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Gate Tunable WSe2-BP van der Waals Heterojunction Devices a
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Peng Chen , Ting Ting Zhang , Jing zhang , Jianyong Xiang , Hua Yu , Shuang Wu , Xiaobo Lu , Received 00th January 20xx, Accepted 00th January 20xx
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Guole Wang , Fusheng Wen , Zhongyuan Liu , Rong Yang , Dongxia Shi & Guangyu Zhang*
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DOI: 10.1039/x0xx00000x www.rsc.org/
Due to the weak screening effect, the concentration and type of charge carriers in 2D semiconductor heterostructures can be effectively tuned by electrostatic gating, enabling us to realize different type heterojunctions in a single device. Such ‘type tunable’ properties are useful for designing novel electrical or optoelectircal devices. Here, we demonstrate a ‘type tunable’ heterojunction device construct with two pieces of ambipolar 2D semiconductors: WSe2 and Black phosphorus (BP). This heterojunction could be tuned to either p-p junction or n-n junction by the gate modulation. The p-p junction shows large current rectification ratio while the n-n junction shows negligible current rectification ratio, indicating a large valence band offset and a small conduction band offset at WSe2/BP interface. In the optoelectrical measurements, We found the amplitude and even the polarity of photocurrent could be modulated by the electrostatic gating. Our study could further enhance the understanding of designing devices based on these ‘type tunable’ van der Waals heterojunctions. Moreover, the properties of the WSe2/BP interface were also experimentally identified through the electrical and optoelectrical meaurements in our study.
Heterojunctions built from different 2D atomic crystals with van der
properties, such as the band offset, band bending slopes and band
Waals (vdW) interactions are fundamentally different from those
bending directions
conventional
vdW-
in p-p junction and n-n junction are determined by conduction band
heterojunctions are free from the atomic interdiffusion thus have
offset and valence band offset, respectively; and the interfacial
covalently-bonded
materials.
These
1-3
10-12
. For example, the charge carriers transport
atomically regulated interfaces and thereby sharp band edges .
band bending directions are completely reversed between p-p
More interestingly, the concentration and even the type of charge
junction and n-n junction. Previous studies have demonstrated the
carriers in these vdW-heterojunctions can be tuned by the gate
heterojunctions
4-9
based
on
n-type
MoS2
and
other
2D
voltage owing to their ultra-thin nature . The gate tunable
semiconductors, such as WSe2 and black phosphorus (BP), etc.
behavior makes it possible to realize different types of
However, due to the Fermi level pinning of MoS2, the gate
heterojunctions in a single device. The electrical and optoelectrical
tunablility of those heterojunction devices is limited
properties are substantially different for different types of
report heterojunctions stacked vertically by two pieces of
heterojunctions, which are determined by their basic interfacial
ambipolar 2D atomic crystals (WSe2 and BP). Owing to unpinned
13-17,34
. Here we
7, 8
Fermi level of both the BP and WSe2 , the heterojunction could be tuned to either p-p junction or n-n junction via electrical gating, and these two kinds of heterojunctions show completely different
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electrical and optoelectrical properties. From the electrical and
(Supplementary Materials Section B). A back gate voltage (VBG) was
optoelectrical measurements of the p-p and n-n junction, the
applied through the P Si to modulate the carrier concentrations in
properties of the WSe2/BP interface were also experimentally
the heterostructure. Fig. 1b shows the band structure of WSe2 and
identified.
BP, respectively. According to previous studies
++
WSe2-BP heterostructures were fabricated by stacking BP thin layer on top of WSe2 thin layer (Supplementary Materials Section A). Fig. 1a presents schematic illustration of a WSe2-BP heterostructure on ++
a Si(P )/SiO2(100nm) substrate. Note that a BP monolayer has a 18
puckered honeycomb structure of P atoms ; and a WSe2 monolayer consists of a hexagonally packed plane of W atoms 5
sandwiched between two planes of Se atoms . An optical image of a
WSe2 (
19, 20
, the bandgap of
) and BP ( ) is ~1.3 eV at 12nm-thickness and ~0.4
eV at 20nm-thickness, respectively; while the electron affinity of WSe2 ( ) and BP ( ) is ~4 eV and ~4.1 eV, respectively. Fig. 1c shows the transfer curve of individual WSe2 and BP, and both of them exhibit ambipolar characteristics. The 12 nm WSe2 shows an on/off ratio of ~105, and the 20 nm BP shows an on/off ratio of ~7. These results are consistent with previous reports7-9, 21.
