bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of the Surface Nanomorphology on the Growth of Al Whiskers Formed by Glancing Angle Deposition on a High Temperature Substrate
Authors: Hamachi Kenji , Suzuki Motofumi , Nakajima Kaoru , Kimura Kenji
Published: April 2011

We have investigated the effect of the substrate-surface morphology on the growth of Al whiskers grown by high temperature glancing angle deposition (HT-GLAD). Before the HT-GLAD of Al at 390 °C, the morphology of the substrate was systematically modified by depositing nanocolumnar SiO2 layer of thickness between 0 and 100 nm on the flat SiO2 layer. Aluminum whiskers with the width of ≈100 nm and the length ≤ μm are found on all the samples. The number of short whiskers, which can be grown from very small nuclei, depends strongly on the thickness of the SiO2 nanocolumnar layer and shows the maximum at SiO2 thickness of 20 nm. On the other hand, the number of long whiskers, which requires extraordinary amount of Al than that deposited on the side surface of the whikers, is almost independent of SiO2 thickness. These facts suggest that the surface roughness of the substrate plays an important role in the nucleation of the whiskers and that there are some transport processes of Al, which are insensitive to the surface morphology.
Nanopattern Formation by Periodic Array of Interfacial Misfit Dislocations in Bi(111)/Si(001) Heteroepitaxy
Authors: Jnawali Giriraj , Hattab H. , Bobisch C. , Bernhart A. , Zubkov E. , zu Heringdorf F.-J. Meyer , Möller R. , Krenzer B. , Horn-von Hoegen M.
Published: April 2011

Despite their large lattice mismatch of 18 %, the lattices of Bi(111) and Si(001) fit surprisingly well. A remaining compressive strain in the Bi film of 2.3 % along the direction is accommodated by the formation of a periodic array of edge-type misfit dislocations confined to the interface. The strain fields surrounding each dislocation interact with each other, producing a quasi-periodic nanopattern of grating-like periodic height undulations on the surface. The separation and the amplitude of the height undulations have been derived by spot profile analyzing LEED and STM surface height profiles. The observed undulations agree well with elasticity theory.
Pattern Formation During Nanowear of Gold Films
Authors: Pendergast Megan E. , Volinsky Alex A. , Pang Xiaolu
Published: April 2011

The effects of water on the wear resistance of 3 μm thick sputtered gold film on silicon substrate using contact AFM and a scanning nanoindenter were investigated. In performing wear tests on gold samples in the presence of water, a significant increase in depth of the wear area was observed compared to the same tests performed in the ambient atmosphere (∼55% humidity). These results were obtained using Hysitron Triboindenter on areas of 10×10 μm2 and an Atomic Force Microscope on areas of 1×1 μm2. Nanowear tests were preformed using silicon nitride cantilevers on the AFM and diamond Berkovich or blunt conical tips on the Hysitron. Normal loads used were 2000 μN and 10 μN, respectively. Tests performed in the ambient atmosphere resulted in a slightly reduced surface roughness, while a much higher wear rate was observed in the wear tests performed in water. Additionally, gold surface ripples formed under certain scanning conditions, in water for the Hysitron Triboindenter and in ambient atmosphere for the AFM. Nanoscale stick slip is being investigated as a possible explanation to the rippling phenomenon, and single scan line tests provide valuable information about the mechanisms and progression of the nanoscale wear.
A General Nonlithographic Method for Producing Nanodots by RIE Etching
Authors: Leach Jacob H. , Morkoç Hadis
Published: April 2011

In this work, thin layers of poly(methyl methacrylate) (PMMA) on Ni on silicon <111> substrates were etched almost completely away by oxygen RIE, leaving only the topmost portion of the roughness, generating nanodots of PMMA approximately 30-40nm or smaller in size. After sufficiently hard baking the samples to promote PMMA adhesion to the Ni and to increase the robustness of the PMMA, the nanodots were used as a mask to etch the thin Ni films, thus generating Ni nanodots on Si. The Ni nanodots were then used as a reactive ion (RIE) etch mask, thereby generating Si nanopillars. With further understanding of the mechanism of the generation of the roughness of the PMMA, or with the use of other polymeric materials suitable as wet etching masks, nanodots of varying size should be attainable. This method represents a very simple, low cost, scalable, and general technique to produce nanodots of various thin metals on various substrates.
Raman Spectrum of Graphene Coated Nano-Holes
Authors: Banerjee Amrita , Li Ruiqiong , Grebel Haim
Published: April 2011

It was assumed in the past that aluminum is not a platform of choice for surface enhanced Raman scattering (SERS) experiments: this was in spite of its large negative permittivity value, larger than gold or silver at optical wavelength. It was also assumed that any oxide on top of metallic platform significantly hinders SERS signals. In addition, graphene has not been studied on perforated substrates. Here we use periodically perforated and oxidized aluminum surfaces to examine electrical and optical properties of multi layered graphene. Linear and nonlinear optical methods were used to characterize single and multi layered structures.
Oriented Film Growth of Organic Semiconductor Sexithiophene on Artificial Periodic Grooves and Electrical Conduction Properties of the Films
Authors: Ikeda Susumu , Wada Yasuo , Inaba Katsuhiko , Terashima Kazuo , Shimada Toshihiro , Saiki Koichiro
Published: April 2011

