bluuebunny/crossref_metadata_2025 huggingface dataset:
New Polaronic-Type Excitons in Ferroelectric Oxides: INDO-Calculations and Experimental Manifestation
Authors: Vikhnin Valentin S. , Eglitis Roberts I. , Kotomin Eugene A. , Kapphan Siegmar , Borstel Gunnar
Published: April 2011

The current experimental and theoretical knowledge of new polaronic-type excitons in ferroelectric oxides – charge transfer vibronic excitons (CTVE) – is discussed. It is shown that Hartree-Fock-type INDO calculations as well as photoluminescence studies in ferroelectric oxygen-octahedral perovskites confirm the CTVE-concept. Single CTVE as well as a new phase of strongly correlated CTVEs are analysed.
Structural and vibrational properties of silicon dioxide thin films densified by medium-energy particles bombardment
Authors: Lefèvre Alexis , Lewis Laurent J. , Martinu Ludvik , Wertheimer Michael R.
Published: April 2011

Using classical molecular-dynamics simulations, we worked out a simple model of Ion Beam Assisted Deposition (IBAD) of silicon dioxide on an amorphous silica substrate, in view to in- vestigate the modifications of the structural and vibrational properties induced by medium-energy bombardment. Atoms are assumed to interact via the two- and three-body potential developed by Nakano et al.[1]. Analysis of the films grown with increasing ratio, R, of medium-(30 eV) to low- (1 eV) kinetic energy SiO2 particles shows that the density rises rapidly, from 1.3 g/cm3 for R =0 to about 2.3 g/cm3 for R = 0.7. This effect can be associated primarily with structural changes occurring at an intermediate length scale (4-10 Å), as it manifests itself by changes in the so-called first sharp diffraction peak (FSDP), the finger-print of medium range order (MRO) in a-SiO2 glass [2, 3]. We found also that the densification results in a significant decrease of the number of “soft” vibrational modes, occurring in the 0.5-3.5 THz frequency range.
Deep levels of antisite defects clusters in ZnGeP2
Authors: Voevodin Valeriy G. , Grinyaev Sergey N.
Published: April 2011

On the basis of the methods of pseudopotential and large unit cell the research of deep lev- els of point and cluster defects in ZnGeP2 (ZGP) was carried out. It was shown that owing to lowering of the crystal lattice symmetry localised states of point defects in the ternary compound essentially differ from such states in its binary analogue GaP. Small quasicubic antisite defects clusters reduce the value of band gap, but render weak influence on the probability of optical transitions. But both single and the cluster antisite defects cause large modifications in deep levels of another point defects closed to them. On an example of phosphorus vacancy it was shown that intensive optical transitions with small energies can appear and cause an absorption in near IR – range because of antisite defects effect.
Applications of Real-Time Multiresolution Analysis for Molecular Dynamics Simulations of Infrequent Events
Authors: Richie David A. , Kim Jeongnim , Wilkins John W.
Published: April 2011

The simulation of defect dynamics (e.g., transient enhanced diffusion of boron in the presence of silicon interstitials) is a technologically relevant challenge for computational materials science. The dynamics of defect structures in bulk unfolds as a sequence of thermally induced structural transitions. Identifying and characterizing reaction paths, as well as extracting dynamical quantities (e.g., diffusion constants) is important for modeling the macroscopic properties of real materials. Applying real-time multiresolution analysis (RTMRA) to various dynamical quantities using simple Haar wavelets, we have developed a computationally cheap data compression scheme to handle the massive data sets generated in molecular dynamics (MD) simulations; data storage has been reduced hundredfold with no loss of relevant information. More importantly, the same RTMRA techniques are developed into a sophisticated event detection scheme capable of solving three major challenges to multiscale MD simulations, specifically, (1) identifying meta-stable structures against the background of thermal vibrations, (2) detecting infrequent events, e.g., structural transitions, in the presence of thermal noise, and (3) accurately identifying transition times to further enhance recently emerging MD acceleration techniques.
Temperature and Strain-Rate Dependent Plastic Deformation of Carbon Nanotube
Authors: Wei Chengyu , Cho Kyeongjae
Published: April 2011

In this work we use classical molecular dynamics to study strain rate and temperature dependent plasticity of carbon nanotube (CNT) under compressive strain. We focus on two types of defects: sp3 bond formation and bond rotation. Our simulation shows that thermal fluctuations help the strained CNT to overcome the local energy barrier to obtain plastic deformation. The yielding strain of a compressed CNT found to be strain-rate and temperature dependent, and low strain rate limit of the yielding strain is estimated to be less that 6%.
Percolation/Diffusion through the Void Space of a Bed of Randomly Packed Particles of Different Sizes
Authors: Riley Daniel P , Belova Irina V , Murch Graeme E
Published: April 2011

In this paper a Monte Carlo method is introduced for determining the diffusivity in the void space of porous media constructed from spheres. We studied ordered and random arrangements of spheres, the latter with different sizes. It is found that the effect of structure on the diffusivity is fairly slight. The empirical curve of Neale and Nader is shown to provide a convenient upper bound. The result of Bruggemann provides a convenient lower bound.
Structure of crystallographically challenged materials by profile analysis of atomic pair distribution functions: study of LiMoS2 and mesostructured MnGe4S10.
Authors: Petkov V. , Rangan K. K. , Kanatzidis M. G. , Billinge S.J.L.
Published: April 2011

