Optical transmission spectroscopy and spectroscopic ellipsometry were used to extract the optical properties of an epitaxially grown quasi-tetragonal BiFeO3 thin film in the near infrared to near ultraviolet range. The absorption spectrum is overall blue shifted compared with that of rhombohedral BiFeO3, with an absorption onset near 2.25 eV, a direct 3.1 eV band gap, and charge transfer excitations that are ~0.4 eV higher than those of the rhombohedral counterpart. We interpret these results in terms of structural strain and local symmetry breaking

Chen, P.

Gopalan, V.

Melville, A.

Musfeldt, J. L.

Podraza, N. J.

Ramesh, R.

Schlom, D. G.

Vlahos, E.

Xu, X. S.

English

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Xiaoshan Xu Papers Research Papers in Physics and Astronomy
2010
Optical properties of quasi-tetragonal BiFeO3 thin
films
P. Chen
University of Tennessee, pchen@ion.chem.utk.edu
N. J. Podraza
Pennsylvania State University
X. S. Xu
University of Nebraska-Lincoln, xiaoshan.xu@unl.edu
A. Melville
Cornell University
E. Vlahos
Pennsylvania State University
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Chen, P.; Podraza, N. J.; Xu, X. S.; Melville, A.; Vlahos, E.; Gopalan, V.; Ramesh, R.; Schlom, D. G.; and Musfeldt, J. L., "Optical
properties of quasi-tetragonal BiFeO3 thin films" (2010). Xiaoshan Xu Papers. 31.
https://digitalcommons.unl.edu/physicsxu/31
Authors
P. Chen, N. J. Podraza, X. S. Xu, A. Melville, E. Vlahos, V. Gopalan, R. Ramesh, D. G. Schlom, and J. L.
Musfeldt
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicsxu/31
Optical properties of quasi-tetragonal BiFeO3 thin films
P. Chen,1,a N. J. Podraza,2 X. S. Xu,1,b A. Melville,3 E. Vlahos,4 V. Gopalan,4
R. Ramesh,5,6 D. G. Schlom,3 and J. L. Musfeldt1
1Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA
2Department of Electrical Engineering, Pennsylvania State University, University Park,
Pennsylvania 16802, USA
3Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA
4Department of Materials Science and Engineering, Pennsylvania State University, University Park,
Pennsylvania 16802, USA
5Department of Materials Science and Engineering, University of California-Berkeley, Berkeley,
California 94720, USA
6Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
Received 2 February 2010; accepted 23 February 2010; published online 1 April 2010
Optical transmission spectroscopy and spectroscopic ellipsometry were used to extract the optical
properties of an epitaxially grown quasi-tetragonal BiFeO3 thin film in the near infrared to near
ultraviolet range. The absorption spectrum is overall blue shifted compared with that of
rhombohedral BiFeO3, with an absorption onset near 2.25 eV, a direct 3.1 eV band gap, and charge
transfer excitations that are 0.4 eV higher than those of the rhombohedral counterpart. We
interpret these results in terms of structural strain and local symmetry breaking. © 2010 American
Institute of Physics. doi:10.1063/1.3364133
The physical properties of polar oxides are of founda-
tional importance for investigations of coupling in functional
materials. This is because the rich tunability in these systems
arises from the interplay between charge, structure, and mag-
netism. In thin film oxides, strain is well known to modify
hybridization and bonding and, as a consequence, the bulk
properties.1–3 It can also stabilize nonequilibrium meta-
stable phases. In this work, we focus on bismuth ferrite, the
only known single phase room temperature multiferroic.
BiFeO3 displays many exotic properties including magneto-
electric coupling,4,5 a large remanent polarization,6,7 and
photovoltaic current,8 and it is already incorporated in polar
oxide-based solar cells and actuators.9,10 In both applications,
the different phases of the parent material play a vital role,
either to achieve better match with the solar spectrum or to
define a morphotropic phase boundary. In this work, we in-
vestigate the optical properties of quasi-tetragonal11 BiFeO3,
a phase stabilized by compressive strain.10
Bulk BiFeO3 is ferroelectric TC1100 K and antifer-
romagnetic TN640 K at 300 K. The structure is that of a
rhombohedrally distorted perovskite12 belonging to the R3c
space group. Each Fe3+ center is coordinated by six O2− ions
Fig. 1a, although the distortion along 111 yields a quasi-
octahedral arrangement. A slight tetragonal distortion re-
duces the space group to P4mm Ref. 14 and modifies the
overall perovskite structure so that the local Fe3+ environ-
ment is quasi-square pyramidal Fig. 1b. Theoretical cal-
culations predict an unusually large tetragonality ratio for
this system.10,14,15 In practice, the nonequilibrium tetragonal
phase can be stabilized by compressive strain, and epitaxial
