Temperature-Dependent and Gate-Tunable Rectification in a Black

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Surfaces, Interfaces, and Applications

Temperature-dependent and Gate-tunable Rectification in a Black Phosphorus-WS van der Waals Heterojunction Diode 2

Ghulam Dastgeer, Muhammad Farooq Khan, Ghazanfar Nazir, Amir Afzal, Sikandar Aftab, Bilal Naqvi, Janghwan Cha, Kyung-Ah Min, Yasir Jameel, Jongwan Jung, Suklyun Hong, and Jonghwa Eom ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00058 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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ACS Applied Materials & Interfaces

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 Naqvi2, Janghwan Cha1, Kyung-Ah Min1, Yasir Jameel3, Jongwan Jung2, Suklyun Hong1 and Jonghwa Eom1* 1

Department of Physics & Astronomy and Graphene Research Institute, Sejong University, Seoul 05006, Korea

2

Department of Nanotechnology & Advanced Materials Engineering and Graphene Research Institute, Seoul 05006, Korea

3

University of Agriculture, Faculty of Sciences, Department of Physics, Faisalabad, Pakistan

*E-mail: [email protected]

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Abstract: Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention due to their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p–n diode functionality. In this study, for the first time, we fabricated a heterojunction p–n diode composed of BP and WS2. The rectification effects are examined for mono-layer, bilayer, tri-layer, and multi-layer WS2 flakes in our BP/WS2 van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6×104, temperature dependence, thickness dependence of rectification and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS2 van der Waals p–n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS2 van der Waals p–n diodes have a profound potential to fabricate rectifiers, solar cells and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

KEYWORDS: Black phosphorus; Tungsten disulfide; Van der Waals p–n diode; Gatemodulated rectification; Photovoltaic effect


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Introduction Conventionally, in semiconductor technology, n-type and p-type semiconductors are used for electronic and optoelectronic devices such as logic circuits, field-effect transistors (FETs), light emitting diodes and photovoltaic circuits1-2. Graphene is the first 2-dimensional (2D) material, which revolutionized the field of electronics and opened new ways for scientists and engineers to develop optoelectronic devices3. Graphene possesses a high charge-carrier concentration and high mobility. These charges can travel up to a thousand interatomic distances without scattering4-6. Despite its great practical potential as 2D electronic material, graphene shows the limitation of functionality due to the lack of band gap. For this reason, graphene is constrained in its potential for use in optoelectronic devices. Although the band gap can be created artificially by chemical doping, nanoribbons, and dual-gated techniques, these processes cause the degradation of graphene’s mobility and ON/OFF ratio7-9. Beyond graphene, numerous 2D transition metal dichalcogenide (TMD) materials such as MoS2, WSe2, WS2 facilitate the physics of electronics to develop flexible, stable, and efficient devices10-11. These 2D TMDs retain considerable direct and indirect band gaps ranging from 1 eV to 2 eV. These TMDs hold a very high ON/OFF ratio and gate-tunable switching. These intrinsic characteristics render these 2D materials more promising and preferable than graphene-based devices in FETs and photodetectors12. As an emerging 2D TMD, tungsten disulfide (WS2) crystals consist of three atomic layers. The tungsten atom is surrounded by two sulfur atoms, and the layers of WS2 are attached to one another by van der Waals interactions. Meanwhile, the monolayer WS2 possesses a direct band gap of 2.1 eV, and the bulk WS2 crystal holds an indirect band gap of 1.4 eV. Monolayer WS2 retains a high mobility and ON/Off ratio10 of up to 107 in electronic devices. The material also retains a high thermal stability through the absence of dangling bonds and electrostatic integrity10, 13 compared with those in MoS2.



Similar to graphite which comprises stacked graphene14, black phosphorous (BP) is composed of stacked phosphorene. In this material, phosphorus atoms are firmly bonded in plane and each phosphorene layer feebly interacts through van der Waals interactions. The band gap of BP increases as the thickness decreases15-17. Bulk BP also shows a band gap of ~0.3 eV, whereas monolayer phosphorene displays a band gap of ~2.1 eV18. Currently, the leading role of BP can be found in p-type FETs, inverters, thermoelectric power generation, light-emitting diodes (LEDs), and controlled switching devices because of its ambipolarity and narrow direct band gap19-20. Scientists and engineers predicted that BP would pave the way for 2D electronics because of the material’s high ON/OFF ratio and high mobility of up to 10,000 cm2/Vs14, 17. The most ubiquitous and fundamental p–n junctions are essential building blocks of electronics and optoelectronic devices. After the advent of atomically thin van der Waals 2D materials, numerous heterostructure-based p–n junctions have been fabricated21-22. Most heterojunctions involving 2D semiconductors are van der Waals junctions formed by 2D–2D semiconductor layers. These van der Waals heterojunctions provide a route for the fabrication the electronic and optoelectronic devices using TMDs21 in accordance with the desires of engineers. After the discovery of 2D materials, van der Waals heterojunctions engender substantial attention in the physics of electronics to develop next-generation nanoelectronic devices

