bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron Spin Resonance Characterization of Defects at Interfaces in Stacks of Ultrathin High-κ Dielectric Layers on Silicon
Authors: Stesmans A. L. , Afanas'ev V.V.
Published: April 2011

Electron spin resonance (ESR) analysis of (100)Si/SiOx/ZrO2, (100)Si/Al2O3 and Si/HfO2 structures with nm-thin dielectric layers deposited by different chemical vapor deposition procedures reveals, after hydrogen detachment, the presence of the trivalent Si dangling-bond-type centers Pb0, Pb1 as prominent defects in all entities. This Pb0, Pb1 fingerprint, generally unique for the thermal (100)Si/SiO2 interface, indicates that the as-deposited (100)Si/metal oxides interface is basically Si/SiO2-like. Though sensitive to the deposition process, the Pb0 density is found to be substantially larger than in standard (100)Si/SiO2. As probed by the Pb- type center properties, the Si/dielectric interfaces of all structures are under enhanced (unrelaxed) stress, typical for low temperature Si/SiO2 growth. Standard quality thermal Si/SiO2 properties in terms of Pb signature may be approached by appropriate annealing (≥ 650°C) in vacuum in the case of (100)Si/SiOx/ZrO2. Yet, O2 ambient appears required for Si/Al2O3 and Si/HfO2. It appears that Si/high-κ metal oxide structures with device grade quality interfaces can be realized with sub-nm thin SiOx interlayers. The density of fast interface states closely matches the Pb0 density variations, suggesting the center as the dominant fast interface trap. They may be efficiently passivated in H2 at 400 °C.
On the interface quality of MIS structures fabricated from Atomic Layer Deposition of HfO2, Ta2O5 and Nb2O5−Ta2O5−Nb2O5 dielectric thin films
Authors: Dueñas S. , Castán H. , García H. , Barbolla J. , Kukli K. , Ritala M. , Leskelä M.
Published: April 2011

A study of metal-insulator-semiconductor (MIS) structures based on atomic layer deposited HfO2, Ta2O5 and Nb2O5−Ta2O5−Nb2O5 thin films is presented. Our attention was focussed on interface quality and defect density in the dielectric. Interface states as well as defects inside the insulator bulk were measured by using capacitance-voltage (C-V), deep level transient spectroscopy (DLTS) and conductance transient (G-t) techniques. Nb2O5−Ta2O5−Nb2O5 based capacitors exhibit the highest interface state density, whereas the minimum is obtained for HfO2. Conductance transients are not observed in Al/HfO2/SiO2/Si stacks, thus indicating that disordered induced gap states (DIGS) are not present in these structures. We also observed that post-metallization annealing in forming gas diminishes the trap interface density at the expense of increasing DIGS in the Al/HfO2/Si cases.
Determination of Nano Fluctuations in Surface Oxides of GaSb With Br-IBAE
Authors: Krishnaswami K. , Krejca B. , Vangala S.R. , Santeufemio C. , Allen L.P. , Ospina M. , Liu X. , Sung C. , Vaccaro K. , Goodhue W.D.
Published: April 2011

High-quality substrates with thin uniform oxide layers are critical to the development of low-power epitaxy-based GaSb electronic and electro-optic devices. In this material system, the thickness and elemental composition of the oxide layers are a strong function of the surface preparation method. Here, a figure of merit for quantifying the thickness and thickness variations of an oxide layer on GaSb has been developed using bromine ion beam assisted etching (Br-IBAE). Furthermore, the overall thickness and composition of the oxide determines the etch depth for a fixed set of Br-IBAE parameters. The etch rate in GaSb was determined to be ∼395nm/min and that of its surface oxides was measured to be between 1.86 to 4.2nm/min. Hence, due to the difference in etch rates, nano fluctuations of the oxide thickness at the oxide surface and at the substrate/oxide interface, result in amplified roughness fluctuations in the etched GaSb substrate. The average roughness of Br-IBAE etched surfaces is used as figures of merit to quantify the uniformity and smoothness of the oxide layer. To demonstrate this, GaSb surfaces with various oxide layers and thicknesses were identically etched and their surface roughness measured using atomic force microscopy (AFM). This novel technique can generally be extended to a number of material systems.
Atomic-scale investigation of the dielectric screening at the interface between silicon and its oxide
Authors: Giustino Feliciano , Pasquarello Alfredo
Published: April 2011

We investigate the dielectric screening across the Si-SiO2 interface using a first-principle approach. By determining the profile of the microscopic polarization and the effective polarizabilities of SiOn (n = 0,‥4) structural units, we show that the variation of the local screening across the interface relates to the chemical grading. The oxide region near the Si substrate shows the same dielectric permittivity as bulk SiO2 as long as the oxide is locally stoichiometric. The suboxide region carries an enhanced permittivity, with a value intermediate between those corresponding to bulk Si and SiO2. The implications of these findings for the scalability of the equivalent oxide thickness in high-κ gate stacks are discussed.
Precise Characterization of Silicon on Insulator (SOI) and Strained Silicon on Si1−xGex on Insulator (SSOI) Stacks with Spectroscopic Ellipsometry
Authors: Sun Lianchao , Fouere Jean-Claude
Published: April 2011

Further improvements in CMOS circuit performance such as switching speed and power reduction will rely on the use of silicon on insulator (SOI) substrates with decreased functional layer thicknesses. According to the International Technology Roadmap for Semiconductors (ITRS), the silicon and buried SiO2 (BOX) layer thicknesses for a fully depleted device should be in the ranges of 10–16nm and 24 – 40nm by 2005, respectively. A key issue for fully-depleted CMOS transistors is control of such ultra-thin layer thicknesses and their uniformity along with other parameters such as surface and interface roughness. This poses a challenge to metrology, especially to conventional reflectometry technique because the layer thicknesses must be determined with angstrom precision for both silicon cap and SiO2 box layer.

