bluuebunny/crossref_metadata_2025 huggingface dataset:
Redeposition of etch products on sidewalls during SiO2 etching in a fluorocarbon plasma. III. Effects of O2 addition to CF4 plasma
Authors: Min Jae-Ho , Hwang Sung-Wook , Lee Gyeo-Re , Moon Sang Heup
Published: August 2003

The effect of added O2 on the etching of SiO2 sidewalls in a CF4 plasma was examined using a transformer-coupled plasma etcher, for two cases when the sidewall was either affected or unaffected by particles emitted from the bottom SiO2 surface. The deposition rate on the sidewall decreased in proportion to the amount of added O2. This occurred because the increased amount of F radicals enhanced the re-etching of SiO2, which is present beneath the surface polymer layer, and of etch products, which are redeposited from the bottom. The surface polymer layer on the sidewall, affected by particles emitted from the bottom, became thin and smooth as a result of the reaction with O radicals. The chemical etch rate of the sidewall, unaffected by energetic ions and bottom-emitted particles, and the bottom etch rates were the highest for oxygen concentrations of 10% and 20%, respectively. These concentrations were lower than an O2 concentration of 30%, which yields the maximum concentration of F radicals. The mismatch in the O2 concentrations arises because the O2 concentration required to obtain the maximum etch rate on the sidewall or bottom surface is determined by competitive reactions among F, CF2, O radicals, and incident ions.
Emission fluctuation and slope–intercept plot characterization of Pt and transition metal carbide field-emission cathodes in limited current regimes
Authors: Mackie W. A. , Southall L. A. , Xie Tianbao , Cabe G. L. , Charbonnier F. M. , McClelland P. H.
Published: August 2003

Our objective was to identify optimum emitter materials for block arrays and individually addressable tip arrays where emission stability and operation in a moderate vacuum is required. In general, transition metal carbide emitters are robust at high current and moderate vacuum levels due to their hard nature. Physical vapor deposition of composite emitters consisting of a carbide cone on a truncated niobium cone was used throughout. Four carbides (HfC, NbC, TiC, and ZrC) and platinum were deposited for the emitter materials in this study. All data were taken in the classically unstable pressures region of 10−7 Torr. Brief current stability measurements were collected along with current–voltage data. HfC, ZrC, and Pt demonstrated good initial stability, though all five materials were similar in emission performance. After a proprietary processing algorithm, TiC gave variations as low as ±1.5% ΔI/I at a 240 nA current level and 7.6×10−8 Torr pressure. An extensive discussion and analysis is given of Fowler–Nordheim slope–intercept plots that reveal information on noise and emitter apex behavior.
Field emission characteristics of multiwalled carbon nanotubes grown at low temperatures using electron cyclotron resonance chemical vapor deposition
Authors: Woo Yun Sung , Jeon Duk Young , Han In Taek , Park Young Jun , Kim Ha Jin , Jung Jae Eun , Kim Jong Min , Lee Nae Sung
Published: August 2003

Vertically aligned multiwalled carbon nanotubes were synthesized by electron cyclotron resonance chemical vapor deposition on Ni-coated glass substrates at temperatures as low as 400 °C. Negative self-biases were induced to the substrates by radio frequency plasma to give ion bombardment to the growing surface. An increase of self-bias voltages from −50 to −200 V resulted in an evolution of the microstructures from amorphous carbon to nanorods, subsequently to nanotubes. Nanotubes grown above −150 V were more straight in morphology and better in crystallinity than nanorods grown at −100 V. In the field emission (FE) measurements, the electric fields to obtain 1 μA/cm2 were 4.6 and 11.1 V/μm for the nanorods and nanotubes grown at −100 and −200 V, respectively. The emission areas, calculated from the Fowler–Nordheim plots, were much larger in the nanorods than the nanotubes. It is considered that a larger amount of crystalline defects in nanorods plays a major role in improving their FE characteristics.
Lithographic characterization of the printability of programmed extreme ultraviolet substrate defects
Authors: Naulleau Patrick , Goldberg Kenneth A. , Anderson Erik H. , Bokor Jeffrey , Gullikson Eric , Harteneck Bruce , Jackson Keith , Olynick Deirdre , Salmassi Farhad , Baker Sherry , Mirkarimi Paul , Spiller Eberhard , Walton Chris , Zhang Guojing
Published: August 2003

Extreme ultraviolet lithography, the leading next-generation lithography candidate, has now entered the commercialization phase. One of the most daunting challenges to the commercial viability of this technology is the issue of defect-free multilayer-coated reflection masks. Of great potential interest are multilayer deposition processes that can effectively reduce the printability of substrate defects, thereby relaxing substrate particle cleanliness requirements and facilitating the precoating substrate-inspection task. In particular, ion-beam deposition with additional ion-assisted polishing has been shown to drastically reduce defect sizes as seen after multilayer coating. Here we report on tests performed to lithographically characterize the effectiveness of defect smoothing and to verify defect printability models. The results show that normally printable 50 nm substrate defects are rendered nonprintable through the smoothing process. Moreover, a programmed defect fabrication method enabling controlled proximity printing tests is presented.
Field emission electroluminescence on diamond and carbon nanotube films
Authors: Kim U. , Aslam D. M.
Published: August 2003

