252 research outputs found
Effect of high temperature deposition on CoSi 2 phase formation
Abstract: This paper discusses the nucleation behaviour of the CoSi to CoSi2 transformation from cobalt silicide thin films grown by deposition at elevated substrate temperatures ranging from 375 °C to 600 °C. A combination of channelling, real-time Rutherford backscattering spectrometry, real-time x-ray diffraction, and transmission electron microscopy was used to investigate the effect of the deposition temperature on the subsequent formation temperature of CoSi2, its growth behaviour, and the epitaxial quality of the CoSi2 thus formed. The temperature at which deposition took place was observed to exert a significant and systematic influence on both the formation temperature of CoSi2 and its growth mechanism. CoSi films grown at the lowest temperatures were found to increase the CoSi2 nucleation temperature above that of CoSi2 grown by conventional solid phase reaction, whereas the higher deposition temperatures reduced the nucleation temperature significantly. In addition, a systematic change in growth mechanism of the subsequent CoSi2 growth occurs as a function of deposition temperature. First, the CoSi2 growth rate from films grown at the lower reactive deposition temperatures is substantially lower than that grown at higher reactive deposition temperatures, even though the onset of growth occurs at a higher temperature, Second, for deposition temperatures below 450 °C, the growth appears columnar, indicating nucleation controlled growth. Elevated deposition temperatures, on the other hand, render the CoSi2 formation process layer-by-layer which indicates enhanced nucleation of the CoSi2 and diffusion controlled growth. Our results further indicate that this observed trend is most likely related to stress and changes in microstructure introduced during reactive deposition of the CoSi film. The deposition temperature therefore provides a handle to tune the CoSi2 growth mechanism
Optimized fabrication of high quality La0.67Sr0.33MnO3 thin films considering all essential characteristics
In this article, an overview of the fabrication and properties of high
quality La0.67Sr0.33MnO3 (LSMO) thin films is given. A high quality LSMO film
combines a smooth surface morphology with a large magnetization and a small
residual resistivity, while avoiding precipitates and surface segregation. In
literature, typically only a few of these issues are adressed. We therefore
present a thorough characterization of our films, which were grown by pulsed
laser deposition. The films were characterized with reflection high energy
electron diffraction, atomic force microscopy, x-ray diffraction, magnetization
and transport measurements, x-ray photoelectron spectroscopy and scanning
transmission electron microscopy. The films have a saturation magnetization of
4.0 {\mu}B/Mn, a Curie temperature of 350 K and a residual resistivity of 60
{\mu}{\Omega}cm. These results indicate that high quality films, combining both
large magnetization and small residual resistivity, were realized. A comparison
between different samples presented in literature shows that focussing on a
single property is insufficient for the optimization of the deposition process.
For high quality films, all properties have to be adressed. For LSMO devices,
the thin film quality is crucial for the device performance. Therefore, this
research is important for the application of LSMO in devices.Comment: Accepted for publication in Journal of Physics D - Applied Physic
Origin of different deactivation of Pd/SnO<sub>2</sub> and Pd/GeO<sub>2</sub> catalysts in methanol dehydrogenation and reforming: A comparative study
Pd particles supported on SnO2 and GeO2 have been structurally investigated by X-ray diffraction, (High-Resolution) transmission and scanning electron microscopy after different reductive treatments to monitor the eventual formation of bimetallic phases and catalytically tested in methanol dehydrogenation/reforming. For both oxides this included a thin film sample with well-defined Pd particles and a powder catalyst prepared by incipient wetness impregnation. The hexagonal and the tetragonal polymorph were studied for powder GeO2. Pd2Ge formation was observed on all GeO2-supported catalysts, strongly depending on the specific sample used. Reduction of the thin film at 573 K resulted in full transformation into the bimetallic state. The partial solubility of hexagonal GeO2 in water and its thermal structural instability yielded Pd2Ge formation at 473 K, at the cost of a structurally inhomogeneous support and Ge metal formation at higher reduction temperatures. Pd on tetragonal GeO2 entered a state of strong metal–support interaction after reduction at 573–673 K, resulting in coalescing Pd2Ge particles on a sintered and re-crystallized support, apparently partially covering the bimetallic particles and decreasing the catalytic activity. Pd2Ge on amorphous thin film and hexagonal GeO2 converted methanol primarily via dehydrogenation to CO and H2. At 573 K, formation of Pd2Sn and also PdSn occurred on the Pd/SnO2 thin film. Pd3Sn2 (and to some extent Pd2Sn) were predominantly obtained on the respective powder catalyst. Strong deactivation with increasing reduction temperature was observed, likely not based on the classical strong metal–support interaction effect, but rather on a combination of missing active structural ensembles on Sn-enriched bimetallic phases and the formation of metallic β-Sn. Correlations to Pd and its bimetallics supported on ZnO, Ga2O3 and In2O3 were also discussed
Proximity induced metal/insulator transition in superlattices