The approach of the atomic pair distribution function (PDF) technique to study the structure of materials with significant disorder is considered and successfully applied to LiMoS2 and mesostructured MnGe4S10. We find that LiMoS2 is built of layers of distorted MoS6 octahedra stacked along the c axis of a triclinic unit cell with well-defined Mo-Mo bonding. Mesostructured MnGe4S10 is a three-dimensional framework of “adamantane-like” [Ge4S10] units bridged by Mn atoms.
Comparison of Crack Geometry Determined with Phase Contrast Radiography and with Microtomography
Authors: Stock S.R. , Ignatiev K. , Lee W.K. , Fezzaa K. , Davis G.R. , Elliott J.C.
Published: April 2011

X-ray microtomography of the three-dimensional spatial distribution of crack opening as a function of applied load has helped clarify important processes of roughness-induced fatigue crack closure. Fracture mechanics dictates sample geometries which limit the crack opening sensitivity that can be obtained with microtomography, and there is a real need to be able to quantify smaller openings. Synchrotron phase contrast radiographs of an AA 2090 T8E41 (central 2 mm of a 12.7 mm thick plate) compact tension sample show the fatigue crack geometry clearly while absorption contrast radiographs of the same sample have difficulty even detecting the crack. The phase contrast expected in the projection images is determined from the microtomography-derived, three-dimensional crack geometry and compared to that observed experimentally. Good agreement is obtained between the phase contrast radiographs and the expected pattern of contrast.
Correlated Local Atomic Displacements: The Microscopic Origins for Macroscopic Phenomena.
Authors: Bridges Frank , Cao Daliang , Booth Corwin H.
Published: April 2011

In many systems for which there are several atoms in the unit cell, the displacements of the atoms may be locally correlated even though there is no long range coherence. Such displacements can play an important role in determining various macroscopic properties. We consider several examples to demonstrate this phenomena - the local distortions in the colossal magneto-resistive (CMR) and charge-ordered manganites, magnetic field induced distortions in CMR materials that are connected to macroscopic magnetostriction, and correlated displacements of tetrahedral units within the negative thermal expansion material ZrW2O8. The distortions in the CMR materials observed using XAFS change rapidly just below Tc and are attributed to the formation of polarons as the temperature is increased through Tc. In the ferromagnetic state, the lattice is more ordered for CMR systems; consequently applying a magnetic field for T ∼ Tc should decrease the local distortions. Such an effect has been observed and is a much larger effect than the measured macroscopic magnetostriction. Finally, in ZrW2O8 the tetrahedral and octahedral units are found to be very rigid as expected. More surprising is that the width of the pair-distance distribution for the W-Zr atom-pair is also quite small, indicating that the W-Zr linkage is stiff. In contrast, for the W-W pair, the distribution width grows rapidly with T, indicating correlated displacements of two WO4 tetrahedral units.
Structural Distortions in the Paramagnetic Insulating Phase of La0.7Ca0.3MnO3
Authors: Nelson C.S. , Kim Y.J. , Hill J.P. , Gibbs Doon , Kiryukhin V. , Koo T.Y. , Cheong S-W.
Published: April 2011

We report x-ray scattering studies of polarons and tilt ordering in the high-temperature, paramagnetic insulating phase of La0.7Ca0.3MnO3. The temperature dependence of scattering due to individual and correlated polarons was measured, and the latter was found to disappear at ∼400 K. The scattering due to tilt ordering, which was observed by tuning the incident photon energy near the La L1 edge, was also measured as a function of temperature. The destruction of tilt ordering at ∼690 K is believed to be associated with the orthorhombic-rhombohedral structural transition.
A silicon structure for electrical characterisation of nanoscale elements
Authors: Sazio P.J.A. , Berg J. , See P. , Ford C.J.B. , Lundgren P. , Greenham N.C. , Ginger D.S. , Bengtsson S. , Chin S.N.
Published: April 2011

The problem of mass manufacturing electrode structures suitable for contacting nanoscale elements lies primarily in the difficulty of fabricating a nanometre-scale gap between two electrodes in a well controlled, highly parallel manner. In ULSI circuit production, the gate and substrate in MOSFETs are routinely fabricated with a precise vertical spacing of 3 nm between them. In this work, we have investigated a number of highly parallel methods for the generation of nanogaps, including reconfiguration of the ubiquitous MOS device structure. The silicon dioxide layer that provides vertical separation and electrical insulation between two regions of silicon (the crystalline substrate and the poly-crystalline gate) gives a leakage current of 1 nA μm−2 at 1 V for an oxide thickness of 2 nm [1]. This will enable objects the size of single molecules that are held across this layer to be detected electrically if they provide currents on the nanoampere scale, assuming a parasitic area for leakage between gate and substrate of order 1 µm2. In the future this kind of device has the potential to provide a bolt-on technology for the fabrication of ULSI circuits in which conventional CMOS devices are directly hybridised with functional nanoscale elements.
Molecular Wires: Charge Injection, Charge Transport and Motion Mechanisms in DNA
Authors: Berlin Yu. A. , Burin A. L. , Nitzan A. , Mujica V. , Xue A. , Ratner M. A.
Published: April 2011

The use of the DNA duplex as a molecular wire is discussed with particular attention to recent experimental findings. Experimental studies of photo-excited hole dynamics in DNA can be understood within the phenomenological hopping model. However a microscopic first principles approach requires taking into account the interaction between charge and duplex degrees of freedom. The nature of possible metallic native DNA behavior is discussed.
MOCVD Growth of Ga(Al)N/InGaN/Ga(Al)N-Heterostructures: Influence of the Buffer Layer Al-Concentration and Growth Duration on the In-Incorporation in InGaN
Authors: Schowalter Marco , Neubauer Brigitte , Rosenauer Andreas , Gerthsen Dagmar , Schön Oliver , Heuken Michael
Published: April 2011