typical WSe2-BP heterojunction device with contact electrodes of
To investigate electrical properties of this vdW-heterojunction,
Ti(6 nm)/Au(50 nm) is also shown in Fig. 1a. The thickness of WSe2
we performed measurements between electrodes D1 (contacted
and BP flake is 12 and 20 nm, respectively, for this particular device
with WSe2) and S1 (contacted with BP). Fig. 2a shows the currentvoltage (IDS-VDS) curves of the WSe2-BP heterojunction at VBG=-35 V, -33 V, and -31 V (Supplementary Materials Section D shows the semi-log plot). The IDS-VDS curves exhibit strong current rectification properties with the forward current (IF) much larger than the reverse current (|I R |). Inset of Fig. 2a shows the current rectification ratio (|IF/IR|) calculated by dividing IF at VDS=3 V by |IR| at VDS=-3 V. The |IF/IR| is ~2.5×103 at VBG=-35 V and decreases to ~96 at VBG =-31V. These results can be understood by a simple model, where the total resistance of the device can be roughly
Fig. 1 (a) Top left: schematic illustration of a WSe2-BP heterojunction device on a Si/SiO2 substrate. Top right: optical image of the fabricated device, where D1 and D2 (S1 and S2) denotes the metal contacted on WSe2 (BP). The scale bar is 5μm. Bottom: atomic view of the WSe2-BP heterostructure. (b) Band structure of the WSe2 and BP, respectively. (c) The transfer curve at VDS=100 mV for individual WSe2 (blue curve, measured between D1 and D2) and BP (red curve, measured between S1 and S2).
divided into three parts: the resistance of the junction near the WSe2/BP interface, the sheet resistances of WSe2 and BP, and the contact resistances of metal/WSe2 and metal/BP. We excluded the current rectification comes from the contacts (Supplementary materials section E), and identified it is origin from the WSe2/BP interface. According to Fig. 1c, both WSe2 and BP are p-type at these back gate voltages. When the p-type WSe2 stacked with the p-type BP, a p-p heterojunction is formed. Based on the band
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ARTICLE 19,20
structure of WSe2 and BP reported in literatures
, we proposed a
rectification effect (Supplementary Materials Section F). Moreover,
reasonable band alignment of p-p junction (zero bias) in Fig. 2c.
the direction of the current rectification is also consistent with the
Since the work function of p-type WSe2 is higher than that of p-type
experimental result shown in Fig.2a. The decrease of current
to the increased sheet resistance, which doesn’t contribute to the current rectification. Note that the MoS2 based 2D p-n heterojunctions have demonstrated large current rectification 13-17
properties
, while our experimental results indicate the strong
current rectification can also be realized in the 2D p-p heterojunctions, and such structures are potentially very fast since 28
only majority-carrier transport is involved .
Fig. 2 (a) IDS-VDS curves measured at VBG=-35 V, -33 V and -31 V. The inset shows current rectification ratio at VDS=±3 V. (b) IDS-VDS curves measured at VBG=20 V, 25 V and 30 V. The inset shows current rectification ratio at VDS=±3 V. (c) Band profile for the p-p junction
The band offset value can be extrapolated according to the Anderson’s rule
10
in which the conduction band offset ∆ is
proposed to be the difference between the electron affinity of BP and WSe2. Thus we get ∆ is ~0.1 eV and ∆ is ~0.8 eV according
at VDS=0. The electrical properties are determined by the valence band offset ∆E . The dashed line denotes the Fermi level. (d) Band profile for the n-n junction at VDS=0. The electrical properties are
to the equation12: ∆ ∆
results in large current rectification ratio in the p-p junction. It should be note that the Anderson’s rule suffers from a number of limitations
determined by the conduction band offset ∆E .
