bluuebunny/crossref_metadata_2025_split huggingface dataset:
Coherent growth of r‐plane GaN films on ZnO substrates at room temperature
Authors: Kobayashi Atsushi , Ueno Kohei , Ohta Jitsuo , Fujioka Hiroshi 
Published: January 2011

AbstractWe grew semipolar r‐plane GaN films on ZnO substrates at room temperature (RT) by pulsed laser depositon (PLD). The structural characteristics of r‐plane GaN films grown on nearly lattice‐matched ZnO substrates at RT were investigated by X‐ray diffraction and reciprocal space mapping (RSM). The r‐plane GaN films grown on ZnO exhibited the narrowest X‐ray rocking curves of any heteroepitaxial semipolar GaN films. The improvement in crystalline quality achieved by the use of ZnO substrates was attributed to the coherent growth of r‐plane GaN under non‐equilibrium conditions by PLD. Symmetric RSM revealed that r‐plane GaN grows on ZnO substrates without the tilting of the r‐plane that was often observed in lattice‐mismatched semipolar growth. These results indicated that the formation of misfit dislocations at the GaN/ZnO interface is suppressed by coherent growth at RT.
Structure characterization of spark plasma sintered alumina by positron annihilation lifetime spectroscopy
Authors: Djourelov Nikolay , Aman Yann , Berovski Kalin , Nédélec Patrick , Charvin Nicolas , Garnier Vincent , Djurado Elisabeth 
Published: January 2011

AbstractPositron annihilation lifetime spectroscopy (PALS) measurements were performed to study the influence of the sintering temperature, heating rate and dwell time on the porosity and on the defect concentration on series of spark plasma sintered alumina samples. Two long‐lived components were found in the PALS spectra and were associated to pick‐off annihilation of ortho‐positronium localized in two types of pores. The same spectra were analysed in the frame of the three‐state‐trapping model and information for the intragranular and at grain boundary defect concentration was extracted. At sintering temperatures below 1200 °C the regime of the high (600 °C/min) heating rate leaded to more efficient defect annealing compared to the regime of the low (8 °C/min) heating rate, while at higher temperatures this difference disappeared. The low heating rate at high temperatures leaded to more compact samples compared to these sintered at the high heating rate as a result of intergranular pore concentration decrease.
Growth temperature altered morphology of Ge nanocolumns
Authors: Khare Chinmay , Gerlach Jürgen W. , Weise Michael , Bauer Jens , Höche Thomas , Rauschenbach Bernd 
Published: January 2011

AbstractGermanium nanocolumns grown by glancing angle deposition on Si(100) substrates and prepatterned substrates comprising of SiO2 nanospheres demonstrated an altered morphology that is greatly influenced by the substrate temperature (TS). Surface diffusion‐driven mass transport and increased adatom mobility at homologous temperatures TS/TM ≈ 0.34 and 0.39 (TM melting temperature of Ge) augmented fibrous‐columnar and intracolumnar growth, respectively. Further increment of the substrate temperature provoked column merging and column broadening. The degree of film crystallinity increased while TS increased from 200 to 340 °C, shifting the preferential crystallite orientation from (111) to (220).
Nonalloyed ohmic contact of AlGaN/GaN HEMTs by selective area growth of single‐crystal n+‐GaN using plasma assisted molecular beam epitaxy
Authors: Zheng Zhi , Seo Huichan , Pang Liang , Kim Kyekyoon (Kevin) 
Published: January 2011

AbstractSelective area growth (SAG) based on plasma assisted molecular beam epitaxy (PAMBE) was demonstrated to be effective to achieve low contact resistance for nonalloyed ohmic metals. An AlGaN/GaN high‐electron mobility transistor (HEMT) using SAG by PAMBE with nonalloyed ohmic metals and recessed drain/source structure exhibited a low specific contact resistance of 3.7 × 10−5 Ω cm2, high‐peak drain current of 604 mA/mm, and small gate leakage current of 3.4 µA. These results demonstrate that SAG by PAMBE produces nonalloyed HEMT with comparable or more favorable electric performance versus conventional alloyed counterpart while avoiding problems of the latter resulting from high‐temperature annealing.
Characteristics of undoped and Sb‐doped ZnO thin films prepared in different atmospheres by pulsed laser deposition
Authors: Zhu B. L. , Zhu S. J. , Zhao X. Z. , Su F. H. , Li G. H. , Wu X. G. , Wu J. 
Published: January 2011

