bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical Thermal Conductivity of Periodic Two-Dimensional Nanocomposites
Authors: Yang Ronggui , Chen Gang
Published: April 2011

A phonon Boltzmann transport model is established to study the lattice thermal conductivity of nanocomposites with nanowires embedded in a host semiconductor material. Special attention has been paid to cell-cell interaction using periodic boundary conditions. The simulation shows that the temperature profiles in nanocomposites are very different from those in conventional composites, due to ballistic phonon transport at nanoscale. The thermal conductivity of periodic 2-D nanocomposites is a strong function of the size of the embedded wires and the volumetric fraction of the constituent materials. At constant volumetric fraction the smaller the wire diameter, the smaller is the thermal conductivity of periodic two-dimensional nanocomposites. For fixed silicon wire dimension, the lower the atomic percentage of germanium, the lower the thermal conductivity of the nanocomposites. The results of this study can be used to direct the development of high efficiency thermoelectric materials.
Synthesis and High Temperature Thermoelectric Properties of Alkaline-Earth Metal Hexaborides MB6 (M=Ca, Sr, Ba)
Authors: Takeda Masatoshi , Kurita Yosuke , Yokoyama Keisuke , Miura Takahiro , Suzuki Tsuneo , Suematsu Hisayuki , Jiang Weihua , Yatsui Kiyoshi
Published: April 2011

Polycrystalline alkaline-earth hexaborides (MB6: M =Ca, Sr, Ba) were synthesized and their thermoelectric and transport properties were examined to discuss their possibility as high temperature thermoelectric materials. Hall measurements showed that carrier concentration of the BaB6 was the highest among the three hexaborides and that of CaB6 was the lowest. Substitution of part of the alkaline earth metals with one of the others changed the carrier concentration of the hexaboride. As the carrier concentration increased, Seebeck coefficient increased and electrical conductivity decreased. These results suggest that the thermoelectric properties of the divalent hexaborides depend largely on the carrier concentration, and optimum carrier concentration which gives maximum power factor was estimated to be approximately 2x1026 m−3. Consequently, such a substitution enables us to control Seebeck coefficient and electrical conductivity of the hexaborides, and will also be effective to reduce the lattice heat conduction due to the alloying effect. A thermoelectric device was fabricated using SrB6 and boron carbide thin films as n-type and p-type elements, respectively. To the best of our knowledge, this is the first demonstration of a thermoelectric device composed of only boron-rich solids.
Evidence of a Order-Disorder Transition in the Crystalline Phase of Cd6Yb, 1/1 cubic approximant of icosahedral Cd 5.7 Yb
Authors: Sorloaica N. , Pope A. L. , Winkler D. W. , Tritt Terry M. , Keppens V. , Mandrus D. , Sales B.
Published: April 2011

The electrical and thermal transport properties (electrical resistivity, thermopower, heat capacity and thermal conductivity) of the crystalline phase of the binary Cd6Yb system has been measured over a temperature range from 10 K to 300 K. Evidence for a phase transition in Cd6Yb is observed in the electrical transport properties with distinct changes in the temperature dependence of the resistivity and thermopower around T ≈ 110K. An anomaly in the heat capacity and a thermal conductivity is also observed at this same temperature. Hysteretic behavior is not evident in the temperature dependence of any of the electrical and thermal transport properties. In addition, the elastic properties using resonant ultrasound (RUS) techniques have been investigated over a similar temperature range. A large “resonance dip” is observed in the RUS data at T ≈ 110K, which is indicative of some type of structural change in the crystalline material at this temperature. These data will be presented and discussed in context of the undergoes reversible order-disorder transition in the 1/1 cubic approximant at about 110 K, which makes the system very interesting compared to the quasicrystal phase Cd5.7Yb
Performance of Integrated Thin-film Thermoelectrics in cooling “hot-spots” on Microprocessors – Experimental setup and Results
Authors: Cancheevaram Jai , Alley Randy , Siivola Edward , Venkatasubramanian Rama
Published: April 2011

The improving performance of microprocessors is increasing the Thermal Design Power [TDP] and cause localized power densities commonly referred to as “Hot-spots”, which reach in excess of 200 W/cm2. The focus of this work is to present Thin-film Superlattice Thermoelectric materials [TFST] as a solution for microprocessor die hot spot cooling. TFST have measured ZT values of ∼2 at 300K, response times of the order of 15μs and potential to pump heat flux of several hundred Watts/cm2. TFST devices appear suitable for managing the junction temperatures at such hot spots such that the recommended maximum operational temperature, Tjmax, is not exceeded. This promotes stable processor performance and increased reliability. To test this theory, hot spots were identified on the microprocessor die by infrared (IR) imaging during operation and a suitable TFST module was mounted onto the processor. Thermal transients at the hot spot show that active heat pumping and spreading by TFST significantly reduces the constraints on the thermal design. This investigation suggests that in the future, with the transition of Superlattice material performance into higher module ZT values; these hot spots may be effectively removed.
High Temperature Measurement System Design for Thermoelectric Materials In Power Generation Application
Authors: Loo Sim , Short Jarrod , Hsu Kuei Fang , Kanatzidis Mercouri , Hogan Tim
Published: April 2011