22, 23
and there are also some uncertainties on the
electron affinity values of WSe2 and BP BP, the holes in WSe2 will transfer to BP due to the alignment of their Fermi levels. As a result, the conduction band edge (CBE) and
. This large ∆
22, 24
; the extrapolated ∆
and ∆ might be not accurate. Nonetheless, we could still use them to analyze our experimental results qualitatively.
valence band edge (VBE) of WSe2 will bend downward, and the CBE
Despite of the p-p junction, we can also tune the device to the
and VBE of BP will bend upward accordingly. Under a reverse bias,
n-n junction under positive VBG modulations. Different from the p-p
the holes will transport from BP to WSe2 and must overcome an
junction, the electrical properties of n-n junction is determined by
interfacial energy barrier ∆ at valence band. Here ∆ denotes
the interfacial barrier ∆ . Fig. 2b shows IDS-VDS curves of the WSe2-
the valence band offset value. Under a positive bias, the holes will
BP heterojunction at VBG=20 V, 25 V, and 30 V. The current
transport from WSe2 to BP, and the energy barrier height will
rectification ratio is close to 1 (Inset of Figure 2c), implying a small
decrease with increased bias voltage. The different barrier heights
∆ at the interface, which is consistent with ∆ ~0.1 eV calculated
for holes under positive and negative bias result to the current
by Anderson’s rule. Fig. 2d shows the proposed band alignment of
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rectification ratio as VBG increases from -35 V to -31 V is attributed
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n-n junction at a zero bias. In contrast to the p-p case, the work 19, 20
. Thus
overlapped regions, and
we
further
confirmed that the
the CBE and VBE of WSe2 bend upward and the CBE and VBE of BP
photocurrent is generated from the WSe2/BP junction with the
bend downward.
spatial resolved photocurrent measurement (Supplementary Materials Section G). Fig. 3a and 3b show the IDS-VDS curves of the pp (VBG=-40 V) and n-n (VBG=31 V) junction under various incident laser powers Popt. The IDS-VDS data obtained in the dark and under 532 nm laser irradiation show a clear photoresponse in both the p-p and n-n junction, and the photoresponse becomes more obvious with increased incident laser power (Fig. 3b and 3d). The p-p junction shows an open-circuit voltage (Voc) of ~0.29 V and shortcircuit current (Isc) of ~-30 nA at Popt=2.6 µw; while the n-n junction shows a completely reversed photoresponse with Voc of ~-0.31 V and Isc of ~35 nA at Popt=2.6 µw. The reversed sign of Voc and Isc is attributed to the opposite interfacial band bending directions of p-p and n-n junction. Fig. 3e and 3f illustrate the photocurrent and
Fig. 3 (a) IDS-VDS curves measured under dark and various of incident
photovoltage generation process at the WSe2-BP interface. The
laser powers popt (VBG=-40 V). (b) laser power dependence of Isc and
incident laser excites electron-hole pairs in WSe2 and BP. For p-p
Voc at VBG=-40 V. (c) IDS-VDS curves measured at dark and various
junction, the CBE and VBE of WSe2 bend downward and the CBE
incident laser powers popt (VBG=31 V). (d) laser power dependence
and VBE of BP bend upward. Hence, the photon excited electrons
of Isc and Voc at VBG=31 V. (e) photocurrent generation at the
will move from WSe2 to BP, and the photon excited holes will move
interface of p-p junction. (f) photocurrent generation at the
from BP to WSe2, resulting to a negative Isc and a positive Voc. For n-
interface of n-n junction. The green arrow denotes the incident
n junction, the band bending direction reversed. Hence, the photon
photons. The blue dashed arrow indicates the electron-hole pairs
excited electrons will move from BP to WSe2, and the photon
generation process. The black arrow denotes the moving direction
excited holes will move from WSe2 to BP, which leads to a positive
of charge carriers. The dashed line denotes the Fermi level.
Isc and a negative Voc. Note that the photocurrent measurements matched well with the band alignments we proposed in Fig.2c and 2d, thus further confirmed their correctness. These experimental
After demonstrated the gate tunable electrical properties of results also indicate the polarity of photoresponse in the WSe2/BP the WSe2-BP heterojunction, we further explored its optoelectrical heterojunction could be externally tuned by the gate voltage. The properties. A 532 nm He-Ne laser was used to illuminate the device. tunable polarity of photocurrent has been also observed in a dualThe spot size of the laser was controlled to be smaller than the
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function of n-type WSe2 is lower than that of n-type BP
junction area in order to exclude the photo response from the non-
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ARTICLE 26
gated graphene-MoS2-graphene heterostructure , while its’ device
gap regime (-30 V~-26 V). Further increasing of the VBG (10 V~ 31 V)
structure is much complicated than our single-gated WSe2/BP
tunes the device into n-n junction and the sign of Voc will be
device.
