bluuebunny/crossref_metadata_2025 huggingface dataset:
Voltage Creep in Holographic PDLC Gratings
Authors: Klosterman A.M. , Pogue R.T. , Schmitt M.G. , Natarajan L.V. , Tondiglia V.P. , Tomlin D. , Sutherland R. L. , Bunning T.J.
Published: April 2011

In holographic Bragg gratings formed by anisotropic photopolymerization of free-radical monomers, an electric field is required to switch the grating between a diffracting and a transmitting state. The voltage necessary to turn the grating completely off is defined as the switching voltage. In this work, we report on the gradual increase of this switching voltage with time after initial fabrication. The switching field increases approximately 60% over a seven-day period for a pentaacrylate/E7 material system. Using dielectric measurements, it is observed that the resistance of the cell does not change over time while the capacitance decreases. FT-IR spectroscopy was also used to explore the continued post-polymerization after fabrication of the gratings. Increases in switching field were observed for samples fully illuminated with light as well as those kept in the dark. We speculate that the rise in the switching voltage is caused by post polymerization of residual reactive moieties located near the polymer/nematic droplet interface. An increase in the amount of interfacial area generated by post-polymerization yields more surface area for the LC molecules to bind; thus a larger field is needed to switch.
In-Situ Observation of Thermal Amplification of Photogenerated Optical Anisotropy of Films of Azobenzene Polymers
Authors: Kidowaki Masatoshi , Fujiwara Takenori , Morino Shin'ya , Ichimura Kunihiro , Stumpe Joachim
Published: April 2011

Thin films of polymers containing p-cyanoazobenzene side chains were studied to reveal its applicability for optical storage media, surface grating optics and aligning films for liquid crystal (LC) display devices. We followed the generation of optical anisotropy of LPLirradiated films of three polymethacrylates with azobenzene side chains exhibiting amorphous, crystalline and liquid-crystalline natures during heat treatment by mean of an ellipsometry and polarized UV-visible spectroscopy. Whereas photogenerated optical anisotropy of amorphous polymer films disappeared above glass transition temperature, the photo-induced anisotropy of films of liquid crystalline and crystalline polymer films was significantly amplified by heat treatment. In case of crystalline polymers amplified optical anisotropy was maintained even at 300°C whereas partial decomposition was observed.
Light Absorption Properties of Anthraquinone Dyes and Guest-Host Liquid Crystal Displays
Authors: Wang Shuangxi , Abeygunaratne Sugat , Chien Liang-Chy
Published: April 2011

The light absorption properties of anthraquinone dye in different hosts are compared in order to demonstrate the influence of the dye-liquid crystal interactions on dye order parameter. Surprisingly, the anthraquinone dye have a higher order parameter in liquid crystalline mixtures of positive dielectric anisotropy as well as in a liquid crystalline mixture of negative dielectric anisotropy (S = 0.71-0.82). We studied the Heilmeier displays use homogeneous alignment. The display is operated at the reverse mode, transparent at zero field. The switching voltage between the bright and dark states for these displays is 2-4 volts, depending on host liquid crystal and cell configuration.
Antiferroelectric Liquid Crystals from Achiral Molecules And A Liquid Conglomerate
Authors: Walba David M. , Körblova Eva , Shao Renfan , Maclennan Joseph E. , Link Darren R. , Clark Noel A.
Published: April 2011

Until recently, it was an empirical fact that creation of a chiral liquid crystal phase required enantiomerically enriched molecules. In addition, to date known ferroelectric and antiferroelectric smectics have also been composed of enantiomerically enriched molecules. Herein are described the first examples of the formation of chiral and antiferroelectric supermolecular liquid crystalline structures from achiral molecules. In one case (apparently metastable) the liquid crystal structure is macroscopically chiral, with samples composed of heterochiral macroscopic domains: a liquid conglomerate.
Rheo-Optics Studies in the Development of Hybrid Liquid Crystalline Based Mwir Polarizers
Authors: Mather P.T. , Barnes W. , Hood P.J. , Bunning T.J.
Published: April 2011

We present here a rheo-optical study of the flow behavior of two cholesteric liquid crystals, one with a large pitch and one with a small pitch. The large pitch compound has been investigated as a possible fixed wavelength polarizer in the mid-wavelength infra-red region (3-5 micron). The investigation of these compounds is driven by their low melt viscosity and ability to vitrify order, and thus functionality, into films with a wide range of thickness. In our attempts to obtain consistent thin films with reproducible contrast ratios, we explored the defect textures of both compounds under a polarizing optical microscope. These materials were sheared at various strain rates and at various temperatures in an attempt to determine the best processing window for defect free films. The pitch lengths of the two materials investigated were 160 and 1330 nm. The flow behavior of the large pitch material resembles a pure nematic with defect refinement taking place under flow. The short pitch material exhibited the typical Grandjean oily streaks upon shearing followed by coarsening. Observations made during this rheological study were used to identify a processing technique for the large pitch materials. Upon application of a conventional buffed alignment layer, films with consistent quality were routinely made. The measured polarization contrast of >70:1 exceeds the values obtained from state-of-the art commercial polarizers in this wavelength regime.
Magnetic Exchange-Coupling In Copt/Co Bilayer Thin Films
Authors: Kim J. , Barmak K. , Lewist L. H. , Crewt D. C. , Welcht D. O.
Published: April 2011

Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase hard/soft composite magnets. CoPt films, with a thickness of 25 nm, were sputter-deposited from a nearly equiatomic alloy target onto oxidized Si wafers. The films were subsequently annealed at 700°C and fully transformed from the FCC phase to the magnetically hard, ordered LI0 phase. The coercivity of the films increased rapidly with annealing time until it reached a plateau at approximately 9.5 kOe. Fully-ordered CoPt films were then used as substrates for deposition of Co layers, with thicknesses in the range of 2.8–225 nm, in order to produce the hard/soft composite bilayers. As predicted by theory, the magnetic coherency between the soft Co phase and the hard, ordered CoPt phase decreased as the thickness of the soft phase increased. This decrease in coupling was clearly seen in the magnetic hysteresis loops of the bilayers. At small thicknesses of Co (a few nanometers), the shape of the loop was one of a uniform material showing no indication of the presence of two phases with extremely different coercivities. At larger Co thicknesses, constricted loops, i.e., ones showing the presence of a mixture of two ferromagnetic phases of different hardnesses, were obtained. The magnetic exchange present in the bilayer samples was qualitatively analyzed using magnetic recoil curves and the dependence of exchange coupling on the soft phase dimension in the bilayer hard/soft composite magnet films is discussed.
The Influence of pH and Temperature on Polish Rates and Selectivity of Silicon Dioxide and Nitride Films
Authors: America W. G. , Srinivasan R. , Babu S.V.
Published: April 2011

Planarization of dielectric films, silicon dioxide, silicon nitride, etc. by Chemical- Mechanical Polishing (CMP) is regulated and moderated by the interaction of the abrasive particles and chemicals in solution with the film surface through complex chemical and physical processes. Changes in the slurry properties have a profound effect on the polishing chemistry and relative removal rates of dielectric films. Common slurry properties include pH, temperature, abrasive particle composition, its size and shape, degree of agglomeration, and weight percent, and chemical composition. While the slurry vendor has control over most slurry properties, the pH and temperature can be controlled during the polishing process by the user and can have a strong influence. Data are presented highlighting the influence of pH and temperature on the CMP of both blanket and patterned silicon dioxide and silicon nitride films.
An Exploration of the Copper CMP Removal Mechanism
Authors: Renteln Peter , Ninh Ton
Published: April 2011

Copper CMP is emerging as the next generation process technology enabling feature size reduction to .15μm and beyond[1]. We propose a copper removal mechanism in the context of a slurry consisting of an oxidizer and an abrasive. The body of evidence suggests that we are polishing in an oxidation complex rate limited regime. We observed low removal rate of copper in the absence of either oxidizer or abrasive, but rate was still dependent on CMP parameters and strongly tied to temperature. Any proposed mechanism must explain the observed dependence of rate on CMP aggressiveness and the role of each of the components. For the slurry used in this work we propose that an increase in temperature resulting from an increase in CMP intensity drives the kinetics of the oxidation reaction, and that the removal process can be classified as temperature-activated, abrasion assisted dissolution.
Process Control Challenges and Solutions: TEOS, W, and CU CMP
Authors: Mendonca J. , Dang C. , Selinidis S. , Angyal M. , Boeck B. , Islam R. , Pettinato C. , Grudowski P. , Cope J. , Smith B. , Kolagunta V.
Published: April 2011

Various options that afford control of the TEOS, W, and Cu/barrier polishes were explored in the building of multilevel dual inlaid structures. Improved tool performance that enables more sophisticated down pressure control with higher resolution backpressure adjustments was employed for the oxide module to control the interlevel capacitances. Planarity at both the global and local levels at the oxide polish affords a good starting point for successive builds without metal pooling. In W CMP, small and controllable oxide erosion and plug recess was obtained with harder polishing pads. In Cu/barrier CMP, the tight overpolish/underpolish margin was maintained by head control and appropriate endpoint algorithms. A six-level build with tight and low sheet resistances and leakages was demonstrated.
Use of Dilute HF With Controlled Oxygen for Post Cu CMP Cleans
Authors: Christenson K. K. , Pizetti C.
Published: April 2011

Wet cleaning of wafers during the semiconductor production process often requires uniform removal of a few nanometers of material. Ideally, a single cleaning chemistry can be found that etches all exposed features at a comparable rate. Etch rates near 1 nm/min are desired for batch process and near 10 nm/min for single-wafer processes. A mixture of 500:1 DHF (dilute HF) with dissolved oxygen controlled near parts-per-million (ppm) levels has been found to meet these requirements for post copper CMP (chemical-mechanical polishing) cleans with exposed SiO2 and Cu metal.
Scanning Force Microscope Studies of Detachment of Nanometer Adhering Particulates
Authors: Dickinson J. T. , Hariadi R. F. , Scudiero L. , Langford S. C.
Published: April 2011

We employ salt particles deposited on soda lime glass substrates as a model system for particle detachment in chemically active environments. The chemical activity is provided by water vapor, and detachment is performed with the tip of a scanning force microscope. The later force required to detach nanometer-scale salt particles is a strong function of particle size and relative humidity. The peak lateral force at detachment divided by the nominal particle area yields an effective interfacial shear strength. The variation of shear strength with particle size and humidity is described in terms of chemically assisted crack growth along the salt-glass interface.
Structured Abrasive CMP: Length Scales, Subpads, and Planarization
Authors: Goetz D. P.
Published: April 2011

Chemical-Mechanical Planarization with structured abrasive uses a subpad to manage the pressure variations due to loading over a range of length scales. The effect of subpad construction on pressure responses related to those scales is illustrated.

