMR spectra of PbSnF4 and BaSnF4 show that the fluoride ions are mobile on the NMR timescale (10−4 s), even at room temperature. In the case of BaSnF4, two different groups of fluoride ions were observed, one group corresponding to fluorine atoms between the layers of Ba2+ cations, and the other set, corresponding to mobile fluoride ions undergoing exchange between sites in the Ba-Sn and Sn-Sn layers. The 119Sn NMR suggests a highly distorted Sn environment in these compounds, consistent with the presence of stereoactive Sn lone pairs. MD simulations, using the Polarizable Ion Model, have been carried out to probe the conduction mechanism. These simulations are able to reproduce elements of the structure such as the reduction in the occupancy of the fluorine ions between the Sn-Sn layers. Anisotropic conductivity, involving primarily motion in the M-Sn layers, is predicted, consistent with the NMR results. In the case of BaSnF4, no motion involving the fluoride ions in the Ba-Ba layers is observed on the simulation timescale (10−12 s) and a cyclic mechanism of fluoride-ion motion involving two types of fluoride ions in the Ba-Sn layers is proposed.
Modified Sol-Gel Synthesis of Vanadium Oxide Nanocomposites Containing Surfactant Ions, and Their Partial Removal
Authors: Dobley Arthur , Zavalij Peter Y. , Schulte Jürgen , Stanley M.
Published: April 2011

Recently, there has been much interest in creating new layered transition metal oxides. Vanadium oxides may be used as sorbents, catalysts, and cathodes in lithium batteries. The modified sol-gel technique allows for some control towards the final structure of the compound. Using this technique, a new layered vanadium oxide phosphate material, containing the surfactant dodecylphosphate, has been synthesized. The compound was analyzed using powder XRD, TGA, SEM, FTIR, TEM, and solid state NMR for both 51V and 31P. V2O3(PO4C12H25)3Na2−xKx(H2O)3.2 is the general formula of the layered product with an interlayer spacing of 36.6 Å The initial compound is composed of a vanadium oxide phosphate layer sandwiched between two hydrocarbon layers. The synthesis, composition, and structure of the initial compound will be discussed. Interestingly, when this compound is calcined to 400°C, the structure changes and is possibly hexagonal. Preliminary results are presented on this calcined material.
A Computational Study of the Translational Motion of Protons in Zeolite H-ZSM-5
Authors: Franke M.E. , Sierka M. , Sauer J. , Simon U.
Published: April 2011

The potential energy profile for proton translational motions between two neighboring Alsites in zeolite H-ZSM-5 is calculated by a combined quantum mechanics-interatomic potential function approach. Thepotential energies of the six stable intermediate proton positions and the five transition structures along this path show an almost symmetrical trend reaching the maximum in the middle between these sites. Therefore, the maximum barrier will decrease with decreasing SiO2/Al2O3 ratio of thezeolite. For the SiO2/Al2O3 ratio examined (190) an activation energy of∼ 210 kJ/mol is calculated. This is much lower than the energy of deprotonation, about 1300 kJ/mol. The deprotonation energy of an Al-OH-Si bridge is obviously partially compensated by the proton affinity of the Si-O-Si bridges.
Synthesis of Tetramethylammonium Polyoxovanadates
Authors: Steunou Nathalie , Bouhedja Laure , Maquet Jocelyne , Livage Jacques
Published: April 2011

Tetramethylammonium (TMA) polyoxovanadates have been precipitated from aqueous solutions aroundpH 7. Decavanadate clusters (TMA)4[H2V10O28]•4H2O are formed at room temperature whereas a layered (TMA)[V4O10] mixed valence compound is formed under hydrothermal conditions.51V NMR spectra recorded on the solution at different temperatures show that upon heating decavanadate clusters are progressively transformed into cyclic [V4O12]4−metavanadates. This suggests that the mixed valence polyoxovanadates are formed via the ring opening polymerization of metavanadate precursors.
Chemical delithiation, thermal transformations and electrochemical behaviour of iron- substituted lithium nickelate.
Authors: Lavela Pedro , Pérez-Vicente Carlos , Tirado José L.
Published: April 2011