The far-infrared dielectric response of superlattices (SL) composed of
superconducting YBaCuO (YBCO) and ferromagnetic La%
CaMnO (LCMO) has been investigated by ellipsometry. A drastic
decrease of the free carrier response is observed which involves an unusually
large length scale of d20 nm in YBCO and d10
nm in LCMO. A corresponding suppression of metallicity is not observed in SLs
where LCMO is replaced by the paramagnetic metal LaNiO. Our data suggest
that either a long range charge transfer from the YBCO to the LCMO layers or
alternatively a strong coupling of the charge carriers to the different and
competitive kind of magnetic correlations in the LCMO and YBCO layers are at
the heart of the observed metal/insulator transition. The low free carrier
response observed in the far-infrared dielectric response of the magnetic
superconductor RuSrGdCuO is possibly related to this effect
Observation and Assignment of Silent and Higher Order Vibrations in the Infrared Transmission of C60 Crystals
We report the measurement of infrared transmission of large C60 single
crystals. The spectra exhibit a very rich structure with over 180 vibrational
absorptions visible in the 100 - 4000 cm-1 range. Many silent modes are
observed to have become weakly IR-active. We also observe a large number of
higher order combination modes. The temperature (77K - 300K) and pressure (0 -
25KBar) dependencies of these modes were measured and are presented. Careful
analysis of the IR spectra in conjunction with Raman scattering data showing
second order modes and neutron scattering data, allow the selection of the 46
vibrational modes C60. We are able to fit *all* of the first and second order
data seen in the present IR spectra and the previously published Raman data
(~300 lines total), using these 46 modes and their group theory allowed second
order combinations.Comment: REVTEX v3.0 in LaTeX. 12 pages. 8 Figures by request. c60lon
Disorder-Driven Pretransitional Tweed in Martensitic Transformations
Defying the conventional wisdom regarding first--order transitions, {\it
solid--solid displacive transformations} are often accompanied by pronounced
pretransitional phenomena. Generally, these phenomena are indicative of some
mesoscopic lattice deformation that ``anticipates'' the upcoming phase
transition. Among these precursive effects is the observation of the so-called
``tweed'' pattern in transmission electron microscopy in a wide variety of
materials. We have investigated the tweed deformation in a two dimensional
model system, and found that it arises because the compositional disorder
intrinsic to any alloy conspires with the natural geometric constraints of the
lattice to produce a frustrated, glassy phase. The predicted phase diagram and
glassy behavior have been verified by numerical simulations, and diffraction
patterns of simulated systems are found to compare well with experimental data.
Analytically comparing to alternative models of strain-disorder coupling, we
show that the present model best accounts for experimental observations.Comment: 43 pages in TeX, plus figures. Most figures supplied separately in
uuencoded format. Three other figures available via anonymous ftp
International Financial Reporting Standards and Earnings Quality: The Myth of Voluntary vs. Mandatory Adoption
We revisit evidence whether incentives or IFRS drive earnings quality changes, analyzing a large sample of German firms in the period from 1998 to 2008. Consistent with previous studies we find that voluntary and mandatory adopters differ distinctively in terms of essential firm characteristics and that size, leverage, age, bank ownership and ownership concentration influenced the decision to voluntarily adopt IFRS. However, regardless of the decision to voluntarily adopt IFRS, we find that conditional conservatism increased under IFRS for both groups of adopters, while evidence does not suggest an increase in value relevance under IFRS. Results on earnings management in the post-adoption period are mixed. While income smoothing decreases for voluntary but not for mandatory adopters, discretionary accruals only decrease for mandatory but not for voluntary adopters. However, further analyses suggest that the capital market environment and the economic cycle during the adoption period seem to be a more powerful explanation for this evidence than voluntary or mandatory IFRS adoption. Therefore, we conclude that incentives to voluntarily adopt IFRS did not unambiguously dominate accounting standards in determining earnings quality in the case of German firms
Earnings Management to Avoid Delisting from a Stock Market
We show that firms 'in danger' of being delisted from a stock market (NASDAQ) report higher performance-adjusted discretionary accruals and the inflated accruals are associated with an increased likelihood of maintained listing. Accruals of firms 'in danger' are less positive in fiscal quarters audited by a Big-4 auditor and after the implementation of SOX. In contrast, accruals are higher for firms that benefit most from public listing and for firms with good future prospects. This suggests that managers consider reputation and litigation risk associated with earnings management and they manage earnings only when they believe the firm will recover in near future. The market can thus interpret discretionary accruals as a signal revealing managers' private information about firm quality. Consistent with the signaling explanation we observe a stronger stock price reaction on the announcement of earnings that contain large accruals in threatened firms
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