Transmission electron microscopy (TEM) has been applied to analyze the thickness and the In-concentration of InGaN layers in GaN/InGaN/GaN- and AlGaN/InGaN/AlGaN-quantum well (QW) structures. Two series of samples were grown by metal organic chemical vapor deposition varying either only the growth duration for the InGaN QW or by changing the Al- concentration in the buffer layers at unaltered InGaN growth conditions. A rising average In- concentration from 6.5 to 15.4 % and a decreasing growth rate are observed with increasing growth duration. The increase of the Al-concentration in the buffer layers from 0 to 36 % strongly affects the In-incorporation during the InGaN growth, which decreases from 17.5 to 2.5 %. All samples are characterized by an inhomogeneous In-distribution containing In-rich agglomerates with a size of only a few nanometers and less pronounced composition fluctuations on a scale of 100 nm.
Infrared absorption and annealing behavior of semi-insulating 4H SiC HTCVD substrates
Authors: Magnusson B. , Ellison A. , Storasta L. , Janzén E.
Published: April 2011

Infrared absorption and resistivity are studied in two different types of semi-insulating (SI) 4H SiC substrates. In type A the silicon vacancy dominates the infrared absorption spectra whereas in type B the UD-1 defect dominates. Both types of substrates are still SI (> 105 Ω cm at 200°C) after annealing at typical CVD growth temperatures (1600°C). The type A material shows a decreasing absorption of the silicon vacancy after 1600°C at the same time as the resistivity decreases from high 107 Ω cm to mid 106 Ω cm measured at 200°C. The thermal activation energy of the resistivity for the type A material is 0.85 eV for temperatures above 250°C both before and after annealing. The type B material does not change with annealing, besides that the absorption increases in intensity with a 1600°C anneal. The resistivity is > 1010 Ω cm at 200°C and the thermal activation energy is 1.4 eV.

The absorption measurements on the silicon vacancy also show that it is still present in the as-grown material after a 1600°C anneal. We interpret this as follows: The silicon vacancy is stable even at 1600°C and does not get mobile at 750°C as earlier suggested.
Spectroscopic Investigation of Traps Producing Current Collapse In AlGaN/GaN Hemt Structures
Authors: Klein P.B. , Binari S.C. , Ikossi K. , Wickenden A.E. , Koleske D.D. , Henry R.L.
Published: April 2011

Photoionization spectroscopy (PS) measurements, previously carried out for the GaN MESFET, have been extended to the more complicated AlGaN/GaN HEMT structures. In all cases, the spectra revealed that the same two traps causing current collapse in the high resistivity (HR) GaN buffer layer of the MESFET were also responsible for current collapse in the HEMT structures. The HR buffer layers supporting the HEMT structures were prepared by MOVPE at varying growth pressures, in order to vary the incorporation of deep trapping centers. Lower growth pressures were observed to correlate with more severe current collapse and with an enhanced incorporation of carbon. Detailed analysis of the PS data suggests that one of the two responsible traps is related to carbon, while the other may be associated with structural defects in the material.
Gadolinium Oxide Gate Dielectrics for GaN MOSFETs
Authors: Gila B.P. , Johnson J.W. , Lee K. N. , Krishnamoorthy V. , Bates S. , Abernathy C. R. , Ren F. , Pearton S. J.
Published: April 2011

Substrate preparation of GaN, both in-situ and ex-situ, and the growth of gadolinium oxide, Gd2O3, by Gas source molecular beam epitaxy (GSMBE) have been investigated. Ex-situ cleaning techniques included wet chemical etching and UV-ozone treatments to remove surface contaminants and the native oxide. In-situ cleaning consisted of thermal treatment with and without exposure to an electron cyclotron resonance (ECR) oxygen plasma. A GaN (1x3) streaky RHEED pattern was the final product of this surface treatment study. Various growth initiation techniques were explored to produce Gd2O3 films with different microstructures as evidenced by RHEED, TEM, and XRD. Gd2O3 films planarized the initial GaN surface and stoichiometry was maintained over a range of substrate temperatures (300° to 650°C). Single crystal gadolinium oxide films were grown at substrate temperatures of 600-650°C. These films exhibited a breakdown field strength (EBD) of ∼1MV/cm, and showed high leakage current at high forward bias due to defects within the oxide. Single crystal oxide films were found to be thermally stable at annealing temperatures up to 1000°C. Quasi-amorphous films were grown at a substrate temperature of 100°C. These films exhibited a higher E BD of ∼3MV/cm and an interface state density of 3 × 1011 cm−2eV−1. However, the quasi-amorphous films were not thermally stable at 1000°C, showing evidence of re-crystallization in x-ray diffraction (XRD) scans.
Current Gain of an AlGaN/GaN Heterojunction Bipolar Transistor
Authors: Zhang Yumin , Ruden P. Paul
Published: April 2011

