bluuebunny/crossref_metadata_2025 huggingface dataset:
Hepatic Resection for Hepatocellular Carcinoma with Obstructive Jaundice due to Biliary Tumor Thrombi
Authors: Yeh Chun‐Nan , Jan Yi‐Yin , Lee Wei‐Chen , Chen Miin‐Fu
Published: April 2004

Hepatocellular carcinoma (HCC) with obstructive jaundice due to biliary tumor thrombi is uncommon, and few studies have examined the outcome of hepatectomy for HCC with this unusual entity. This study examined the clinicopathologic factors influencing the outcomes of 17 HCC patients with obstructive jaundice due to biliary tumor thrombi undergoing hepatectomy. The clinical features of 17 HCC patients with obstructive jaundice due to biliary tumor thrombi (group A) undergoing hepatectomy from 1986 to 1998 were reviewed. The clinical features and factors influencing the outcome of 555 HCC patients without biliary tumor thrombi (group B) undergoing hepatectomy were used for comparison. Of 572 patients with surgically resected HCCs, 17 (3.0%) were classified into group A. Right upper quadrant pain, physical signs of jaundice, low albumin level, elevated bilirubin level, small tumor size, more vascular invasion, and tumor rupture were characteristic of group A patients. Multivariate stepwise logistic regression analysis revealed no independently significant factor differentiating group A patients from group B patients. The disease‐free survival was similar between the group A and B patients, although group B patients exhibited significantly better overall survival (p = 0.014). Vascular invasion may adversely influence overall survival in group A patients undergoing hepatic resection (p = 0.0709). When feasible, hepatic resection is the preferred treatment for HCC patients with obstructive jaundice due to biliary tumor thrombi. It can achieve a disease‐free survival comparable to that of HCC patients without biliary tumor thrombi. However, HCC patients with biliary tumor thrombi had significantly worse overall survival than did those without biliary tumor thrombi, especially those with concomitant vascular invasion.
Procedures of Choice for Resection of Primary and Recurrent Liver Metastases from Colorectal Cancer
Authors: Tanaka Kuniya , Shimada Hiroshi , Ohta Mitsuyoshi , Togo Shinji , Saitou Shuji , Yamaguchi Shigeki , Endo Itaru , Sekido Hitoshi
Published: April 2004

Although liver resection offers the only realistic chance of cure for patients with liver metastases from colorectal cancer, no consensus exists as to the procedure of choice for managing these tumors. Data from 193 patients who underwent hepatectomy for liver metastases from colorectal cancer and 26 of 193 patients who underwent repeat hepatectomy for recurrent metastases were collected. The suitability of resection was evaluated retrospectively based on known risk factors for recurrence and patterns of recurrence. On multivariate analysis, a positive surgical margin (SM+) was the only risk factor for recurrence after the initial resection (p < 0.01). SM+ (p < 0.01) and nonanatomic resection (p < 0·05) that was less than a sectionectomy (p < 0.05) were risk factors for recurrence after repeat hepatectomy. Multiple tumors (four or more) was the most common pattern of recurrence after initial hepatectomy, and recurrence close to the line of resection was most common after repeat hepatectomy. Based on tumor doubling times, recurrence after initial hepatectomy seemed to originate from the primary colorectal lesion, whereas recurrence after repeat hepatectomy was derived from a hepatic metastasis. Retrospective analysis suggests that hepatectomy with clear surgical margins is more important than anatomic resection for initial hepatectomy, and at least sectionectomy is necessary for repeat hepatectomy.
Recovery of Bone Mineral Density in 126 Patients after Surgery for Primary Hyperparathyroidism
Authors: Nordenström Erik , Westerdahl Johan , Bergenfelz Anders
Published: April 2004

Primary hyperparathyroidism (pHPT) is associated with increased fracture risk and decreased bone mass. The recovery of bone mass after surgery varies; therefore tests that predict the increase in bone mass after parathyroidectomy would be desirable. Preoperatively and at 1 year after surgery bone mineral content (BMC) in the distal radius and bone mineral density (BMD) in the lumbar spine and hip, as well as biochemical variables, were measured in 126 pHPT patients (95 women, 31 men). The mean ± SD age of the patients was 63 ± 15 years. The mean ± SD serum calcium level was 2.78 ± 0.16 mmol/L. Altogether, 60% of the patients had a low oral calcium intake, and 18% had a 25‐hydroxyvitamin D3 deficiency. Preoperatively, postmenopausal women had lower Z‐scores for BMD in the hip (p < 0.001) and lumbar spine (p < 0.05) than did premenopausal women. One year after surgery the bone density had increased in about 50% of the patients. The multiple logistic regression analysis showed that there was a weak association between the change in BMD in the hip, the serum 1,25‐dihydroxyvitamin D3 level (p < 0.05), and renal function (p < 0.05), respectively. We concluded that about 50% of patients have increased bone mass after pHPT surgery, but the increase in the bone density is difficult to predict for the individual patient. Because many pHPT patients have low oral calcium intake and a vitamin D deficiency, it would be of interest to evaluate the role of postoperative calcium/vitamin D supplements.
Distribution patterns of metapopulation determined by Allee effects
Authors: Hui Cang , Li Zizhen
Published: April 2004