Field emission electroluminescence (FEEL), observed during field emission from polycrystalline diamond films for the first time, is initiated at field emission currents above 1 μA. The electroluminescence spectra exhibit the main luminescence peak at 482 nm. Typically, the luminescence intensity increases with the emission current. Although the FEEL observed in diamond and carbon nanotube films is poorly understood, it seems to be related to certain types of defects in the films.
Rapid thermal annealing effects on the electrical behavior of plasma oxidized silicon/silicon nitride stacks gate insulators
Authors: San Andrés E. , del Prado A. , Mártil I. , González-Dı́az G. , Martı́nez F. L.
Published: August 2003

We present a comparative study of the electrical and structural characteristics of metal–insulator–semiconductor (MIS) devices using SiN1.55:H or SiN1.55:H/SiOx stacks as gate dielectrics, with the aim of improving the thermal stability of the SiN1.55:H/Si interface. The dielectrics were grown on Si by the electron cyclotron resonance plasma method. The stacks were produced by plasma oxidation of the Si surface, resulting in a thin layer of SiOx (PO–SiOx), followed by deposition of the SiN1.55:H layer. Afterwards, the samples were rapid thermally annealed (RTA) at temperatures ranging from 300 to 1000 °C. Some representative samples were studied by Fourier transform infrared spectroscopy to characterize the bonding structure of the SiN1.55:H/PO–SiOx stack and its evolution with the annealing temperature. The results were in good agreement with the well known model for suboxide evolution with the temperature: the formation of highly defective nanocrystalline Si clusters inside a high quality SiO2 matrix. This process takes place for RTA temperatures higher than 700 °C. This model also explains the results derived from C–V measurements, which show improvement of the PO–SiOx/Si interface as a result of the formation of the high quality SiO2 matrix. Additionally, the thermal stability improved with respect to the SiN1.55:H/Si interface, which suffers significant degradation when annealed above 600 °C, while devices made with the stacks are able to hold annealing temperatures up to 900 °C. On the other hand, the formation of nc-Si results in degradation of the reliability of the MIS devices.
Selective growth of TiO2 thin films on Si(100) surfaces by combination of metalorganic chemical vapor deposition and microcontact printing methods
Authors: Kang B.-C. , Lee J.-H. , Chae H.-Y. , Jung D.-Y. , Lee S.-B. , Boo J.-H.
Published: August 2003

We successfully patterned TiO2 thin films by metalorganic chemical vapor deposition (MOCVD) on Si(100) substrates where the surface was first modified by an organic thin film. The organic thin film [octadecyltrichlorosilane (OTS)] of self-assembled monolayers (SAMs) was deposited by microcontact printing. Selective deposition of a 130 nm thick TiO2 film was done on a 300–500 °C surface prepared by MOCVD without any carrier or bubbler gas. Auger electron spectroscopy and x-ray diffraction analyses showed that the deposited TiO2 material was stoichiometric, polycrystalline, and consisted of anatase phase. Alpha-step profile and optical-microscopic images also showed that the boundaries between the OTS SAMs and selectively deposited TiO2 thin film areas are definite and sharp. Capacitance–voltage measurement of a TiO2 thin film yielded a dielectric constant of 29, suggesting possible application to electronic materials.
Synthesis of a material for semiconductor applications: Boron oxynitride prepared by low frequency rf plasma-assisted metalorganic chemical vapor deposition
Authors: Chen G. C. , Lim D.-C. , Lee S.-B. , Boo J.-H. , Kim Y. J. , Hong B. Y.
Published: August 2003

We have synthesized a material, boron oxynitride (BON), deposited as films on Si(100) substrates by low frequency rf-derived plasma-assisted metalorganic chemical vapor deposition (MOCVD), and have studied the electrical and optical properties of these films. The effects of growth conditions such as the flux of the feed gas and growth time on these properties are investigated. Our data show that the electrical resistance decreases with an increase in nitrogen flux and growth time. Amorphous BON thin films grown at relatively low temperatures have higher resistance than microcrystal-containing films deposited at high temperature. Thus by controlling the nitrogen content of the film we can make BON thin films that are either semiconducting or insulating. We also monitored optical emission spectra (OES) during MOMCVD to analyze reaction of the gas phase in the plasma. Based on the OES result we confirm that BON thin films can be prepared under a nitrogen plasma.
Field emission from screen-printed carbon nanotubes irradiated by tunable ultraviolet laser in different atmospheres
Authors: Zhao W. J. , Kawakami N. , Sawada A. , Takai M.
Published: August 2003

A tunable ultraviolet (UV) laser was used to irradiate the screen-printed carbon nanotube (CNT) samples in air and in high vacuum. UV laser lights with wavelengths of 349 and 266 nm from a tunable laser were used with average energy densities of 20.3, 10.2, and 2.25 mJ/cm2, respectively, with different irradiation times. The field emission characteristics of the CNT cathodes irradiated in air were drastically improved, whereas those of the samples irradiated in vacuum were slightly improved. Irradiation by a 266 nm laser light resulted in more obvious improvement in the emission characteristics than that by 349 nm. The maximum emission current density at an applied electric field of 6.2 V /μm at an energy density of 20.3 mJ/cm2 increased from 0.006 to 20.15 mA/cm2 after laser irradiation with a wavelength of 266 nn, and the turn-on field decreased from 3.7 to 1.2 V/μm.
Field emission induced fabrication of nanostructures on Au thin films using a noncontact mode atomic force microscope
Authors: Park Kang-Ho , Kim Jeongyong , Ha Jeong Sook , Song Ki-Bong
Published: August 2003

