Surface Structure Analysis of Atomically Smooth BaBiO$_3$ Films
Surface Structure Analysis of Atomically Smooth BaBiO3 Films
A. Gozar∗, G. Logvenov, V. Y. Butko, and I. Bozovic
Brookhaven National Laboratory, Upton, New York 11973-5000, USA
Using low energy Time-of-Flight Scattering and Recoil Spectroscopy (TOF-SARS) and Mass
Spectroscopy of Recoiled Ions (MSRI) we analyze the surface structure of an atomically smooth
BaBiO3 film grown by molecular beam epitaxy. We demonstrate high sensitivity of the TOF-SARS
and MSRI spectra to slight changes in the orientation of the ion scattering plane with respect to the
crystallographic axes. The observed angle dependence allows us to clearly identify the termination
layer as BiO2. Our data also indicate that angle-resolved MSRI data can be used for high resolution
studies of surface structure of complex oxide thin films.
PACS numbers: 68.03.Hj, 68.47.Gh, 68.49.Sf, 81.15.Hi
Introduction - The plethora of remarkable electrical
and magnetic properties of transition metal oxides made
them both a focus of basic research and very appeal-
ing candidates for integration in electronic devices. For
the latter, it is important to reproducibly synthesize and
characterize atomically perfect surfaces and interfaces.
However, this goal is difficult to attain in complex ox-
ides because of the complicated phase diagrams and high
sensitivity to growth conditions.1 This explains the large
disparity between what is known about the structure of
the very top atomic layers in these materials compared
to widely used semiconductor or metal surfaces.
In this work we provide a new route to circumvent
these problems. This is done by combining the power
of atomic layer-by-layer molecular beam epitaxy (ALL-
MBE)2, enabling production of films with perfect sur-
faces, with the extreme surface sensitivity of the low en-
ergy Time-of-Flight Scattering and Recoil Spectroscopy
(TOF-SARS)3,4 and the Mass Spectroscopy of Recoiled
Ions (MSRI)5 techniques. Angle-Resolved (AR) TOF-
SARS could determine in principle inter-atomic spac-
ings with a resolution approaching 0.01 Å, comparable
to lateral values obtained in surface X-ray crystallogra-
phy and even better in the direction perpendicular to the
surface.4 This comparison is to be judged also from the
perspective of having a table-top experimental setup as
opposed to the requirement of very intense synchrotron
light. However, in order to reach such accuracy one
needs samples with very smooth surfaces. For this rea-
son TOF-SARS and MSRI data in oxides have been pri-
marily used for monitoring surface composition rather
than the structure.6 Exceptions are very few materials
like Al2O3 or SrTiO3 which are commercially available
and commonly used as single crystal substrates for film
growth.7,8
Ba1−xKxBiO3 is a family of superconductors with the
maximum Tc ≃ 32 K (at x = 0.4) being the highest in an
oxide material without copper.9,10 BaBiO3 , the parent
compound, is insulating and non-magnetic. These inter-
esting properties are believed to arise due to a charge-
density-wave instability11 which leads to the lowering of
the crystal symmetry from the simple cubic perovkite
structure, see Fig. 1. The driving force of this transi-
tion and the persistence of the charge order in supercon-
ducting Ba1−xKxBiO3 are still a matter of debate.
11,12,13
This is largely due to the difficulty in obtaining high
quality single crystals from these materials. A recent
study brought to the forefront the important problem
of dimensionality in Ba1−xKxBiO3 suggesting that this
compound has in fact a layered structure, in analogy
to the high Tc superconducting cuprates.
14 In the con-
text of superconducting electronics, the interest in un-
doped and K doped BaBiO3 stems from succesful fabrica-
tion of superconductor-insulator-superconductor tunnel
junctions, a task that has bee quite ellusive with either
cuprates or MgB2 superconductors. For these reasons
it is critically important to understand and control the
surface properties in this family of compounds.
