bluuebunny/crossref_metadata_2025 huggingface dataset:
In Situ STM Observation of the Spiral Growth in the Epitaxial Fe Films on MgO(001)
Authors: Subagyo Agus , Sueoka Kazuhisa , Mukasa Koichi
Published: April 2011

We reported a scanning tunneling microscopy(STM) observation on the growth mode transition from 2D-nucleation to spiral growth in the epitaxial Fe films on MgO(001). As the growth temperature is increased to above 493 K, a temperature region where the Schwoebel barrier is overcome, the Fe films grow in a 2D-nucleation and growth mode formed atomically flat films. The 2D-nucleation transformed into a spiral growth as increasing film thickness. At a growth temperature of 493 K, the transition of 2D nucleation to the spiral growth was observed at a film thickness of 75 Å. The critical thickness of the emergence of growth transition decreased as the growth temperature is increased.
The Effect of Low-Energy Nitrogen Ions on the Growth Modes of Nitrides on Polymers used in the Microelectronics Industry
Authors: Abramowitz P. , Kiene M. , Ho P.
Published: April 2011

Ultra-thin titanium and tantalum nitride layers grown on three different dielectrics were studied to examine how low-energy ions change the chemical composition at and near their interface. Comparisons were made by growing titanium and tantalum nitride under similar conditions both with (ion-assisted) and without (reactive) nitrogen ions. Although the chemical reactions between the nitrides and the three dielectrics under both growth conditions depend on the type of dielectric used, a few general observations were seen. In comparison with the reactively grown samples, all of the ion-assisted growths show a significant increase in the amount of nitride in the nitride layer at and near the nitride/dielectric interface. Moreover, the amount of chemical binding between the titanium nitride and dielectric is increased when low-energy ions are used. Angle resolved x-ray photoemission determined that the enhancement in the deposition process from low-energy ions occurs without inducing significant intermixing between the nitride layer and dielectric. Although thicker layers of titanium nitride show a difference in the grain structure from ion deposition1, the ultra-thin layers grown in this work do not have any dependence with ion-assisted growth for the samples measured.
Incoherent High-Resolution Z-Contrast Imaging of Silicon and Gallium Arsenide Using HAADF-STEM
Authors: Kotaka Y , Yamazaki T. , Kikuchi Y , Watanabe K.
Published: April 2011

The high-angle annular dark-field (HAADF) technique in a dedicated scanning transmission electron microscope (STEM) provides strong compositional sensitivity dependent on atomic number (Z-contrast image). Furthermore, a high spatial resolution image is comparable to that of conventional coherent imaging (HRTEM). However, it is difficult to obtain a clear atomic structure HAADF image using a hybrid TEM/STEM. In this work, HAADF images were obtained with a JEOL JEM-2010F (with a thermal-Schottky field-emission) gun in probe-forming mode at 200 kV. We performed experiments using Si and GaAs in the [110] orientation. The electron-optical conditions were optimized. As a result, the dumbbell structure was observed in an image of [110] Si. Intensity profiles for GaAs along [001] showed differences for the two atomic sites. The experimental images were analyzed and compared with the calculated atomic positions and intensities obtained from Bethe's eigen-value method, which was modified to simulate HAADF-STEM based on Allen and Rossouw's method for convergent-beam electron diffraction (CBED). The experimental results showed a good agreement with the simulation results.
Cutting Single Wall Carbon Nanotubes
Authors: Stepanek I. , Maurin G. , Bernier P , Gavillet J. , Loiseau A.
Published: April 2011

In this contribution, we report on a mechanical method to cut and open single walled carbon nanotubes. This technique is based on using an abrasive material (diamond powder) without any chemical treatments or oxidation in air at high temperature. We present highresolution transmission electron microscopy micrographs, which show firstly that the tubes are unambiguously opened and secondly, that the nanotubes have not suffered the treatment. x-ray diffraction pattern confirms a well-defined bundle organisation. A breaking mechanism of the nanotube bundles is proposed. Nitrogen adsorption measurements at 77K reveal the presence of new microporosities ranging in the average nanotube diameter, which confirm the opening of some tubes.
Plasma Diagnostics During Pulsed Laser Deposition of Diamond-Like Carbon Using Single Crystal Graphite and Amorphous Carbon
Authors: Yamagata Y , Sharma A. , Narayan J. , Mayo R. M. , Newman J. W. , Ebihara K.
Published: April 2011

Optical emission study of ablation plasma plumes from single crystal graphite (SCG) and amorphous carbon (a-C) targets during the preparation of diamond-like carbon (DLC) films by KrF excimer pulsed laser deposition (PLD) has been investigated. The C I emission intensity increases with laser energy density (EL) increase, while the C2 emission changes drastically with EL for both ablated plasma plumes. The C2/C emission intensity ratio for the a-C plume decreases with EL increase, while the C2/C ratio for the SCG plume decreases with EL increase up to 3.0 J/cm2, then increases slightly with further EL increase Nanohardness of the DLC films decreases with the C2/C ratio increase. It is suggested that the C2 molecule in the plasma plume does not play an important role in producing high quality DLC films, and that the C2/C ratio is a good parameter to monitor the PLD process.
Dielectric Properties of Amorphous Carbon Nitride Films
Authors: Aono M. , Katsuno T. , Nitta S. , Itoh T. , Nonomura S.
Published: April 2011

