CHARACTERIZATION OF SUPERCONDUCTORS
BY ELECTRON BACKSCATTERED DIFFRACTION IN SEM
Z. Barkay1, E. Grunbaum2, A. Gholinia 3, S .Reich4
1
Wolfson Applied Materials Research Center, Tel-Aviv University, Tel-Aviv, Israel
Department of Physical Electronics, Faculty of Engineering, Tel-Aviv University,
Tel-Aviv, Israel
3
Oxford Instruments HKL A/S, Hobro, Denmark
4
Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot,
Israel
2
Microtexture characterization by electron backscattered diffraction (EBSD) in
the scanning electron microscope (SEM) provides a complementary method to
conventional surface and element analysis methods in SEM. EBSD theoretical and
experimental background is discussed with emphasis on phase identification and
orientation mapping at high lateral resolution. We show an application of EBSD for
electronic materials and particularly for low Tc and high Tc superconductors. The
EBSD method is demonstrated on thermal vapour grown CsXWO3 0.005≤x≤0.3
crystals using HKL CHANNEL5 EBSD system in Quanta 200FEG ESEM. EBSD
patterns showed phase transformation upon Cs doping to nominal concentrations of
x=0.005, x=0.05 and x=0.3. In particular, the 2D superconducting crystals of x=0.005
nominal concentration, were of inhomogeneous crystallographic phase depending on
the local Cs doping. The superconducting Cs-doped regions of the hexagonal phase
were shown to be epitaxially grown on the WO3 monoclinic crystal surface with the
(0001) planes parallel to the (001) planes in the WO3 monoclinic crystal and
perpendicular to the sample surface. The utility of EBSD for high Tc superconductor
texture characterization is demonstrated on YBa2Cu307 superconducting thin films.
INTRODUCTION
Electron backscatter diffraction (EBSD) in the scanning electron microscope
(SEM) provides crystallographic information from the sample upper surface. In
stationary mode, an electron beam strikes a tilted crystalline sample and the diffracted
electrons form an EBSD pattern on a fluorescent screen. In mapping mode, EBSD
patterns are accumulated from a scanned region for providing information on grain
boundary misorientations, local crystalline perfection and preferred crystal
orientations. The correlation between the microstructure and the crystallographic
directions gives rise to microtexture information [1-2].
The mechanism by which the EBSD patterns are formed is based on scattering
of electrons with a small energy loss close to the surface of the material forming a
divergent source of electrons. Some electrons on either side of the divergent beam are
arriving at Bragg angle to the crystallographic planes, producing two reflected cones
for each crystallographic plane. Due to the short electron wavelength the diffracted
electrons form large angle cones, which intersect the detector fluorescence screen as
nearly straight parallel lines named as Kikuchi lines after its discoverer [3]. Hence the
EBSD bridges the two traditional methods of studying crystalline materials: X-ray
diffraction (XRD) and transmission electron diffraction (TED) in transmission
electron microscope (TEM), providing the possibility of high spatial resolution in
bulk specimen. The lateral and depth resolutions of the EBSD method are dependent
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on the sample composition, the e-beam diameter and interaction volume and are [4]
both of the order of 10nm for field emission gun (FEG) SEM.
Low Tc and high Tc superconductor cryoelectronic devices both strongly
depend on the development of reproducible high quality bulk and thin films. Most
applications of YBa2Cu307 (YBCO) utilize conduction in the CuO2 layers i.e.,
conduction along the a-b crystal directions. This requires that films should be oriented
with the c direction normal to the surface, while misorientation in grain boundaries
influences detrimentally the transport critical current density [5].
Our work discusses the utility of EBSD for investigation of high Tc YBCO
thin superconducting films and in particularly focuses on low Tc superconductors
made of Cs doped WO3 crystals. It was shown [6-8] that CsxWO3, 0.3x0.19, is a
3D superconductor. Below x=0.19, a transition from a metal to an insulator occurs
and no superconductivity is observed. For x0.05 superconductivity reappears with
2D superconductivity at particularly low doping levels of x=0.005.
Local phase identification and texture orientation were studied in YBCO high
Tc superconductors (HTSC) by other groups [9-10] using the EBSD method in SEM.
Following our previous study [11-12], we will discuss texture analysis using the
EBSD method in SEM for both families of low Tc and high Tc superconducting
materials.