A novel hybrid model and simulation results for an advanced, graded base AlGaN/GaN heterojunction bipolar transistor structure are presented. The base of the n-p-n HBT examined has two parts, a linearly graded AlGaN layer on the emitter side and a heavily p-doped GaN layer on the collector side. In the hybrid model developed here the potential profile is first calculated self-consistently in the biased state taking into account ionized impurity charges, polarization charges, and majority carrier charges. The minority carrier transport is examined subsequently. Injection of electrons from the emitter is modeled as a thermionic emission process. The minority electron transport process in the graded region is drift-dominated due to the large built-in effective field strength. In the low-field GaN layer of the base, electron transport is assumed to be diffusion-dominated. High-level injection effects are modeled in the framework of the Gummel-Poon model. Example structure design parameters are presented and it is found that the calculated current gain can be greater than 25, with a collector current density of 104A/cm2.
Ab-Initio Calculation of the β-SiC/Ni Interface
Authors: Blasetti A. , Profeta G. , Picozzi S. , Continenza A. , Freeman A. J.
Published: April 2011

We investigate the adsorption of a Ni monolayer on the β-SiC(001) surface by means of highly precise first-principles all-electron FLAPW calculations. Total energy calculations for the Si- and C-terminated surfaces reveal high Ni adsorption energies, with respect to other metals, confirming the strong reactivity and the stability of the transition metal/SiC interface. These high binding energies, about 7.3-7.4 eV, are shown to be related to strong p-d hybridization, common to both surface terminations and different adsorption sites, which, despite the large mismatch, may stabilize overlayer growth. A detailed analysis of the bonding mechanism, in terms of density of states and hybridization of the surface states, reveals the strong covalent character of the bonding. We also calculate and discuss the Schottky barrier heights at the Ni/SiC junction for both terminations.
Effect of Composition and Bead Settling on Debonding of Underfill Layers
Authors: Wang Lorraine C. , Dauskardt Reinhold H.
Published: April 2011

With the increased complexity of microelectronic devices continuing to drive chip packages to smaller sizes, the need for reliable high-density flip chip packaging is increasingly important. In these packages, epoxy underfills are used to surround a solder ball grid array in order to support thermomechanical stresses in the system. This research focuses on determining the mechanisms that cause debonding at interfaces in these packages, particularly between the underfill and chip passivation. A fracture mechanics approach is used to study the adhesion of Si3N4to two model underfills, one based on aliphatic epoxy and the other based on bisphenol F epoxy. Interestingly, these two underfills, which are similar mechanically, appear to show very different critical and subcritical behavior. The aliphatic underfill exhibits subcritical debond growth under constant displacements, behavior typically seen in interface systems, while the bisphenol F underfill surprisingly shows little susceptibility to stress corrosion. Although both underfills show evidence of silica bead settling, the addition of filler has opposing effects on their critical adhesion values. The effects of varying filler content in these underfills as well as environmental effects on the critical and subcritical behavior of these systems will be examined.
Characterization and Soldering of Organic Modified Aluminum Thin Films and Bond Pads
Authors: Fang Rui , Dahlgren Eric , O'Keefe Matthew J. , O'Keefe Thomas J. , Shih Wu-Sheng , Gamota Daniel R. , Zhang Jie
Published: April 2011

An immersion plating process using organic solutions was used to deposit a seed layer of copper and to activate the surface of thin film aluminum surfaces and bond pads. The seeded surfaces were subsequently electroless copper plated and tested for compatibility with soldering assembly. It was demonstrated that the copper particles deposited from the organic solution onto sputter deposited aluminum thin films were able to act as nucleation sites for the subsequent build up of adherent, continuous copper films from conventional electroless plating baths. In addition, the metal deposition from organic solution process was found to be very selective on patterned test vehicles and integrated circuits as metal deposition occurred only on the exposed aluminum bond pads and not on any passivated surfaces. Tests were conducted to determine the solderability of the samples after only seeding in the organic solution as well as the combination of seeding plus electroless copper deposition. In these tests, all attempts to directly solder without any pre-treatment were unsuccessful. Results on solder adherence varied with different substrates. Nevertheless, the concept of depositing metal seed layers in these applications is feasible. Additional work is needed, however, to optimize the deposition process for each particular surface to be soldered.
Using the Surface Potential Decay Method to probe electrical conduction in epoxy resins used for packaging
Authors: Sylvestre Alain , Gonon Patrice , Teysseyre Jérôme , Prior Christophe
Published: April 2011

In this work we discuss the usefulness of the Surface Potential Decay (SPD) method to investigate electrostatic charges deposited on epoxy resins which are used for electronic packaging. Corona discharges were applied on epoxy resins with different degrees of Post Mold Curing (PMC). SPD studies were conducted at room temperature under a relative humidity of about 45%. In the first part of this study we analyze the flow of electrostatic charges as a function of time. Using the SPD method we evidence that 10% of the electrical charge is still present at the surface three hours after the corona discharge has been applied. We find that the surface potential decay is more important for samples with longer PMC duration. In the second part of this work we provide a physical interpretation of the surface charge decay. We suggest that it is related to polarization mechanisms within the bulk of the material.
Fatigue Processes in Silicon MEMS Devices
Authors: Renuart Emily D. , Fitzgerald Alissa M. , Kenny Thomas W. , Dauskardt Reinhold H.
Published: April 2011

MEMS devices may experience significant alternating loads during service, associated with both applied and vibrational loading. Long-term reliability and lifetime predictions require understanding of possible fatigue mechanisms in these structures. Although silicon is not generally considered susceptible to fatigue crack growth, recent studies suggest that there may be fatigue processes in silicon MEMS structures. The phenomenon, however, has still not been extensively studied. In this work, we used a compressive double cantilever beam geometry to examine stable crack growth. Crack length and loads were carefully monitored throughout the test in order to distinguish between the apparent role of environmentally assisted crack growth (stress corrosion) and mechanically induced fatigue. Results revealed similar step-like crack extension versus time for the cyclic and monotonic tests. The fatigue crack-growth curve extracted from the crack extension data exhibited a nearly vertical slope with no evidence of fatigue crack-growth. Fracture surfaces for the monotonic and cyclic tests were similar, further suggesting that a true mechanical fatigue crack-growth mechanism did not occur.
Dislocation Walls in Finite Media: The Case of an Infinite Slab
Authors: Surh M. , Wolfer W. G.
Published: April 2011