A minimum length scale for the effect of the subpad is established via contact mechanics. Differences between one- and two-layer subpads are shown. Uniform compression, point loading, and edge exclusion are considered briefly. A model of the subpad as a plate on an elastic foundation is applied to the problem of die doming. The roles of process pressure, die size, and subpad construction are illustrated. Planarization at the intra-die, die, and wafer scales are related to the subpad construction.
Passivation of (111) Si/SiO2 Interfacial Defects by Molecular Hydrogen
Authors: Gheorghita Ligia , Ogryzlo Elmer
Published: April 2011

The reaction of molecular hydrogen with Pb defects at the (111) Si/SiO2 interface has been studied using a remote radio frequency (RF) probe to continuously monitor the rate of removal of electrically active defects. The kinetic parameters calculated from these experiments are compared with the earlier work of Brower and Stesmans. Although the data confirms the non-exponential nature of the reaction reported by Stesmans the activation energy of the process is found to be somewhat different.
Effects of Inversion Layer Quantization and Polysilicon Gate Depletion on Tunneling Current of Ultra-Thin SiO2 Gate Material
Authors: Saha S. , Srinivasan G. , Rezvani G. A. , Farr M.
Published: April 2011

We have investigated the impact of inversion layer quantization and polysilicon-gate depletion effects on the direct-tunneling gate-leakage current and reliability of ultra-thin silicon-dioxide gate dielectric. The gate-leakage current was measured for nMOSFET devices with gate oxide thickness down to 3 nm. A simulation-based methodology was used to determine the physical oxide thickness from the measured capacitance data, and the corresponding effective gate oxide thickness at inversion was computed from the simulation data obtained with and without the quantum mechanical and polysilicon depletion effects. The simulation results indicate that the effective gate oxide thickness is significantly higher than the physically grown oxide thickness due to inversion layer quantization and polysilicon depletion effects. The increase in oxide thickness is strongly dependent on the supply voltage and is more than 0.6 nm at 1 V. Our data, also, show that in order to maintain a leakage current ≥ 1 A/cm2 for 1 V operation, the effective gate oxide thickness must be ≥ 2.2 nm.
Soft Breakdown in Ultra-Thin Oxides
Authors: Weir B. E. , Silverman P. J. , Alers G. B. , Monroe D. , Alam M. A. , Sorsch T. W. , Green M. L. , Timp G. L. , Ma Y. , Frei M. , Liu C. T. , Bude J. D. , Krisch K. S.
Published: April 2011

An understanding of dielectric breakdown mechanisms is critical for continued oxide scaling. Although working transistors have been demonstrated with sub-2nm SiO2 gate dielectrics, the manufacturability of such devices hinges on the reliability of the oxide. As oxides become thinner and operating voltages become lower, a fundamentally different mode of dielectric breakdown occurs. This has been called soft breakdown and is considered to be the formation of a small, localized tunneling path through a dielectric. For transistors with 2-nm gate oxides, threshold voltage and maximum transconductance are not affected by soft breakdown, implying that circuits may continue to operate after soft breakdown. The increase in gate current or voltage noise associated with soft breakdown is not a limiting factor for many applications. However, some cases will be shown in which soft breakdown does degrade device function.

In order to make comparisons of ultra-thin oxide quality, it is important to be able to reliably detect soft breakdown. J-ramp, a commonly used ramped-current measurement to determine oxide quality, is unable to detect soft breakdown in ultra-thin oxides. We will demonstrate the incorporation of noise measurements in a commercial J-ramp algorithm.
Structure and Bonding in Nitrided Oxide Films by Sims and Xps
Authors: Novak S. W. , Shallenberger J. R. , Cole D. A. , Marino J.W.
Published: April 2011

The N distribution and bonding in five types of oxynitride films have been investigated using SIMS and XPS. Films were grown using N2O, NO-O2 and NH3 gas sources, a remote plasma N source and a Helicon plasma source. The SIMS measurements show different N distributions for each type of sample. XPS measurements show only N≡Si3 bonding in the gas source films, N≡Si3 and O-N-Si2 bonding in the remote plasma sample, and N≡Si3, O-N≡Si2, and O2≡N-Si bonding in the Helicon plasma sample. Angle-resolved XPS measurements show that the O2≡N-Si bonding is deepest in the sample whereas the O-N≡Si2 bonding is associated with a surface oxide.
Gate-Prior-To-Isolation Cmos-Technology with Through-The-Gate-Implanted Ultra-Thin Gate Oxides
Authors: Schwalke Udo , Gruensfelder Christian , Gschwandtner Alexander , Innertsberger Gudrun , Kerber Martin
Published: April 2011

We have realized direct-tunneling gate oxide (1.6nm) NMOS and PMOS transistors by means of through-the-gate-implantation in a comer parasitics-free shallow-trench-isolation CMOS technology. In order to take full advantage of in-situ cluster-tool processing and to preserve initial wafer-surface quality, the essential part of the MOS gate is fabricated prior to device isolation and through-the-gate-implantation is utilized for well- and channel doping. In addition, a fully-reinforced-gate-oxide-perimeter is provided and trench comer parasitics are eliminated by the advanced process architecture EXTIGATE without increasing process complexity.
The Effect of Grown-In Point Defects on Sb Diffusion in MBE-Grown Si and Sige
Authors: Bonar J.M. , Mcgregor B.M. , Willoughby A.F.W. , Paine A.D.N.
Published: April 2011

