Supporting Information
Temperature-Dependent and Gate-Tunable Rectification in a Black Phosphorus/WS2 van der Waals Heterojunction Diode Ghulam Dastgeer1, Muhammad Farooq Khan1, Ghazanfar Nazir1, Amir Muhammad Afzal1, Sikandar Aftab1, Bilal Abbas Naqvi2, Janghwan Cha1, Kyung-Ah Min1, Yasir Jamil3, Jongwan Jung2, Suklyun Hong1 and Jonghwa Eom1* 1
Department of Physics & Astronomy and Graphene Research Institute, and 2Department of
Nanotechnology & Advanced Materials Engineering and Graphene Research Institute, Sejong University, Seoul 05006, Korea 3
Faculty of Sciences, Department of Physics, University of Agriculture, Faisalabad 38000, Pakistan
*E-mail:
[email protected]
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Figures S1a-b are displaying the optical images of van der Waals heterojunction of few layer BP with mono-layer and bi-layer WS2. The underneath WS2 flakes of mono-layer and bilayer thickness show tunneling-like effects in the reversed bias regime as shown in Figure 3a in the main text. When we increase the thickness of WS2 flakes to tri-layer, the tunneling effect is suppressed and the rectification effect becomes dominant. The rectification effect is enhanced until WS2 flake reaches a certain level of thickness.1
(b)
(a)
Figure S1. Optical images of BP/WS2 van der Waals heterojunction p-n diodes. (a) Optical image of van der Waals heterojunction of multi-layer BP with underneath mono-layer WS2. (b) Optical image of van der Waals heterojunction of multi-layer BP with underneath bilayer WS2.
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The nature of n-type WS2 and p-type black phosphorus (BP) is verified by their transfer curves2-3 keeping Vds = 0.5 V constant. Figure S2 shows the corresponding transfer curves. While the number of holes increases in p-type BP as Vg is increased in the negative range of voltage (Vg < 0 V), the number of electrons increases in n-type WS2 as Vg is increased in the positive range of voltage (Vg > 0 V). The charge carrier concentration of holes in p-type BP and electrons in n-type WS2 is extracted by using the formula4 n = Cg(Vg-Vth)/e . The concentration of holes in BP at Vg= 50 V is calculated 2.58×1012/cm2 and concentration of electrons in n-type WS2 is calculated 2.01×1012/cm2 at Vg= +50 V. In addition, we estimate the field-effect mobility of electrons and holes of 7.6 nm-thick WS2 and 9.5 nm-thick BP to be 192.5 cm2/Vs and 62.3 cm2/Vs, respectively.
100
Transfer curve of p-type BP Transfer curve of n-type WS2
Ids (A)
10 1
0.1 0.01 1E-3 -60 -45 -30 -15
0
15
30
45
60
Vg (V)
Figure S2. Transfer curves of BP and WS2 in logarithmic scale demonstrate their intrinsic natures. BP flake is p-type, whereas WS2 flake is n-type.
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We draw schematics of energy band diagrams to explain briefly the band bending in Figure S3. The energy levels were aligned according to our density functional theory calculations in Figure 3. The conduction band minimum (CBM) of WS2 was set to 0 eV in the energy band diagram, because WS2 was grounded in our experiments. The difference between CBMs of BP and WS2 decreases as the back-gate voltage (Vg) increases from a negative value to a positive value. Since more electrons accumulate at the interface between WS2 and BP in case of the larger difference of CBM, the higher barrier height builds up at the interface for Vg < 0. In this way, the barrier height is highest for Vg < 0, and it decreases as Vg increases from a negative to a positive value as shown in Figure S3.
Figure S3. Schematic band diagram of BP/WS2 van der Waals heterojunction at (a) Vg < 0, (b) Vg = 0, and (c) Vg > 0.
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We used Keithley 2400 to apply the source to drain voltage (Vds) and back gate voltage (Vg). Output current was measured by using pico-ammeter Keithley 6485. The output characteristics (I-Vds) were measured at different Vg to examine the effect of Vg on the characteristics of the WS2/BP van der Waals heterojunction diode. Vg values were changed from -50 V to +50 V with a step of 10 V as shown in Figure S4.