The Allee effect, the social dysfunction and failure to mate successfully when population density falls below a certain threshold, is one of the most important phenomena in ecology that can profoundly affect metapopulation persistence. We have developed a continuous dynamic model by pair approximation and two derived spatial lattice models to describe the influences of Allee effects on the distribution and dynamics of metapopulation. Analytical results of pair approximation show that the initial global stable equilibrium of metapopulation size turns into a local stable equilibrium with Allee effects and sensitivity to the initial situations that can incur a threshold phenomenon in dynamics. When the intensity of the Allee effect varies within a certain range, a new positive local stable equilibrium appears. This new equilibrium has the same local intensity as the initial one and a smaller metapopulation size. However, a metapopulation with a too strong Allee effect is doomed. Simulation results from the lattice models reinforce these findings and show that the new equilibrium forms a static distribution border in space. Hence, an Allee effect with moderate intensity can incur three distribution patterns that are sensitive to the initial metapopulation size and the spatial configuration of local populations: aggregation, circumscription and extinction. The pattern of circumscription may be a new explanation for the species’ current distributional range. The relationships between distribution patterns (such as random, uniform, aggregation, circumscription and extinction) and other factors (such as mean‐field assumption, local interaction, demographic stochasticity and Allee effect) are also discussed.
VICARIOUS LIABILITY FOR VIOLENT EMPLOYEES
Authors: Weekes Robert
Published: April 2004

A series of recent cases have extended the liability of employers for

intentional wrongdoing by their employees. The article explores the

implications of these decisions, arguing that they evidence the revival of

the “master’s tort” theory of vicarious liability, and have the potential

for a further, undesirable, widening of liability rules.
Analytic continuation of the Birkhoff–Rott equation in complex-time domain
Authors: SAKAJO TAKASHI
Published: April 2004

A vortex sheet is a surface across which the velocity field of incompressible and inviscid flows has a jump discontinuity. Mathematical and numerical studies reveal that a two-dimensional vortex sheet, which is governed by the Birkhoff–Rott equation, acquires a singularity in finite time without forming rolling-up spiral. On the other hand, numerical computation of a regularized Birkhoff–Rott equation shows that the vortex sheet evolves into a rolling-up doubly branched spiral. Because of the finite-time singularity, it is impossible to regard the rolling-up spiral as a solution of the Birkhoff–Rott equation as long as time is real. However, it may be possible to analytically continue the equation to the spiral along a path to get around the singularity in complex-time plane. In the present article, we consider singularities in complex-time plane for the regularized Birkhoff–Rott equation by numerical means. Distribution of the complex singularities and their limiting behaviour indicate that it is absolutely impossible to perform analytic continuation in complex-time domain to the spiral solution. Furthermore, we propose a simple model of a doubly branched spiral and investigate it mathematically. The model is successful in approximating the rolling-up motion of the vortex sheet. Comparing the vortex-sheet motion with the model indicate that the doubly branched spiral with infinite windings at the centre could be a solution of the Birkhoff–Rott equation beyond the singularity time.
Motion of bubbles towards the boundary for the Cahn–Hilliard equation
Authors: ALIKAKOS N. D. , FUSCO G. , KARALI G.
Published: April 2004

In this work we describe some aspects of the dynamics of the Cahn–Hilliard equation. In particular, we consider the dynamics of ‘bubble’ solutions that is spherical interfaces which move superslowly towards the boundary without changing their shape. We show for the Cahn–Hilliard that the bubble drifts towards the closest point on the boundary provided it is sufficiently small. This is contrasted with the related mass conserving Allen–Cahn equation where size is not an issue.
Hele–Shaw flow with a point sink: generic solution breakdown
Authors: CUMMINGS L. J. , KING J. R.
Published: April 2004

Recent numerical evidence [8, 28, 33] suggests that in the Hele–Shaw suction problem with vanishingly small surface tension $\gamma$, the free boundary generically approaches the sink in a wedge-like configuration, blow-up occurring when the wedge apex reaches the sink. Sometimes two or more such wedges approach the sink simultaneously [33]. We construct a family of solutions to the zero-surface tension (ZST) problem in which fluid is injected at the (coincident) apices of an arbitrary number $N$ of identical infinite wedges, of arbitrary angle. The time reversed suction problem then models what is observed numerically with non-zero surface tension. We conjecture that (for a given value of $N$) a particular member of this family of ZST solutions, with special complex plane singularity structure, is selected in the limit $\gamma\,{\to}\,0$.
Influence of volcanic activity at Mauna Loa, Hawaii, on earthquake occurrence in the Kaoiki Seismic Zone
Authors: Walter Thomas R. , Amelung Falk
Published: April 2004

The Kaoiki Seismic Zone at Mauna Loa's east flank is the locus of some of the largest earthquakes on Hawaii. In this paper we examine the link of those earthquakes to volcanic activity at Mauna Loa. We calculate the changes of Coulomb failure stress along the Kaoiki faults for typical volcanic events at Mauna Loa volcano. We simulate the dislocation associated with the 1950 eruption at the Southwest Rift Zone, the 1984 eruption at the Northeast Rift Zone, and the subsequent replenishment of the shallow magma chamber. Our model calculations suggest that the earthquake occurrence in the Kaoiki Seismic Zone strongly depends on the type of volcanic activity. Kaoiki earthquakes are encouraged by dike intrusions into the Southwest Rift Zone of Mauna Loa, but discouraged by dike intrusion into the Northeast Rift Zone. Inflation of a shallow magma chamber may again encourage earthquakes.
Timing of volcanism along the northern East Pacific Rise based on paleointensity experiments on basaltic glasses
Authors: Carlut J. , Cormier M.‐H. , Kent D. V. , Donnelly K. E. , Langmuir C. H.
Published: April 2004

