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Article
Two-dimensionally layered p-black phosphorus/ n-MoS/p-black phosphorus Heterojunctions 2
Geonyeop Lee, Stephen J. Pearton, Fan Ren, and Jihyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19334 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018
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Two-dimensionally layered p-black phosphorus /n-MoS2/p-black phosphorus Heterojunctions Geonyeop Lee1, Stephen J. Pearton2, Fan Ren3 and Jihyun Kim*,1 1
Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Korea 2
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA
3
Department of Chemical Engineering, University of Florida, Gainesville, Florida, 32611, USA
Abstract Layered heterojunctions are widely applied as fundamental building blocks for semiconductor devices. For the construction of nanoelectronic and nanophotonic devices, the implementation of two-dimensional materials (2DMs) is essential. However, studies of junction devices composed of 2DMs are still largely focused on single p-n junction devices. In this study, we demonstrate a novel pnp double heterojunction fabricated by the vertical stacking of 2DMs (black phosphorus (BP) and MoS2) using dry-transfer techniques, and the formation of high quality p-n heterojunctions between the BP and MoS2 in the vertically stacked BP/MoS2/BP structure. The pnp double heterojunctions allowed us to modulate the output currents by controlling the input current. These results can be applied for the fabrication of advanced heterojunction devices composed of 2DMs for nano(opto)electronics.
Keywords black phosphorus, molybdenum disulfide, heterostructure, p-n junction, two-dimensional materials
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Introduction There are many technological and basic science reasons to fabricate layered 2D structures.1-3 The ability to realize heterostructures is necessary for fabricating photonic devices such as lasers, photodetectors, and light-emitting diodes and electronic devices such as junction rectifiers and bipolar transistors with improved performance over homojunction structures.4-6 The transport of carriers across heterojunctions and the bias-control of effective band offsets as barriers to current flow are keys to achieving optimal device performance.7 In addition, the engineering of these structures provides a rich platform to study the physics of quantum phenomena in van der Waals heterostructures that are not present in single materials.8,9 These include interlayer electron-electron and electron-phonon interactions that influence the physical properties of van der Waals heterostructures.10,11 Two-dimensional
materials
(2DMs)
such as graphene and transition metal
dichalcogenides (TMDCs) have received considerable attention as candidates for nextgeneration semiconductor materials owing to their excellent electrical, mechanical, and optical properties.2,12,13 2DMs with weak van der Waals interactions between the layers can not only be easily separated into individual layers, but the layers can also be combined with other 2DMs.5 In particular, the atomically sharp interface and no dangling bonds in each layer of 2DMs make them suitable materials for use in the fabrication of heterostructure devices.10 When 2DMs are combined in a heterostructure, problems such as undesired atomic diffusion, dislocation propagation, and lattice mismatch do not occur.6,10 Moreover, a combination of 2DMs may form an abrupt heterojunction, which prevents intervalley transfer through a launching ramp and facilitates the construction of high-frequency devices.14 There is a large variety of 2DMs with diverse electrical, optical, and optoelectronic properties. Their unique energy bandgap, electron affinity, and carrier mobility features can be modulated by tuning their thickness or by chemical doping.15-17 Therefore, the diversity and flexibility of 2DMs enable the fabrication of heterostructured devices without requiring an ultra-high vacuum chamber for epitaxy.18 Numerous studies have investigated various heterostructured devices constructed based on the unique advantages of 2DMs.8,19-22 Lee et al. fabricated h-BN encapsulated MoS2 field-effect transistors (FETs) that have graphene contact electrodes and demonstrate enhanced device performance compared to that of MoS2/SiO2 structures.19 Peng et al. fabricated p-n diodes from WSe2-MoS2 and analyzed their electrical and optical properties.20 In contrast to the studies on various forms of heterostructures, studies on 2 ACS Paragon Plus Environment