Recent interest in thermoelectric materials for power generation applications has initiated the development of a measurement system in our laboratory for characterization of materials in the 80K to 800K temperature range. This system has been specifically designed for measuring thermoelectric power and electrical conductivity as needed for determining the power factor of the measured samples. This is a single sample measurement system based on a continuous flow cryostat. Significant effort has gone into the computer controlled data acquisition and PID controlled temperature stabilization. Investigation of the influence of temperature stability on the measured data will be presented along with important aspects of the system design, development, and testing. Data collected on reference materials and new thermoelectric materials of interest will be presented.
Modelling and Optimal Design of Multilayer Cantilever Microactuators
Authors: Han L. H. , Lu T. J.
Published: April 2011

Thermal expansion is an important actuation mechanism for microelectromechanical system (MEMS). We investigate multi-layered cantilever microactuators actuated by resistive heating with an electric current. The actuator consists of films and sandwiched microheaters. An electrothermal-elastic model for characterizing microactuators is developed. The general analytical expressions relating tip deflection with an applied electric field, temperature change, the film thickness and the applied load at the tip are obtained. The large deflection is considered in the model. The width effect of the cantilever is also investigated using 3D finite element analysis. Based on the analytical solutions and a mixed variable programming (MVP) algorithm, the optimal design of the actuators is presented. The algorithms optimize simultaneously with respect to the number of films and the material choice from a list of candidate materials. The minimum weight and maximum deflection are chosen as the design objectives.
Bulge Test on Free Standing Gold Thin Films
Authors: Li Yawen , Cima Michael J.
Published: April 2011

Bulge test was performed on gold and gold/nitride composite films with in plane sizes ranging from 20 to 200μm. The films were prepared using the microfabrication process of a drug delivery MEMS device, and the bulge test setup was constructed using some of the packaging components of the device. Incremental pressure at 5psi interval was applied to the films, whose deflection was measured using interferometry. The extracted gold film properties from bulge test on either pure gold films or composite films yielded comparable values. The lower modulus (126∼168 GPa) comparing with the bulk gold could be due to the less dense microstructure of the evaporated gold film.
Indentation and Finite Element Modelling Investigations of the Indentation Size Effect in Aluminium Coatings on Borosilicate Glass Substrates
Authors: Spary I. , Jennett N. M. , Bushby A. J.
Published: April 2011

Nanoindentation is one of the few available methods and the most commercially widespread technique for investigating the elastic and plastic properties of small volumes such as thin films. Quantitative methods for obtaining the indentation (plane strain) modulus and hardness of a coating have been published and finite element models (FEM) of the elastic-plastic response of indentation have been developed. Comparison of the FEM output with actual indentation data has shown that, as the indentation size reduces, the apparent yield stress of the material increases. We have shown that the increase in yield stress is predictable and falls on a master curve (MRS Symp. Proc., Vol 788, p123, 2003). Predictions have been tested and agree for a range of metals (Cu, Al, W, Ir). This points to there being a fundamental length scale for dislocation-based deformation and raises the question as to whether the yield stress of thin films may be altered by reducing thickness. This study therefore investigates the indentation response of Al coatings on Borosilicate glass as a function of coating thickness and indentation depth. FEM of the indentation contact will be compared with indentation data and AFM measurements of the surface profile to investigate the relative contributions of the indentation size effect and the effect of hardening due to the additional constraint of substrate proximity to the plastic zone.
Indentation Curve Analysis for Pile-up, Sink-in and Tip-Blunting Effects in Sharp Indentations
Authors: Choi Yeol , Lee Baik-Woo , Lee Ho-Seung , Kwon Dongil
Published: April 2011

Hardness and elastic modulus can be derived from instrumented sharp indentation curves by considering the effects of materials pile-up and sink-in and tip blunting. In particular, this study quantifies pile-up or sink-in effects in determining contact area based on indentation-curve analysis. Two approaches, finite-element simulation and theoretical modeling, were used to describe the detailed contact morphologies. The ratio of contact depth to maximum indentation depth was proposed as a key indentation parameter and was found to be a material constant independent of indentation load. In addition, this parameter can be determined strictly in terms of indentation-curve parameters, such as loading and unloading slopes at maximum depth and indentation energy ratio. This curve-analysis method was verified by finite-element simulations and nanoindentation experiments.
Determining Stress-strain Curves for Thin Films by Experimental/Computational Nanoindentation
Authors: Lee Baik-Woo , Choi Yeol , Lee Yun-Hee , Kim Ju-Young , Kwon Dongil
Published: April 2011

The nanoindentation technique has great promise in evaluating mechanical properties such as nanohardness and elastic modulus at micrometer or nanometer scales, since sample preparation and testing procedures are very easy. However, the nanohardness and elastic modulus cannot be directly related to basic material flow properties. Here a novel and simple experimental/computational method is proposed to extract stress-strain curves based on finite-element modeling (FEM) of nanoindentation. This method was verified for bulk Al by comparing the stress-strain curves extracted with those obtained from tensile testing, and was applied to Al thin films (0.5 μm and 1 μm) deposited on a Si substrate.
Thermomechanical Behavior of Tantalum Thin Films: The Effects of Oxygen and the β–α Phase Transformation
Authors: Knepper Robert , Stevens Blake , Baker Shefford P.
Published: April 2011

