bluuebunny/crossref_metadata_2025 huggingface dataset:
The Role of Friction in the Mechanics of Non-bonded Random Fiber Networks
Authors: Subramanian Gopinath , Picu R. Catalin
Published: August 2010

The mechanics of systems of non-cross-linked fiber networks is studied in this work using a computational model inspired from the bead-spring models of polymeric melts. The fibers have random orientation and distribution in space, interact via stiff repulsive potentials and are characterized by their axial and bending stiffness. Fiber-fiber Coulombian friction is considered. The system is subjected to isostatic compression (strain control) and various statistical measures are evaluated. As the system is compacted, a critical density is reached at which stiffness develops. At this stage there is in average one fiber-fiber contact per fiber and the fiber free segment length has a uniform probability distribution function. Upon further compaction, the number of contacts per fiber increases and the segment length distribution becomes exponential. The respective cross-over densities depend on the fiber length and the friction coefficient. Significant hysteresis is observed upon loading-unloading in the total energy and the number of contacts per fiber. It is also observed that the distribution of contact energies in the range of densities where the system forms a topological network is a power law.
Biosensor Capture Kinetics Model of Nanocube-Augmented Carbon Nanotube Networks
Authors: Claussen Jonathan , Porterfield David Marshall , Fisher Timothy
Published: August 2010

Au-coated Pd (Au/Pd) nanocubes (˜250 nm in width) connected via a network of single-walled carbon nanotubes (SWCNTs) have been employed as an electrochemical biosensor. As previously reported, these in situ Au/Pd nanocube SWCNT networks are capable of ultrasensitive amperometric sensing of glucose, with a sensitivity, detection limit, and linear sensing range greater than similar CNT-based glucose biosensors. The 3D mass diffusion of glucose molecules to the Au/Pd nanocube surfaces, forced convection environment of the testing vial, and Brownian motion of the Au/Pd nanocubes are all likely factors contributing to the strong electrochemical performance of the Au/Pd-SWCNT biosensor. In an effort to elucidate the effects of these contributing factors, this work demonstrates an analytical biosensor capture kinetics model that analyzes the analyte-biosensor mass transfer by molecular diffusion and convection due to both the fluid motion within the test vial and the Brownian motion of the Au/Pd nanocubes themselves.

The biosensor capture kinetics model incorporates a quasi steady-state integrated incident flux equation to model mass diffusion of biomolecules to the surface of a 1D planar, 2D nanowire, and 3D nanosphere surface in Cartesian, cylindrical, and spherical coordinates respectively. A Burgers vortex model is introduced to analyze the biosensor diffusion boundary layer within a test vial that experiences fluid downwelling and upwelling within the vial center and boundaries due to the rotation of a magnetic stir bar. Finally the convection-diffusion equation simplified by Stokes flow is utilized to model the diffusion boundary layer of the Au/Pd nanocubes experiencing Brownian motion.

Several key conclusions can be interfered from this model. First, a biosensor experiencing 3D mass diffusion will exhibit a greater analyte concentration flux of at least one order of magnitude greater than a biosensor experiencing 2D diffusion and 1D mass diffusion in quiescent and convective fluid environments. Additionally, mass transfer by convection increases the concentration flux to the biosensor by inhibiting the continued advancement of the analyte depletion layer around the biosensor. Furthermore, the Brownian motion model of the Au/Pd nanocubes is shown to improve the mass transfer to the biosensor surface, portraying a substantial increase in amperometric current signal output as compared to a similar electrochemical-based biosensor with stationary Au/Pd nanocubes. In summary, the results of the biosensor capture kinetics model corroborate the high sensitivities and low detection limits previously observed experimentally by Au/Pd nanocube-SWCNT biosensors.
Exploiting Phosphate Dependent DNA Immobilization on HfO2, ZrO2and AlGaN for Integrated Biosensors
Authors: Fahrenkopf Nicholas M , Jindal Vibhu , Tripathi Neeraj , Oktyabrsky Serge , Shahedipour-Sandvik Fatemeh , Tokranova Natalya , Bergkvist Magnus , Cady Nathaniel C
Published: August 2010

A significant challenge for high throughput nucleic acid analysis and sequencing is to increase both throughput and sensitivity. Electrical detection methods are advantageous since they can be easily scaled to high density arrays, are highly sensitive, and do not require bulky optical equipment for readout. A focus of most nucleic acid based sensors is the detection of sequence-specific hybridization events between complementary strands of DNA or RNA. These hybridization events can be detected electrically, due to the intrinsic negative charge associated with the phosphate-rich nucleic acid backbone. Field effect transistors (FETs) and high electron mobility transistors (HEMTs) are ideal devices for detecting such hybridization events, due to their high sensitivity to changes in electrical field strength. A key concern for the construction of DNA-based FET and HEMT biosensors is the immobilization of probe oligonucleotides on the active region of the sensor. In previous work, our group has shown that single stranded DNA can be directly immobilized onto semiconductor materials without the need for complex surface chemistry or crosslinking strategies. In the present work, we have shown that the immobilization of single stranded DNA onto these materials is influenced by the terminal phosphate group of the DNA molecule, independent of backbone phosphates. This agrees with previous studies in which phosphates and phosphonates exhibited strong attachment to a variety of metal oxides. We have also shown that surface-immobilized DNA is available for hybridization and that hybridization is sequence specific. Phosphate-dependent immobilization was demonstrated for HfO2, AlGaN, and ZrO2surfaces using optical detection of DNA-DNA hybridization, as well as x-ray photoelectron spectroscopy (XPS) analysis of DNA-modified surfaces.
Optimized in situ DNA synthesis on patterned glass
Authors: Saaem Ishtiaq , Ma Kuosheng , Tian Jingdong
Published: August 2010

