bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular Dynamics and Relaxation Methods in the Stability Calculations for the Study of Distortions of Confined Nematic Liquid Crystals
Authors: Calles A. , Valladares R.M. , Castro J.J.
Published: April 2011

We present a calculation using a relaxation method for the study of the orientational ordering of a nematic liquid crystal near the surfaces confining the system. We comment on the advantage of using this method as compared with molecular dynamics simulation. The system is simulated by a lattice model with a superposition of isotropic and anisotropic intermolecular interaction of the Maier-Saupe and induced dipole-induced dipole type force for the bulk nematic phase. For the nematic confining surface we consider a Rapini-Papoular interaction. We present simulations for negative dielectric anisotropy.
Ordered Morphologies of Confined Diblock Copolymers
Authors: Tsori Yoav , Andelman David
Published: April 2011

We investigate the ordered morphologies occurring in thin-films diblock copolymer. For temperatures above the order-disorder transition and for an arbitrary two-dimensional surface pattern, we use a Ginzburg-Landau expansion of the free energy to obtain a linear response description of the copolymer melt. The ordering in the directions perpendicular and parallel to the surface are coupled. Three dimensional structures existing when a melt is confined between two surfaces are examined. Below the order-disorder transition we find tilted lamellar phases in the presence of striped surface fields.
Spectral Properties of Various Cerium Doped Garnet Phosphors for Application in White GaN-based LEDs
Authors: Wu Jennifer L. , Denbaars Steven P. , Srdanov Vojislav , Weinberg Henry
Published: April 2011

Recently, a renewed research interest has emerged for the development of visible light, down-converting phosphors for application in white light emitting diodes (LEDs). In such devices, a blue GaN LED can act as the primary light source, exciting photoluminescence in a phosphor with subsequent broad-band emission occurring at lower energies. It was recently reported that use of a combinatorial approach to synthesize and screen potential inorganic phosphors for such an application could aid in identifying improved phosphors for blue to yellow down conversion. Using solution chemistry techniques developed by Symyx Technologies, solid state thin-film arrays (libraries) based on the garnet structure (A1-xBx)3(C1-yDy)5O12:Ce3+, where x and y = 0 to 1.0 and A, B = Y, Gd, or Lu; C., D = Al or Ga, were synthesized. X-ray diffraction was used to select library samples of the crystalline garnet phase. Libraries of these various garnets were then characterized, and their spectral properties compared to traditionally prepared bulk powder phosphors of similar composition. Emission and excitation trends show that as larger cations are substituted for Y (A = Y), emission and excitation are red-shifted and as larger cations are substituted for Al (C = Al), emission and excitation are blue-shifted. If smaller cations are substituted for Y and Al, an opposite trend is observed.
Origin of Ferroelectricity in Aurivillius Compounds
Authors: Y. Donají , Reaney Suárez Ian M. , Lee William E.
Published: April 2011

The structures and microstructures of a range of Aurivillius phases have been investigated using transmission electron microscopy. Superlattice reflections arising from rotations of octahedra around the c-axis have been identified and their intensity at room temperature has been shown to diminish as the tolerance factor of the perovskite blocks increases. The paraelectric to ferroelectric phase transition temperature (Tc) has been monitored using permittivity measurements as a function of temperature and Tc has also been shown to decrease as tolerance factor increases. It is proposed that the onset of octahedral tilting and Tc are related in Aurivillius phases.
Investigation of Lead Borosilicate Glass Structure With 207Pb and 11B Solid-State NMR
Authors: Gibson James M. , Vogt Frederick G. , Barnes Amy S. , Mueller Karl T.
Published: April 2011

A series of three lead borosilicate glasses were synthesized and analyzed for structural information with both 11B and 207Pb solid-state nuclear magnetic resonance (NMR) spectroscopic methods. Results showed that increasing lead content caused lead to take a more active role in the network as a former and that the populations in these sites can be approximately quantified. 207Pb phase-adjusted-spinning sidebands (PASS), 11B magic-angle spinning (MAS), and 11B multiple-quantum MAS (MQMAS) experiments were used to determine structural parameters for the two nuclei. The 207Pb PASS experiment showed that at higher lead content, more covalent bonding was present. This principle was demonstrated in both an overall shift of the spectral resonances and a quantitative change in site ratios. The 11B MAS experiment showed that the ratio of BO3 to BO4 units was dependent on the amount of lead and boron, consistent with previous studies. Preliminary 11B MQMAS experiments failed to detect any BO3– units, previously hypothesized to exist in this system.
Crystal Chemistry of Colloids Obtained by Hydrolysis of Fe(III) in the Presence of SiO4 Ligands
Authors: Doelsch Emmanuel , Masion Armand , Rose Jérôme , Stone William E.E. , Bottero Jean Yves , Bertsch Paul M.
Published: April 2011

The crystal chemistry of a number of Fe-Si systems (Si/Fe 0-4, pH 3-10) was investigated by combining local scale spectroscopic methods (EXAFS, FTIR and NMR) and at the semi local scale (SAXS). The Fe clusters within the precipitates have two growth regimes depending on the Si/Fe ratio: the growth is three and two dimensional for Si/Fe fl and Si/Fe ≤ 1 respectively. The presence of Fe-O-Si bonds within the precipitated phases has been demonstrated. Their formation and relative proportion was found to be very dependent on the pH and Si concentration The size of silica domains within the precipitates was shown to increase with increasing Si/Fe and/or decreasing pH. The high fractal dimension (Df) of the aggregates is attributed to the presence of the SiO4 ligands, but the evolution of Df linearly depends on the polymerization state of iron.
Slater Transition in the Pyrochlore Cd2Os2O7
Authors: Mandrus D. , Thompson J. R. , Woods L. M.
Published: April 2011

