Electrical resistivity of thermally evaporated bismuth telluride thin films

Semiconducting chalcogenide thin films have been receiving considerable attention in the recent years because of their wide applications in the various fields of science and technology. The studies of the electronic properties of semiconductors have been largely stimulated by attractive micro-electronic device applications. Among the various V–VI compounds, Bismuth Telluride (Bi₂Te₃) is an established low-temperature thermo electric material and is widely employed in thermoelectric generators and coolers. The present work deals with the structural and the electrical characterization of Bi₂Te₃ thin films vacuum deposited on well-cleaned glass substrates. A constant rate of deposition was maintained through out the process. To obtain uniform and homogeneous film thickness through out on all the substrates a rotary drive was employed. Quartz crystal thickness monitor was used to measure the thickness of the samples. From the X-ray diffractogram the Bi₂Te₃ films are found to be amorphous at lower thicknesses and posses hexagonal polycrystalline structure at higher thickness, having lattice parameters a=4.44 Å and c=29.40 Å. The grain size of the Bi₂Te₃ thin films before annealing and after annealing are found to be 100 and 160 Å, respectively. The micro-strain and the dislocation density are found to decrease after annealing. The thermogravimetry–differential thermal analysis (TG–DTA) studies revealed that the Bi₂Te₃ films are non-decomposable. Electrical resistivity, TCR measurements have been carried out as a function of varying temperatures in the range 303–453 K are found to show the size effect. Analyzing the size dependence of electrical resistivity it is found that the electrical resistivity is a linear function of the reciprocal of thickness of the film. The energy gap of Bi₂Te₃ thin film was calculated from the graph ln ρ vs. 1/T and it is found that the energy gap decreases with increasing thickness. From the negative values of TCR, it is inferred that Bi₂Te₃ films exhibit semiconducting behavior.     

Cognitive benefits and costs of bilingualism in elementary school students: The case of mathematical word problems.

Previous research has emphasized the importance of language for learning mathematics. This is especially true when mathematical problems have to be extracted from a meaningful context, as in arithmetic word problems. Bilingual learners with a low command of the instructional language thus may face challenges when dealing with mathematical concepts. At the same time, speaking two languages can be associated with cognitive benefits with regard to attentional control processes, although such benefits have only been found in highly proficient bilinguals. In the present study, we attempted to disentangle the effects of bilingual proficiency on mathematical problem solving in Turkish–German bilingual elementary school students. We examined whether the positive cognitive effects of bilingualism could be found not only in highly proficient bilinguals but also in students with an immigrant background and a low command of the instructional or native language. Our findings emphasize the importance of language proficiency for mathematics problem solving, as shown by the predictive value of students' proficiency in the language of testing (German/Turkish) for their performance on mathematical word problems. No additional effect of the language of instruction (German) was found for problem solving in the bilingual students' native language (Turkish). Furthermore, bilinguals gained scores comparable to those of their monolingual peers on word problems that required attentional control skills although performing significantly below their monolingual classmates on ordinary word problems, suggesting that bilinguals have an advantage when it comes to attentional control. Finally, bilingual students with a relatively high command of the instructional language performed better on word problems presented in German than on those presented in Turkish, thus facing cognitive costs when transferring knowledge from one language to the other. Implications of our findings for bilingual education are discussed. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Rotationally invariant similarity measures for nonlocal image denoising

Many natural or texture images contain structures that appear several times in the image. One of the denoising filters that successfully take advantage of such repetitive regions is NL means. Unfortunately, the block matching of NL means cannot handle rotation or mirroring. In this paper, we analyse two natural approaches for a rotationally invariant similarity measure that will be used as an alternative to, respectively a modification of the well-known block matching algorithm in nonlocal means denoising. The first approach is based on moment invariants whereas the second one estimates the rotation angle, rotates the block via interpolation and then uses a standard block matching. In contrast to the standard method, the presented algorithms can find similar regions or patches in an image even if they appear in several rotated or mirrored instances. Hence, one can find more suitable regions for the weighted average and yield improved results.     