β-Ta films were prepared in an ultra-high vacuum sputter deposition system, and the stresses that arose during thermal cycles to 750° C were measured using an in situ substrate curvature measurement system. The oxygen content in the environment was controlled during both deposition and thermomechanical testing. In films with no added oxygen, phase transformation from β to α takes place in distinct “jumps.” Addition of controlled amounts of oxygen to the annealing atmosphere led to increases in compressive stress, inhibited the phase transformation, and altered the magnitudes of the “jumps” in stress. The stresses in Ta films were found to be sensitive to oxygen content as well as thermal history.
Thin Film Material Parameters Derived from Full Field Nanometric Displacement Measurements in Non-uniform MEMS Geometries
Authors: Cárdenas-García Jaime F. , Cho Sungwoo , Chasiotis Ioannis
Published: April 2011

MEMS-scale polycrystalline silicon 2 μm thin film specimens fabricated via the Multi User MEMS Processes (MUMPs) have been employed to obtain the non-uniform nanometric displacement fields in the vicinity of prefabricated circular and elliptical micron-sized perforations. For the hole diameter-to-specimen width ratios considered in this work, and for all practical purposes, the displacement solution for a hole in an infinite plate is applicable. This method requires the ability to reliably and repeatably acquire nanometer level displacements on freestanding thin films. These tensile tests were conducted by a custom microtensile tester with the aid of Atomic Force Microscopy (AFM) and a special gripper that makes use of electrostatically assisted UV adhesion to handle and load miniature MEMS specimens. This non-conventional procedure for material parameter determination relies on Digital Image Correlation (DIC) to compare two AFM images, one before and one after specimen loading, and thus compute the nanometer level displacement fields (<50 nm global displacements) in 15×15-μm2 (or smaller) areas, with better than 2–3 nm resolution in displacements. By posing and solving a non-linear least squares inverse problem where, for known applied loads and measured displacement fields in an infinite plate with either a circular or an elliptical hole, it is possible to recover the elastic modulus (E). The main advantage of this approach is the full utilization of high-resolution displacement measurements over a specific specimen area, using only measurements acquired at one load level. Further statistical measurements of material properties may be obtained at varying load levels.
Monotonic Testing and Tension-Tension Fatigue Testing of Free-standing Al Microtensile Beams
Authors: Barbosa Nicholas , El-Deiry Paul , Vinci Richard P.
Published: April 2011

Free-standing Al thin films were loaded statically and dynamically through the use of a custom-built microtensile system. The system is capable of performing monotonic loading/unloading up to 50 μm/s (10-1/s) and tension-tension fatigue experiments at 100 Hz on 600 μm long, 100 μm wide, and 1 μm thick free-standing Al microtensile beams. Monotonic loading/unloading and stress relaxation experiments have been performed. The microtensile beams show plasticity as well as a relaxation dependence on strain rate and stress level. Displacement controlled tension-tension fatigue experiments have also been performed. A trend of decreasing cycles to failure with increasing displacement amplitude and increasing mean displacement has been noted but requires further experimental exploration.
Effect of Monazite Coating on Tensile Properties of Nextel 720 Fibers, Tows and Minicomposites
Authors: Pai Devdas M. , Yarmolenko Sergey , Sankar Jagannathan , Kailasshankar Balasubramanian , Murphy Christopher , Freeman Edwardo , Zawada Larry P.
Published: April 2011

The tensile strengths of Nextel™ 720 fibers as well as tows and minicomposites made of these fibers were measured to study the effects of monazite coating at room temperature (RT) and 1200°C (HT). Tests were conducted on as-received materials (called unsoaked) as well as materials that had been thermally soaked at HT for 100 hours. Weibull analysis of fiber strength data shows close agreement between the Weibull modulus obtained from the statistical fracture distribution and the modulus obtained from a study of the gage length dependence of strength. The monazite coating does not change the RT strength of unsoaked single fibers, but has a beneficial effect in terms of strength retention under HT testing. This beneficial effect of the coating disappears for soaked samples. The results for tows and minicomposites also indicate that for the unsoaked condition, tensile failure appears to be driven by fiber surface flaws and the presence of the monazite coatings clearly improves both RT and HT strength of fibers, tows and minicomposites. However, the beneficial effects of the coating disappear after long term exposure to HT.
Fracture Behavior of Micro-Sized Specimens Prepared from a TiAl Thin Foil
Authors: Takashima K. , Halford T. P. , Rudinal D. , Higo Y. , Bowen P.
Published: April 2011

Fracture tests have been carried out on micro-sized specimens prepared from a fully lamellar γ-TiAl based alloy thin foil. Micro cantilever beam type specimens with dimensions = 50 × 10 × 20 μm were prepared from one lamellar colony of the thin foil by focused ion beam machining. Notches with a width of 0.5 μm and a depth of 10 μm were also introduced into the micro-sized specimens by focused ion beam machining. Notch directions were introduced into samples in order to select the trans- and inter-lamellar directions, respectively. Fracture tests were carried out using a mechanical testing machine for micro-sized specimens. Fracture tests for the micro-sized specimens were performed successfully, showing the fracture behaviour to be dependent upon the notch orientation. The fracture toughness of specimens with a notch direction perpendicular to the lamellar direction was 4.7 – 6.9 MPam1/2, while that with a notch direction in the inter-lamellar direction was 1.4 – 2.7 MPm1/2. This indicates that the orientation of the lamellar microstructure greatly affects the fracture properties of micro-sized components prepared from fully lamellar γ-TiAl based alloy thin foils. It is required to consider the results obtained in this investigation when designing actual micro scale structures using TiAl thin foils.
Strain control in SrRuO3 thin films by using a lattice constant tunable buffer
Authors: Terai K. , Ohnishi T. , Lippmaa M. , Koinuma H. , Kawasaki M.
Published: April 2011

