Spontaneous Polarization and Bulk Photovoltaic Effect Driven by Polar Discontinuity in ${\mathrm{LaFeO}}_{3}/{\mathrm{SrTiO}}_{3}$ Heterojunctions

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Spontaneous Polarization and Bulk Photovoltaic Effect Driven by Polar Discontinuity in ${LaFeO}_{3} / {SrTiO}_{3}$ Heterojunctions

M. Nakamura, F. Kagawa, T. Tanigaki, H. S. Park, T. Matsuda, D. Shindo, Y. Tokura, and M. Kawasaki

Phys. Rev. Lett. 116, 156801 – Published 11 April 2016

Abstract

Structurally coherent and chemically abrupt interfaces formed between polar and nonpolar perovskite oxides provide an ideal platform for examining the purely electronic reconstruction known as the polar catastrophe and the emergence of mobile or bound charges at the interface. The appearance of mobile charges induced by the polar catastrophe is already established in the ${LaAlO}_{3} / {SrTiO}_{3}$ heterojunctions. Although not experimentally verified, the polar catastrophe can also lead to the emergence of spontaneous polarization. We report that thin films of originally nonpolar ${LaFeO}_{3}$ grown on ${SrTiO}_{3}$ are converted to polar as a consequence of the polar catastrophe. The induced spontaneous polarization evokes photovoltaic properties distinct from conventional $p − n$ junctions, such as a switching of the photocurrent direction by changing the interfacial atomic sequence. The control of the bulk polarization by engineering the interface demonstrated here will expand the possibilities for designing and realizing new polar materials with photovoltaic functions.

Received 27 November 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.156801

Physics Subject Headings (PhySH)

Ferroelectricity

Interfaces Oxides Photovoltaic absorbers Two-dimensional electron gas

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

M. Nakamura^1,*, F. Kagawa¹, T. Tanigaki^1,†, H. S. Park^1,‡, T. Matsuda², D. Shindo^1,3, Y. Tokura^1,4, and M. Kawasaki^1,4

¹RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
²Japan Science and Technology Agency (JST), Kawaguchi 332-0012, Japan
³Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
⁴Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan

^*masao.nakamura@riken.jp
^†Present address: Research and Development Group, Hitachi, Ltd., Hatoyama, Saitama 350-0395, Japan.
^‡Present address: Department of Materials Science and Engineering, Dong-A University, Busan 604-714, Republic of Korea.

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Vol. 116, Iss. 15 — 15 April 2016

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Figure 1
(a) A schematic of the elemental stacking in the $LFO / STO$ junctions. The interfacial atomic layers were controlled and were either ${TiO}_{2}$ or ${FeO}_{2}$ layers. (b) Elemental distribution mappings for a ${TiO}_{2}$ junction (upper panel) and a ${FeO}_{2}$ junction (lower panel) taken with a STEM, where atomic resolution energy-dispersive x-ray spectroscopy images are superimposed on high-angle annular dark images. Line profiles of the elemental distribution are shown in the Supplemental Material [20]. (c) Current density ( $J$ )–voltage ( $V$ ) properties of the $LFO / Nb : STO$ junctions upon shinning laser light. The wavelength of the laser is 473 nm. The positive direction of the applied voltage is defined by the schematics illustrated in the inset.
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Figure 2
(a) A schematic of the piezoresponse force microscope observation for a LFO film grown on a Nb doped STO substrate having both ${TiO}_{2}$ and SrO surface terminated regions on the surface. (b) Observation of artificially tailored polarization domains by the piezoresponse force microscope as a phase image. The boundary of the opposite polarization domains is coincident with that of the interface termination.
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Figure 3
(a) Mappings of the electrostatic potentials derived from electron holography for two differently interface terminated $LFO / STO$ junctions. Here, we show the potentials for an electron. (b) Line profiles of the potentials along the dashed lines in (a). (c) Possible band structure in the $LFO / STO$ junctions deduced from the potential profiles in (b) and capacitance measurements [20]. (d) A model for electrostatic conditions in a ${TiO}_{2}$ junction. $σ_{0}$ denotes the bound charges stemming from the polar discontinuity between LFO and STO. $P$ is an electric polarization induced in LFO to partially screen $σ_{0}$ . $E$ is a built-in electric field originating from the remnant interface charges ( $σ_{0} - P$ ).
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Figure 4
(a) Schematic of two possible origins of the photocurrent in the $LFO / STO$ junction. The shift current ( $J_{shift}$ ) is induced by spontaneous polarization while the drift current ( $J_{drift}$ ) is induced by the internal electric field. (b) Photocurrent action spectra for ${TiO}_{2}$ junctions with different LFO thicknesses. (c) Band structures and photocurrent generation in the $LFO / STO$ junctions in three thickness ranges. $Δ$ and $ϕ$ stand for the band gap of LFO and the potential elevation, respectively. Case (i): the slope of the electrostatic potential is constant and the origin of the photocurrent is $J_{drift}$ only. Case (ii): when $t > t_{c}$ , $ϕ$ is constant and $J_{drift}$ and $J_{shift}$ coexist. Case (iii): at $t = t_{c}^{'} > t_{c}$ , $J_{shift}$ becomes more dominant than $J_{drift}$ . (d) Calculated thickness dependence of the spontaneous polarization ( $P_{s}$ ) and dielectric polarization ( $P_{d}$ ) on the basis of the band structure. The solid (dashed) lines correspond to the case of $ϕ = 0.17$ (2.2) eV. (e) Magnitude of the photocurrent as a function of the thickness of LFO in ${TiO}_{2}$ junctions measured at 2.8 and 3.5 eV.
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Physical Review Letters

Spontaneous Polarization and Bulk Photovoltaic Effect Driven by Polar Discontinuity in LaFeO3/SrTiO3 Heterojunctions

M. Nakamura, F. Kagawa, T. Tanigaki, H. S. Park, T. Matsuda, D. Shindo, Y. Tokura, and M. Kawasaki

Phys. Rev. Lett. 116, 156801 – Published 11 April 2016

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Spontaneous Polarization and Bulk Photovoltaic Effect Driven by Polar Discontinuity in ${LaFeO}_{3} / {SrTiO}_{3}$ Heterojunctions