Spectroscopic ellipsometry (SE) is an optical and non destructive technique for determining thin film thickness and material optical properties. Because ellipsometry measures the change in the polarization state for both the amplitude ratio of the p to s polarizations, and in the phase retardation, it provides a precise way to characterize such ultra thin SOI stacks. Comprehensive characterization results for a number of thin and ultra thin SOI stacks with different thickness ranges will be presented together with measurement repeatability results relevant to the film thickness process tolerances. In addition, characterization results for advanced device applications such as strained silicon-on- Si1−xGex-on-insulator (SSOI) will also be shown, demonstrating the use and capability of spectroscopic ellipsometry for precise determination of layer thickness, material composition, interfacial layers, etc. Principles and advantages of the technique will also be discussed in the presentation.
A New Mechanistic Diagram for Molecularly Imprinted Polymers
Authors: Spivak David A.
Published: April 2011

A new mechanistic diagram describing the non-covalent molecular imprinting process is put forth in the text. A significant consequence of the new mechanistic picture is that the pre-polymer complex structure does not necessarily reflect the structure of the final binding sites in the polymer. Two independent studies are presented in combined form that support the suggested changes to the mechanistic diagram. In the first study, the maximum number of functional groups surrounding the template molecule in solution are shown to be less than the average number of functional groups in the binding sites of the polymers. In the second study, shape selectivity is shown to be an important contributor to molecular recognition by the imprinted polymers; which is significant because contributions of shape cannot be predicted by the solution phase pre-polymer complex.
Development of a high throughput synthesis technique for the optimization of MIPs for 17β-Estradiol
Authors: Dirion B. , Schillinger E. , Sellergren B.
Published: April 2011

A high throughput synthesis technique was developed for the combinatorial optimization of polymers imprinted with 17□-Estradiol. This incorporated a liquid handling robot for the rapid dispensing of monomers, templates, solvents and initiator into the reaction vessels of a 96-well plate. A library of 80 polymers, each ca 80mg, was prepared using functional monomers containing acid functionalities (methacrylic acid (MAA), trifluoromethylacrylic acid (TFMAA)), basic functionalities (2-vinylpyridine (2-Vpy), 4-vinylpyridine (4-Vpy), diethyl-2-aminoethylmethycrylate (DEAEMA)) or neutral functional groups (2-hydroxyethylmethacrylate (HEMA), methacrylamide (MAAM), N-vinylpyrrolidone (NVP), acetoxystyrene (AST)). Four replicas each of imprinted and nonimprinted polymers were made for each functional monomer using ethyleneglycoldimethacrylate (EDMA) as crosslinking monomer and acetonitrile as solvent. Polymer washing and exchange of incubation solutions could be effectively carried out after transfer of the polymers to solvent resistant filter plates. The binding properties of the polymers could then be rapidly assessed in the batch mode by quantifying non-bound fractions in parallel using a fluorescence monochromator plate reader. These data showed an acceptable agreement with the data from a corresponding HPLC evaluation of the rebinding solutions. In agreement with other reports, the acid functional monomers MAA and TFMAA, the basic DEAEMA and the neutral MAAM exhibited the highest imprinting factors.
Phase Inversion Molecularly Imprinting of Uracil Targeted Membranes Made of Polyacrylonitrile Copolymers Having Methacrylic Acid and Acrylic Acid Segments for Recognition and Permselective Binding
Authors: Xia Shao Ling , Wang Hong Ying , Kobayashi Takaomi
Published: April 2011

Polyacrylonitrile having methacrylic acid [P(AN-co-MAA)] and acrylic acid [P(AN-co-AA)] was used for phase inversion imprinting of uracil (URA). Resultant imprinted membranes had porous morphology and showed permselective binding of URA, when 32 μM URA aqueous solution was permeated under pressure gradient of 100 Pa across the membrane. Under the circumstance, the imprinted P(AN-co-MAA) membrane bound 7.9 μmol/g of URA with 8.4×10−6 m3/m2·s of volume flux. The imprinted P(AN-co-AA) membrane showed loose selectivity relative to the P(AN-co-MAA) membrane. Binding behavior of URA and dimethyluracil (DMURA) was also compared in both imprinted membranes. Evidence was presented that P(AN-co-MAA) with methacrylic segments was effective for URA imprinting and resulted in high URA selectivity in the permselective binding experiments.
Control of Morphology and Growth Direction of Gallium Nitride Nanostructures
Authors: Bae Seung Yong , Seo Hee Won , Park Jeunghee
Published: April 2011