Samples from two adjacent and contrasting ridge segments along the East Pacific Rise were measured for their magnetic paleointensity in order to further explore the possibilities of dating very young volcanic samples using secular variations in the Earth's magnetic field. The ridge segment north of the Orozco transform fault (15°22′–16°20′N) is the shallowest and broadest along more than 5000 km of the East Pacific Rise, whereas the adjacent segment to the north (16°16′–18°N) has a “typical” morphology for its intermediate spreading rate. Both ridge segments were densely sampled during the PANR01MV cruise and 36 samples of axial lava flows, consisting mainly of glasses from the rims of the flows and some fragments of lobate basalts, were selected from this collection for paleointensity experiments. The Coe version of the Thellier double‐heating procedure (in air) was used. Twenty‐seven units provide internally consistent paleointensity estimates leading to precise estimates of the paleofield, which range between 8 μT and 57 μT. Comparisons with reference paleointensity curves compiled from subaerial flows, archeomagnetic data and sedimentary records projected to the sampling site coordinates show that the measured values can be used to constrain the volcanic history of the ridge segments over the past few thousand years. A good agreement was found between apparent “freshness” of the glasses, the geochemistry of the lavas, and their magnetic paleointensity values. The inflated southern segment seems characterized by recent activities as indicated by numerous flows with paleointensities clustering around today's value (39 μT) or around the high values typical of 2000–3000 years ago (∼55 μT). We interpret this distribution to indicate the flooding by effusive lava flows of the entire axial plateau some 2000–3000 years ago, followed by a volcanic phase producing smaller volume lava flows confined to the innermost 200 m of the ridge axis. The northern ridge segment is characterized by dispersed paleointensity values consistent with a series of small eruptions of diverse ages. Samples collected at the tips of both ridge segments across the 16°20′N axial discontinuity have the lowest paleointensities and are thus thought to be significantly older, consistent with models advocating reduced magmatism near ridge axis discontinuities. This study demonstrates the strong potential of paleointensity measurements as a tool to help constrain volcanic history at ridge axes.
Stratigraphic correlations between the European Project for Ice Coring in Antarctica (EPICA) Dome C and Vostok ice cores showing the relative variations of snow accumulation over the past 45 kyr
Authors: Udisti R. , Becagli S. , Castellano E. , Delmonte B. , Jouzel J. , Petit J. R. , Schwander J. , Stenni B. , Wolff E. W.
Published: April 2004

High‐resolution chemistry analysis and electrical measurements performed on two ice core records (European Project for Ice Coring in Antarctica (EPICA) Dome C and Vostok) spanning the last 45 kyr allow stratigraphic correlations by matching volcanic events. Several common events were identified along the two ice cores on the basis of acidity and sulphate spikes in snow layers. Timescales were matched through comparison with isotope (δD) profiles and using the Antarctic cold reversal (ACR) minimum, a 10Be peak, and a dust spike as temporal checkpoints. Ratios of relative snow accumulation at the two sites during the Holocene, in the glacial‐interglacial transition and in the last part of the glacial period, were reconstructed by finding the best fit between Dome C and Vostok depths recording the same events. After accounting for thinning of the layers as they are buried within the glacier, the Dome C‐Vostok accumulation ratio, expected to be roughly constant from the conventional accumulation‐temperature‐isotope approach, is 1.12 for the glacial period but increases to as much as 1.44 for a large part of the Holocene. Glaciological effects, mainly related to the geographic origin of the Vostok ice along the Ridge B‐Vostok axis, can account for only a minor fraction of this change. Instead, we argue that accumulation variability between the cores stems from differential changes in atmospheric circulation during these respective climatic periods at the two sites. Regional changes in atmospheric circulation are proposed with a negative anomaly in Dome C, a positive accumulation anomaly in Vostok, or a combination of both during glacial climate. Our approach may help to improve ice core dating by: (1) revealing anomalies in accumulation‐rate estimation based on the classical thermodynamic method and (2) supporting the necessity to take into account contributions due to changes in atmospheric circulation processes.
Minimum energy as the general form of critical flow and maximum flow efficiency and for explaining variations in river channel pattern
Authors: Huang He Qing , Chang Howard H. , Nanson Gerald C.
Published: April 2004

Although the Bélanger‐Böss theorem of critical flow has been widely applied in open channel hydraulics, it was derived from the laws governing ideal frictionless flow. This study explores a more general expression of this theorem and examines its applicability to flow with friction and sediment transport. It demonstrates that the theorem can be more generally presented as the principle of minimum energy (PME), with maximum efficiency of energy use and minimum friction or minimum energy dissipation as its equivalents. Critical flow depth under frictionless conditions, the best hydraulic section where friction is introduced, and the most efficient alluvial channel geometry where both friction and sediment transport apply are all shown to be the products of PME. Because PME in liquids characterizes the stationary state of motion in solid materials, flow tends to rapidly expend excess energy when more than minimally demanded energy is available. This leads to the formation of relatively stable but dynamic energy‐consuming meandering and braided channel planforms and explains the existence of various extremal hypotheses.
Upscaling hydraulic conductivity by means of entropy of terrain curvature representation
Authors: Niedda Marcello
Published: April 2004