AbstractUndoped and 0.56 at.% Sb‐doped ZnO thin films were prepared by pulsed laser deposition (PLD) under vacuum and an oxygen pressure of 0.2 Pa with sintered ceramic as targets. The effects of Sb doping and deposition atmosphere on structure and optical–electrical properties of the films were studied by X‐ray diffraction (XRD), scanning probe microscopy (SPM), Hall Effect measurement, transmittance spectra, and photoluminescence (PL) spectra. The results showed that undoped and Sb‐doped films deposited under vacuum had better crystallinity, higher carrier concentration, lower bandgap (Eg), and single violet emission as compared with the films deposited in an oxygen pressure of 0.2 Pa. Compared with undoped ZnO film, Sb‐doped ZnO film had higher carrier concentration and almost uniform Eg in both atmospheres, and it exhibited obviously improved crystallinity and green emission under an oxygen pressure of 0.2 Pa. The results implied that the deposition atmosphere strongly affected the growth kinetics of the films and intrinsic defect in the films, and Sb doping seemed also to affect the growth kinetics of the films under certain conditions and introduced SbZn defects and possibly SbZn‐2VZn defects, thus the structure and optical–electrical properties of the films were modified by the deposition atmosphere and Sb doping.
Effective control of polarity in Bi0.9La0.1FeO3 thin films by dopant‐related internal bias
Authors: Wang Yang , Chen Weigang , Chen Zuhuang , Rami Naidu Chukka , Wang Junling , Wang John , Chen Lang 
Published: January 2011

AbstractThe as‐grown polarizations of Mn‐ and Ti‐doped Bi0.9La0.1FeO3 thin films (BLFMn and BLFTi) are demonstrated to align in an upward direction due to the occurrence of an internal bias field Ei that points up. On the basis of the conductivity and XPS as well as leakage current measurements, Ei in BLFMn is attributed to an asymmetric distribution of charged defects, while the elevated Fermi level and broadened positively charged depletion zone are supposed to induce Ei in BLFTi. The different physical origins of Ei are intimately connected with the nature of aliovalent acceptor and donor dopants of Mn and Ti ions, which is further verified by the consistent voltage offsets in the piezoelectric hysteresis loops. This suggests an effective way to control the polarity of ferroelectric thin films through the tuning of chemical doping levels.
Fluctuation‐induced conductivity of melt‐textured Y0.95Er0.05Ba2Cu3O7−δ superconductor prepared in air
Authors: Pinheiro Lincoln Brum de Leite Gusmão , Costa Rosângela Menegotto , Júnior Pedro Rodrigues , da Cruz Gerson Kniphoff , Jurelo Alcione Roberto 
Published: January 2011

AbstractSystematic conductivity measurements near TC in melt‐textured Y0.95Er0.05Ba2Cu3O7−δ superconductors, prepared using the top‐seeding growth technique, are presented. Measurements were performed both for current applied parallel and perpendicular to the c‐axis. Samples with different disorder states were studied and the influences of the erbium addition on conductivity fluctuations are discussed. The microstructural characteristics of samples were analyzed by scanning electron microscopy and X‐ray diffraction, while the superconducting properties were determined by resistivity and dc magnetization measurements. To identify power‐law divergences of the conductivity, the results were analyzed in terms of the temperature derivative of the resistivity (${\rm d}{\rho} /{\rm d}T$) and of the logarithmic temperature derivative of the conductivity (${-} {\rm d(ln}{\Delta} {\sigma} {\rm )}/{\rm d}T$). The fluctuations above the critical temperature are highly anisotropic: Gaussian regimes were observed for the $J//a(b)c$ configuration and critical regimes for the $J//ab$ configuration. Also, for the more disordered state, the data evidenced a two‐stage transition.
Lateral IBIC characterization of single crystal synthetic diamond detectors
Authors: Lo Giudice Alessandro , Olivero Paolo , Manfredotti Claudio , Marinelli Marco , Milani Enrico , Picollo Federico , Prestopino Giuseppe , Re Alessandro , Rigato Valentino , Verona Claudio , Verona‐Rinati Gianluca , Vittone Ettore 
Published: January 2011