Dielectric properties of amorphous carbon nitride films made by the layer-by-layer method, (LLa-CNx) consisting of a cyclic process with (A) the deposition of an amorphous carbon nitride layer by sputtering and with (B) the etching by atomic hydrogen, have been compared with that of amorphous carbon nitride (a-CNx) films prepared without the hydrogen etching process. It is found that the nitrogen in a-CNx and LLa-CNx could decrease the refractive index n and the dielectric constant ε at 1 MHz. The nitrogen could create less dense structure in a-CNx and LLa-CNx compared to diamond-like carbon (DLC). The sp3 bonds between carbon atoms seem to increase by the atomic hydrogen treatment in LLa-CNx. The refractive index of LLa-CNx increases with sp3content. Preliminary experiments adding oxygen in amorphous carbon nitride to create a-CNxOy film are tried and discussed briefly.
Hyperbranched Conductive Polymers Constituted of Triphenylamine
Authors: Tanaka S. , Takeuchi K. , Asai M. , Iso T. , Ueda M.
Published: April 2011

A hyperbranched conjugated polymer containing triphenylamine was prepared by the Grignard reaction of tris(4-bromophenyl)amine 1, via the coupling of N, N-bis(4-bromophenyl)-N-(4-bromomagnesiophenyl)amine 2 with the catalytic amount of Ni(acac)2. Grignard reagent 2 reacted as an AB2-type monomer to give hyperbranched conjugated polymer 3 in a one-step process. Polymer 3 was also obtained via the Pd-catalyzed coupling of N, N-bis(4-bromophenyl)-4-animobenzeneboronic acid 4. Polymer 3 had an average molecular weight of 4.0–6.3×103 and was found to be soluble in organic solvents such as THF and CHC13. A cast film had an anodic peak at 0.95–1.20 V vs. Ag wire. It was dark blue above the oxidation potential and brown-yellow in the neutral state. When polymer 3 was doped with iodine, its conductivity rose to 0.8–3.0 S/cm
The Constitutive Response of Active Polymer Gels
Authors: Marra S. P. , Ramesh K. T. , Douglas A. S.
Published: April 2011

Active polymer gels can achieve large, reversible deformations in response to environmental stimuli, such as the application of an electric field or a change in pH level. Consequently, great interest exists in using these gels as actuators and artificial muscles. The goal of this work is to characterize the mechanical properties of ionic polymer gels and to describe how these properties evolve as the gel actuates. Experimental results of uniaxial tests on poly(vinyl alcohol)-poly(acrylic acid) gels are presented for both acidic and basic environments. These materials are shown to be to be slightly viscoelastic and compressible and capable of large recoverable deformations. The gels also exhibit similar stress in response to mechanical deformation in both the acid and the base.
Structural Change in Ferroelectric Phase Transition of Vinylidene Fluoride Copolymers as Studied by Waxs, Saxs, IR, Raman, and Computer Simulation Techniques
Authors: Tashiro K.
Published: April 2011

Structure changes in the ferroelectric phase transitions of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers were investigated on the basis of a set of experimental data of WAXS, SAXS, IR and Raman measurements. An intimate relation has been clarified between the structural changes in the crystal lattice and the morphological changes. These structural changes were found to originate ultimately from the remarkable change in the molecular conformation between trans and gauche forms. In order to extract the essentially important factors governing this trans-gauche conformational change in the phase transition, the molecular dynamics calculation was carried out, giving relatively good reproduction of the observed structural changes.
Manufacturing Aluminum Sandwich Structures by Means of Superplastic Forming
Authors: Impiö P. , Pimenoff J. , Hänninen H. , Heinäkari M.
Published: April 2011

This paper examines the possibilities of manufacturing large-scale aluminium sandwich structures using superplastic forming. The materials tested were Mg-alloyed production quality aluminium (Al 5083-0, Al 5083-H321) and Aluminium 1561. Tensile tests at elevated temperatures were performed in order to establish the suitability of the test materials for superplastic forming. The microstructural changes in the test material specimens were examined. Thereafter numerical simulation of the Al 5083-0 forming process was conducted based on the tensile test results. The numerical simulation results were subsequently used to estimate forming parameters and the feasibility of manufacturing large-scale structures by superplastic forming.

The results indicate that higher strain can be reached at higher temperatures for the test materials. Aluminium alloy 1561 exhibited the largest elongation to fracture and Al 5083-H321 the smallest. Strain appeared to be temperature dependent but not much affected by the strain rate. Metallographic examination clarified that Al-5083-0 and AI-5083-H321 showed susceptibility for cavity forming whereas Aluminium 1561 formed relatively few voids. The numerical simulation indicated that Al 5083-0 can be superplastically formed using relatively low forming pressure (0.9 – 1.4 bar).
A TEM Study of Dynamic Continuous Recrystallization in a Superplastic AL-4MG-0.3SC Alloy
Authors: Dougherty L.M. , Robertson I.M. , Vetrano J.S.
Published: April 2011