Various shaped single-crystalline gallium nitride (GaN) nanostructures were produced by chemical vapor deposition method in the temperature range of 900–1200 °C. Scanning electron microscopy, transmission electron microscopy, electron diffraction, x-ray diffraction, electron energy loss spectroscopy, Raman spectroscopy, and photoluminescence were used to investigate the structural and optical properties of the GaN nanostructures. We controlled the GaN nanostructures by the catalyst and temperature. The cylindrical and triangular shaped nanowires were synthesized using iron and gold nanoparticles as catalysts, respectively, in the temperature range of 900 – 1000 °C. We synthesized the nanobelts, nanosaws, and porous nanowires using gallium source/ boron oxide mixture. When the temperature of source was 1100 °C, the nanobelts having a triangle tip were grown. At the temperature higher up to 1200 °C the nanosaws and porous nanowires were formed with a large scale. The cylindrical nanowires have random growth direction, while the triangular nanowires have uniform growth direction [010]. The growth direction of the nanobelts is perpendicular to the [010]. Interestingly, the nanosaws and porous nanowires exhibit the same growth direction [011]. The shift of Raman, XRD, and PL bands from those of bulk was correlated with the strains of the GaN nanostructures.
Surface States in Passivated, Unpassivated and Core/Shell Nanocrystals: Electronic Structure and Optical Properties
Authors: Bryant Garnett W. , Jaskolski W.
Published: April 2011

Surface effects significantly influence the functionality of semiconductor nanocrystals. A theoretical understanding of these surface effects requires models capable of describing surface details at an atomic scale, passivation with molecular ligands, and few-monolayer capping shells. We present an atomistic tight-binding theory of the electronic structure and optical properties of passivated, unpassivated and core/shell nanocrystals to study these surface effects.
Degree of Asymmetry of CdSe Quantum Dots Grown in Glass Probed by Four Wave Mixing
Authors: Filin A. I. , Babocsi K. , Schmitt M. , Persans P. D. , Kiefer W. , Kulakovskii V. D.
Published: April 2011

Information about quantum dot (QD) asymmetry is derived by analyzing the polarization properties of the time-integrated four wave mixing (FWM) signal. The lowering of QD symmetry results in the splitting of bright J= +/-1 exciton states. This causes the polarization oscillation of circularly excited excitons between these two split states. In a QD ensemble with a random distribution in the exciton level splitting, this results in the decay of the difference in FWM signals observed in scattering of σ+ and σ+ polarized light on the population grating created by two σ+ pulses, the decay time reflecting the degree of QD asymmetry. We have investigated the decay time of the difference in two polarized signals for quantum dots of equal size, grown in a glass matrix under different conditions. Increasing growth temperature and decreasing growth time lead to lowering of QD symmetry. We discuss this experimental result in terms of kinetics of nanoparticle growth in glass.
Vapor-Phase Synthesis and Surface Functionalization of ZnSe Nanoparticles in a Counterflow Jet Reactor
Authors: Sarigiannidis Christos , Petrou Athos , Mountziaris T. J.
Published: April 2011

Compound semiconductor nanocrystals (quantum dots) exhibit unique size-dependent optoelectronic properties making them attractive for a variety of applications, including ultrasensitive biological detection, high-density information storage, solar energy conversion, and photocatalysis. There is presently a great need for developing scalable techniques that allow efficient synthesis, size control, and functionalization of quantum dots, without a loss of the desirable optical properties. We report experimental results on the properties and surface modification of ZnSe nanoparticles grown by a continuous vapor-phase technique utilizing an axisymmetric counterflow jet reactor. Luminescent ZnSe nanocrystals were obtained at room temperature by reacting vapors of dimethylzinc:triethylamine adduct with hydrogen selenide, diluted in a hydrogen carrier gas. The two reactants were supplied from opposite inlets of the counterflow jet configuration and initiated particle nucleation in a region near the stagnation point of the laminar flow field. Surface modification of nanoparticles by adsorption of 1-pentanethiol was used to control the rate of particle coalescence. The counterflow jet technique can be scaled up for commercial production and is compatible with other vapor-phase processing techniques used in the microelectronics industry.
Synthesis and Size Control of Luminescent II-VI Semiconductor Nanocrystals by a Novel Microemulsion-Gas Contacting Technique
Authors: Karanikolos Georgios N. , Alexandridis Paschalis , Petrou Athos , Mountziaris T. J.
Published: April 2011

A scalable, room-temperature technique for controlled synthesis of luminescent II-VI nanocrystals has been developed by using the dispersed phase of stable, well-characterized microemulsions as templates for nanoparticle synthesis. The microemulsions were formed by self-assembly of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) amphiphilic block copolymer and heptane in formamide. By adjusting the surfactant to dispersed phase ratio, stable microemulsions were obtained with droplet diameter of ∼40nm. These microemulsions avoid problems or rapid droplet-droplet coalescence that hamper reverse micelles and lead to polydisperse particle populations. Luminescent ZnSe quantum dots were synthesized by reacting diethylzinc (dissolved in the heptane dispersed phase) with hydrogen selenide gas (diluted in hydrogen). The gas was bubbled through the microemulsion, dissolved in the formamide, and diffused to the nanodroplet interfaces to react with diethylzinc. The experiments indicate that a single nanocrystal is formed in each nanodroplet by coalescence of clusters (nuclei) and smaller crystals. The energy released during coalescence is sufficient to anneal the clusters into high-quality crystals. The process allows precise control of nanocrystal size by adjusting the initial concentration of diethylzinc in heptane. The “as grown” nanocrystals exhibit size-dependent luminescence, narrow and symmetric emission, good monodispersity (confirmed by TEM analysis), and excellent photochemical stability. The technique is currently being extended to the synthesis of CdSe nanocrystals with promising preliminary results.
Preparation of Tungsten Bronze Nanowires
Authors: Qi Hang , Wang Cuiying , Liu Jie
Published: April 2011