23-24

. Such van der Waals heterojunctions play a vital role in the physics of electronics

to fabricate highly efficient and flexible FETs21, 25. In these FETs, different charges migrate from one region to another in opposite directions during forward biasing1, 21-22, 26-27. However, most previous studies adopted additional processes to make TMDs p-type, such as chemical doping, special metal contact, or electrical gating methods to create p−n junctions which possess problematic and annoying factors for practical applications of van der Waals p−n junctions28-30. For example, to acquire p-type MoS2, a complex chemical doping


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process must be involved for its strong Fermi-level pinning effect23, 31, which could degrade the carrier mobility as a result of scattering from the dopant. Herein, we report gate-modulated and temperature-dependent rectification in a BP/WS2 van der Waals heterojunction p–n diode. Gate modulated p-n diodes in this paper have a built-in potential at Vg = 0 V and the doping concentration in WS2 and BP can be further tuned by the back-gate voltage. These both aspects make our BP/WS2 van der Waals diode more efficient and tunable. The rectification of the BP/WS2 van der Waals heterojunction p–n diode shows a high ON/OFF output current ratio of ~2.6×104 which is much higher than previously reported studies16, 32-33 and strongly depends on gate voltage and temperature. The rectification effects are closely examined for mono-layer, bi-layer, tri-layer, and multi-layer WS2 flakes in our BP/WS2 p-n diodes. We also report that how rectification varies with changing the thickness of underneath WS221,

34-35

. For the first time, using BP as a p-type semiconductor both Zener

breakdown and avalanche breakdown have been observed in the same device. High photoresponsivity (R) and superior external quantum efficiency (EQE) as compared to previously reported papers21-22, 29-30, 32, 36-37 have been found in photovoltaic measurement.

Experimental Section Device Fabrication: We transferred a multi-layer WS2 flake on a p-doped silicon wafer with a 285 nm-thick silicon oxide (SiO2) layer by mechanical exfoliation. Subsequently, a multi-layer flake of BP was mechanically exfoliated from a high-quality single crystal bulk BP on transparent polydimethylsiloxane (PDMS) stamp and transferred onto the top of the WS2 by controlling the motion of the micro-aligner stage, being monitored by charge coupled device (CCD) camera image. As a result, a heterojunction was formed through van der Waals interaction. This formation led to the generation of a p–n junction as presented in Figure 1a. By photolithography and e-beam lithography, we patterned Cr/Au contacts on p-BP and n-WS2



flakes directly in a high vacuum chamber of 10-7 Torr. The real and final optical microscope image of the device is displayed in Figure 1b. The Raman peaks in Figure 1c reveal the basic characteristics of BP and WS2, respectively. These results are consistent with those of previously reported papers10, 38-40. The Figure 1d-e divulge that the thicknesses of WS2 and BP flakes are ~7.6 nm and ~9.5 nm respectively, determined by atomic force microscopy (AFM) measurements. The interface between the TMDs was cleaned, residue-free and sharp by thermal annealing41 in a high vacuum of order 10-7 Torr at 250 oC for 2 hours. Using atomic layer deposition (ALD), a thin passive layer of Al2O3 (20 nm thick) was deposited on all devices to circumvent BP from oxidation and to reduce the interface trap density defects42. The passivation process preserves the electrical properties of BP and enhances the built-in potential and stability43-44. The Al2O3 passivation layer also screens the Coulomb potential from the charge impurities45. Multiple devices were measured and all the electrical measurements were performed in a vacuum to avoid the electrical degradation of the device from an ambient environment24-25, 39, 46-47.

Electrical and Photovoltaic Measurements: The I-V curves at different Vg were measured by using Keithley 2400 as a source meter and pico-ammeter (Keithley 6485) to measure drain current. The photoresponsivity and EQE were measured under a light source of wavelength λ = 600 nm and 1.2 µW irradiation power.

Density Function Theory (DFT) Calculations: Density functional theory (DFT) calculations are performed within generalized gradient approximation (GGA) for exchange-correlation (xc) functionals48-49, implemented in the Vienna ab initio simulation package (VASP)50-51. The kinetic energy cut-off is set to 400 eV, and electron-ion interactions are represented by the projector augmented wave (PAW) potentials52-53. Grimme’s DFT-D3 method54 based on a semi-


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empirical GGA-type theory is used for van der Waals corrections. In the calculations, an (1x5) unit cell of few-layer BP is adsorbed on an (1x4) unit cell of few-layer WS2. For the Brillouinzone integration, we use (12x3x1) grid for atomic optimizations of BP on WS2, in the Gamma centered scheme. Atomic coordinates are fully optimized until the Hellmann-Feynman forces are less than 0.01 eV/Å.