AbstractIn order to evaluate the charge collection efficiency (CCE) profile of single‐crystal diamond devices based on a p‐type/intrinsic/metal configuration, a lateral Ion Beam Induced Charge (IBIC) analysis was performed over their cleaved cross sections using a 2 MeV proton microbeam. CCE profiles in the depth direction were extracted from the cross‐sectional maps at variable bias voltage. IBIC spectra relevant to the depletion region extending beneath the frontal Schottky electrode show a 100% CCE, with a spectral resolution of about 1.5%. The dependence of the width of the high efficiency region from applied bias voltage allows the constant residual doping concentration of the active region to be evaluated. The region where the electric field is absent shows an exponentially decreasing CCE profile, from which it is possible to estimate the diffusion length of the minority carriers by means of a drift–diffusion model. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effect of III‐nitride polarization on VOC in p–i–n and MQW solar cells
Authors: Namkoong Gon , Boland Patrick , Bae Si‐Young , Shim Jae‐Phil , Lee Dong‐Seon , Jeon Seong‐Ran , Foe Kurniawan , Latimer Kevin , Doolittle W. Alan 
Published: January 2011

AbstractWe performed detailed studies of the effect of polarization on III‐nitride solar cells. Spontaneous and piezoelectric polarizations were assessed to determine their impacts upon the open circuit voltages (VOC) in p–i(InGaN)–n and multi‐quantum well (MQW) solar cells. We found that the spontaneous polarization in Ga‐polar p–i–n solar cells strongly modifies energy band structures and corresponding electric fields in a way that degrades VOC compared to non‐polar p–i–n structures. In contrast, we found that piezoelectric polarization in Ga‐polar MQW structures does not have a large influence on VOC compared to non‐polar MQW structures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Annual cycle of the West African monsoon: regional circulations and associated water vapour transport
Authors: Thorncroft Chris D. , Nguyen Hanh , Zhang Chidong , Peyrillé Philippe 
Published: January 2011

AbstractAnalysis of the annually varying regional circulations and their relationship to surface conditions and water vapour transport in the West African region is presented. The progression of the West African monsoon is described in terms of four key phases: (i) an oceanic phase between November and mid‐April when the rain band is broad with peak values just north of the Equator (∼1°N); (ii) a coastal phase between mid‐April and the end of June when the rainfall peak is in the coastal region around 4°N (over the ocean); (iii) a transitional phase during the first half of July when the rainfall peak decreases; and (iv) a Sahelian phase between mid‐July and September when the rainfall peak is more intense and established in the Sahelian region around 10°N. The annual evolution of the moisture fluxes, associated convergence, and rainfall is strongly impacted by the Atlantic cold tongue (cool water close to the Equator between boreal spring and summer) and the Saharan heat‐low. The cold tongue strongly regulates the timing and intensity of the coastal rainfall in spring. The heat‐low and its associated shallow meridional circulation strongly affect the profile in moisture flux convergence north of the main rain‐band maximum; in particular it is responsible for the establishment of a second peak in column moisture flux convergence there (approximately 8° poleward of the rainfall peak).Particular emphasis is given to the coastal rainfall onset in April. A key aspect of this onset is acceleration of low‐level cross‐equatorial southerly winds, important for establishing the cold tongue, discouraging convection near the Equator and transporting moisture towards the coast. We argue that the rainfall peak is maintained at the coast, rather than steadily moving inland with the solar insolation, due to persistent warm water in the coastal region together with frictionally induced moisture convergence there. Copyright © 2011 Royal Meteorological Society
The use of potential vorticity inversion to evaluate the effect of precipitation on downstream mesoscale processes
Authors: Baxter Martin A. , Schumacher Philip N. , Boustead Joshua M. 
Published: January 2011