Two simple techniques to prepare tungsten bronze nanowires are reported in this paper. Tungsten bronze nanowires with a diameter of less than 100nm and a length of more than 10 μm have been successfully prepared by employing these techniques. A Vapor-Liquid-Solid (VLS) mechanism has been proposed to explain the growth of these tungsten bronze nanowires.
A Novel Tick Film Nanocrystalline Y2O3:Eu3+ Phosphor Synthesized by Post-Dispersion Treatment at Low Temperature
Authors: Hong Sung-Jei , Kwak Min-Gi , Moon Dae-Gyu , Kim Won-Keun , Han Jeong-In
Published: April 2011

In this study, a novel thick film nanocrystalline phosphor is synthesized by using postdispersion treatment at low temperature. The post-dispersion treatment is to prevent the agglomeration between the precursors by mixing the organic dispersing agent with them before heat-treatment. The mean size of the particle heat-treated at 500°C is 4nm. Also, europium is uniformly distributed in the yttrium oxide with deviation below 0.5%. The nanocrystalline phosphor is principally composed of cubic structure having preferred orientation of <222>, <440>, and <400>. The photoluminescence properties of the nanocrystalline phosphor characterized with monochrometric systems reveals that the main PL peak is detected at 611 nm, and thick film nanocrystalline phosphor with 15wt% europium exhibits better properties.
Quantum Calculations of Carbon Nanotube Charging and Capacitance
Authors: Pomorski Pawel , Pastewka Lars , Roland Christopher , Guo Hong , Wang Jian
Published: April 2011

Although it has long been known that the classical notions of capacitance are altered at the nanoscale, few first principles calculations of these properties exist for real material systems. With a recently developed ab initio formalism, which combines nonequilibrium Greens function techniques with real-space density functional calculations, we have investigated charging effects for carbon nanotube systems, which are described by the capacitance coefficients. Specifically, the capacitance matrix of two nested nanotube armchair nanotubes, the insertion of one nanotube into another, and the properties of a nanotube acting as a probe over a flat aluminum surface were considered.
Design of Water-Soluble Quantum Dots with Novel Surface Ligands for Biological Applications
Authors: Tetsuo Uyeda H. , Medintz Igor L. , Mattoussi Hedi
Published: April 2011

We have designed a series of organic oligo- and polyethylene glycol (PEG) based surface capping ligands that allow for QD manipulation in aqueous media. We utilized readily available thioctic acid and various oligo- and polyethylene glycols in simple esterification schemes, followed by reduction of the dithiolane to produce multi-gram quantities of capping substrates. Cap exchange of the native trioctyl-phosphine and -phosphine oxide based ligands with the PEG-terminated dithiol-alkyl cap readily resulted in aqueous dispersions of QDs that were homogeneous and stable in various pH ranges over an extended period of time. Mixed surface capping strategies utilizing ratios of dihydrolipoic acid to the pegylated dihydrolipoic acid were also prepared. We anticipate that such systems should allow one to covalently attach amine containing biomolecules to nanoparticle systems bearing carboxylates, employing known coupling agents, such as (dimethylamino) propyl-3-ethyl-carbodiimide (EDC). This design and conjugation strategy may facilitate the development of a new generation of QD-bioconjugates, which can be directly utilized in bio-related applications such as sensing and cellular imaging.
Optimization of Quantum Dot – Nerve Cell Interfaces
Authors: Winter Jessica , Schmidt Christine , Korgel Brian
Published: April 2011

Nerve cells communicate by passing electrical signals through extensions from their cell bodies. Micron-scale devices such as microelectrode arrays and field effect transistors have already been used to externally manipulate these signals. However, these devices are almost as large as the cell body of the neuron. In order to make functional electrical connections to nerve extensions or their component ion channels that propagate signals, it will be necessary to utilize increasingly smaller components. We propose the use of semiconductor quantum dots, which can be optically activated, as a potential means of perturbing the nerve membrane potentials. As a first step, we have created qdot-neuron interfaces with cadmium sulfide quantum dots. The qdots may be attached to cells either non-specifically or through selected interactions exploiting biorecognition molecules attached to the qdot particle surface. We have investigated the effect of altered synthesis conditions including pH, concentration, reactant ratio, ligand length, ligand R group, and ligand concentration on the nature and quality of qdot-cell binding. We discuss the effect of these altered synthesis conditions on particle fluorescence intensity and color. Additionally, we studied the interaction of these particles with cells and determined that larger particles are more likely to bind non-specifically than smaller particles produced with the same amount of passivating ligand. It is possible this results from reduced surface area coverage of passivating chemical. Finally, we have produced particles passivated with the biorecognition peptide CGGGRGDS in the absence of other chemical stabilizers and characterized their surfaces using NMR and IR.
Growth Structure, and Optical Properties of III-Nitride Quantum Dots
Authors: Morkoç Hadis , Neogi Arup , Kuball Martin
Published: April 2011