This paper describes studies of patterned arrays on glass surfaces and their use as spatially separated reactors for in situ synthesis of DNA using an inkjet synthesizer. Photolithographic methods were employed to fabricate arrays composed of homogenous circular features containing a hydroxyl-terminated silane coupled to the surface of the glass via a siloxane bond. Features are embedded within a background matrix composed of a fluorosilane attached to the glass. Due to the differential wettability of the two silanes, whereby the hydroxyl-terminated silane and fluorosilane are hydrophilic and hydrophobic respectively because of their head groups, the patterned circular features are able to constrain liquid within a defined site. The silanization result was analyzed using X-ray photoelectron spectroscopy (XPS) to optimize silanization time and solvent. Synthesis was then performed using a custom-built inkjet system using phosphoramidite chemistry. Base-by-base analysis using fluorescent labeling showed consistent coupling efficiency on synthesis of a 50-mer homopolymer.
The Combination of Drug or Gene Delivery System Responding to Cellular Signals (D-RECS) and Sonoporation System for Effective and Safe Gene Delivery
Authors: Tsuchiya Akira , Mori Takeshi , Naritomi Yuki , Kang Jeong-Hun , Asai Daisuke , Niidome Takuro , Endo Yoko , Suzuki Ryo , Negishi Yoichi , Maruyama Kazuo , Katayama Yoshiki
Published: August 2010

We have developed new gene expression-regulating polymer that can activate transgene expression in response to target intracellular signals. Here, we tried applying sonoporation system to this gene regulation system to enhance the gene expression efficacy. Sonoporation is the method for effective gene transfection in vitro and in vivo. Therefore, the method might enhance the transfection efficiency in our polymer and realize an efficient and safe gene delivery system. Results suggested that the combination of our polymer and sonoporation could improve the gene expression compared to the system using only our polymer that transfers genes into cells via endocytosis. It also kept the ability of the gene regulation responding to cellular signals.
Effects of Magnetic Nanoparticles on Magnetic Resonance and Spin Relaxation in Systems of Different Viscosity
Authors: Noginova Natalia , Andreyev Aleksandr , Noginova Julia , Hall Joseph C , Ramesh Vani , Atsarkin Vadim A
Published: August 2010

Nuclear Magnetic Resonance (NMR) technique is a convenient method to monitor magnetic nanoparticles in different biomedical applications and observe changes induced by the particles. To better understand the specifics of the magnetic resonance and spin relaxation in the systems with magnetic nanoparticles, the NMR spectra and magnetization dynamics of the host protons are studied in the model systems of different viscosity and some biological systems in the presence of magnetic nanoparticles. The results confirmed that nanoparticles affect the proton relaxation kinetics of liquid solutions, changing the relaxation time (T1) significantly, whereas in systems of high viscosity the relaxation times are unchanged. The kinetics in intermediate systems is multi-exponential. A complicated picture is observed in biological systems, demonstrating contributions of liquid-like and solid-like behavior.
Novel Gelation System For Fabricating 3-D Structures via Ink Jet Printing
Authors: Saunders Rachel E. , Liem Kwan P , Mart Robert J , Webb Simon J , Derby Brian
Published: August 2010

Drop-on-demand inkjet printing is a fabrication technique that is capable of depositing materials layer-by-layer to form complex 3-dimensional (3-D) constructs. Here we present a new single drop delivery method in which both the matrix and cross-linker are present but separated through the use of vesicle packaging. Changing the printing parameters has little effect on the integrity of the calcium(II)-loaded vesicles, with calcium(II) released selectively by warming after printing. Alginate solutions containing calcium(II)-loaded vesicles were successfully printed and the printed layers were shown to gel on demand at 37 °C. The printed alginate layers were evaluated with regards to their potential to provide 3-D structures for cell culture.
Impedance Spectroscopy and Structural Studies on Silver Doped Hydroxyapatite
Authors: Singh Brajendra , Kumar Samayendra , Basu Bikramjit , Gupta Rajeev
Published: August 2010

We report the structural transformation and the transport studies of Silver doped hydroxyapatites Ca10-xAgx(PO4)6(OH)2 (0.0 ≤ x ≤ 1.5). A dramatic increase in the conductivity by two orders of magnitude for hydroxyapatite in presence of silver ions is recorded using impedance spectroscopy measurements in the temperature range of 450°C to 650°C. The characteristic surface plasmon resonance effect is used to explore the presence of silver nano-particles, and Ag+ ions in hydroxyapatite using optical absorption measurements. The activation energy has been found to be 0.07 eV in silver doped composition in comparison to 0.39 eV for the parent hydroxyapatite. The sintering temperature dependence and compositional variation on the structural transformations from hydroxyapatite into tricalcium phosphate phases have been explored using Raman Spectroscopy and X-ray diffraction techniques.
Pulsed-DC Electrospray for Biopolymer Particle Production
Authors: Lim Cho-Hui , Schierbeek Kiersten R , Mullins Michael E
Published: August 2010