AbstractThe influence of precipitation near the surface warm front on precipitation downstream is evaluated. A two‐way nested simulation is computed for two selected cases: 13–15 February 2003 and 3–5 January 2005. The results of the simulation are compared with output from the North American Regional Reanalysis dataset, which is used as a high‐resolution proxy for true atmospheric conditions. The relevant physical processes in the generation and maintenance of precipitation poleward of other precipitation areas are analysed, along with perturbation potential vorticity (PV) fields. Piecewise PV inversion is used to determine height and wind fields associated with the perturbation PV. These height and flow fields are used to compute derived quantities such as balanced deformation, balanced temperature perturbation, and balanced moisture flux. Both cases illustrate that the role of rainfall near surface warm fronts on precipitation downstream is complex, and is associated with more than just moisture flux. The strength, depth and orientation of diabatically generated PV anomalies within these areas of rainfall are associated with mesoscale processes relevant to the maintenance of downstream precipitation. Lastly, this work presents a methodology for analysing the role of precipitation in individual events. Copyright © 2011 Royal Meteorological Society
The dynamics of heat lows over elevated terrain
Authors: Smith Roger K. , Spengler Thomas 
Published: January 2011

AbstractOrographic effects on the dynamics of heat lows are investigated in an idealized flow configuration using a hydrostatic numerical model. The behaviour of the heat low that forms over a plateau‐like orography on a circular island is compared with that when the island is flat, and that when the plateau is surrounded by land instead of sea. In all cases, a broad‐scale, negative radial gradient of potential temperature forms in the daytime mixed layer over land. The presence of orography enhances the broad‐scale baroclinicity over the orographic slope due to the identical heating of a column of air with a reduced mass, i.e. lower surface pressure. In the absence of sea, the baroclinicity is solely confined to the slope of the orography. The broad‐scale potential temperature gradient results in an overturning circulation in the lowest few kilometres, which is separate from the shallower and more intense sea breeze circulation in the island cases. The presence of orography leads to a stronger overturning circulation via enhanced baroclinicity. In the case without sea, both the overturning circulation and tangential circulation are closely tied to the orography. The overturning circulation advects absolute angular momentum inwards to spin up the low‐level circulation, despite some frictional loss of angular momentum en route. During the night, radiative cooling over the land leads to a strong nocturnal low‐level jet that amplifies the spin‐up process. During the daytime, the cyclone weakens as the angular momentum is convectively mixed through a deep layer. The study extends an earlier one of Reichmann and Smith and corrects some details in their model. The results offer a refined interpretation of the Atlantic inflow to the Saharan heat low, described recently by Grams et al., and emphasize the role of orography in the formation of the inland baroclinic zone, which is a feature thereof. Copyright © 2011 Royal Meteorological Society
The definition of ‘truth’ for Numerical Weather Prediction error statistics
Authors: Frehlich Rod 
Published: January 2011

AbstractA consistent definition of ‘truth’ is presented to define the errors in a numerical weather prediction (NWP) forecast, analysis and observations resulting from the unresolved turbulent field. ‘Truth’ is defined as the convolution of the continuous atmospheric variables by the effective spatial filter of an NWP model. Direct measurements of atmospheric variables are represented as an instrument error and a convolution of the continuous atmospheric variables by the observation sampling function. This clearly separates the instrument error from the observation sampling error that describes the mismatch between the NWP model effective spatial filter and the observation sampling function. The ensemble average that defines error statistics is defined by an infinite number of atmospheric realizations with statistically similar random fluctuations in the unresolved model field. This results in large spatial variations in the observation sampling errors due to the atmospheric variations in turbulence statistics. Two approaches are discussed to describe these spatial variations: one that defines observation error referenced to each model coordinate and one that assigns observation error referenced to each observation coordinate. The observation‐error statistics depend on the observation sampling function, the local spatial statistics of the turbulence field and the NWP model filter. The effects of imprecise knowledge of the shape of the model filter on observation sampling error are small for rawinsonde measurements and for observations that produce a linear average along a track. The modifications to data‐assimilation algorithms (the maximum‐likelihood (ML) method, minimum mean‐square‐error algorithms, Kalman filtering, variational data assimilation and ensemble data assimilation) to include the spatial variations in observation‐error statistics are discussed. In addition, the generation of ensemble forecast members should be consistent with the spatial variations in total observation error. A rigorous definition of error statistics is essential for evaluating the many different types of current and future observing systems. Copyright © 2010 Royal Meteorological Society
A case‐study of mesoscale spectra of wind and temperature, observed and simulated
Authors: Larsén Xiaoli Guo , Larsen Søren , Badger Merete 
Published: January 2011