Heteroepitaxial oxide thin films are usually grown on single crystal substrate which offer a similar lattice constant as the target material. In general, there are no substrates that are suitable for film fabrication and have a good lattice matching. In our previous report, we succeeded in fabricating a lattice constant tunable buffer by using a Ba1-xSrxTiO3 / BaTiO3 bilayer structure on SrTiO3. The in-plane lattice constant of the buffer layer can be tuned from 3.91 to 3.99 Å and the lattice constant is determined only by the Ba/Sr ratio in the Ba1-xSrxTiO3 layer. The buffer is suitable for growing both strain-free films and strained films. In this report we demonstrate the use of the lattice constant tunable buffer for strain control in SrRuO3 magnetic thin films. SrRuO3 has a magnetic anisotropy which changes under lattice strain. We show that the easy axis of magnetization is parallel to the film surface under tensile strain on a Ba0.5Sr0.5TiO3 buffer. The easy axis is perpendicular to the surface in compressively strained films. The tensile strain in a film also results in an increase of the ferromagnetic ordering temperature from a bulk value of 160 K to 164 K.
Stress and Defect Generation in Si Epitaxy
Authors: Wang Tien , Carlson Douglas
Published: April 2011

A quasi-static model is proposed to describe wafer deformation due to stress build up in the Si epitaxy process. The analysis takes into account temporal stress variations as the wafer sags into the dish-shaped pocket of the susceptor at a deposition temperature above 1100 °C. It is shown that the magnitude of the negative bending moment at the wafer supporting edge decreases over time. As a result, the maximum deflection at the wafer center and the tensile stress at the wafer supporting edge also decrease over time. The generation of slip defects near the wafer edge as the wafer responds the thermal, gravitational and contact stress is explored. A new test methodology is developed to track the time-varying wafer deflection. It allows for the measurement of quality of the susceptor and its fitness for use in the reactor system.
Evaluation on Stress and Optical Property of Thin Films Used in Optical MEMS Device
Authors: Sun Lianchao , Hou Ping
Published: April 2011

Control of the film stress and optical property has long been considered as an issue in the tunable optical MEMS (Micro-Electro-Mechanical Systems) devices. In this paper, the atmospheric evolution of Titanium Dioxide (TiO2) and Silicon Dioxide (SiO2) thin films for the optical MEMS devices were studied. These films were prepared by ion-assisted e-beam evaporation. It is found that as-deposited SiO2 films exhibit compressive stress; whereas, it is tensile in the TiO2 films under present processing conditions. When annealed at 150 °C, both SiO2 and TiO2 films show slight changes in stress with annealing time. However, increasing the anneal temperature to 250°C caused an apparent change of film stresses with time, in which SiO2 film turns into less compressive and TiO2 film appears to be more tensile. The optical properties after annealing were also investigated by measuring the thickness and the refractive index changes using the spectroscopic ellipsometry technique. At both experimental temperatures, the film thickness increases slightly and the refractive index at 1550 nm decreases a little at the initial annealing stage for SiO2 films. For TiO2 films, it is found that the refractive index increases after annealing at 250°C. This might be caused by the TiO2 film densification process during amorphous-to-crystalline phase transformation. Because most of the significant film evolutions occur during the initial 12 hours of annealing, a practical way of stabilizing the film properties in a MEMS device is to pre-anneal the as-deposited thin films.
Investigation of mechanical properties of mesoporous methyl-substituted SiO2 thin films (Methylsilesquioxane materials).
Authors: Verdier M. , Montagnat M. , Maîtrejean S. , Passemard G.
Published: April 2011

In ULSI circuit, interlayer thin film with low dielectric constant shall be used in order to reduce the interconnection RC delay. One promising material is methylsilesquioxane (MSQ) deposited by spin coating with controlled porosity. High porosity combined with matrix weakness result in low mechanical properties that could be detrimental to circuit reliability.