The effects of topography resolution on upscaling point‐scale processes and parameters on watershed hydrology numerical routing are investigated. Parsimonious continuous simulation was applied to two forested catchments with shallow and sloping soils, one medium‐sized (123 km2) and one small‐sized (4.5 km2), where saturation excess runoff prevails. The computed discharge showed highest sensitivity to spatial resolution, due to smoothing effects during aggregation of the digital elevation model caused by a coarse grid. The loss of information content of terrain curvature as consequence of the averaging procedure was related to the amplification factor required for the soil hydraulic conductivity to compensate the resulting retardation of the runoff hydrograph. A scaling relation has been developed that links soil hydraulic conductivity measured at the point‐scale with that required at the typically much coarser modeling scale. The entropy concept for the measurement of information loss could be a good index for parameter rescaling of other basins where the terrain curvature is similarly scale‐dependent.
Oceanic export of relict carbon by small mountainous rivers
Authors: Komada Tomoko , Druffel Ellen R. M. , Trumbore Susan E.
Published: April 2004

Small mountainous rivers (SMR) export globally significant quantities of particulate organic carbon (POC) to the oceans, the composition of which may be distinct from POC delivered by major world rivers. To better constrain the controls on the Δ14C signatures of POC suspended in SMR, we examined the temporal variability of Δ14C and δ13C of POC suspended in the Santa Clara River, a SMR in California, USA. Our data suggest mixing of bedrock and contemporary soil, which can be applied to other SMR in contrasting climate zones and with varying land cover. The data also suggest that chemical denudation is incomplete, but effective to the point that only the most recalcitrant moieties of the parent POC are exported. We further propose sediment yield as a potentially useful proxy for the Δ14C of riverine POC. A preliminary analysis of the existing data for a spectrum of rivers supports this hypothesis.
On smoothing potentially non‐stationary climate time series
Authors: Mann Michael E.
Published: April 2004

A simple approach to the smoothing of a potentially non‐stationary time series is presented which provides an optimal choice among three alternative, readily motivated and easily implemented boundary constraints. This method is applied to the smoothing of the instrumental Northern Hemisphere (NH) annual mean and cold‐season North Atlantic Oscillation (NAO) time series, yielding an objective estimate of the smoothed decadal‐scale variations in these series including long‐term trends.
An extreme anomaly in stratospheric ozone over Europe in 1940–1942
Authors: Brönnimann S. , Luterbacher J. , Staehelin J. , Svendby T. M.
Published: April 2004

Reevaluated historical total ozone data reveal extraordinarily high values over several European sites in 1940–1942, concurrent with extreme climatic anomalies at the Earth's surface. Using historical radiosonde data, reconstructed upper‐level fields, and total ozone data from Arosa (Switzerland), Dombås, and Tromsø (Norway), this unusual case of stratosphere‐troposphere coupling is analyzed. At Arosa, numerous strong total ozone peaks in all seasons were due to unusually frequent upper troughs over central Europe and related ozone redistribution in the lower stratosphere. At the Norwegian sites, high winter total ozone was most likely caused by major stratospheric warmings in Jan./Feb. 1940, Feb./Mar. 1941, and Feb. 1942. Results demonstrate that the dynamically driven interannual variability of total ozone can be much larger than that estimated based on the past 25–40 years.
Extreme cross‐shelf transport induced by eddy interactions southwest of Iberia in winter 2001
Authors: Peliz Álvaro , Santos A. Miguel P. , Oliveira Paulo B. , Dubert Jesus
Published: April 2004

Unusually high for winter time satellite‐derived chlorophyll‐a concentrations southwest of Iberia were detected in February 2001. Analysis of satellite data (SeaWiFS, AVHRR, sea level anomaly from TOPEX/POSEIDON and ERS) revealed that the phytoplankton‐rich waters were associated with an extremely long filament (up to 400 km) induced by eddy‐eddy interactions occurring in the vicinity of the continental margin. The eddy interaction occurred simultaneously with an accumulation of coastal fresh water over the shelf. The event promoted an export of biogenic material offshore along with an estimated shelf flushing in a 5 to 6 day period that resulted in a dramatic offshore growth of phytoplankton. A description of the event is given and preliminary estimates of the cross‐shelf transport are presented and discussed.
Influence of hydrogen on the stability of iron phases under pressure
Authors: Skorodumova N. V. , Ahuja R. , Johansson B.
Published: April 2004

The influence of hydrogen presence on the stability of iron phases (bcc, hcp, dhcp, fcc, simple cubic) in a wide pressure interval at 0 K has been studied by the first‐principles projector augmented‐wave (PAW) method. Hydrogen is shown to occupy different interstitial lattice positions depending on the type of structure and pressure. An introduction of hydrogen impurities (∼6 at. %) leads to a stabilization of the close‐packed iron structures, shifting the calculated pressure of the bcc‐hcp transition from ∼9 GPa for pure iron to 7 GPa for Fe (6 at. % H). This tendency is further enhanced in the iron hydride structures. The iron hydrides in the close‐packed structures (hcp, dhcp, fcc) are essentially degenerate in energy and found to be most stable in the whole pressure range.
Routes of inoculation and the immune response to a resolving genital flavivirus infection in a novel murine model
Authors: Burke Shannon A , Wen Li , King Nicholas JC
Published: April 2004

