RESEARCH ARTICLE
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Dramatically Enhanced Valley-Polarized Emission by
Alloying and Electrical Tuning of Monolayer WTe
2
x
S
2(1-
x
)
Alloys at Room Temperature with 1T
′
-WTe
2
-Contact
Wei-Hsiang Lin,* Chia-Shuo Li, Chih-I Wu, George R. Rossman, Harry A. Atwater,*
and Nai-Chang Yeh*
Monolayer ternary tellurides based on alloying different transition metal
dichalcogenides (TMDs) can result in new two-dimensional (2D) materials
ranging from semiconductors to metals and superconductors with tunable
optical and electrical properties. Semiconducting WTe
2
x
S
2(1-
x
)
monolayer
possesses two inequivalent valleys in the Brillouin zone, each valley coupling
selectively with circularly polarized light (CPL). The degree of valley
polarization (DVP) under the excitation of CPL represents the purity of valley
polarized photoluminescence (PL), a critical parameter for opto-valleytronic
applications. Here, new strategies to efficiently tailor the valley-polarized PL
from semiconducting monolayer WTe
2
x
S
2(1-
x
)
at room temperature (RT)
through alloying and back-gating are presented. The DVP at RT is found to
increase drastically from
<
5% in WS
2
to 40% in WTe
0.12
S
1.88
by Te-alloying to
enhance the spin-orbit coupling. Further enhancement and control of the DVP
from 40% up to 75% is demonstrated by electrostatically doping the
monolayer WTe
0.12
S
1.88
via metallic 1T
′
-WTe
2
electrodes, where the use of
1T
′
-WTe
2
substantially lowers the Schottky barrier height (SBH) and weakens
the Fermi-level pinning of the electrical contacts. The demonstration of
drastically enhanced DVP and electrical tunability in the valley-polarized
emission from 1T
′
-WTe
2
/WTe
0.12
S
1.88
heterostructures paves new pathways
towards harnessing valley excitons in ultrathin valleytronic devices for RT
applications.
W.-H.Lin,H.A.Atwater
Department of Applied Physics
California Institute of Technology
Pasadena, CA 91125, USA
E-mail: whlin@alumni.caltech.edu;haa@caltech.edu
C.-S.Li,C.-IWu
GraduateInstituteofPhotonicsandOptoelectronicsandDepartmentof
ElectricalEngineering
NationalTaiwanUniversity
Taipei,Taiwan106,P.R.China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/advs.202304890
© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.
DOI: 10.1002/advs.202304890
1. Introduction
Monolayer 1H-phase transition metal
dichalcogenides (TMDs) such as WS
2
are direct band gap semiconductors that
consist of an atomic layer of tungsten sand-
wiched between a top and a bottom layer of
sulfur atoms that are arranged in their re-
spective hexagonal lattice structure.
[1,2]
The
band structures of monolayer TMDs
[3–7]
consist of two inequivalent K (
−
K) valleys
in the hexagonal Brillouin zone. The strong
spin-orbit coupling and broken inversion
symmetry in monolayer TMDs result in
a large energy splitting between the top
spin-up (spin-down) valence band and the
bottom spin-down (spin-up) valence band
in the K (
−
K) valley via the preservation
of time reversal symmetry.
[8–13]
Given that
both of the Berry curvature and orbital
magnetic moment are odd under the
time-reversal symmetry operation, one can
selectively populate excitons in different
valleys (K or
−
K) by means of circularly
polarized light (CPL), where CPL with posi-
tive helicity (
휎
+
) couples to the K valley and
that of the negative helicity (
휎
−
) couples to
the
−
K valley according to valley-dependent
G. R. Rossman
Department of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA 91125, USA
N.-C. Yeh
Department of Physics
California Institute of Technology
Pasadena, CA 91125, USA
E-mail: ncyeh@caltech.edu
N.-C.Yeh
KavliNanoscienceInstitute
CaliforniaInstituteofTechnology
Pasadena,CA91125,USA
Adv. Sci.
2024
,
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, 2304890
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optical selection rules.