Film growth of organic semiconductor α-sexithiophene (6T) was studied using substrates with artificial periodic grooves. The grooves were fabricated on thermally oxidized silicon substrates by electron beam lithography. Based on the orientation analysis by atomic force microscopy and grazing-incidence x-ray diffraction, part of the 6T grains grew having the in-plane orientational relationship with the artificial grooves. This phenomenon corresponds to “ggraphoepitaxy” which has been well known in inorganic materials research field. As well as the in-plane preferred orientation, one-dimensional structure consisting of connected 6T grains was observed. The electrical conduction properties of the films were also evaluated.
Geometric confinement governs the rupture strength of H-bond assemblies at a critical length scale
Authors: Keten Sinan , Buehler Markus J.
Published: April 2011

Using theoretical considerations and MD simulation techniques, we show that the rupture strength of H-bond assemblies is governed by geometric confinement effects, suggesting that clusters of at most 3-4 H-bonds break concurrently, even under uniform shear loading of a much larger number of H-bonds. This universally valid result leads to an intrinsic strength limitation that suggests that shorter strands with less H-bonds achieve the highest shear strength. Our finding explains how the intrinsic strength limitation of H-bonds is overcome by the formation of a nanocomposite structure of H-bond clusters, thereby enabling the formation larger, much stronger beta-sheet structures. Our results explain recent experimental proteomics data, suggesting a correlation between the shear strength and the prevalence of beta-strand lengths in biology.
Protein Fingerprinting Using Flat-Surface Electrophoresis
Authors: Ramasamy Perumal Radha , Elmaghrabi Raafat , Halada Gary , Rafailovich Miriam
Published: April 2011

Developments in the field of proteomics are highly encouraging for medical researchers. While gel electrophoresis offers a successful means for protein recognition, it requires the use of a relatively large system and a rather long period of time. Here, a protocol is developed for electrophoresis of proteins using flat surfaces based on the principles of electrophoresis of DNA on flat surfaces. By further adapting this system, it is hoped to create a portable device for protein electrophoresis. Droplets of fluorescently tagged proteins such as albumin, casein, poly-L-lysine and their mixtures were placed on glass surfaces in an electrophoretic cell and allowed to dry. TBE buffer was added to the cell and the migration of the salt complexes was monitored using confocal microscopy. We show that different protein - salt complexes have different mobilities on a flat surface. The shape and size distributions of the protein-salt complexes and their mixtures on surfaces were also studied using atomic force microscopy and were found to be dependent upon the proteins. It is observed that the native charge of the proteins play a dominant role in the migration of the protein-salt complexes in the electrophoretic cell. From the morphology of the protein droplets it is observed that large aggregates are formed when oppositely charged proteins are mixed. Light scattering measurements and zeta potential measurements confirm the difference in the size and shape of the aggregates in solution leading to different mobilities of the protein-salt aggregates during electrophoresis.
Asymptotic Strength Limit of Hydrogen Bond Assemblies in Proteins at Vanishing Pulling Rates
Authors: Keten Sinan , Buehler Markus J.
Published: April 2011

Experimental and computational studies on mechanical unfolding of proteins suggest that rupture forces approach a limiting value of a few hundred pN at vanishing pulling velocities. We develop a fracture mechanics based theoretical framework that considers the free energy competition between entropic elasticity of polypeptide chains and rupture of peptide hydrogen bonds, which we use here to provide an explanation for the intrinsic strength limit of proteins. Our analysis predicts that individual protein domains stabilized by hydrogen bonds can not exhibit rupture forces larger than approximately ≈200 pN, regardless of the presence of a large number of hydrogen bonds. This result explains a wide range of experimental and computational observations.
The Detection and Characterization of Nanoparticulate Heavy Metals in Epithelial Tissues in Patients with Nephrogenic Fibrosing Dermopathy
Authors: Ayers Reed , High Whitney , Chandler John , Ranville Jim
Published: April 2011

Certain diseases have been associated with the administration of heavy elements as contrast agents to patients undergoing medical imaging procedures. Recently, the presence of gadolinium (Gd) administered as a paramagnetic contrast agent for MRI contrast studies was associated with the incidence of Nephrogenic Fibrosing Dermopathy (NFD), also called Nephrogenic Systemic Fibrosis (NSF). To determine specific causation, Gd and other metallic nanoparticles in various tissues must be detected directly and characterized in-situ. This is done to develop specific mechanisms for the chemical modification of the metal elements as the result of a biologic response. Fixed biopsies embedded in paraffin were sectioned at 3-5 μm thick, deparaffinized by hand (xylene and 100% ethyl alcohol), placed on carbon planchettes, and allowed to air dry. Deparaffinized tissues were examined using a field emission SEM (FE-SEM) to directly detect and image the presence of Gd as well as other metals. Backscatter electron (BSE) imaging (20kV) was used to discern metal particles within tissues. Energy dispersive spectroscopy (EDS) (15kV) was used to verify the specific elements present. This allowed for the spatial characterization of the nanoparticles within the tissues but due to the physical limitations of SEM/EDS, quantification of the amount of metal was not possible. Mass concentration of the metal elements was determined using inductively coupled plasma mass spectrometry (ICP-MS) on digested tissues. Thick tissue sections, >30 μm, were used for ICP-MS to provide enough mass for detection. These sections were taken from the histology blocks adjacent to the thin sections used in the FE-SEM. Gadolinium was detected in skin, heart, lung and liver tissues. The highest concentrations were found in heart and skin; both had average tissue concentrations greater than 200μg/g (100-450μg/g range). In skin, gadolinium nano-particulates were readily seen near cell body locations in autopsy samples and within the cells in biopsy samples. The cells where gadolinium was most easily found were along blood vessels. In the cells the agglomerates appear granular with a size of less than 100 nm. They are diffused throughout the cell but as of this time not associated with any particular cell structure. Subsequent work using TEM will examine that aspect as well as the specific ultrastructure and chemistry of the nanoparticles. In this investigation, gadolinium was detected in the tissues of a number of patients with NSF. Although neither dispositive of a pathophysiologic mechanism, nor proof of causation, the detection and quantification of gadolinium within tissues of NSF patients is supportive of the epidemiologic association between exposure to gadolinium containing contrast material and development of the disease.
Device Structure for enhancement of DNA hybridization kinetics by Electrodeless Dielectrophoresis
Authors: Swami Nathan , Chou Chia-fu , Camacho-Alanis Fernanda
Published: April 2011