reversed. Note that the Voc is almost unchanged when VBG is far
at interface is almost unchanged at this regime. This might be heterojunction, we studied the gate dependence of photovoltage attributed to the maturity of the p-p and n-n junction and the and photocurrent at various incident laser power (Fig. 4a and 4b). increased screening of electrical field by the carriers in bottom According to Fig. 1c, we could divide the VBG into three regimes: the 19
WSe2 layer . The gap regime is a transition regime of p-p junction p-p regime (-40 V~-26 V), the gap regime (-20 V~1 V), and the n-n and n-n junction, and Voc will be minimum. The Isc of both p-p regime (10 V~ 31 V). Within the p-p regime, both WSe2 and BP are junction and n-n junction decreases monotonously as VBG p-type; within the gap regime, WSe2 is insulating and BP is weakly approaches to the gap region. This can be attributed to the doped; within the n-n regime, both WSe2 and BP are n-type. When increased sheet resistance and decreased Voc of the device. Based VBG increases from -40 V to -26 V, the Voc changes little at first (-40 on the photocurrent response and input laser power, we can V~-30 V) then decreases rapidly when V BG is close to the determine the external quantum efficiency (EQE, η) of the device. The EQE is defined as the number of carriers produced per photon, or
ƞ
Where Iph is the photocurrent, ϕ is the photon flux (Popt/hν), h is Planck’s constant, ν is the frequency of light, q is the electron charge and Popt is the optical power25. As shown in Fig. 4c, the EQE of the p-p junction increases from ~0.6% at VBG=-26 V to ~2.7% at VBG=-40 V; and the EQE of n-n junction increases from ~1% at Fig. 4 (a) Gate dependence of Voc under various of laser powers. VBG=10 V to ~3.1% at VBG=31 V. The 3.1% EQE is ten times larger Two dashed lines divide the VBG into p-p regime, gap regime and n-n
than that observed in the MoS2/BP heterojunction devices15, and it
regime. (b) Gate dependence of Isc at various laser powers. (c) The indicates the WSe2/BP heterojunction could be potentially used for gate dependence of EQE at Popt=2.6 µw. (c) The laser power photo detection. Several approaches could be used to further dependence of EQE at VBG=-40 V (blue square) and 31 V (red dot). enhance the EQE, such as improving the sample quality by fabricating the device in the inert atomosphere29,30, reducing the contact resistance by using graphene as electrodes31-33, and
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away from the gap regime, indicating that the band bending slope To have a better understanding of the photoresponse in WSe2-BP
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facilitating the carrier collection by designing vertical sandwiched
1.
structure13,26. Fig. 4d shows the laser power dependence of EQE at VBG=-40 V and VBG=31 V, respectively. It was found that the EQE
2.
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decreases with increasing excitation laser power. The decrease of EQE with increasing laser power could be attributed to the absorption saturation of the heterojunction and the increased
3.
surface recombination process 26, 27.
In summary, heterojuntion stacked by two ambipolar 2D
4.
semiconductors (WSe2 and BP) demonstrates promising gate
5.
tunable properties. We have shown this heterojunction could be
6. 7.
tuned freely to either p-p junction or n-n junction by the 8. electrostatic gating. By tuning the type of the vdW-heterojunction, 9. both the current rectification and photocurrent polarity of the device could be completely changed. The interfacial properties of
10. 11. 12.
WSe2/BP heterojunction at both the valence band edge and 13. conduction band edge were also experimentally identified, and can be used as a reference the other TMDCs/BP heterojunctions, due to 14.
5
the similar band structures of TMDCs family . Our study could 15. provide a better understanding to design novel heterojunction devices based on 2D semiconductors in the future.
16.
Acknowledgements
17.
This work was supported by the National Natural Science Foundation of China (NSFC) under the grant No. 61325021, the National Basic Research Program of China (973 Program) under the grant Nos. 2013CB934500, 2012CB921302, and NSFC under the grant Nos. 51572289,91323304 and Strategic Priority Research Program（B）of the Chinese Academy of Sciences (Grant No. XDB07010100).
18.
19. 20. 21. 22. 23. 24.