[14–16]
However, the degree of valley polar-
ized emission in monolayer TMDs under CPL depends strongly
on the intervalley scatterring time and exciton lifetimes. There-
fore, understanding the processes that govern the exciton life-
times and the associated degree of valley polarization is essential
for assessing the emergent applications of valley-polarized exci-
tons in devices.
[17]
Various strategies aiming at enhancing the
valley polarization by further breaking the spatial-inversion sym-
metry have been proposed, including applying magnetic fields,
chemical doping of magnetic elements, and employing magnetic
proximity effects.
[17–22]
However, with respect to these methods
for enhancing the valley polarization, the efficiency of applying
an external magnetic field is extremely low for valley polariza-
tion (
≈
0.3 meV per Tesla); magnetic doping suffers from the
formation of inhomogeneously distributed dopant clusters; and
magnetic proximity effects are easily diminished by the valley
submergence. Alternative approaches by electrical and optical
control of the valley polarization in TMDs at room temperature
and under off-resonance conditions would be more practical and
desirable.
[13,17]
Towardsthisgoal,carrierdoping,includingchem-
ical and physical approaches, appears to be an efficient way to
manipulate the valley polarization, because excess carriers intro-
duced in the TMDs not only tailor the exciton species but also
modify the valley polarization dynamics considerably. Chemi-
cal doping is known to be an effective and convenient method
to modify the carrier concentrations and electronic bandstruc-
tures in monolayer TMD materials, which can induce shifts in
the Fermi level as well as modifications to the electronic, opti-
cal, and valley polarization properties. Physcial doping, via ei-
ther electrostatic carrier doping by gating or light excitation by
creating electron-hole pairs, can also induce valley polarization
enhancement through stronger screening of the Coulomb inter-
action by excess carriers, which helps suppress the intervalley
scattering.
[17]
For effective manipulation of the valley degrees of freedom in
semiconducting monolayer TMDs by electrostatic doping, it is
essential to address the interfacial issues of Fermi level pinning
and Schottky barrier heights when making electrical contacts to
the TMDs. To date, several approaches to circumvent these is-
sues in TMD-based field effect transistors (FETs) have been re-
ported, including the use of a low work function (WF) metal for
the electrical contact,
[23,24]
the use of a Fermi-level de-pinning
layer,
[25,26]
and various techniques of molecule/chemical doping
of TMDs.
[27–32]
Developing heterostructures that consist of a two-
dimensional (2D) van der Waals (vdW) metal as the top contact
material on a 2D semiconductor is another approach to lower the
Schottky barrier height (SBH).
[33–36]
For this purpose, a natural
material for consideration is graphene.
[37]
However, deposition of
another metallic layer on graphene is required for electrical char-
acterizations, and the carrier injection efficiency generally varies,
depending on the metal deposited. Alternatively, the metallic 1T
′
-
phase WTe
2
with a low WF
[38]
and a vdW clean surface
[39]
may be
considered as an efficient electron-type (
n
-type) contact material
for 2D semiconductors. However, there have not been extensive
studies to date on using the 1T
′
-phase WTe
2
as the metal contact
to lower the contact resistance of TMD-based devices due to the
challenges of material preparation and material stability.
[40–43]
In this study, ternary WTe
2
x
S
2(1-
x
)
(0
≤
x
≤
1) alloys were syn-
thesized via chemical vapor deposition in a one-step synthesis
process to produce high-quality 2D semiconductors of tunable
bandgaps for high-performance eletronic devices. By alloying Te
into tungsten disulfide WS
2
, the WF of the ternary WTe
2
x
S
2(1-
x
)
(0
≤
x
≤
1) alloy could be tuned to match that of the 2D contacts
as the source (S) / drain (D) electrodes in the FET structure to re-
duce the SBH. These monolayer ternary WTe
2
x
S
2(1-
x
)
(0
≤
x
≤
1)
alloys evolved from the semiconducting 1H phase to the metallic
1T
′
phase, depending on the Te concentration (
x
). X-ray photo-
electron spectroscopic (XPS) characterizations confirmed the ex-
istence of W, S, and Te with controlled ratios. The optical bandgap
of the WTe
2
x
S
2(1-
x
)
alloy could be tuned from 2 to 1.65 eV in the
1H semiconducting phase and then dropped down to 0 in the 1T
′
metallic phase. The FET devices based on monolayer WTe
2
x
S
2(1-
x
)
alloys revealed characteristics that confirmed the 1H phase being
n-type semiconductors and the 1T
′
phase being a metal. More-
over, the use of WTe
2
metallic contacts with a WF close to the
band edge of the WTe
0.12
S
1.88
alloy resulted in WTe
2
x
S
2(1-
x
)
-based
FETs with excellent electronic characteristics, including a high
electron carrier mobility up to 50 cm
2
V
−
1
S
−
1
and an on/off cur-
rent ratio up to 10
6
. Furthermore, it has been reported that valley
polarization can be tuned by doping,
[44]
defects,
[13,45]
and alloying
engineering.