The dislocation microstructure observed in solids exhibits cellular patterns. The interiors of these cells are depleted of dislocations while the walls contain dense bundles including the geometrically necessary dislocations leading to misorientations of the crystal lattice on either side. This clustering is the result of short-range interactions which favor the formation of dislocation dipoles or multipoles and tilt and twist boundaries. While this short-range ordering of dislocations is readily understood, the long-range pattern formation is still being studied. We examine finite tilt boundaries in an infinite medium, a model grain, and a free slab to investigate the conditions for long-range stress interactions. We find that finite tilt walls in a larger medium generally possess a long-range stress field because the local bending at the tilt wall is constrained by the surrounding material.
A Generalized Model of the Yield Drop for Impure Semiconductors
Authors: Petukhov Boris V.
Published: April 2011

The Alexander-Haasen theory describing the deformation behavior of low-dislocated semiconductor crystals is generalized taking into account the dynamic ageing of dislocations due to the impurities dragging. The constitutive equations describing kinetics of the plastic deformation are modified for the case of a spectrum of age-dependent internal stresses. Besides the solution hardening, the theory developed explains a number of qualitative distinctions of the elastic-plastic transition in silicon crystals grown by the Czochralski and the float zone methods. Particularly, this concerns a dependence of the yield drop on the initial dislocation density, and a weakening of the strain rate sensitivity of the yield stress.
Radiation Hardening and Plastic Instability in Neutron Irradiated CuCrZr
Authors: Edwards Dan. J. , Singh Bachu. N.
Published: April 2011

The effect of post-irradiation annealing at 300°C for 50 hours on the microstructure and mechanical properties of CuCrZr irradiated to 0.3 dpa at 100°C has been evaluated. The post- irradiation annealing restores some ductility and work hardening to the material as well as lowers the yield strength, however it does not completely remove the effects of irradiation. A comparison of the microstructural features and mechanical properties in the as-irradiated condition and in the post-irradiation annealed case highlights the fact that the observed microstructure does not necessarily correlate with the changes in tensile behavior, most notably in the removal of the yield point and lowering of the yield stress after annealing.
Modeling of Dislocation Mobility in Metals: Effect of Obstacles and Thermal Processes
Authors: Hiratani Masato , Zbib Hussein M.
Published: April 2011

Thermally activated dislocation glide velocity through weak point obstacle arrays is studied analytically and computationally. Thermal activation rate is estimated using the modified Friedel relations under the weak obstacle approximation. The average flight velocity after an activation event as a function of stress and temperature is obtained by the discrete dislocation dynamics (DD). This numerical calculation includes the effect of self-stress, interaction with electrons and phonons, and the inertial effect. These results are implemented into a phenomenological model to study the dislocation velocity under various conditions. The model can reproduce both obstacle control and drag control motion in low and high velocity regions, and a flow stress anomaly at transient regions.
Marquette's Engineering Student Volunteer Outreach Program to Fifth Graders in the Public Schools, Choice Schools, and a Charter School in Milwaukee
Authors: Brower William E.
Published: April 2011

Educational efforts by the MSE profession can effect society in several ways: by producing more and better MSE graduates, by enhancing the Materials backgrounds of students from other engineering disciplines, by continuing education programs, and, in a nontraditional sense, by impacting the non-engineering community with outreach programs. I suggest that the effect on society of working with disadvantaged elementary school children, such as inner city fifth graders, is potentially far greater than efforts spent on non-engineering students already in college, high school students, or even middle school students. The Fifth Grade Volunteer Teaching Program (5GVP) at Marquette has attempted over the last ten years to inform Milwaukee's inner city fifth graders of the existence of an engineering career while showing them that science is something that they can actually do themselves and that the doing is fun. Maybe some of the roughly 5,000 students taught have stayed in the college prep track in math and science as a result of contact with our enthusiastic engineering student volunteers. Our lessons are self contained and heavy with things for the fifth graders to do themselves. One lesson, which portends Materials Science and Engineering, is called, “Phases of Matter”. This year I have expanded the 5GVP Program to include three of the Choice Schools operating in Milwaukee. These schools differ from the Milwaukee Public Schools (MPS) and from each other significantly. Class sizes in the Choice Schools vary from much smaller than MPS to about the same size as MPS. The school buildings range from ramshackle to very modern. The Charter School has science labs and the Choice Schools do not. The Charter School is for profit and these two Choice Schools are not for profit.
The Macroscope: A Macroscopic Tool for Validating Microscopic Force Relationships
Authors: Guerra-Vela Claudio , Zypman Redy R.
Published: April 2011

The Force Macroscope (FM) was presented at the MRS-Spring-2000 as a laboratory teaching tool to introduce students to concepts of Scanning Force Microscopy (SFM). It is a macroscopic version of the SFM. The FM pedagogical advantage over the SFM is its size: students relate to the FM, only a few grasp at once the concepts for the 100 [.proportional]m-long SFM cantilever. In this work we will show how we take advantage of the FMs large size to teach concepts of force reconstruction.
Role of Fluorite Formation in Orientation Selection in Sol-Gel Derived Pb(Zr,Ti)O3 Films on Pt Electrode Layers
Authors: Norga G. J. , Fè L. , Vasiliu F. , Wouters D. J. , Biest O. Van der
Published: April 2011

