bluuebunny/crossref_metadata_2025 huggingface dataset:
Changes in HRM and job satisfaction, 1998–2004: evidence from the Workplace Employment Relations Survey
Authors: Brown Andrew , Forde Chris , Spencer David , Charlwood Andy
Published: June 2008

This paper examines the relationship between human resource management practices and job satisfaction, drawing on data from the 1998 and 2004 Workplace Employment Relations Surveys. The paper finds significant increases in satisfaction with the sense of achievement from work between 1998 and 2004; a number of other measures of job quality are found to have increased over this period as well. It also finds a decline in the incidence of many formal human resource management practices. The paper reports a weak association between formal human resource management practices and satisfaction with sense of achievement. Improvements in perceptions of job security, the climate of employment relations and managerial responsiveness are the most important factors in explaining the rise in satisfaction with sense of achievement between 1998 and 2004. We infer that the rise in satisfaction with sense of achievement is due in large part to the existence of falling unemployment during the period under study, which has driven employers to make improvements in the quality of work.
An analysis of the use and success of online recruitment methods in the UK
Authors: Parry Emma , Tyson Shaun
Published: June 2008

The use of online recruitment methods is now widespread among UK organisations, but has not dominated the recruitment market in the way that was predicted by the popular media. This may be because organisations experience mixed success in using online methods of recruitment. This article reports on the use of a large‐scale, longitudinal survey of recruitment activity to investigate the usage and perceived success of both corporate and commercial web sites by employers. In addition, 20 interviews with users and providers of online recruitment were conducted in order to provide a deeper exploration of the factors that may affect the success of these methods. The results provide valuable insights into the use and success of online recruitment in the UK and have strong implications for practitioners.
HR business partner competency models: re‐contextualising effectiveness
Authors: Caldwell Raymond
Published: June 2008

The rise of ‘HR business partnering’ over the last decade has led to an enormous growth in HR competency models. Competencies are believed to provide a mechanism for reinventing traditional HR roles and improving the effectiveness of HR professionals as business partners. But are they effective? The survey and interview evidence presented here suggest that the effectiveness of competency models is disappointing, and they appear to be particularly weak at predicting performance in a business partner role. There are also significant contextual variations in effectiveness based on the degree of change experienced by the HR function, how consistently business partnering is implemented, overall levels of reduction in transactional HR, and the patterns of centralisation–decentralisation of the HR function. The findings highlight the intrinsic limitations of competency models, as well as the powerful influence of contextual factors, and they raise important questions about the future direction of HR business partnering.
Careers and temporal flexibility in the new economy: an Anglo‐Dutch comparison of the organisation of consultancy work
Authors: Donnelly Rory
Published: June 2008

Knowledge workers are widely considered to represent the vanguards of a new employment era, characterised by a greater degree of balance in the relationship between the employer and the knowledge worker. This fundamental shift in power relations is expected to lead to the emergence of a new group of so‐called ‘free’ or empowered portfolio workers, who share boundaryless career arrangements and internationally ‘universal’ forms of flexibility. However, such propositions have been made without extensive comparative research. This paper therefore examines the organisation of knowledge worker careers and the nature of the temporal flexibility available to knowledge workers through an Anglo‐Dutch case‐study of a multinational consultancy firm. The data collected by this study provides little evidence to support the notion of free/boundaryless career models and demonstrates that national context has a significant impact on the construction of consultancy careers and the availability, level and form of flexibility offered to consultants.
Effects of Abiotic Stressors on Synthesis of the Mycosporine‐like Amino Acid Shinorine in the Cyanobacterium Anabaena variabilis PCC 7937
Authors: Singh Shailendra P. , Klisch Manfred , Sinha Rajeshwar P. , Häder Donat‐P.
Published: June 2008

In the present investigation we show that the cyanobacterium Anabaena variabilis PCC 7937 produces a single mycosporine‐like amino acid (MAA), shinorine (retention time = 2.3 min and absorption maximum at 334 nm) when isolated and purified by HPLC. Although there was significant induction of MAA synthesis from its initial value under 395 or 320 nm cutoff filters, MAA induction was significantly more pronounced in samples covered with 295 nm cutoff filters after 72 h of exposure. Heat as a stress factor had no effect on MAA induction with or without UV radiation. In contrast, salt and ammonium treatment had synergistic effects with UV stress. MAA synthesis was also induced by salt and ammonium in a concentration‐dependent manner without UV stress in samples covered with 395 nm cutoff filters. The results indicate that MAAs may have other functions in addition to photoprotection in this organism.
A Role for Internal Water Molecules in Proton Affinity Changes in the Schiff Base and Asp85 for One‐way Proton Transfer in Bacteriorhodopsin†
Authors: Morgan Joel E. , Gennis Robert B. , Maeda Akio
Published: June 2008