Results and Discussion Rectification Effect and Electrical Transport in BP/WS2 van der Waals Heterojunction pn Diodes: Each transfer curve of BP and WS2 reveals the clear p-type and n-type nature, respectively, as shown in Figure S2 of Supporting Information. In our experiment, the source contact was connected to p-type BP, whereas WS2 was connected to the drain contact (grounded) shown in Figure 1b. The source-to-drain voltage (Vds) was applied to the upper BP flake. The conduction level difference increased for a reverse bias (Vds < 0V) and yielded an increase in barrier height across the BP/WS2 junction. The rectifying effect of the BP/WS2 van der Waals heterojunction p–n diode is strongly dependent on the number of layers of underneath WS2 flake as shown in Figure 2a. High tunneling and negligible rectification ratio are observed in van der Waals heterojunction of multi-layer BP with mono-layer and bi-layer WS2. Since both multi-layer BP and mono-layer WS2 have a direct band gap, electrons tunnel through the overlapped active region of multi-layer BP and mono-layer WS2, which can be described as direct band-to-band tunneling21, 55. Moreover, the barrier width is so small between monolayer WS2 and multi-layer BP56 that electrons from WS2 can easily tunnel through the thin depletion region to the multi-layer BP. Therefore, no rectification occurs except for tunneling. It is confirmed in Figure 2a that the tunneling current is suppressed by increasing the number of WS2 layers23, so the rectification effect becomes prominent21, 57.



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The I–V characteristics for multi-layer-BP and multi-layer-WS2 van der Waals heterojunction, in linear scale, are shown in Figure S4 of supporting Information. The output characteristics of van der Waals heterojunction p–n diodes can be examined by the I-V relation used for conventional p-n diodes composed of bulk semiconductors1, 58. The current I through diode is given by the following equation, = exp

ɳ

− 1

(1)

where Io is the saturation current, ɳ = ( )( ) is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, and q is the electron charge. The ideality factor is extracted from Figure 2b by fitting the I–V curves in a small range of forward-bias regime59. To check the rectification effect, we define the rectification ratio as a ratio between the forward-bias current (If) and reverse-bias leakage current (Ir). The rectification ratio at different Vg is shown in Figure 2c. The rectification ratio is improved from 4×103 to 2.6×104 as Vg is tuned from 50 V to -50 V respectively. The rectification ratio is boosted more than two times as compare to the previously reported BP-MoS2 van der Waals heterojunctions18,

37, 60

for the

negative Vg because Ir decreases more effectively in the negative Vg. There are two reasons which are attributed to the high rectification. First, a low reverse-bias leakage current is attributed to a higher barrier height for Vg < 0 V. The barrier height is extracted1 by Φ =

∗

ln

varies from 0.49 eV to 0.55 eV as Vg changes from +50 V to -50 V. Here A is

area of heterojunction region and A* is Richardson’s constant. Expectedly, for the positive Vg, the barrier height between the BP/WS2 junction decreases and consequently reduces the depletion width. Similarly, a negative Vg enlarges the depletion width and barrier height increases. The other reason for the high rectification ratio is the gate voltage modulation of Schottky barrier at the metal/TMDs contact57. For more detailed analysis see Figures S6 and S7 of Supporting Information. We conclude that both (Cr/Au)/WS2 contact and BP/WS2 junction


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contribute to the rectification effect that we observed in this experiment. A further experiment is needed to address the issue which junction plays more dominant role for the rectification effect. Output characteristics of the BP/WS2 heterojunction p-n diodes strongly depend on Vg. Figure 2d shows η as a function of Vg at room temperature. When Vg changes from 50 V to -50 V, η decreases monotonically from 4.8 to 1.7. At negative Vg, the diode behaves more like an ideal diode with an ideality factor of 1.7, which is closer to the ideal value than the previously reported BP/MoS2 van der Waals heterojunction22, gate doping (η >> 1.9)22,

36

or bulk

semiconductor (η >> 2)61 diodes. The band gap of multi-layer WS2 (~1.4 eV) is much larger than BP (∼0.3 eV). Therefore, the electrostatic doping effect will be more effective in BP than WS2. When a large negative gate voltage is applied, BP remains p-type, whereas the doping type of WS2 is slightly changeable by the electrostatic doping effect. Therefore, high rectification ratio of 2.6×104 was acquired at Vg = −50 V in this experiment. The small band gap of the BP permits electrostatic inversion from p-type to n-type slightly, when a large positive gate voltage is applied. Subsequently, it yields to an n-n junction showing a poor rectification ratio for Vg = 50 V in this experiment. To obtain the band alignments of the van der Waals heterojunction for BP-WS2 depending on Vg, we have performed density functional theory (DFT) calculations by applying the external electric field (E) to the BP/WS2 system. In the calculations, tri-layer BP is chosen to simulate the multi-layer system of BP since the electronic structure of tri-layer BP reasonably follows that of the bulk system62. We also chose tri-layer WS2 in DFT calculation. The contact distance between BP and WS2 is considered to be 3.27 Å, suitable for van der Waals interaction between them. Figures 3a-c show the density of states of BP and WS2 comprising the heterojunction for E = −1.0, 0.0, and +1.0 V/nm, respectively. Here, the conduction band minimum of WS2 is set to 0 eV. In the heterojunctions under the gate electric field, the energy band gap of BP is about 0.1 eV and those of WS2 are 1.10 ~ 1.22 eV, as clearly shown in



Figure S3 of Supporting Information. Figure S3 demonstrates schematic diagrams of band alignment between BP and WS2 for Vg < 0, Vg = 0, and Vg > 0 V. As seen from Figure 3, the density of states of BP are shifted upward with respect to that of WS2 as E changes from negative to positive values. Due to such shift, the conduction band minimum of BP becomes close to the conduction band minimum of WS2 under the positive gate electric field, as shown in Figure 3c. This situation can lead to the reduction of the barrier height and thus results in increased source-drain current as measured in the experiment, which makes the gate dependence of the rectifying effect in BP/WS2 p–n diode.