An Al-4Mg-0.3Sc alloy, aged at 280°C for 8 hours and cold rolled to a 70% reduction, exhibited dynamic recrystallization during superplastic forming at 460°C and at a strain rate of 10−3sec−1. To understand the progression of recrystallization during forming, specimens were deformed under these same conditions to 0.1, 0.2, 0.4 and 0.8 true strain and studied postmortem using optical microscopy, transmission electron microscopy and orientation imaging microscopy. The microstructural evolution that occurred between each strain state was directly observed during deformation experiments at a nominal temperature of 460°C in the transmission electron microscope. These in-situ experiments showed the migration, coalescence, disintegration and annihilation of subboundaries. This combination of post-mortem analysis of specimens strained in bulk with real time observations made during these in-situ experiments allows the mechanisms operating during dynamic continuous recrystallization to be ascertained.
Creep deformation in a 3mol% Y2O3-Stabilized Tetragonal Zirconia
Authors: Morita K. , Hiraga K. , Sakka Y.
Published: April 2011

The high temperature deformation behavior of a 3mol% yttria-stabilized tetragonal zirconia (3Y-TZP) is examined. The analysis of creep strain rate, which was monitored directly with an optical extensometer and corrected for the current grain size, reveals a sigmoidal feature in the true stress-creep rate relationship. At lower stresses, a stress exponent n of ∼1.6 incorporated with a grain size exponent of 2.0 suggests the intervention of a lattice diffusion creep mechanism. For higher stresses where n ∼2.7, evidence of intragranular dislocation activities suggests that dislocations may play an important role in the accommodation process for grain boundary sliding in 3Y-TZP.
A Mechanism for Magnetically Driven Shape Memory Alloys
Authors: Ferreira P. J. , Vander Sande J. B.
Published: April 2011

A mechanism for shape memory alloys driven by a magnetic field is proposed. The mechanism involves the motion of twin dislocations in response to the application of a magnetic field. As a consequence, twin variants oriented favorably with respect to the magnetic field direction will grow. The maximum pressure that can be exerted at the twin dislocations is when the magnetic field is at angle . The shape memory effect is significantly affected by the presence of impurities, second-phase particles and grain boundaries
Magnetoimpedance: a Choice Property for Smart Materials
Authors: García K.L. , Valenzuela R.
Published: April 2011

A phenomenon known as Giant Magnetoimpedance (GMI) is attracting interest both for its technological applications in magnetic field sensors and for its physical basis. In this paper, a review of GMI is presented: first, a brief description of the main characteristics of GMI is discussed, an overview of the main applications is presented, and finally, an interpretation of the physical basis underpinning GMI is proposed
Structural Changes on Photochromism of Organic-Inorganic Hybrid Materials
Authors: Kuboyama Keiichi , Matsushige Kazumi
Published: April 2011

Some transition metal oxides are known to exhibit the reversible coloration phenomena. Tungsten oxide is one of such materials and exhibits the photochromism and the electrochromism. It is known that the coloration phenomena in the tungsten oxide hydrate are caused by the redox reaction. We found that the photochromic efficiency became extremely higher by addition of some organic materials to the tungsten oxide hydrate and we have studied the mechanism of such a remarkable photochromic enhancement. In some spectroscopic measurement as FT-IR (Fourier transform infrared spectroscopy) and XPS (X-ray photoelectron spectroscopy), we obtained interesting features as follows. The addition of an organic material leads to reducing the tungsten oxide hydrate to smaller pieces, that is, the surface area of the pieces that can react with the additive increases. Moreover, it was found that specific sites in the additive are oxidized when the sample colors. The fact suggests that the additives having such specific sites can enhance the photochromism of the tungsten oxide hydrate
Indentation of Piezoelectric Ceramics: Theory, Experiments and Applications
Authors: Suresh Subra , Giannakopoulos Antonios E. , Sridhar S. , Ramamurty U.
Published: April 2011

A new method is proposed for the characterization of properties of piezoelectric materials using depth-sensing indentation involving both mechanical and electrical measurements. First, a rigorous general theory is presented for axisymmetric indentation of piezoelectric solids with anisotropic properties. The theoretical results facilitate the prediction of the indentation load versus the depth of penetration of indenter into the substrate, as well as some transient electrical effects for different electrical boundary conditions. Used in conjunction with instrumented indentation experiments at the nanoscopic, microscopic or macroscopic size scales, these results lead to the prediction of some of the elastic, dielectric and piezoelectric constants as well as the activation energy for depolarization. The predictions of the theory as well as the validity of the approach have been substantiated further with detailed indentation experiments on PZT-4 and barium titanate using either a conducting indenter or an insulated indenter. The theoretical predictions of the coupled electrical-mechanical indentation of piezoelectric solids have also been checked with finite element analyses. The implications of the proposed method for the design and characterization of piezoelectric materials and for quality control in commercial production are also addressed.
New Concept for Resonant Longitudinal-Shear Ultrasonic Motor
Authors: Bouchilloux P. , Koc B. , Uchino K.
Published: April 2011

This paper presents an original linear piezoelectric motor for applications in space and robotics. The originality of this motor consists in the combination of longitudinal and shear modes of vibration of the stator. These two modes are mixed to produce an elliptical vibration of the surface of the stator in contact with a slider. The motion of the linear slider is obtained through friction forces that develop at the interface between the stator and the slider.