films with different c /a ratios have been reported.10,15,16
In order to investigate the impact of strain on the charge
excitations of a model polar oxide, we measured the optical
properties of tetragonal BiFeO3 and compared the results
with rhombohedral films17–20 and recent first principles elec-
tronic structure calculations.14,15 We find that the optical re-
sponse of tetragonal BiFeO3 is overall blue shifted, with an
absorption onset near 2.25 eV and a direct band gap and
charge transfer excitations that are 0.4 eV higher than
those of the rhombohedral material. These results can be
understood in terms of symmetry breaking and local struc-
ture strain.
A series of tetragonal BiFeO3 films with thicknesses
between 23 and 38 nm were deposited on 110 YAlO3
substrates using conventional and laser molecular-beam
epitaxy.10 The pure tetragonal phase with small monoclinic
distortion is stabilized by the large epitaxial stain due to lat-
tice mismatch with the substrate.10 X-ray results show only
substrate and film peaks, and these films have a c /a ratio of
1.27. Rocking curves show partial relaxation but with a full
width at half max that is virtually the same as the substrate.
We employed both optical transmission spectroscopy and
spectroscopic ellipsometry to elucidate the optical properties
of this nonequilibrium phase. A combination of techniques
was employed due to the challenges inherent in a thin film
measurement. These include i important reflectance contri-
butions in a normal-incidence transmittance measurement
when the absorption coefficient is low and ii surface rough-
aElectronic mail: pchen@ion.chem.utk.edu.
bPresent address: Oak Ridge National Laboratories, Oak Ridge, TN.
(a) (b)
3.96 Å a = 3.70 Å
c = 4.69 Å
c/a = 1.27
FIG. 1. Color online Schematic view of the local structure around the Fe3+
center of a rhombohedral Ref. 13 and b tetragonal Ref. 14 BiFeO3.
APPLIED PHYSICS LETTERS 96, 131907 2010
0003-6951/2010/9613/131907/3/$30.00 © 2010 American Institute of Physics96, 131907-1
ness sensitivity and the difficulty in characterizing low
absorption coefficients via ellipsometry.21,22 Room tempera-
ture transmittance and reflectance spectra were collected us-
ing a Perkin-Elmer Lambda-900 spectrometer 3000–190
nm; 0.41–6.53 eV. We calculated the absorption coefficient
E from combined transmittance and reflectance
measurements.23 Ellipsometric spectra in  , were ex-
tracted at four angles of incidence, =50°, 60°, 70°, and
80°, using a variable-angle dual rotating-compensator multi-
channel spectroscopic ellipsometer24 over a spectral range
from 0.75 to 5.15 eV. E was also determined from spec-
troscopic ellipsometry analysis and compared with the
complementary data obtained from optical transmittance.
Figure 2a displays the 300 K absorption spectrum of
the tetragonal BiFeO3 film compared with that of the rhom-
bohedral analog.17 We assigned the observed excitations
based on recent first-principles calculations.14,15,25 Although
the overall absorption profiles are similar, the familiar 3.2
and 4.5 eV electronic excitations of the rhombohedral mate-
rial, assigned as minority channel dipole-allowed charge
transfer features,17,18,25 are blue shifted by 0.4 eV in te-
tragonal BiFeO3. The blue shift of these strongly hybridized
O p to Fe d excitations is a consequence of the strained na-
ture of the nonequilibrium tetragonal phase. The quasi-
tetragonal film displays an additional electronic feature cen-
tered at 6.2 eV that we assign as a strongly hybridized
majority channel O p+Fe d→Bi p state excitation. The di-
rect band gap is extracted by a linear extrapolation of an
 ·E2 versus E plot to zero Fig. 2b.21 We find the band
gap in tetragonal BiFeO3 to be 3.100.02 eV, 0.4 eV
larger than that in rhombohedral BiFeO3 2.670.02 eV.
This result is different from the recent theoretical
calculations14,15 that predict a smaller band gap in tetragonal
BiFeO3. The 2.5 eV sub band gap absorption feature in the
rhombohedral system17 is also present in the tetragonal ma-
terial. This feature contributes to the onset of optical absorp-
tion at 2.25 eV and is probably a collective excitation.26
The experimental ellipsometric spectra were fit to a pa-
rameterized optical and structural model using a least-
squares regression analysis with an unweighted error
function27 in order to extract the complex dielectric function
spectra and microstructural parameters. The latter include
bulk film and surface roughness thicknesses. The optical
properties of the surface roughness layer were represented by
the Bruggeman effective medium approximation consisting
of fractions of bulk material and void.28 The fitting param-
eters of both materials are given in Table I. The average
unweighted error function obtained from the least-squares