Quasi-zero-dimensional (0D) semiconductors have been the subject of considerable interest which is stemmed from their unique physical properties which in turn are conducive to devices such as low threshold lasers and light polarization insensitive detectors, in addition to exciting basic physical phenomena. A laboratory analogue of 0D systems is semiconductor quantum dots (QDs) wherein the electronic states are spatially localized and the energy is fully quantized, loosely similar to an atomic system, making it more stable against thermal perturbations. In addition, the electronic density of states near the band gap is higher than in 3D and 2D systems, leading to a higher probability for optical transitions. Furthermore, the electron localization may dramatically reduce the scattering of electrons by bulk defects and reduce the rate of non-radiative recombination. Semiconductor based and metal based dots have been produced, the former via self-assembly and also by lithographic methods in many II-VI, III-V, and group IV semiconductor. The aim of this paper is focused on III-Nitride based quantum dots covering their production and optical properties, as well as reporting on the GaN quantum dots produced by molecular beam epitaxy utilizing standard, ripening, metal spray followed by nitridation methods.
Anisotropy in Thermoelectric Properties of CsBi4Te6
Authors: Chung Duck-Young , Mahanti S. D. , Chen Wei , Uher Citrad , Kanatzidis Mercouri G.
Published: April 2011

CsBi4Te6 (ZT ∼ 0.8 at 225 K) shows highly anisotropic features in its crystal morphology and structure as expressed by the parallel infinite [Bi4Te6] rods which are linked via Bi-Bi bonds. Band calculations also point to a significant anisotropy in the carrier effective masses, and for this reason we examined the anisotropic thermoelectric properties of CsBi4Te6. The electrical conductivity, thermopower and thermal conductivity were measured along the three different crystallographic directions of the monoclinic structure of CsBi4Te6. These measurements were performed on samples with different degrees of doping. The strong charge transport anisotropy of these samples was confirmed and also observed that the thermopower values along the c-axis direction (which is perpendicular to the layer of Cs atoms) was negative (-80 μV/K) while those along the needle direction (b-axis) and parallel to the [Bi4Te6] layers (a-axis) were p-type (50–100 μV/K at room temperature. Other anisotropic features in the crystal growth habit, electronic band structure, and electrical and thermal conductivities are also presented.
Electronic Structure and Thermoelectric Properties of AxMo3Sb5Te2
Authors: Soheilnia Navid , Kleinke Holger
Published: April 2011

Mo3Sb7 may be chemically modified to become semiconducting by replacing two Sb atoms with two Te atoms (per formula unit). This material may be an attractive candidate for the thermoelectric energy conversion, as its thermal conductivity may be lowered by creating the rattling effect upon intercalation of small cations, and its band structure may be tailored, i.e. the band gap size modified. The higher the Te content and the higher the cation amount, the smaller is the band gap, which can virtually reach any value below 0.5 eV.
Electronic Structure Of (AgSb)xPbn-2xTen
Authors: Bilc Daniel I , Mahanti S.D. , Kanatzidis M.G.
Published: April 2011

Complex quaternary chalcogenides (AgSb)xPbn-2xTen (0<x<n/2) are thought to be narrow band-gap semiconductors which are very good candidates for room and high temperature thermoelectric applications. These systems form in the rock-salt structure similar to the well known two component system PbTe (x=0). In these systems Ag and Sb occupy Pb sites randomly although there is some evidence of short-range order. To gain insights into the electronic structure of these compounds, we have performed electronic structure calculations in AgSbTe2 (x=n/2). These calculations were carried out within ab initio density functional theory (DFT) using full potential linearized augmented plane wave (LAPW) method. The generalized gradient approximation (GGA) was used to treat the exchange and correlation potential. Spinorbit interaction (SOI) was incorporated using a second variational procedure. Since it is difficult to treat disorder in ab initio calculations, we have used several ordered structures for AgSbTe2. All these structures show semimetallic behavior with a pseudogap near the Fermi energy. Te and Sb p orbitals, which are close in energy, hybridize rather strongly indicating a covalent interaction between Te and Sb atoms.
Thermoelectric Properties of TiS2 type materials
Authors: Abbott Edward E. , Kolis Joseph W. , Lowhorn Nathan D. , Sams William , Tritt Terry M
Published: April 2011

TiS2 belongs to a family of layered compounds that displays promise as a thermoelectric material. At room temperature the thermopower (a) of TiS2 displays an n-type behavior, with a magnitude of ≈ -200 μV/K. The electrical resistivity (ρ), is on the order of 1 mΩ-cm at room temperature and displays a “metallic-like” behavior with dR/dT > 0 from 300 K to 10 K. Thus, these compounds exhibit relatively large power factors (PF = α2/ρ) with a PF ∼30 μW/K2 cm at T = 300 K, which are comparable to the state-of-the art Bi2Te3 type materials, which have a PF ∼40 μW/K2, at T = 330K. These values suggest that further investigations of these systems could be profitable. Thin plate-like crystals of TiS2 are grown by the iodine vapor transport method with planar dimensions of 1 cm and thicknesses of 20 μm or more. In this synthetic approach some dopants can be integrated into the parent compound, effectively providing a route for the tuning of electronic properties. We present here some effects of elemental doping on the electronic properties in these TiS2 based materials.
Thermoelectric performance of textured Bi2Te 3-based sintered materials prepared by spark plasma sintering
Authors: Chen Lidong , Jiang Jun , Shi Xun
Published: April 2011