Cd2Os2O7 crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. Here we present resistivity, heat capacity, and magnetization results on Cd2Os2O7. Both single crystal and polycrystalline material were examined. We also present LAPW electronic structure calculations on Cd2Os2O7. We interpret the results in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition--unlike a Mott transition--is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below TMIT.
Metal Cyanide Networks Formed at an Air-Water Interface: Structure and Magnetic Properties
Authors: Culp Jeffrey T. , Morgan A. Nicole , Meisel Mark W. , Talham Daniel R.
Published: April 2011

Extended two-dimensional coordinate covalent networks have been formed at an air-water interface by condensing a Langmuir monolayer of an amphiphillic pentacyanoferrate(III) complex and aqueous nickel(II) ions. The cyanide-bridged arrays were transferred to solid supports by the Langmuir-Blodgett (LB) technique. The resulting thin films were structurally characterized by UV-Vis and FT-IR spectroscopies, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and SQUID magnetometry. Magnetic measurements on the thin films transferred to Mylar show evidence for ferromagnetic exchange interactions between the S = 1/2 Fe(III) and S = 1 Ni(II) centers.
Decoration of silicon carbide nanotubes by CoFe2O4 Spinel nanoparticles.
Authors: Estournès Claude , Pham-Huu Cuong , Keller Nicolas , Ledoux Marc J.
Published: April 2011

Silicon carbide nanotubes have been prepared and decorated with CoFe2O4 spinel. These nanocomposites were prepared by incipient wetness impregnation of the dried SiC support with a stoichiometric aqueous solution of the cobalt and iron nitrates. After drying, subsequent annealing treatments were performed under air in the temperature range 873 to 1073 K. X-ray diffraction and high resolution transmission electron microscopy reveal the presence of spinel nanoparticles at the surface of the tubes. Magnetic properties of the nanocomposites show in the magnetic field explored (up to 5 T) that saturation magnetization is not observed neither at room temperature nor at 5Kelvin. The nanometric character and the nature of the spinel particles are also confirmed by the coercive field (1.6 T) observed at low temperature and the curie temperature.
Synthesis of Large ZSM-5 Crystals under High Pressure
Authors: Wang Xiqu , Jacobson Allan J.
Published: April 2011

The synthesis temperature for silicalite-I (MFI) can be raised to 300°C by applying high pressure to stabilize the structure-directing organic template. The elevated temperature and pressure favor theformation of crystals with improved quality. Prismatic silicalite-I crystals with a uniform size of about 0.7× 0.2 × 0.2 mm have been obtained by heating a gel prepared from TMA-silicate solution, TPABr andsodium hexafluorosilicate at 250°C under a pressure of 80 MPa. The influence of synthesis conditions on the crystal sizes has been studied by systematically changing temperature, pressure and gel compositions. Underthe specific conditions of 250°C and 80 MPa, a strong correlation was found between the crystal size andthe F/Si mole ratio of the starting gel, which enables the preparation of uniform crystals of silicalite-I with preset dimensions.
7Li MAS NMR Studies of Lithiated Manganese Dioxide Tunnel Structures: Pyrolusite and Ramsdellite
Authors: Paik Younkee , Lee Young J. , Wang Francis , Bowden William , Grey Clare P.
Published: April 2011

The one-dimensional 1×1 and 1×2 tunnel structures of manganese dioxides, pyrolusit(β-MnO2) and ramsdellite (R-MnO2), respectively, were chemically intercalated with LiI. Two 7Li resonances were observed in lithiated pyrolusite. One isotropic resonance arising at 110 ppm shows a short spin-lattice relaxation time (T1 ∼ 3 ms) and was assigned to Li+ ions in the 1×1 tunnel structure. The other isotropic resonance arising at 4 ppm shows a long spin-lattice relaxation time (T1 ∼ 100 ms) and was assigned to Li+ ions in diamagnetic local environments in the form of impurities such as Li2O or on the surface of the MnO2 particles. Three 7Li resonances were observed in lithiated ramsdellite at very different frequencies (600, 110 and 0 ppm). The resonance at 600 ppm, which is observed at low lithium intercalation levels, is assigned toLi+ ions coordinated to both Mn(III) and Mn(IV) ions in the 1×2 tunnels, while the resonanceat 110 ppm is due to Li+ ions coordinated to Mn(III) ions and appears at higher Li levels. The resonance at 0 ppm is associated with a long spin-lattice relaxation time (T1 ∼ 100 ms) and is also assignedto Li+ ions in diamagnetic impurities.
Microporous Ruthenium Oxide Films for Energy Storage Applications
Authors: Zheng J.P. , Fang Q.L.
Published: April 2011