Signal transduction for purposes of biomolecular sensing can be enhanced through miniaturization (nanoscale sensors such as nanowires, cantilevers, etc), but this may impose insurmountable limitations on transport of analyte molecules to the surface. In order to improve biomolecular detection sensitivity, this study aims to develop electrodeless dielectrophoresis (EDEP) methods for the selective transport and trapping of analytes in close proximity to the sensor surface. A device design based on microscale dielectric constrictions of the fluidic channel and nanoscale metal electrode edges was used to locally enhance electric field gradients and trap biomolecules. The resulting electric field focusing effect was enhanced as a result of miniaturization and an instantaneous ten-fold EDEP preconcentration was obtained in high-salt buffers (50 mM NaCl) that enabled an equivalent improvement in DNA hybridization kinetics.
Carbon Nanospheres for Biomedical Applications
Authors: Styres Courtney , Stanishevskaya Inessa , Nasti Tahseen , Yusuf Nabiha , Everts Maaike , Yockell-Lelievre Helene , Stanishevsky Andrei
Published: April 2011

Carbon nanostructures, e.g., nanotubes, fullerenes, carbon blacks, etc., are being extensively explored for numerous biomedical applications. The most of such studies, however, deal with carbon nanotubes, and comparatively less is known on the biomedical potential of other nanosize carbon particles. In the present work, carbon and metal/carbon core/shell spherical nanoparticles have been prepared using the decomposition of monosaccharide-based compositions under hydrothermal conditions with or without the presence of metal seed particles. The effects of different process conditions on the particle size, structure, and composition have been examined using TEM, XRD, UV-Vis, FTIR and Raman spectroscopies. The nearly perfect spherical particles with the dimensions in the range of 20 – 100 nm have been obtained depending on the process parameters such as precursor concentration, presence of seed particles and polymeric additives, process temperature and time. The particles prepared with 5 – 20 nm gold seeds clearly showed the core/shell structure with the thickness of carbon shell in the range of 10 – 50 nm. The FTIR experiments have indicated a strong effect of the processing conditions on the chemical activity of nanoparticle surfaces in the attachment of the additional surface functional groups and organic molecules. It has been found that the both hydrothermally prepared carbon and metal/carbon core/shell nanoparticles possess very good dispersibility and stability in the both water and simulated body fluids in the most of experiments. The particles have been successfully functionalized with several molecules such as polyethyleneglucol and biotin. Selected samples of well-dispersed carbon nanospheres with different concentrations have been tested for their interaction with several cultured cell lines including epidermal keratinocytes, fibroblasts, and dog macrophages.
Detection of Viral Proteins using Human Receptor Functionalized Carbon Nanotubes
Authors: Chen Michelle , Khamis S.M. , Datta S.S. , Zhang Y.-B , Kanungo M. , Ho A.J. , Freimuth P. , van der Lelie Daniel , Johnson A.T. , Misewich J.A. , Wong S.S.
Published: April 2011

We present proof-of-concept experiments for developing a highly-sensitive and fast-response miniaturized single-walled carbon nanotube field-effect transistor (SWNT-FET) biosensor for electrically detecting adenovirus using ligand-receptor-protein specificity. SWNTs are mildly oxidized to form carboxylic groups on the surfaces without compromising the electronic integrity of the nanotubes. Then the human coxsackievirus and adenovirus receptor (CAR) is covalently functionalized onto the nanotube surface via diimide-activated amidation process. Upon exposure of the device to adenovirus protein, Ad12 Knob (Knob), specific binding of Knob to CAR decreases the current that flows through the SWNT-FET device. For control experiment, the CAR-SWNT device is exposed to YieF, which is a virus protein that does not bind specifically to CAR, and no current change is observed. The biological activity of the CAR and Knob proteins that are immobilized on SWNTs has been confirmed by previous fluorescence studies [1]. AFM analysis is done to show height increase of a few nanometers at specific spots where the CAR-Knob complex are covalently linked to the nanotube surface. Therefore, our results show that the human receptor protein CAR does immobilize on SWNT surface while fully retains its biological activity. Moreover, the specific binding of CAR to its complementary adenovirus Knob can be electrically detected using individual SWNT-FET devices. These findings suggest that CAR-functionalized SWNT-FETs can ably serve as biosensors for detection of environmental adenoviruses.
Crystallinity Uniformity of Microcrystalline Silicon Thin Films Deposited in Large Area Radio Frequency Capacitively-coupled Reactors
Authors: Strahm Benjamin , Howling Alan A. , Hollenstein Christoph
Published: April 2011