We report a reliable nanofabrication on Au cluster films using an atomic force microscope (AFM) without the occurrence of tip damage or serious mass transfer between tip and sample. Noncontact mode of AFM equipped with W2C coated tip is used, and the field emission current amounting to ∼500 pA is observed, indicating the noncontact nature of our nanofabrication method. We reproducibly create dots and lines having widths as small as 50 nm. The mechanism is explained by effective local heating due to the low thermal conductivity of granular Au films and by concurrent field induced diffusion of Au clusters.
Nature of electrical contacts in a metal–molecule–semiconductor system
Authors: Hsu J. W. P. , Loo Y. L. , Lang D. V. , Rogers J. A.
Published: August 2003

The effects of the top electrode fabrication method on the transport properties of Au–alkanedithiol–GaAs junctions are examined. We found that diodes made by evaporating Au on 1,8-octanedithiol monolayer behave similarly to Au–GaAs junctions without the molecular layer. The direct Au–GaAs contact dominates the electrical conduction in these evaporated devices despite the presence of the molecular layer. In contrast, Au–dithiol–GaAs diodes made by nanotransfer printing (nTP) exhibit no direct contact between Au and GaAs. Electrical transport in the nTP junctions occurs through the 1,8-octanedithiol insulating layer. The low current levels and the high barrier-height values in the current–voltage results and the exponential energy dependence in the photoreponse yield that are observed in the nTP diodes support this conclusion. Since transport through the molecular layer is much less conductive, even a miniscule region of direct contact between the two electrodes will make it impossible to observe electrical transport through molecules. Fractional amounts of the different types of contacts for each kind of sample were obtained from the experimental results.
Influence of AlN buffer on electronic properties and dislocation microstructure of AlGaN/GaN grown by molecular beam epitaxy on SiC
Authors: Simpkins B. S. , Yu E. T.
Published: August 2003

Electronic and structural properties of AlGaN/GaN heterostructures grown by molecular beam epitaxy on semi-insulating 4H–SiC substrates with and without an initial AlN nucleation layer are studied. Differences in microstructure were examined using scanning capacitance microscopy, which reveals negatively charged dislocations through capacitance variations, conductive atomic force microscopy, used to quantify the density of dislocation-related reverse-bias leakage paths, and x-ray diffraction (XRD). Samples grown without the AlN buffer exhibited features consistent with the commonly observed coalesced island structure, with negatively charged features, commonly associated with dislocations, occurring at the domain boundaries. Samples grown with an AlN buffer layer showed a more random distribution of negatively charged features, much broader symmetric XRD peak widths, and about 1 order of magnitude greater density of localized reverse-bias leakage paths. This difference in microstructure is expected to have significant influence on carrier mobilities and optical efficiencies, and may suggest improved nucleation schemes for device-quality nitride material.
Pulsed laser deposition growth of Fe3O4 on III–V semiconductors for spin injection
Authors: Preisler E. J. , Brooke J. , Oldham N. C. , McGill T. C.
Published: August 2003

We report on the growth of thin layers of Fe3O4 on GaAs and InAs by pulsed laser deposition. It is found that Fe3O4 grows epitaxially on InAs at a temperature of 350 °C. X-ray photoelecton spectroscopy (XPS) studies of the interface show little if any interface reaction resulting in a clean epitaxial interface. In contrast, Fe3O4 grows in columnar fashion on GaAs, oriented with respect to the growth direction but with random orientation in the plane of the substrate. In this case XPS analysis showed much more evidence of interface reactions, which may contribute to the random-in-plane growth.
Structural and magnetic properties of (Ga,Mn)N layers grown on SiC by reactive molecular beam epitaxy
Authors: Ploog K. H. , Dhar S. , Trampert A.
Published: August 2003

Incorporation of Mn into hexagonal GaN does not lead to ferromagnetic semiconductors, as predicted, but to insulating spin glass materials. Structural characterization by high-resolution transmission electron microscopy reveals that samples with Mn content below 10% are uniform ternary alloys, while in samples with higher Mn content we find Mn-rich clusters which are embedded in the (Ga,Mn)N alloy matrix. At low temperature (<10 K) the magnetic behavior of the samples is similar showing antiferromagnetic characteristics with a spin-glass transition. However, the samples with higher Mn content additionally exhibit ferromagnetic properties even far above room temperature. This ferromagnetism originates from Mn-rich clusters in the ternary alloy.
X-ray diffraction study of InGaN/GaN superlattice interfaces
Authors: Kusakabe Kazuhide , Ohkawa Kazuhiro
Published: August 2003