Temperature Dependent I-V Characteristics of BP/WS2 van der Waals Heterojunction p-n Diodes: Temperature dependence is also investigated on the BP/WS2 van der Waals heterojunction p-n diodes. We found that BP/WS2 van der Waals junction resistance decreases with increasing temperature alike conventional p–n junctions18. The temperature dependence of the junction resistance is explained by fact that additional electron-hole pairs are spawned at a high temperature in semiconductor materials. Moreover, the band gap of the semiconductor shrinks and semiconductors decreases its resistance as temperature increases2, 63. In a conventional p–n diode, a breakdown occurs as reverse bias increases beyond a certain voltage. At the breakdown voltage, the leakage current exponentially increases. We also observed the breakdowns of the p–n diode in our BP/WS2 van der Waals heterojunction. Figure 4a shows breakdowns, but interesting phenomena were found under detailed examination. As we increased the magnitude of Vds in the reverse-bias regime, Zener breakdown was observed initially, followed by avalanche breakdown as Vds further increased. The Zener breakdown voltages were found around -0.6 V and -4 V at 10 K, which are attributed to the tunneling of accelerated charge carriers through the depletion region. The Zener breakdown voltages were observed at less than 4Eg/q; otherwise, avalanche breakdown occurs1, 64. Here, Eg is the energy

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gap, which is 0.3 eV for BP and 1.4 eV for WS2. While the breakdown around -0.6 V is due to BP, the breakdown around -4 V is attributed to WS2. The temperature dependence of the Zener breakdown is illustrated in Figure 4b. As the temperature increases, the magnitude of Zener breakdown voltage (ZBV) decreases. By contrast, the avalanche breakdown voltage (ABV) is observed around -9 V as shown in Figure 4c, which is much larger than 4Eg/q. At such a high reverse voltage the electrostatic field strength becomes so high that it can pull out the electrons from valence band within the depletion region. However, the temperature dependence of ABV shows a trend opposite to that of ZBV as shown in Figure 4b and c. The temperature coefficient of ZBV is negative, whereas the temperature coefficient of ABV is positive. These findings are consistent with the conventional p–n junction diode1, 64. We note that both ZBV as well as ABV were observed in a single BP/WS2 p-n junction. In addition, the temperature dependence of the rectification of BP/WS2 p–n diode was examined by the ideality factor. The temperature dependence of the ideality factor is associated with the barrier height at the interface, which increases as the

temperature is lowered58-59. The relation for the ideality factor, ɳ = ( )( ), also indicates "# that η changes inversely with changing temperature. Figure 4d reveals that η changes from 5.2 to 2.3 when the temperature increases from 10 K to 200 K.

Optoelectronic Characteristics of BP/WS2 van der Waals Heterojunction p-n Diodes: The optoelectronic characteristics of devices composed of 2D materials have constantly served as a major research theme. The BP/WS2 p–n diode is illuminated under the light, having wavelength λ = 600 nm and 1.2 μW irradiation power. At Vg = 0 our device exhibits a short circuit current (Isc) of 0.6 μA and open circuit voltage (Voc) of 0.35 V at room temperature as demonstrated in Figure 5a. The photoresponsivity (R) is extracted by formula, $ =


%& '(

=

)* +, '(

, where -. is the


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photocurrent, is the dark current, and /0 is the incident power of light. The photoresponsivity of the BP/WS2 van der Waals p–n diode is measured 500 mA/W, which is hundred times higher than previously reported heterojunctions of BP with other TMDs22, 32. The external quantum efficiency (EQE) was extracted 30, 65 by the following equation; 121 (%) = $

ℎ5 67

.