The microcrystalline silicon (μc-Si:H) intrinsic layer for application in micromorph tandem photovoltaic solar cells has to be optimized in order to achieve cost-effective mass production of solar cells in large area, radio frequency, capacitively-coupled PECVD reactors. The optimization has to be performed with regard to the deposition rate as well as to the crystallinity uniformity over the substrate area. The latter condition is difficult to achieve since the optimal solar grade μc-Si:H is deposited at the limit between a-Si:H and μc-Si:H material, where the film crystallinity is very sensitive to the plasma process. In this work, a controlled RF power nonuniformity was generated in a large area industrial reactor. The resulting film uniformity was studied as a function of the deposition regimes. Results show that the higher the input silane concentration, the more the uniformity of the crystallinity is sensitive to the RF power nonuniformity for films deposited at the limit between a-Si:H and μc-Si:H. The effect of the input silane concentration on the microstructure uniformity could be explained on the basis of an analytical plasma chemistry model. This result is important for reactor design. In reactors generating nonuniform plasma the input silane concentration has to be limited to low values in order to deposit films with uniform microstructure. To benefit from the high silane flow rate utilization fraction encountered only for higher input silane concentration, the RF power distribution has to be as uniform as possible over the whole substrate area.
Cone Kinetics Model: Insights into the Morphologies of Mixed-phase Silicon Film Growth
Authors: Branz Howard M. , Stradins Paul , Teplin Charles W.
Published: April 2011

The ‘cone kinetics’ model, which explains development of cone-shaped inclusions during nanocrystalline silicon film growth and during low-temperature silicon epitaxy breakdown, is extended to protocrystalline (‘edge’) amorphous silicon, Si heterojunctions and other Si film morpholologies. We generalize the physics underlying cone formation and present diagrams that delineate the deposition regimes giving rise to the different film morphologies; these regimes are determined by the nucleation rate of the second phase and the relative growth rate of the phases present. The model predicts cone growth during thin-film deposition by plasma-enhanced and other chemical vapor deposition techniques when isotropic growth is coupled with the isolated nucleation of a second phase with a higher growth rate. Protocrystalline amorphous silicon and other embedded crystallite phases are formed when a second phase successfully nucleates but has a smaller growth rate than the surrounding amorphous film. The cone kinetics phase diagram provides a simple explanation of the various nanocrystalline film morphologies observed when silane precursors are diluted with different concentrations of hydrogen gas.
Metastable Defects in Light Soaked Amorphous Silicon at 77 K
Authors: Ju Tong , Stradins Paul , Taylor P. Craig
Published: April 2011

We have observed the growth of defects caused by optical illumination in liquid nitrogen. We kept the sample in liquid nitrogen over one year. After one year and half the ESR signal reached ∼1018 cm−3 with no evidence of saturation. After that, we step-wise annealed isochronal the sample up to room temperature, where two thirds of original defects were annealed out. After room temperature, the sample was annealing isothermally around 300 K for several months. At this temperature, the defects slowly anneal. After a hundred of hours at 295K, the defect density decreased 10x from its original value at 77K.
Studies on the Surface Reactions of Substituted Disilanes with Silica Surface
Authors: Blomberg Tom , Matero Raija , Haukka Suvi , Root Andrew
Published: April 2011

Both CVD and ALD deposition techniques benefit from a detailed understanding of the reaction mechanisms of the precursor molecules with the surface. In this paper, the reactions of hexakis ethylaminodisilane (AHEAD™), hexamethoxydisilane and hexamethyldisilane were studied on high surface area silica granules at 200-375 °C. Silica was heat treated at 200-820 °C to control the number of surface Si-OH groups. The samples were characterized by FTIR and solid state NMR spectroscopy. After the chemisorption of the precursors with silica, Si-H bonds, not originally present in the molecules, were identified for AHEAD and hexamethoxydisilane, but not for hexamethyldisilane. It is suggested that with AHEAD and hexamethoxydisilane, cleavage of the Si-Si bond takes place during the chemisorption with Si-OH sites. Since no reaction for hexamethyldisilane at the studied temperatures was observed, a prerequisite for the reaction with Si-OH groups seems to be the presence of electronegative O or N atoms in the ligands. In the paper, possible reaction mechanisms with the various surface species are discussed.
Photoluminescence of Different Phase Si Nanoclusters in Amorphous Hydrogenated Silicon
Authors: Torchynska Tetyana V.
Published: April 2011

This paper presents the results of XRD and PL spectrum studies for Si nano-clusters embedded in amorphous silicon matrix. Investigated layers were deposited by the hot-wire CVD method on glass substrates at the wafer temperature 300°C and different filament temperatures from the range 1735-1885°C. It was shown that variation of filament temperatures allows producing the films with desirable parameters. Using of X-ray diffraction and photoluminescence methods the correlation between the intensity of some photoluminescence bands and the concentrations of Si nanocrystallites and amorphous Si nanoclusters has been shown. The nature of light emission is discussed.
On determination of properties of ultrathin and very thin silicon oxide layers by FTIR and X - ray reflectivity
Authors: Kopani Martin , Jergel Matej , Kobayashi Hikaru , Takahashi Masao , Brunner Robert , Mikula Milan , Imamura Kentarou , Jurecka Stanislav , Pincik Emil
Published: April 2011