Real space crystallography using TOF-SARS is based
on the concepts of shadowing and blocking cones.3,15
TOF-SARS is sensitive to both ions and neutrals so it is
not dependent on charge exchange processes at the sur-
face. The drawback is that these spectra display broad
’tails’ at high times (see Fig. 2) associated with multi-
ple scattering events that are difficult to analyze quan-
titatively. This drawback is eliminated by MSRI which
achieves ’time focussing’ according to t = t0+k(M/e)
i.e. the flight time of the ions is only a function of their
mass to charge ratio. The broad continua seen in TOF-
SARS are turned into very sharp features allowing iso-
topic mass resolution5, see Fig. 3. MSRI is thus easier to
interpret and one is tempted to use AR MSRI for surface
structure analysis. However, because MSRI detects only
ions, one should worry about possible anisotropic neu-
tralization effects which makes a quantitative interpreta-
tion problematic.16 In fact this aspect is a long standing
problem in ion based mass spectrometry affecting both
compositional and structural studies.
Here we report on succesful and reproducible synthe-
sis of large area single crystal thin films of BaBiO3 using
ALL-MBE. This opens the path of improved basic exper-
iments including high resolution surface crystallography
in oxides based on ion scattering. Next we present results
of surface analysis of a BaBiO3 film using AR TOF-SARS
and MSRI. We show that atomically smooth surfaces lead
to high sensitivity of both type of spectra with respect
http://arxiv.org/abs/0704.1501v1
0 25 50 m
[100]
[210]
[110]
Roughness
Rms < 0.4 nm
0 20 40 60 80 100
2  (degrees)
(001)
(002)
(003)
(004)
1.2 10
0.6 0.8 1 1.2 1.4
Incident Angle (deg.)
(a) (b)
FIG. 1: (a) The cubic perovskite structure of undistorted
BaBiO3 . (1) and (2) correspond to K trajectories as described
in the text. (b) A 50 × 50 µm AFM image of the BaBiO3 film.
(c) ω − 2θ scan of the BaBiO3 film; STO labels Bragg peaks
from the SrTiO3 substrate. The inset shows X-ray reflectance
intensity oscillations at grazing incidence.
to as small as one degree variation in the azimut angle.
Comparison between the AR TOF-SARS and MSRI data
shows that the latter can be a powerful tool for quan-
titative surface structure analysis. To the best of our
knowledge this result has not been reported before. The
angular dependence of the spectra along with numerical
simulations allows us to unambiguously determine that
the BaBiO3 film terminates by a BiO2 layer.
Experimental - A BaBiO3 thin film was grown on a
SrTiO3 substrate. The thickness of 96 nm was deter-
mined from low angle X-ray reflectometry, a value in
very good agreement with the prediction from the pro-
grammed growth rate. Atomic force microscopy data
show absence of any secondary phase precipitates and a
roughness Rms that is below 4 Å in a typical 25µm2 area,
see Fig. 1b. This was expected based on our observation
of strong specular reflection and undamped RHEED os-
cillations during growth. The pronounced finite thickness
reflectance oscillations (Fig. 1c - inset) clearly demon-
strate atomically flat film interfaces. The high quality
of the film is furthermore reflected in the sharp Bragg
diffraction peaks seen in the ω−2θ scans shown in Fig. 1c.
The lattice constants of the bulk (96 nm thick) film at
T = 300 K were determined to be c = 4.334(2) Å while
in-plane a ≈ b ≃ 4.353(4) Å. Note also that the unit cell
of our BaBiO3 film appears to be very close to the sim-
ple perovskite structure shown in Fig. 1b, which is not
the case for the body-centered monoclinic structure char-
acteristic of single crystals at this temperature.11 The
experimental observation of excellent epitaxy is intrigu-
ing taking into account the relatively large mismatch,
of about 11%, between the in-plane lattice constants of
SrTiO3 and bulk single crystals of BaBiO3 .
The ion scattering data were taken using a recently
built Ionwerks TOF system, based on principles de-
scribed in Refs.5,17 We used a K+ ion source tuned to
provide a monochromatic beam of 10 keV. Microchannel
plate detectors were mounted at θ = 270 and 370 total
scattering angles. The incident angle was α = 150. Time
resolved spectra were achieved by pulsing the source
beam at a 20 kHz over a 0.5×2 mm aperture. The typical
pulse width was 12 ns. This, together with the I ≈ 0.1µA
value of the ion current on the aperture, allows us to esti-
mate an ion dose of about 3×1011 ions/cm2 per spectrum.