High-resolution x-ray diffractometry was employed to investigate crystallinity and interface properties of InGaN/GaN multiple quantum wells (MQWs) heterostructures grown by metalorganic vapor phase epitaxy on (0001) sapphire substrates. To obtain a complimentary picture, coherent-grown and relaxed InGaN/GaN MQW structures were investigated. In addition, thermal annealing at 1000 °C was subjected to samples to estimate their thermal stability. A structural variation by thermal process was observed only for the coherent-grown InGaN/GaN MQWs due to an interdiffusion of indium and gallium atoms at InGaN/GaN interfaces. Photoluminescence (PL) experiment at room temperature revealed that a postgrowth annealing also affected the optical properties. For relaxed InGaN/GaN MQWs, a reduction of PL intensity was confirmed with a factor of 1/2 while a slight decrease of PL intensity was shown by coherent-grown InGaN/GaN MQWs. The effects of thermal annealing on the structural and optical properties were depended on the strain conditions that the thermal annealing was made the interface profile unclear for coherent-grown InGaN/GaN MQWs, and degraded the emission efficiency for relaxed InGaN/GaN MQWs.
Towards understanding epitaxial growth of alternative high-K dielectrics on Si(001): Application to praseodymium oxide
Authors: Fissel A. , Osten H. J. , Bugiel E.
Published: August 2003

First investigations demonstrate that crystalline Pr2O3 on Si(001) is a promising candidate for highly scaled gate insulators, displaying a sufficiently high-K value of around 30, ultralow leakage current density, good reliability, and high electrical breakdown voltage. Here, we report on molecular beam epitaxial growth of crystalline praseodymium oxide (as Pr2O3 in the bixbyite or manganese oxide structure) on Si(001) substrates. The Pr2O3 was found to grow as (110)-single-crystalline domains, with two orthogonal in-plane orientations. Investigations of the initial growth phase indicate that the occurrence of these domains is due to the nucleation on neighboring terraces with Si dimer rows (2×1 reconstruction) perpendicular to each other. We postulate the formation of a layer consisting of very small Pr2O3 islands on top of the Si dimers in the initial stage of growth. This interface layer acts as a coincidence lattice on which further growth in the (110) orientation can occur. X-ray photoelectron spectroscopy investigations indicate that the formation of the interface layer is accompanied by oxygen accumulation in the interface.
Silicon carbide formation by methane plasma immersion ion implantation into silicon
Authors: An Zhenghua , Fu Ricky K. Y. , Chen Peng , Liu Weili , Chu Paul K. , Lin Chenglu
Published: August 2003

Silicon carbide films were synthesized by methane plasma immersion ion implantation into silicon and their properties were investigated. The molecular ions dissociate upon entry into the sample surface and our simulation results show that the implanted hydrogen peak is located at about twice as deep as the implanted carbon. The films undergo a transformation from hydrogenated amorphous silicon carbide to β-SiC with increasing annealing temperature. The amount of Si–C bonds increases with annealing temperature whereas the C–C bonds change in an opposite manner. At high temperature, a large amount of β-SiC forms and graphitization takes place within the remaining carbon clusters. For the sample annealed at 1300 °C, Si–C bonds are detected by Raman spectroscopy, and our data indicate that β-SiC grains may contribute to the strong photoluminescence behavior.
Polyelectrolyte effects in model photoresist developer solutions
Authors: Prabhu Vivek M. , Jones Ronald L. , Lin Eric K. , Wu Wen-li
Published: August 2003

We demonstrate that the deprotected photoresist poly(4-hydroxy styrene) is a polyelectrolyte when dissolved in aqueous base solutions. This polyelectrolyte effect manifests itself by the well-known monomer–monomer correlations as measured by small-angle neutron scattering. The correlation peak at the finite wave vector is a function of the solution ionic strength and polymer concentration. The weakening of the polyelectrolyte effects with added salts and excess base is also demonstrated. These studies emphasize the role of salt additives and aqueous base concentration and their influence on equilibrium solution properties such as the second virial coefficient and single chain radius of gyration. The fundamental role of these equilibrium properties with respect to the dissolution process is discussed.
High current diamond field emission diode
Authors: Wisitsora-at A. , Kang W. P. , Davidson J. L. , Howell M. , Hofmeister W. , Kerns D. V.
Published: August 2003

This article presents fabrication and characterization results for a diamond field emission diode with high emission current. Techniques such as high-temperature diamond brazing and a special anode with an “indented-edge” design were developed to achieve a microsized vacuum diode with high electron emission current. A diamond field emission diode with a moderately low turn-on electric field of 6 V/μm and a high emission current of ∼22 mA at an electric field of ∼16 V/μm has been achieved. The high total emission current capability of this diamond vacuum diode is attributed to the ability to produce a uniform diamond emitter configuration by the molding method, the utilization of a high-temperature diamond brazing technique and the indented-edge anode design, along with the inherent high thermal conductivity of diamond material. These factors enable the high emission current potential of diamond field emitters to begin to be observed. This work suggests that diamond pyramidal microtip arrays are promising candidates for high emission current cathodes.
Method to enhance atomic-layer deposition of tungsten–nitride diffusion barrier for Cu interconnect
Authors: Sim Hyun Sang , Kim Seong-Il , Kim Yong Tae
Published: August 2003