We analyze properties of ultra-thin SiO2 + very thin SiOx double layer structure formed on high-doped n-type Si (100) wafers using FTIR, X-ray reflectivity and AFM methods. The observed absorption band around 1230 cm−1 is attributed to the longitudinal optical mode of SiOx precipitates incorporated in silicon matrix. In particular, the corresponding peak positions indicate that there are precipitates of SiOx with x >1.8. The absorption band around 1070 cm−1 is attributed to the Si–O–Si stretching bond. This position is characteristic for stoichiometric SiO2. From the results it can be concluded that differently shaped particles co-exist in the samples. This assumption is supported by the oxide density measurements performed by FTIR and X-ray reflectivity. We determined density of oxide layers, roughness of corresponding interfaces, and surface roughness by the X-ray reflectivity. The obtained values were compared with those determined by FTIR and AFM. Additionally, we present the results of multifractal analysis on a complete set of six samples.
Voids in Hydrogenated Amorphous Silicon: A Comparison of ab initio Simulations and Proton NMR Studies
Authors: Chakraborty Sudeshna , Bobela David C , Taylor P C , Drabold D. A.
Published: April 2011

Recently, a new hydrogen NMR signal has been observed in a number of PECVD prepared hydrogenated amorphous silicon (a-Si:H) films of varying quality. It is speculated that the signal is the consequence of a dipolar-coupled hydrogen pair separated, on average, by 1.8 ± 0.1 Å. To elucidate the possible bonding configurations responsible for the NMR data of ref. [1], we have used ab initio simulation methods to determine a set of relaxed structures of a-Si:H with varying void sizes and H-concentrations. Models containing two isolated hydrogen atoms indicate a preferred H-H distance of approximately 1.8 Å when the two atoms bond to nearest neighbor silicon atoms. This separation also occurs for models containing small, hydrogenated voids, but the configurations giving rise to this H-H distance do not appear to be unique. For larger voids, a proton separation of about 2.4Å is seen, as noted previously [2]. There appears to be consistency between the computed structures and the NMR data for configurations consisting of isolated hydrogen pairs or for clusters of an even number of hydrogen atoms with the constraint that the average H-H distance is 1.8 Å. In this paper, we will discuss the most probable bonding configurations of clustered hydrogen based upon the extent of the NMR data and simulated structures.
Study of Large Area a-Si:H and nc-Si:H Based Multijunction Solar Cells and Materials
Authors: Xu Xixiang , Yan Baojie , Beglau Dave , Li Yang , DeMaggio Greg , Yue Guozhen , Banerjee Arindam , Yang Jeff , Guha Subhendu , Hugger Peter G. , Cohen J. David
Published: April 2011

Solar cells based on hydrogenated nanocrystalline silicon (nc-Si:H) have demonstrated significant improvement in the last few years. From the standpoint of commercial viability, good quality nc-Si:H films must be deposited at a high rate. In this paper, we present the results of our investigations on obtaining high quality nc-Si:H and a-Si:H films and solar cells over large areas using high deposition rate. We have employed the modified very high frequency (MVHF) glow discharge technique to realize high-rate deposition. Modeling studies were conducted to attain good spatial uniformity of electric field over a large area (15”×1”) MVHF cathode for nc-Si:H deposition. A comparative study has been carried out between the RF and MVHF plasma deposited a-Si:H and nc-Si:H single-junction and a-Si:H/nc-Si:H double-junction solar cells. By optimizing the nc-Si:H cell and the tunnel/recombination junctions, we have obtained an initial aperture-area (460 cm2) efficiency of 11.9% for a-Si:H/nc-Si:H double-junction cells using conventional RF (13.56 MHz) plasma deposition. The deposition rate was 3 Å/sec. Results on solar cells made with MVHF will also be presented.
The Effect of Electrical Stress on the New Top Gate N-type Depletion Mode Polycrystalline Thin Film Transistors Fabricated by Alternating Magnetic Field Enhanced Rapid Thermal Annealing
Authors: Lee Won-Kyu , Han Sang-Myeon , Park Sang-Geun , Choi Sung-Hwan , Choi Joonhoo , Han Min-Koo
Published: April 2011

We have fabricated the new top gate depletion mode n-type alternating magnetic field enhanced rapid thermal annealing (AMFERTA) polycrystalline silicon (poly-Si) thin film transistors (TFTs), which show the excellent electrical characteristics and superior stability compared with hydrogenated amorphous silicon (a-Si:H) TFTs and excimer laser crystallized (ELC) low temperature polycrystalline silicon (LTPS) TFTs. The fabricated AMFERTA poly-Si TFTs were not degraded under hot-carrier stress, and highly biased vertical field stress. The considerably large threshold voltage shift (ΔVTH) and trap state density reducing were occurred when the gate bias and drain bias were both large enough. The dominant mechanism of instability in the fabricated depletion mode AMFERTA poly-Si TFTs may be due to carrier induced donor-like defects reduction within the channel layer, especially near the drain junction.
Negative Bias Temperature Instability for P-channel of LTPS Thin Film Transistors with Fluorine Implantation
Authors: Kao Chyuan-Haur , Sung W. H.
Published: April 2011

This paper studies the impact of LTPS (low temperature polycrystalline silicon) TFTs with fluorine implantation under NBTI (Negative bias temperature instability) stress. The fluorinated TFTs' devices can obtain better characteristics with samller threshold voltage shift, lower trap states and lower subthreshold swing variation. Therefore, the fluorine implantation does not only improve initial electrical characteristics, but also suppresses the NBTI-induced degradation.
Noise Analysis of Image Sensor Arrays for Large-Area Biomedical Imaging
Authors: Lai Jackson , Striakhilev Denis , Vygranenko Yuri , Heiler Gregory , Nathan Arokia , Tredwell Timothy
Published: April 2011