Light‐induced proton pumping in bacteriorhodospin is carried out through five proton transfer steps. We propose that the proton transfer to Asp85 from the Schiff base in the L‐to‐M transition is accompanied by the relocation of a water cluster on the cytoplasmic side of the Schiff base from a site close to the Schiff base in L to the Phe219‐Thr46 region in M. The water cluster present in L, formed at 170 K, is more rigid than that at room temperature. This may be responsible for blocking the conversion of L to M at 170 K. In the photocycle at room temperature, this water cluster returns to the site close to the Schiff base in N, with a rigid structure similar to that of L at 170 K. The increase in the proton affinity of Asp85, which is a prerequisite for the one‐way proton transfer in the M‐to‐N transition, is suggested to be facilitated by a structural change which disrupts interactions between Asp212 and the Schiff base, and between Asp212 and Arg82. We propose that this liberation of Asp212 is accompanied by a rearrangement of the structure of water molecules between Asp85 and Asp212, stabilizing the protonated Asp85 in M.
Induction of Persistent Double Strand Breaks Following Multiphoton Irradiation of Cycling and G1‐arrested Mammalian Cells—Replication‐induced Double Strand Breaks
Authors: Harper Jane V. , Reynolds Pamela , Leatherbarrow Emma L. , Botchway Stanley W. , Parker Anthony W. , O’Neill Peter
Published: June 2008

DNA double strand breaks (DSBs) are amongst the most deleterious lesions induced within the cell following exposure to ionizing radiation. Mammalian cells repair these breaks predominantly via the nonhomologous end joining pathway which is active throughout the cell cycle and is error prone. The alternative pathway for repair of DSBs is homologous recombination (HR) which is error free and active during S‐ and G2/M‐phases of the cell cycle. We have utilized near‐infrared laser radiation to induce DNA damage in individual mammalian cells through multiphoton excitation processes to investigate the dynamics of single cell DNA damage processing. We have used immunofluorescent imaging of γ‐H2AX (a marker for DSBs) in mammalian cells and investigated the colocalization of this protein with ATM, p53 binding protein 1 and RAD51, an integral protein of the HR DNA repair pathway. We have observed persistent DSBs at later times postlaser irradiation which are indicative of DSBs arising at replication, presumably from UV photoproducts or clustered damage containing single strand breaks. Cell cycle studies have shown that in G1 cells, a significant fraction of multiphoton laser‐induced prompt DSBs persists for >4 h in addition to those induced at replication.
Monitoring ALA‐induced PpIX Photodynamic Therapy in the Rat Esophagus Using Fluorescence and Reflectance Spectroscopy
Authors: Kruijt Bastiaan , De Bruijn Henriette S. , Van Der Ploeg‐van den Heuvel Angelique , De Bruin Ron W. F. , Sterenborg Henricus J. C. M. , Amelink Arjen , Robinson Dominic J.
Published: June 2008

The presence of phased protoporphyrin IX (PpIX) bleach kinetics has been shown to correlate with esophageal response to 5‐aminolevulinic acid‐based photodynamic therapy (ALA‐PDT) in animal models. Here we confirm the existence of phased PpIX photobleaching by increasing the temporal resolution of the fluorescence measurements using the therapeutic illumination and long wavelength fluorescence detection. Furthermore fluorescence differential pathlength spectroscopy (FDPS) was incorporated to provide information on the effects of PpIX and tissue oxygenation distribution on the PpIX bleach kinetics during illumination. ALA at a dose of 200 mg kg−1 was orally administered to 15 rats, five rats served as control animals. PDT was performed at an in situ measured fluence rate of 75 mW cm−2 using a total fluence of 54 J cm−2. Forty‐eight hours after PDT the esophagus was excised and histologically examined for PDT‐induced damage. Fluence rate and PpIX photobleaching at 705 nm were monitored during therapeutic illumination with the same isotropic probe. A new method, FDPS, was used for superficial measurement on saturation, blood volume, scattering characteristics and PpIX fluorescence. Results showed two‐phased PpIX photobleaching that was not related to a (systematic) change in esophageal oxygenation but was associated with an increase in average blood volume. PpIX fluorescence photobleaching measured using FDPS, in which fluorescence signals are only acquired from the superficial layers of the esophagus, showed lower rates of photobleaching and no distinct phases. No clear correlation between two‐phased photobleaching and histologic tissue response was found. This study demonstrates the feasibility of measuring fluence rate, PpIX fluorescence and FDPS during PDT in the esophagus. We conclude that the spatial distribution of PpIX significantly influences the kinetics of photobleaching and that there is a complex interrelationship between the distribution of PpIX and the supply of oxygen to the illuminated tissue volume.
Evolutionary origins of invasive populations
Authors: Lee Carol Eunmi , Gelembiuk Gregory William
Published: June 2008

What factors shape the evolution of invasive populations? Recent theoretical and empirical studies suggest that an evolutionary history of disturbance might be an important factor. This perspective presents hypotheses regarding the impact of disturbance on the evolution of invasive populations, based on a synthesis of the existing literature. Disturbance might select for life‐history traits that are favorable for colonizing novel habitats, such as rapid population growth and persistence. Theoretical results suggest that disturbance in the form of fluctuating environments might select for organismal flexibility, or alternatively, the evolution of evolvability. Rapidly fluctuating environments might favor organismal flexibility, such as broad tolerance or plasticity. Alternatively, longer fluctuations or environmental stress might lead to the evolution of evolvability by acting on features of the mutation matrix. Once genetic variance is generated via mutations, temporally fluctuating selection across generations might promote the accumulation and maintenance of genetic variation. Deeper insights into how disturbance in native habitats affects evolutionary and physiological responses of populations would give us greater capacity to predict the populations that are most likely to tolerate or adapt to novel environments during habitat invasions. Moreover, we would gain fundamental insights into the evolutionary origins of invasive populations.
Restricted N‐terminal truncation of cardiac troponin T: a novel mechanism for functional adaptation to energetic crisis
Authors: Feng Han‐Zhong , Biesiadecki Brandon J. , Yu Zhi‐Bin , Hossain M. Moazzem , Jin J.‐P.
Published: June 2008