The prolonged, abnormal immune response patterns produced by many sexually transmitted viruses have been intensively studied. Because normal antiviral immune responses in the vagina are less well‐defined, we developed a resolving murine model using vaginal inoculation with the flavivirus, West Nile virus. Infection resulted in 12% mortality, with sterile protective immunity to vaginal or systemic re‐challenge. B‐cell numbers increased in the vaginal mucosa from day 1−7 after primary infection, while similar increases in B220+, CD4+ and CD8+ lymphocytes in the draining lymph node were delayed by 48 h. By day 4 postinfection, a MHC‐II+ dendritic cell population became depleted from the stroma and formed aggregates below the basement membrane at points of demonstrable epithelial infection. In contrast, primary systemic or intradermal inoculation resulted in 80−90% mortality, but also conferred protective sterile immunity to vaginal West Nile virus re‐challenge. Intravaginal and intradermal immunization elicited comparable, accelerated accumulation of larger B‐cell numbers in the mucosa and draining lymph node upon intravaginal re‐challenge than systemic immunization. However, accumulation of CD4+ T cells in both sites in the intradermally immunized group was significantly greater than in intravaginally or systemically immunized mice. Accelerated accumulation of dendritic cells occurred at periodic sub‐basement membrane sites in the absence of detectable virus 1 day after vaginal re‐challenge, irrespective of the route of immunization. These data illustrate the diversity of possible effective immune responses to West Nile virus in the vaginal mucosa. They show primary vaginal inoculation produces effective immunity to flavivirus infection with lower mortality than other routes and suggest a local role for vaginal mucosal dendritic cells in both primary and secondary responses.
Room-temperature photoluminescence and electroluminescence properties of sputter-grown gallium nitride doped with europium
Authors: Kim Joo Han , Holloway Paul H.
Published: April 2004

We have studied the room-temperature photoluminescence (PL) and electroluminescence (EL) properties of europium (Eu)-doped gallium nitride (GaN) thin films grown by radio frequency planar magnetron cosputter deposition. X-ray photoelectron core level spectra collected for Eu 4d revealed that the Eu ions in the GaN host were mainly in the trivalent state. A series of PL peaks were observed in the region of 530–700 nm, with the most intense peak at 614 nm. They were assigned to the radiative transitions between the 4f6 energy levels of the Eu3+ ion, specifically DJ5→7FJ′ (J=0,1;J′=1,2,3,4). The maximum PL intensity was obtained at a Eu concentration of ∼1.8 at. %. Electroluminescent devices were fabricated with an Al electrode, Eu-doped GaN light-emitting layer, Al2O3–TiO2 dielectric layer over an indium-tin-oxide electrode on a Corning 7059 glass substrate. The EL emission spectra from the fabricated devices were almost identical to the PL spectra. Higher electron energies (higher applied voltages) were needed to excite Eu3+ ground-state electrons into the higher-lying D15 state, which is consistent with the EL excitation mechanism being direct impact by hot electrons.
Pressure effect on crystallization temperature in Zr70Pd30 metallic glass
Authors: Jiang J. Z. , Jeppesen S. , Saida J. , Lathe C.
Published: April 2004

The pressure effect on amorphous-to-quasicrystalline-to-intermetallic phase transformations in a Zr70Pd30 metallic glass has been investigated by in situ x-ray diffraction measurements using synchrotron radiation. It is found that the glass crystallizes in two steps: (1) amorphous-to-icosahedral quasicrystalline and (2) icosahedral quasicrystalline-to-intermetallic Zr2+xPd alloy. The intermetallic alloy has a tetragonal structure with lattice parameters, a=3.310(1) Å and c=10.974(1) Å, and a space group of I4/mmm. External pressure enhances the onset temperatures for the formation of quasicrystalline phase and intermetallic compound with rates of 11±3 and 9±4 K/GPa, respectively, while the temperature interval for the stability of quasicrystals remains almost unchanged in the pressure range of 0–3 GPa. External pressure does not affect the phase-selection sequence. The enhancement of the onset temperature for the formation of quasicrystals has been further discussed with the nucleation theory.
In situ second-harmonic generation measurements of the stability of Si(111)–H and kinetics of oxide regrowth in ambient
Authors: Bodlaki D. , Borguet E.
Published: April 2004

The oxidation of H terminated silicon surfaces is a significant and controversial problem in silicon device fabrication. Second-harmonic generation rotational anisotropy (SHG–RA) provides a convenient means to monitor the chemical state of the Si surfaces, and to follow the conversion of H terminated surface to SiO2 by oxidation as a function of time in ambient. The change in SHG–RA of Si(111)–H was shown to correlate well with the ellipsometric thickness. SHG is sensitive to the initial stage of oxidation (induction period) as well as to the logarithmic oxide growth. SHG is sensitive to the electronic properties of the surface, therefore it is a sensitive probe of the quality of H terminated Si(111) surface. Under ambient conditions, (20% relative humidity, 23 °C) the initial oxidation rate is at most 2×10−6 ML/s.
Contact mechanisms and design principles for nonalloyed ohmic contacts to n-GaN
Authors: Mohammad S. Noor
Published: April 2004