Notes and references
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Gate Tunable WSe2-BP van der Waals Heterojunction Devices a
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Peng Chen , Ting Ting Zhang , Jing zhang , Jianyong Xiang , Hua Yu , Shuang Wu , Xiaobo Lu , Received 00th January 20xx, Accepted 00th January 20xx
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Guole Wang , Fusheng Wen , Zhongyuan Liu , Rong Yang , Dongxia Shi & Guangyu Zhang*
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DOI: 10.1039/x0xx00000x www.rsc.org/
Due to the weak screening effect, the concentration and type of charge carriers in 2D semiconductor heterostructures can be effectively tuned by electrostatic gating, enabling us to realize different type heterojunctions in a single device. Such ‘type tunable’ properties are useful for designing novel electrical or optoelectircal devices. Here, we demonstrate a ‘type tunable’ heterojunction device construct with two pieces of ambipolar 2D semiconductors: WSe2 and Black phosphorus (BP). This heterojunction could be tuned to either p-p junction or n-n junction by the gate modulation. The p-p junction shows large current rectification ratio while the n-n junction shows negligible current rectification ratio, indicating a large valence band offset and a small conduction band offset at WSe2/BP interface. In the optoelectrical measurements, We found the amplitude and even the polarity of photocurrent could be modulated by the electrostatic gating. Our study could further enhance the understanding of designing devices based on these ‘type tunable’ van der Waals heterojunctions. Moreover, the properties of the WSe2/BP interface were also experimentally identified through the electrical and optoelectrical meaurements in our study.
Heterojunctions built from different 2D atomic crystals with van der
properties, such as the band offset, band bending slopes and band
Waals (vdW) interactions are fundamentally different from those
bending directions
conventional
vdW-
in p-p junction and n-n junction are determined by conduction band
heterojunctions are free from the atomic interdiffusion thus have
offset and valence band offset, respectively; and the interfacial
covalently-bonded
materials.
These
1-3
10-12
. For example, the charge carriers transport
atomically regulated interfaces and thereby sharp band edges .
band bending directions are completely reversed between p-p
More interestingly, the concentration and even the type of charge
junction and n-n junction. Previous studies have demonstrated the
carriers in these vdW-heterojunctions can be tuned by the gate
heterojunctions
4-9
based
on
n-type
MoS2
and
other
2D
voltage owing to their ultra-thin nature . The gate tunable
semiconductors, such as WSe2 and black phosphorus (BP), etc.
behavior makes it possible to realize different types of
However, due to the Fermi level pinning of MoS2, the gate
heterojunctions in a single device. The electrical and optoelectrical
tunablility of those heterojunction devices is limited
properties are substantially different for different types of
report heterojunctions stacked vertically by two pieces of
heterojunctions, which are determined by their basic interfacial
ambipolar 2D atomic crystals (WSe2 and BP). Owing to unpinned
13-17,34
. Here we
7, 8
Fermi level of both the BP and WSe2 , the heterojunction could be tuned to either p-p junction or n-n junction via electrical gating, and these two kinds of heterojunctions show completely different
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electrical and optoelectrical properties. From the electrical and
(Supplementary Materials Section B). A back gate voltage (VBG) was
optoelectrical measurements of the p-p and n-n junction, the
applied through the P Si to modulate the carrier concentrations in
properties of the WSe2/BP interface were also experimentally
the heterostructure. Fig. 1b shows the band structure of WSe2 and
identified.
BP, respectively. According to previous studies
++
WSe2-BP heterostructures were fabricated by stacking BP thin layer on top of WSe2 thin layer (Supplementary Materials Section A). Fig. 1a presents schematic illustration of a WSe2-BP heterostructure on ++
a Si(P )/SiO2(100nm) substrate. Note that a BP monolayer has a 18
puckered honeycomb structure of P atoms ; and a WSe2 monolayer consists of a hexagonally packed plane of W atoms 5
sandwiched between two planes of Se atoms . An optical image of a
WSe2 (
19, 20
, the bandgap of
) and BP ( ) is ~1.3 eV at 12nm-thickness and ~0.4
eV at 20nm-thickness, respectively; while the electron affinity of WSe2 ( ) and BP ( ) is ~4 eV and ~4.1 eV, respectively. Fig. 1c shows the transfer curve of individual WSe2 and BP, and both of them exhibit ambipolar characteristics. The 12 nm WSe2 shows an on/off ratio of ~105, and the 20 nm BP shows an on/off ratio of ~7. These results are consistent with previous reports7-9, 21.