The N‐terminal variable region of cardiac troponin T (TnT) is a regulatory structure that can be selectively removed during myocardial ischaemia reperfusion by μ‐calpain proteolysis. Here we investigated the pathophysiological significance of this post‐translational modification that removes amino acids 1–71 of cardiac TnT. Working heart preparations were employed to study rat acute myocardial infarction and transgenic mouse hearts over‐expressing the N‐terminal truncated cardiac TnT (cTnT‐ND). Ex vivo myocardial infarction by ligation of the left anterior descending coronary artery induced heart failure and produced cTnT‐ND not only in the infarct but also in remote zones, including the right ventricular free wall, indicating a whole organ response in the absence of systemic neurohumoral mechanisms. Left ventricular pressure overload in mouse working hearts produced increased cTnT‐ND in both ventricles, suggesting a role of haemodynamic stress in triggering an acute whole organ proteolytic regulation. Transgenic mouse hearts in which the endogenous intact cardiac TnT was partially replaced by cTnT‐ND showed lowered contractile velocity. When afterload increased from 55 mmHg to 90 mmHg, stroke volume decreased in the wild type but not in the transgenic mouse hearts. Correspondingly, the left ventricular rapid‐ejection time of the transgenic mouse hearts was significantly longer than that of wild type hearts, especially at high afterload. The restricted deletion of the N‐terminal variable region of cardiac troponin T demonstrates a novel mechanism by which the thin filament regulation adapts to sustain cardiac function under stress conditions.
On Modeling the Energetics of the Ridging Process
Authors: Hopkins M. A. , Hibler W. D.
Published: June 2008

A two-dimensional particle simulation model of the sea-ice ridging process is developed. In this model, ridges are formed from a floating layer of disk-shaped rubble compressed between converging multi-year floes. The energy consumed in ridge growth, including dissipation, is explicitly calculated. The results of numerical experiments using this model indicate that the amount of energy required to ridge ice may be two to three times larger than previously thought.
Drag Forces and Flow-Induced Vibrations of a Long Vertical Tow Cable—Part I: Steady-State Towing Conditions
Authors: Yoerger D. R. , Grosenbaugh M. A. , Triantafyllou M. S. , Burgess J. J.
Published: June 2008

The analysis of data from a unique experiment on the quasistatics and vortex-induced dynamics of a long vertical tow cable is presented. The experiment consisted of measuring simultaneously the vortex-induced motions and the steady-state configuration of a tow cable. The measured steady-state configuration of the cable was used to calculate the cable’s average drag coefficient which ranged from 2.2 to 2.5. It was observed that the vortex-induced motion of the cable was modulated by a beat due to the presence of a shear current. This is confirmed here by spectral and time domain analysis of the acceleration records. The rms motion of the cable varies with depth, which indicates that the drag coefficient of the cable is spatially varying.
Statistical Uncertainties in Wave Heights and Combined Loads on Offshore Structures
Authors: Winterstein S. R. , Haver S.
Published: June 2008

Probabilistic models of combined environmental variables are shown, and their effect on the probability distribution of annual maximum base shear is estimated. A new “generalized Gumbel” model is introduced for the critical wave height parameter. By preserving higher statistical moments, this model better follows extreme storm events. Uncertainty in this model is included through statistical uncertainty in these moments. Corresponding reliability confidence intervals are also shown as a function of the sample size of hindcast data. Finally, models of the non-Gaussian crest and the drag parameter are found to be of similar importance in predicting the 100-yr base shear.
Response of Dolos Concrete Armor Units to Impact Loads
Authors: Tedesco J. W. , McGill P. B. , McDougal W. G.
Published: June 2008

A finite element analysis is conducted to determine the critical impact velocities for concrete dolos. The model formulation includes deformations at the contact surface and nonlinear material properties. Two dolos orientations are considered: vertical fluke seaward and horizontal fluke seaward. In both cases, the larger units fail at lower angular impact velocities. It is also shown that doubling the concrete strength increases the impact resistance by approximately 40 percent.
Minimum Weight Optimization of Offshore Jackets
Authors: Murty V. S. R. , Rao C. L. , Rajagopalan K.
Published: June 2008

Offshore steel jackets are extensively used all over the world for the extraction of hydrocarbons from the reservoirs lying underneath the seabed. In this paper, the results of research carried out by the authors on the minimum weight optimization of the jacket for wave forces have been described. The gradient projection method has been used in the optimization. The jacket is modeled as a space truss and the cross-sectional areas of the tubular members have been the variables in the optimization process. The optimization histories with different sets of group area variables have been presented. The minimum weight design has been repeated over a range of ocean wave parameters, such as wave period, wave height, and wave direction, and the results are presented to indicate their interplay in the optimal picture.
Numerical Modeling of Dynamic Inelastic Response of Clamped Rectangular Plates Impacted by a Knife-Edge Indentor
Authors: Zhu L. , Faulkner D.
Published: June 2008