PLLA microparticles were successfully fabricated via pulsed-DC electrospray. In this study, we investigated the effect of the pulsed voltage characteristics (e.g. pulse frequency, pulse amplitude and pulse width) on the particle’s size. We found that pulse frequency, pulse amplitude, pulse width, and the combinations of these factors had a statistically significant effect on the particle’s size. The process conditions to obtain smaller particles with uniform shape and size are a low pulse frequency, high pulse amplitude, and long pulse width (or a high duty cycle).
3D-printing of Urethane-based Photoelastomers for Vascular Tissue Regeneration
Authors: Baudis Stefan , Pulka Thomas , Steyrer Bernhard , Wilhelm Harald , Weigel Guenter , Bergmeister Helga , Stampfl Juergen , Liska Robert
Published: August 2010

The mechanical properties of materials designated for vascular tissue replacement are of crucial importance. The elastic modulus, the tensile strength as well as the suture tear resistance have to be adjusted. Our approach is to use photopolymers for artificial vascular grafts. Via the layer-by-layer photopolymerization of suitable resin formulations as performed in additive manufacturing (AM) very complex structures are realizable. Hence AM offer the possibility to create cellular structures within the artificial grafts that might favor the ingrowth of new tissue. Commercially available urethane acrylates (UA) were chosen as base monomers since urethane groups are known to have good cell-adhesion behavior and poly-UAs show adequate mechanical performance. The mechanical properties of the photoelastomers can be tailored by addition of reactive diluents (e.g. 2-hydroxyethyl acrylate, HEA) and thiols (e.g. 3,6 dioxa-1,8-octane-dithiol) as chain transfer agents to comply with the mechanical properties of natural blood vessels. To examine the suture tear resistance a new testing method has been developed. Finally, a formulation containing 30 wt% UA and 70 wt% HEA complies with the mechanical properties of natural blood vessels, shows good biocompatibility in in-vitro tests and was successfully 3D-printed with digital light processing AM.
Fabrication of Highly Conductive Pedot Nanofibers
Authors: Laforgue Alexis , Robitaille Lucie
Published: August 2010

Poly(3,4-ethylenedioxythiophene) (PEDOT) nanofibers were obtained by the combination of electrospinning and vapour-phase polymerization. The fibers had diameters around 350 ± 60 nm, and were soldered at every intersection, ensuring superior dimensional stability of the mats. The nanofiber mats demonstrated very high conductivity (60 ± 10 S/cm, the highest value reported so far for polymer nanofibers) as well as very interesting electrochemical properties, due to the porous and nanostructured nature of the electrospun mats. The mats were incorporated into all-solid flexible supercapacitors that showed interesting performances for applications where flexible and lightweight energy storage devices are required.
Ion Exchange Functional Nanofibers
Authors: Patra Prabir K , Sengupta Sukalyan
Published: August 2010

We have synthesized a series of ion exchange functionalized fibers (IXF) from polystyrene (PS) and polyacrylonitrile (PAN). To obtain strong-acid cation exchange fibers, polystyrene was sulfonated using specific sulfonation protocols. Micron sized fibers (average diameter of 100m) were then produced from the functionalized polystyrene using a single-screw extruder equipped with a 30 hole spinneret with orifice diameter of 0.5 mm with a precise screw speed of 5 rpm, pump speed of 15 rpm, and with a feed rate of 2.4 cc/min. The extruder zone temperature was kept at 250 – 270 °C. Fiber was drawn at 120 degree with a draw ratio of 2. Electrospinning of functionalized polystyrene was also carried out to produce ultrafine functionalized fibers of 100 nm in average diameter. We have also electrospun polystyrene and polyisoprene blended nanofibers to increase the strength of the resulting blend nanofibers compared to pure PS nanofibers. To synthesize weak-acid cation exchange fibers polyacrylonitrile (PAN) was electrospun and the nanofibers obtained were alkaline hydrolyzed with 2 N NaOH for 20 minutes at room temperature to convert nitrile bonds to carboxylate. Cation exchange capacity (CEC) of the microfibers and nanofibers was determined. Sulfonated PS microfibers show high CEC of 4.0 meq/gm compared to that of nanofibers with 2.5 meq/gm. CEC of blended nanofibers of PS and polyisoprene was 2.0 meq/gm. In case of PAN fibers, nanosized electrospun fibers were found to show a CEC of 1.5 meq/gm. Weak-base anion exchange fiber synthesis was undertaken using appropriate protocol and its CEC was measured. For all IXF synthesized, fiber diameter was measured using SEM, degree of functionalization was qualitatively determined using FTIR and ion exchange capacity was computed after mass balance on a binary exchange system after equilibrium.
Chiral CdTe Quantum Dots
Authors: Moloney Mícheál P , Gallagher Shane A , Gun'ko Yurii K
Published: August 2010

Quantum dots (QDs) are fluorescent semiconductor (e.g. II-VI) nanocrystals, which have a strong characteristic spectral emission. This emission is tunable to a desired energy by selecting variable particle size, size distribution and composition of the nanocrystals. QDs have recently attracted enormous interest due to their unique photophysical properties and range of potential applications in photonics and biochemistry.The main aim of our work is develop new materials based chiral quantum dots (QDs) and establish fundamental principles influencing the structure and properties of chiral QDs. Here we report the quantum efficiency control in cysteine capped CdTe quantum dots (QDs) by varying ratios of enantiomeric stabilizers. We also demonstrate that the circular dichroism (CD) of CdTe QDs can be introduced by utilizing the mixture of penicilamine and cysteine stabilizers of the same chirality. This approach results in QDs with the enhanced CD activity, but causes a decrease in the quantum yield and widening of the emission due to the presence of chiral defects at the nanoparticle surface. We believe that these new QDs could find important applications as fluorescent assays and sensors (or probes) in asymmetric synthesis, catalysis, enantioseparation, biochemical analysis and medical diagnostics.
From R&D to Large-Area Modules at Oerlikon Solar
Authors: Meier Johannes , Benagli Stefano , Bailat Julien , Borello Daniel , Steinhauser Jerome , Hötzel Jochen , Castens Lucie , Orhan Jean-Baptiste , Djeridane Yassine , Vallat-Sauvain Evelyne , Kroll Ulrich
Published: August 2010