typical WSe2-BP heterojunction device with contact electrodes of
To investigate electrical properties of this vdW-heterojunction,
Ti(6 nm)/Au(50 nm) is also shown in Fig. 1a. The thickness of WSe2
we performed measurements between electrodes D1 (contacted
and BP flake is 12 and 20 nm, respectively, for this particular device
with WSe2) and S1 (contacted with BP). Fig. 2a shows the currentvoltage (IDS-VDS) curves of the WSe2-BP heterojunction at VBG=-35 V, -33 V, and -31 V (Supplementary Materials Section D shows the semi-log plot). The IDS-VDS curves exhibit strong current rectification properties with the forward current (IF) much larger than the reverse current (|I R |). Inset of Fig. 2a shows the current rectification ratio (|IF/IR|) calculated by dividing IF at VDS=3 V by |IR| at VDS=-3 V. The |IF/IR| is ~2.5×103 at VBG=-35 V and decreases to ~96 at VBG =-31V. These results can be understood by a simple model, where the total resistance of the device can be roughly
Fig. 1 (a) Top left: schematic illustration of a WSe2-BP heterojunction device on a Si/SiO2 substrate. Top right: optical image of the fabricated device, where D1 and D2 (S1 and S2) denotes the metal contacted on WSe2 (BP). The scale bar is 5μm. Bottom: atomic view of the WSe2-BP heterostructure. (b) Band structure of the WSe2 and BP, respectively. (c) The transfer curve at VDS=100 mV for individual WSe2 (blue curve, measured between D1 and D2) and BP (red curve, measured between S1 and S2).
divided into three parts: the resistance of the junction near the WSe2/BP interface, the sheet resistances of WSe2 and BP, and the contact resistances of metal/WSe2 and metal/BP. We excluded the current rectification comes from the contacts (Supplementary materials section E), and identified it is origin from the WSe2/BP interface. According to Fig. 1c, both WSe2 and BP are p-type at these back gate voltages. When the p-type WSe2 stacked with the p-type BP, a p-p heterojunction is formed. Based on the band
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ARTICLE 19,20
structure of WSe2 and BP reported in literatures
, we proposed a
rectification effect (Supplementary Materials Section F). Moreover,
reasonable band alignment of p-p junction (zero bias) in Fig. 2c.
the direction of the current rectification is also consistent with the
Since the work function of p-type WSe2 is higher than that of p-type
experimental result shown in Fig.2a. The decrease of current
to the increased sheet resistance, which doesn’t contribute to the current rectification. Note that the MoS2 based 2D p-n heterojunctions have demonstrated large current rectification 13-17
properties
, while our experimental results indicate the strong
current rectification can also be realized in the 2D p-p heterojunctions, and such structures are potentially very fast since 28
only majority-carrier transport is involved .
Fig. 2 (a) IDS-VDS curves measured at VBG=-35 V, -33 V and -31 V. The inset shows current rectification ratio at VDS=±3 V. (b) IDS-VDS curves measured at VBG=20 V, 25 V and 30 V. The inset shows current rectification ratio at VDS=±3 V. (c) Band profile for the p-p junction
The band offset value can be extrapolated according to the Anderson’s rule
10
in which the conduction band offset ∆ is
proposed to be the difference between the electron affinity of BP and WSe2. Thus we get ∆ is ~0.1 eV and ∆ is ~0.8 eV according
at VDS=0. The electrical properties are determined by the valence band offset ∆E . The dashed line denotes the Fermi level. (d) Band profile for the n-n junction at VDS=0. The electrical properties are
to the equation12: ∆ ∆
results in large current rectification ratio in the p-p junction. It should be note that the Anderson’s rule suffers from a number of limitations
determined by the conduction band offset ∆E .