EXPERIMENTAL
Crystals of CsXWO3 0.005x0.3 were prepared by thermal vapor growth
process as described elsewhere [13]. We will refer to crystals of nominal
concentrations of x=0.3, x=0.05 and x=0.005. The process for YBCO thin film growth
is described elsewhere [14].
EBSD analysis was performed at the conventional specimen configuration
[15] with 70 tilt position relative to the main beam and with 15mm working distance.
The EBSD system was HKL-Oxford Channel5 with Nordlys II detector mounted on
Quanta 200FEG in the CsXWO3 study and on LEO Supra 55VP SEM in the YBCO
study. All the analysis was carried out at 20kV acceleration voltage with a few nA
beam current, without surface preparation. EBSD was locally correlated with element
composition measurement using Energy Dispersive Spectroscopy (EDS).
The EDS system for element composition measurement was nitrogen-cooled
Oxford INCA with 133eV resolution. The EDS analysis was performed at the system
optimum configuration of horizontal sample position at 10mm working distance. The
EBSD lateral resolution (mentioned above) is about order of magnitude better than the
EDS micro-scale resolution, which provides local and comparable information.
RESULTS AND DISCUSSIONS
A typical secondary electron (SE) image of the thermal vapor grown CsXWO3
crystals is shown at fig. 1. The crystals are typically of 100m size and tend to
agglomerate within mm size volume with their crystal facets inclined in various
directions. The sample was a challenge for EBSD analysis due to the inhomogenueity
in Cs concentration on the surface, and due to the requirement of selecting a facet
with the appropriate inclination to the EBSD detector.
In an enlarged SE image (fig. 2a) of CsXWO3 crystal with nominally x=0.005,
two locations have been chosen for EBSD analysis. Figure 2b shows the EBSD
pattern at point 1, which fits to the Cs-doped hexagonal phase (6/mmm Laue group,
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a=7.42Å, b=7.42Å, c=7.57Å). Figure 2c shows the EBSD pattern at point 2, which
fits the WO3 monoclinic phase (2/m Laue group, a=5.28Å, b=5.16Å, c=7.66Å); this
corresponds also to undoped WO3 crystal samples. The comparison of EBSD patterns
for the other Cs doped samples showed the hexagonal phase structure for x=0.05, and
the trigonal phase (-3m Laue group with a=7.26Å, b=7.26Å, c=110.58Å) for x=0.3.
Table 1 summarizes the surface crystallographic phases at the Cs rich regions versus
the nominal doping of the CsXWO3 crystals.
Nominal doping
(x)
0
Crystallographic
phase
Monoclinic
0.005
Hexagonal
0.05
Hexagonal
0.3
Trigonal
Table 1:
Surface crystallographic phase at the
agglomeration
Cs-rich regions versus the nominal
doping of the CsXWO3 crystals.
Fig. 1.
Typical SE image of an
of CsXWO3 crystals.
EBSD mapping in areas 1 and 2 showed texture orientation of each phase. The
analysis in fig. 2a includes 40% patterns corresponding to the monoclinic phase (fig.
2c) and the other 60% corresponding to the hexagonal phase (fig. 2b). The location of
each phase is shown in the phase-map (fig. 2d), where the left side (in red)
corresponds to the Cs-doped hexagonal phase and the right side (in blue) corresponds
to the WO3 monoclinic phase. Correlation with the SE image at fig. 2a shows that
each of the two phases in the adjacent planes occupies about 10m2. Missing points
within the mapping frame region actually constitute unresolved EBSD patterns due to
the topographic shadow effect at the slope of the plane and to incomplete flatness of
the surface.
EDS analysis of x=0.005 crystals at the Cs-rich regions yielded concentrations
which are order of magnitude higher than the nominal value of 0.5 wt%. In particular,
EDS analysis from the left side of fig. 2d showed Cs enhancement to 8.5 wt% in
correlation with the EBSD phase-map. Similar concentration enhancement was
obtained by EDS analysis for crystals of x=0.05 nominal concentration, whilst
CsXWO3 crystals of x=0.3 yielded values of 15-20 wt% at the Cs-doped regions.