This motor is being developed for applications in active truss members of variable geometry structures, such as those used in space. Currently, these structures employ stepper motors. It is expected that replacing these electromagnetic devices with ultrasonic motors will offer the following advantages: position-locking without power supply, compactness, nonmagnetic operation, and lower power consumption.

This paper details the design of this motor and emphasizes the novel concept that was developed to combine the longitudinal and shear modes of vibration.
Characterization of NiTi Materials Using a Novel Aces Methodology
Authors: Mizar S. P. , Pech-Canul M. I. , Pryputniewicz R J.
Published: April 2011

Effective use of Shape Memory Alloys (SMAs) depends upon detailed knowledge of their thermiomechanical behavior. In this paper, thermomechanical behavior of SMAs is studied using a novel ACES methodology. More specifically, variation of modulus of elasticity of equiatomic NiTi is studied, as a function of temperature. The results show that the modulus of elasticity of the NiTi studied herein varies from 38 GPa to 72 GPa as the temperature changes from –15 °C to 190°C, corresponding to phase transition from martensite to austenite. Comparison of Analytical, Computational, and Experimental Solutions (ACES) results showed good correlation.
Suppression of Stiction in MEMS
Authors: Mastrangelo C. H.
Published: April 2011

Stiction failures in microelectromechanical systems (MEMS) occur when suspended elastic members are unexpectedly pinned to their substrates. This type of device failure develops both in fabrication and during device operation, being a dominant source of yield loss in MEMS. Stiction failures require first a collapse force that brings the elastic member contact with the substrate followed by an intersolid adhesion sufficiently large to overcome the elastic restoring force. Stiction failure mechanisms have been studied extensively elsewhere [1]. This paper briefly summarizes these mechanisms in a the practical way. Over the last decade, stiction failure rates in MEMS have been minimized using a wide variety of processing, surface treatment, and physical schemes. An update of these methods is provided.
Selective W for Coating and Releasing MEMS Devices
Authors: Mani S. S. , Fleming J. G. , Sniegowski J. J. , Boer M. P. de , Irwin L. W. , Walraven J. A. , Tanner D. M. , Van D. A. La
Published: April 2011

Two major problems associated with Si-based MEMS (MicroElectroMechanical Systems) devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in micromechanical structures to solve these problems. In this paper, we will present a CVD (Chemical Vapor Deposition) process that selectively coats MEMS devices with tungsten and significantly enhances device durability. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable. This selective deposition process results in a very conformal coating and can potentially address both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through the silicon reduction of WF6. The self-limiting nature of this selective W deposition process ensures the consistency necessary for process control. The tungsten is deposited after the removal of the sacrificial oxides to minimize stress and process integration problems. Tungsten coating adheres well and is hard and conducting, requirements for device performance. Furthermore, since the deposited tungsten infiltrates under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release stuck parts that are contacted over small areas such as dimples. The wear resistance of selectively coated W parts has been shown to be significantly improved on microengine test structures.
Testing of Critical Features of Polysilicon MEMS
Authors: Lavan D. A. , Buchheit T. E.
Published: April 2011

The behavior of MEMS devices is limited by the strength of critical features such as thin ligaments, oxide cuts joining layers, pin joints and hinges. Devices fabricated at Sandia's Microelectronic Development Laboratory have been successfully tested to investigate these features. A series of measurements were performed on samples with gage lengths of 15 to 1000 microns, using conventional and tungsten coated samples as well as samples that include the critical features of standard components in the test section. Specimens have a freely moving pin joint on one end that anchors the sample to the silicon die to allow rotation to reduce effects of bending. Each sample is loaded in uniaxial tension by pulling laterally with a flat tipped diamond in a computer-controlled Nanoindenter. Load is calculated by resolving the measured lateral and normal forces into the applied tensile force and frictional losses. The specimen cross section and gage length dimensions were verified by measuring against a standard in the SEM. Multiple tests can be programmed at one time and performed without operator assistance allowing the collection of significant populations of data.
Micromachined Polysilicon Resonating Xylophone Bar Magnetometer: Resonance Characteristics
Authors: Lamb J. L. , Wickenden D. K. , Champion J. L. , Givens R. B. , Osiander R. , Kistenmacher T. J.
Published: April 2011

A miniaturized, polysilicon version of a xylophone bar magnetometer (with dimensions on the order of microns) has been fabricated by microelectromechanical systems (MEMS) processing techniques. All devices tested to date have performed extremely well in static magnetic fields and exhibit mechanical quality factors, Q, of up to 30,000 at reduced pressures. The resonance frequency of the fundamental flexural mode of vibration of these polysilicon xylophone bars has been found to be a sensitive function of the torsional stiffness of the support arms, in quantitative accord with an analytical model based on Bernoulli-Euler beam theory.
Molecular Beam Mass Spectrometry Studies of the Thermal Decomposition of Tetrakis(dimethylamino)Titanium
Authors: Amato-Wierda Carmela C. , Norton Edward T. , Wierda Derk A.
Published: April 2011