The dynamic behavior of a fully clamped rectangular plate impacted by a rigid knife-edge is studied in this paper. Based on the variational finite difference method, a numerical model of dynamic inelastic response of plates impacted by a rigid knife-edge indentor is being developed, which includes the influence of finite transverse displacements, axial restraints, bending moments, material elasticity and strain hardening. The struck plate and rigid striker were coupled in the numerical simulation, with the deformation, strain, stress and impact force as outputs of the calculation. Numerical results were given on a small-scale ship model test, which provides a full picture of the response of the clamped rectangular plate under dynamic load. It provides information on the impact force, deformation, stress and strain everywhere in the plate. Such data could not be obtained experimentally or by simple plastic formulations.
Metallurgical Phenomena Modeling in Friction Stir Welding of Aluminium Alloys: Analytical Versus Neural Network Based Approaches
Authors: Fratini Livan , Buffa Gianluca
Published: June 2008

In this paper, the metallurgical phenomena occurring in friction stir welding processes of AA6082-T6 and AA7075-T6 aluminum alloys are investigated. In particular, to predict the local values of the average grain size, either a simple analytical expression depending on a few material constants or a properly trained neural network is linked to the finite element model of the process. The utilized tools, which take as inputs the local values of strain, strain rate, and temperature, were developed starting from experimental data and numerical results.
Failure of Cylindrical Brittle Deposits Impacted by a Supersonic Air Jet
Authors: Eslamian M. , Pophali A. , Bussmann M. , Cormack D. E. , Tran H. N.
Published: June 2008

The failure of a cylindrical brittle material impacted by a supersonic air jet is investigated. Gypsum was cast around steel tubes to simulate the deposit formed on tube surfaces in industrial boilers. The breakup behavior of two deposit sizes, positioned at several distances from the nozzle exit, was visualized and documented using a high-speed video camera. Three deposit failure behaviors were observed: (i) crack formation and propagation along the longitudinal axis of the cylinder, (ii) surface pitting followed by axial crack formation, and (iii) surface pitting followed by spalling. These types of failure depend on the ratio of jet diameter to deposit diameter, which affects the magnitudes of compressive, tensile, and shear forces induced within the material. By analyzing the breakup movies, characteristics of the broken deposits, such as the breakup duration and the amount of deposit removed, were measured. Also, the effects of deposit thickness and distance from the nozzle exit on these characteristics were investigated.
A Study on the Design and Mechanical Adhesion of Polymer Foam-Metal Joints
Authors: Ahmed Ahsan , Fahim Atef , Naguib Hani
Published: June 2008

The design of metal-polymer foam adhesion and load transfer characteristics is carried out in this research work. The metal inserts are used as the load transfer component, while the foam is used as the structural element of the system. The inserts are embedded in the foam during the foaming process. Flexural testing was conducted on different metal foam configurations to establish the typical interaction trends. The load-deflection response and the mode of failure of the structure were documented. Moduli of elasticity of the system for various geometries and embedded lengths were evaluated, and behavior patterns were gleaned. Rectangular, circular, and triangular (taper-/wedgelike) inserts were used. Results show that simple taper inserts embedded in foam slabs perform better than the other shapes. Finite element analyses of the interaction under different loads were carried out. The modeling results coincided with the experimental ones hence validating the model.
Reducing Residual Stress in 7050 Aluminum Alloy Die Forgings by Heat Treatment
Authors: Robinson J. S. , Tanner D. A.
Published: June 2008

Aerospace aluminum alloy forgings can have the residual stresses arising from heat treatment reduced by modification to the quench cooling rates and subsequent aging treatments. A series of propeller hubs usually made from the alloy 2014 have been closed die forged from the less quench sensitive alloy 7050. These forgings have been subjected to various quenching and aging treatments in an attempt to improve the balance of mechanical properties with the residual stress magnitudes. These forgings were not amenable to stress relieving by cold compression or stretching. Warm water (60°C) and boiling water quenches are investigated in addition to quenching into molten salt (200°C) and uphill quenching from −196°C. Various dual aging treatments including retrogression and reaging have been evaluated in an attempt to optimize low residual stress magnitudes with mechanical properties. Residual stresses determined by the center hole-drilling strain-gauge method are reported in addition to electrical conductivity, stress corrosion cracking, fracture toughness, initiation fatigue, and tensile mechanical property variations. It was found that quenching into boiling water and salt at 200°C did substantially reduce the residual stress but had only a small detrimental effect on the majority of the properties measured. However, the influence of quench rate on fracture toughness was much more significant. This is attributed to both coarse grain boundary precipitation and heterogeneous precipitation of η on Al3Zr dispersoids within the grains, which promotes easier crack propagation.
Constitutive Behavior of AA5754 Sheet Materials at Large Strains
Authors: Kang Jidong , Wilkinson David S. , Wu P. D. , Bruhis Mike , Jain Mukesh , Embury J. David , Mishra Raja K.
Published: June 2008

The constitutive behavior of both direct-chill cast (DC) and continuous cast (CC) AA5754 sheet materials has been investigated up to large strains using a modified shear test. The modified shear sample prevents rotation of the shear zone compared to the ASTM standard B831 test (2005, “Standard Test Method for Shear Testing of Thin Aluminum Alloy Products,” 2005 Annual Book of ASTM Standards, West Conshohocken, PA, Vol. 02.02, pp. 601–603). The results show that the effective stress and effective strain curves from shear tests match those obtained from uniaxial tension, but only by incorporating the material anisotropy using the Barlat–Lian yield function. The flow stresses of both materials saturate at large strains; however, the fracture strain of the CC material is significantly lower than that of the DC material.
Densification, Microstructure, and Behavior of Hydroxyapatite Ceramics Sintered by Using Spark Plasma Sintering
Authors: Li Shufeng , Izui Hiroshi , Okano Michiharu
Published: June 2008