We have suggested an atomic layer deposition (ALD) method for preparing W–N films on Si and non-Si surfaces by using WF6 and NH3. It is very difficult to deposit W–N films with sequential exposures of WF6 and NH3 because WF6 either reacts with Si quickly due to the catalytic reaction of Si, forming a thick W layer instead of the W–N, or does not adhere to the non-Si surface at 200–400 °C. In this method, during the ALD cycle we have introduced NH3 gas with pulse rf power, resulting in a NH3 pulse plasma that modified Si and SiO2 surfaces to become Si–N and Si–O–N surfaces. With this method, we can obtain a uniformly distributed N concentration in the W–N films deposited on the Si and non-Si surfaces with the deposition rate of ∼2.2 Å/cycle at 350 °C. As a diffusion barrier for the Cu interconnect, high-resolution transmission electronic microscopy reveals that after annealing at 700 °C for 30 min, 22 nm thick W–N successfully prevents Cu diffusion.
Effects of treatment parameters in electric-field-enhanced postexposure bake
Authors: Cheng Mosong , Poppe Jacob , Neureuther Andrew
Published: August 2003

This article demonstrates the improved lithographic performance of Shipley UVIIHS with an electric-field treatment during postexposure bake (PEB) for KrF optical exposures. Linewidth trends with treatment are given and explained through simulation of the acid motion. An upward electric field of approximately 10 V/μm was applied during the first 7 s of PEB to crack the surface inhibition, and thereafter the field polarity was altered every 7 s to enhance the acid-catalyzed reactions. Compared with standard PEB, the electric-field treatment resulted in deeper trenches, wider openings and significant compensation for underexposure. In some cases, electric-field-enhanced PEB led to more profile tapering, indicating that a larger downward direct electric field is needed to enhance the deprotection reactions in the lower half of the resist.
Model parameter extraction for nonlinear Fowler–Nordheim field emission data
Authors: Nicolaescu D. , Nagao M. , Filip V. , Kanemaru S. , Itoh J.
Published: August 2003

The traditional approach of analyzing field emission (FE) data using the “emission area” and “field enhancement” factors is faulty. Instead, the emission current I should be computed through integration of the tunneling current density over the emitter surface, taking into account the local variation of the electric field. As a consequence, FE data represented as Fowler–Nordheim (FN) plots ln(I/V2) versus 1/V are nonlinear regardless of the model used to derive the electric field and the modified FN plot ln(I/V3) versus 1/V is more suitable for representing the FE data in a linear way. In this article, a general approach for extraction of model parameters using wide-range FE data is proposed. A nonlinear least-square fitting procedure is applied to emission data based on the statistical gated emitter/triode model tailored for the particular emitter configuration. Uniform distributions for the emitter radius R and work function φ in the array are considered, although other distributions may be included in the model. Deriving an accurate analytical formula for computing the electric field on the emitter tip is a crucial step for obtaining good modeling results. At least two model parameters from the set comprising R, φ and their dispersion range ΔR, Δφ can thus be derived. Comparison with experimental results is provided.
Optimization and characterization of III–V surface cleaning
Authors: Liu Zhi , Sun Yun , Machuca Francisco , Pianetta Piero , Spicer William E. , Pease R. F. W.
Published: August 2003

Achieving clean surfaces is a major and challenging requirement for the study of surfaces and surface reactions. Nondestructive cleaning is a crucial step in semiconductor manufacturing, growth of materials, and processing. We use photoemission spectroscopy (PES) to systematically study the core and valence band electronic structure of various chemical treatments of InP(100), GaAs(100), and GaN(0001). These surfaces undergo wet chemical cleaning of H2SO2/H2O2/H2O followed by thermal heating. In order to achieve the necessary surface sensitivity and spectral resolution, synchrotron radiation in the energy range of 60–1000 eV is used for PES. In tuning the sulfuric acid based chemistry, we achieve oxygen free GaAs and InP surfaces, as shown in our valence band and core level PES analysis. Furthermore, core level PES shows oxygen coverage of the GaN surface is reduced to less than 0.1 monolayer (ML). The carbon coverage is also reduced dramatically for the III–V surfaces, <10% ML for InP and GaAs and approximately 1% ML for GaN. The chemical reactions and species at different cleaning stages are determined and cleaning mechanisms are proposed. Our study shows that material similarities do not imply exact correlation to the chemical cleaning properties among III–V materials.
Low resistance Ohmic contacts to n-GaN by Ar plasma and forming gas ambient treatments
Authors: Lee C. C. , Lee C. P. , Yeh M. H. , Lee W. I. , Kuo C. T.
Published: August 2003

In this article, a scheme for fabricating low resistance Ohmic contacts to n-GaN was developed. This approach takes advantage of Ar plasma treatment and thermal annealing in forming gas ambient. As a result, the adjustment of Ar flow rate was very effective in improving the contact resistance. After proper Ar plasma treatment, the contact resistance and specific contact resistance of as-deposited Ohmic contacts were reduced to 0.362 Ω mm and 3.9×10−5 Ω cm2, respectively. Low contact resistance (0.103 Ω mm) and specific contact resistance (3.2×10−6 Ω cm2) were obtained after annealing in N2 gas ambient. By performing thermal annealing in forming gas ambient, even lower contact resistance (0.093 Ω mm) and specific contact resistance (2.6×10−6 Ω cm2) were successfully achieved, indicating that the electrical characteristics of Ohmic contacts would not be affected by the effect of hydrogen passivation of dopants in n-GaN.
Development and evaluation of a three‐dimensional hyperthermia applicator with water‐coated antennas (WACOA)
Authors: Nadobny Jacek , Wlodarczyk Waldemar , Westhoff Lothar , Gellermann Johanna , Rau Beate , Mönich Gerhard , Wust Peter
Published: August 2003