300
Ids (A)
250 200 Vg= +50 V
150 100 50
Vg= -50 V
0 -5
-4
-3
-2
-1
0
1
2
3
4
5
Vds (V) Figure S4. I-Vds curves of BP/WS2 heterojunction p-n diode at different back gate voltages. The forward bias current increases monotonically with back gate voltage.
Photoresponses of BP and WS2 flakes are demonstrated in Figure S5. We used a light with power 1.2 μW and 600 nm wavelength. When BP/WS2 van der Waals heterojunction diode was exposed to light, the photocurrent suddenly increases and then saturates5. If the light is turned OFF,
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then current begins to decrease gradually as shown in Figure S5a-b. The arrows in both figures indicate the time when the light source is turned ON and OFF. The excitation and decay response of WS2 is faster than BP. The slow photoresponse of BP is related with oxidation of its surface6-7, since it is very sensitive and reactive in the ambient environments.
57 56
(a)
13.7
Light OFF
13.6
Light OFF
13.5
Iph (A)
55
IPh (A)
(b)
54 53 52
13.4 13.3 13.2
Light ON
51
13.1
50
13.0
49
Light ON
12.9 0
100
200
300
400
0
Time (s)
100
200
300
400
Time (s)
Figure S5. (a) Photocurrent of pristine BP with light ON and OFF. (b) Photocurrent of pristine WS2 with light ON and OFF at Vg = 0 for Vds = 1 V.
We have fabricated a multi-layer WS2 (~7 nm) device with Cr/Au (6/60 nm) and Al (60 nm) contacts. Since Al has low work function (4.3 eV) as compared to Cr (4.5 eV), Au (5.1 eV), it makes ohmic contact with WS2 as reported8. The interface between Cr/Au and WS2 is acting as a Schottky diode, which limits the reverse bias current as shown in Figure S6. The forward bias direction is from Cr/Au to WS2 in the Schottky junction, whereas Al contact to WS2 is considered to be ohmic. The logarithmic plots of I-V characteristics of (Cr/Au)/WS2 Schottky junction at S‐6
different Vg are shown in Figure S7. As Vg is increased to more positive values, the rectification ratio decreases. While the maximum rectification of ~103 is observed at Vg = -20 V, the minimum rectification of ~10 is observed at Vg = 40 V. We believe that the rectification effect is due to not only BP/WS2 van-der Waals heterojunction but also (Cr/Au)/WS2 Schottky junction, which contributes to increasing the rectification ratio9. However, the rectification of (Cr/Au)/WS2 at Vg > 0 is qualitatively different from the observed rectification of BP/WS2 as shown in Figure 2b. The reverse bias current at Vg > 0 in Figure 2b is much smaller than that of (Cr/Au)/WS2 junction in Figure S7. The rectification ratio of the BP/WS2 device remains ~5×103 even at Vg = 40 V, which is much higher than that of (Cr/Au)/WS2 in Figure S7.
3 Al-source (Cr/Au)-drain (Cr/Au)-source Al-drain
2
Ids (A)
1 0 -1 -2 -3 -4 -3
-2
-1
0
1
2
3
Vg (V)
Figure S6. I-V characteristics of multi-layer WS2 with Cr/Au and Al contacts at 300 K, showing the Schottky diode behavior at Vg = -20 V. Redline corresponds to the measurement configuration of Cr/Au as a source contact, whereas black line corresponds to the measurement configuration of Al as source contact.
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100 10
Ids (A)
1 0.1 0.01
Vg= -40V Vg= -20V
1E-3
Vg= 0V
1E-4
Vg= +20V Vg= +40V
1E-5 -3
-2
-1
0
1
2
3
Vg (V)
Figure R7. The logarithmic plot of I-V characteristics of multi-layer WS2 with Cr/Au and Al contacts at 300 K, showing the Schottky diode behavior at various gate voltages.
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