AbstractThe spectra of the zonal and meridional winds and temperature over the mesoscale range of length‐scales (10−5<k<10−3rad m−1, with k the radian wave number) are examined through a case‐study using measurements and simulations. The measurements include 1 s and 10 min average winds and temperature, from which the temporal spectra are obtained by Fourier transform. The mesoscale Weather Research and Forecasting (WRF) model is used to simulate the weather for four days when gravity waves are observed. The four‐day dataset also includes a period with unstable conditions and free of gravity waves. The simulation provides a possibility to study the spatial spectra and, together with the measurements, it provides an opportunity to examine the validity of the Taylor hypothesis for transforming between temporal and spatial spectra. We examined these issues under stable and unstable conditions, both in the absence and presence of gravity waves. It was found that, in the absence of gravity waves, the spectral behaviour of wind and temperature (in terms of the form of the spectra and the energy distribution) is similar to the literature, and the Taylor hypothesis is valid. When the gravity waves are present, it was found that the Doppler‐shifted frequency has to be taken into account when converting the spectra between time and space. The simulation suggests that, in the presence of gravity waves, the kinetic energy is not evenly distributed between the zonal and meridional winds, rather the wind component along the main wave propagation direction contains larger variance. Copyright © 2011 Royal Meteorological Society
Classifying severe rainfall events over Italy by hydrometeorological and dynamical criteria
Authors: Molini L. , Parodi A. , Rebora N. , Craig G. C. 
Published: January 2011

AbstractRaingauge data over Italy for the period January 2006–February 2009 have been used to classify severe rainfall events into two types using a recently developed methodology. The types are defined as either long‐lived and spatially distributed (Type I) if lasting more than 12 h and larger than 50 × 50 km2or brief and localized (Type II) if having shorter duration or smaller spatial extent. A total of 81 events were identified, with 51 classified as Type I and 30 as Type II. The work presented here examines the hypothesis that the two types of event are associated with different dynamical regimes distinguished by differing degrees of control of convective precipitation by the synoptic‐scale flow. For each of the 81 events, a time‐scale for convective adjustment is computed, based on gridded hourly precipitation rates derived from rain‐gauge data and ECMWF analysis (ERA‐Interim) of convective available potential energy (CAPE). Values of the convective adjustment time‐scale,τc, shorter than 6 h indicate convection that is responding rapidly to to the synoptic environment (equilibrium), while slower time‐scales indicate that other, presumably local, factors dominate. It was anticipated thatτc> 6 h would correspond to brief and localized Type II events, whileτc< 6 h would indicate Type I events. This hypothesis was largely confirmed, with 45 of the 51 Type I events having time‐scales shorter than 6 h and 20 of the 30 Type II events having time‐scales longer than 6 h. Copyright © 2011 Royal Meteorological Society
Improving the use of observations to calibrate ensemble spread
Authors: Flowerdew Jonathan , Bowler Neill E. 
Published: January 2011

AbstractThe Met Office Global and Regional Ensemble Prediction System (MOGREPS) uses an online inflation factor calculation to calibrate the spread of the ensemble in space and time and counteract the tendency of the Ensemble Transform Kalman Filter (ETKF) to underestimate analysis uncertainty. Until 2008, this calibration mechanism relied entirely on sonde and wind profiler data, and was only applied locally in the Extratropics. By producing more appropriate estimates of the error variance of ATOVS brightness temperature observations, it has become possible to include these in the inflation factor calculation. This in turn provides sufficient data to apply localisation uniformly over the globe. The new scheme improves the latitudinal distribution of spread in comparison to forecast error, especially in the Tropics. Issues remain with the vertical distribution of spread, which are addressed by work to be reported in a future paper. © 2011 Crown Copyright, the Met Office. Published by John Wiley & Sons, Ltd.