(2)

Here h is Plank's constant, c is the speed of light, e is a charge of electron and λ is the wavelength of light. Comparing with all the previously reported heterojunctions with BP we attained the highest EQE (%) of 103%. The increase in the reverse- and forward-bias currents, shown in Figure 5a and b, is associated with the electron-hole pair generation caused by illumination under the light. Under illumination, the charges are separated by the interface 22, 66, such that the holes reside on the lower face of BP and the electrons accumulate on the upper face of the WS2. As a result, dipoles are formed within the junction area and these dipoles then induce more electron-hole pair generation 67-68. The variation of photocurrent in a sequent light ON and OFF is demonstrated in Figure 5b. The photocurrent shows ON and OFF switching behavior indicating a possible photo-switching device. The efficient photoresponsivity and high EQE of our BP/WS2 heterojunctions have demonstrated a profound potential towards both electronic and optoelectronic devices. The high 121 = $

.8 9:

in our BP/WS2 heterojunction can be attributed to the high

photoresponsivity R= 500 mA/W due to the small band gap (~1.4 eV) of WS2 as n-type material. For other reason, the built-in potential at the interface22, 64, 69 also gives rise to a high EQE. There is smaller energy-level difference between the conduction band maximum (CBM) of BP and CBM of WS2 as compare to BP/mono-layer MoS2. Due to the small difference between CBMs of BP and WS2 the electrons-holes recombine in larger amount at the interface between WS2 and BP, which increases the built-in potential. Finally it is also expected that the


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screening against coulomb impurities is enhanced in the heterojunction made of thick 2D materials and hence yields to a higher EQE69.

Conclusions In conclusion, we fabricated a stable BP/WS2 van der Waals heterojunction p–n diode on Si/SiO2. The device exhibits the rectifying effect, which efficiently depends on the thickness of TMDs, the back-gate voltage, and temperature. We observed that the rectification ratio is enhanced and the ideality factor also improves as Vg decreases from 50 V to -50 V. The electrical characteristics of the device also strongly depended on temperature. The ideality factor changed from 5.2 to 2.3 as the temperature rose from 10 K to 200 K. Both Zener and avalanche breakdowns were observed in the reverse-bias regimes at various temperatures in a single device. The highest EQE of 103% and efficient photoresponsivity of 500 mA/W are observed, which are not yet obtained in any other van der Waals heterojunction consisting of BP. By employing BP and WS2 as p- and n-type 2D materials, respectively, one can attain a van der Waals heterojunction p–n diode that provides the rectifying function in flexible and transparent electronics. In the future, BP-based heterojunctions would provide various FETs, photodetectors, solar cells and high-performance photovoltaic cells in the field of nanoelectronic and optoelectronic devices.

Acknowledgements This work was supported by the Priority Research Center Program (2010-0020207) and the Basic Science Research Program (2016R1D1A1A09917762, 2017R1A2B2010123) through the National Research Foundation of Korea grant funded by the Korea government (Ministry of Education, Ministry of Science and ICT).



Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Optical images of BP/WS2 van der Waals heterojunction p-n diodes, transfer curves of BP and WS2 in logarithmic scale, schematic band diagram of BP/WS2 van der Waals heterojunction, I-Vds curves of BP/WS2 heterojunction p-n diode at different back gate voltages.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jonghwa Eom: 0000-0003-4031-4744 Notes The authors declare no competing financial interest.

References: 1. Sze, S. M.; Ng, K. K., Semiconductor Devices: Physics and Technology. 2006; p 568568. 2. Neamen, D. A., Semiconductor Physics and Devices: Basic Principles. 2003; p 729-729. 3. Geim, A. K.; Novoselov, K. S., The rise of graphene. Nature Mater. 2007, 6 (3), 183191. 4. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K., Two-dimensional atomic crystals. Proceedings of the National Academy of Sciences of the United States of America 2005, 102 (30), 10451-10453. 5. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A., Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005, 438 (7065), 197-200. 6. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A., Electric field effect in atomically thin carbon films. Science (New York, N.Y.) 2004, 306 (5696), 666-669.


