PHYSICAL REVIEW B 91, 165309 (2015)
Band-gap engineering of functional perovskites through quantum confinement and tunneling
Ivano E. Castelli,* Mohnish Pandey, Kristian S. Thygesen, and Karsten W. Jacobsen
Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
(Received 30 December 2014; revised manuscript received 25 March 2015; published 24 April 2015)
An optimal band gap that allows for a high solar-to-fuel energy conversion efficiency is one of the key factors to
achieve sustainability.We investigate computationally the band gaps and optical spectra of functional perovskites
composed of layers of the two cubic perovskite semiconductors BaSnO3 and BaTaO2N. Starting from an indirect
gap of around 3.3 eV for BaSnO3 and a direct gap of 1.8 eV for BaTaO2N, different layerings can be used to
design a direct gap of the functional perovskite between 2.3 and 1.2 eV. The variations of the band gap can be
understood in terms of quantum confinement and tunneling.We also calculate the light absorption of the different
heterostructures and demonstrate a large sensitivity to the detailed layering.
DOI: 10.1103/PhysRevB.91.165309 PACS number(s): 68.35.bg, 73.21.Ac, 73.20.−r, 78.20.−e
I. INTRODUCTION
Functional oxides form a fascinating class of materials
exhibiting a large range of phenomena and with great potential
for technological applications. Some of their properties include
high-temperature superconductivity, multiferroic and halfmetallic
behavior, thermoelectric, magnetocaloric, and photoconductivity
effects, transport phenomena, and catalytic properties
1. The oxides in the perovskite structure constitute an
interesting subclass with high stability and new underexplored
possibilities for producing layered heterostructures with atomically
well-defined interfaces. Effects of quantum confinement
in atomically layered perovskites have been discussed in
several different heterosystems 2. Yoshimatsu et al. 3–5
have studied quantum wells of the metal SrVO3 embedded
in an insulator, SrTiO3, with photoemission, demonstrating
that modifications of the electronic structure develop below
six layers of SrVO3 and that for a single layer a substantial
gap appears. Other studies include confinement effects on
the magnetic structure of LaMnO3/SrMnO3 superlattices 6
and recent investigations of how non-Fermi-liquid behavior
appears when a SrTiO3 quantum well embedded in SmTiO3
is sufficiently thin 7. More recently, Grote et al. 8 have
investigated how to tune the band gap of tin- and lead-halide
perovskites through effects of atomic layering and quantum
confinement.
In the present work, we investigate the band gaps and the
light-absorption properties of functional perovskites obtained
by stacking cubic perovskite planes, with general formula
ABO3, in one direction (say, the z axis) while the other two
directions preserve the cubic symmetry, as shown in Fig. 1. The
possibilities for producing such structures are numerous, but
little is known about the potential for systematic, quantitative
control of their properties.We showthat a large variation of the
band gap can be obtained and that the size of the band gap for
a particular stacking sequence can be understood in terms of
confinement and tunneling behavior. Using these ingredients,
an engineering of the band gap can be pursued to tune the gap
*ivca@fysik.dtu.dk. Present address: Theory and Simulation of
Materials (THEOS) and National Center for Computational Design
and Discovery of Novel Materials (MARVEL), ´ Ecole Polytechnique
F´ed´erale de Lausanne, CH-1015 Lausanne, Switzerland.
to a desired window. This approach could potentially be used
to achieve high efficiencies in light-harvesting devices.
More specifically, we consider combinations of the two
cubic perovskite semiconductors BaSnO3 and BaTaO2N,
indicated with α and β in Fig. 1, respectively 9. The choice
of these two materials as building blocks is based on the fact
that both BaSnO3 and BaTaO2N have been previously selected
as good materials for light harvesting and photocatalytic water
splitting 10,11 and their crystal lattices are rather similar,
with the consequence that the obtained layered structure 12
will not be subjected to high stress.
All of the calculations presented in this work are performed
in the framework of density functional theory (DFT) using
the electronic structure code GPAW 13,14. Due to the wellknown
problem of standard DFT with the underestimation
of the band gaps, the gaps have been calculated using the
GLLB-SC potential by Gritsenko, van Leeuwen, van Lenthe,
and Baerends (GLLB) 15, modified by Kuisma et al. 16
to include the correlation for solids (-SC). This potential
has been shown to provide realistic estimates of band gaps
when compared with other more advanced computational
methods and experiments for a range of semiconductors and
insulators including oxides without too strong correlation
effects 10,17–19. One reason for the favorable comparison
is the addition to the DFT Kohn-Sham gap of the so-called
derivative discontinuity, which is explicitly calculated in the
GLLB-SC approach. We have furthermore performed hybrid
calculations using the functional proposed by Heyd, Scuseria,
and Ernzerhof (HSE06) 20,21 as a comparison for a subset
of the layered materials investigated in this work.
II. BAND GAPS
The compounds that we study here are all obtained by
stacking nα layers of α with nβ layers of β, where 1
nα(β) 6, and then repeating this unit periodically. The lattice
parameter is taken equal to the average value of the lattices
of α and β (4.1A° 22). BaSnO3 and BaTaO2N have been
frozen in their perfect cubic perovskite symmetry, i.e., without
any distortion. Even though distortions usually have large
effects on the band gaps, BaSnO3 and BaTaO2N have a high
cubicity so that the changes in the band gaps are expected to
be small. Keeping the structures frozen in the cubic symmetry
furthermore allows us to analyze the changes in the electronic
1098-0121/2015/91(16)/165309(6) 165309-1 ©2015 American Physical Society