The aim of this study is to investigate the deformation behavior of mesoporous MSQ thin films by nanoindentation and by AFM analysis of residual indents. At first, proper measurement conditions were explored by modulating the film thickness from 100 nm up to 600nm for a fixed film density: the very low elastic modulus of the material on stiff substrate requires great care in the raw data analysis. Next, the density of these mesoporous films has been adjusted and the response to indentation is compared with dense bulk fused silica. AFM investigations lead to the sequence of fracture processes and to the sensitivity to hydrostatic pressure on the deformation of these materials.
Stress and texture in sputter deposited Cr films
Authors: Grachev S. Yu. , Kamminga J.-D. , Janssen G. C. A. M.
Published: April 2011

Intrinsic stress in coatings is often responsible for its performance. We studied tensile stress in sputter deposited chromium films as a function of film thickness and Ar pressure during deposition. We correlate the stress evolution to the grain growth in the polycrystalline films. Both grain growth and stress evolution obey the same power law dependence on thickness. We conclude that the tensile stress is generated at the grain boundaries. The power law exponent did not depend on pressure of Ar and remained 0.36. However, texture and microstructure in the layers changed when pressure was increased from 2×10-2 to 6×10-2 mbar. Texture switched from 110 to 111 fiber type. Grooves and sharp star-like grains were observed at higher pressure. We explain changes in terms of suppressed shadowing and less surface diffusion.
Effect of Dielectric Materials on Stress-Induced Damage Modes in Damascene Cu Lines
Authors: Paik Jong-Min , Park Hyun , Park Ki-Chul , Joo Young-Chang
Published: April 2011

Various low-k materials are being pursued as dielectric materials for future interconnects. However, poor thermo-mechanical properties of low-k materials cause tremendous reliability concerns, thus the proper materials for integration with Cu are not suggested yet. In this study, the line width and spacing dependence of damascene Cu lines embedded by TEOS and low-k materials (CORAL) was analyzed using x-ray diffraction. Generally, the hydrostatic stress of Cu/TEOS was greater than that of Cu/CORAL, while the opposite for von-Mises stress. Using a three-dimensional finite analysis (FEA), the effect of low-k materials on the stress and its distribution in via-line structures of dual damascene Cu interconnects was studied. In the case of Cu/TEOS, the hydrostatic stress was concentrated at the via and on the top of the lines, where it was suspected that the void would nucleate. On the other hand, in the via-line structures integrated with organic low-k materials, large von-Mises stress was maintained in the via. Therefore, the deformation of via, rather than voiding, may be the main failure mode in the interconnects with low-k materials.
Stretchable conductors: thin gold films on silicone elastomer
Authors: Lacour Stéphanie P. , Wagner Sigurd , Suo Z.
Published: April 2011

Thin stripes of gold deposited onto elastomeric substrates can be stretched reversibly by more than 20 % while remaining electrically conducting. We are developing such stripes to serve as electrical interconnects on stretchable electronic skins. The gold layers are 25-nm to 500-nm thick. We observe two different film morphologies: the stripe is either buckled and continuous, or flat and contains micrometer-long cracks. Stretchability is correlated with the thickness and initial topography of the gold layer. Stripes thicker than 100-nm fail electrically at tensile strain of ∼ 1 %, while thinner stripes remain conducting up to much larger strain. Upon stretching the buckled stripes flatten and break into islands of 1 to 100 micrometers on a side, while the initially microcracked stripes retain their micrometer scale structure. The electrical resistance of the buckled stripes is the lowest but the micro-textured stripes can be stretched more.
Effects of the substrate on the determination of hardness of thin films by the nanoscratch and nanoindentation techniques: A comparative study for the cases of soft film on hard substrate and hard film on soft substrate.
Authors: Tayebi Noureddine , Polycarpou Andreas A. , Conry Thomas F.
Published: April 2011

Hardness values of Au/Fused Quartz and SiO2/Al systems, which correspond to the cases of soft film on hard substrate and hard film on soft substrate, were measured using both the nanoindentation and nanoscratch techniques. The effect of substrate interaction on the measurement of hardness using the nanoscratch technique is found to be much less pronounced compared to that of the nanoindentation technique. Such reduction in substrate effect is attributed to the features used in the nanoscratch analysis: (a) direct imaging of residual profile allows for the effect of pile-up/sink-in to be considered, (b) lower normal loads applied as compared to the nanoindentation, (c) effect of elastic recovery of the plastically deformed surfaces is included in the nanoscratch analysis, whereas the nanoindentation analysis is based solely on the load-displacement data. Moreover, experiments with residual scratch depths as shallow as 3 nm are used to estimate hardness of thin films; a promising indication for the use of such technique in the measurements of ultra-thin films.
Cracking and Phase Transformation in Silicon During Nanoindentation
Authors: Jang Jae-il , Wen Songqing , Lance M. J. , Anderson I. M. , Pharr G. M.
Published: April 2011

Nanoindentation experiments were performed on single crystals of (100) Si using a series of triangular pyramidal indenters with centerline-to-face angles in the range 35.3° to 85.0°. The influences of the indenter geometry on cracking and phase transformation during indentation were systematically studied. Although reducing the indenter angle reduces the threshold load for cracking and increases the crack lengths, c, at a given indention load, P, the frequently observed relation between P and c3/2 is maintained for all of the indenters over a wide range of load. Features in the nanoindentation load-displacement curves in conjunction with Raman spectroscopy of the crystalline and amorphous phases in and around the contact impression show that the indenter geometry also plays a role in the phase transformation behavior. Results are discussed in relation to prevailing ideas about indentation cracking and phase transformation in silicon.
Effects of Process Parameters on the Adhesion of Copper Film on Polyethylene Tetrephthalate(Pet) Substrate Prepared by ECRMOCVD Coupled with a Periodic DC Bias
Authors: Hyun Jin , Jeon Bup Ju , Byun Dongjin , Lee Joong Kee
Published: April 2011