The diffusion of antimony in Si and SiGe under the influence of grown-in point defects is studied in this work. The layers were grown by very low temperature MBE, and the diffused profiles measured using SIMS analysis. Fast diffusion of Sb is observed in both Si and SiGe. and is thought to be caused by grown-in point defects, specifically vacancies, arising from the very low temperature MIBE growth. The vacancies may be removed by heat treatments. and it was found that the processing the samples received before the heat treatments caused the most substantial diffusion. Comparison of the diffusivities for inert diffusion at 850°C following the heat treatments suggests 30 minutes at 800°C is sufficient to remove the majority of the grown-in defects. The diffusion of Sb in Si is thus demonstrated to be a very sensitive monitor of grown-in vacancies, and dopants which diffuse at least partly by vacancy mediated processes would be expected to be affected by this effect.
Diffusion of Phosphorus in Strained Si/SiGe/Si Heterostructures
Authors: Kuznetsov A. Yu. , Christensen J. S. , Linnarsson M. K. , Svensson B. G. , Radamson H. H. , Grahn J. , Landgren G.
Published: April 2011

Phosphorus diffusion in a biaxially compressed Si0.87Ge0.13 film has been investigated in the temperature range of 810–900°C. A significant enhancement of the P diffusion in the biaxially compressed Si0.87Ge0.13 in comparison with P diffusion in Si is observed. Injection of Si self-interstitials (I) during oxidation of a Si-cap in Si/Si0.87Ge0.13/Si heterostructures is used to characterize the atomic mechanism of P diffusion in Si0.87Ge0.13. It is found that the upper limit of the interstitial fraction of the P diffusion in Si0.87Ge0.13 is 0.87 of that in Si. A comparison between B and P diffusivities in SiGe supports the hypothesis of the pairing-controlled mechanism for the diffusion of B in SiGe.
Ab-Initio Pseudopotential Calculations of Boron Diffusion in Silicon
Authors: Windl W. , Bunea M. M. , Stumpf R. , Dunham S. T. , Masquelier M. P.
Published: April 2011

In this work we investigate boron diffusion as a function of the Fermi-level position in crystalline silicon using ab-initio calculations and the nudged elastic band method to optimize diffusion paths. Based on our results, a new mechanism for B diffusion mediated by Si self-interstitials is proposed. We find a two-step diffusion process for all Fermi-level positions, which suggests a kick-out with a directly following kick-in process without extensive B diffusion on interstitial sites in-between. Our activation energy of 3.47 – 3.75 eV and diffusion-length exponent of -0.55 to -0.18 eV are in excellent agreement with experiment.
In Situ Etch Rate Measurements by Alpha-Particle Energy Loss
Authors: Levy Y. , Ballestad A. , Davies M. , Feng Y. , Kelson I. , Mandeville W.J. , Pacradouny V. , Schmalz A. , Tiedje T. , Young J.F.
Published: April 2011

When alpha-particles pass through thin films they lose an amount of energy proportional to the film thickness with the proportionality constant depending on the film composition. Thus, by measuring this energy loss one can determine the film thickness. We have applied this technique to measurements of the etch rate of various III-V semiconductor layers grown by molecular beam epitaxy. Prior to film growth, GaAs substrates were recoil-implanted with the alpha-emitting 224Ra isotope by exposure to a 5µCi source of 228Th. The implanted isotope decays with a half-life of 3.7 days, which allows measurements to be done for up to about two weeks after implantation. Following growth, the samples were etched in an electron cyclotron resonance etcher using a Cl2/BCl3/Ar gas mixture. As the film is etched the energy of the alpha-particles emitted from the surface increases. By introducing a high resolution Si detector into the etcher we are able to measure changes in the alpha-emission spectrum without removing the sample from the etcher. Thickness changes with an uncertainty of 5–10nm are obtained in 5 minute measurements at the end of each etch step. Some of the samples were also measured by SEM, yielding results in good agreement with values obtained by the alpha-particle measurements. As an example of an application of the technique we will describe measurements of the temperature dependence of the etch rate of GaAs in the 15–150 °C temperature range using optical bandgap thermometry to determine the substrate temperature. In a second example, we explore the application of the technique to etch rate of short pitch (250–500nm) grating. In this case the shape of the alpha-spectrum is sensitive to the profile of the etched trenches.
Extension of Multichannel Spectroscopic Ellipsometry into the Ultraviolet for Real Time Characterization of the Growth of Wide Bandgap Materials from 1.5 to 6.5 eV
Authors: Zapien J. A. , Collins R. W. , Messier R.
Published: April 2011

In this article, we report the results of a successful effort to extend rotating-polarizer multichannel ellipsometry into the ultraviolet (uv) spectral region above 5 eV. Key modifications over previous system designs include (i) incorporation of a see-through deuterium (D2) lamp that allows a tandem Xe/D2 source configuration for a usable spectral output from 1.5 to 6.5 eV, (ii) MgF 2 Rochon polarizers for high transmission in the uv without the need for optical activity corrections, and (iii) a spectrograph with a concave grating blazed at 2500 A and internally mounted order-sorting filters. With these modifications, we can collect 132-point ellipsometric spectra over the range from 1.5 to 6.5 eV with a minimum acquisition time of 24.5 ms (one optical cycle). For averages over two and eighty optical cycles (requiring 49 ms and 1.96 s, respectively), the standard deviations in (Ψ, Δ) are (0.04°, 0.08°) and (0.008°, 0.015°), respectively, measured on a thermally-oxidized silicon wafer at a photon energy of 6 eV. In this first report, we briefly describe problems faced in the development of the new instrument. As an example of the applications of the instrument, we present the results of a real time study of the growth of a hexagonal boron nitride (BN) thin film by rf magnetron sputtering on a Si wafer substrate at a temperature of 250°C. The instrument is expected to be useful in BN film growth studies since the additional data in the uv assist in discriminating the wide band gap (Eg) hexagonal (4 < Eg < 5.8 eV) and cubic (Eg > 6 eV) phases of this material.
Electrical Injection and Transport in Films of Semiconductor Nanocrystals
Authors: Ginger D.S. , Greenham N.C.
Published: April 2011