Highly porous ruthenium oxide films were prepared using ruthenium ethoxide solution at low temperatures. The specific capacitance of the ruthenium oxide film electrode made with ruthenium ethoxide solution is much higher than that made using the traditional ruthenium chloride solution. It was found that amorphous ruthenium oxide films with high porosity could be formed at temperatures of 100-200 °C. At temperatures above 250 °C, crystalline ruthenium oxide films were formed. Electrochemical capacitors were made with ruthenium oxide film electrodes and were tested under constant current charging and discharging. The maximum specific capacitance of 593 F/g was obtained from the electrode prepared at 200 °C. The interfacial capacitance increased linearly with increasing film thickness. A value of interfacial capacitance as high as 4 F/cm2 was obtained from the electrode prepared at 200 °C.
Formation and Erosion of WC under W+ Irradiation of Graphite
Authors: Roth J. , Toussaint U. v. , Schmid K. , Luthin J. , Eckstein W. , Zuhr R. A. , Hensley D.K.
Published: April 2011

The bombardment of C with 100 keV and 1 MeV W at normal incidence is studied as a function of the incident W fluence experimentally and by computer simulation with the program TRIDYN. Calculated oscillations in the amount of retained W and in the target weight change are confirmed experimentally for 100 keV at room temperature. XPS investigations show W2C formation during ion implantation already at room temperature. RBS depth profiles for 1 MeV bombardment show W mobility and surface segregation even at liquid nitrogen (LN2) temperatures. At elevated temperatures W clusters to form nano-particles at the surface and the oscillations in the retained amount of W disappear.
Thermodynamic Model of the Role of Hydrogen Dilution in Plasma Deposition of Microcrystalline Silicon
Authors: Robertson J
Published: April 2011

Hydrogen dilution is used to promote the nucleation and growth of microcrystalline Si (μc-Si) by plasma enhanced chemical vapour deposition (PECVD). The free energy of μc-Si and hydrogenated amorphous silicon (a-Si:H) is analysed as a function of Si:H composition in order to derive the effect of hydrogen dilution. It is shown that increasing the hydrogen content of the a-SiHx precursor phase increases the relative stability of μc-Si slightly, but strongly increases the driving force for nucleation. The higher stability of μc-Si is the fundamental origin of the higher etch rates of a-Si:H, while surface mobility models do not account for sub-surface nucleation of μc-Si.
The Staebler-Wronski Effect and 1/f Noise in Amorphous Silicon
Authors: Belich T. J. , Kakalios J.
Published: April 2011

Experimental measurements of the Q-function (a comparison of the conductivity and thermopower activation energies) and the non-Gaussian statistical character of the 1/f noise in n-type doped a-Si:H as a function of light soaking (the Staebler-Wronski effect) are reported. There is a significant decrease in the non-Gaussian statistical character of the 1/f noise following light soaking of a device quality film, consistent with a slight increase in the long-range disorder. However, there is no change in the Q-function following light exposure, indicating that there is no significant increase in the long-range disorder.
Assessment of the Use of Microcrystalline Silicon Materials Grown at Rates Near 15 Å/s as i-layer Material for Single and Multi-Junction Solar Cells
Authors: Jones S.J. , Crucet R. , Capangpangan R. , Izu M. , Banerjee A.
Published: April 2011

A microwave-based technique has been used to prepare microcrystalline Si (µc-Si) materials rates near 15 Å/s. The use of these materials as intrinsic layers (i-layers) for single and multi-junction devices has been assessed. Since the high deposition rates allow for fabrication of the required thicker µc-Si i-layers in a similar amount of time to that used for high quality a-SiGe i-layers (rates of 1-3 Å/s), the materials are attractive, low cost replacements for a-SiGe bottom cell i-layers in a-Si/a-SiGe and a-Si/a-SiGe/a-SiGe multi-junction cells. Single-junction nip, a-Si/µc-Si and a-Si/a-SiGe/µc-Si devices have been fabricated. For these devices, the doped and amorphous layers were deposited using conventional rf glow discharge processes and deposition equipment separate from that used to fabricate the µc-Si materials. 7.0% efficiencies have been achieved for single-junction devices while pre-light soaked 9.8 and 11.4% efficiencies have been achieved for the tandem and triple-junction devices, respectively. The single-junction devices exhibit a degradation of only 0-2% after long term (1000 hrs.) of light soaking demonstrating a high degree of stability. Based on the present status, the µc-Si material prepared at high rates qualifies as a reasonable candidate for the i-layer of a bottom cell of a triple-junction device. Improvements in the performance, particularly the FF, will be needed before use in single-junction and tandem devices can be considered.
Improvement In Electrical Properties of Polycrystalline Silicon Films by The H2O Vapor Annealing Method
Authors: Sameshima Toshiyuki , Asada Katsimi , Tsunoda Yoshiyuki , Kaneko Yoshiyasu
Published: April 2011

Improvement of electrical properties by heat treatment with high-pressure H2O vapor was discussed for 7.4×1017cm−3 phosphorus-doped pulsed laser crystallized silicon films. The analysis of the electrical conductivity resulted in that 1.3×106-Pa-H2O vapor annealing at 270°C for 3 h reduced the density of defect states from 5.5 ×1018 cm−3(as crystallized) to 5.0×1017 cm−3 for tail states and from 1.0×1018 cm−3(as crystallized) to 3.0×1017 cm−3 for deep level defect states. The potential barrier height at grain boundaries decreased from 0.34 eV (as crystallized) to 0.05 eV by the heat treatment. The combination of oxygen plasma with high-pressure H2O vapor annealing effectively reduced the densities of defect tail sates as well as deep level defect states. It achieved a high performance of thin film transistors with a threshold voltage of 1.3 V and an effective mobility of 160 cm2/Vs.
Towards an All-Hot-Wire TFT: Silicon Nitride and amorphous Silicon deposited by Hot-Wire Chemical Vapor Deposition
Authors: Stannowski B. , Veen M.K. van , Schropp R.E.I.
Published: April 2011