Characteristics of Cu/C:H films on the PET substrate prepared by ECR-MOCVD coupled with a DC bias under Cu(hfac)2-Ar-H2 was investigated with special attention to process parameters. Our results showed that the Cu/C:H film has strongly adhere with the polymer substrate by chemical bonding and exhibited wide range of electrical conductivity that could be controlled by process parameters. The increase in H2/Ar ratio, microwave power and negative DC bias voltage brought on the higher pulling strength between Cu/C:H films and PET substrate. The effects of surface pretreatments of polymer substrate on pulling strength were insignificant in the range of our experimental range.
Adhesion Strength of Polymer Coatings studied by Laser Induced Delamination
Authors: Fedorov A. , van Veen A. , van Tijum R. , de Hosson J. Th. M.
Published: April 2011

Laser Induced Delamination is a novel technique aimed at quantification of adhesion of thin polymer films to a metal substrate. In this technique a high power IR pulsed laser beam (maximum 500 mJ in 5 ns) is used to create the primary delaminated area in the form of a blister. The blister is formed due to heating of the metal substrate and because of partial evaporation of the polymer material adjacent to the interface. By varying the power of the laser pulse the gas pressure inside the blister is increased until further delamination takes place. This critical pressure is related to the strength of adhesion of the polymer to the substrate. The shape of the blister (radius and height) is controlled with a stylus profiler and with an optical confocal microscope.

In the present work 30 μm thick PET films on steel substrate were studied. From the shape of the obtained blisters the strength of adhesion of the polymer is estimated. Various models describing blister formation and growth are discussed.
An Atomistic View of Interface-Mediated Dislocation Plasticity in Thin Metal Films
Authors: Ege E. S. , Shen Y.-L.
Published: April 2011

Atomistic simulations using molecular statics are carried out to study dislocation plasticity in thin metal films attached to stiff substrates. The analysis utilizes a sample two-dimensional crystal, with an embedded initial point defect used for triggering dislocation activities in a controlled manner. The existence of an interface between the film and the substrate is shown to delay plastic yielding and lead to film strengthening. The capability of atoms to slide along the interface plays a crucial role in determining the macroscopic stress-strain response and the microscopic dislocation activities. Within the modeling framework we examine the quantitative interfacial sliding behavior and the resulting dislocation-interface interactions and their consequences.
Electrical and Photonic Functions in Transparent Oxide Semiconductors: Utilization of Built-in Nanostructure
Authors: Hosono Hideo
Published: April 2011

We review distinct photonic/electronic properties onginating from built-in nano-structures in transparent oxide based matenals, emphasizing potential of nanostructures hidden in crystal structure. Matenals focused are oxychalcogenides LaCuOCh (Ch=chalcogen ion) and homologous oxides InGaO3(ZnO)m(m=integer) having naturally formed multi-quantum well structures. Novel functions and devices ansing from the built-in nanostructure are: (1) modulation doping of positive holes and room temperature stable exciton in LaCuOCh, (2) high performance transparent field-effect transistor fabncated in InGaO3(ZnO)5 epitaxial thin films, and (3) conversion of insulator to persistent electronic conductor by carner doping in 12CaO 7A12O3 (C12A7).
Surface Electronic Structure of Nitric-oxide-treated Indium Tin Oxide
Authors: Jianqiao Hu , Jisheng Pan , Zhu Furong , Hao Gong
Published: April 2011

The surface electronic properties of the nitric oxide (NO) treated indium tin oxide (ITO) are examined in-situ by a four-point probe and X-ray photoelectron spectroscopy (XPS). The XPS N1s peak emerged at a high binding energy of 404 eV indicating that NO is reactive with ITO. NO adsorption induces an increase of film sheet resistance, arising from an oxygen rich layer near the ITO surface region, with approximately 2.5 nm thick. This implies that the interaction of NO with ITO is occurred around surface region. Valence band maximum measured for NO-absorbed ITO was shifted to the low binding energy side. This is related to the upward surface band bending.
Emission of Rare Earth Ions Incorporated into Metal Oxide Thin Films and Fibres
Authors: Sildos Ilmo , Lange Sven , Tätte Tanel , Kiisk Valter , Kirm M. , Aarik J.
Published: April 2011

Photoluminescence (PL) of undoped and Sm-doped TiO2, ZrO2and HfO2thin films and fibers was investigated at temperatures ranging from 6 to 300 K. The thin films were grown by the atomic layer deposition (ALD) technique and doped by using the ion implantation method. The fibers were prepared by using the sol-gel method whereas anin-situdoping was used to obtain the required concentration of Sm3+ions in the films. In undoped as well as doped materials, PL was efficiently excited via band-to-band transitions. The emission of undoped materials was attributed to the radiative recombination of self-trapped excitons (STE). In doped materials, intense emission of Sm3+was recorded. It is proposed, that there exists a concurrence between the radiative recombination of bound excitonic states and the energy transfer to Sm3+ions, particularly at lower temperatures.
Correlations Between Conditions of Synthesis, Phase Composition and Luminescent Properties of Eu - Polytantalate
Authors: Vasilyev Vladimir , Drehman Alvin , Bouthillette Lionel
Published: April 2011