Large area digital imaging made possible by amorphous silicon thin-film transistor (a-Si TFT) technology, coupled with a-Si photo-sensors, provides an excellent readout platform to form an integrated medical image capture system. Major development challenges evolve around optimization of pixel architecture for detector fill factor, and manufacturability, while suppressing noise stemming from pixel array and external electronics. This work discusses the behavior and modeling of system noise that arises from imaging array operations. An active pixel sensor (APS) design with on-pixel amplification is studied. Our evaluation demonstrates that a 17 inch by 17 inch array can achieve system noise as low as 1000 electrons through proper design and optimization.
Fluorescence Detection of DNA Hybridization Using an Integrated Thin-Film Amorphous Silicon n-i-p Photodiode
Authors: Pimentel Alexandra , Cabeça R. , Rodrigues M. , Prazeres D.M.F. , Chu V. , Conde J. P.
Published: April 2011

This paper presents the fluorescence detection of DNA hybridization with a surface immobilized probe using an hydrogenated amorphous silicon photosensor. This sensor integrates a SiO2 layer for DNA probe immobilization, a p-i-n amorphous silicon (a-Si:H) photodiode for fluorescence detection and a fluorescence filter of amorphous silicon carbon (a-SiC:H) to cut the excitation light. With this integrated photosensor system, a five order of magnitude difference was obtained in the signal measured at the emission wavelength and that measured at the excitation wavelength for the same incident photon flux. The fluorophore Alexa Fluor 430 was used to label the DNA target molecules and a laser at 405 nm and a photon flux of 5.7×1016 cm−2.s−1 was used as the excitation light source. The detection limit achieved for fluorophores in solution in contact with the device and for fluorophores immobilized on the device surface is 5×10−9 M and 0.4 pmol/cm2, respectively. The fluorescence detection of the DNA target hybridization with a covalently or electrostatically immobilized probe was successfully detected at a surface density of ∼3 pmol/cm2.
Noise Characterization of Polycrystalline Silicon Thin Film Transistors for X-ray Imagers Based on Active Pixel Architectures
Authors: Antonuk L. E. , Koniczek M. , McDonald J. , El-Mohri Y. , Zhao Q. , Behravan M.
Published: April 2011

An examination of the noise of polycrystalline silicon thin film transistors, in the context of flat panel x-ray imager development, is reported. The study was conducted in the spirit of exploring how the 1/f, shot and thermal noise components of poly-Si TFTs, determined from current noise power spectral density measurements, as well as through calculation, can be used to assist in the development of imagers incorporating pixel amplification circuits based on such transistors.
1T Memory Cell Based on PVDF-TrFE Field Effect Transistor
Authors: Salvatore Giovanni Antonio , Bouvet Didier , Ionescu Mihai Adrian , Riester Sebastian , Stolichnov Igor , Gysel Roman , Setter Nava
Published: April 2011

Interest in PVDF-TrFE copolymers as ferroelectric material for Memory application is driven by the prospect of having low cost, low operating voltage and fully organic device. Some previous studies reported FET designs using copolymers [refs 1,2] but none of these structures were fully integrated on silicon wafers and using a MOSFET fabrication process. We present for the first time the integration of a PVDF-TrFE (70%-30%) layer into a standard n-MOS transistor through a quasi-standard semiconductor technology. This allows us to achieve a Non Volatile Memory cell and at the same time to compact capacitor-transistor ferroelectric cell into a one-transistor memory cell.
High-resolution Photoinduced Transient Spectroscopy of Defect Centers in Undoped Semi-Insulating 6H-SiC
Authors: Kaminski Pawel , Kozlowski Roman , Miczuga Marcin , Pawlowski Michal , Kozubal Michal , Zelazko Jaroslaw
Published: April 2011

High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to studying defect centers controlling the charge compensation in semi-insulating (SI), vanadium-free, bulk 6H- SiC. The photocurrent relaxation waveforms were digitally recorded in the temperature range of 50 − 750 K and a new approach to extract the parameters of defect centers from the temperature-induced changes in the time constants of the waveforms has been implemented. It is based on a two-dimensional analysis using the numerical inversion of the Laplace transform. As a result, the images of spectral fringes depicting the temperature dependences of the emission rate of charge carriers for defect centers are created. Using the new procedure for the analysis of the photocurrent relaxation waveforms and the new way of the visualization of the thermal emission rate dependences, a number of shallow and deep defect levels ranging from 80 to 1900 meV have been detected. The obtained results indicate that defect structure of undoped SI bulk 6H-SiC is very complex and the material properties are affected by various point defects occupying the hexagonal and quasi-cubic lattice sites.
Point Defects in SiC
Authors: Gali Ádám , Bockstedte Michel , Son Ngyen Tien , Janzén Erik
Published: April 2011

Tight control of defects is pivotal for semiconductor technology. However, even the basic defects are not entirely understood in silicon carbide. In the recent years significant advances have been reached in identification of defects by combining the experimental tools like electron paramagnetic resonance and photoluminescence with ab initio calculations. We summarize these results and their consequences in silicon carbide based technology. We show recent methodological developments making possible the accurate calculation of absorption and emission signals of defects.
Improved SiC Epitaxial Material for Bipolar Applications
Authors: Bergman Peder , Hassan Jawad ul , Ellison Alex , Henry Anne , Godignon Philippe , Brosselard Pierre , Janzén Erik
Published: April 2011