We present thin-film transistors with both amorphous silicon and silicon nitride deposited by hot-wire chemical vapor deposition. Hot-wire amorphous silicon with good electrical properties was deposited from the decomposition of silane at a substrate temperature of 250°C. For Hot-wire silicon nitride we used silane and ammonia at a substrate temperature of 340°C. In this paper we address structural and electrical properties of this material. A high ammonia flow results in porous films that exhibit post-deposition oxidation. By limiting the ammonia/silane ratio to 30, compact layers with a hydrogen content of only 10 at.% and a refractive index of 1.95 are obtained. Using this layer as gate dielectric results in thin-film transistors with good switching behavior and a field-effect mobility of 0.3 cm2/Vs.
Jet-printed Fabrication of a-Si:H Thin-film Transistors and Arrays
Authors: Wong W.S. , Ready S. , Matusiak R. , White S.D. , Lu J.-P. , Lau R. , Ho J. , Street R.A.
Published: April 2011

Phase-change wax-based printed masks were used to fabricate a-Si:H thin-film transistors (TFTs) in place of conventional lithography. Wax-mask features with a minimum feature size of ∼20 [.proportional]m was achieved using an acoustic-ink-printing process. Bottom-gate TFTs with source-drain contacts overlapping the channel were created using a four-mask process. The TFTs have I-V characteristics comparable to photolithographically patterned devices, with mobility of 0.6-1 cm2/V·s, threshold voltage of 2-3 V, and on/off ratios exceeding 107, for devices with channel lengths below 50[.proportional]m.
Illumination Dependence of Microcrystalline PIN Diodes
Authors: Brammer Torsten , Birmans Franz , Krause Mathias , Stiebig Helmut , Wagner Heribert
Published: April 2011

Numerical simulations of the current-voltage characteristics of PECVD-microcrystalline silicon based p-i-n diodes were performed to study the affect of defect density and mobility on solar cell performance. Depending on the combination of both parameters the ideality factor increases or decreases with applied forward bias. The reason is the variable contribution of volume recombination to the total diode current and space charge stored in defect states. The decrease in dark current with reduced hydrogen dilution can partly be attributed to a decrease in recombination centers by the same factor as predicted for midgap defect states by the analytic diode theory. Microcrystalline silicon solar cells deposited in the highly crystalline regime (high H-dilution) are limited by recombination of photogenerated carriers and high dark current. Both can be attributed to a large number of recombination centers. The fill factor of our state-of-theart solar cell is limited by the dark current for small illumination intensities, by series resistance for high illumination levels and by both at its maximum under AM1.5 illumination. Short-circuit current and open-circuit voltage pairs measured under intensities from 10-6 to 30 suns reveal a diode characteristic corresponding to an ideality factor of one at large forward bias.
Ex situ and in situ defect density measurements of a-Si:H by means of the cavity ring down absorption technique
Authors: Smets A.H.M. , van Helden J.H. , van de Sanden M.C.M.
Published: April 2011

For the first time it is demonstrated that the surface defect density can be measured using the cavity ring down (CRD) absorption technique and it is shown that CRD is more sensitive for surface defects than dual beam photoconductivity (DBP) technique. Ex situ measurements have shown that the surface defects of the oxidized a-Si:H surface are distributed over a surface region with thickness W(d). The obtained surface defect density is 1.0 × 1012 up to 1.4 × 1012 cm-2. During growth the a-Si:H surface defect density region has at least a thickness of 15 nm. During deposition of 15 nm a-Si:H the surface density increases up to a not yet saturated value of 1×1013 cm-2.
Study of Defects in Hydrogenated Amorphous Silicon by Constant Photocurrent Method and Positron Annihilation
Authors: Amaral A. , Lavareda G. , de Carvalho C. Nunes , Brogueira P. , Gordo P. M. , Subrahmanyam V. S. , Gil C. Lopes , Naia M.. Duarte , de Lima A. P.
Published: April 2011

The influence of the power density, PD, on the density and structure of defects of undoped a-Si:H thin films, deposited by rf-PECVD, is studied by constant photocurrent method, CPM, and by slow positron beam spectroscopy, respectively. Deep defect density, NDD, remains approximately constant at 10-16 cm-3, typical of device quality material, for PD in the range 7 - 20 mW.cm-3 calculated from CPM. NDD increases roughly one order of magnitude for both low and for high power densities. Positron annihilation spectroscopy reveals the presence of two kinds of vacancy type defects in the films: large vacancy clusters or voids for PD ∼ 7 mW.cm-3 and small vacancy type defects as PD increases to ∼30 mW.cm-3. The relative abundance of the structural defects is shown to be controlled by the power density used during the deposition without affect the electronic deep defect density, significantly.
Charge carrier transport in a-Si:H/c-Si heterojunctions
Authors: Aichberger Susanne von , Wünsch Frank , Kunst Marinus
Published: April 2011