The contact mechanism and design principles for nonalloyed ohmic contacts are investigated. Illustrative studies of various contacts show wide validity of the design principles. Experimental measurement suggests that reactive ion etching (RIE) of the n-GaN surface creates extra electrons. There is an optimum etching time that produces the highest electron concentration. RIE creates disturbances in the N–Ga bonds. This, together with metal deposition such as Ti/Al/Ni/Au and Ti/Au/Pd/Au depositions on RIE-treated samples, promotes the formation of TiN. N vacancies, thus created in the GaN subsurface, act as donor atoms. There occurs structural, atomic, and/or bonding reorganization at the metal/semiconductor (M/S) interface during postdeposition cooling yielding the M/S barrier height lower than the Schottky–Mott barrier height. Band gap narrowing and/or image force lowering resulting from heavy doping lead to further reduction of the barrier height. As a result thermionic emission, rather than tunneling, may be the primary mechanism for the low resistivity of many contacts.
Comparison of texture evolution in Ag and Ag(Al) alloy thin films on amorphous SiO2
Authors: Kim H. C. , Theodore N. D. , Alford T. L.
Published: April 2004

The thermal stability of Ag metallization on SiO2 was improved by using Ag(Al) alloy films instead of pure Ag thin films which agglomerate at high temperature. X-ray diffraction techniques were used to study the texture evolution of Ag and Ag(Al) films. Different behaviors were observed in texture evolution of Ag and Ag(Al) films. Detailed information about the texture of Ag and Ag(Al) films was obtained by performing pole-figure analysis. Two different changes in texture are explained using a grain-growth mechanism and an agglomeration phenomenon, both of which work to reduce the total free energy of the system at high temperature. Based on the data obtained in this study, the texture evolution of Ag film, induced by agglomeration, and Ag(Al) film is discussed.
Writing submicrometric metallic patterns by ultraviolet synchrotron irradiation of nanostructured carbon and TiOx–carbon films
Authors: Caruso T. , Agostino R. G. , Bongiorno G. , Barborini E. , Piseri P. , Milani P. , Lenardi C. , La Rosa S. , Bertolo M.
Published: April 2004

Nanostructured carbon and carbon–TiOx films produced by supersonic cluster beam deposition have been characterized by spatially resolved (0.5 μm lateral resolution) photoemission spectroscopy with synchrotron light. We have found that pure and TiOx-containing nanostructured carbon is modified under high flux UV exposure showing the onset of a metallic character near the Fermi level. We have studied the spectroscopic features, the spatial confinement, and the kinetics of this metallization process of the irradiated regions. This effect allows one to write metallic patterns in the TiOx-containing films with a submicrometric resolution and with a fast kinetic.
Wet thermal oxidation of AlAsSb alloys lattice matched to GaSb
Authors: Meneou K. , Lin H. C. , Cheng K. Y. , Kim J. G. , Martinelli R. U.
Published: April 2004

Lateral wet oxidation of thin (1000 Å) films of AlAs0.1Sb0.9 was performed at temperatures ranging from 300 to 450 °C. Oxidation kinetics and morphology were studied to assess the utility of an AlAs0.1Sb0.9 oxidation process. Oxidation rates up to 2.15 μm/min and maximum oxidation depths up to 130 μm were achieved at temperatures below 350 °C. The oxidation starts out reaction rate limited, becoming diffusion rate limited at longer oxidation times and higher oxidation temperatures. At higher temperatures, the apparent diffusion constant decreases, limiting maximum oxide depth in a phenomenon called self-limiting oxidation, as has also been observed during oxidation of AlAs0.56Sb0.44. Surface and oxidation front morphology are studied to help understand the transport of reactants through the oxide. Composition and structure of the oxide are studied to shed more light on the oxidation kinetics. In particular, we suggest a physical explanation for how self-limiting oxidation occurs in this material.
Room-temperature tunneling magnetoresistance in La0.7Sr0.3MnO3 step-edge junctions
Authors: Wang L. M. , Liu Chen-Chung , Yang H. C. , Horng H. E.
Published: April 2004

La 0.7 Sr 0.3 MnO 3 tunneling magnetoresistance (TMR) junctions have been fabricated on step-edge (001) SrTiO3 substrates with a high step-edge angle. The step-edge junctions show nonvanishing TMR spikes in R(H) curves above room temperature (RT). The resistance, dynamic conductance, and TMR ratio are characterized to explore the possible conduction mechanism for the step-edge junctions. The temperature dependence of surface magnetization MS(T) deduced from the spin polarization P(T), following a (Tc−T)0.92±0.02 dependence, agrees with the theoretical models for MS(T). From these results, we conclude that tunneling is the dominating mechanism and that the charge carriers at the surface boundary govern the tunneling conductivity.
Donor-acceptor pair emission near 0.55 eV in CdGeAs2
Authors: Bai Lihua , Giles N. C. , Schunemann P. G. , Pollak T. M. , Nagashio K. , Feigelson R. S.
Published: April 2004

A photoluminescence (PL) study has been performed from 5 to 200 K on a series of p-type bulk CdGeAs2 single crystals. Within our large set of samples, the peak position of an emission band at 5 K was observed to vary from about 0.54 to 0.58 eV when using 1064 nm excitation with a power density of 2.5 W/cm2. The variations of the PL peak position with excitation power and sample temperature have been measured and are consistent with donor-acceptor pair (DAP) recombination in the presence of potential fluctuations. We find that the position of the DAP emission in a particular crystal depends on the hole carrier concentration and the level of compensation. Thermal quenching activation energies of 14 and 120 meV are determined for the donor and acceptor states, respectively. The acceptor defect involved in this radiative recombination is the primary center responsible for the commonly observed room-temperature absorption band at 5.5 μm in p-type CdGeAs2 crystals.
High temperature transport properties of partially filled CaxCo4Sb12 skutterudites
Authors: Puyet M. , Lenoir B. , Dauscher A. , Dehmas M. , Stiewe C. , Müller E.
Published: April 2004