This paper discusses the dependence of the mechanical properties and microstructure of sintered hydroxyapatite (HA) on the sintering temperature and pressure. A set of specimens was prepared from as-received HA powder and sintered by using a spark plasma sintering (SPS) process. The sintering pressures were set at 22.3MPa, 44.6MPa, and 66.9MPa, and sintering was performed in the temperature range from 800°Cto1000°C at each pressure. Mechanisms underlying the interrelated temperature-mechanical and pressure-mechanical properties of dense HA were investigated. The effects of temperature and pressure on the flexural strength, Young’s modulus, fracture toughness, relative density, activation energy, phase stability, and microstructure were assessed. The relative density and grain size increased with an increase in the temperature. The flexural strength and Young’s modulus increased with an increase in the temperature, giving maximum values of 131.5MPa and 75.6GPa, respectively, at a critical temperature of 950°C and 44.6MPa, and the fracture toughness was 1.4MPam1∕2 at 1000°C at 44.6MPa. Increasing the sintering pressure led to acceleration of the densification of HA.
Fatigue of AL6XN Stainless Steel
Authors: Kalnaus Sergiy , Jiang Yanyao
Published: June 2008

Tension-compression, torsion, and axial-torsion fatigue experiments were conducted on the AL6XN alloy to experimentally investigate the cyclic plasticity behavior and the fatigue behavior. The material is found to display significant nonproportional hardening when the equivalent plastic strain amplitude is over 2×10−4. In addition, the material exhibits overall cyclic softening. Under tension-compression, the cracking plane is perpendicular to the axial loading direction regardless of the loading amplitude. The smooth strain-life curve under fully reversed tension-compression can be described by a three-parameter power equation. However, the shear strain-life curve under pure torsion loading displays a distinct plateau in the fatigue life range approximately from 20,000 to 60,000 loading cycles. The shear strain amplitude corresponding to the plateau is approximately 1.0%. When the shear strain amplitude is above 1.0% under pure shear, the material displays shear cracking. When the shear strain amplitude is below 1.0%, the material displays tensile cracking. A transition from shear cracking to tensile cracking is associated with the plateau in the shear strain-life curve. Three different multiaxial fatigue criteria were evaluated based on the experimental results on the material for the capability of the criteria to predict fatigue life and the cracking direction. Despite the difference in theory, all the three multiaxial criteria can reasonably correlate the experiments in terms of fatigue life. Since the cracking mode of the material subjected to pure torsion is a function of the loading magnitude, the prediction of cracking orientation becomes rather challenging.
The Effect of Cold Expansion on the Fatigue Life of the Chamfered Holes
Authors: Jang Jae-Soon , Kim Dave , Cho Myoung-Rae
Published: June 2008

The cold expansion method is one of the most popular techniques in the fatigue enhancement processes, and it has been widely used as a means of improving the fatigue resistance for aircraft structures with holes. Cold expanded holes have lower compressive residual stresses on the entry surface rather than the middle and exit surfaces. Due to the nonuniform residual stress distribution, fatigue crack initiation often occurs on the entry surface. This study proposes a new approach to increase the compressive residual stress magnitude at the entry of the hole. The new method is to apply chamfers into holes before the cold expansion process. Split mandrel process was used to cold work the hole with and without chamfers. Both numerical and experimental studies were done to verify the effects of hole chamfers on the residual stress distribution of the cold expanded holes. Finite element analysis (FEA) was conducted in order to see the effects of the chamfer geometries on the residual stress distributions. The FEA results showed an improvement of compressive residual stress magnitudes at the entry position of the cold expanded hole. The numerical results were compared with X-ray diffraction measurements. Fatigue tests were done to compare the fatigue life of the holes with various chamfer sizes and angles. The cold expansion chamfered holes showed a clear improvement in fatigue life over cold expanded holes without chamfers.
nBn detectors based on InAs∕GaSb type-II strain layer superlattice
Authors: Bishop G. , Plis E. , Rodriguez J. B. , Sharma Y. D. , Kim H. S. , Dawson L. R. , Krishna S.
Published: June 2008

We report on a type-II InAs∕GaSb strain layer superlattice photodetector using a nBn design with cutoff wavelength of ∼4.8μm at 250K. The surface component of dark current was eliminated. Using a shallow isolation etch, low temperature dark current was reduced by two orders of magnitude compared with conventional photodiode processing. Dark current densities were equal to 2.3×10−6 and 3.1×10−4A∕cm2 (Vb=0.1V, T=77K) for detectors with shallow isolation etch and conventional defined mesa, respectively. Quantum efficiency, responsivity, and spectral detectivity D* of the device are presented.
Growth and characterization of GaAs1−xSbx barrier layers for advanced concept solar cells
Authors: Bremner S. P. , Liu G. M. , Faleev N. , Ghosh K. , Honsberg C. B.
Published: June 2008