Contactless transient photoconductivity measurements of a-Si:H / c-Si heterojunctions are presented. It is shown that n a-Si:H / n c-Si junctions furnish an excellent passivation of the n c-Si surface. For i a-Si:H / n c-Si junctions the passivation is better for thicker films, whereas for very thin films (10 nm or less) a deviating behaviour probably due to inhomogeneities is observed. For the p a-Si:H / n c-Si junction separation of excess charge carriers in the space charge region is observed leading to a slowly decaying tail of the signal. This separation is also observed in thick i a-Si:H / n c-Si samples where electrons are injected from the a-Si:H film in the c-Si substrate.
High Electron Mobility TFTs of Nanocrystalline Silicon Deposited at 150°oC on Plastic Foil
Authors: Cheng I-Chun , Wagner Sigurd , Bae Sanghoon , Fonash Stephen J.
Published: April 2011

Top gate n-channel thin film transistors (TFTs) of nanocrystalline silicon (nc-Si:H) were fabricated on Kapton polyimide film substrates at a maximum process temperature of 15°C. These are thefirst nc-Si:H TFTs ever made on a plastic substrate. Both intrinsic and n+ source/drain layers were directly deposited at 80 MHz excitation frequency for high growth rate. Coplanar TFTs with top source/drains and staggered TFTs with bottom source/drains were made. The coplanar top gate and top source/drain structures have linear electron mobility of ∼ 30 cm2V-1s-1 and ON/OFF ratio up to ∼ 105. TFTs with the top gate and bottom source/drain structure made to date have linear mobility of up to ∼ 12 cm2V-1s-1 and ON/OFF ratio of ∼105.
T-Site Trapped Molecular Hydrogen in Hot Wire a-Si:H
Authors: Herberg J. , Fedders P. A. , Leopold D. J. , Norberg R. E. , Schropp R. E.
Published: April 2011

Proton NMR and proton-29Si double resonance NMR have been performed on hot wire a-Si:H films deposited from SiH4. Results are compared with those from conventional plasma enhanced chemical vapor deposition a-Si:H. Proton nutational angle studies and proton-29Si spin-echo double resonance (SEDOR) signals at 80 and 294 K indicate that a significant proton resonance population arises from T-site-trapped molecular H2. The hot wire films also display a ≥80 kHz FWHM uperbroad proton line and a sharp feature diamagnetically shifted by 25 ppm.
High rate growth of device grade silicon thin films for solar cells
Authors: Kondo M. , Suzuki S. , Nasuno Y. , Matsuda A.
Published: April 2011

We have developed a plasma enhanced chemical vapor deposition (PECVD) technique for high-rate growth of µc-Si:H at low temperatures using hydrogen diluted monosilane source gas under high-pressure depletion conditions. It was found that material qualities deteriorate, e.g. crystallinity decreases and defect density increases with increasing growth rate mainly due to ion damage from the plasma. We have found that deuterium dilution improves not only the crystallinity but also defect density as compared to hydrogen dilution and that deuterium to hydrogen ratio incorporated in the film has a good correlation with crystallinity. The advantages of the deuterium dilution are ascribed to lower ion bombardment due to slower ambipolar diffusion of deuterium ion from the plasma. Further improvement of material quality has been achieved using a triode technique where a mesh electrode inserted between cathode and anode electrodes prevents from ion bombardment. In combination with a shower head cathode, the triode technique remarkably improves the crystallinity as well as defect density at a high growth rate. As a consequence, we have succeeded to obtain much better crystallinity and uniformity at 5.8 nm/s with a defect density of 2.6×1016cm−3. We also discuss the limiting factors of growth rate and material quality for µc-Si solar cells.
Directional Field Aided Lateral Crystallization of Amorphous Silicon Thin Films
Authors: Izmajlowicz Marek A. T. , Morrison Neil A. , Flewitt Andrew J. , Milne William I.
Published: April 2011

For application to active matrix liquid crystal displays (AMLCDs), a low temperature (< 600 °C) process for the production of polycrystalline silicon is required to permit the use of inexpensive glass substrates. This would allow the integration of drive electronics onto the display panel. Current low temperature processes include excimer laser annealing, which requires expensive equipment, and solid phase crystallization, which requires high temperatures. It is known that by adding small amounts of metals such as nickel to the amorphous silicon the solid phase crystallization temperature can be significantly reduced. The rate of this solid phase metal induced crystallization is increased in the presence of an electric field. Previous work on field aided crystallization has reported crystal growth that either proceeds towards the positive terminal or is independent of the direction of the electric field. In this work, extensive investigation has consistently revealed directional crystallization, from the positive to the negative terminal, of amorphous silicon thin films during heat treatment in the presence of an electric field. This is the first time that this phenomenon has been reported. Models have been proposed for metal induced crystallization with and without an applied electric field in which a reaction between Ni and Si to produce NiSi is the rate-limiting step. The crystallization rate is increased in the presence of an electric field through the drift of positive Ni ions.
Ge Island evolution during growth, in-situ anneal, and Si capping in an industrial CVD reactor
Authors: Loo Roger , Meunier-Beillard Philippe , Dentel Didier , Goryll Michael , Vanhaeren Danielle , Vescan Lili , Bender Hugo , Caymax Matty , Vandervorst Wilfried
Published: April 2011