We study injection and transport in thin disordered films of CdSe nanocrystals between metal electrodes, We investigate the current-voltage characteristics of these devices as a function of electrode material, nanocrystal size, and temperature. We also measure the photocurrent response of these devices, and find that the photocurrent action spectra follow the quantum-confined absorption spectra of the nanocrystals. For dissimilar top and bottom electrodes, we find that the devices are highly rectifying. By studying space charge limited currents in these devices, we are able to place a lower bound on the effective carrier mobility in such films, and we find that the effective mobility is strongly field dependent. We find that the conductivity is strongly temperature dependent, and is qualitatively consistent with an activated hopping process at temperatures above 180 K.
Synthesis and Raman Spectra of Cupric Oxide Quantum Dots
Authors: Xu J.F. , Ji W. , Shen Z.X. , Tang S.H. , Ye X.R. , Jia D.Z. , Yin X.Q.
Published: April 2011

We have synthesised CuO quantum dots by using a method of one-step solid state reaction under ambient conditions, and investigated them by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman scattering technique. The XRD shows that the sample is composed of single phase CuO with a monoclinic structure. The particle size estimated from the x-ray diffraction peaks is about 12 nm, consistent with the TEM result. The Raman spectra show that there are three Raman peaks at 282, 332 and 618 cm−1, which are much broader and shifted several cm−l to lower frequencies in comparison with those of bulk CuO crystal. The temperature dependence of the Raman spectra in the range 77–873 K is also presented.
Low Temperature Si Dot Thin-Film-Transistor Memory
Authors: Nomoto K. , Gosain D. P. , Noguchi T. , Usui S. , Mori Y.
Published: April 2011

We report a novel poly-Si-thin-film-transistor based memory with Si-nano-crystals (Si dots) floating gate fabricated on a quartz substrate at a temperature below 400°C. Novel techniques of Si-dot and tunnel-oxide formation using excimer laser annealing were performed. A preliminary device shows a threshold voltage shift larger than 1 V with 20 V, 10 ms write/erase (W/E) operation and a retention time of 103 s at room temperature. The device operates 104 W/E cycles without significant degradation.
Size Dependence of Second Harmonic Generation in Cdse Nanocrystals
Authors: Jacobsohn M. , Banin U.
Published: April 2011

Second Harmonic Generation in CdSe nanocrystal quantum dots is observed by Hyper-Rayleigh scattering. We use a Ti-Sapphire femtosecond laser at 820 nm to induce the nonlinear optical response of the nanocrystals in solution. The unit cell normalized second harmonic coefficient βn, shows a substantial systematic enhancement in small sizes. The observed size dependence of the second harmonic generation process, is explained assuming two contributions. The first is a bulk-like contribution, from the non-centrosymmetric nanocrystal core, and the second, a contribution from the particle surface. The latter contribution is most significant in small nanocrystals with a substantial proportion of surface atoms.
Mechanism of the Preferential Edge-Positioning of Self-Organized Ge Quantum Dots on Si Mesas
Authors: Jin G. , Tang Y.S. , Liu J.L. , Thomas S.G. , Wang K.L.
Published: April 2011

We have investigated the preferential positioning of self-organized Ge quantum dots on Si (001) mesas fabricated in a selective epitaxial growth (SEG) mode of gas-source molecular beam epitaxy. The surface morphology was evaluated by atomic force microscopy, and for the Si SEG structures, showed mass accumulation at the periphery. Using micro-Raman spectroscopic imaging, the strain-field distribution of Si structures was studied before Ge deposition, and the results suggest a tensile strain at/near the periphery of the structure while a compressive strain at the center. These structures were subsequently used as a template for Ge dots, which tended to align along the mesa edge. The strain field distribution on Si mesas is speculated to be the driving force for preferential nucleation of Ge dots.
Growth Evolution of (Zn,Cd)Se Quantum Dots Deduced from Spatially Resolved Structural and Optical Characterization
Authors: Leonard K. , Hommel D. , Stockmann A. , Selke H. , Seufert J. , Weigand R. , Bacher G. , Forchel A.
Published: April 2011

The growth mode of CdSe layers grown by migration enhanced epitaxy between ZnSe barriers has been investigated. In situ reflection high-energy electron diffraction shows a gradual transition to a three-dimensional growth mode which, however, is not accompanied by a change of the surface lattice constant. High-resolution transmission electron micrographs reveal a strong Cd diffusion, leading to ternary ZnCdSe quantum wells. Furthermore. composition fluctuations perpendicular to the growth direction on a nanometer scale are found already prior to the beginning of the growth mode transition. In the case of heterostructures containing a CdSe layer that has undergone the growth mode transition, micrographs show Cd-rich quantum dots with diameters of around 8 nm and heights of around 1.5 nm within a ternary quantum well. By spatially resolved photoluminescence the emission from single quantum dots could be observed. The polarization dependence of the emission from single dots indicates an asymmetric shape of the dots with certain preferential orientations along the [110] and [110] directions.
Growth and Microstructure of SrBi2Ta2O9 thin films
Authors: Srinivas S. , Katiyar Ram S. , Kim Won-Jeong , Kircohefer Steven W. , Horwitz James S. , Chrisey Douglas B.
Published: April 2011