Tetrakis(dimethylamino)titanium (TDMAT) is an important precursor for TiN, TiCN, and TiSiN thin films in chemical vapor deposition. In order to better understand how the gas phase chemistry influences the formation of these films, the decomposition of TDMAT has been studied in a high-temperature flow reactor (HTFR) by molecular beam mass spectrometry (MBMS). Two kinetic regimes have been observed as a function of temperature. Rate expressions and mechanistic implications will be presented. Further studies are in progress to identify the gas phase species relevant to the decomposition mechanism of TDMAT.
Sol-Gel Synthesis of BaTiO3 Based Films for Photonic Applications
Authors: Fuflyigin V. , Jiang H. , Wang F. , Yip P. , Vakhutinsky P. , hao J.
Published: April 2011

High quality barium titanate films were grown on (001)LaAlO3 and r-Al2O3 in a wide thickness range of 0.5-10 µtm by sol-gel technique. Significant improvement of the films' crystallinity and optical quality was observed if acetate precursors are used vs. alkoxide precursors, and in the presence of lead oxide. The material is transparent at 350-2000 nm, indicating the possibility of its application in light controlling devices at wavelengths used in optical communication: 1300 and 1500 nm. Maximum field induced relative phase shift of 0.22 radian was measured in the film with composition of Ba0.9Pb0.1TiO3 under a field strength of 3.106 V/cm.
β-Acetoxyethyl Silsesquioxanes: Chloride-Free Precursors for SiO2 Films Via Staged Hydrolysis
Authors: Ezbiansky Karin A. , Arkles Barry , Composto Russell J. , Berry Donald H.
Published: April 2011

Silsesquioxanes containing β-acetoxyethyl (BAE) groups are processible resins that can be employed as spin-on-glass precursors to dielectric silica films. Thermal treatment >250 °C results in extrusion of ethylene from the CH2CH2OCOCH3 moiety with formation of Si- OCOCH3 groups, which undergo facile hydrolysis to a silica network. A minor pathway involving extrusion of acetic acid leaves some silicon vinyl groups, leading to residual organic carbon in the material. However, addition of a fluoride ion catalyst greatly accelerates the major reaction, resulting in lower conversion temperatures (<200 °C), quantitative extrusion of ethylene, and essentially pure silica. Alternatively, BAESSQs can be processed photochemically (λ <200 nm) to cleanly yield silica at ambient temperature.
High Density Plasma Etching of Ta2O5-Selectivity to Si and Effect of UV Light Enhancement
Authors: Lee K.P , Cho H. , Singh R. K. , Pearton S. J. , Hobbs C. , Tobin P.
Published: April 2011

Etch rates up to 1200 Åmin−1 for Ta2O5 were achieved in both SF6/Ar and Cl2/Ar discharges under Inductively Coupled Plasma conditions. The etch rates with N2/Ar or CH4/H2/Ar chemistries were an order of magnitude lower. There was no effect of post deposition annealing on the Ta2O5 etch rates, at least up to 800 °C. Selectivities to Si of ∼1 were achieved at low source powers, but at higher powers the Si typically etched 4-7 times faster than Ta 20 5. UV illumination during ICP etching in both SF6/Ar and Cl2/Ar produced significant enhancements (up to a factor of 2) in etch rates due to photo-assisted desorption of the TaFx products. The UV illumination is an alternative to employing elevated sample temperatures during etching to increase the volatility of the etch products and may find application where the thermal budget should be minimized during processing.
High-Power Mid-IR Interband Cascade Lasers Based on Type-II Heterostructures
Authors: Yang Rui Q. , Bruno J. D. , Bradshaw J. L. , Pham J. T. , Wortman D. E.
Published: April 2011

The interband cascade lasers (IC) represent a new class of mid-IR light sources, which take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions from serially connected active regions. Here, we describe recent progress in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.8-4 µm; these semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Also, these lasers were able to operate at temperatures up to 217 K, which is higher than the previous record (182 K) for an IC laser at this wavelength. We observed from several devices at temperatures above 80 K a slope efficiency of ∼800 mW/A per facet, corresponding to a differential external quantum efficiency of /500%. A peak optical output power exceeding 4 W/facet and peak power efficiency of /7% were observed from a device at 80 K. Also, we report the first cw operation of IC lasers.
Type-II InAs/InGaSb Superlattices on Compliant GaAs Substrates
Authors: Brown G. J. , Mahalingam K. , Saxler A. , Linville R. , Szmulowicz F. , Seaford M. L. , Tomich D. H. , Lin Chih-Hsiang , Kuo C. H. , Hwang W. Y.
Published: April 2011

Type-II InAs/GaInSb superlattices of different designs have been grown by molecular beam epitaxy on wafer bonded InGaAs on GaAs, and on standard GaSb substrates. The extremely thin (∼100Å) InGaAs layer is loosely bonded to the GaAs substrate and serves as a compliant layer for subsequent epitaxy of a larger lattice constant material. The effects of these substrates on the optical, electrical, and structural properties of the superlattice were studied. The superlattices grown on bonded substrates were found to have uniform layers, with broader x-ray linewidths than superlattices grown on GaSb. The photoresponse results for the superlattices on the bonded 2 inch diameter substrates, with the InGaAs compliant layer, were not as favorable as early work on similar, smaller area, bonded substrates. However, refinements to the wafer bonding process to eliminate microvoids between the bonded layers will provide higher quality superlattices.
Pseudopotential Methods for Superlattices: Applications to Mid-Infrared Semiconductor Lasers
Authors: Dente G. C. , Tilton M. L.
Published: April 2011