The surface density is about 1015 atoms/cm2 indicating
that the technique was not invasive for the duration of
the experiment. More important, measurements from
pristine regions of the 1 cm2 sample during and after the
experiment ensured that the surface was neither dam-
aged nor charged. The TOF-SARS and MSRI spectra
were collected at T ≈ 6000 C in ozone atmosphere at a
pressure p ≃ 5×10−6 Torr. In the following KS(X) de-
notes K+ ions undergoing single scattering events from
the element X on the surface while the notation XR
stands for particles (X = Ba,Bi or O) recoiled from the
BaBiO3 surface. The numerical calculations were per-
formed using the Scattering and Recoil Imaging Code
(SARIC) which is a classical trajectory simulation pro-
gram based on the binary collision approximation.18
Surface structure analysis of BaBiO3 - Fig. 2a illus-
trates the dependence of the TOF-SARS spectra on the
azimuth angle Φ, defined as the angle between the scat-
tering plane and the [100] direction of the cubic structure.
One minute long scans taken with Φ varied in 10 incre-
ments between 3.50 and 10.50 are shown. The continuum
above t ≥ 9.9 µs is due to K multiple scattering. Based
on the predictions of elastic binary collision the two sharp
features in Fig. 2a can be immediately assigned to single
scattering events of K ions. The peak at t = 9.75 ns cor-
responds toKS(Bi) and the one at t ≃ 9.8 ns to KS(Ba).
Dramatic changes are seen in the behavior of the higher
time KS(Ba) peak as Φ is varied.
Surface roughness typically smears out the structure in
AR scattering or recoil features. The strong sensitivity of
the spectra in Fig. 1 to small variations of the azimuthal
angle is a consequence (and a direct proof) of high surface
quality. The angle independent intensity of the KS(Bi)
peak along with the strong variation in the KS(Ba) fea-
ture suggest that the film terminates with BiO2 planes
which shadow the subsurface BaO layers. We show be-
low that this is fully supported by more detailed analysis
including numerical simulations.
Information about the surface arrangement and dy-
namics can be obtained by studying the details of the
spectral shapes. For example the inset of Fig. 2a, which
shows a zoomed in region around the KS(Bi) peak at
Φ = 450, reveals a shoulder on the low time side. Indeed,
9.7 9.8 9.9 10
time ( Sec)
9.72 9.75 9.78
 = 45
time ( Sec)
17 18 19
time ( Sec)
recoil
FIG. 2: (Color online) Time-of-Flight K scattering data from
BaBiO3 recorded at θ = 27
0 total scattering angle. (a) The
spectra are taken for azimuthal angles Φ from 3.50 to 10.50.
The arrow indicates the direction of increasing angle. The
peaks at t = 9.74 µs and t ≈ 9.8 µs correspond to KS(Bi)
and KS(Ba) respectively. Inset: zoomed in area around the
KS(Bi) peak for Φ = 45
0. The line through the data points
is a two Gaussian peaks fit. These two peaks, denoted by (1)
and (2), correspond to the K trajectories shown in Fig. 1a.
(b) The BiR peak at higher times for Φ = 0
0, 150 and 210.
the data can be well fitted by two Gaussian peaks which
correspond to trajectories involving single and double
scattering events denoted by (1) and (2) in Fig. 1a. This
assignment to events involving only K and Bi atoms along
the [110] direction is based on the elastic binary collision
model which predicts a difference of 33 ns between these
two trajectories, in good agreement with the experimen-
tal value δt = 30 ns. We do not observe the low time
feature for Φ = 00; this is understood as the effect of
the intervening O atom along [100] direction. Since the
intensity of peak (2) depends strongly on the atomic ar-
rangement at the surface, angular dependencies of the
relative intensity of these two peaks could be used to get
information about the symmetry and vibration ampli-
tudes of the top layer atoms. Shown in Fig. 2b is the
BiR peak which is found around t ≃ 18µs.
We turn now to the question whether AR MSRI can
be a quantitative probe for high resolution surface study.
It is natural to ask (a) if one can see structure in the AR
MSRI data and (b) whether such dependence, if present,
provides quantitative information about the surface. The
latter problem is related to the fact that it is hard to
quantify the yield of ionic fraction which, moreover, could
be itself an anisotropic function with respect to the ori-
entation of the crystallographic axes.16
The answer to the first question is given in Fig. 3. The
main panel shows a MSRI spectrum taken at Φ = 00 and
the inset shows the Ba isotopes region for three azimuths.