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26. Bernardi, M.; Palummo, M.; Grossman, J. C., Extraordinary sunlight absorption and one nanometer thick photovoltaics using two-dimensional monolayer materials. Nano Letters 2013, 13 (8), 3664-3670. 27. Roy, T.; Tosun, M.; Kang, J. S.; Sachid, A. B.; Desai, S. B.; Hettick, M.; Hu, C. C.; Javey, A., Field-effect transistors built from all two-dimensional material components. ACS Nano 2014, 8 (6), 6259-6264. 28. Huang, C.; Wu, S.; Sanchez, A. M.; Peters, J. J. P.; Beanland, R.; Ross, J. S.; Rivera, P.; Yao, W.; Cobden, D. H.; Xu, X., Lateral heterojunctions within monolayer MoSe2–WSe2 semiconductors. Nat Mater 2014, 13 (12), 1096-1101. 29. Duan, X.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Li, H.; Wu, X.; Tang, Y.; Zhang, Q.; Pan, A.; Jiang, J.; Yu, R.; Huang, Y.; Duan, X., Lateral epitaxial growth of two-dimensional layered semiconductor heterojunctions. Nat Nano 2014, 9 (12), 1024-1030. 30. Pospischil, A.; Furchi, M. M.; Mueller, T., Solar-energy conversion and light emission in an atomic monolayer p-n diode. Nat Nano 2014, 9 (4), 257-261. 31. Zhang, X.-Q.; Lin, C.-H.; Tseng, Y.-W.; Huang, K.-H.; Lee, Y.-H., Synthesis of Lateral Heterostructures of Semiconducting Atomic Layers. Nano Letters 2015, 15 (1), 410-415. 32. Chen, P.; Zhang, T. T.; zhang, J.; Xiang, J.; Yu, H.; Wu, S.; Lu, X.; Wang, G.; Wen, F.; Liu, Z.; Yang, R.; Shi, D.; Zhang, G., Gate tunable WSe2-BP van der Waals heterojunction devices. Nanoscale 2016, 8 (6), 3254-3258. 33. Pezeshki, A.; Shokouh, S. H. H.; Nazari, T.; Oh, K.; Im, S., Electric and Photovoltaic Behavior of a Few‐Layer α‐MoTe2/MoS2 Dichalcogenide Heterojunction. Advanced Materials 2016. 34. Chen, L.-M.; Li, G.-C.; Zhang, Y.; Guo, Y.-F., Film Thickness Dependence of Rectifying Properties of La 1.85 Sr 0.15 CuO 4 /Nb-SrTiO 3 Junctions. Chinese Physics Letters 2010, 27 (7), 077401. 35. Yu, W. J.; Li, Z.; Zhou, H.; Chen, Y.; Wang, Y.; Huang, Y.; Duan, X., Vertically stacked multi-heterostructures of layered materials for logic transistors and complementary inverters. Nat Mater 2013, 12 (3), 246-252. 36. Jariwala, D.; Sangwan, V. K.; Wu, C.-C.; Prabhumirashi, P. L.; Geier, M. L.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C., Gate-tunable carbon nanotube–MoS2 heterojunction p-n diode. Proceedings of the National Academy of Sciences 2013, 110 (45), 18076-18080. 37. Hong, T.; Chamlagain, B.; Wang, T.; Chuang, H.-J.; Zhou, Z.; Xu, Y.-Q., Anisotropic photocurrent response at black phosphorus-MoS2 p-n heterojunctions. Nanoscale 2015, 7 (44), 18537-18541. 38. Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Ye, P. D., Phosphorene: An unexplored 2D semiconductor with a high hole mobility. ACS Nano 2014, 8 (4), 4033-4041. 39. Wan, B.; Yang, B.; Wang, Y.; Zhang, J.; Zeng, Z.; Liu, Z.; Wang, W., Enhanced stability of black phosphorus field-effect transistors with SiO 2 passivation. Nanotechnology 2015, 26 (43), 435702-435702. 40. Island, J. O.; Steele, G. A.; van der Zant, H. S.; Castellanos-Gomez, A., Environmental instability of few-layer black phosphorus. 2D Materials 2015, 2 (1), 011002. 41. Fang, H.; Battaglia, C.; Carraro, C.; Nemsak, S.; Ozdol, B.; Kang, J. S.; Bechtel, H. A.; Desai, S. B.; Kronast, F.; Unal, A. A., Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides. Proceedings of the National Academy of Sciences 2014, 111 (17), 6198-6202. 42. Na, J.; Lee, Y. T.; Lim, J. A.; Hwang, D. K.; Kim, G.-T.; Choi, W. K.; Song, Y.-W., Few-Layer Black Phosphorus Field-Effect Transistors with Reduced Current Fluctuation. ACS Nano 2014, 8 (11), 11753-11762.