Amorphous silicon single-junction p-i-n and Micromorph tandem solar cells are deposited in KAI-M reactors on in-house developed LPCVD ZnO front TCO's. An a-Si:H p-i-n cell with a stabilized efficiency of 10.09 % on 1 cm2 has been independently confirmed by NREL. An alternative ZnO/a-Si:H cell process with an intrinsic absorber of only 180 nm has reached 10.06 % NREL confirmed stabilized efficiencies as well. Up-scaling of such thin cells to 10x10 cm2 mini-modules has led to an aperture module efficiency of stabilized 9.20 ± 0.19 % as well independently confirmed by ESTI of JRC Ispra.

Micromorph tandem cells with stabilized efficiencies of 11.0% have been achieved on as-grown LPCVD ZnO front TCO at bottom cell thickness of just 1.3 μm in combination with the in-house developed AR concept. Applying an advanced LPCVD ZnO front TCO stabilized tandem cells of 10.6 % have been realized at a bottom cell thickness of only 0.8 μm. Implementing in-situ intermediate reflectors in Micromorph tandems on LPCVD ZnO reached in a stabilized cell efficiency of 11.3% with a bottom cell thickness of 1.6 μm.
Uniformity and Quality of Monocrystalline Silicon Passivation by Thin Intrinsic Amorphous Silicon in a New Generation Plasma-enhanced Chemical Vapor Deposition Reactor
Authors: Strahm Benjamin , Andrault Yoann , Bäetzner Derk , Lachenal Damien , Guérin Chloé , Kobas Manuela , Mai Joachim , Mendes Bruno , Schulze Thomas , Wahli Guillaume , Buechel Arthur
Published: August 2010

This work reports the first results of a new generation plasma-enhanced chemical vapor deposition (PECVD) reactor manufactured by Roth and Rau. This large area parallel plate reactor has been especially designed for the manufacturing of silicon heterojunction solar cells which are made of very thin amorphous silicon films over monocrystalline silicon substrates. Layer thickness uniformity below ± 3 % is reported for both intrinsic and doped layer over a 400 × 400 mm2 area. Moreover, it is shown that the passivation quality is excellent with life-times up to 4.15 ms on n-type FZ silicon substrates. A ± 0.6 % uniformity in open circuit voltage (mean value of 701.4 mV) is achieved over 32 devices having a 4 cm2 area and an average conversion efficiency of 19.5 %.
Microstructure Effects in Amorphous and Microcrystalline Ge:H Films
Authors: Beyer Wolfhard , Einsele Florian , Kondo Michio , Matsui Takuya , Pennartz Frank
Published: August 2010

The characterization of void-related microstructure in amorphous and microcrystalline Ge:H films is reported. Various methods are applied including effusion measurements of hydrogen and of implanted helium and neon, measurements of the infrared absorption of C-H bonds due to in-diffusion of contaminants and of the stretching modes of bonded hydrogen. Several microstructure effects like interconnected voids and isolated voids and a quite different material homogeneity are detected and are found to depend on the preparation conditions. Amorphous Ge:H can be prepared with a (largely) homogeneous structure while microcrystalline Ge:H tends to consist of compact grains surrounded by more or less open voids. Enhanced substrate temperatures (Ts ≈ 250°C) favour the growth of more compact material.
Effects of Grain Boundaries on Performance of Hydrogenated Nanocrystalline Silicon Solar Cells
Authors: Su Tining , Bobela David , Xu Xixiang , Ehlert Scott , Beglau Dave , Yue Guozhen , Yan Baojie , Banerjee Arindam , Yang Jeff , Guha Subhendu
Published: August 2010

We investigate the effect of hydrogenation of grain boundaries on the performance of solar cells for hydrogenated nanocrystalline silicon (nc-Si:H) thin films. Using hydrogen effusion, we found that the amplitude of the lower temperature peak in the H-effusion spectra is strongly correlated to the open-circuit voltage in solar cells. This is attributed to the hydrogenation of grain boundaries in the nc-Si:H films.
Material Properties of a-SiGe:H Solar Cells as a Function of Growth Rate
Authors: Hugger Peter , Lee JinWoo , Cohen J. David , Yue Guozhen , Xu Xixiang , Yan Baojie , Yang Jeff , Guha Subhendu
Published: August 2010

