BAND-GAP ENGINEERING OF FUNCTIONAL . . . PHYSICAL REVIEW B 91, 165309 (2015)
TABLE I. Gap (in eV) and photon-absorption efficiency η (in
%, for light polarized in the xy and z directions) of the sequences
αβnβ and αnαβ , calculated for a thickness of 10−7 m. The efficiency
calculated for the pure α and β cubic perovskites is included for
comparison.
Gap ηxy ηz Gap ηxy ηz
α 3.33 0.1 0.1 β 1.84 14.9 6.4
αβ 2.26 4.3 0.9 αβ 2.26 4.3 0.9
αβ2 2.10 7.0 1.5 α2β 1.57 5.3 2.1
αβ3 2.04 8.9 2.4 α3β 1.41 4.3 2.0
αβ4 2.00 10.0 3.0 α4β 1.27 4.1 2.0
αβ5 1.98 10.7 3.5 α5β 1.40 2.8 1.5
αβ6 1.97 11.3 3.8 α6β 1.35 2.6 1.5
band-edge states have to be localized in the same layers to
obtain efficient absorption.
IV. CONCLUSIONS
In this work, we have investigated the electronic properties
of perovskite heterostructures obtained by stacking BaSnO3
and BaTaO2N layers. The band gap is seen to be tunable
over the wide range of around 1 eV and the variation
can be understood in terms of quantum confinement and
tunneling. Confinement leads to up-shifts of the conductionband
minimum and thus to increase of the band gap, while
tunneling effects reduce the confinement and lead to lower
band gap. The tunneling effects are seen to decay over a few
perovskite unit cells. The systems studied here are close to
cubic and with similar lattice constants, but in general bandgap
formation in layered perovskites can be expected to depend
sensitively also on strain and lattice distortions/reconstructions
28.
The calculated optical absorption spectra for the heterostructures
indicate that high absorption is only obtained
if the VBM and CBM states are localized in the same spatial
region. The design of heterostructures for efficient visible-light
absorption therefore requires not only appropriate band
gaps, but also tailored band-edge states with proper spatial
overlap.
The stacking of BaSnO3 and BaTaO2N layers that we have
described here is a type-II heterojunction with the conduction
band of BaSnO3 above the valence band of BaTaO2N. A type-I
heterojunction can be designed using different perovskites.
One example is LaAlO3 (as α) and LaTiO2N (as β), with a
calculated indirect band gap between and R points of 6.11
and a direct gap at the point of 1.49 eV, respectively, and
where the band edges of LaTiO2N are placed in between the
edges of LaAlO3. Preliminary results showthat due to the large
band gap of α, there is no tunneling through the α layer and
there is already a full confinement of the β layers with a single
α 29. In addition, the band gaps of the layered combinations
are direct, with the VBM formed by N2p orbitals and the CBM
composed of Ti3d . Also in this case, the stacking has the effect
of placing the VBM and CBM closer together spatially. This
fact might increase the absorption properties of the materials
and, together with the possibility of tuning the band gap using
quantum confinement and tunneling, can be used to design
novel light-harvesting heterojunctions.
ACKNOWLEDGMENTS
The authors acknowledge support from the Catalysis for
Sustainable Energy (CASE) initiative funded by the Danish
Ministry of Science, Technology and Innovation, and from
the Center on Nanostructuring for the Efficient Energy Conversion
(CNEEC) at Stanford University, an Energy Frontier
Research Center founded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences under
Award No. DE-SC0001060. K.S.T. acknowledges support
from the Danish Council for Independent Research Sapere
Aude Program through Grant No. 11-1051390. The Center for
Nanostructured Graphene is sponsored by the Danish National
Research Foundation, Project No. DNRF58.
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