Epitaxial growth on Si-face nominally on-axis 4H-SiC substrates has been performed using horizontal Hot-wall chemical vapor deposition system. The formation of 3C inclusions is one of the main problem with growth on on-axis Si-face substrates. In situ surface preparation, starting growth parameters and growth temperature are found to play a vital role in the epilayer polytype stability. High quality epilayers with 100% 4H-SiC were obtained on full 2″ substrates. Different optical and structural techniques were used to characterize the material and to understand the growth mechanisms. It was found that the replication of the basal plane dislocation from the substrate into the epilayer can be eliminated through growth on on-axis substrates. Also, no other kind of structural defects were found in the grown epilayers. These layers have also been processed for simple PiN structures to observe any bipolar degradation. More than 70% of the diodes showed no forward voltage drift during 30 min operation at 100 A/cm2.
Deep-Level Defects in Nitrogen-Doped 6H-SiC Grown by PVT Method
Authors: Kaminski Pawel , Kozubal Michal , Grasza Krzysztof , Tymicki Emil
Published: April 2011

An effect of the nitrogen concentration on the concentrations of deep-level defects in bulk 6H-SiC single crystals is investigated. Six electron traps labeled as T1A, T1B, T2, T3, T4 and T5 with activation energies of 0.34, 0.40, 0.64, 0.67, 0.69, and 1.53 eV, respectively, were revealed. The traps T1A (0.34 eV) and T1B (0.40 eV), observed in the samples with the nitrogen concentration ranging from ∼2×1017 to 5×1017 cm−3, are attributed to complexes formed by carbon vacancies located at various lattice sites and carbon antisites. The concentrations of traps T2 (0.64 eV) and T3 (0.67 eV) have been found to rise from ∼5×1015 to ∼1×1017 cm−3 with increasing the nitrogen concentration from ∼2×1017 to ∼2.0×1018 cm−3. These traps are assigned to complexes involving silicon vacancies occupying hexagonal and quasi-cubic sites, respectively, and nitrogen atoms. The trap T4 (0.69 eV) concentration also substantially rises with increasing the nitrogen concentration and it is likely to be related to complexes formed by carbon antisites and nitrogen atoms. The midgap trap T5 (1.53 eV) is presumably associated with vanadium contamination. The presented results show that doping with nitrogen involves a significant change in the defect structure of 6H-SiC single crystals.
High-frequency electron paramagnetic resonance study of the as deposited and annealed carbon-rich hydrogenated amorphous silicon-carbon films
Authors: Vasin A. V. , Kalabukhova E. N. , Lukin S. N. , Savchenko D. V. , Lysenko V. S. , Nazarov A. N. , Rusavsky A. V. , Koshka Y.
Published: April 2011

Three paramagnetic defects were revealed in amorphous hydrogenated carbon-rich silicon-carbon alloy films (a-SiC:H), which were attributed to the to silicon (Si) dangling bonds (Si DB), carbon-related defects (CRD), and bulk Si DB defect bonded with nitrogen atoms Si-N2Si. The effect of thermal annealing on a-SiC:H films was studied. It was established that annealing at high temperatures leads to formation of graphite-like carbon clusters in a-SiC:H films and strong increase of the concentration of CRD, while EPR signal from Si DB disappeared. A dependence of the resonance positions for both Si DB and CRD signals on the orientation of the magnetic field relative to the a-SiC:H film plane was found at Q-band and D-band frequencies and was explained by the influence of demagnetizing fields, which is becoming significant at high spin density of the paramagnetic centers, higher microwave frequency, and low temperature. A demagnetizing field of 11 Gs was found in annealed a-SiC:H film at 140 GHz and 4.2 K.
Impact Ionization in Ion Implanted 4H-SiC Photodiodes
Authors: Loh Wei Sun , Goh Eric Z. J. , Vassilevski Konstantin , Nikitina Irina , David John P. R. , Wright Nick G , Johnson C Mark
Published: April 2011

Hole dominated avalanche multiplication and thus breakdown characteristics of ion implanted 4H-SiC p+-n−-n+ photodiodes were determined by means of photomultiplication measurements using 325 nm UV light. All the tested diodes exhibited low reverse leakage current and reasonably uniform avalanche breakdown. With avalanche widths of 0.2 µm to 1.5 µm and the capability to measure multiplication factor as low as 1.001, the room temperature impact ionization coefficients were precisely deduced from these 4H-SiC diodes using a local ionization model for electric fields ranging from 1.25 MV/cm to 2.8 MV/cm. The results agree with those reported by Ng et al. and are within the accuracy of both the C-V measurements and electric-field determinations.
Spontaneous and Piezoelectric Polarization Effects on the Frequency Response of Wurtzite Aluminium Gallium Nitride / Silicon Carbide Heterojunction Bipolar Transistors
Authors: Praharaj Choudhury
Published: April 2011