We have examined a series of a Si,Ge:H alloy devices deposited using both RF and VHF glow discharge in two configurations: SS/n+/i (a-SiGe:H)/p+/ITO nip devices and SS/n+/i (a-SiGe:H)/Pd Schottky contact devices, over a range of deposition rates. We employed drive-level capacitance profiling (DLCP), modulated photocurrent (MPC), and transient junction photo-current (TPI) measurement methods to characterize the electronic properties in these materials. The DLCP profiles indicated quite low defect densities (mid 1015 cm-3. to low 1016 cm-3 depending on the Ge alloy fraction) for the low rate RF (∼1Å/s) deposited a-SiGe:H materials. In contrast to the RF process, the VHF deposited a-SiGe:H materials did not exhibit nearly as rapid an increase of defect density with the deposition rate, remaining well below 1017 cm-3. up to rates as high as 10Å/s. Simple examination of the TPI spectra on theses devices allowed us to determine valence band-tail widths.. Modulated photocurrent (MPC) obtained for several of these a-SiGe:H devices allowed us to deduce the conduction band-tail widths. In general, the a-Si,Ge:H materials exhibiting narrower valence band-tail widths and lower defect densities correlated with the best device performance.
Improving Performance of Amorphous Silicon Solar Cells Using Tungsten Oxide as a Novel Buffer Layer between the SnO2/p-a-SiC Interface
Authors: Fang Liang , Baik Seung Jae , Lim Koseng Su , Yoo Seung Hyup , Seo Myung Soo , Kang Sang Jung
Published: August 2010

A thermally evaporated p-type amorphous tungsten oxide (p-a-WO3) film was introduced as a novel buffer layer between SnO2 and p-type amorphous silicon carbide (p-a-SiC) of pin-type amorphous silicon (a-Si) based solar cells. By using this film, a-Si solar cells with a p-a-WO3/p-a-SiC double p-layer structure were fabricated and the cell photovoltaic characteristics were investigated as a function of p-a-WO3 layer thickness. By inserting a 2 nm-thick p-a-WO3 layer between SnO2 and a 6 nm-thick p-a-SiC layer, the short circuit current density increased from 9.73 to 10.57 mA/cm2, and the conversion efficiency was enhanced from 5.17 % to 5.98 %.
Preparation of Narrow-gap a-Si:H Solar Cells by VHF-PECVD Technique
Authors: Kim Do Yun , Yunaz Ihsanul Afdi , Kasashima Shunsuke , Miyajima Shinsuke , Konagai Makoto
Published: August 2010

Optical, electrical and structural properties of silicon films depending on hydrogen flow rate (RH), substrate temperature (TS), and deposition pressure (PD) were investigated. By decreasing RH and increasing TS and PD, the optical band gap (Eopt) of silicon thin films drastically declined from 1.8 to 1.63 eV without a big deterioration in electrical properties. We employed all the investigated Si thin films for p-i-n structured solar cells as absorbers with i-layer thickness of 300 nm. From the measurement of solar cell performances, it was clearly observed that spectral response in long wavelength was enhanced as Eopt of absorber layers decreased. Using the solar cell whose Eopt of i-layer was 1.65 eV, the highest QE at long wavelength with the short circuit current density (Jsc) of 16.34 mA/cm2 was achieved, and open circuit voltage (Voc), fill factor (FF), and conversion efficiency (η) were 0.66 V, 0.57, and 6.13%, respectively.
A Highly Sensitive Integrated a-Si:H Fluorescence Detector for Microfluidic Devices
Authors: Kamei Toshihiro , Shikanai Amane
Published: August 2010

Most of micromachined and/or integrated fluorescence detectors suffer from high limit of detection (LOD) compared to conventional optical system that consists of discrete optical components, which is mainly due to higher laser light scattering of integrated optics rather than detector sensitivity. In this work, we have reduced background (BG) photocurrent of an integrated hydrogenated amorphous Si (a-Si:H) fluorescence detector due to laser light scattering by nearly one order magnitude, significantly improving a LOD. The detection platform comprises a microlens and the annular fluorescence detector where a thick SiO2/Ta2O5 multilayer optical interference filter (>6 μm) is monolithically integrated on an a-Si:H pin photodiode. With a microfluidic capillary electrophoresis (CE) device mounted on the platform, the system is demonstrated to separate DNA restriction fragment digests (LOD: 58 pg/μL) as well as 2 nM of fluorescein-labeled oligomer (LOD: 240 pM) with high speed, high sensitivity and high separation efficiency. The integrated a-Si:H fluorescence detector exhibits high sensitivity for practical fluorescent labeling dyes as well as feasibility of monolithic integration with a laser diode, making it ideal for application to point-of-care microfluidic biochemical analysis.
Reviewing Photo-sensing Devices Using a-SiC Based Materials
Authors: Vieira Manuela , Fernandes Miguel , Louro Paula , Fantoni Alessandro , Vieira M. A. , Costa João , Barata Manuel
Published: August 2010

In this paper a double pi'n/pin a-SiC:H voltage and optical bias controlled device is presented and it behavior as image and color sensor, optical amplifier and multiplex/demultiplex device discussed. The sensing element structure (single or tandem) and the light source properties (wavelength, intensity and frequency) are correlated with the sensor output characteristics (light-to-dark sensivity, resolution, linearity, bit rate and S/N ratio). Depending on the application, different readout techniques are used. When a low power monochromatic scanner readout the generated carriers the transducer recognize a color pattern projected on it acting as a color and image sensor. Scan speeds up to 104 lines per second are achieved without degradation in the resolution. If the photocurrent generated by different monochromatic pulsed channels is readout directly, the information is multiplexed or demultiplexed. It is possible to decode the information from three simultaneous color channels without bit errors at bit rates per channel higher than 4000bps. Finally, when triggered by appropriated light, it can amplify or suppress the generated photocurrent working as an optical amplifier. An electrical model is presented to support the sensing methodologies. Experimental and simulated results show that the tandem devices act as charge transfer systems. They filter, store, amplify and transport the photogenerated carriers, keeping its memory (color, intensity and frequency) without adding any optical pre-amplifier or optical filter as in the standard p-i-n cells.
A New Perspective on an Old Problem: The Staebler-Wronski Effect
Authors: Fritzsche Hellmut
Published: August 2010