Based on the EBSD patterns, which were collected for crystal phase mapping
(fig. 2d), pole figures are derived for crystal orientation mapping of both the Cs-doped
hexagonal phase (fig. 3a) and the monoclinic WO3 phase (fig. 3b) regions. The
intensity in the pole figures is presented in multiples of uniform density (MUD) units.
We use a 10º half width plane spread angle and 5º clustering of data points for
increased MUD weighting. The pole figures are presented in X-Y sample coordinates
while the Z direction is perpendicular to the surface.
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a
Fig. 2, CsXWO3 crystal of x=0.005 nominal concentration: (a) SE image (b) EBSD
pattern of crystal in fig. 2a at location 1 - Cs-doped hexagonal phase (c) EBSD pattern
of crystal in fig. 2a at location 2 - WO3 monoclinic phase (d) EBSD phase-map
corresponding to the framed region of fig. 2a: red – Cs-doped hexagonal phase, blue WO3 monoclinic phase.
Fig. 3. Pole figures of CsXWO3 crystal of x=0.005 nominal concentration: (a) (0001)
plane orientation of the hexagonal Cs-doped phase (b) (001) plane orientation of the
monoclinic WO3 phase.
Alignment within each phase is clearly derived from the corresponding pole
figures for each of the crystallographic planes. In particular, the (0001)
crystallographic plane of the Cs-doped hexagonal phase lies within the X-Y sample
plane, and is parallel to the (001) crystallographic plane of the monoclinic WO3
phase. Texture alignment was also observed within the Cs-doped regions of x=0.3 and
x=0.05 CsXWO3 crystals.
The superconducting YBCO has an orthorhombic structure, which is built
from three perovskite unit cells, where the difference between a, b and c axes is about
1%. Such small differences give rise to pseudo-symmetry problems in the orientation
analysis. The EBSD pattern was indexed using the YBCO-tetragonal crystal structure
to distinguish between the c and a axis (4/mmm Laue group a=b=3.88Å, c=11.79Å).
Figure 4a shows the band contrast image provided by EBSD mapping. The (001)
plane alignment with respect to the x, y and z directions of the sample is shown by
inverse pole figure (IPF) map (fig. 4b). The pole-figure for the (001) plane is shown
(fig. 4c) with a schematic diagram (fig. 4d) referring to the alignment of the unit cell.
The microtexture analysis in the YBCO thin film shows that most of the sample
exhibit c-axis orientation, with tiny elongated grains of a-axis and b-axis orientations.
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a
b
c
d
Fig. 4. The EBSD data shown in a) band-contrast map, b) IPF map, c) (001) pole
figure and d) color key showing the alignment of the YBCO crystal structure, where
the x-axis of the sample is rotated 45 with respect to the horizontal of the map.
CONCLUSIONS
EBSD provided the possibility to reveal phase and texture information on both
low Tc crystal superconductors and high Tc thin superconducting film with submicron lateral resolution and high surface sensitivity. The EBSD results on CsXWO3
crystals support previous observations by scanning tunneling microscopy (STM)
indicating the existence of a local surface superconducting phase within the x=0.005
CsXWO3 crystals. In addition, it confirmed the existence of a small amount of
hexagonal phase previously observed by XRD. Apart from showing the correlation
between the crystallographic phases and doping levels, we showed a high texture
alignment for the 2D superconducting phase of x=0.005 nominal concentration. In
particular, the Cs-doped WO3 (0001) plane was in alignment with the (001) WO3
crystal plane, which indicated an epitaxial growth of the superconducting phase due to
Cs doping.
As complimentary information we introduced the analysis of high Tc
superconductors by EBSD. The orientation of a and c axis growth directions in the
YBCO film were shown at sub-micron resolution. The band-contrast and IPF images
both show that the YBCO grain shapes are either flat or of needle shape aligned in the
horizontal or vertical directions. In the flat grains, the YBCO structure was found to
have c-axis perpendicular to the sample surface, where as in the needle shaped grains
the c-axis was in the sample plane.
Microtexture analysis could be further correlated with macroscopic
measurements of critical current density in case of cryoelectronic device applications.
The applications of EBSD in SEM could be extended to the enlarged family of alkali
doped WO3 crystals as well as to other HTSC materials.
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ACKNOWLEDGEMENT
Acknowledgement is given to G. Leibovitch and Prof. G. Deutscher from the physics
department in Tel-Aviv University for providing the thin YBCO film for EBSD
measurement.
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