Single phase, (001) oriented SrBi2Ta2O9(SBT) thin films have been deposited on (100) MgO substrates using pulsed laser deposition(PLD) technique. In order to study the influence of the growth conditions on the microstructure and stoichiometry, SBT film growth was carried out under different deposition conditions, using an Kr:F excimer laser (248 nm, 30 ns FWHM). Effect of laser fluence(0.75–2 J/cm2), frequency of the laser pulse (4–10 Hz), substrate temperature (700–850°C), oxygen partial pressure(150–450 mTorr) have been studied using X-ray diff-raction(XRD), Scanning Electron Microscope(SEM), X-ray energy dispersive analysis (EDAX), atomic force microscopy(AFM). It was found that small variation in growth temperature and oxygen pressure has a large influence on the average grain size (50–180 nm) and surface roughness (0.14–0.3 nm) respectively. Outgrowths on the film surface were observed at high substrate temperatures, high laser energy and with higher frequencies (>6) of the laser pulse. Highly c-axis oriented SBT thin films with homogeneous surface morphology and with an average surface roughness of 0.14 nm were deposited at 750 °C, 260 mTorr O2 pressure and 1.25 J/cm2 laser fluence. These films have a tangent loss around 0.05 and dielectric constant around 417 with a tunability of 4% in the microwave frequency region (1–20 GHz).
Growth Mechanisms and Surface Morphology of Ybco Based High-Tc Thin Films and Heterostructures for sns Junctions
Authors: Rao R. A. , Eom C. B.
Published: April 2011

We have controlled the nanoscale growth mechanism and surface morphology of YBa2Cu3O7 (YBCO) based high-Tc thin films and heterostructures, using miscut SrTiO3 substrates. On exact (001) SrTiO3 substrates, the YBCO films grow in a screw dislocation growth mode. The barrier layers (La6.4Sr1.6Cu8O20 and PrBa2Cu3O7) grown on top of such a YBCO film also show spiral growth features, indicating pseudomorphic growth. On miscut substrates (with miscut angle ≥ 4° toward [010]) the YBCO films grow by step-flow. However, the La6.4Sr1. 6Cu8O20 layers grown on such YBCO bottom electrodes, show a high degree of step bunching with rough surface. In contrast, the PrBa2Cu3O7 layers show clear step-terrace surface morphology similar to the underlying YBCO bottom electrode, suggesting the existence of periodic nanoscale steps at the S-N interface. These heterostructures can be used for the fabrication of SNS Josephson junctions to take advantage of the proximity effect coupling at the nanoscale steps at the interface.
Low Magnetic Field Response of 2d-Array of Weakly Coupled Ferromagnets
Authors: Ivanov Z. G. , Chakalov R. A. , Claeson T.
Published: April 2011

We demonstrated a low magnetic field response in 2D array of weakly coupled ferromagnets created by bi-epitaxial growth of La0.7Sr0.3MnO3 film on multilayer template of MgO seed and CeO2 buffer layer deposited on SrTiO3 substrate. To form the array we etched the MgO seed layer into a chess board pattern. The chess board fields, where the substrate surface is disclosed, initiate a 45° in-plate rotated growth of the CeO2 buffer layer while it does not rotate on the neighboring fields covered by MgO seed layer. The La0.7Sr0.3MnO3 film inherits the template orientation of the buffer layer forming misoriented at 45° domains and a well defined 2D array of 45° grain boundaries is created. The size of the domains, correspondingly the number of grain boundaries, can be varied by changing the dimensions of the chess board fields. The multilayer structures were investigated by θ–2θ and φ-scan x-ray diffraction. A magnetoresistance of 25% by 0.5 T was measured and ascribed to the properties of the 2D array of grain boundaries.
Dry Etching to form Submicron Features in CMR Oxides: (Pr,Ba,Ca)MnO3 and (La,Sr)MnO3
Authors: Lee K. P. , Jung K. B. , Cho H. , Kumar D. , Pietambaram S. V. , Singh R. K. , Hogan P. H. , Dahmen K. H. , Hahn Y. B. , Pearton S. J.
Published: April 2011

Effective pattern transfer into (Pr,Ba,Ca)MnO3 and (La,Sr)MnO3 has been achieved using Cl2/Ar discharges operated under Inductively Coupled Plasma conditions. Etch rates up to 900 Å-min−1 for (La,Sr)MnO3 and 300 Å-min−1 for (Pr,Ba,Ca)MnO3 were obtained, with these rates being a strong function of ion flux, ion energy and ion-to-neutral ratio. The etching is still physically-dominated under all conditions, leading to significant surface smoothing on initially rough samples. Sub-micron (0.35 μm) features have been produced in both materials using SiNx as the mask.
Recent Advances in ab initio time—dependent Hartree-Fock theory and their applications to predict nonlinear optical properties of semiconductor nanoclusters
Authors: Karna Shashi P. , Korambath Prakashan P.
Published: April 2011