Photo-induced structural changes and defect creation are common phenomena in a large variety of polymeric and non-crystalline semiconductors. The photo-induced degradation of a-Si:H and its alloys, discovered by Staebler and Wronski in 1977, belongs to a special category with quite unique features, which so far has resisted an explanation. Part of the problem is that the near 4-fold coordinated network does not naturally form an amorphous material. It is over-constrained and forms a stress relief void structure. While reviewing the experimental evidence it will be argued that some of our commonly held views regarding the underlying mechanisms of the Staebler-Wronski effect (SWE) may have to be abandoned. First, the internal void surfaces seem to be the principal locations of the photo-structural changes. Second, non-radiative bimolecular recombinations of photo carriers do not seem to be the driving force of defect creation at helium temperatures. Alternative pathways for the photo-induced processes will be suggested.
A Thermodynamic Model for the Laser Fluence Ablation Threshold of PECVD SiO2 on Thin a-Si:H Films Deposited on Crystalline Silicon
Authors: Mangersnes Krister , Foss Sean Erik
Published: August 2010

We have developed a thermodynamic model that predicts the heat distribution in a stack of PECVD SiO2 and a-Si:H on crystalline Si after laser irradiation. The model is based on solving the total enthalpy heat equation with a finite difference scheme. The laser used in the model is a frequency doubled Nd:YVO4 green laser with pulse duration in the nanosecond range. The modeling was done with the aim of getting a better understanding of our newly developed laser ablation process for making local contacts on back-junction silicon solar cells. Lasers with pulse duration within the nanosecond range are usually believed to induce too much thermal damage into the underlying silicon to make them suitable for high efficiency solar cells. In our case, insertion of a thin layer of a-Si:H between the SiO2 and the Si absorbs much of the laser irradiation both optically and thermally. This makes it possible to form local contacts to Si in a damage-free way. In addition, the residual a-Si:H serves as an excellent surface passivation layer for the Si substrate. We have also developed a simple static model to determine the onset of SiO2 ablation on a-Si:H layers of varying thickness. The models, both the static and the dynamic, are in good agreement with experimental data.
Low Temperature Dopant Activation Using Variable Frequency Microwave Annealing
Authors: Alford Terry L. , Sivaramakrishnan Karthik , Indluru Anil , Ahmad Iftikhar , Hubbard Bob , David Theodore
Published: August 2010

Variable frequency microwaves (VFM) and rapid thermal annealing (RTA) were used to activate ion implanted dopants and re-grow implant-damaged silicon. Four-point-probe measurements were used to determine the extent of dopant activation and revealed comparable resistivities for 30 seconds of RTA annealing at 900 °C and 6-9 minutes of VFM annealing at 540 °C. Ion channeling analysis spectra revealed that microwave heating removes the Si damage that results from arsenic ion implantation to an extent comparable to RTA. Cross-section transmission electron microscopy demonstrates that the silicon lattice regains nearly all of its crystallinity after microwave processing of arsenic implanted silicon. Secondary ion mass spectroscopy reveals limited diffusion of dopants in VFM processed samples when compared to rapid thermal annealing. Our results establish that VFM is an effective means of low-temperature dopant activation in ion-implanted Si.
Deep-Level Defects in Electron Irradiated 6H-SiC
Authors: Kozubal Michal Sebastian , Kaminski Pawel , Warchol Stanislaw , Racka-Dzietko Katarzyna , Grasza Krzysztof , Tymicki Emil
Published: August 2010

An effect of electron irradiation on the concentrations of deep-level centers in C-rich and Si-rich 6H-SiC wafers is investigated. In the former material, the main deep-level centers with activation energies of Ec-0.50, Ec-0.64 and Ec-0.67 eV are found to be related to dicarbon interstitials and CiNC complexes located in hexagonal and quasi-cubic lattice sites, respectively. In the latter material, the dicarbon interstitials are dominant after the irradiation with 1.5-MeV electrons. At the energy of bombarding electrons equal to 0.3 and 0.7 MeV, the activation energies of the dominant deep-level centers are Ec-0.38 and Ec-0.52 eV, respectively. The first center is related to carbon vacancies and the second to silicon interstitials.
Optimization of Poly-silicon Process for 3C-SiC Based MOS Devices
Authors: Esteve Romain , Schoner Adolf , Reshanov Sergey A. , Zetterling Carl-Mikael
Published: August 2010

Cubic 3C-SiC is regarded as a perfect material for medium power MOSFETs with blocking voltages of around 1500V and current handling of 100A and more. One of the main issues to realize such power MOSFETs is the improvement of the MOS gate to ensure low on-state resistance operation.

The benefits of the implementation of an advanced oxidation process combining PECVD SiO2 deposition and short post-oxidation steps in wet oxygen has been previously demonstrated [1]. The concentrations of fixed and mobile charges in the oxide and at the SiO2/SiC interface were significantly reduced. But the optimization of the gate material is still an issue.

The experiences from silicon technology point in the direction of using a poly-Si gate for MOS controlled devices. Significant improvements in terms of gate oxide reliability could be achieved by applying a poly-Si gate. But the usage of hydrogen for passivation of defects in oxides grown on 3C-SiC gives restrictions to the poly-Si deposition and activation process conditions. A reduced thermal budget is required to preserve the high electrical properties of the oxide.