Chemical deintercalation in Fe-substituted lithium nickelate and its effects on the thermal stability and electrochemical behaviour are studied. A sample with Fe:Ni ratio of 1:9 was used as the starting material. Chemical deintercalation of the ceramic product was achieved by acid treatment with 0.6 M aqueous hydrochloric acid solutions at room temperature. The atomic Fe:Ni ratio remained unaffected while the Li:(Fe+Ni) ratio decreased significantly down to ca. 0.5 after acid treatment. Infrared spectroscopy was used to discard a proton exchange side reaction. The initial open circuit voltage (OCV) of non-aqueous electrolyte lithium cells using the chemically deintercalated solids was ca. 3.7 V, while 3.0 V were obtained with the pristine oxide. Heat treating the deintercalated solids lead to oxygen evolution at 230°C with the formation of spinel rock-salt structure solids at 600°C. The improved thermal stability as compared with iron-free lithium nickelate is an interesting factor for battery safety.
In Situ Probing and Atomistic Simulation of a-Si:H Plasma Deposition
Authors: Aydil Eray S. , Maroudas Dimitrios , Marra Denise C. , Kessels W. M. M. , Agarwal Sumit , Ramalingam Shyam , Sriraman Saravanapriyan , Van de Sanden M. C. M. , Takano Akihiro
Published: April 2011

Hydrogenated amorphous silicon thin films deposited from SiH4 containing plasmas are used in solar cells and thin film transistors for flat panel displays. Understanding the fundamental microscopic surface processes that lead to Si deposition and H incorporation is important for controlling the film properties. An in situ method based on attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy was developed and used to determine the surface coverage of silicon mono-, di-, and tri-hydrides as a function of deposition temperature and ion bombardment flux. Key reactions that take place on the surface during deposition are hypothesized based on the evolution of the surface hydride composition as a function of temperature and ion flux. In conjunction with the experiments, the growth of a-Si:H on H-terminated Si(001)-(2×1) surfaces was simulated through molecular dynamics. The simulation results were compared with experimental measurements to validate the simulations and to provide supporting evidence for radical-surface interaction mechanisms hypothesized based on the infrared spectroscopy data. Experimental measurements of the surface silicon hydride coverage and atomistic simulations are used synergistically to elucidate elementary processes occurring on the surface during a-Si:H deposition.
Amorphous Silicon and Silicon Germanium Alloy Solar Cells Deposited by VHF at High Rates
Authors: Yang Jeffrey , Yan Baojie , Smeets Jozef , Guha Subhendu
Published: April 2011

A modified very high frequency (MVHF) glow discharge technique is used to deposit amorphous silicon (a-Si) and amorphous silicon-germanium (a-SiGe) alloy solar cells at high deposition rates. High quality a-Si alloy solar cells have been obtained by using MVHF at deposition rates up to ∼10 Å/s. The cells show good initial and stabilized efficiencies comparable to those obtained from conventional radio-frequency (RF) glow discharge deposition at low rates (∼1 Å/s). However, high quality a-SiGe alloy solar cells are more difficult to achieve at high deposition rates. In this paper, we present the progress made on a-SiGe alloy solar cells by incorporating bandgap profiling and appropriate buffer layers. Using the improved a-SiGe alloy solar cells, a-Si/a-SiGe tandem configurations are made and results presented.
Carrier Transport in Ultra-Thin Nano/Polycrystalline Silicon Films and Nanowires
Authors: Kamiya Toshio , Tan Yong T. , Furuta Yoshikazu , Mizuta Hiroshi , Durrania Zahid A.K. , Ahmed Haroon
Published: April 2011

Carrier transport was investigated in two different types of ultra-thin silicon films, polycrystalline silicon (poly-Si) films with large grains > 20 nm in size and hydrogenated nanocrystalline silicon (nc-Si:H) films with grains 4 nm – 8 nm in size. It was found that there were local non-uniformities in grain boundary potential barriers in both types of films. Single-electron charging effects were observed in 30 nm × 30 nm nanowires fabricated in 30 nm-thick nc-Si:H films, where the electrons were confined in crystalline silicon grains encapsulated by amorphous silicon. In contrast, the poly-Si nanowires of similar dimensions showed thermionic emission over the grain boundary potential barriers formed by carrier trapping in grain boundary defects.
Nanocrystalline Silicon TFTs With 50 nm Thick Deposited Channel Layer, 10 cm2/Vs Electron Mobility and 108 On/Off Current Ratio
Authors: Min Robert B. , Wagner Sigurd
Published: April 2011