We present theoretical calculations for the effect of spontaneous and piezoelectric polarization on the base resistance and frequency response of wurtzite Aluminium Gallium Nitride / Silicon Carbide Heterojunction Bipolar Transistors. Heterojunction Bipolar Transistors ( HBTs ) built using wide band gap semiconductors with AlGaN emitter and SiC base/collector hold the promise of high-power and high-frequency operation due to lower impact ionization coefficients and higher breakdown voltages. Further, Silicon Carbide has an indirect bang gap, and a large lifetime of minority carriers compared to most other compound semiconductors, which tend to have direct band gaps. This reduces the base recombination factor when the base is made from SiC, and helps to achieve higher overall current gain. Spontaneous and piezoelectric polarizations of the order of 1013 electrons per cm2 exist in wurtzite wide band-gap semiconductors. This has a non-trivial effect on band profile, charge transport and overall device characteristics since the polarization-induced charges are of the same order of magnitude as the total dopant charge content of critical device layers, and can significantly affect the amount of mobile charge depletion or accumulation in these layers. We calculate the effect of this polarization for both very thin pseudomorphic emitters and for relaxed emitter structures. We present calculations for the cases of Si-face and C-face SiC, since the signs of polarization-induced charges are different for the two cases. The intrinsic base resistance near emitter flat-band conditions is changed by a factor of 10 depending on the alloy composition of the emitter and the polarity of growth. The maximum frequency of oscillation under emitter flat-band conditions can also be modulated by the polarization-induced charges by up to 60%. Our calculations show that the technologically less prevalent C-face SiC can give a higher advantage for frequency response, especially when the emitter thickness is larger than the critical thickness.
Characterization and Growth Mechanism of B12As2 Epitaxial Layers Grown on (1-100) 15R-SiC
Authors: Chen Hui , Wang Guan , Dudley Michael , Xu Zhou , Edgar James. H. , Batten Tim , Kuball Martin , Zhang Lihua , Zhu Yimei
Published: April 2011

A systematic study is presented of the heteroepitaxial growth of B12As2 on m-plane 15R-SiC. In contrast to previous studies of B12As2 on other substrates, including (100) Si, (110) Si, (111) Si and (0001) 6H-SiC, single crystalline and untwinned B12As2 was achieved on m-plane 15R-SiC. Observations of IBA on m-plane (1100)15R-SiC by synchrotron white beam x-ray topography (SWBXT) and high resolution transmission electron microscopy (HRTEM) confirm the good quality of the films on the 15R-SiC substrates. The growth mechanism of IBA on m-plane 15R-SiC is discussed. This work demonstrates that m-plane 15R-SiC is potentially a good substrate choice to grow high quality B12As2 epilayers.
Long-Term Characterization of 6H-SiC Transistor Integrated Circuit Technology Operating at 500 °C
Authors: Neudeck Philip , Spry David J. , Chen Liang-Yu , Chang Carl W. , Beheim Glenn M. , Okojie Robert S. , Evans Laura J. , Meredith Roger D. , Ferrier Terry L. , Krasowski Michael J. , Prokop Norman F.
Published: April 2011

NASA has been developing very high temperature semiconductor integrated circuits for use in the hot sections of aircraft engines and for Venus exploration. This paper reports on long-term 500 °C electrical operation of prototype 6H-SiC integrated circuits based on epitaxial 6H-SiC junction field effect transistors (JFETs). As of this writing, some devices have surpassed 4000 hours of continuous 500 °C electrical operation in oxidizing air atmosphere with minimal change in relevant electrical parameters.
3D Thermal Stress Models for Single Chip SiC Power Sub-Modules
Authors: Tsao Bang-Hung , Lawson Jacob , Scofield James
Published: April 2011

Three dimensional models of single chip SiC power sub-modules were generated using ANSYS in order to simulate the effects of various substrate materials, heat fluxes, and heat transfer coefficients on temperature and thermal stress contours. Silicon nitride, aluminum-nitride, alumina were compared as substrates with or without an additional layer of CVD diamond on either top or bottom of the surfaces. Simulated heat fluxes of 100 to 300 watts/cm2 resulted in device junction temperatures in the range of 377 to 535 K. With modest cooling, represented by a heat transfer coefficient (hconv) of 3350 watts/m2 K, SiC chips operated at 300 watts/cm2 power density maintained junction temperatures Tj < 535 K. Both the maximum and minimum chip temperature decreased with increasing heat transfer coefficient from 50 to 5000 watts/m2 K. In the applied heat flux range, the minimum and maximum Von Mises stress of a simulated single SiC device sub-module was between 946 MPa to 1.31GPa. If consistent with simulation results, CVD diamond integrated substrates should be superior to those comprised of only AlN, Al2O3, and Si3N4. Experimental validation of ANSYS results and more extensive multiple-chip power module simulations will also be explored.
Micro-uniformity during laser anneal : metrology and physics
Authors: Vandervorst W. , Rosseel E. , Lin R. , Petersen D. H. , Clarysse T. , Goossens J. , Nielsen P. F. , Churton K.
Published: April 2011

Maintaining or improving device performance while scaling semiconductor devices, necessitates the development of extremely shallow (< 20 nm) source/drain extensions with a very high dopant concentration and electrical activation level. Whereas solutions based on RTA with cocktail implants have been proposed in previous generations, sub-45 nm technologies will require even shallower junctions which motivates the research effort on milli-second anneal approaches as these hold the promise of minimal diffusion coupled with high activation levels [1]. Laser annealing is one of these concepts proposed to achieve the junction specifications and is typically described as a msec anneal process. Different from lamp based concepts which illuminate a full wafer simultaneously, the laser has an illuminated area which is much smaller than the wafer size thus necessitating a dedicated scanning pattern. In such a case one is potentially faced with areas subject to multiple overlaps and/or different temperatures and thus issues related to within wafer and within die uniformity need to be addressed.

In this work we use optimized metrology to probe such macro- and micro non-uniformity and determine the origin of the various components contributing to the observed non-uniformity patterns (laser stitching patterns, laser beam uniformity, optical path) and their impact on the local sheet resistance.