Calculations of optoelectronic properties for superlattice materials require accurate subband energies, wavefunctions and radiative matrix elements. We have recently begun using a solution method based on the Empirical Pseudopotential Method, or EPM. This method shows particular strength in analyzing structures with short periods or thin layers, for which the standard method, based on k,p perturbation theory and the envelope function approximation, may be problematical. We will describe the EPM applied to bulk solids and then demonstrate our direct generalization of the method for applications to superlattice structures. Finally, we will apply the EPM method to several type II superlattice samples and compare the predictions to absorbance spectroscopy data.
Progress in Monolithic Photonic Integration Using Quantum Well Shape Modification Enhanced by Ion Implantation
Authors: Koteles Emil S.
Published: April 2011

Progress in the development of ion implantation enhanced quantum well shape modification as a technique for monolithically integrating optoelectronic devices of varying functionalities is reviewed. Fundamental issues related to the physics of the technique, both material issues and device issues, are considered and the performance of both discrete and integrated devices is discussed. The main conclusion of this review is that there are no inherent drawbacks to the utilization of this technique and some serendipitous advantages but that more work on reproducibility and reliability is required to ensure commercial viability.
AlGaAs/GaAs Distributed Feedback Quantum Cascade Lasers
Authors: Schrenk W. , Finger N. , Gianordoli S. , Strasser G. , Gornik E.
Published: April 2011

We report on the realization of distributed feedback quantum cascade lasers in the GaAs/AIGaAs material system. The use of a metallized surface relief grating for feedback allows a fabrication process without regrowth. A feature of this laser is that either single mode or double mode emission at λ ≍ 10 µm is achieved, which is typical for index coupled lasers. The coupling coefficient is measured from the mode spacing for double mode emission of a strong overcoupled laser to be κ ≍ 24 cm"−1. The emission wavenumber can be continuously tuned with the temperature at a rate of dv / dT ≍ 0.048 cm−1/K, which is in close agreement to the temperature dependence of the refractive index of GaAs.
Low Frequency Noise and Random Telegraph Signal in a Multiple Quantum Well Infrared Photodetector
Authors: Kore L. , Bosman G.
Published: April 2011

Current-voltage, voltage-time, and noise measurements were taken on a three well, twenty period, InGaAs/A1GaAs quantum well infrared photodetector at low temperature and low constant current bias. Data from the voltage-time measurements exhibited random telegraph signal (RTS) which is a manifestation of a single electron capture/emission event. It thus contributes to generation-recombination noise of the device. A model is proposed that allows extraction of number of mobile carriers, continuum state electron mobility, electron emission times and trap activation energy from the RTS data. These parameters may then be utilized to predict the excess noise spectrum. Data from the noise spectral density measurements show 1/f like noise at frequencies below I kHz. It can be shown to be a sum of Lorenztian curves with various electron emission times.

The model proposed predicts that the local number of mobile charge carriers in any barrier section is less than 5 (total in the device around 300), there is nearly ballistic electron transport (mobility values around 3×105cm2/Vs), and the traps causing the RTS are in the barriers of the device and there are two of them in the device tested. The model applies to the linear mode of operation and assumes that the well regions are electron reservoirs, i.e. contacts, for the resistive barrier regions. The device is thus divided into a number of resistive portions in series. Each resistive portion is statistically independent from the others at low temperatures since the time the electrons spend in the wells is long compared to intra-well scattering times. This allows the electrons to fully thermalize and lose any statistical information from their previous continuum state trajectory.
Electronic Structures of Shallow Acceptors Confined in Si/SiGe Quantum Well Structures
Authors: Zhao Q.X. , Willander M.
Published: April 2011

Energy levels of the shallow acceptor states have been calculated for center-doped Si/Si1−xGex/Si quantum wells (QWs). The impurity states were calculated using an effective-mass theory that accounts for valence-band mixing as well as the mismatch of band parameters and dielectric constants between well and barrier materials. Acceptor binding energies and splitting between the acceptor 1S3/2(Γ7) and 1S3/2(Γ6) ground states were studied at various Ge concentrations and well widths. The results are discussed in comparison with the recent experimental data from the lateral transport measurements in boron-doped Si/SiGe quantum wells.
Effect of Fast Electron Irradiation on Electrical and Optical Properties of CdGeAs2 and ZnGep2
Authors: Zwieback I. , Maffetone J. , Perlov D. , Harper J. , Ruderman W. , Bachmann K. , Dietz N.
Published: April 2011

We report on the effects of fast electron irradiation on the optical absorption (α) of CdGeAs2and ZnGeP2 and on the electrical properties of CdGeAs2. In p-CdGeAs2 irradiation led to the reduction in α and an increase in the electrical resistivity. The lowest values of α (about 0.1 cm' at 5µm<λ<10µm) were obtained on irradiated crystals of p-type with the highest degree of compensation. Further accumulation of the electron dose caused conversion to n-type and deterioration of the optical transmission. In ZnGeP2 irradiation caused a decrease in a at λ>0.85 λm and increase in α at λ<0.85 μm. At λ=2.05 μm, α for the o-ray could be reduced to less than 0.08 cm−1. At higher doses, saturation in α was observed. The effects of irradiation are discussed in connection with possible mechanisms of optical absorption in CdGeAs2 and ZnGeP2.
Optical Diagnostics of Microstructures Fabricated Using Quantum Well Intermixing
Authors: Saher Helmy A. , Bryce A.C. , Ironside C.N. , Aitchison J.S. , Marsh J.H. , Ayling S.G.
Published: April 2011