The InAs∕GaAsSb material system is a promising medium for the implementation of a quantum dot solar cell due to a favorable valence band alignment. The quantum dot solar cell requires a highly dense, highly ordered array of quantum dots for overlap of wave functions to form a band in the band gap of the host material. Since the GaAsSb barriers are a III-V-V ternary the alloy composition is particularly sensitive to variations in temperature. We have studied the variation in Sb content for thin layers of GaAsSb in GaAs for various fluxes of Sb. The purpose was to be able to predict the required Sb flux at a particular temperature to obtain a desired Sb composition. The target Sb composition for this study was 12% with the composition obtained confirmed by x-ray diffraction. By also studying the reciprocal space maps of the 12% samples, it is inferred that the composition can be maintained for a large temperature range. The implications of these results for the growth of InAs quantum dots on GaAsSb barriers are briefly discussed.
Improved quantum confinement of self-assembled high-density InAs quantum dot molecules in AlGaAs∕GaAs quantum well structures by molecular beam epitaxy
Authors: Swe N. Chit , Tangmattajittakul O. , Suraprapapich S. , Changmoang P. , Thainoi S. , Wissawinthanon C. , Kanjanachuchai S. , Ratanathammaphan S. , Panyakeow S.
Published: June 2008

Self-assembled, multistack InAs quantum dot molecules (QDMs) were grown by a modified molecular beam epitaxial (MBE) technique, which involves multiple stacking and multiple cycling of the thin-capping-and-regrowth process, so as to obtain a large volume density of quantum dots on the sample. Furthermore, the high-density InAs QDMs were also grown sandwiched either between a double heterostructure (DHS) or between a quantum-well (QW) structure. It was found from microphotoluminescence (μ-PL) measurements that the QDMs sandwiched between these structures give broader PL spectra than those of the as-grown QDMs. The broadening of the PL spectra is associated with the poorer dot size uniformity, which arises from the long and complicated MBE growth processes. However, comparing between the QDMs in the DHS and in the QW structure, the latter give narrower PL spectra. The narrower PL spectra for the QDM-in-QW structure is attributed to the improved quantum confinement effect arising from the use of the QW.
In
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As
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P
buried-heterostructure laser diodes grown on InP using molecular beam epitaxy and metal-organic chemical vapor deposition
Authors: Pickrell G. W. , Zhang H. L. , Ren H. W. , Zhang D. , Xue Q. , Anselm K. A. , Hwang W. Y.
Published: June 2008

Buried-heterostructure (BH) laser diodes were fabricated using both molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). MBE was used to grow the active region and the p-type cladding and contact structures while MOCVD was used for the Fe-doped, blocking layer structures. Using this method, devices were fabricated which showed good performance over the temperature range of −20–85°C. Devices were demonstrated at several wavelengths including 1550 and 1610nm. Room temperature operation of the unpackaged 1610nm devices, with coated facets, demonstrated threshold currents as low as 6mA and efficiencies of 0.3mW∕mA. The same devices operating at 85°C showed threshold currents as low as 22mA and efficiencies of 0.2mW∕mA. Coaxial, fiber pigtailed devices consistently demonstrated OC-48 dispersion power penalties of less than 1.5dB for fiber spans of 100km. Preliminary device reliability shows minimal degradation. TO-56 packaged devices operating at a constant power of 6mW at 90°C for over 4000h have shown a maximum of 3% increase in operating current. The performance and excellent reliability of these devices indicate that this method can be used for the low-cost manufacture of BH distributed feedback lasers.
Direct electron beam patterning and molecular beam epitaxy growth of InAs: Site definition of quantum dots
Authors: Yokota H. , Tsunashima K. , Iizuka K. , Okamoto H.
Published: June 2008

An electron beam (EB) lithography technique to form patterns directly on a semiconductor crystal surface, without using a resist or an oxide film, combined with molecular beam epitaxy (MBE) growth is described for potential applications to site definition of quantum dots. Periodically arranged submicrometer protrusions on a GaAs wafer surface were observed by EB irradiation from a scanning electron microscope with moderately high vacuum. This protrusion has a caldera shape with a concave on the top. On the other hand, when the GaAs was irradiated with an EB, although not well focused, from reflection high energy electron diffraction gun in an ultrahigh vacuum annealing chamber of MBE, a pit with concave shape was obtained. Clear photoluminescence emission was observed from an InAs ultrathin layer overgrown on the pit area.
In situ molecular beam epitaxial growth of SiN films using a source port compatible electron-gun for silicon evaporation
Authors: Hoke W. E. , Kennedy T. D. , Torabi A. , Vandermeulen K. Y. , Mosca J. J.
Published: June 2008

SiN films were grown by molecular beam epitaxy using an electron beam heated silicon rod and a nitrogen rf plasma. The silicon source mounts on a standard furnace flange with the conventional source-substrate geometry. Films were grown under nitrogen-rich conditions and were resistive with refractive indexes close to 2.0. SiN growth rates over 600Å∕h were achieved. The uniformity of films deposited on 4in. diameter silicon substrates had thickness and refractive index variations less than 1%. The films contained less than 0.5% hydrogen content and exhibited low etch rates in buffered HF. SiN films deposited on silicon substrates were under a compressive stress of (0.7–1.4)×109N∕m2.
High-quality nanothick single-crystal Y2O3 films epitaxially grown on Si (111): Growth and structural characteristics
Authors: Lee Y. J. , Lee W. C. , Nieh C. W. , Yang Z. K. , Kortan A. R. , Hong M. , Kwo J. , Hsu C.-H.
Published: June 2008

