This is a repository copy of WSe2 Light-Emitting Tunneling Transistors with Enhanced
Brightness at Room Temperature.
White Rose Research Online URL for this paper:
http://eprints.whiterose.ac.uk/94114/
Version: Accepted Version
Article:
Withers, F., Del Pozo-Zamudio, O., Schwarz, S. et al. (15 more authors) (2015) WSe2
Light-Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature.
Nano Letters, 15 (12). pp. 8223-8228. ISSN 1530-6984
https://doi.org/10.1021/acs.nanolett.5b03740
eprints@whiterose.ac.uk
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1
WSe
2
light-emitting tunneling transistors with
enhanced brightness at room temperature
F. Withers
1,
2
†, O. Del Pozo-Zamudio
3
†, S. Schwarz
3
, S. Dufferwiel
3
, P. M. Walker
3
, T.
Godde
3
,A. P. Rooney
4
, A. Gholinia
4
, C. R. Woods
1
, P. Blake
1,2
, S. J. Haigh
4
, K. Watanabe
5
, T.
Taniguchi
5
, I. L. Aleiner
6,2
, A. K. Geim
1
, V. I. Fal’ko
1,2
, A. I. Tartakovskii
3
, K. S. Novoselov
1,2
*
1
School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester, M13
9PL, UK
2
National Graphene Institute, University of Manchester, Oxford Road, Manchester, M13 9PL,
UK
3 School of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
4
School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
5
National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
6
Physics Department, Columbia University, New York, New York 10027, USA
†Authors contributed equally
KEYWORDS: electroluminescence, photoluminescence, transition metal dichalcogenides,
tungsten diselenide, hexagonal boron nitride, graphene, van der Waals heterostructure.
Abstract
Monolayers of molybdenum and tungsten dichalcogenides are direct bandgap semiconductors,
which makes them promising for opto-electronic applications. In particular, van der Waals
heterostructures consisting of monolayers of MoS
2
sandwiched between atomically thin
hexagonal boron nitride (hBN) and graphene electrodes allows one to obtain light emitting
quantum wells (LEQW’s) with low-temperature external quantum efficiency (EQE) of 1%.
However, the EQE of MoS
2
and MoSe
2
-based LEQW’s shows behavior common for many other
materials: it decreases fast from cryogenic conditions to room temperature, undermining their
practical applications. Here we compare MoSe
2
and WSe
2
LEQW’s. We show that the EQE of
WSe
2
devices grows with temperature, with room temperature EQE reaching 5%, which is
250x more than the previous best performance of MoS
2
and MoSe
2
quantum wells in ambient
conditions. We attribute such a different temperature dependences to the inverted sign of spin-
2
orbit splitting of conduction band states
in tungsten and molybdenum
dichalcogenides, which makes the
lowest-energy exciton in WSe
2
dark.
Introduction
Recently molybdenum and tungsten
dichalcogenides
1
(referred below as
MoX
2
and WX
2
(X= S,Se),
respectively) have attracted
considerable attention following the
discovery of the indirect-to-direct
bandgap transition
1-4
and the coupling
of the spin and valley degrees of
freedom in atomically thin layers
5, 6
.
Both in WX
2
and MoX
2
electrons and
holes form strongly bound excitons
7-9
which are stable at room temperature
2-4,
10, 11
. Such properties are very attractive
for optoelectronic applications
including photovoltaics
12-15
and light
emitting diodes
10, 16-23
as well as optical
micro-cavity devices
24, 25
including
lasers and exciton-polariton
structures
26
. Furthermore, a strong
spin-orbit interaction in these
compounds, has been predicted by
density functional theory
27, 28
: in WX
2
(MoX
2
) the lowest energy states in the
conduction band and the highest energy
states in the valence band have the
opposite (same) spin orientations,
preventing (enabling) their
recombination with emission of a
photon (see Fig.1F). Thus, according to
the theoretically predicted spin-state
ordering, the lowest-energy excitonic
sub-band in WX
2
corresponds to dark
Figure 1. (A) Schematic of the device architecture.
(B) High resolution transmission electron
microscopy image of a cross-sectional slice of a
WSe
2
LEQW on a DBR substrate. (C) Band
alignment at high bias of a WSe
2
LEQW. (D)
Schematic representation of the band structure of
WSe
2
. Red and blue arrows denote spin orientation.
(E) Current density vs bias voltage V
b
for the
presented devices. (F) 50x magnification
monochrome image of a WSe
2
LEQW device with
an applied bias of V
b
=2 V and current of 2 A taken
in ambient conditions with weak backlight
illumination. Red false color: Au contacts to bottom
graphene, Blue false color: Au contacts to top
graphene (Central white region corresponds to strong
electroluminescence) . See supplementary
information for fabrication details.