Si based nanostructures such as Ge or Si1-xGex dots embedded in Si receive a lot of attention. This interest is driven by the reduction of device sizes as well as by their possible use in opto-electronic applications, as a possible solution to improve the radiative light emission. In this paper we give a detailed overview of the growth kinetics as observed for Ge growth in a standard production oriented chemical vapor deposition system. The island morphology and density are controlled by varying the growth conditions or by applying a thermal anneal after the island growth. Island densities up to 2.3x1010 cm−2 have been obtained for depositions at 650°C. With increasing deposition time, the usual change-over from monomodal to bimodal island distribution is pointed out and this change-over depends on the critical island diameter, which decreases with decreasing growth temperature. Applying a thermal budget after the island growth initiates Ge surface diffusion and Si diffusion from the substrate through the islands. This results in an enhancement of the island diameter and height, and also in a reduction of island density. Furthermore, depending on the island distribution after Ge deposition, a transition from pyramid to dome or visa versa is observed after the in-situ anneal. Optical device structures require a Si cap layer. However, Si capping at 700°C, leads to a nearly total dissolution of small islands and a truncation of bigger dome-shaped islands. This can be prevented by reducing the deposition temperature and by changing the Si gas source. Clear island luminescence, is observed up to 200K and lies in the spectral range of 1.35-1.50μm, which is interesting from the point of view of applications. In particular, this shows the potential of this material system for opto-electronic device applications. In spite of the fact that the observed PL intensity is comparable to the best reported values, we could further enhance it by a thermal treatment in a H2 plasma.
Morphology, Phonon Confinement and Properties of a-Si:H Films
Authors: Ligatchev Valeri
Published: April 2011

It is usually assumed that optical and electrical properties of amorphous semiconductors are determined by the short-range-order (SRO) parameters of atomic structure. The SRO parameters behavior is considered here as governed by morphology geometrical parameters (MGP) variations. Generalized Skettrup model is developed for a quantitative description of the MGP influence on the density of electron states distribution N(E). Common N(E) relation for the power, exponential and the ‘defect’ regions of the dependence is derived both for adiabatic (optical) and non-adiabatic (thermal) electron excitation processes. The simulated (by the N(E) convolution) spectral dependence of the opti-cal absorption coefficient contains the Tauc, Urbach and the ‘defect’ parts. The optical gap and the Urbach tail slope energies are typical for the ‘device quality’ a-Si:H films at the average morphology geometrical parameters values of order of 1 μm. Nearly linear the optical gap versus the Urbach tail slope energies dependence is obtained at the mor-phology geometrical parameters and temperature changes. Good agreement of experi-mental and the simulation results is achieved for a-Si:H films, prepared both by the RF sputtering and by the silane decomposition.
Nanoparticle Oxides Precursor Inks for Thin film Copper Indium Gallium Selenide (CIGS) Solar Cells
Authors: Kapur Vijay K. , Fisher Matthew , Roe Robin
Published: April 2011

The paper describes ISET's patented non-vacuum process for low cost mass production of CIGS solar cells. In this process, the water based precursor inks of mixed oxides are deposited on various conducting substrates by a variety of non-vacuum coating techniques. The oxides are converted to CIGS by annealing and the device is completed by deposition of CdS by CBD followed by ZnO deposition by MOCVD. Small area solar cells with efficiency >13% have been fabricated by this process. The advantages of this non-vacuum process are: high compositional control of the absorber layer, high materials utilization and low cost.
Fabrication of Light weight Flexible CIGS solar Cells for Space Power Applications
Authors: Kapur Vijay K. , Fisher Matthew , Roe Robin
Published: April 2011

A novel, non-vacuum and a patented process developed by ISET for manufacturing terrestrial CIGS solar cells was successfully applied to fabricate CIGS solar cells on flexible 50 μm thick Mo foil for space power applications. Solar cells of area 5.0 cm2 were fabricated with their AM0 efficiency approaching 9.0% with an estimated power density of 250 watts/kg. With further improvements it is possible to fabricate CIGS solar cells with AM0 efficiencies approaching 13%. This process can be used to fabricate low cost, light-weight CIGS solar cells on a variety of flexible substrates much lighter than Mo foil to achieve power densities approaching 1.0 kW/kg.
Low Temperature Growth of CIGS Thin Films for Flexible Solar Cells
Authors: Rudmann D. , Haug F.-J. , Kaelin M. , Zogg H. , Tiwari A.N. , Bilger G.
Published: April 2011

Low substrate temperatures have to be used for polymer substrates. Therefore, using soda- lime glass (SLG) substrates with and without an alkali barrier (Al2O3), a three-step CIGS coevaporation process for a substrate temperature of 450 °C has been developed and compared to film deposition with constant evaporation rates. The three-step process was found to enhance grain nucleation. An efficiency of 14.0 % has been achieved with this process for solar cells on SLG. Since polymers in general do not contain Na, a way of Na addition to the absorber is needed. It is shown that NaF coevaporation can be used to control the Na content in CIGS. Also incorporation of Na in CIGS by diffusion from a NaCl layer through a polyimide is demonstrated. With such SLG/NaCl/polyimide structures flexible solar cells can be obtained using a lift-off process. A cell efficiency of 11.6 % (0.99 cm2 area) has been achieved.
Structural and Electronic Properties of Polycrystalline Cu(In,Ga)(S,Se)2 Alloys
Authors: Kötschau I. M. , Turcu M. , Rau U. , Schock H. W.
Published: April 2011