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43. Chen, Y.; Ren, R.; Pu, H.; Chang, J.; Mao, S.; Chen, J., Field-effect transistor biosensors with two-dimensional black phosphorus nanosheets. Biosensors and Bioelectronics 2017, 89, Part 1, 505-510. 44. Rehman, A. u.; Khan, M. F.; Shehzad, M. A.; Hussain, S.; Bhopal, M. F.; Lee, S. H.; Eom, J.; Seo, Y.; Jung, J.; Lee, S. H., n-MoS2/p-Si Solar Cells with Al2O3 Passivation for Enhanced Photogeneration. ACS Applied Materials & Interfaces 2016, 8 (43), 29383-29390. 45. Moon, B. H.; Han, G. H.; Kim, H.; Choi, H.; Bae, J. J.; Kim, J.; Jin, Y.; Jeong, H. Y.; Joo, M.-K.; Lee, Y. H., Junction-Structure-Dependent Schottky Barrier Inhomogeneity and Device Ideality of Monolayer MoS2 Field-Effect Transistors. ACS Applied Materials & Interfaces 2017. 46. Wood, J. D.; Wells, S. A.; Jariwala, D.; Chen, K. S.; Cho, E.; Sangwan, V. K.; Liu, X.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C., Effective passivation of exfoliated black phosphorus transistors against ambient degradation. Nano Letters 2014, 14 (12), 6964-6970. 47. Castellanos-Gomez, A., Black Phosphorus: Narrow Gap, Wide Applications. The Journal of Physical Chemistry Letters 2015, 6 (21), 4280-4291. 48. Kohn, W.; Sham, L. J., Self-consistent equations including exchange and correlation effects. Physical review 1965, 140 (4A), A1133. 49. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Physical review letters 1996, 77 (18), 3865. 50. Kresse, G.; Furthmüller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Computational Materials Science 1996, 6 (1), 15-50. 51. Kresse, G.; Furthmüller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Physical review B 1996, 54 (16), 11169. 52. Blöchl, P. E., Projector augmented-wave method. Physical review B 1994, 50 (24), 17953. 53. Kresse, G.; Joubert, D., From ultrasoft pseudopotentials to the projector augmentedwave method. Physical Review B 1999, 59 (3), 1758. 54. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. The Journal of chemical physics 2010, 132 (15), 154104. 55. Appenzeller, J.; Lin, Y.-M.; Knoch, J.; Avouris, P., Band-to-band tunneling in carbon nanotube field-effect transistors. Physical review letters 2004, 93 (19), 196805. 56. Appenzeller, J.; Yu-Ming, L.; Knoch, J.; Zhihong, C.; Avouris, P., Comparing carbon nanotube transistors - the ideal choice: a novel tunneling device design. IEEE Transactions on Electron Devices 2005, 52 (12), 2568-2576. 57. Zhou, R.; Ostwal, V.; Appenzeller, J., Vertical versus Lateral Two-Dimensional Heterostructures: On the Topic of Atomically Abrupt p/n-Junctions. Nano Letters 2017, 17 (8), 4787-4792. 58. Kumar, A.; Vinayak, S.; Singh, R., Micro-structural and temperature dependent electrical characterization of Ni/GaN Schottky barrier diodes. Current Applied Physics 2013, 13 (6), 1137-1142. 59. Schroder, D. K., Semiconductor material and device characterization. John Wiley & Sons: 2006. 60. Peng, C.; Jianyong, X.; Hua, Y.; Jing, z.; Guibai, X.; Shuang, W.; Xiaobo, L.; Guole, W.; Jing, Z.; Fusheng, W.; Zhongyuan, L.; Rong, Y.; Dongxia, S.; Guangyu, Z., Gate tunable MoS 2 –black phosphorus heterojunction devices. 2D Materials 2015, 2 (3), 034009. 61. Shah, J. M.; Li, Y.-L.; Gessmann, T.; Schubert, E., Experimental analysis and theoretical model for anomalously high ideality factors (n≫ 2.0) in AlGaN/GaN pn junction diodes. Journal of Applied Physics 2003, 94 (4), 2627-2630. 17Environment ACS Paragon Plus


62. Tran, V.; Soklaski, R.; Liang, Y.; Yang, L., Layer-controlled band gap and anisotropic excitons in few-layer black phosphorus. Physical Review B 2014, 89 (23), 235319. 63. Piprek, J., Semiconductor optoelectronic devices. Introduction to Physics and Simulation. 2003; p 296-296. 64. Lan, C.; Li, C.; Wang, S.; He, T.; Jiao, T.; Wei, D.; Jing, W.; Li, L.; Liu, Y., Zener Tunneling and Photoresponse of a WS2/Si van der Waals Heterojunction. ACS Applied Materials & Interfaces 2016, 8 (28), 18375-18382. 65. Jin, Y.; Keum, D. H.; An, S. J.; Kim, J.; Lee, H. S.; Lee, Y. H., A Van Der Waals homojunction: ideal p–n diode behavior in MoSe2. Advanced Materials 2015, 27 (37), 55345540. 66. Baeg, K. J.; Binda, M.; Natali, D.; Caironi, M.; Noh, Y. Y., Organic light detectors: Photodiodes and phototransistors. 2013; Vol. 25, pp 4267-4295. 67. Yim, C.; O'Brien, M.; McEvoy, N.; Riazimehr, S.; Schäfer-Eberwein, H.; Bablich, A.; Pawar, R.; Iannaccone, G.; Downing, C.; Fiori, G.; Lemme, M. C.; Duesberg, G. S., Heterojunction Hybrid Devices from Vapor Phase Grown MoS2. Scientific reports 2014, 4, 5458-5458. 68. Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Van Der Zant, H. S. J.; Castellanos-Gomez, A., Fast and broadband photoresponse of few-layer black phosphorus fieldeffect transistors. Nano Letters 2014, 14 (6), 3347-3352. 69. Wang, F.; Wang, Z.; Xu, K.; Wang, F.; Wang, Q.; Huang, Y.; Yin, L.; He, J., Tunable GaTe-MoS2 van der Waals p–n junctions with novel optoelectronic performance. Nano letters 2015, 15 (11), 7558-7566.


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Figures

(a)

(b)

BP 2 Bg

1 Ag

Intensity (a.u.)