Partially filled CoSb3 skutterudite compounds are emerging materials for thermoelectric energy conversion at high temperature. CaxCo4Sb12 with different Ca contents has been prepared by the conventional metallurgical route. The temperature dependences of the electrical resistivity, Seebeck coefficient, and thermal conductivity have been measured on these compounds in the 300–800 K temperature range. These measurements have identified Ca as being a true n-type filler atom and offer, for this family of skutterudite, several valuable insights into the potential of Ca to provide good thermoelectric performance.
Structural evolution of Ti/TiC multilayers
Authors: Dahan I. , Frage N. , Dariel M. P.
Published: April 2004

Hard coatings based on metal/ceramic multilayers with periods in the nanometer range have been shown to possess some potential for improved tribological and mechanical properties. The present work is concerned with the structural evolution of (Ti/TiC) multilayers. Two kinds of multilayers consisting of 30 equithick (40 nm)TiC layers and 20 and 60 nm thick Ti layers, respectively, were sputter deposited on Mo substrates. The structural and the compositional evolution of these multilayers were examined by x-ray diffraction, transition electron microscopy (TEM), high-resolution TEM, Auger electron microscopy spectroscopy and differential thermal analysis (DTA), in the as-deposited state and after various heat treatments up to 500 °C. Initially, the Ti layers had a crystalline columnar grain structure displaying a (002) texture. The TiC layers displayed weak crystallinity with a pronounced (111) texture. In the course of the heat treatments, carbon diffused from the carbide layer into the adjacent Ti layers transforming the latter into off-stoichiometric TiCx with x≈0.5 and simultaneously depleting the carbon content of the initial carbide layer. The formed TiCx layers maintained the textural relationship with the neighboring TiC layers, consistent with a transformation that involved only a ABAB to ABC stacking change of the Ti sublattice. Increased mobility of the Ti atoms in carbon-depleted original TiC layers led to their full or partial recrystallization. The thermal effects associated both with the transformation of Ti layers into TiC, due to the influx of carbon atoms, and with the recrystallization of the original TiC layers were clearly revealed by the DTA measurements.
Analysis of hydroxyl group controlled atomic layer deposition of hafnium dioxide from hafnium tetrachloride and water
Authors: Puurunen Riikka L.
Published: April 2004

Atomic layer deposition (ALD) has recently gained interest because of its suitability for the fabrication of conformal material layers with thicknesses in the nanometer range. Although the principles of ALD were realized 30 to 40 years ago, the description of many physicochemical processes that occur during ALD growth is still under development. “Substrate-inhibited (SI)” ALD growth is one phenomenon not yet well understood. In SI-ALD, the growth-per-cycle (GPC) increases in the beginning of the growth, goes through a maximum, and levels off to a constant value. The origin of SI growth is investigated in this work with two recent models of ALD: Model A of Puurunen [Chem. Vap. Deposition 9, 249 (2003)] and Model B of Alam and Green [J. Appl. Phys. 94, 3403 (2003)]. The hafnium tetrachloride/water ALD process, of interest for gate dielectric applications, is taken to represent typical SI growth. The possible reaction chemistry is evaluated with two models: Model C of Ylilammi [Thin Solid Films, 279, 124 (1996)] and Model A. Model A seemingly allows higher amounts of species adsorbed at saturation than Model C. The ligand exchange reaction of hafnium tetrachloride with one surface hydroxyl (OH) group is chosen as the chemical basis of the modeling. Models A and B are, despite their apparent dissimilarity, found to treat the GPC identically when the same chemical reactions are assumed. According to Model A, the maximum in the GPC of hafnium dioxide ALD originates from a maximum in the surface concentration of OH groups, whereas according to Model B, the maximum is caused by a sudden decrease in the fraction of OH groups reacted with hafnium tetrachloride. The physical picture obtained with Model A is in better agreement with other investigations. Analysis of Model B reveals that OH surface concentrations produced by the model are systematically too high and that the numerical solution of Model B is based on an assumption not valid for the hafnium oxide ALD process. In addition, Model B is constructed assuming that ALD is a continuous process. A theoretical example of random deposition as a growth mode in ALD compared to continuous deposition illustrates that the noncontinuous, discrete nature of ALD affects the resulting mathematic equations.
Improving plasma uniformity using lens-shaped electrodes in a large area very high frequency reactor
Authors: Schmidt H. , Sansonnens L. , Howling A. A. , Hollenstein Ch. , Elyaakoubi M. , Schmitt J. P. M.
Published: April 2004

Experiments using a lens-shaped circular electrode are described to measure the correction of plasma nonuniformity due to the standing wave effect in a large area very high frequency plasma reactor. This work is the experimental verification of the theoretical reactor design in cylindrical geometry recently presented by L. Sansonnens and J. Schmitt, Appl. Phys. Lett. 82, 182 (2003). It is found that the lens-shaped electrode effectively compensates the standing wave effects by creating a uniform rf vertical electric field in the plasma volume. The plasma is uniform, except for edge effects, for a wide range of parameters and consequently the design is suitable for plasma processing.
From large to low height dispersion for self-organized InAs quantum sticks emitting at 1.55 μm on InP (001)
Authors: Gendry M. , Monat C. , Brault J. , Regreny P. , Hollinger G. , Salem B. , Guillot G. , Benyattou T. , Bru-chevallier C. , Bremond G. , Marty O.
Published: April 2004

