Visualizing giant ferroelectric gating eﬀects in large-scale WSe 2BiFeO 3heterostructures Raphaël SalazarykSara VarottozkCéline VergnaudVincent Garciaz

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Visualizing giant ferroelectric gating eﬀects in

large-scale WSe2/BiFeO3heterostructures

Raphaël Salazar,∗,†,kSara Varotto,‡,kCéline Vergnaud,¶Vincent Garcia,‡

Stéphane Fusil,‡Julien Chaste,§Thomas Maroutian,§Alain Marty,¶Frédéric

Bonell,¶Debora Pierucci,§Abdelkarim Ouerghi,§François Bertran,†Patrick Le

Fèvre,†Matthieu Jamet,∗,¶Manuel Bibes,∗,‡and Julien Rault†

†Synchrotron SOLEIL, L’Orme des Merisiers, Départementale 128, F-91190 Saint-Aubin,

France

‡Unité Mixte de Physique, CNRS, Thales, Université Paris-Saclay, 1 avenue Augustin

Fresnel, 91767, Palaiseau France

¶Univ. Grenoble Alpes, CEA, CNRS, Grenoble INP, IRIG-SPINTEC, 38000 Grenoble,

France

§Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, 91120,

Palaiseau, France

kContributed equally to this work

E-mail: raphael.salazar@synchrotron-soleil.fr; matthieu.jamet@cea.fr;

manuel.bibes@cnrs-thales.fr

Abstract

Multilayers based on quantum materials (complex oxides, topological insulators,

transition-metal dichalcogenides, etc) have enabled the design of devices that could

revolutionize microelectronics and optoelectronics. However, heterostructures incorpo-

arXiv:2210.14786v1 [cond-mat.mtrl-sci] 26 Oct 2022

rating quantum materials from diﬀerent families remain scarce, while they would im-

mensely broaden the range of possible applications. Here we demonstrate the large-scale

integration of compounds from two highly-multifunctional families: perovskite oxides

and transition-metal dichalcogenides (TMDs). We couple BiFeO3, a room-temperature

multiferroic oxide, and WSe2, a semiconducting two-dimensional material with poten-

tial for photovoltaics and photonics. WSe2is grown by molecular beam epitaxy and

transferred on a centimeter-scale onto BiFeO3ﬁlms. Using angle-resolved photoemis-

sion spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe2

and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polar-

ization direction in the underlying BiFeO3. Such a strong shift opens new perspectives

in the eﬃcient manipulation of TMDs properties by proximity eﬀects.

Transition metal dichalcogenides (TMDs) constitute a class of materials that have gath-

ered a tremendous interest from the solid-state physics community focusing on two-dimensional

(2D) materials. Schematically, TMDs of general formula MX2present a X-M-X conﬁgura-

tion, where a metal is covalently bonded to two chalcogens (M = Mo, W; X = S, Se, Te).

In TMDs, the global structure is made of several MX2planes bonded to one another by van

der Waals (vdW) interactions. They hold promise for numerous applications in photonics

and electronics4,5 owing to their large exciton binding energies,6–8 a theoretical ambipolar-

ity originating from the X-M-X stacking and a transition from an indirect to a direct band

gap when the material reaches the monolayer limit.9,10 They also exhibit a strong spin-orbit

coupling11–13 and a nonlinear and anisotropic Rasbha spin splitting has been predicted by ab

initio calculations14 which are promising properties for spintronics. Owing to their ultrathin

character, their electronic properties are easily tunable by proximity eﬀects or any external

stimuli. The engineering of the electronic and spin properties of TMDs is currently a very

dynamical research area. Various methods based on strain engineering,15 hybrid TMDs het-

erostructures, hybrid dimension (2D/3D) heterostructures, or the exploitation of the twist

degree of freedom have been reported to substantially modify their band structure.16–19

Until recently, most studies on TMDs have focused on micrometer-sized ﬂakes exfoliated

from bulk samples.20,21 Extensive eﬀorts are beginning to enable the growth of wafer-scale

TMD single-crystal ﬁlms.22–24 Yet, a remaining challenge is the transfer of these ﬁlms onto

wafers based on functional materials, notably from diﬀerent structural and chemical families.

A particularly appealing direction consists in coupling TMDs with electrically polarized

materials such as ferroelectrics. This would enable a remanent modulation of their electronic

properties and pave the way towards non-volatile memory devices such as ferroelectric ﬁeld

eﬀect transistors (FeFET) with giant OFF/ON ratios.