A novel twelve‐channel three‐dimensional (3‐D) hyperthermia applicator has been developed and evaluated, which consists of twelve separate WAter COated Antenna (WACOA) modules. The modules are arranged in three transversal antenna rings (sub‐arrays) and are placed into an acrylic applicator frame as cartridge‐like elements in a staggered arrangement. The operating frequency is 100 MHz. For the design of the applicator, the finite‐difference time‐domain (FDTD) method was used. The applicator's dimensions allow its placement into the gantry of a magnetic resonance (MR) tomograph. The WACOA modules are designed as MR‐compatible specially shaped metallic cylindrical dipole structures that are placed into hermetically closed water‐filled cassettes. Due to the design of the dipole structures, only a conventional coaxial feed circuitry is needed, and no external impedance matching networks are necessary. Instead, fine on‐line impedance matching is realized using adjustable tuning rods and matching rings, both elements being parts of the radiating antenna structure. Experimental and numerical evaluations demonstrate a good stability of impedance matching, a low inter‐channel coupling of less thanand a good ability of field pattern steering.
Angular correction in reconstruction of electron spectra from depth dose distributions
Authors: Chvetsov Alexei V. , Sandison George A.
Published: August 2003

Techniques for reconstruction of electron spectra from the depth‐dose curves used to date have ignored the angular distribution of incident electrons scattered at large angles. The approximation adopted is to employ a database of monoenergetic depth‐dose curves generated for normal incidence of electrons at the surface. This approximation is acceptable for direct electrons with small angular spread. However, electrons scattered from the treatment head and collimating system may have large average angles of incidence which affects the depth‐dose distribution significantly at shallow depths by increasing energy deposition close to the surface. We show that ignoring the electron incident angular distribution leads to systematic errors in the low energy region of reconstructed electron spectra. We propose a simple 1‐D model to correct for these systematic errors using only electron angular distribution at the central beam axis. This model provides reconstructed spectra in excellent agreement with Monte Carlo simulation in the low energy region.
More Than a Hot Flash
Authors: Davenport R. John
Published: August 2003

After humans pass their reproductive primes, estrogen and testosterone concentrations wane. The decline appears to have far-reaching health consequences--on the brain, bones, muscle, and the circulatory system. As scientists tease apart the ramifications of sex hormone loss, they hope to fine-tune therapeutic replacement of those hormones.
The Quest for a General Theory of Aging and Longevity
Authors: Gavrilov Leonid A. , Gavrilova Natalia S.
Published: August 2003

Extensive studies of phenomena related to aging have produced many diverse findings, which require a general theoretical framework to be organized into a comprehensive body of knowledge. As demonstrated by the success of evolutionary theories of aging, quite general theoretical considerations can be very useful when applied to research on aging. In this theoretical study, we attempt to gain insight into aging by applying a general theory of systems failure known as reliability theory. Considerations of this theory lead to the following conclusions: (i) Redundancy is a concept of crucial importance for understanding aging, particularly the systemic nature of aging. Systems that are redundant in numbers of irreplaceable elements deteriorate (that is, age) over time, even if they are built of elements that do not themselves age. (ii) An apparent aging rate or expression of aging is higher for systems that have higher levels of redundancy. (iii) Redundancy exhaustion over the life course explains a number of observations about mortality, including mortality convergence at later life (when death rates are becoming relatively similar at advanced ages for different populations of the same species) as well as late-life mortality deceleration, leveling off, and mortality plateaus. (iv) Living organisms apparently contain a high load of initial damage from the early stages of development, and therefore their life span and aging patterns may be sensitive to early-life conditions that determine this initial damage load. Thus, the reliability theory provides a parsimonious explanation for many important aging-related phenomena and suggests a number of interesting testable predictions. We therefore suggest adding the reliability theory to the arsenal of methodological approaches applied to research on aging.
Phylogenetics and the Cohesion of Bacterial Genomes
Authors: Daubin Vincent , Moran Nancy A. , Ochman Howard
Published: August 2003

Gene acquisition is an ongoing process in many bacterial genomes, contributing to adaptation and ecological diversification. Lateral gene transfer is considered the primary explanation for discordance among gene phylogenies and as an obstacle to reconstructing the tree of life. We measured the extent of phylogenetic conflict and alien-gene acquisition within quartets of sequenced genomes. Although comparisons of complete gene inventories indicate appreciable gain and loss of genes, orthologs available for phylogenetic reconstruction are consistent with a single tree.
Heterochromatin and Epigenetic Control of Gene Expression
Authors: Grewal Shiv I. S. , Moazed Danesh
Published: August 2003

Eukaryotic DNA is organized into structurally distinct domains that regulate gene expression and chromosome behavior. Epigenetically heritable domains of heterochromatin control the structure and expression of large chromosome domains and are required for proper chromosome segregation. Recent studies have identified many of the enzymes and structural proteins that work together to assemble heterochromatin. The assembly process appears to occur in a stepwise manner involving sequential rounds of histone modification by silencing complexes that spread along the chromatin fiber by self-oligomerization, as well as by association with specifically modified histone amino-terminal tails. Finally, an unexpected role for noncoding RNAs and RNA interference in the formation of epigenetic chromatin domains has been uncovered.
Le terrane de Slide Mountain (Cordillères canadiennes) : une lithosphère océanique marquée par des points chauds
Authors: Tardy M , Lapierre H , Bosch D , Cadoux A , Narros A , Struik L C , Brunet P
Published: August 2003