We show how the height dispersion of self-organized InAs/InP(001) quantum islands emitting at 1.55 μm was reduced by optimizing the epitaxial growth parameters. Low height dispersion was obtained when the InAs deposit thickness was much greater than the critical thickness for two-dimensional/three-dimensional growth mode transition, and when adatom surface diffusion was favored by increasing the growth temperature or reducing the arsenic pressure during the InAs growth. When these growth conditions are not respected, the multicomponent photoluminescence spectrum obtained is explained through the common interpretation of island height varying with monolayer fluctuation. In optimized growth conditions, the multicomponent spectrum obtained is interpreted as emission from fundamental and excited levels of InAs islands with low height dispersion. Transmission electron microscopy (TEM) imaging shows that these InAs islands are stick-like, 50–100 nm in length and 22±1.2 nm in width. Cross-sectional TEM reveals flat islands, shaped like truncated pyramids, with a very homogeneous height measured at 2.4 nm. A fundamental level linewidth of 22 meV at 8 K is associated to this very narrow height distribution. Such low photoluminescence linewidth values are believed to be mainly due to the propensity of the InAs/InP(001) system to produce flat InAs islands with discrete height fluctuation.
Noncontact atomic force microscopy study of copper-phthalocyanines: Submolecular-scale contrasts in topography and energy dissipation
Authors: Fukuma Takeshi , Kobayashi Kei , Yamada Hirofumi , Matsushige Kazumi
Published: April 2004

Copper-phthalocyanine (CuPc) thin films on MoS2 surfaces were investigated by noncontact atomic force microscopy (NC-AFM). Submolecular resolution was successfully obtained in both topographic and dissipation images of CuPc monolayers. For topographic contrasts, the influence of short-range chemical interactions is particularly considered while the dissipation contrasts are discussed in relation to the tip-induced molecular fluctuations. Molecularly-resolved NC-AFM image was also obtained on CuPc multilayer, which revealed the structural difference between the monolayer and multilayer surfaces. The energy dissipation measured on these surfaces showed distinctive difference reflecting the different structural stabilities in the films. Furthermore, local surface modification of CuPc monolayer was demonstrated by NC-AFM. This is a direct evidence for the existence of energy transfer from the vibrating cantilever to the molecules through dissipative tip–sample interactions.
Systematic characterization of low-pressure capacitively coupled hydrogen discharges
Authors: Salabas A. , Marques L. , Jolly J. , Gousset G. , Alves L. L.
Published: April 2004

This paper presents a systematic characterization of pure hydrogen capacitively coupled discharges, produced in a parallel plate cylindrical setup. A two-dimensional, time-dependent fluid model is used to describe the production, transport, and destruction of electrons and positive ions H+, H2+, and H3+, at different frequencies (13.56–60 MHz), pressures (0.2–8 Torr), rf applied voltages (50–450 V) and geometric dimensions (1.6–12.8 cm radii and 1.6–6.4 cm interelectrode distances). A good agreement is found between calculation results and experimental measurements for the coupled electrical power, the plasma potential, and the self-bias potential, at various frequencies and rf applied voltages. However, the model generally underestimates the electron density with respect to its measured values. The paper discusses different space-time events, such as the development of double-ionization structures or the occurrence of field inversion and field reversal phenomena. The dependencies on pressure and frequency of the time-average electric field distribution are analyzed and related to the electron displacement within space-charge sheaths. This study is later used to understand the variations of the hydrogen dissociation rate, with changes in discharge operating conditions. The influence of reactor dimensions on the spatial profiles of the plasma potential, the rf electric field, the electron density, and the electron mean energy are analyzed in terms of discharge symmetry. An investigation of the space-time averaged rf electric field variations, with changes in the applied voltage, pressure, and geometric dimensions is carried out. These variations are shown to follow a universal similarity curve, if an adequate normalization is used when plotting the rf electric field as a function of pressure. This innovative representation of rf discharges allows a univocal definition of a reactor working point, for given operating conditions.
Matrix and thermal effects on photoluminescence from PbS quantum dots
Authors: Kang K. , Daneshvar K.
Published: April 2004

Lead sulfide nanoquantum dot samples were prepared with sol gel materials, a capping agent, and with various concentrations of PbS quantum dots. The –OH group absorption was significantly reduced during the heating process. The calculated quantum dot sizes were 1.2 nm for undoped sol gel and 0.97 nm for zirconium-doped sol gel. Without a capping agent, no photoluminescence was observed for a wide range of PbS particle concentrations. Strong photoluminescence was achieved with a capping agent when excited by a 480 nm wavelength xenon light source at room temperature. Luminescence quenching was observed with an extra –SH group from the matrix and excess H2S exposure. The quenching process typically broadened the spectra and blueshifted the peak when there was excess sulfur. The band gap energy was shifted from 1.64 to 1.68 eV for the zirconium-doped samples and 1.54–1.56 eV for the zirconium-free samples at room temperature. The thermal process significantly enhanced the luminescence intensity and redshifted the peak. The peak shift was 1.64–1.53 eV for zirconium-doped samples and 1.54–1.49 eV for zirconium-free samples.