Recent advancements in ab initio time-dependent Hartree-Fock (TDHF) theory have made it a technique of choice for modeling nanoscale nonlinear optical (NLO) materials from first-principles. We have used this method to study structure-NLO property relationships of GaN, GaP and GaAs clusters. The geometry of the clusters used in the study was optimized by ab initio Hartree Fock (HF) calculations with the use of even tempered Gaussian (ETG) basis set. The clusters used in this study are of the type Gam Xn (M = 1,3,4,7 and n = 1,3,4,7) where X=N, P, and As. The GamXn clusters are in a charge neutral (q = 0) state for m = n and in appropriately charged state for m ∦ n. The magnitude of the calculated (hyper)polarizabilities appears to strongly depend on the composition of the cluster. For the same composition of heteroatoms, the hyperpolarizability depends on the size as well as the geometry of the cluster. The cluster size-dependence of calculated (hyper)polarizabilities is more pronounced for the first-hyperpolarizability. β than for the polarizability, α The calculated β(–ωμ,ωl,ω2) corresponding to various second order effects shows the following trend β(–2ω; ω,ω) > β(–ω; 0, ω) >β(0;0,0).
Subband Structure and Optical Transitions in a Superlattice with Electron Quasi-Localised States in Unit Cell
Authors: Dmitriev A.V. , Makeev V.V.
Published: April 2011

We studied theoretically the electron spectrum and infrared transitions in a superlattice with a unit cell allowing for quasi-localised carrier states. The dispersion relation and the band structure of such a system have been found. We also calculated the dipole matrix element for inter-subband carrier infrared transitions. The wave functions and the electron spectrum in this superlattice show a peculiarity when the energy of a band state approaches the energy of the quasi-localised state in the single cell. In particular, the absorption strength peaks up at the respective frequencies.
Semiempirical Self-Consistent Field Modeling of the Ce+3 4f AND 5d Energy Levels in Solid State Luminescent Materials
Authors: Rack Philip D. , Holloway Paul H. , O'Brien Ted A , Zerner Michael C.
Published: April 2011

Rare earth doped semiconductors are known to be efficient luminescent materials. In particular, SrS and SrxS and SrxGa2S4 doped with Ce+3 have recently been explored as efficient blue cathodoluminsecent and electroluminescent materials for flat panel display applications. The blue emission in these materials is due to Ce+3 5d-4f transitions, and the transition energies are sensitive to the local chemistry of the Ce+3 ion. To understand the effect that the local chemistry has on the Ce+3 4f–5d emission spectrum, we used a self-consistent-field configuration interaction (SCF/CI) model to calculate the electronic spectrum of the Ce−3 ion embedded in a cluster representation of the semiconductor lattice. The effects of changing nearest neighbor anions and cations have been modeled and are in excellent agreement with the experimental spectroscopy. While vibronic transitions are not calculated in this model, the calculated line transitions were fit to Gaussian peaks to generate absorption and emission spectra. Calculated and experimental Ce3+ 4f–5d emission spectra are compared for different materials, and the observed spectral changes are correlated to an analysis of the magnitude of the ligand field splitting and the nephelauxetic effect in various host lattices. Computed atomic orbital electron populations support these arguments.
Computation of the Porous Silicon Dielectric Function in the Supercell Model and Comparison with Experiment
Authors: Tagüeeña-Martínez J. , Rubo Y.G. , Beltrán M. , Wang C. , Cruz M.
Published: April 2011

We present the results for the imaginary part of the dielectric function of porous silicon, which were obtained with the tight-binding 128–atom supercell model for different porosities. The supercells have been chosen to allow the interconnection of the Si skeleton. We have analyzed also the effects of pore morphology. We have found that, at a fixed porosity, the developing of the surface, resulting in the increase of saturating hydrogen atoms, leads to a noticeable blueshift of the absorption edge.
Polymeric Carbon Dioxide
Authors: Yoo Choong-Shik
Published: April 2011

Synthesis of polymeric carbon dioxide has long been of interest to many chemists and materials scientists. Very recently we discovered the polymeric phase of carbon dioxide (called CO2-V) at high pressures and temperatures. Our optical and x-ray results indicate that CO2-V is optically non-linear, generating the second harmonic of Nd: YLF laser at 527 nm and is also likely superhard similar to cubic-boron nitride or diamond. COz-V is made of CO4 tetrahedra, analogous to SiO2 polymorphs, and is quenchable at ambient temperature at pressures above 1 GPa. In this paper, we describe the pressure-induced polymerization of carbon dioxide together with the stability, structure, and mechanical and optical properties of polymeric CO2-V. We also present some implications of polymeric CO2 for high-pressure chemistry and new materials synthesis.
A Method of Incorporating the Composition into the Calculation of the Si Kβ X-ray Emission Spectrum of the SiO2 — CaO Binary Glass with the DV-X α Molecular Orbital Calculation
Authors: Morishita M. , Koyama K. , Kikko T.
Published: April 2011

Si KB X-ray emission spectra of SiO2—CaO binary glasses simulated with the DV-X α molecular orbital calculation were compared with ones measured by EPMA. The composition of the glass has been incorporated in the calculation of the Si 3p partial density of states (PDOS). The Si 3p PDOS, as a function of the CaO content, agreed well with the measured Si Kβ X-ray emission spectra of the 62.4mol%SiO2 – 37.6mol%CaO and 43.3mol%SiO2 – 56.7mol%CaO binary glasses. The present method was found to provide a very useful means of clarifying the change in the ionic configuration of the glass with the metal oxide content.
Influence of Vacancies on Optical Properties of Sic Nanocrystallites
Authors: Plucinski K.J. , Kityk I.V. , Kassiba A. , Charpentier C.
Published: April 2011