Correlations between synthesis conditions, phase composition, and spectral properties of the sintered ceramic, thin films and single crystals of EuTa7O19 phosphors have been studied using x-ray diffractometry and temperature dependent photoluminescence (PL) spectroscopy at temperatures between 18 K and 650 K. From the PL spectra of Eu3+, one can obtain information about the area of homogeneity of phases, their temperature transformations, and changes in the bonding character in the neighborhood of the luminescent ion. As a result, this information helps to optimize the synthesis parameters for luminescent materials.
Study of ZnTe:Cu/Metal Interfaces in CdS/CdTe Phovoltaic Solar Cells
Authors: Gessert T. A. , Perkins C. L. , Asher S. E. , Duda A. , Young M. R.
Published: April 2011

The present model for current transport at the CdTe/p-ZnTe:Cu/metal back contact assumes that the CdTe and ZnTe valence bands align, while current transport at a highly doped ZnTe and a metal interface proceeds by tunneling. To test part of this model, we have investigated the electrical and material properties of CdS/CdTe devices where the outer metal is either Ti or Ni. Our results show that differences in device series resistance are not linked simply to metal/ZnTe:Cu interfacial contact resistance, but that metallization-induced diffusion remains a more likely cause of significant performance distinctions.
The Organic/Inorganic Interface in Micro and Nano Composite Materials
Authors: Tang Zhexiong , Wan Hui , Clark Robert , Yang Sze C.
Published: April 2011

We report the synthesis of new inorganic/organic composite particles with a core/shell structure. The core component is an inorganic oxide (e.g. TiO2, CeO2 or MoO3), and the shell component is a double-strand polyaniline. Three methods for material synthesis were examined. The electrochemical properties of one type of the composite particles show electronic interaction between the organic conducting polymer shell and its inorganic core. The double-strand polyaniline in the composite shows better pH stability of the conductive form than that of the corresponding single-strand polyaniline in a similar core/shell composite.
High Current Injection to a UV-LED grown on a Bulk AlN Substrate
Authors: Nishida Toshio , Ban Tomoyuki , Saito Hisao , Makimoto Toshiki
Published: April 2011

We applied a bulk AlN substrate to an AlGaN-based ultraviolet light emitting diode (UV-LED) and found that this combination enables high injection current, which shows the LED's potential for large ultraviolet flux extraction. Heat dissipation is an important issue for LEDs. Bulk AlN substrate has high thermal conductivity, a wurtzite crystal symmetry the same as that of nitride emitters, and transparency in the ultraviolet wavelength range. An UV-LED grown on a bulk AlN substrate shows output power linearity up to high injection current up to 300 mA, whereas a similar device grown on an AlN-template formed on a sapphire substrate only shows linearity up to an injection current of about 150 mA. It also showed very stable emission peak wavelength. For example, the emission peak shift is less than 2 nm in spite of the large injection current of 200 mA. Both findings are attributed to the heat dissipation afforded by the high thermal conductivity of the bulk AlN. This LED still suffers from internal absorption loss caused by the residual color centers in the AlN at present. However, further improvement of bulk AlN substrates will lead to high flux and highly efficient ultraviolet sources.
Growth and Characterization of AlN and GaN Thin Films Deposited on Si(111) Substrates Containing a Very Thin Al Layer
Authors: Reitmeier Zachary J. , Davis Robert F.
Published: April 2011

AlN films and GaN films with AlN buffer layers were deposited via metalorganic vapor phase epitaxy on Si(111) substrates previously exposed to trimethylaluminum for increasing times. Atomic force microscopy (AFM) was used to determine the influence of Al pre-flow time on the nucleation and surface morphology of the AlN and GaN films. When preceded by a 10 second Al pre-flow, AlN films feature an increased and more uniform nucleation density as compared to films deposited without Al pre-flows. Ten second Al pre-flows were also found to result in a reduction of the RMS roughness for 100 nm thick AlN films from 3.6 nm to 1.0 nm. AFM of 0.5 μm thick GaN films deposited on AlN buffers with varying pre-flow times showed reduced roughness and decreased pit density when using Al pre-flows of 10 or 20 seconds. High resolution x-ray diffraction of the GaN films showed a reduction in the average full-width halfmaximum (FWHM) of the GaN (00.2) reflection from 1076 arcsec to 914 arcsec when the AlN buffer layer was initiated with a 10 second Al pre-flow. Increasing the pre-flow time to 20 seconds and 30 seconds resulted in average (00.2) FWHM values of 925 arcsec and 928 arcsec, respectively. Similar behavior of the peak widths was observed for the (30.2) and (10.3) reflections when the pre-flow times were varied from 0 to 30 seconds.
MBE Grown AlN Films on SiC for Piezoelectric MEMS Sensors
Authors: Doppalapudi Dharanipal , Mlcak Richard , Chan Jeffrey , Tuller Harry , Bhattacharya Anirban , Moustakas Theodore
Published: April 2011