We present a systematic study on the polycrystalline Cu(In,Ga)(S,Se)2 alloys with a gallium to indium ratio of Ga/(Ga+In)<0.3 and a sulfur to selenium ratio varying in the range between 0<S/(S+Se)<1. All samples were grown by coevaporation of the elements at constant rates under high vacuum conditions. The formation of island-like (Cu,S,Se) segregations correlate with the sulfur to selenium ratio in the layer and are found in the growth region near the copper rich phase boundary. These segregations are related to a preferred incorporation of sulfur in the copper rich growth mode. We obtain solar cell grade material from an indium rich growth mode up to a sulfur to selenium ratio of S/(S+Se)=0.9. A detailed analysis of the electronic and optical properties of Mo/CIGSSe/CdS/ZnO:Al heterojunctions allows us to determine the energetic position of the bands within the Cu(In,Ga)(S,Se)2 alloy system. We find that contrary to the Cu(In,Ga)(Se)2 alloy system the valence band position is significantly lowered with increasing bandgap.
Impedance Spectroscopy and Hall Measurements on CdTe Thin Polycrystalline Films
Authors: Gilmore A.S. , Kaydanov V. , Ohno T.R. , Grecu D. , Rose D.
Published: April 2011

Measurements of film impedance in a wide frequency range (impedance spectroscopy, IS) were used to evaluate the grain boundary (GB) and the bulk material (B) resistance. Undoped films demonstrated very low carrier concentration and negligible GB resistance, both due to depletion of the bulk material by the GB deep states. Doping the film with Cu led to a significant increase in the GB resistance, in addition to the expected increase in carrier concentration. Illumination of doped films reduced GB resistance by orders of magnitude. This effect was used for the analysis of Hall concentration and Hall mobility data, aimed at exclusion of GB influence on measured parameters, and was used to determine the carrier concentration and mobility in the grain bulk. Hall studies on undoped films under illumination were also used to estimate the lifetime of the photogenerated carriers.
Solar Cells Based on PVD Grown CuGaSe2 – Absorber and Device Properties
Authors: Schuler S. , Nishiwaki S. , Dziedzina M. , Klenk R. , Siebentritt S. , Lux-Steiner M.Ch.
Published: April 2011

For solar cells based on CuGaSe2(CGS) absorber layers physical vapour deposition (PVD) was employed to deposit polycrystalline CGS thin films. Efficiencies up to 7.9% were achieved by applying a two-stage deposition process for absorber preparation. The high structural quality of the absorbers is shown. For highest cell efficiencies our standard recipe for the CdS buffer deposition had to be re-adapted. Voltage dependent spectral response measurements of the device in conjunction with absorption measurements of the absorber layers allow calculation of the minority carrier diffusion length, space charge region (SCR) width, absorber carrier concentration and the built-in voltage. The defect state density at the buffer/absorber interface is estimated. It can be shown that the benefical effect of the modified buffer deposition is mainly due to a reduced acceptor concentration at the absorber/buffer interface.
Challenges in Integrating the High-K Gate Dielectric Film to the Conventional Cmos Process Flow
Authors: Agarwal Avinash , Freiler Michael , Lysaght Pat , Perrymore Loyd , Bergmann Renate , Sparks Chris , Bowers Bill , Barnett Joel , Riley Deborah , Kim Yudong , Nguyen Billy , Bersuker Gennadi , Shero Eric , Lim Jae E. , Lin Steven , Chen Jerry , Murto Robert W. , Huff Howard R.
Published: April 2011

ZrO2 and HfO2 and their alloys with SiO2 are currently among the leading high-k materials for replacing SiOxNy as the gate dielectric for the sub-100 nm technology nodes. International SEMATECH (ISMT) is currently investigating integration issues associated with this required change in materials. Our work has focused on the integration of ALCVD deposited ZrO2 and HfO2 with an industry standard conventional MOSFET process flow with poly-Si electrode. Since the impact of contamination by these new high-k materials introduced in a production fab has not yet been established, it becomes very critical to prevent cross- contamination through the process tools in the fab. A baseline study was completed within ISMT's fab and appropriate protocols for handling high-k materials have been established. The integrated high-k gate stack in a conventional transistor flow should not only meet all the performance requirements of scaled transistors, but the gate dielectric film should be able withstand high-temperature anneal steps. Reactions between ZrO2 and Si have been observed at temperatures as low as 560°C (during the amorphous Si deposition process). Various wet chemistries were also evaluated for removing the high-k film inadvertently deposited on wafer backside, and it was found that ZrO2 etches at extremely slow rates in the majority of the common wet etch chemistries available in a fab. A new hot HF based process was found to be successful in lowering Zr contamination on the wafer backside to as low as 1.8 E10 atoms/cm2. The patterning of a high-k gate stack with poly-Si electrode is another area that required considerable focus. Various dry (plasma) etch and wet etch chemistries were evaluated for etching ZrO2 using both blanket films as well as wafers with patterned poly-Si gate over the high-k films. On the full CMOS flow device wafers, most of these wet chemistries resulted in severe pitting in the ZrO2 film remaining over the source/drain (S/D) areas, as well as in the Si substrate and the field oxide. A poly-Si gate over ZrO2 gate dielectric film was successfully patterned using the standard poly-Si gate etch (Cl2/HBr) for the main etch, followed by a combination of HF and H2SO4 clean for removing all of the ZrO2 remaining over the S/D area. This allowed the fabrication of low-resistance contacts to transistor S/D areas, which ultimately resulted in demonstration of functional transistors with high-k gate dielectric films.
Atomic Scale Nitridation of Silicon Oxide Surfaces by Remote-Plasma-Excited Nitrogen
Authors: Saito Yoji , Tokuda Koichi
Published: April 2011

We have incorporated several percent of nitrogen only near the top surfaces of thermally grown oxides by exposure to fluorine gas at room temperature followed by an atomic nitrogen treatment at 550°C. The depth profiles and the bonding of incorporated nitrogen atoms have been studied by angle-resolved x-ray photoelectron spectroscopy.