In this paper we shall discuss techniques for accurate, non-destructive, optical characterisation of structures fabricated using quantum well intermixing (QWI). Spatially resolved photoluminescence and Raman spectroscopy were used to characterise the lateral bandgap profiles produced by impurity free vacancy disordering (IFVD) technology. Different features were used to examine the spatial resolution of the intermixing process. Features include 1:1 gratings as well as isolated stripes. From the measurements, the spatial selectivity of IFVD could be identified, and was found to be ∼4.5 μm, in contrast with the spatial resolution of the process of sputtering induced intermixing, which was found to be ∼2.5 μm. In addition, PL measurements on 1:1 gratings fabricated using IFVD show almost complete suppression of intermixing dielectric cap gratings with periods less than 10 microns. Finally, some insight into the limitations and merits of PL and Raman for the precision characterisation of QWI will be presented.
X-RAY Photoemission Spectroscopic Study of Light-Induced Structural Changes in Amorphous Silicon
Authors: Sheng Shuran , Sacher Edward , Yelon Arthur
Published: April 2011

Light- and annealing-induced structural changes in undoped hydrogenated amorphous silicon (a-Si:H), pure amorphous silicon (a-Si) and crystalline silicon (c-Si) have been investigated in detail by X-ray photoemission spectroscopy (XPS). Both the Si2s and Si2p peaks in a-Si:H films were found to shift simultaneously to lower binding energies by the same amount with illumination time, and nearly reach saturation at about 0.06 eV after one hour of light-soaking at the intensity used. In contrast to the metastable changes in electronic properties [Staebler-Wronski effect (SWE)], the light-induced shifts in both peaks are unstable even at room temperature and can be reversed by annealing with a lower activation energy than that for the SWE. The absence of metastable XPS changes in pure a-Si and c-Si suggests that hydrogen is actively involved in the light-induced structural changes. Furthermore, visible light exposure produces XPS changes in a-Si:H less effectively than X-ray irradiation, despite its much higher incident intensity, indicating a high-energy photon irradiation effect. Our present results suggest that essentially the whole Si network structure is affected by light-soaking or X-ray irradiation, and becomes more stable after repeated irradiation-annealing training. These structural changes may be an independent metastable phenomenon or a precursor process of the SWE.
Improved Resolution in A P-I-N Image Sensor by Changing the Structure of the Doped Layers
Authors: Vieira M. , Fernandes M. , Martins J. , Louro P. , Maçarico A. , Schwarz R. , Schubert M.
Published: April 2011

An amorphous ZnO/p-i-n/Al imager that uses a small-signal scanning beam to read out the short circuit current signal is presented. An analysis of the image geometric distortion, restoration, and enhancement is performed. A simple image-processing algorithm is used to recover main features of projected images. Modifications of the transducer structure are proposed for improving the sensor performance.The effect of the doped layers and image intensity on the sensor output characteristics are analysed. Results show that a trade-off between the sensor structure and contact geometry is needed for a correct read-out. Algorithms and tools are proposed for image analysis.
Study of the Amorphous-to-Microcrystalline Transition during Silicon Film Growth at Increased Rates: Extensions of the Evolutionary Phase Diagram
Authors: Ferlauto Andre S. , Rovira Pablo I. , Koval Randy J. , Wronski Christopher R. , Collins Robert W.
Published: April 2011

The microstructural and phase evolution of silicon films (Si:H) prepared by low temperature (200°C) rf plasma-enhanced chemical vapor deposition (PECVD) with hydrogen dilution of silane has been studied using real time spectroscopic ellipsometry (RTSE) and atomic force microscopy (AFM). Both RTSE and AFM support the concept of an evolutionary phase diagram that describes different regimes of bulk layer thickness and H2-dilution ratio R within which predominantly amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) films are obtained. In this study, the evolutionary phase diagram has been extended to identify four separate growth regimes: (i) a-Si:H with a smooth surface and a stable roughness layer thickness, (ii) a-Si:H with a rougher surface and an unstable roughness layer thickness, (iii) mixed-phase (a+μc)-Si:H, and (iv) fully-coalesced (single-phase) μc-Si:H. Based on this framework, the effect of increased rf power to achieve higher deposition rates in the rf PECVD process was investigated.
Comparative Fluctuation Microscopy Study of Medium-Range Order in Hydrogenated Amorphous Silicon Deposited by Various Methods
Authors: Voyles P. M. , Treacy M. M. J. , Jin H-C. , Abelson J. R. , Gibson J. M. , Yang J. , Guha S. , Crandall R. S.
Published: April 2011