We investigated the electrical properties of MOS structures prepared with poly-Si gates. The poly-Si layer was deposited by the LPCVD technique mixing Si2H6 and PH3 at 380°C. The poly-Si activation has been carried out with five different methods. The influence of the following two main parameters has been considered: the process duration (thermal annealing or rapid thermal annealing) and the gas atmosphere (argon, dry or wet oxygen).

MOS capacitors were fabricated on the oxidized free-standing n-type 3C-SiC (001) wafers with 5μm low nitrogen doped (5×1015 cm-3) epitaxial layers. The MOS capacitors were characterized on the wafer level (about 200 MOS structures per wafer) by capacitance-conductance-voltage (C-G-V) measurements using a HP4284A LCR meter in the frequency range of 100 Hz to 1 MHz. The measurements were performed at room temperature in a light-tight and electrically shielded environment. The interface trap densities Dit were extracted by the conductance method. To assess the oxide reliability, time-zero dielectric breakdown (TZDB) measurements were conducted on the fabricated MOS structures.

The optimized poly-Si activation process based on RTA in argon has minimal thermal budget and preserves the oxide and interface quality. The fabricated MOS structures demonstrate high electrical properties and reliability of the oxide: A small negative flat band voltage shift of -1V and an interface state density Dit of 7.4×1010 eV-1cm-2 at 0.63 eV below the conduction band. The TZDB measurements revealed an average breakdown electric field of 9.4 MV/cm.
Epitaxial Deposition of 3C-SiC on Si Using Unconventional Sputtering of a Hollow Cathode
Authors: Huguenin-Love James L , Soukup Rodney J , Ianno Natale J , Lauer Nole T
Published: August 2010

Epitaxial 3C-SiC was grown using an unconventional technique for epitaxial growth. Thin films of 3C-SiC were deposited onto the (111) and (110) faces of Si using pulsed DC sputtering of a high purity, hollow cathode SiC target. The films were studied using x-ray diffraction (XRD), reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM), and auger electron spectroscopy (AES) techniques. XRD results presented as Bragg diffraction spectra and pole figures, and electron diffraction patterns verify crystal orientation and epitaxy. In addition, AES profiles identify the compositional integrity of the deposited films.
SiC Bipolar Power Transistors - Design and Technology Issues for Ultimate Performance
Authors: Ostling Mikael , Domeij Martin , Zaring Carina , Konstantinov Andreij , Ghandi Reza , Buono Benedetto , Hallen Anders , Zetterling Carl-Mikael
Published: August 2010

Silicon carbide (SiC) semiconductor devices for high power are becoming more mature and are now commercially available as discrete devices. Schottky diodes have been on the market since a few years but also bipolar junction transistors (BJTs), JFETs and MOSFETs are now reaching the market. The interest is rapidly growing for these devices in high power and high temperature applications. The BJTs have low conduction losses, fast switching capability, operate in normally-off mode, have high radiation hardness, and can handle high power density.

This paper will review the current state of the art in active switching device performance with special emphasis on BJTs. Device performance has been demonstrated over a wide temperature interval. A very important feature in high power switch applications is the low on-resistance of a device. Better material quality and epi processes suppress the amount of basal plane dislocations to avoid stacking fault formation generated during high current injection. This has long been a concern for bipolar SiC devices but several research reports and long term reliability measurements of pn-junctions show that the bipolar degradation problem can be solved by a fine-tuned epitaxial technique. A discussion on surface passivation control is included.

Finally, an example of a power switching module is given also demonstrating the excellent paralleling capability of BJTs.
Bandstructure Manipulation of Epitaxial Graphene on SiC(0001) by Molecular Doping and Hydrogen Intercalation
Authors: Starke Ulrich
Published: August 2010

Graphene, a monoatomic layer of graphite hosts a two-dimensional electron gas system with large electron mobilities which makes it a prospective candidate for future nanocarbon devices. Grown epitaxially on silicon carbide (SiC) wafers, large area graphene samples appear feasible and integration in existing device technology can be envisioned. A precise control of the number of graphene layers and growth of large homogeneous graphene samples can be achieved. However, as-grown epitaxial graphene on SiC is electron doped due to the influence of the reconstructed interface layer present between graphene and SiC. Covalent bonds between SiC and the first carbon layer grown on top induce a dipole layer which induces charges into the graphene. As a result, the Dirac point energy where the π-bands cross is shifted away from the Fermi energy, so that the ambipolar properties of graphene cannot be exploited. How this effect can be overcome by a precise control and manipulation of the electronic structure of the π-bands is demonstrated by two methods. On the one hand, transfer doping of the epitaxial graphene surfaces with the strong acceptor molecule tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) allows for a fine tuning of the doping level. Charge neutrality can be achieved for mono- and bilayer graphene. On bilayer samples the magnitude of the existing bandgap can be increased up to more than double of its initial value. On the other hand, the impact of the SiC-graphene interface can be completely eliminated by annealing the samples in molecular hydrogen. The hydrogen atoms migrate through the graphene layers, intercalate between the SiC substrate and the interface layer and bind to the Si atoms of the SiC(0001) surface. Thus the interface layer, decoupled from the SiC substrate, is turned into a quasi-free standing graphene monolayer. Similarly, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation process represents a highly promising route towards epitaxial graphene based nanoelectronics.
Graphene Growth on SiC and Metal Surfaces by Solid Source Carbon Deposition
Authors: Mitchel W. C. , Park J. H. , Smith H. E. , Grazulis L.
Published: August 2010