We present evidence that the microstructure of the metastable fluorite phase Pb2(Zr,Ti)2O6, which forms during the pyrolysis treatment of sol-gel PZT layers by homogeneous nucleation throughout the film, has a larger impact on orientation selection than has previously been realized. Elaborate TEM studies demonstrate that by varying the pyrolysis time, temperature and duration, the crystallinity of the transient fluorite phase can be greatly influenced. Pyrolysis at 350°C leads to the formation of a well crystallized fluorite phase with a cubic structure (Fd3m space group) and a= 10.5-10.6 Å. Rapid crystallization of the transient fluorite phase is attributed to the low oxygen partial pressure conditions, prevailing during burnoff of acetate groups originating from the lead starting compound. A possible connection between the fluorite formation kinetics and orientation selection in the layers is discussed.
Precise Control of Nucleation and Growth in Lead Zirconate Titanate Thin Films by Scanning Rapid Thermal Annealing
Authors: Lee Jang-Sik , Joo Seung-Ki
Published: April 2011

In this paper, a novel annealing method, scanning-rapid thermal annealing (RTA), for the selectively nucleated lateral crystallization (SNLC) of Pb(Zr,Ti)O3 (PZT) thin films is discussed. The effects of lamp power and scan speed on the SNLC were investigated. It was found that scanning-RTA was very effective method for SNLC in reducing the process time and preventing undesirable nucleation at other than pre-determined positions.
Epitaxial Growth of LaVO3/(Ba,Sr)TiO3/(Pb,La)(Zr,Ti)O3/(La,Sr)CoO3 Semiconductor/Ferroelectric/Conductor Heterostructures
Authors: Choi Woong , Sands Tim
Published: April 2011

The feasibility of using LaVO3 as a semiconducting layer in all-oxide semiconductor/ferroelectric/conductor heterostructures was explored by growing epitaxial LaVO3/(Ba,Sr)TiO3/(Pb,La)(Zr,Ti)O3/(La,Sr)CoO3 heterostructures on (001) LaAlO3 single crystal substrates by pulsed laser deposition. A high degree of c-axis orientation and strong in-plane texture revealed by x-ray diffraction indicated the epitaxial crystallographic relationships between the layers. With a 30 nm layer of (Ba,Sr)TiO3, the heterostructure showed optimal ferroelectric hysteresis with remanent polarizations over 30 C/cm2. The capacitance measurement as a function of voltage might reveal the modulation of the depletion layer in the semiconducting LaVO3.
Al2O3/Si3N4 Buffer Layer for High Performance MFIS (Metal-Ferroelectric-Insulator-Semiconductor) Transistors
Authors: Fujisaki Yoshihisa , Ishiwara Hiroshi
Published: April 2011

The Al2O3 thin films were deposited on the Si3N4 (radical- Si3N4) prepared by the direct nitridation of a Si substrate using atomic nitrogen radicals. Because the radical- Si3N4 is highly resistive against the oxidation process, we can completely eliminate the metal phase in Al2O3 that degrades the insulating properties of Al2O3. The 1.3nm thick Al2O3 film was found to have a permittivity of 9.76 in this stacked Al2O3/Si3N4 structure. It was also found that no oxidation took place at the interface of Si3N4 and the Si substrate during the deposition and post-oxidation of the Al2O3 film on the radical-Si3N4/Si substrates. As a result, highly capacitive MIS (Metal-Insulator-Semiconductor) structures were realized using the Al2O3/Si3N4 stacked insulator. The MIS diode with Al2O3/Si3N4 is found to have 105 times less leakage current compared to the diode with Si3N4 insulator film with similar capacitance density. We have shown that the insulating properties of the Al2O3/radical-Si3N4 stack are suitable for the MFIS (Metal-Ferroelectric-Insulator-Semiconductor) transistors.
Raman Studies of the Soft Phonon Modes in BaxSr1−x.TiO3 thin Films
Authors: Tenne D. A. , Clark A. M. , James A. R. , Soukiassian A. , Chen K. , Xi X. X.
Published: April 2011

The vibrational properties of barium strontium titanate thin films were studied by Raman spectroscopy in the temperature range from 5 to 300 K. The films were grown by pulsed laser deposition on SrTiO3 and LaAlO3 substrates with SrRuO3 buffer layers. Soft phonons are observed in Raman spectra of BaxSr1−xTiO3 films with Ba contents x = 0.05, 0.1, 0.2 and 0.5. The temperature dependence of the soft phonon frequencies and the splitting of the triply degenerated soft mode into two components of A and E symmetries indicate the ferroelectric phase transition. The E soft mode line is overdamped over a broad range of temperatures near the ferroelectric phase transition, while it is clearly seen in the spectra at temperatures away from the phase transition. The relative Raman intensity of the A soft mode and hard modes decreases gradually over a broad range of temperatures. This indicates a broad ferroelectric phase transition in the thin films. Comparison of Raman spectra for films grown on SrTiO3 and LaAlO3 substrates shows the influence of strain on the temperature of the ferroelectric phase transition.
Fabrication and Characterization of a PZT thin Film Actuator for a Micro Electromechanical Switch Application
Authors: Hoffmann Marcus , Leuerer Timm , Krüger Clemens , Böttger Ulrich , Mokwa Wilfried , Waser Rainer
Published: April 2011

Micromachined silicon cantilever beams actuated by the converse piezoelectric effect are of great interest for actuator applications, e.g. micro relays or micro mirrors. For the miniaturization and cost saving aspects the combination of silicon bulk micromachining and chemical solution deposition (CSD) technique for the ceramic thin films is very promising.