Thermoelectric performance of polycrystalline materials is greatly influenced by their microstructures including grain sizes, grain boundaries, grain orientations in anisotropic compounds, etc. The material microstructures are sensitive to the preparation processes and the starting materials. In the present study, n-type and p-type Bi2Te3-based sintered materials with highly preferred grain orientations have been fabricated through a spark plasma sintering (SPS) technique, by controlling the particle sizes of the starting powder and other sintering process parameters. The obtained textured Bi2Te3-based materials show a high mechanical strength as 80MPa in bending strength, which is 7 to 8 times as that of the melted ingot materials, and a significant anisotropy in thermoelectric transport properties. The optimal figure of merit (ZT) of the sintered materials in the direction perpendicular to the pressing direction (with c-axis preferred orientation) is comparable to that of the zone-melted ingots in the same crystallographic orientation.
Thermal and thermoelectric properties of III-nitride and III-oxynitride films prepared by reactive RF sputtering
Authors: Izaki R. , Kaiwa N. , Sugimura S. , Yamaguchi S. , Yamamoto A.
Published: April 2011

Towards fabricating a thermoelectric power device using III-nitrides and III-oxynitrides, we studied their thermal and thermoelectric properties and, in particular, we have focused on their thermal conductivity. AInN and AlInON were prepared by reactive radio-frequency sputtering method. We measured the thermal diffusivity and specific heat using the ac calorimetric method, and we estimated values of the thermal conductivity to be 2.2 W/mK (at 773 K) for AlInN and 3.3 W/mK (at 673 K) for AlInON. These values are much smaller than expected considering the large thermal conductivity of nitrides. As for a device composed of 20-pair Chromel-AlInN films, the maximum output power was 0.36 μW at the temperature difference of 180 K, and the open voltage was 0.10 V.
Guided Formation of Nanostructures in Thin Films
Authors: Lu Wei , Kim Dongchoul
Published: April 2011

A thin two-component epilayer grown on a solid surface may separate into distinct phases. Sometimes the phases select sizes about 10 nm, and order into an array of stripes or disks. However, the pattern types are limited and the location of the features is not controlled. This paper develops a dynamic model to simulate guided self-assembly. In particular, we look at the effect of surface chemistry on the pattern formation process. The simulations suggest that diverse patterns may be produced by tuning the surface chemistry of a substrate. In addition, the self-assembled features may be anchored at specific locations.
Mechanical Properties and Morphology of Polycrystalline 3C-SiC Films Deposited on Si and SiO2 by LPCVD
Authors: Fu Xiao-an , Dunning Jeremy , Rajgopal Srihari , Zhang Ming , Zorman Christian A. , Mehregany Mehran
Published: April 2011

Poly-SiC films were deposited on Si and SiO2 substrates in a high-throughput, low pressure chemical vapor deposition (LPCVD) furnace using dichlorosilane (DCS) and acetylene precursors. The deposition temperature and pressure were fixed at 900°C and 2 Torr, respectively, while the flow rate of DCS was varied between 18 and 54 sccm. Poly-SiC deposition rates on both Si and SiO2 were nearly identical to each other and increased as a function of DCS flow rate. Consistent with both substrate materials, the following observations were made. A slope change of the deposition rate versus DCS flow rate was observed around a DCS flow rate of 35 sccm. Residual stress varied with respect to the deposition rate, with tensile stresses occurring at lower deposition rates and compressive stresses at higher deposition rates. The tensile-to-compressive stress transition corresponded to the slope change of the deposition rate versus DCS flow rate. The surface morphology consisted of pyramidal grains, as observed under an SEM. TEM analysis for poly-SiC films grown on Si substrates showed that microstructural differences exist for poly-SiC films having tensile and compressive stresses.
Measurement of Thin-Film Stress, Stiffness, and Strength Using an Enhanced Membrane Pressure-Bulge Technique
Authors: Chalekian Aaron J. , Engelstad Roxann L. , Lovell Edward G.
Published: April 2011

Accurate mechanical properties of thin films are essential for viable design and fabrication of semiconductor devices and microelectromechanical systems. Relevant properties of thin films such as intrinsic stress, biaxial modulus, and fracture strength can be significantly different than their corresponding bulk values, and much more difficult to measure. However, such data can be obtained from the pressure-deflection response of clamped freestanding membranes, i.e., the so-called pressure-bulge test. Experimental challenges include membrane leakage prevention, ensuring proper structural boundary conditions, and accurately measuring applied pressure and transverse displacements simultaneously. In addition to these issues, most previously-developed pressure-bulge instruments rely on vacuum pump loadings. Such tools are limited by the one-atmosphere differential pressure over the membrane, which is inadequate for burst testing of high-strength films. Consequently, an enhanced pressure-bulge tool has been developed and will be described in this paper. It incorporates positive pressure to overcome the one-atmosphere load limitation, improved edge constraints, and the ability to test an array of membrane windows across a single substrate.
A Study on Selective Etching of SiGe Layers in SiGe/Si Systems for Device Applications
Authors: Yamazaki Takashi , Sekikawa Tomohide , Morita Shinya , Hakamada Yoshitaka , Ohri Hiroyuki , Ohmi Shun-ichiro , Sakai Tetsushi
Published: April 2011