Calcite Reinforced Silica–Silica Joints in the Biocomposite Skeleton of Deep‐Sea Glass Sponges

The hierarchically structured glass sponge Caulophacus species uses the first known example of a silica and calcite biocomposite to join the spicules of its skeleton together. In the stalk and body skeleton of this poorly known deep‐sea glass sponge siliceous spicules are modified by the addition of conical calcite seeds, which then form the basis for further silica secretion to form a spinose region. Spinose regions on adjacent spicules are then joined by siliceous crosslinks, leading to unusually strong cross‐spicule linkages. In addition to the biomaterials implications it is now clear, from this first record of a biomineral other than silica, that the hexactinellid sponges are capable of synthesizing calcite, the ancestral skeletal material. We propose that, while the low concentrations of calcium in deep sea waters drove the evolution of silica skeletons, the brittleness of silica has led to retention of the more resilient calcite in very low concentrations at the skeletal joints.     

Impulsive control for fast nanopositioning

In this paper we present a non-linear control scheme for high-speed nanopositioning based on impulsive control. Unlike in the case of a linear feedback controller, the controller states are altered in a discontinuous manner at specific instances in time. Using this technique, it is possible to simultaneously achieve good tracking performance, disturbance rejection and tolerance to measurement noise. Impulsive control is demonstrated experimentally on an atomic force microscope. A significant improvement in tracking performance is demonstrated.     

Scanning probe microscopy based on magnetoresistive sensing

Integrated sensors are essential for scanning probe microscopy (SPM) based systems that employ arrays of microcantilevers for high throughput. Common integrated sensors, such as piezoresistive, piezoelectric, capacitive and thermoelectric sensors, suffer from low bandwidth and/or low resolution. In this paper, a novel magnetoresistive-sensor-based scanning probe microscopy (MR-SPM) technique is presented. The principle of MR-SPM is first demonstrated using experiments with magnetic cantilevers and commercial MR sensors. A new cantilever design tailored to MR-SPM is then presented and micromagnetic simulations are employed to evaluate the achievable resolution. A remarkable resolution of 0.84?? over a bandwidth of 1?MHz is estimated, which would significantly outperform state-of-the-art optical deflection sensors. Due to its combination of high resolution at high bandwidth, and its amenability to integration in probe arrays, MR-SPM holds great promise for low-cost, high-throughput SPM.     

Cleaved-edge-overgrowth nanogap electrodes

We present a method to fabricate multiple metal nanogap electrodes of tailored width and distance in parallel, on the cleaved plane of a GaAs/AlGaAs heterostructure. The three-dimensional patterned structures are obtained by a combination of molecular-beam-epitaxial regrowth on a crystal facet, using the cleaved-edge-overgrowth (CEO) method, and subsequent wet selective etching and metallization steps. SEM and AFM studies reveal smooth and co-planar electrodes of width and distance of the order of 10 nm. Preliminary electrical characterization indicates electrical gap insulation in the 100 MΩ range with kΩ lead resistance. We propose our methodology to realize multiple electrode geometries that would allow investigation of the electrical conductivity of complex nanoscale objects such as branched organic molecules.     

Extending acoustic microscopy for comprehensive failure analysis applications

In manufacturing of microelectronic components, non-destructive failure analysis methods are important for quality control. These non-destructive methods enable rapid defect localization which then guides micro-structural investigations involving destructive sample preparation. Scanning acoustic microscopy (SAM) is a powerful tool for the inspection of internal structures in optically opaque materials. Depth-specific information can be extracted and applied to create two- and three-dimensional images without the need for time consuming tomographic scan procedures. While traditional SAM imaging of the signal intensity is very valuable, it leaves most of the potential of acoustic microscopy unused. The aim of the current work was to develop comprehensive analysis algorithms to utilize the full potential of SAM and thus to extend the range of its applications. Examples representing different application fields were investigated in the current study. The examples include advanced flip-chip devices, bonded wafer pairs, solder tape connectors of a photovoltaic solar panel and high density chip-to-chip interconnects relevant for 3D integration. Progress achieved during this work can be divided into four categories: Signal Analysis and Parametric Imaging, Signal Analysis and Defect Evaluation, Image Processing and Resolution Enhancement and acoustic GHz microscopy (GHz-SAM). For the first three categories, data acquisition was performed using a commercially available scanning acoustic microscope equipped with several ultrasonic transducers covering the frequency range from 15 to 175 MHz. In the fourth category, data acquisition was performed employing a prototype of a novel acoustic GHz microscopy tool which is currently under development into a commercial system. In the first three categories, recorded acoustic data were subjected to sophisticated algorithms operating in time, frequency and spatial domains for performing signal and image analysis. Acoustic microscopy, combined with such advanced signal and image processing algorithms, proved to be a powerful tool for non-destructive inspection.