Graphene has been grown by direct deposition of carbon from solid sources on both SiC and Ta films on SiC in an MBE environment. Carbon fluxes were obtained from thermally evaporated C60 and from a heated graphite filament. The graphene films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Graphene films on Si-face SiC grown by carbon source MBE (CSMBE) were compared with graphene grown by the standard epitaxial graphene process using SiC thermal decomposition. CSMBE on SiC was found to grow at lower temperatures (1200°C) and to have fewer pits and a more uniform surface. Uniform graphene films were found to grow on Ta films after exposure to both carbon sources at 1200°C but Raman measurements showed no signs of graphene on films exposed to the same temperature without a carbon flux.
Cu CMP Edge Uniformity Improvement Studies for 32 nm Technology Node and Beyond
Authors: Zhang John H , Economikos Laertis , Tseng Wei-tsu , Choi Jihong , Fang Qiang , Tang Teck Jung , Salfelder Joe , Truong Connie
Published: August 2010

Studies of the wafer edge uniformity step by step, from hard mask deposition, reactive ion etch, electroplating to post Cu CMP had been done using scanning electron microscopy (SEM) measurements, showed that the major wafer non-uniformity comes from the Cu CMP step. Improvement of Cu CMP edge uniformity had been achieved through engineering of platen 1 using real time profile control as well as CMP head zone pressure adjustment and platen 3 slurry optimizations
Slurry Particle Agglomeration Model for Chemical Mechanical Planarization (CMP)
Authors: Johnson Joy Marie , Boning Duane
Published: August 2010

In this work we propose a particle agglomeration model for chemical mechanical planarization (CMP) under the primary motivation of understanding the creation and behavior of the agglomerated slurry abrasive particles during the CMP process, which are a major cause of defectivity and poor consumable utility due to sedimentation.

The proposed model considers the slurry composition as a colloidal suspension of charged colloidal silica in an electrically neutral aqueous electrolyte. First, a theoretical relationship between the measurable chemical parameters of the slurry's aqueous electrolyte, the surface potential of the abrasive particles, and corresponding zeta potential between the agglomerated abrasive particles is presented. Secondly, this zeta potential is used in a modified DVLO interaction potential model to determine the particle interaction potentials due to both the attractive van Der Waals forces and repulsive electrostatic interactions. Finally, the total interaction potential created is then used to define a stability ratio for slow versus fast agglomeration and corresponding agglomeration rate equations between particles; these are used in a discrete population balance framework to describe the final particle size distribution with respect to time and agglomerate composition.

The proposed model will provide both a qualitative and quantitative description of agglomeration of abrasive slurry particles during CMP that can be extended to account for slurry composition or abrasive particle type, enabling more accurate process control, increased consumable utility, and possible defectivity reduction.
High Quality NH2SAM (Self Assembled Monolayer) Diffusion Barrier for Advanced Copper Interconnects
Authors: Maestre Caro Arantxa , Zhao L. , Maes Guido , Borghs Gustaaf , Beyer Gerald , Tőkei Zsolt , Armini Silvia , Travaly Youssef
Published: August 2010

The trend for future integrated circuits (IC) is decreasing in size beyond the conventional limits. The recent transition from aluminum to copper as the interconnect material for IC is due to copper's higher resistance to electromigration and its lower resistivity. Unfortunately, copper has high mobility in Si and SiO2 and may cause destruction of electrical connections on the chip. Hence, there is a significant necessity in finding ultra thin, thermally stable, high quality and good adhered diffusion barriers. The most widely used barrier is pure Ta films or layer stacks consisting of Ta and TaN. These have excellent conformality, very good uniformity and high thermal stability. But The continuous scaling down of the interconnect dimensions lead to an essential decrease in the barrier layer effective thickness to less than 5nm; coupled with the replacement of silicon oxide by advanced low-k dielectrics it demand further improvements of the diffusion barrier performance. For that reason Self-assembled monolayers (SAMs), with thicknesses of 2nm or less, have been propose for copper diffusion barrier application. By tailoring the structure of these monomolecular organic films, atomic scale properties can be controlled and selective surfaces and interfaces can be engine as desired for a specific application. In the presented work, the quality of an amino-terminated SAM barrier (NH2SAM) is tested. A high density and the absence of pinholes in the barrier layer are essential for a good barrier performance. First, the macroscopic quality of the NH2SAM barrier has been characterized by Water contact angle (CA) and High resolution AFM (HR-AFM). Secondly, the density and the presence and/or absence of pinholes have been tested by Ellipsometry and Cylic Voltametry (CV). Finally, the intrinsic barrier performance in form of Time- dependent dielectric breakdown (TDDB) lifetime has been extracted from planar capacitor structures that permitted to measure the leakage/Cu diffusion through barrier in the vertical direction. The Contact angle of layers formed at different deposition times show a variation of the hydrophilic SiO2 substrate to hydrophobic already with 1min deposited NH2SAM layer. A 15min deposited NH2SAM (~1nm), results in a continuous and pinhole free layer observed by HR-AFM. The refraction index (η) calculated by ellipsometry, indicates an increase in the density of the layer with the deposition time. On the other hand, cyclic voltametry shows inhibition of the electrochemical reduction of Fe3+ specimen to Fe2+ when NH2SAM are formed on ~2nmSiO2/Si electrodes. A decrease in the capacitive current is observed by increasing the layer thickness and density. The intrinsic barrier performance of the NH2SAM barrier by TDDB is demonstrated with an increase of 10 times the capacitor lifetime by comparing with no barrier system.