High-quality single-crystal nanothick Y2O3 films have been grown epitaxially on Si (111) despite a lattice mismatch of 2.4%. The films were electron beam evaporated from pure compacted powder Y2O3 target in ultrahigh vacuum. Y2O3 3nm thick exhibited a bright, sharp, streaky reconstructed (4×4) reflection high energy electron diffraction pattern. Structural studies carried out by x-ray diffraction with synchrotron radiation and high-resolution transmission electron microscopy show that the films have the cubic bixbyite phase with a remarkably uniform thickness and high structural perfection. Two Y2O3 domains of B-type Y2O3[21¯1¯]∥Si[112¯] and A-type Y2O3[21¯1¯]∥Si[21¯1¯] coexist in the initial film growth with B type predominating over A type in thicker films as studied using x-ray diffraction. The narrow full width at half maximum of 0.014° in the ω-rocking curve is the characteristic of excellent crystalline films. High-resolution transmission electron microscopy and fast Fourier transform analysis show atomically sharp interface and strain relaxation in thicker films.
Improved epitaxy of barium titanate by molecular beam epitaxy through a single crystalline magnesium oxide template for integration on hexagonal silicon carbide
Authors: Goodrich T. L. , Cai Z. , Losego M. D. , Maria J.-P. , Fitting Kourkoutis L. , Muller D. A. , Ziemer K. S.
Published: June 2008

Crystalline MgO(111) has the potential to be an effective template for the heteroepitaxial integration of BTO(111) and other functional oxides on 6H-SiC(0001). Deposition of MgO on 6H-SiC(0001) at 140°C resulted in a twinned structure with only (111) orientation. By heating the MgO(111) after deposition to 650°C at a background pressure of 1.0×10−9Torr or depositing the MgO at 650°C, the twinned structure can be minimized, resulting in a reflection high energy electron diffraction (RHEED) pattern characteristic of random epitaxial islands. The use of a 2nm MgO(111) template layer proved optimal for deposition of crystalline BTO(111) by molecular beam epitaxy on 6H-SiC(0001). The BTO was found to be twinned with a 60° in-plane rotation. The grain formation of the BTO resulted in a transmission dominated RHEED pattern. Deposition of BTO at a higher substrate temperature resulted in large grain formation, 50nm in size, but an increased surface roughness of 1.4±0.1nm over a 1μm2 area. In order to integrate BTO in a multiferroic device with multilayers of BTO(111) and hexagonal ferrites, it will be necessary to optimize the processing conditions to establish a smoother BTO surface for the subsequent heteroepitaxy of the magnetic film.
In
Ga
Al
As
∕
In
Ga
As
strain-balanced multi-quantum-well laser/semiconductor optical amplifiers operating at excited transitions
Authors: Feng David J. Y. , Chiu C. L. , Lin S. H. , Lay T. S. , Chang T. Y.
Published: June 2008

We report the design, growth, and fabrication of strain-balanced n-type modulation-doped InGaAlAs∕InGaAs multiple quantum well laser/semiconductor optical amplifiers on InP. The quantum well contains a lattice-matched InGaAs core, a compressive-strained InGaAs padding, and a tensile-strained InGaAlAs spacer. Electroluminescence spectra show higher optical gain for the quantum well e1-hh2 transition at λ=1460nm than the e1-hh1 transition at λ=1550nm. Ridge-waveguide lasers of Fabry-Pérot (FP) type and tilted-end-facet (TEF) type were fabricated. The FP laser shows a lasing peak of λ=1514nm at threshold. Additional lasing wavelengths at λ=1528 and 1545nm were observed sequentially as the injection current increased. For the TEF laser, only the emission at λ=1511nm was observed. These TE-polarized lasing wavelengths are consistent with the δ-like absorption peaks in photocurrent spectra. The lasing performance is attributed to optical transitions within quantum dots/wires which are formed by the strain-field profile and alloy segregation/migration.
Microscopic investigation of the CdS buffer layer growth on Cu(In,Ga)Se2 absorbers
Authors: Sadewasser S. , Bremsteller W. , Plake T. , Kaufmann C. A. , Pettenkofer Ch.
Published: June 2008

To obtain highly efficient thin film solar cell devices, a thin buffer layer of CdS is deposited onto a Cu(In,Ga)Se2 absorber layer. Here, the authors report a photoemission electron microscopy study investigating the growth of evaporated CdS buffer layers. For thin CdS layers (<1nm thickness), they observed enhanced CdS growth on some grains, while other grains remain uncovered or only weakly covered. For thicker CdS layers, they observed a more homogeneous growth. After annealing these layers, an increase in the detailed In and Se structure is observed as well as the reappearance of the inhomogeneous Cd distribution. This indicates an intermixing of CdS and Cu(In,Ga)Se2. The influence of such interdiffusion on solar cell efficiency is discussed.
Microelectromechanical system microhotplates for reliability testing of thin films and nanowires
Authors: Aceros Juan C. , McGruer Nicol E. , Adams George G.
Published: June 2008