WS2

(c)

2 Ag

(d)

1 E2g A1g

350

400

450 -1

Raman shift (cm

500

)

20

(e) ~ 9.5 nm BP

16 12 8

~7.6 nm WS2

300

Height [nm]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 0 4.5

5.0

5.5

6.0

6.5

7.0

Distance [µm]


7.5


Figure 1. Fabrication of exfoliated van der Waals heterojunction composed of multi-layer BP and WS2 flakes. (a) A multi-layer WS2 flake was transferred on SiO2/Si substrate and then a multi-layer BP flake was transferred over it. (b) The optical microscope image of the final BP/WS2 van der Waals heterojunction p-n diode. The source contact (S) is connected to the BP flake and a drain contact (D) is connected to the WS2 flake. (c) Raman spectra of BP and WS2 flakes show their corresponding peaks. (d) Atomic force microscope (AFM) image of the BP/WS2 heterojunction p-n diode. (e) Height profile along the black line in the AFM image. The thickness of the BP flake is 9.5 nm and the thickness of the WS2 flake is 7.6 nm.


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-3

10

(a)

-4

10

Mono-layer WS2

(b)

Vg= 50V

-4

Bi-layer WS2

10

Tri-layer WS2 -5

Vg= -50V

10

-5

Ids (A)

Ids (A)

10

-6

-6

10

-7

10

10

-8

10 -7

10

-9

10 -5

-4

-3

-2

-1

0

1

2

3

4

-4.5

5

-3.0

-1.5

Vds (V)

0.0

1.5

3.0

4.5

Vds (V) 5.0

(c)

4

2.5x10

4

2.0x10

4

1.5x10

4

1.0x10

3

5.0x10

4.5

Ideality factor (η)

Rectification ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(d)

4.0 3.5 3.0 2.5 2.0

Rectification ratio = If /Ir

1.5 -60

-40

-20

0

20

40

60

-60

-40

-20

0

20

40

60

Vg (V)

Vg (V)

Figure 2. Output characteristics (I-Vds) of BP/WS2 van der Waals heterojunction p-n diode. (a) I-Vds curves of multi-layer BP with mono-layer, bi-layer, and tri-layer WS2 flakes, showing the increase of rectification as the thickness of WS2 flake is changed from mono-layer to tri-layer. (b) I-Vds curves of multi-layer BP/multi-layer WS2 van der Waals heterojunction are plotted at different Vg. (c) Rectification ratio (If/Ir) as a function of Vg at room temperature. (d) Ideality factor as a function of Vg at room temperature. The ideality factor of BP/WS2 van der Waals heterojunction p-n diode approaches ~1.7 as Vg is lowered to -50 V.



Figure 3. Density of states of BP/WS2 van der Waals heterojunction at different external electric fields (E). (a) E = −1.0 V/nm, which corresponds to a negative back-gate voltage. (b) E = 0.0 V/nm (zero back-gate voltage). (c) E = +1.0 V/nm (a positive back-gate voltage). Red lines represent the conduction band minima of WS2 in the heterojunctions.


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500

(a)

Zener breakdown

200

(b)

-500 10 K 50 K 100 K 150 K 200 K

-1000

Avalanche breakdown

-1500 -9

-8

-7

-6

-5

-4

-3

-2

-1

0

-100 -200 -4

0

-3

Vds (V) -300

10 K 50 K 100 K 150 K 200 K

Zener breakdown

100

Ids (nA)

Ids (nA)

0

-2

-1

0

Vds (V)

(c)

(d)

5

Ideality factor (η)

-600

Ids (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 K 50 K 100 K 150 K 200 K

-900

-1200

4

3

2

Avalanche breakdown -1500 -10

-9

-8

-7

-6

-5

-4

0

Vds (V)

50

100

150

200

Temperature (K)

Figure 4. Temperatures dependent I-V characteristics of BP/WS2 van der Waals heterojunction p-n diode. (a) I-V curves showing the Zener breakdown and the avalanche breakdown effects at Vg = 0. (b) I-V curves in a small range of Vds to show Zener breakdown voltage. The violet circle indicates the Zener tunneling in BP, whereas the yellow circle indicates the Zener tunneling in WS2. (c) I-V curves in a small range of Vds to show avalanche breakdown voltage. (d) Ideality factor as a function of temperature, varying from 10 K to 200 K.



2

24.2

(a)

(b)

Photo current Dark current

Lamp OFF

24.0

Iph (µA)

1

Ids (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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23.8 23.6

0

Isc

Voc

23.4 Lamp ON

-1

23.2 -0.6

-0.3

0.0

0.3

0.6

0

Vds (V)

100

200

300

400

Time (s)

Figure 5. Photoresponse of BP/WS2 van der Waals heterojunction p-n diode. (a)The I-V characteristics at Vg = 0 in dark and under light showing Isc and Voc at room temperature. (b) Photocurrent with light ON and OFF at Vg = 0 for Vds = +1 V. The arrows indicate ON and OFF state of the light source with wavelength λ = 600 nm and 1.2 μW irradiation power.


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Table of Contents

500

2 Photo current Dark current

Zener breakdown 0

Ids (µA)

1

Ids (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-500 10 K 50 K 100 K 150 K 200 K

-1000

Avalanche breakdown

-1500 -9

-8

-7

-6

-5

-4

-3

0

Voc

Isc -1

-2

-1

0

-0.6

Vds (V)


-0.3

0.0

Vds (V)

0.3

0.6

Temperature-Dependent and Gate-Tunable Rectification in a Black

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