This paper presents the results of such a fabrication process for a PbZr0.45Ti0.55O3 (PZT) thin film micro actuator for a switch application. The actuator was designed with lengths of 190-990 μm, widths of 60-120 μm, and a complete thickness of 1.5 μm. Wherein the piezoelectric PZT function layer has a thickness of 350 nm. For a distance of 10 μm between the switch contacts and an applied voltage of 10 V a finite element analysis simulation (FEA) was carried out to obtain the principal stress contribution, the optimum cantilever length, the sensitivity, the resonance frequency and the switch contact force. The bending beams were characterized by laser interferometry, resonance frequency, and force measurements. These characterization results are compared to the FEM analyses and to an analytical approach.
Infrared Spectroscopy of Epitaxial Antimony Sulpho Iodide Thin Films
Authors: Kotru S. , Surthi S. , Pandey R. K. , Donnelly D.
Published: April 2011

Thin films of antimony sulpho iodide (SbSI) were grown on platinized silicon substrates by the pulsed laser deposition method. As grown films were amorphous and annealing at 250 °C for 5 minutes introduced crystallinity in the films. Infrared reflectance measurements were done in the frequency range ∼ 500 – 5000 cm−1 (wavelength ∼ 2–20 μm). The reflectance measurements were taken at temperatures above and below the ferroelectric transition of SbSI ∼ 20° C. The index of refraction for a (121) oriented film was determined to be 2.83 ± 0.35 at a temperature of 25.6 °C, and 2.80 ± 0.35 at a temperature of 9.6 °C. For a (002) oriented film, the index was 3.82 ± 0.48 at a temperature of 26.5 °C, and 3.76 ± 0.48 at a temperature of 8.0 °C. Pyro-optic coefficients of 1.5 × 10−3 °C−1 for the (121) oriented film, and 3.2 × 10−3 °C−1 for the (002) oriented film were obtained. These results are consistent with measurements done in the visible region, and demonstrate the potential of SbSI as an infrared detector material.
PZT and PMN-PT Thin Film Cantilevers: Comparison between Monomorph and Bimorph Structures
Authors: Hoffmann Marcus , Kügeler Carsten , Böttger Ulrich , Waser Rainer
Published: April 2011

Piezoelectric and electrostrictive materials are potential candidates for integrated micro systems. They can be used in cantilever laminated structures for different applications, e.g. active vibration control or agile transducers [1-3]. Due to the necessity of miniaturization of MEMS devices and the reduction of process costs and time, the use of chemical solution deposition (CSD) technique with microlithography and reactive ion etching (RIE) are essential.

Within this work we used these techniques in combination with silicon bulk micro machining technique to fabricate piezoelectric Pb(Zr,Ti)O3 (PZT) and electrostrictive Pb(Mg1/3,Nb2/3)O3-PbTiO3 (PMN-PT) coated micro cantilevers with different lengths which can be used in micro switch or micro mirror applications. In general PMN shows no elastic hysteresis and a better aging behavior than PZT ceramics. Since the electrostrictive effect is smaller than the piezoelectric effect the tip deflection of PMN-PT coated beams is much lower. Cantilevers with two ceramic thin film layers and an internal electrode (bimorph) were designed and compared to such with single ceramic thin film layers (monomorph). For fabrication control and electrical characterization SEM, polarization hysteresis-, and CV-measurements were performed. Laser interferometry measurements were used to characterize the electromechanic performance of the ceramic thin films and cantilevers.
Hinge Sensitivity in a Micro-Rotating Structure for predicting Induced Thermo Mechanical Stress in Integrated Circuit Metal Interconnects
Authors: dos Santos J. M. M. , Wang K. , Soare S. M. , Bull S. J. , Horsfall A. B. , Wright N. G. , O'Neill A. G. , Terry J. G. , Walton A. J. , Gundlach A. M. , Stevenson J. T. M.
Published: April 2011

The process-induced stress in interconnects within integrated circuits (IC) has a direct influence on the mean time to failure of the devices. Since measurement of stress in individual metallised lines is not possible by existing techniques, another approach has been adopted where a test structure is generated during fabrication based on a micro-rotating cantilever sensor. To support the design, finite element modeling (FEM) has been performed. By comparing the rotation predicted by FEM simulations and that observed experimentally, a clear discrepancy is observed which is critically dependent on the details of the sensor design, the pattern transfer of the lithographic process and on the dry etching processing.
Anti-Weak-Localization Study of Rashba Spin-Splitting Energy as a Function of Well Asymmetry in InAlAs/InGaAs/InAlAs Quantum Wells
Authors: Koga Takaaki , Nitta Junsaku , Akazaki Tatsushi , Takayanagi Hideaki
Published: April 2011

We have investigated the values of the Rashba spin-orbit coupling constant α in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells using an anti-weak-localization (AWL) analysis as a function of quantum well asymmetry. We have found that the deduced α values have strong correlations with the structural inversion asymmetry of the pertinent quantum wells as predicted theoretically. This fact suggests that the AWL analysis provides a reliable way to deduce α values experimentally for designing future spintronics devices.
Imaging Self-Organized Domains at the Micron Scale in Antiferromagnetic Elemental Cr Using Magnetic X-ray Microscopy
Authors: Evans P. G. , Isaacs E. D. , Aeppli G. , Cai Z.-H. , Lai B.
Published: April 2011