Since most technologically-relevant ferroelectrics are perovskite oxides such as BaTiO3

(BTO), Pb(Zr, Ti)O3(PZT) and BiFeO3(BFO), there have been attempts to combine

TMDs with such materials, albeit almost exclusively using exfoliated ﬂakes. Several exper-

iments showed a dependence of the photoluminescence (PL) response on the ferroelectric

polarization direction in MoS2/PZT25 or MX2/BTO.26–29 Chen et al. also deﬁned a p-n

homojunction on micron-sized WSe2ﬂakes deposited on a BFO ﬁlm with pre-poled up and

down polarization domains.5However, these studies could not bring insight into the inﬂuence

of ferroelectricity on the electronic structure of the TMD.

In this work, we present a direct measurement of the modulation of the electronic band

structure of a TMD by ferroelectric polarization using angle-resolved photoelectron emis-

sion spectroscopy (ARPES). We work with high-quality WSe2grown by molecular beam

epitaxy (MBE) and transferred using chemical methods onto BFO ﬁlms grown by pulsed

laser deposition (PLD) with diﬀerent as-grown polarization states. First, we demonstrate

that the single crystalline character of WSe2layers are preserved after transfer on BFO.

Then, we show that 1 to 3 monolayers of WSe2exhibit a giant rigid band shift (ca. 750 meV

for a trilayer) when deposited on upward vs downward polarized BFO. This unprecedented

value oﬀers new opportunities to manipulate the electronic properties of TMDs by proximity

eﬀects.

Epitaxial thin ﬁlms of BFO (001) were prepared by PLD on (110)o-oriented DyScO3

(DSO) (the "o" subindex indicates orthorhombic notation) covered with bottom electrodes

of SrRuO3(SRO) or La0.7Sr0.3MnO3(LSMO). Following Yu et al.,31 the choice of the bottom

electrode sets the direction of the out-of-plane component of the ferroelectric polarization of

the BFO ﬁlm. Fig. 1a presents piezoresponse force microscopy (PFM) phase images of the

out-of-plane piezoresponse signal. In the virgin state, the contrast was homogeneous for both

samples and could be switched reversibly by applying dc voltages higher than the coercive

voltages (cf. concentric square patterns). Before switching, the piezoresponse occurred in

phase with the excitation for the LSMO sample and with a phase shift of 180°for the SRO

samples, indicating that the out-of-plane component of the polarization was pointing up and

down, respectively.

To circumvent the temperature incompatibility for the direct growth of good quality

WSe2ﬁlms on BFO, we ﬁrst grow WSe2ﬁlms on mica substrates.30,32 Reﬂection high-energy

electron diﬀraction (RHEED) indicates the epitaxial growth on the single-crystalline mica

substrate (Fig. 1b). For the trilayer sample (3L), the atomic force microscopy (AFM) image

of Fig. 1c sample reveals a continuous coverage with 22% of the surface corresponding to

3 ML (trilayer), 70% to 2 ML (bilayer) and 8% to 1 ML (monolayer). The area fraction of

uncovered mica substrate (0L) is negligible. The line proﬁle extracted on Fig. 1d shows that

the layers are 0.7 nm thick as expected for bulk WSe2. For the bilayer sample (2L), AFM

images (see Supplementary Information) give a composition of 71% 1 ML and 29% 2 ML.

The WSe2ﬁlms were then wet-transferred on the BFO samples.30 In the case of the 3L

sample, the whole surface was covered (cf. Fig. 1f). X-ray diﬀraction before and after the

transfer shows that the WSe2keeps its crystalline properties, i.e. the substrate does not

induce additional strain or defects in the layer. The in-plane lattice parameter for WSe2on

BFO is a= 0.3284 nm, compared to a= 0.3272 nm on mica. A value of a=0.3282 nm for

bulk WSe2is reported in Ref.?This implies a 0.06% strain for WSe2on BFO comparatively

to a 0.3% strain on mica, meaning that the WSe2layer is nearly relaxed on BFO. Azimuthal

X-ray diﬀraction demonstrates that the WSe2was grown with little angular dispersion and

was transferred in exact conformity as only a little fraction of misoriented grains indicated

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Visualizinggiantferroelectricgatingeectsinlarge-scaleWSe2/BiFeO3heterostructuresRaphaëlSalazar,,y,kSaraVarotto,z,kCélineVergnaud,{VincentGarcia,zStéphaneFusil,zJulienChaste,xThomasMaroutian,xAlainMarty,{FrédéricBonell,{DeboraPierucci,xAbdelkarimOuerghi,xFrançoisBertran,yPatrickLeFèvre,yMatthieuJam...

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Visualizing giant ferroelectric gating eﬀects in large-scale WSe 2BiFeO 3heterostructures Raphaël SalazarykSara VarottozkCéline VergnaudVincent Garciaz

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