Clearly, the intensities of the corresponding peaks vary
substantially when this angle is changed. Note also the
sharpness of the peaks which allows for easy separation of
atomic isotopes. The mass resolution, m/∆m ≈ 380, is
about one order of magnitude better than that obtained
20 40 60 80 100 120 140 160 180 200
Mass (AMU)
  = 0
134 135 136 137 138 139 140
Mass (AMU)
FIG. 3: (Color online) The main panel displays a MSRI
spectrum taken at Φ = 00 azimuth. The inset shows a zoomed
in area of the Ba isotopes region from the main panel, for three
values of Φ: 00, 50 and 200.
in typical TOF-SARS spectra.
The second question is addressed in the top panel of
Fig. 4 where two data sets are compared. One data set
corresponds to the Φ dependence of the intensities of
MSRI BaR peaks derived from the spectra shown in the
inset of Fig. 3. The second data set refers to BaR peak
from the TOF-SARS spectra taken with the detector at
the same total scattering angle θ = 370. The latter data
set was taken with the time focussing analyser not biased,
making it sensitive to both ion and neutral particles. The
two dependencies are essentially identical. Equally good
agreement is also observed if the BiR feature is consid-
ered instead of BaR. We conclude that anisotropic neu-
tralization effects are not important which proves that
AR MSRI can be used as a quantitative probe for sur-
face analysis. The insulating nature of the film and the
use of alkali ion source beam could be responsible for this
effect.16 Note also that the steep decrease in the BaR sig-
nal between Φ = 50 and 100 seen in Fig. 3 is consistent
with the disappearance of the KS(Ba) peak at t ≃ 9.8µs
in the TOF-SARS data at θ = 270 from Fig. 2a.
We address now the problem of film surface termina-
tion. The spectra shown in the bottom panel of Fig. 4
provide the answer to this question. The experimental
points correspond to the Φ dependence of the integrated
intensity of the BiR peak shown in Fig. 2b. For every
data point we subtracted the background due to the high
time tail associated with K multiple scattering (as seen in
Fig. 2a). The results of two SARIC simulations are also
shown in Fig. 4. The solid and dashed lines correspond to
assumed BiO2 and BaO terminations respectively. The
simulation based on BiO2 termination reproduces well
the main features of the experiment, i.e. the two peaks
around 200 and 330. In contrast, the lower angle fea-
ture is absent if BaO is assumed to be the topmost layer.
These numerical simulations clearly show that the film
terminates in a BiO2 surface.
One advantage of real space structure analysis is the
intuitive picture it immediately provides: the dips in the
300 TOF-SARS  37
MSRI  37
Recoil
0 5 10 15 20 25 30 35 40 45
azimuth  (degrees)
simulations
TOF-SARS 27
experiment
Recoil
FIG. 4: (Color online) Top: azimuth dependence of the inte-
grated intensity of the Ba recoil peak from two sets of mea-
surements taken at θ = 370 scattering angle: MSRI (squares)
and TOF-SARS spectra (circles). The curves are matched
at Φ = 50. Bottom: Φ dependence of the experimental in-
tegrated intensity of the BiR peak from TOF-SARS data of
Fig. 2b. The solid and dashed lines correspond to SARIC
simulations assuming BiO and a BaO film terminations re-
spectively. The data in this panel are matched at Φ = 450.
azimuth scans generally correspond to low index crys-
tallographic directions in the material. In Fig. 4b the
minima in the experimental scan found at Φ = 00, 270
and 450 are associated with the [100], [210] and [110] di-
rections of the cubic structure respectively, see Fig. 1.
Because these effects strongly depend on the inter-
atomic distances as well as the type of atoms on the
surface, the magnitude of these minima can also be ex-
plained qualitatively. The severe shadowing and bloking
due to both Bi and O atoms lying along the [100] azimuth
are the cause of the absence of BiR signal for Φ = 0
Although Oxygen atoms contribute to this effect, due to
their lower mass they cannot completely shadow or block
the incoming K ions or the recoiled Bi along [210] direc-
tion. As a result only a shallow minimum is seen around
Φ = 270. This is not the case for Bi atoms which are
responsible for the more pronounced dip at Φ = 450 in
spite of the larger interatomic separation along the [110]
direction. Similarly, the absence of a peak for Φ ≃ 200 in
the simulated BiR intensity for assumed BaO termina-
tion (dashed line in Fig. 4b) can be easily understood in
terms of Ba shadowing effects on Bi atoms along the [141]
direction and the value α = 150 for the incidence angle.