The domains of antiferromagnetic order in elemental chromium can be observed with spatial resolution that is improved by orders of magnitude in comparison with previous techniques using magnetic x-ray scattering with an incident x-ray beam focused to a submicron spot. This use of magnetic x-ray microscopy takes advantage of the incommensurate spin density wave order in Cr to isolate magnetic scattering. The spin polarization dependence of the magnetic x-ray scattering cross section allows the first order spin-flip transition near 120 K to be imaged directly.
High Luminescence Efficiency from GaAsN Layers Grown by MBE with RF Nitrogen Plasma Source
Authors: Ustinov Victor M. , Cherkashin Nikolai A. , Bert Nikolai A. , Tsatsul'nikov Andrei F. , Kovsh Alexei R. , Wang Jyh-Shang , Wei Li , Chi Jim Y.
Published: April 2011

(In)GaAsN based heterostructures have been found to be promising candidates for the active region of 1.3 micron VCSELs. However, (In)GaAsN bulk layers and quantum wells usually demonstrate lower photoluminescence intensity than their nitrogen-free analogues. Defects associated with lower temperature growth and N-related defects due to plasma cell operation and possible nonuniform distribution of nitrogen enhance the non-radiative recombination in N-contained layers. We studied the photoluminescence intensity of GaAsN layers as a function of N content in MBE grown samples using rf-plasma source. Increasing the growth temperature to as high as 520 °C in combination with the increase in the growth rate allowed us to avoid any N-related defects up to 1.5% of nitrogen. Low-temperature-growth defects can be removed by post-growth annealing. We achieved the same radiative efficiency of GaAsN samples grown at 520°C with that of reference layer of GaAs grown at 600°C. Compositional fluctuations in GaAsN layers lead to characteristic S-shape of temperature dependence of photoluminescence peak position and this feature is the more pronounced the higher the amount of nitrogen in GaAsN. Annealing reduces compositional fluctuations in addition to the increase in the photoluminescence intensity. The results obtained are important for further improving the characteristics of InGaAsN lasers emitting at 1.3 micron.
The Role of Nitrogen-Induced Localization and Defects in InGaAsN (? 2% N): Comparison of InGaAsN Grown by Molecular Beam Epitaxy and Metal-Organic Chemical Vapor Deposition
Authors: Kurtz Steven R , Allermana A. A. , Klem J. F. , Sieg R. M. , Seager C. H. , Jones E. D.
Published: April 2011

Nitrogen vibrational mode spectra, Hall mobilities, and minority carrier diffusion lengths are examined for InGaAsN (≈ 1.1 eV bandgap) grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Independent of growth technique, annealing promotes the formation of In-N bonding, and lateral carrier transport is limited by large scale (Ęmean free path ) material inhomogeneities. Comparing solar cell quantum efficiencies for devices grown by MBE and MOCVD, we find significant electron diffusion in the MBE material (reversed from the hole diffusion occurring in MOCVD material), and minority carrier diffusion in InGaAsN cannot be explained by a “universal”, nitrogen-related defect.
Thermal Anneal Effects on Carbon-Hydrogen LVMs In AlGaN
Authors: Manasreh M. O. , Weaver B. D.
Published: April 2011

Thermal annealing effects on carbon-hydrogen (C-H) complexes defects in AlGaN grown on sapphire by metalorganic chemical vapor deposition (MOCVD) technique have been investigated using Fourier transform infrared spectroscopy (FTIR). The CH complexes in AlGaN, formed either during growth or by proton irradiation, exhibit five local vibrational modes (LVMs) due to the symmetric and asymmetric vibrational stretching modes of C-H in CHn (n=1–;3) defect complexes. It was found that the annealing temperature (Ta) of 500°C is sufficient enough to dissociate most of the C-H complexes in AlGaN samples. A turning point annealing temperature is found around 300°C for un-irradiated Mg-doped sample, below which the total integrated area of the C-H LVMs continued to increase with increasing annealing temperature and reach the maximum value around 300°C. At Ta > 300°C, the total integrated area of the C-H LVMs starts to decrease and the C-H complexes seem to be completely depleted at Ta > 600°C. The depleted C-H LVMs were observed to partially recover after thermal annealing at Ta > 500°C and waiting for aging periods of several days. This recovery behavior is explained in terms of the hydrogen being remained inside the crystal after the dissociation of C-H complexes, subsequent diffusion and recombining again with carbon atom to reform C-H complexes.
Irradiation Effects in Space Solar Cells Made of Multiple Absorbers
Authors: Romero M. J. , Walters R. J. , Al-Jassim M. M. , Messenger S. R. , Summers G. P.
Published: April 2011

Solar cells made of multiple absorbers are a commonly used approach for improving efficiency due to their extended range of spectral sensitivity. Indeed, efficiencies nearing the theoretical maximum have been achieved with a triple-junction device made of In0.51Ga0.49P (InGaP2), GaAs, and Ge solar cells connected in series. For extraterrestrial applications, there is the added requirement of radiation tolerance. The main challenge for space power-generation is therefore the development of highly efficient and radiation-tolerant devices. We have investigated several aspects of the radiation response of solar cells made of multiple absorbers, such as multijunction devices and quantum-well solar cells. Novel possibilities such as quantumdot solar cells and ordered-disordered heterostructures are proposed.