Selective etching of a SiGe layer for Si/SiGe/Si stacked layers was investigated for device applications. The SiGe layer is selectively and laterally etched by a HF:HNO3:H2O solution after patterning the layers. The selective etching rate of the SiGe layer increased as the Ge ratio increased. The etching rate of a SiGe layer annealed at 800°C did not change from an as-grown sample, and the Ge diffusion into Si was very small. However, the etching rate of a layer annealed at 1000°C was found to have decreased, and Ge diffused into Si in large amounts. This indicates that annealing at less than 800°C is suitable for device applications.
Asymmetry in Multilayer Adhesion: the Zinc Oxide / Silver Interface
Authors: Barthel E. , Nael P. , Kerjan O. , Nadaud N.
Published: April 2011

We have developed a wedge-loaded double-cantilever beam adhesion measurement set-up for thin films deposited on glass by magnetron sputtering. Results on the Glass/ZnO/Ag/ZnO multilayer evidence that the upper Ag/ZnO interface is tougher than the lower ZnO/Ag interface. This asymmetry in the adhesion energy between identical materials is not expected from first principles. It results from different non-equilibrium interfacial structures.
Dislocation Image Stresses at Free Surfaces by the Finite Element Method
Authors: Tang Meijie , Xu Guanshui , Cai Wei , Bulatov Vasily
Published: April 2011

The finite element method has been routinely used to calculate the image stresses of dislocation segments. When these segments intersect with surfaces, the image stresses at the surfaces diverge singularly. At the presence of these singularities, both convergence and accuracy of using the finite element method need to be examined critically. This article addresses these issues with the aim toward the application of dislocation dynamics simulations in thin films.
Understanding adhesion failure in low-k dielectric stack during Chemical-Mechanical Polishing
Authors: Iacopi F. , Degryse D. , Vos I. , Patz M. , Maex K.
Published: April 2011

Low-k dielectrics are currently starting to replace SiO2 in advanced on-chip interconnects. Being low surface energy materials, a major concern for the integration of such dielectrics in Cu damascene processes is given by adhesion failure at the interfaces of the low-k films with cap or liner layers during Chemical Mechanical Polishing (CMP). A cross-comparison between interface fracture energies as measured through four-point bending, CMP experiments and stress fields computed by Finite Element Modeling, shows that it is not possible to predict failures by setting a ‘universal’ threshold on adhesion strength.

In particular, we report on the critical role of CMP –induced shear stress on the onset of interface debonding, and the way the shear load is transmitted to the critical interfaces. The top-down load transmission depends on the position of the weak interfaces, and on the way the intermediate films in the stack can confine such load through their thickness and mechanical (elastic and plastic) properties.
Effects of Varying Mean Stress and Stress Amplitude on the Fatigue of Polysilicon
Authors: Kahn H. , Ballarini R. , Heuer A. H.
Published: April 2011

Polycrystalline silicon (polysilicon) single edge-notched fatigue specimens with micrometer-sized dimensions were macromachined and subjected to cyclic loading using an integrated electrostatic actuator. The effects of fatigue were determined by comparing the monotonic bend strength measured after cyclic loading to the monotonic bend strength of specimens that received no cycling. Both strengthening and weakening were observed, depending on the levels of mean stress and fatigue stress amplitude during the cyclic loading. Monotonic loading with similar stress levels prior to bend strength measurements had no effect on measured bend strength. Possible physical mechanisms responsible for this fatigue behavior are discussed.
In Situ, Real-Time Curvature Imaging During Chemical Vapor Deposition
Authors: Boyd David A. , Tripathi Ashok B. , El-Naggar Mohamed , Goodwin David G.
Published: April 2011

Coherent Gradient Sensing (CGS) is a full-field optical technique that produces real-time images of macroscopic wafer curvature, which, for thin films, can be related to stress through Stoney's equation. Here we describe the use of CGS as an in situ diagnostic to observe film stress distributions during chemical vapor deposition. The application of this method to measure oxygen diffusion rates in thin film YBa2Cu3O6+δ(YBCO) and stresses in thin film PbxBa1-xTiO3 (PBT) under chemical vapor deposition (CVD) conditions will be discussed.
Determination of Residual-Stress-Free State and Mapping of Residual Stress Fields Using Speckle Interferometry and Thermal Relaxation
Authors: Kim Dong-Won , Kim Jong-jin , Son Dongil , Lee Nak-Kyu , Na Kyung-Hoan , Kwon Dongil
Published: April 2011

We used an electronic speckle pattern interferometer (ESPI) for nondestructive measurement in-situ displacement fields in microsystems. A four-step phase-shift technique and magnifier with long working distance were adopted to increase displacement resolution to ∼10−2 μm and spatial resolution to ∼2 μm. A thermal vacuum chamber was designed to induce thermal treatments, including annealing. From the identification of the residual-stress-free state, we quantitatively modeled thermal strains/stress fields, relaxation stresses during annealing, and residual stress fields. Thermoelasticity theory was applied to model the relationship between the relaxation stresses and the displacements measured by ESPI during the evolution of the residual-stress-free state. We assessed the surface residual stress fields of indented bulk Cu; a Fe-Ni lead frame of 100 μm width; and 0.5 μm Au film. In the indented Cu, the normal and shear residual stresses around the indented point range from –1.7 GPa to 700 MPa and –800 MPa to 600 MPa, respectively, and the residual stress in the bending area of the Fe-Ni lead frame was estimated at 148 MPa and verified using beam-bending theory. In the Au film, tensile residual stresses are uniformly distributed from 500 MPa to 800 MPa as verified by X-ray diffraction.
Dislocation Nucleation and Segregation in Nano-scale Contact of Stepped Surfaces
Authors: Yu H. H. , Shrotriya P. , Wang J. , Kim K.-S.
Published: April 2011