To investigate the origin of vacancies in SiC nanopowders, complex theoretical and experimental investigations of SiC nanopowders of different geometry sizes were carried out. Our theoretical approach included ab initio band energy calculations and molecular dynamics simulations with dynamic boundary conditions. Experimental methods included EPR spectra. The significant influence of nanocrystallite and grain sizes on the concentration and type of vacancies is demonstrated. On the other hand, our comparison of experimental data and theoretical simulations show that vacancies influence the optical properties of SiC nanocrystallites.
Finite Element Analysis of 2-D Dielectric Waveguides with Helmholtz Vector Equations and Virtual Exterior Elements
Authors: Zheng Tieyu , Bao Nal-Keng , Chiang Kin-Seng , Chung Po-Sheun
Published: April 2011

This paper reports a finite-element-method formulation, which is based on the Helmholtz vector equations in terms of transverse electrical field components to characterize two-dimensional dielectric waveguides with an arbitrary continuous permittivity profile. A virtual exterior element iteration technique is associated with the finite-element analysis to process the remote boundary condition. This technique achieves both excellent computational efficiency and accuracy when studying the dielectric propagation constants and cutoff frequencies. The spurious-infection-free modal electrical field can be obtained directly. Numerical results of circular optical fibers with the parabolic permitivity profile are presented to show the validity of this technique.
Estimation of The Critical Radius for The Nucleation of the C54 Phase in C49 TiSi2: Role of The Difference in Density
Authors: Migas D.B. , Iannuzzi M. , Miglio Leo , La Via F. , Grimaldi M.G.
Published: April 2011

We discuss the rather scattered measurements of the lattice parameters for C49 TiSi2, which are reported in literature, along with new and accurate X-ray diffraction measurements and ab-initio calculations. Both agree in indicating that the density of the metastable C49 structure cannot be much smaller than the one for the polymorphic C54 phase, as it is commonly reported. We conclude by demonstrating that only in the case of such a smaller difference in density between the two phases, the elastic strain contribution to the nucleation energy of the C54 structure in the C49 matrix can be neglected. The estimation of the critical radius strongly depends on this issue.
Microstructure Evaluation for Time Dependent Nucleation Protocols in KJMA Kinetics
Authors: Pineda Eloi , Crespo Daniel
Published: April 2011

The microstructure developed in a first order phase transformation is obtained using a populational extension of the Kolmogorov, Johnson-Mehl and Avrami (KJMA) model, PKJMA. PKJMA allows one to determine the grain size distribution resulting from nucleation and growth kinetics. PKJMA is grounded on the mean field hypothesis that the free space around the growing grains is randomly distributed, independent of the grain radius. Although this approach is perfectly valid for the case of constant nucleation, a detailed analysis of the model shows that this hypothesis does not hold in the case of time dependent nucleation protocols or pre-existing nuclei. In this work, the PKJMA model has been improved by estimating the average free surface around the grains as a function of their radius and the time elapsed since nucleation. The resulting model gives quantitative determination of the microstructure developed under a variety of nucleation and growth processes: pre-existing nuclei, constant nucleation, and the combination of both mechanisms, constant and radius dependent growth rates. Comparison with Monte Carlo simulations, showing a quantitative agreement will be presented.
Growth, Microstructure and Micro-Raman Studies of Rf Magnetron Sputter Deposited SrBi2Ta2O9 and SrBi2TaNbO9 Films
Authors: Srinivas S. , Jain Menka , Katiyar Ram S.
Published: April 2011

A systematic study is undertaken on the effect of growth conditions, on the microstructure and other properties of radio frequency (RF) magnetron sputter deposited SrBi2Ta2O9 (SBT) and SrBi2TaNbO9 (SBTN) films. Polycrystalline SBT and SBTN films were deposited on Si (100), platinized Si, and MgO substrates using stoichiometric targets. Cross sectional scanning electron microscopy reveals the tendency for columnar growth in the films deposited at high substrate temperatures and at high partial pressures of sputtering gas mixtures. AFM studies show that the grain size and the homogeneous surface morphology depends on the post annealing time. XPS studies reveal that oxygen vacancies lie in the Bi2O2 planes. The compositional dependence of the rigid-sublattice mode and the symmetric stretching mode of oxygen octahedrons in the Raman spectra of these materials have been discussed.
Short-Range Order and Nanocrystallization in Amorphous Zr-Ti-Cu-Ni-Al
Authors: Xing L. Q. , Gu Xiaofeng , Lusby T. A. , Melmed A. J. , Hufnagel T. C.
Published: April 2011

We have examined the effect of Ti content on the crystallization kinetics of Zr-based amorphous alloys. In Zr59Ti3Cu20Ni8Al10, annealing above the glass transition temperature produces 50-100 nm crystalline precipitates, as seen in transmission electron microscope images. In contrast, TEM images and diffraction patterns from annealed Zr54.5Ti7.5Cu20Ni8Al10 show no evidence of crystalline phase formation. Structural changes upon annealing do occur in this alloy, however, as revealed by field ion microscopy. The effect of Ti is to favor the formation of clusters of short-range order; this tendency for clustering is apparently the cause of difference in crystallization behavior between the two alloys.