Miniaturized piezoelectric sensors based on Microelectromechanical Systems (MEMS) offer the advantages of reduced size, reduced power consumption, increased sensitivity coupled with the ability to form compact multi-sensor arrays. Fabrication of such sensors from single crystal materials further insure more highly reproducible and stable devices with improved performance. In this paper, we describe the integration of MBE grown AlN films onto photoelectrochemically machined SiC microcantilevers and membranes. AlN exhibits excellent piezoelectric properties, including an electromechanical coupling coefficient of 0.088 and a high in-plane acoustic velocity (∼5700m/sec) as well as excellent thermal-mechanical compatibility with SiC. The fabrication of AlN-SiC-based microresonators and flexural plate wave devices, and their application to chemical, biological and fluid sensing, are reported.
Gate Leakage Suppression and Contact Engineering in Nitride Heterostructures
Authors: Wu Yuh-Renn , Singh Madhusudan , Singh Jasprit
Published: April 2011

We present a self-consistent approach to examine current flow in a general metal-polar heterostructure junction. The approach is applied to examine properties of two classes of junctions that are important in devices: (i) GaN/AlGaN/high-κ insulator structures for potential applications in very small gate devices to suppress gate tunneling current; (ii) GaN/AlGaN/LiNbO3 junctions for both n-type and p-type semiconductors with practical application for low source resistance regions. The physical parameters used for high-κ dielectrics and polarization charges reflect values typically found in ferroelectric materials. Our studies indicate that tailoring of junction properties is possible if polar oxides as thin as ∼ 20Å can be achieved.
N-rich GaNAs with High As Content Grown by Metalorganic Vapor Phase Epitaxy
Authors: Kimura Akitaka , Tang H. F. , Paulson C. A. , Kuech T. F.
Published: April 2011

GaN1-yAsy epitaxial alloys on the N-rich side with high As content were grown by metalorganic vapor phase epitaxy. They had specular surfaces and the single-phase epitaxial nature was confirmed by X-ray diffraction. The As incorporation increased through both a decrease in the growth temperature and V/III ratio. These trends were similar to that found in other III-V alloy systems which exhibit a large miscibility gap and the anion incorporation was considered to have been limited kinetically under the conditions of the low V/III ratio. The range of achieved As content was extended up to y=0.067, which is a composition well within the miscibility gap. The As-content dependence of the band gap energy was determined by optical absorption measurements and large bowing parameter of 16.8 ± 0.9 eV was determined.
Curvature and strain in thick HVPE-GaN for quasi-substrate applications
Authors: Roder Claudia , Böttcher Tim , Paskova Tanya , Monemar Bo , Hommel Detlef
Published: April 2011

Uncracked HVPE-GaN layers of different thicknesses were investigated using high resolution x-ray diffraction at variable temperatures. The absolute lattice parameters as well as the wafer curvature were measured simultaneously at different temperatures. All samples were found to be under biaxial tension at growth temperature. At room temperature the GaN film is under biaxial compression while the substrate is tensile strained. A simulation of the stress and curvature of the heterostructures was performed for different thicknesses of the films and substrates and a good agreement was found with the experimentally determined values.
Crucible Selection in AlN Bulk Crystal Growth
Authors: Dalmau Rafael , Raghothamachar Balaji , Dudley Michael , Schlesser Raoul , Sitar Zlatko
Published: April 2011

Growth of AlN bulk single crystals by sublimation of AlN powder was carried out in a resistively heated reactor at temperatures up to 2300°C. A variety of crucible materials, such as BN, W, Ta, Re, ZrO2, TaN, and TaC, were evaluated. Our studies have shown that the morphology of crystals grown by spontaneous nucleation strongly depends on the growth temperature and contamination in the reactor. Crucible selection had a profound effect on contamination in the crystal growth environment, thus influencing nucleation, coalescence, and crystal morphology. Spontaneously grown single crystals up to 15 mm in size were characterized by x-ray diffraction (XRD), x-ray topography (XRT), glow discharge mass spectrometry (GDMS), and secondary ion mass spectrometry (SIMS). Average dislocation densities were on the order of 103 cm−3, with extended areas virtually dislocation-free, while high-resolution XRD showed rocking curves as narrow as 7 arcsec.
Electrical and Optical Characteristics of Delta Doped AlGaN Cladding Layer Materials for Highly Efficient 340nm Ultra Violet LEDs
Authors: Xin H. P. , Flynn J. S. , Dion J. A. , Hutchins E. L. , Antunes H. , Fieschi-Corso L. , Van Egas R. , Brandes G. R. , LeBoeuf S. F. , Cao X. A. , Garrett J. L. , Rowland L. B.
Published: April 2011

In this paper, we report the electrical and optical characteristics of Si delta-doped AlGaN cladding layers, p-cladding structure optimization and the impact on the efficiency of 340nm AlGaN UV LEDs. Compared to the uniformly doped n-AlGaN layer, adding Si Δ-doping layers reduced the sheet resistance by improving both the Hall mobility and carrier concentration. Increasing the number of Si Δ-doped layers further lowered the sheet resistance without cracking the material. The Δ-doped layers in n-Al0.3Ga0.7N improved the optical properties by enhancing near band edge emission as much as 2-fold relative to deep level emission. Additionally, Δ-doping in n-AlGaN layers had no detrimental effect on the optical transparency of the LEDs. The p-cladding layer was found to have a strong absorption at 340nm. Reducing the p-GaN cap layer from 35nm to 10nm tripled the light emission intensity. By optimizing the n- and p-AlGaN cladding layers, a highly efficient UV LED at 340nm was achieved with 1mW output under 800mA/mm2 DC drive current.