512 research outputs found
Low temperature specific heat of vanadium carbide
Low temperature specific heat measurements on vanadium carbide crystal
Superconducting magnesium diboride films on Silicon with Tc0 about 24K grown via vacuum annealing from stoichiometric precursors
Superconducting magnesium diboride films with Tc0 ~ 24 K and sharp transition
\~ 1 K were successfully prepared on silicon substrates by pulsed laser
deposition from a stoichiometric MgB2 target. Contrary to previous reports,
anneals at 630 degree and a background of 2x10^(-4) torr Ar/4%H2 were performed
without the requirement of Mg vapor or an Mg cap layer. This integration of
superconducting MgB2 films on silicon may thus prove enabling in
superconductor-semiconductor device applications. Images of surface morphology
and cross-section profiles by scanning electron microscopy (SEM) show that the
films have a uniform surface morphology and thickness. Energy dispersive
spectroscopy (EDS) reveals these films were contaminated with oxygen,
originating either from the growth environment or from sample exposure to air.
The oxygen contamination may account for the low Tc for those in-situ annealed
films, while the use of Si as the substrate does not result in a decrease in Tc
as compared to other substrates.Comment: 5 pages, 4 figures, 15 references; due to file size limit, images
were blure
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Hyperthermal Pulsed-Laser Ablation Beams for Film Deposition and Surface Microstructural Engineering
This paper presents an overview of pulsed-laser ablation for film deposition and surface microstructure formation. By changing the ambient gas pressure from high vacuum to several Torr (several hundred Pa) and by selecting the pulsed-laser wavelength, the kinetic energy of ablated atoms/ions can be varied from several hundred eV down to {approximately}0.1 eV and films ranging from superhard to nanocrystalline may be deposited. Furthermore, cumulative (multi-pulse) irradiation of a semiconductor surface (e.g. silicon) in an oxidizing gas (0{sub 2}, SF{sub 6}) et atmospheric pressure can produce dense, self-organized arrays of high-aspect-ratio microcolumns or microcones. Thus, a wide range of materials synthesis and processing opportunities result from the hyperthermal flux and reactive growth conditions provided by pulsed-laser ablation
An improved continuous compositional-spread technique based on pulsed-laser deposition and applicable to large substrate areas
A new method for continuous compositional-spread (CCS) thin-film fabrication
based on pulsed-laser deposition (PLD) is introduced. This approach is based on
a translation of the substrate heater and the synchronized firing of the
excimer laser, with the deposition occurring through a slit-shaped aperture.
Alloying is achieved during film growth (possible at elevated temperature) by
the repeated sequential deposition of sub-monolayer amounts. Our approach
overcomes serious shortcomings in previous in-situ implementations of CCS based
on sputtering or PLD, in particular the variations of thickness across the
compositional spread and the differing deposition energetics as function of
position. While moving-shutter techniques are appropriate for PLD-approaches
yielding complete spreads on small substrates (i.e. small as compared to
distances over which the deposition parameters in PLD vary, typically about 1
cm), our method can be used to fabricate samples that are large enough for
individual compositions to be analyzed by conventional techniques, including
temperature-dependent measurements of resistivity and dielectric and magnetic
and properties (i.e. SQUID magnetometry). Initial results are shown for spreads
of (Sr,Ca)RuO.Comment: 6 pages, 8 figures, accepted for publication in Rev. Sci. Instru
Observed Effects of a Changing Step-Edge Density on Thin-Film Growth Dynamics
We grew SrTiO3 on SrTiO3 [001] by pulsed laser deposition, while observing
x-ray diffraction at the (0 0 .5) position. The drop dI in the x-ray intensity
following a laser pulse contains information about plume-surface interactions.
Kinematic theory predicts dI/I = -4sigma(1-sigma), so that dI/I depends only on
the amount of deposited material sigma. In contrast, we observed experimentally
that |dI/I| < 4sigma(1-sigma), and that dI/I depends on the phase of x-ray
growth oscillations. The combined results suggest a fast smoothing mechanism
that depends on surface step-edge density.Comment: 4 figure
Growth mechanism of superconducting MgB2 films prepared by various methods
The growth mechanisms of MgB2 films obtained by different methods on various
substrates are compared via a detailed cross-sectional scanning electron
microscopy (SEM) study. The analyzed films include (a) samples obtained by an
ex-situ post-anneal at 900 degree of e-beam evaporated boron in the presence of
an Mg vapor (exhibiting bulk-like Tc0 about 38.8 K), (b) samples obtained by
the same ex-situ 900 degree anneal of pulsed laser deposition (PLD)-grown Mg+B
precursors (exhibiting Tc0 ~ 25 K), and (c) films obtained by a low-temperature
(600 - 630 degree) in-situ anneal of PLD-grown Mg+B precursors (with Tc0 about
24 K). A significant oxygen contamination was also present in films obtained
from a PLD-grown precursors. On the other hand, it is clearly observed that the
films obtained by the high-temperature reaction of e-beam evaporated B with Mg
vapor are formed by the nucleation of independent MgB2 grains at the film
surface, indicating that this approach may not be suitable to obtain smooth and
(possibly) epitaxial films.Comment: 1 table, 4 figures, 17 references, 16 pages PDF fil
Superconducting MgB(2) films via precursor post-processing approach
Superconducting MgB(2) films with Tc = 38.6 K were prepared using a
precursor-deposition, ex-situ post-processing approach. Precursor films of
boron, ~0.5 micrometer thick, were deposited onto Al(2)O(3) (102) substrates by
e-beam evaporation; a post-anneal at 890 deg C in the presence of bulk MgB(2)
and Mg metal produced highly crystalline MgB(2) films. X-ray diffraction
indicated that the films exhibit some degree of c-axis alignment, but are
randomly oriented in-plane. Transport current measurements of the
superconducting properties show high values of the critical current density and
yield an irreversibility line that exceeds that determined by magnetic
measurements on bulk polycrystalline materials.Comment: PDF file with 10 pages total, including 4 figure
Effective Vortex Pinning in MgB2 thin films
We discuss pinning properties of MgB2 thin films grown by pulsed-laser
deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are
identified that contribute most effectively to the pinning of vortices in
randomly oriented films. The EB process produces low defected crystallites with
small grain size providing enhanced pinning at grain boundaries without
degradation of Tc. The PLD process produces films with structural disorder on a
scale less that the coherence length that further improves pinning, but also
depresses Tc
Field Emission and Nanostructure of Carbon Films
The results of field emission measurements of various forms of carbon films are reported. It is shown that the films nanostructure is a crucial factor determining the field emission properties. In particular, smooth, pulsed-laser deposited amorphous carbon films with both high and low sp3 contents are poor field emitters. This is similar to the results obtained for smooth nanocrystalline, sp2-bonded carbon films. In contrast, carbon films prepared by hot-filament chemical vapor deposition (HE-CVD) exhibit very good field emission properties, including low emission turn-on fields, high emission site density, and excellent durability. HF-CVD carbon films were found to be predominantly sp2-bonded. However, surface morphology studies show that these films are thoroughly nanostructured, which is believed to be responsible for their promising field emission properties
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