Microelectromechanical system microhotplates fabricated using silicon-on-insulator wafers were designed and fabricated for accelerated lifetime testing of thin films and nanowires. Thin films of Cr∕Ru∕Au, Cr∕Ru, and Ru were examined as well as nanowires of Ru and RuOx. Rapid thermal cycling at a 1kHz pulse frequency was employed for testing the thin films. Nanowires were characterized using 1kHz and 500Hz pulse frequencies. A 40nm layer of sputtered Au deposited on 12nm of sputtered Ru and 8nm of sputtered Cr displayed a sheet resistance of 1.18Ω∕◻ and a lifetime of 267h before failure when cycled with a peak temperature of 692K. Ru thin films with a thickness of 12nm displayed a sheet resistance of 51Ω∕◻ and a lifetime of 10h under the same conditions. Sputtered Ru and RuOx nanowires with thicknesses of 15 and 32nm, respectively, displayed resistivities of 72×10−8 and 320×10−8Ωm. It was found that the cycling frequency did not influence the nanowire lifetimes in air or in N2. Ru nanowires tested in N2 displayed lifetimes of 450min followed by Ru nanowires tested in air with lifetimes of 340min and finally by RuOx nanowires tested in air with lifetimes of 45min. This latter behavior is attributed to a higher surface diffusion in RuOx compared to Ru. An activation energy of 0.29eV was observed for failure of Ru nanowires tested in air conditions. Biased percolating behavior with critical exponents ranging from 0.5 to 0.7 was found to describe the electrical behavior for all the nanowires.
Field emission for cantilever sensors
Authors: Yang C. K. , le Fèbre A. J. , Pandraud G. , van der Drift E. , French P. J.
Published: June 2008

Field emission provides an alternative sensing solution in scaled electromechanical systems and devices, when typical displacement detection techniques fail in submicron and nanodimenions. Apart from its independency from device dimension, it has also a high response, integration and high compatibility benefits. In this work, we propose using two modes of detection (fixed current and fixed bias) on two sensing methods: static sensing and dynamic resonance sensing. We measured the characteristic of the two modes and proved that field emission is a viable cantilever displacement detection technique. Customized tip on a fixed substrate has been fabricated and loaded to a UHV atomic force microscopy scanning tunneling microscopy system providing us a field emission environment with precise distance controls without the effects of cantilever bending. Thus, we are able to measure and determine the relationship of emission electric field to the electrode distance, as well as the relationship of the emission current to the electrode distance. The sensitivity obtained in our work for the static mode is 0.5V∕nm. In dynamic mode, we successfully measured a resonance of a piezoactuated cantilever at 162.2kHz. Characterizing these relations enabled us to propose the possibility of using field emission as a cantilever displacement sensing technique.
Field-emission cascades prepared by boron nitride cluster beam deposition
Authors: Song Fengqi , Zhou Feng , Han Min , Wan Jianguo , Liu Zongwen , Zhou Jianfeng , Wang Guanghou
Published: June 2008

Boron nitride nanogranular films on n-type silicon substrates were prepared and their field-emission performance was tested. The films started their electron emission at a mean electric field of 4.0 V/μm. The current density reached 1 mA/cm2 at 6.2 V/μm. The Fowler-Nordheim plot gave an enhancement factor β of 360. The films were boron-rich. As measured by scanning electron microscopy and atomic force microscopy, the films contained the cone-shaped islands with the diameter of 20–40 nm, which were believed to enhance the local electric field. Their performance parameters were comparable to the values of the samples with micrometer roughness.
Entangled Images from Four-Wave Mixing
Authors: Boyer Vincent , Marino Alberto M. , Pooser Raphael C. , Lett Paul D.
Published: June 2008

Two beams of light can be quantum mechanically entangled through correlations of their phase and intensity fluctuations. For a pair of spatially extended image-carrying light fields, the concept of entanglement can be applied not only to the entire images but also to their smaller details. We used a spatially multimode amplifier based on four-wave mixing in a hot vapor to produce twin images that exhibit localized entanglement. The images can be bright fields that display position-dependent quantum noise reduction in their intensity difference or vacuum twin beams that are strongly entangled when projected onto a large range of different spatial modes. The high degree of spatial entanglement demonstrates that the system is an ideal source for parallel continuous-variable quantum information protocols.
Micelles Protect Membrane Complexes from Solution to Vacuum
Authors: Barrera Nelson P. , Di Bartolo Natalie , Booth Paula J. , Robinson Carol V.
Published: June 2008

The ability to maintain interactions between soluble protein subunits in the gas phase of a mass spectrometer gives critical insight into the stoichiometry and interaction networks of protein complexes. Conversely, for membrane protein complexes in micelles, the transition into the gas phase usually leads to the disruption of interactions, particularly between cytoplasmic and membrane subunits, and a mass spectrum dominated by large aggregates of detergent molecules. We show that by applying nanoelectrospray to a micellar solution of a membrane protein complex, the heteromeric adenosine 5′-triphosphate (ATP)–binding cassette transporter BtuC
2
D
2,
we can maintain the complex intact in the gas phase of a mass spectrometer. Dissociation of either transmembrane (BtuC) or cytoplasmic (BtuD) subunits uncovers modifications to the transmembrane subunits and cooperative binding of ATP. By protecting a membrane protein complex within a
n
-dodecyl-β-
d
-maltoside micelle, we demonstrated a powerful strategy that will enable the subunit stoichiometry and ligand-binding properties of membrane complexes to be determined directly, by precise determination of the masses of intact complexes and dissociated subunits.