We have characterized by fluctuation electron microscopy the medium-range order of hydrogenated amorphous silicon thin films deposited by a variety of methods. Films were deposited by reactive magnetron sputtering, hot-wire chemical vapor deposition, and plasma enhanced chemical vapor deposition with and without H2 dilution of the SiH4 precursor gas. All of the films show the signature of the paracrystalline structure typical of amorphous Si. There are small variations in the degree of medium-range order with deposition method and H content. The PECVD film grown with high H2 dilution contains Si crystals ∼5 nm in diameter at a density of ∼109 cm−2. The amorphous matrix surrounding these crystals shows no difference in mediumrange order from the standard PECVD film. This supports explanations of the resistance of the H-dilution material to light-induced degradation that depend only on the presence of crystalline grains without modifications of the amorphous matrix.
Topological Signatures of Medium Range Order in Amorphous Semiconductor Models
Authors: Treacy M. M. J. , Voyles P. M. , Gibson J. M.
Published: April 2011

The topological local cluster (or Schläfli cluster) concept of Marians and Hobbs is used to detect topologically crystalline regions in models of disordered tetrahedral semiconductors. We present simple algorithms for detecting both Wells-type shortest circuits and O'Keeffe-type rings, which can be used to delineate alternative forms of the Schläfli cluster in models.
Influence of the Fermi Energy on Si–H Vibrational Modes in Amorphous and Microcrystalline Silicon
Authors: Nickel N. H. , Lengsfeld P.
Published: April 2011

The influence of boron and phosphorous doping and the effect of structural changes on hydrogen bonding in a-Si:H and µc-Si are investigated using Raman spectroscopy. Information on H bonding is obtained from Si-H stretching local vibrational modes (LVM's). In n-type and undoped a-Si:H the LVM's of isolated Si-H near 2000 cm−1 and clustered Si-H around 2100 cm−1 can be clearly distinguished. In heavily B doped a-Si:H, however, the LVM at 2100 cm−1 disappears. A structural change from a-Si:H to µc-Si gives rise to an additional mode centered at 1911 cm−1. Similar LVM's are observed in poly-Si containing platelets.
T-Site-Trapped Molecular Hydrogen in a-Si:H
Authors: Norberg R. E. , Leopold D. J. , Fedders P. A. , Borzi R. , Chan P. H. , Herberg J. , Tomic N.
Published: April 2011

Proton-29Si double resonance NMR measurements on high quality plasma-enhanced chemical vapor deposition a-Si:H deposited from SiH4 show that more than one third of the contained hydrogen is present as H2 molecules residing in the amorphous equivalent of T sites. The NMR signal from these trapped H2 appears in the narrow 4 kHz proton line, which arises from the less clustered hydrogen population. Very little of the molecular component is in the broad ~24 kHz line, which arises mostly from clustered hydrogen tightly bonded to silicon.
A Study of Non-Infrared-Active Hydrogen Bonding in a-Si:H Thin Film using combined Calibrated Temperature Desorption Spectroscopy and FTIR
Authors: Santjojo DJ. , Cornish J.C.L. , Talukder M.O.G.
Published: April 2011

Non-infrared-active hydrogen bonding species were investigated by analyzing the infrared spectra and the calibrated temperature desorption spectroscopy (CTDS) spectra of hydrogen released during degassing of hydrogenated amorphous silicon thin films. Samples were degassed gradually using a linear temperature ramp (0.5°C/s). Each stage corresponds to a temperature at which the hydrogen effusion peaks can be found (~ 340°C, ~ 500°C and ~ 610°C). Differences in the amounts of hydrogen obtained from the FTIR spectra and the CTDS measurement correspond to the non-infrared-active, occluded hydrogen.
Influence of the Grain Boundary Band Offset on Charge Transport Mechanism in Microcrystalline Silicon Analysed by Numerical Simulation
Authors: Fantoni Alessandro , Vieira Manuela , Schwarz Reinhard
Published: April 2011

Microcrystalline silicon is a two-phase material composed of grains of crystalline silicon embedded in an amorphous silicon tissue. The boundary regions between the crystalline grains and the amorphous matrix are treated similarly to a heterojunction interface.The band offset at the grain boundaries causes the appearance of local electric field peaks, variables in intensity and direction. We present results obtained with two dimensional simulations of a μc-Si:H p-i-n junction in short circuit condition. Charge transport mechanism is described by the internal electric configuration assumed by the junction in thermodynamic equilibrium and illuminated with monochromatic radiations. Different configurations of the band offset at the grain boundaries are also considered and related to the transport properties in microcrystalline silicon
On the Role of Charged Defect States and Deep Traps in the Photocarrier Drift and Diffusion in a-Si:H
Authors: Stradins Paul , Matsuda Akihisa
Published: April 2011

The drift and diffusion in the presence of charged defects and photocarriers trapped in the tail states is re-examined. In continuity equations, diffusive and drift currents are related to free particles while the Poisson equation includes all charges. In order to make use of ambipolar diffusion approximation, the mobilities and diffusion coefficients should be attributed to the total electron and hole populations making them strongly particle-number dependent. Due to the asymmetry of the conduction and valence band tails, almost all trapped electrons reside in negatively charged defects (D−). A simple model of photocarrier traffic via tail and defect states allows to establish the effective mobility values and coefficients in Einstein relations. In a photocarrier grating experiment, grating of D− is counterbalanced by the grating of trapped holes. Nevertheless, electrons remain majority carriers, allowing the measurement of minority carrier diffusion length, but analysis is needed to relate the latter with μτ product.