The Slide Mountain Terrane consists of Devonian to Permian siliceous and detrital sediments in which are interbedded basalts and dolerites. Locally, ultramafic cumulates intrude these sediments. The Slide Mountain Terrane is considered to represent a back-arc basin related to the Quesnellia Paleozoic arc-terrane. However, the Slide Mountain mafic volcanic rocks exposed in central British Colombia do not exhibit features of back-arc basin basalts (BABB) but those of mid-oceanic ridge (MORB) and oceanic island (OIB) basalts. The N-MORB-type volcanic rocks are characterized by light rare-earth element (LREE)-depleted patterns, La/Nb ratios ranging between 1 and 2. Moreover, their Nd and Pb isotopic compositions suggest that they derived from a depleted mantle source. The within-plate basalts differ from those of MORB affinity by LREE-enriched patterns; higher TiO2, Nb, Ta, and Th abundances; lower εNdvalues; and correlatively higher isotopic Pb ratios. The Nd and Pb isotopic compositions of the ultramafic cumulates are similar to those of MORB-type volcanic rocks. The correlations between εNdand incompatible elements suggest that part of the Slide Mountain volcanic rocks derive from the mixing of two mantle sources: a depleted N-MORB type and an enriched OIB type. This indicates that some volcanic rocks of the Slide Mountain basin likely developed from a ridge-centered or near-ridge hotspot. The activity of this hotspot is probably related to the worldwide important mantle plume activity that occurred at the end of Permian times, notably in Siberia.
Paleoweathered surfaces on granitoids of southern Nova Scotia: paleoenvironmental implications of saprolites
Authors: O'Beirne-Ryan Anne Marie , Zentilli Marcos
Published: August 2003

In addition to minor postglacial weathering effects, a complex history of three distinct weathering events has been identified within the granitoids of the South Mountain Batholith of southwestern Nova Scotia. Weathering prior to deposition of Carboniferous strata produced a well-developed saprolite, an in situ weathering profile. Feldspars and micas are altered to clay minerals, and only weak preservation of granitic texture is evident. Subsequent burial and relithification have masked the original weathering mineralogy in this horizon. Beneath Triassic clastic sedimentary rocks is a thick horizon (30 m) of weathered granitoid. This argillaceous saprolite with relict biotite exhibits an intensifying-upward weathering zonation typical of weathered horizons, with increased proportions of clay minerals at the upper levels. A younger arenaceous saprolite of pre-Pleistocene age can be found beneath glacial till at a number of locations throughout the batholith. These paleoweathered horizons attest to relatively warmer climates at several times in the geologic past. Their patterns of preservation imply some were much thicker and more widely distributed before partial erosion. The eroded materials may have formed the sediment for quartz sand in the Carboniferous sequence, clay and silica sand deposits of Cretaceous age, and a sandy till of Pleistocene age. The remnants preserved in situ today serve as significant conduits for water, and the geochemistry, mineralogy, and textures of these weathered profiles may enhance mobility and migration of U and Rn. These weathered horizons are mechanically weak, and the possibility of their presence should be considered when selecting construction and waste-disposal sites.
Benthic microbial utilization of differential dissolved organic matter sources in a forest headwater stream
Authors: Kreutzweiser David P , Capell Scott S
Published: August 2003

Streamside mesocosm experiments were conducted in a low-order forest watershed to directly examine responses by microbial communities on standardized substrates to different terrestrial and aquatic sources of dissolved organic matter (DOM). Community respiration (oxygen uptake), microbial density (colony-forming units on agar plates), leaf decomposition, and community metabolic profiles (metabolism patterns in sole carbon source utilization assays) were measured. Stream benthic microbial communities responded immediately and positively to increases in terrestrially derived DOM. Respiration activity and density estimates increased significantly, but there was no significant change in community metabolic profile. Responses were greater to DOM extracted from upper soil horizons than from deeper soils. Community respiration and bacterial abundance also increased in response to an aquatic DOM source, but were accompanied by a significant change in community metabolic profiles. Results provide direct experimental evidence that benthic microbial communities of forest headwater streams are able to rapidly utilize terrestrial DOM.
A chronosequence of wood decomposition in the boreal forests of Russia
Authors: Yatskov Mikhail , Harmon Mark E , Krankina Olga N
Published: August 2003

Coarse woody debris (CWD), represented by logs and snags >10 cm in diameter and >1 m in length, was sampled at eight sites in Russian boreal forests to determine the specific density of decay classes and decomposition rates. Tree species sampled included Abies siberica Ledeb., Betula pendula Roth., Betula costata Trautv., Larix siberica Ledeb., Larix dahurica Turcz., Picea abies (L.) Karst., Picea obovata Ledeb., Picea ajanensis Fisch., Pinus koraiensis Sieb. et Zucc., Pinus siberica Ledeb., Pinus sylvestris L., and Populus tremula L. The mean densities for decay clas ses 1 through 5 ranged from 0.516 to 0.084 g·cm3, respectively. Annual decomposition rates varied among the species, and for logs, decomposition rates ranged from 4.2 to 7.8% for B. pendula, 2.6 to 4.9% for Picea spp., 2.7 to 4.4% for Pinus sylvestris, 1.5 to 3.1% for Larix spp., and 1.5 to 1.9% for Pinus koraiensis and Pinus siberica. Logs decomposed faster than snags. Among the sites examined, temperature and precipitation did not correlate with decomposition rates, which is consistent with other studies in the boreal region. Globally, a positive correlation between decomposition and mean annual temperatures was found, with decay-resistant trees less responsive than those with low decay resistance.
Growth recovery in young, plantation white spruce following artificial defoliation and pruning
Authors: Piene Harald
Published: August 2003