A detailed analysis of the lattice constants and possible
surface relaxation based on numerical calculations of the
shadowing and blocking cones are the purpose of future
work.
Conclusions - Achieving atomically smooth surfaces is
proven to have a great impact on the possibility to use AR
TOF-SARS and MSRI for structure and interface anal-
ysis in complex oxides. This is a stepping stone for fu-
ture characterization of artificially layered superconduct-
ing compounds like Ba1−xKxBiO3 or the cuprates. The
quantitative agreement between TOF-SARS and MSRI
spectra of a BaBiO3 film show that AR MSRI can be a
powerful tool for high resolution surface analysis. Data
and simulations allowed us to clearly identify that the
BaBiO3 film terminates in a BiO2 layer.
Acknowledgements - This work was supported by DOE
grant DE-AC-02-98CH1886. We thank W. J. Rabalais
for providing us with the SARIC simulation code and
J. A. Schultz for useful discussions.
* E-mail: agozar@bnl.gov
1 I. Bozovic, IEEE Trans. Appl. Supercond. 11, 2686 (2001)
2 I. Bozovic, G. Logvenov, I. Belca, B. Narimbetov, and I.
Sveklo, Phys. Rev. Lett. 89, 107001 (2002).
3 J. W. Rabalais, Principles and Applications of Ion Scat-
tering Spectrometry, Wiley-Interscience, 2003.
4 V. Bykov, L. Houssiau, and J. W. Rabalais, J. Phys. Chem.
104, 6340 (2000).
5 V. S. Smentkowski, A. R. Krauss, D. M. Gruen, J. C. Hole-
cek, and J. A. Schultz, J. Vac. Sci. Technol. A 17, 2634
(1999).
6 O. Auciello, J. Appl. Phys. 100, 051614 (2006) and refer-
ences therein.
7 J. Ahn, and J. W. Rabalais, Surf. Sci. 388, 121 (1997).
8 P. A. W. van der Heide, Q. D. Jiang, Y. S. Kim, and J.
W. Rabalais, Surf. Sci. 473, 59 (2001).
9 L. F. Maththeiss, E. M. Gyorgy, and D. W. Johnson, Jr.,
Phys. Rev. B 37, R3745 (1988).
10 R. J. Cava, B. Batlogg, J. J. Krajewski, R. Farrow, L.
W. Rupp, A. E. White, K. Short, W. F. Peck, and T.
Kometani, Nature 332, 814 (1988).
11 S. Pei, J. D. Jorgensen, B. Dabrovski, D. G. Hinks, D. R.
Richards, A. W. Mitchell, J. M. Newsam, S. K. Sinha, D.
Vaknin, and A. J. Jacobson, Phys. Rev. B 41, 4126 (1990).
12 T. Thonhauser, and K. M. Rabe, Phys. Rev. B 73, 212106
(2006).
13 A. V. Puchkov, T. Timusk, M. A. Karlow, S. L. Cooper, P.
D. Han, and D. A. Payne, Phys. Rev. B 54, 6686 (1996).
14 L. A. Klinkova, M. Uchida, Y. Matsui, V. I. Nikolaichik,
and N. V. Barkovskii, Phys. Rev. B 67, R140501 (2003).
15 M. Aono, Y. Hou, C. Oshima, and Y. Ishizawa, Phys. Rev.
Lett. 49, 567 (1982).
16 H. Niehus, W. Heiland, and E. Taglauer, Surf. Sci. Rep.
17, 213 (1993).
17 A. R. Krauss, Y. Lin, O. Auciello, G. J. Lamich, D. M.
Gruen, J. A. Schultz, and R. P. H. Chang, J. Vac. Sci.
Technol. A 12, 1943 (1994); J. A. Schultz, S. Ulrich, L.
Woolverton, W. Burton, K. Waters, C. Kleina, and H.
Wollnik, Nucl. Intrum. Methods Phys. Res. B 118, 758
(1996).
18 V. Bykov, C. Kim, M. M. Sung, K. J. Boyd, S. S. Todorov,
and J. W. Rabalais, Nucl. Intrum. Methods Phys. Res. B
114, 371 (1996).