A myriad of engineering applications involve contact between two surfaces, which induces localized plastic deformation near the surface asperities. As a generic problem in studying nanometer scale plastic deformation of solid surfaces, a unit process model of dislocation formation near a surface step under contact loading of a flat rigid surface is considered. The driving force on the dislocation is calculated using conservation integrals. The effect of surface adhesion, step size and lattice resistance on the dislocation driving force are analyzed in a continuum dislocation model, while the nucleation process is simulated atomistically. The driving force formula is used for a dislocation nucleation criterion and to get the equilibrium distance traveled by the dislocation away from the surface step. Results of the unit process model show that under a normal contact load dislocations nucleated in certain slip planes can only stay in a thin layer near the surface, while dislocations nucleated along other slip planes easily move away from the surface into the bulk material. The former dislocation is named anti-load dislocation and the latter dislocation is called pro-load dislocation. Embedded atom method (EAM) is utilized to perform the atomistic simulation of the unit-process model. As predicted by the continuum dislocation model, the atomistic simulations also indicate that surface adhesion plays significant role in dislocation nucleation process. Varying the surface adhesion leads to three different regimes of load-deflection instabilities, namely, just dislocation nucleation instability for no adhesive interaction, two distinct surface adhesion and dislocation nucleation instabilities for weak adhesive interaction and a simultaneous surface adhesion and dislocation nucleation instability for strong adhesive interaction. The atomistic simulations provide additional information on dislocation nucleation and growth near the surface steps. The results of dislocation segregation predict existence of a thin tensile-stress layer near the deformed surface and the results on the adhesion effect provides a cold-welding criterion.
Subcritical Delamination of Dielectric and Metal Films from Low-k Organosilicate Glass (OSG) Thin Films in Buffered pH Solutions
Authors: Lin Y. , Vlassak J. J. , Tsui T. Y. , McKerrow A. J.
Published: April 2011

Understanding subcritical fracture of low-k dielectric materials and barrier thin films in buffered solutions of different pH value is of both technical and scientific importance. Subcritical delamination of dielectric and metal barrier films from low-k organosilicate glass (OSG) films in pH buffer solutions was studied in this work. Crack path and subcritical fracture behavior of OSG depends on the choice of barrier layers. For the OSG/TaN system, fracture takes place in the OSG layer near the interface, while in OSG/SiNx system, delamination occurs at the interface. Delamination behavior of both systems is well described by a hyperbolic sine model that had been developed previously based on a chemical reaction controlled fracture process at the crack tip. The threshold toughness of both systems decreases linearly with increasing pH value. The slopes of the reaction-controlled regime of the crack velocity curves for both systems are independent of pH as predicted by the model. Near transport-controlled regime behavior was observed in OSG/TaN system.
Tuning the Mechanical Properties of SiO2 Thin Film for MEMS Application
Authors: Su Wang-Shen , Fang Weileun , Tsai Ming-Shih
Published: April 2011

This study reported a novel method for tuning thin film mechanical properties by means of plasma surface modification. In order to demonstrate the feasibility of this approach, various plasma treatments, including O2, H2, NH3 atmospheres, were implemented to tune the Young's modulus and residual stress of SiO2 film. Without plasma treatment, the static tip deflection of 200μm long SiO2 cantilever was 9.01μm. After treatment with H2, O2, and NH3 plasma, the tip deformation of the treated cantilevers became 10.22μm, 8.28μm, and -6.84μm respectively. The Young's modulus of the SiO2 cantilever without plasma treatment was 76.3GPa. After treated with H2, O2, NH3 plasma, the Young's modului of those treated cantilevers became 70.8 GPa, 74.7 GPa, and 71.4 GPa, respectively. Hence, after H2 and NH3 plasma treatment, the equivalent elastic modulus of SiO2 cantilever could be reduced about 7%.
Determination of mechanical parameters for rotating MEMS structures as a function of deposition method
Authors: Soare S. , Bull S. J. , Oila A. , O'Neill A. G. , Wright N. , Horsfall A. , dos Santos J.
Published: April 2011

The dimensions of microelectronic devices are constantly being reduced due to the increasing operational demands imposed such as higher working frequencies, higher component density and lower power consumption. This affects the geometrical dimensions of the metallisation, i.e. its width and thickness. The mechanical properties of very thin films are considerably different from those of bulk materials and, also, the deposition method may influence the mechanical behaviour of the components. In order to obtain reliable metallisation it is therefore important to assess accurately the mechanical parameters of the interconnecting lines. As part of designing, developing and manufacturing of a stress micro-sensor there is a need to extract properties useful for performance prediction such as yield stress or creep relaxation behaviour. Part of the data may be obtained by nanoindentation but to have a more complete view, finite element analysis of the indentation cycle has to be employed. In this study nanoindentation testing was carried out at various depths on sputtered and evaporated aluminium layers with different thicknesses deposited on (100) silicon. The loading curves were then simulated by FEA and the results compared to identify the yield properties of the coating. Modelling data for thicker samples closely follows experimental data but for thinner coatings there is a considerable gradient in properties through the film thickness. By incorporating a peak load hold the creep behaviour of the metallization can also be assessed and modelling parameters developed.