22, 23
and there are also some uncertainties on the
electron affinity values of WSe2 and BP BP, the holes in WSe2 will transfer to BP due to the alignment of their Fermi levels. As a result, the conduction band edge (CBE) and
. This large ∆
22, 24
; the extrapolated ∆
and ∆ might be not accurate. Nonetheless, we could still use them to analyze our experimental results qualitatively.
valence band edge (VBE) of WSe2 will bend downward, and the CBE
Despite of the p-p junction, we can also tune the device to the
and VBE of BP will bend upward accordingly. Under a reverse bias,
n-n junction under positive VBG modulations. Different from the p-p
the holes will transport from BP to WSe2 and must overcome an
junction, the electrical properties of n-n junction is determined by
interfacial energy barrier ∆ at valence band. Here ∆ denotes
the interfacial barrier ∆ . Fig. 2b shows IDS-VDS curves of the WSe2-
the valence band offset value. Under a positive bias, the holes will
BP heterojunction at VBG=20 V, 25 V, and 30 V. The current
transport from WSe2 to BP, and the energy barrier height will
rectification ratio is close to 1 (Inset of Figure 2c), implying a small
decrease with increased bias voltage. The different barrier heights
∆ at the interface, which is consistent with ∆ ~0.1 eV calculated
for holes under positive and negative bias result to the current
by Anderson’s rule. Fig. 2d shows the proposed band alignment of
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rectification ratio as VBG increases from -35 V to -31 V is attributed
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n-n junction at a zero bias. In contrast to the p-p case, the work 19, 20
. Thus
overlapped regions, and
we
further
confirmed that the
the CBE and VBE of WSe2 bend upward and the CBE and VBE of BP
photocurrent is generated from the WSe2/BP junction with the
bend downward.
spatial resolved photocurrent measurement (Supplementary Materials Section G). Fig. 3a and 3b show the IDS-VDS curves of the pp (VBG=-40 V) and n-n (VBG=31 V) junction under various incident laser powers Popt. The IDS-VDS data obtained in the dark and under 532 nm laser irradiation show a clear photoresponse in both the p-p and n-n junction, and the photoresponse becomes more obvious with increased incident laser power (Fig. 3b and 3d). The p-p junction shows an open-circuit voltage (Voc) of ~0.29 V and shortcircuit current (Isc) of ~-30 nA at Popt=2.6 µw; while the n-n junction shows a completely reversed photoresponse with Voc of ~-0.31 V and Isc of ~35 nA at Popt=2.6 µw. The reversed sign of Voc and Isc is attributed to the opposite interfacial band bending directions of p-p and n-n junction. Fig. 3e and 3f illustrate the photocurrent and
Fig. 3 (a) IDS-VDS curves measured under dark and various of incident
photovoltage generation process at the WSe2-BP interface. The
laser powers popt (VBG=-40 V). (b) laser power dependence of Isc and
incident laser excites electron-hole pairs in WSe2 and BP. For p-p
Voc at VBG=-40 V. (c) IDS-VDS curves measured at dark and various
junction, the CBE and VBE of WSe2 bend downward and the CBE
incident laser powers popt (VBG=31 V). (d) laser power dependence
and VBE of BP bend upward. Hence, the photon excited electrons
of Isc and Voc at VBG=31 V. (e) photocurrent generation at the
will move from WSe2 to BP, and the photon excited holes will move
interface of p-p junction. (f) photocurrent generation at the
from BP to WSe2, resulting to a negative Isc and a positive Voc. For n-
interface of n-n junction. The green arrow denotes the incident
n junction, the band bending direction reversed. Hence, the photon
photons. The blue dashed arrow indicates the electron-hole pairs
excited electrons will move from BP to WSe2, and the photon
generation process. The black arrow denotes the moving direction
excited holes will move from WSe2 to BP, which leads to a positive
of charge carriers. The dashed line denotes the Fermi level.
Isc and a negative Voc. Note that the photocurrent measurements matched well with the band alignments we proposed in Fig.2c and 2d, thus further confirmed their correctness. These experimental
After demonstrated the gate tunable electrical properties of results also indicate the polarity of photoresponse in the WSe2/BP the WSe2-BP heterojunction, we further explored its optoelectrical heterojunction could be externally tuned by the gate voltage. The properties. A 532 nm He-Ne laser was used to illuminate the device. tunable polarity of photocurrent has been also observed in a dualThe spot size of the laser was controlled to be smaller than the
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function of n-type WSe2 is lower than that of n-type BP
junction area in order to exclude the photo response from the non-
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ARTICLE 26
gated graphene-MoS2-graphene heterostructure , while its’ device
gap regime (-30 V~-26 V). Further increasing of the VBG (10 V~ 31 V)
structure is much complicated than our single-gated WSe2/BP
tunes the device into n-n junction and the sign of Voc will be
device.