Defoliation by the spruce budworm (Choristoneura fumiferana (Clem.)) was simulated by artificially defoliating trees in a plantation of 12-year-old white spruce (Picea glauca (Moench) Voss) over a 2-year period and then allowing the trees to recover for 3 years. Four treatments were applied: control (C); removal of 50% of the current-year foliage (50); removal of all current-year shoots (100P); and removal of all current-year shoots and some older foliage age-classes (100P+). All treatments increased shoot production. Trees in the 100P treatment completely recovered their foliage mass after 1 year, but trees in the 50 treatment were still affected after 3 years of recovery. Trees in the 100P+ treatment showed poor recovery rates in foliage mass. Only the trees in the 50 treatment completely recovered height growth. After 2 years of defoliation, specific volume increment was reduced by 21.3, 58.1, and 75.3% for the 50, 100P, and 100P+ treatments, respectively. After 3 years of recovery, specific volume increment in the 50 treatment recovered completely, while the 100P and 100P+ treatments were reduced by 34.2 and 79.9%, respectively. Because of the release of suppressed buds following both needle loss only and shoot loss, white spruce may be a reforestation candidate for areas having a high probability of budworm outbreaks.
Several Necessary Conditions for the Evolution of Complex Forms of Life in an Artificial Environment
Authors: Suzuki Hideaki , Ono Naoaki , Yuta Kikuo
Published: August 2003

In order for an artificial life (Alife) system to evolve complex creatures, an artificial environment prepared by a designer has to satisfy several conditions. To clarify this requirement, we first assume that an artificial environment implemented in the computational medium is composed of an information space in which elementary symbols move around and react with each other according to human-prepared elementary rules. As fundamental properties of these factors (space, symbols, transportation, and reaction), we present ten criteria from a comparison with the biochemical reaction space in the real world. Then, in the latter half of the article, we take several computational Alife systems one by one, and assess them in terms of the proposed criteria. The assessment can be used not only for improving previous Alife systems but also for devising new Alife models in which complex forms of artificial creatures can be expected to evolve.
Speculum of Being Two
Authors: Giardini Federica
Published: August 2003

Nearly 30 years after the first edition of Speculum, Irigaray's present work can be read from a different position. Accordingly, the main political and civil concerns of the author are examined in their relation to a theory of sexual difference. A new insight can emerge, as women realize that their own position is also made by Irigaray's work. This produces new configurations between politics and theory, embodied experiences and relations and their formalization, in a move that is both a rooting in and a departure from Irigaray's work.
Becoming Woman: Or Sexual Difference Revisited
Authors: Braidotti Rosi
Published: August 2003

This article revisits Irigaray's theory of sexual difference in the light of more contemporary developments in terms of nomadic becomings and non-unitary subjectivity, especially in Deleuze. It defends the notion of embodied materiality on philosophical grounds, by linking it to the issues of power, access, hegemony and exclusion, which are central to post-structuralism. Through a detailed analysis of the sexual politics of difference feminism, the author argues for a non-reactive redefinition of the feminine as a project of becoming, and connects it firmly to feminist discussions about gender and queer theory.
Women and Men in the Project of Reform
Authors: Stewart Debra W. , Siemienska Renata , Sprinthall Norman
Published: August 2003

Are there politically relevant differences between female and male local government officials in postcommunist Poland? In structured interviews with 485 local officials in two provinces, the authors explored differences in terms of ethical reasoning and attitudes toward anomalous groups. Women more strongly favored a model of ethical reasoning characterized by a concern for abstract principles of social cooperation than did their male counterparts. On attitudes toward anomalous groups, women were more temperate than men on the issues of participation by former communists in contemporary government and men had a less favorable view than women on participation of women in politics generally. Based on this study, gender may make a difference in the practice of government in contemporary Poland.
The New Structuralism in Organizational Theory
Authors: Lounsbury Michael , Ventresca Marc
Published: August 2003

Over the past decade, a new structuralism has begun to emerge in organizational theory. This exciting new research program draws inspiration from the social structural tradition in sociology, but extends that tradition by more broadly conceptualizing social structure as comprised of broader cultural rules and meaning systems as well as material resources—revealing the subtleties of both overt and covert power. Building on the insights of Bourdieu and related work in social theory and cultural sociology, new structuralist empirical research focuses on concrete manifestations of culture in everyday practice and has pioneered the measurement of cultural aspects of social structure using a variety of relational methods. In this essay, we revisit mid-century social structural approaches to organizations, review the development of organization theory as a management subfield that increasingly focused on instrumental exchange, highlight key aspects of the new structuralism in organizational theory, and discuss promising new research directions.