regression fitting is sufficiently low with a value of 6.4
10−3, indicating that the fit using this model is reasonable.
Dielectric functions were extracted by numerical inversion
from each  , spectra using the bulk film and surface
roughness thicknesses determined from the parameterized fit.
Figure 3 displays 1E and 2E for the quasi-tetragonal
BiFeO3 film compared to similar results on the rhombohedral
material.19 Tetragonal BiFeO3 exhibits higher energy charge
transfer excitations that are also broadened compared with
the rhombohedral phase, a result that is overall consistent
with the absorption spectra in Fig. 2c.
FIG. 2. Color online a Absorption coefficient of tetragonal BiFeO3 com-
pared with that of the rhombohedral system Ref. 17. b Direct band gap
analysis of both materials. The triangle marks the 2.670.02 eV charge
gap of the rhombohedral film whereas the square marks the 3.100.02 eV
gap of tetragonal BiFeO3. c E obtained by optical transmittance com-
pares well with that obtained by ellipsometry.
TABLE I. Dielectric function parameters for rhombohedral and tetragonal
BiFeO3. All Tauc–Lorentz TL oscillators Ref. 29 share a common Tauc
gap Eg=2.14, =2.24 for the rhombohedral film and Eg=2.30, =2.49 for
the tetragonal film, where  is the high frequency permittivity a constant
additive term to 1. SO, E0, A, and 	 are, respectively, the Sellmeier oscil-
lator Ref. 30, resonance energy, amplitude, and broadening. All parameters
are in units of energy electron volts.
Oscillator
Rhombohedral R3c a Tetragonal P4mm
E0 A 	 E0 A 	
TL1 2.39 9.54 0.31 3.01 1.05 0.24
TL2 2.97 36.10 0.70 3.37 33.50 0.98
TL3 4.19 44.03 1.63 4.91 47.03 1.77
TL4 /SO 6.15 38.66 3.47 6.50 6.04 0
aReference 20.
FIG. 3. Color online Complex dielectric function spectra =1+ i2 vs
photon energy E obtained for tetragonal BiFeO3 solid line. 1E and
2E of rhombohedral BiFeO3 dashed line are shown for comparison
Ref. 19.
131907-2 Chen et al. Appl. Phys. Lett. 96, 131907 2010
We can also compare the optical properties of tetragonal
BiFeO3 with a local structure analysis Fig. 1, simple crystal
field theory, and first principles electronic structure
calculations.14,15 We begin by considering the FeO6 structure
in rhombohedral BiFeO3 as an ideal octahedron. It is well-
known that, in an octahedral crystal field, the 3d orbital
states split into t2g and eg levels. Strain has important conse-
quences for the local structure. In tetragonal BiFeO3, one of
the Fe–O axial distances increases to 2.80 Å, and the
Fe–O equatorial bond lengths are reduced to 1.93 Å.14
These modifications split the t2g states into a doubly-
degenerate pair dxz ,dyz and a singly-degenerate dxy level,
and the eg states break symmetry to yield a lower energy dz2
state and a higher energy dx2−y2 level. The net effect is to
move the empty Fe3+ 3d states closer to the occupied ones.
Band structure calculations14,15 confirm this trend, predicting
that the unoccupied Fe3+ 3d states broaden and move closer
to the Fermi level in the quasi-tetragonal system. At the same
time, the calculations predict that the O2− 2p states move
further away from the Fermi level.14 Together, these compet-
ing density of states shifts define the predicted charge gap. In
other words, this is a joint density of states effect. Based
upon our measurement of a larger band gap in quasi-
tetragonal BiFeO3 compared to that in the rhombohedral ana-
log, the calculations either overestimate the strain-induced
shift in the Fe3+ 3d states or underestimate the shift of the
O2− 2p states.
To summarize, both optical transmission spectroscopy
and spectroscopic ellipsometry were used to reveal the opti-
cal properties of quasi-tetragonal BiFeO3. Due to the struc-
ture strain and symmetry breaking induced distortions of the
local Fe3+ crystal field and shifts in the O 2p states, the band
gap and charge transfer excitations are blue shifted compared
with those in the rhombohedral material. Although compres-
sive strain stabilizes a non-equilibrium phase, its optical
properties are unfortunately a less attractive match with the
solar spectrum. On the other hand, an applied electric field
can drive a morphotropic phase transition,10 a result that may
allow the development of an electro-optic switch.
This work was supported by the U.S. DOE Grant Nos.
DE-FG02-01ER45885 at UT and DE-AC02-05CH1123 at
Berkeley, NSF Grant Nos. DMR-0820404 and DMR-
0908718 at PSU and CU, and ARO Grant No. W911NF-
08-2-0032 at CU. We thank H. M. Tütüncü for useful dis-
cussions.
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Optical properties of quasi-tetragonal BiFeO3 thin films

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Optical properties of quasi-tetragonal BiFeO3 thin films

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