Thin film transistors were made using 50 nm thick directly deposited nanocrystalline silicon channel layers. The transistors have coplanar top gate structure. The nanocrystalline silicon was deposited from discharges in silane, hydrogen and silicon tetrafluoride. The transistors combine a high electron field effect mobility of ∼ 10 cm2/Vs with a low “off” current of ∼ 10−14 A per µm of channel length, and an “on”/“off” current ratio of ∼ 108. This result shows that directly deposited silicon can combine high mobility with low “off” currents.
Identification of Growth Precursors In Hot Wire CVD of Amorphous Silicon Films
Authors: Duan H. L. , Zaharias G. A. , Bent Stacey F.
Published: April 2011

A soft ionization laser-based technique using 10.5 eV photon energy has been used to probe radical growth precursors in the hot wire chemical vapor deposition (HW-CVD) of a-Si:H. Using a Re filament, it is shown that Si, SiH3, and Si2H6are the major silicon-containing species formed from the hot wire dissociation of silane, and SiH2is at most a very minor product. However, chamber history is found to influence the radical species produced; i.e. SiH3 and Si2H6are largely related to the chamber wall and filament conditions. The gas species produced by W and Re filaments at wire temperatures between 1000oC and 2000oC have been studied and compared. Heating the filament to higher temperatures increases the flux of Si, SiH3 and Si2H6 in a similar fashion for both filament materials. Above 1800oC, the Si intensity saturates, while SiH3 and Si2H6show monotonic increase without saturation up to 2000oC.
Super Poly-Si And Transistor Formed by Nickel-Induced-Lateral-Crystallization
Authors: Cheng C.F. , Leung T.C. , Poon M.C.
Published: April 2011

Nickel-Induced-Lateral-Crystallization (NILC) was carried out on a 6000 Å amorphous Si (a-Si) layer. 6000 Å NILC poly-Si was formed and the upper 5000 Å Si layer was then removed. When compared to the conventional 1000 Å NILC poly-Si, the bottom 1000 Å NILC poly-Si layer was found to have relatively larger grains, less grain boundaries and better transistor mobility. The new high quality (super) NILC poly-Si layer can potentially provide greater contributions for novel device and circuit applications.
Microstructure Characterization of Hydrogenated Amorphous Silicon Films by Rare Gas Effusion Studies
Authors: Beyer Wolfhard
Published: April 2011

The effusion of the rare gases neon and helium, as well as of hydrogen, was studied for plasma deposited (boron-doped and undoped) hydrogenated amorphous silicon films, grown at various substrate temperatures. Rare gas atoms were incorporated into the material during the growth process or by ion implantation. The results suggest that helium and neon effusion spectra give information on the material microstructure.
Effects of Hydrogen Dilution on a-Si:H and its Solar Cells Studied by Raman and Photoluminescence Spectroscopy
Authors: Yue Guozhen , Owens Jessica M. , Weinberg-Wolf Jennifer , Han Daxing , Yang Jeffrey , Lord Kenneth , Yan Boajie , Guha Subhendu
Published: April 2011

a-Si:H films and their n-i-p solar cells were prepared using plasma-enhanced CVD. The samples were prepared with no-, low-, standard, and high-H dilution. Raman and photoluminescence (PL) were used to characterize the i-layer. The main results are (a) Raman shows typical a-Si:H mode except for a c-Si peak in the 450 nm-thick film with high-H dilution, and (b) PL shows two regimes. (I) Below the onset of microcrystallinity characterized by x-ray diffraction, a blue-shift of the 1.4 eV PL peak energy and a decrease of the band width occur. (II) Above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4-5, and the PL peak energy is red-shifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.