[46]
In particular, alloying with heavier elements can
modify the valley polarization by enhancing the spin-orbit cou-
pling (SOC). Therefore, it is worth investigating how the valley
polarized emission from WTe
2
x
S
2(1-
x
)
alloys under CPL evolves
with the concentration of Te. We note that the degree of valley
polarization (DVP) for as-grown monolayer WS
2
is typically very
low (
<
5%) at RT due to significant phonon- and defect-induced
inter-valley scattering (
Figure 1
a), where the DVP value (
P
DVP
)is
defined by the following expression:
P
DVP
=
I
(
휎
+
)
−
I
(
휎
−
)
I
(
휎
+
)
+
I
(
휎
−
)
(1)
with
I
(
휎
+
)and
I
(
휎
−
) denoting the right-handed (RH) and left-
handed (LH) circular polarization-resolved photoluminescence
(PL) intensity, respectively. In contrast, the
P
DVP
values in mono-
layer ternary alloys WTe
2
x
S
2(1-
x
)
with
x
>
0werefoundtobe
tunable and were enhanced up to 40% under the excitation
of right-handed circularly polarized (RCP) light. The underly-
ing mechanism for tailoring the valley-polarized PL of mono-
layer 1H-ternary WTe
2
x
S
2(1-
x
)
alloys may be attributed to the
enhanced SOC strength and broken mirror symmetry by mix-
ing the Te-S species, as schematically shown in Figure 1b. The
stronger SOC of the Te atoms than that of the S atoms can in-
crease the spin-orbit energy splitting (
Δ
SO
) so that
Δ
SO
(WTe
2
)
=
484 meV and
Δ
SO
(WS
2
)
=
412 meV.
[42]
Additionally, by ap-
plying a back-gated voltage
V
G
to WTe
2
x
S
2(1-
x
)
-based FETs with
1T
′
-WTe
2
as the contact electrodes, the resulting DVP values
were found to be further enhanced from 40% for
V
G
=
0up
to
≈
75% for
V
G
=−
20 V. This finding suggests that modu-
lating the carrier doping level can enhance the valley polar-
ization by screening the long-range electron-hole exchange in-
teractions, thus reducing the momentum-dependent intervalley
scattering, as shown in Figure 1c. Overall, we have demonstrated
successfully tuning and drastically enhancing the DVP values in
semiconducting monolayer 1H-TMDs at RT by combined strate-
gies of chemically alloying and electrically gating the monolayer
TMD-based FETs with electrodes of reduced SBH and weakened
Adv. Sci.
2024
,
11
, 2304890
© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH
2304890 (2 of 15)
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Figure 1.
Proposed mechanism for tailoring the valley polarized PL of gated WTe
2
/6%-WTe
2
x
S
2(1-
x
)
heterostructure: a) Schematics of the energy bands
of monolayer WS
2
under right-handed CPL with significant intervalley scattering and therefore comparable decay rates for both
휎
+
and
휎
−
excitons. b)
Schematics of the energy bands of monolayer 6%-WTe
2
x
S
2(1-
x
)
under right-handed CPL, showing a significantly increased decay rate for
휎
+
excitons. c)
Schematics of the energy bands of a gated 6%-WTe
2
x
S
2(1-
x
)
/WTe
2
heterostructure, showing both an increased decay rate of
휎
+
excitons and suppressed
intervalley scattering due to carrier doping-induced screening of the long-range electron-hole interaction.
Adv. Sci.
2024
,
11
, 2304890
© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH
2304890 (3 of 15)