3
excitons, separated from the bright exciton sub-band by the energy combined from the electron
spin-orbit splitting
SO
(of the order of 30-40 meV
27-30
) and electron-hole exchange interaction
energy. As we show here, such band-structure properties of WX
2
strongly impact on the LEQW
performance, leading to a significant enhancement of the room temperature EQE of the WSe
2
LEQW’s in the electroluminescence (EL) regime. This is in contrast to a more common behavior
observed in MoX
2
LEQW’s
10
where the EL intensity falls by up to 100 times when the
temperature is varied from 6 to 300 K leading to significant reduction of the EQE.
Experimental procedure
The main component of our stacked-layer van der Waals heterostructure LEQWs is a light-
emitting monolayer of WSe
2
encapsulated between thin (2-5 monolayers) hexagonal boron
nitride (hBN) barriers with top and bottom transparent graphene electrodes for vertical current
injection (Fig. 1). The layer stacks in the van der Waals structure were manufactured using
multiple ‘peel/lift’ procedure
31, 32
in ambient conditions. The high quality of the samples is
confirmed by cross-sectional TEM measurements (Fig. 1B), which demonstrate micron scale
absence of contamination between the layers occurring as a result of the self-cleansing effect
32, 33
(see supplementary information for AFM and dark field optical microscopy of different devices).
We also fabricate similar LEQW structures comprising MoSe
2
monolayers. Optical properties of
the LEQW devices are studied using micro-photoluminescence (PL) at low bias voltages,
typically V
b
<1.8 V (or micro-electroluminescence (EL) measured at larger biases, typically
V
b
>1.8 V) with samples placed in a variable temperature cryostat (see Methods).
By applying a bias voltage, V
b
, between the two graphene electrodes we are able to pass a
tunnel current through the device (Fig. 1E), with the magnitude of the current determined by the
largest thickness of one of the hBN barriers. Fig.1E shows the current density (J) vs bias voltage
(V
b
) for four devices having different hBN barrier thicknesses. At high bias we are able to
simultaneously inject electrons and holes into the transition metal dichalcogenide (TMDC) layer,
Fig. 1C, which is observed as a strong increase of the tunnel conductivity. The lifetime of the
injected carriers within the active region of the quantum well is expected to vary as
-N
where
is the probability of an electron tunneling a single layer of hBN and N is the number of hBN
atomic layers
34-36
(denoted L below). If the lifetime is long enough then excitons can form within
the TMDC and recombine radiatively (Fig.1F). For hBN thickness below 2 L a high proportion
of current will be created by direct carrier tunneling through the whole heterostructure leading to
a reduction of the current-to-light conversion efficiency. For thicker barriers the lifetime of the
carriers increases, leading to improved light emission efficiency. However, in this case the
maximum current density drops leading to dimmer LEQW’s. We find that 2-3 layers of hBN is
an optimal compromise between overall brightness and efficiency (eN
ph
/I) of our devices.
Results
4
Typical light-emission behavior of a WSe
2
LEQW at T = 4.2 K is shown in Fig. 2. PL bias-
dependence is shown in Fig. 2B where at zero bias we measure a spectrum shown in Fig. 2E
exhibiting several peaks. We use notation adopted in You et al
37
, and similarly observe neutral
exciton X
0
(1.725eV) and trion X
-
(1.69 eV) peaks as well as a number of additional features,
with the most pronounced peaks P
1
(1.669 eV) and P
2
(1.649 eV) and a band at photon energies
below 1.64 eV denoted P
3
. We would like to stress that, although features P
1
-P
3
are always
present in all our samples, their relative intensity varies from device to device. We attribute the
low energy peaks to excitons localized on defects in the TMDC: This is in agreement with
theoretical prediction, that their intensity decays faster upon heating than the intensity of the
trion line
38
For biases |V
b
|>2V, we observe strong electroluminescence (EL). In Figs. 2A,C,D,F the peaks
in EL can be easily traced to the peaks in PL spectra, as only insignificant energy shifts of ~3
Figure 2. Contour maps of the EL spectra from a 5+/- 1L hBN- WSe
2
-3L hBN
LED at T=
4.2 K for negative (A) and positive (C) bias voltage. (B) PL contour map as a function of
V
b
, measured at an excitation power of 10 W and excitation energy of 1.95 eV. Current
density vs bias voltage for this device is shown in Fig. 1E (black curve) (E) PL spectrum at
V
b
=0V. (D), (F) Bias dependence of the EL for negative and positive polarities respectively
(Low bias spectra are presented more clearly in the supplementary materials).