reversed. Note that the Voc is almost unchanged when VBG is far
at interface is almost unchanged at this regime. This might be heterojunction, we studied the gate dependence of photovoltage attributed to the maturity of the p-p and n-n junction and the and photocurrent at various incident laser power (Fig. 4a and 4b). increased screening of electrical field by the carriers in bottom According to Fig. 1c, we could divide the VBG into three regimes: the 19
WSe2 layer . The gap regime is a transition regime of p-p junction p-p regime (-40 V~-26 V), the gap regime (-20 V~1 V), and the n-n and n-n junction, and Voc will be minimum. The Isc of both p-p regime (10 V~ 31 V). Within the p-p regime, both WSe2 and BP are junction and n-n junction decreases monotonously as VBG p-type; within the gap regime, WSe2 is insulating and BP is weakly approaches to the gap region. This can be attributed to the doped; within the n-n regime, both WSe2 and BP are n-type. When increased sheet resistance and decreased Voc of the device. Based VBG increases from -40 V to -26 V, the Voc changes little at first (-40 on the photocurrent response and input laser power, we can V~-30 V) then decreases rapidly when V BG is close to the determine the external quantum efficiency (EQE, η) of the device. The EQE is defined as the number of carriers produced per photon, or
ƞ
Where Iph is the photocurrent, ϕ is the photon flux (Popt/hν), h is Planck’s constant, ν is the frequency of light, q is the electron charge and Popt is the optical power25. As shown in Fig. 4c, the EQE of the p-p junction increases from ~0.6% at VBG=-26 V to ~2.7% at VBG=-40 V; and the EQE of n-n junction increases from ~1% at Fig. 4 (a) Gate dependence of Voc under various of laser powers. VBG=10 V to ~3.1% at VBG=31 V. The 3.1% EQE is ten times larger Two dashed lines divide the VBG into p-p regime, gap regime and n-n
than that observed in the MoS2/BP heterojunction devices15, and it
regime. (b) Gate dependence of Isc at various laser powers. (c) The indicates the WSe2/BP heterojunction could be potentially used for gate dependence of EQE at Popt=2.6 µw. (c) The laser power photo detection. Several approaches could be used to further dependence of EQE at VBG=-40 V (blue square) and 31 V (red dot). enhance the EQE, such as improving the sample quality by fabricating the device in the inert atomosphere29,30, reducing the contact resistance by using graphene as electrodes31-33, and
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away from the gap regime, indicating that the band bending slope To have a better understanding of the photoresponse in WSe2-BP
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facilitating the carrier collection by designing vertical sandwiched
1.
structure13,26. Fig. 4d shows the laser power dependence of EQE at VBG=-40 V and VBG=31 V, respectively. It was found that the EQE
2.
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decreases with increasing excitation laser power. The decrease of EQE with increasing laser power could be attributed to the absorption saturation of the heterojunction and the increased
3.
surface recombination process 26, 27.
In summary, heterojuntion stacked by two ambipolar 2D
4.
semiconductors (WSe2 and BP) demonstrates promising gate
5.
tunable properties. We have shown this heterojunction could be
6. 7.
tuned freely to either p-p junction or n-n junction by the 8. electrostatic gating. By tuning the type of the vdW-heterojunction, 9. both the current rectification and photocurrent polarity of the device could be completely changed. The interfacial properties of
10. 11. 12.
WSe2/BP heterojunction at both the valence band edge and 13. conduction band edge were also experimentally identified, and can be used as a reference the other TMDCs/BP heterojunctions, due to 14.
5
the similar band structures of TMDCs family . Our study could 15. provide a better understanding to design novel heterojunction devices based on 2D semiconductors in the future.
16.
Acknowledgements
17.
This work was supported by the National Natural Science Foundation of China (NSFC) under the grant No. 61325021, the National Basic Research Program of China (973 Program) under the grant Nos. 2013CB934500, 2012CB921302, and NSFC under the grant Nos. 51572289,91323304 and Strategic Priority Research Program（B）of the Chinese Academy of Sciences (Grant No. XDB07010100).
18.
19. 20. 21. 22. 23. 24.
Notes and references
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