MOS devices were fabricated using the nitrided oxide with boron doped polycrystalline silicon gate. From the capacitance-voltage measurements we confirmed that the nitrided oxide would prevent the boron penetration in comparison with the conventional oxide films. The proposed technique identifies a unique process for obtaining high quality ultrathin dielectrics.
Influence of Film/Substrate Interface Structure on Plasticity in Metal Thin Films
Authors: Dehm G. , Inkson B.J. , Balk T.J. , Wagner T. , Arzt E.
Published: April 2011

In-situ transmission electron microscopy studies of metal thin films on substrates indicate that dislocation motion is influenced by the structure of the film/substrate interface. For Cu films grown on silicon substrates coated with anamorphousSiNxdiffusion barrier, the transmission electron microscopy studies reveal that dislocations are pulled towards the interface, where their contrast finally disappears. However, in epitaxial Al films deposited on single-crystalline α- Al2O3substrates, threading dislocations advance through the layer and deposit dislocation segments adjacent to the interface. In this latter case, the interface is between twocrystallinelattices. Stresses in epitaxial Al films and polycrystalline Cu films were determined by substrate- curvature measurements. It was found that, unlike the polycrystalline Cu films, the flow stresses in the epitaxial Al films are in agreement with a dislocation-based model.
Thickness-fringe Contrast Analysis of Defects in GaN
Authors: Farrer Jeffrey K. , Carter C. Barry , Mao Z. , McKernan Stuart
Published: April 2011

The analysis of thickness-fringe contrast in weak-beam transmission electron microscope (TEM) images has been shown to be a reliable method for the complete determination of the character, as well as the magnitude, of a dislocation Burgers vector. By selecting multiple diffraction conditions and, for each condition, determining the number of terminating thickness fringes at the exit of a dislocation from a wedge-shaped sample, the Burgers vector can be unambiguously determined. Defect analysis of GaN pyramids grown on (111)Si by the lateral epitactic overgrowth (LEO) technique reveals a core region which contains a relatively high density of dislocations and a lateral-growth region where the defect density is decreased. The thickness-fringe contrast technique was used in the lateral growth regions of the pyramids to analyze the dislocation Burgers vectors.
Dislocations in thin metal films observed with X-ray diffraction
Authors: Seijbel Léon J. , Delhez Rob
Published: April 2011

X-ray diffraction has been used to measure the stress, the crystallite size and the dislocation distribution in thin metal layers. By measuring two orders of a reflection, the contribution of the size distribution to the diffraction line broadening can be eliminated. A model equation is fitted to the strain Fourier coefficients of the diffraction line from which the dislocation arrangement can be obtained. For untextured nickel on steel or on silicon the dislocation densities have been obtained. It is demonstrated that for highly textured layers more information can be obtained than for untextured layers. It was found that a heated molybdenum layer on oxidized silicon showed only inclined screw dislocations.
Integration of Materials and Device Research Enabled by Wafer Bonding and Layer Transfer
Authors: Tong Q.-Y.
Published: April 2011

Wafer bonding and layer transfer technology has emerged as a versatile approach for integrated research and development of materials and devices. It does not only provide a flexible way to prepare integrated materials but also breaks the barrier between materials science and device engineering since it appears to be one of the fundamental technologies for 3-D device fabrication and for integration of partially or fully processed dissimilar functional layers. The device performance can be significantly enhanced when both sides of device layers can be processed such as in the double gate CMOS and in HBTs. The processed device layers can be considered as unique materials layers and can also be integrated. The integration of dissimilar integrated circuits provides a promising solution to realize micro-systems or system on a 3-D chip in which multi-functional layers are interconnected (3-D SOC). The main issues are to develop methods that form strong bond between required materials at low temperatures and to cut the device layer without compromising the device integrity by using VLSI compatible processes. The advances in low temperature bonding of hydrophilic and hydrophobic silicon, compound semiconductors and other materials in ambient are reviewed, and application examples are discussed.
TEM measurement of hydrogen pressure within a platelet
Authors: Grisolia J. , Assayag G. Ben , de Mauduit B. , Claverie A. , Kroon R.E. , Neethling J.H.
Published: April 2011

Proton implantation and thermal annealing of silicon result in the formation of a specific type of extended defect involving hydrogen, named “platelets”. These defects have been related to the exfoliation mechanism on which a newly developed process to transfer thin films of silicon onto various substrates is based. In a previous paper, we have shown that these platelets undergo a quasi-conservative Ostwald ripening upon annealing. The measurement of the pressure within such pressurised gas-filled cavities is important to understand and simulate both the growth of these defects and the exfoliation mechanism. To extract this pressure from TEM studies, we have developed and tested an analogy between the platelets and a well-known 2D defect: a dislocation loop. The comparison between simulations of the image of the strain field surrounding a fictitious dislocation loop and experimental TEM images of the platelets shows that the platelets can be described by a Burgers vector of about 0.6nm. Moreover, this vector can be used to deduce the pressure of the molecular hydrogen within a platelet. A typical value of 10 GPa is found for a platelet of 20 nm in diameter at room temperature. Consequently, the atomic density of hydrogen within a platelet and the total number of hydrogen trapped by a population of platelet can be calculated and give reasonable values when compared to the implanted dose.