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heterojunction-based devices have largely focused on investigating fundamental structures such as a single p-n junction diode, while active devices like heterojunction bipolar transistors, tunnel diodes and junction field-effect transistors have not yet been widely studied.10,22-24 This study presents novel vertical pnp heterojunctions fabricated by stacking 2DMs using a dry-transfer technique and analyzes their resulting electrical transport properties. Black phosphorus (BP), a p-type material, was stacked vertically with MoS2, an n-type material, to fabricate a strain-free heterostructure at room temperature and atmospheric pressure. BP is an intrinsic p-type material with a high hole mobility (~1000 cm2/V·s) and a thickness-dependent bandgap that can vary significantly, from 0.3 eV for the bulk state to 2.0 eV for the monolayer.25-28 MoS2 is one of the most common n-type TMDCs and has a high electron mobility (~700 cm2/V·s) and bandgap that is tunable from 1.2 eV (bulk) to 1.8 eV (monolayer).29-31 Deng et al. demonstrated that p-n junction diodes composed of BP and MoS2 have excellent rectifying behavior.32 In addition, other studies on the p-n junction using BP and MoS2 have suggested that BP and MoS2 p- and n-type materials that could be adapted for double heterostructures.33-36 This study systematically evaluated the electrical, morphological, and material characteristics of fabricated BP/MoS2/BP pnp heterostructures with techniques including transmission electron microscopy (TEM), Raman microscopy, and atomic force microscopy. This analysis confirmed the formation of a p-n junction between BP and MoS2 through the resultant rectifying behavior and ideality factor.
Experimental methods Micrographs illustrating the fabrication stages for pnp double heterojunctions from BP/MoS2/BP flakes are shown in Fig. 1 and Fig. S1. BP flakes were mechanically exfoliated from bulk BP crystals (Smart Elements) with an adhesive tape in an argon-filled glove box and immediately dry-transferred to a transparent gel film (Gel-pak). MoS2 flakes separated from bulk MoS2 crystals (Graphene supermarket) were also transferred to a gel film by the same method. The back-side of SiO2/p++-Si substrate (300 nm/525 μm) was wet-etched to remove the back oxide layer with the front SiO2 protected by photoresist, followed by depositing the back-gate electrode (Ti/Au, 20/80 nm) using an electron beam evaporator. The bottom p-type BP flakes were dry-transferred onto the SiO2/p++-Si substrates (Fig. 1(a)), after 3 ACS Paragon Plus Environment
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which n-type MoS2 (Fig. 1(b)) and p-type BP (Fig. 1(c)) flakes were stacked vertically with a micro-manipulator in an air ambient, ensuring that they were well-aligned vertically with the previously transferred flakes. Electrodes (Ti/Au, 20/80 nm) for the p-BP layers and the nMoS2 were fabricated using a standard electron-beam lithography, electron-beam evaporation and lift-off process, as shown in Fig. 1(d). Once the fabrication process was completed, the heterojunction devices were characterized within 48 hours. During each fabrication process, the samples were stored in a vacuum-sealed box to prevent their degradation. The thickness of each exfoliated flake was measured by using atomic force microscopy (AFM; Bruker) in the tapping mode. Raman spectra were obtained with a 532 nm diodepumped solid-state laser (Omicron) under a back-scattering geometry. The stacked structure of the BP/MoS2/BP flakes was investigated with scanning transmission electron microscopy (STEM; JEM-2100F, JEOL); the specimen was prepared using the focused ion beam (FIB) technique (Quanta 2003D, FEI) after platinum coating for sample protection. The electrical properties of the double heterojunctions were measured using a semiconductor parameter analyzer (Agilent 4155C) connected to a low-vacuum probe station (~15 mTorr).
Results and discussion A schematic of the fabricated pnp double heterojunction is shown in Fig. 2(a). The pnp double heterojunction was fabricated by vertically stacking two BP flakes (p-type) and a MoS2 flake (n-type) using a dry-transfer technique. Figure 2(b) shows an AFM image of one of the pnp double heterojunctions fabricated with the dry-transfer process shown in Fig. 1. The green, blue, and red dotted lines correspond to the bottom BP, MoS2, and top BP flake, respectively, and indicate that the arrangement of the flakes is well-aligned. Note that the top BP is isolated from the bottom BP by the MoS2 layer. The thickness of each flake is given in Fig. 2(c), as measured along the black dotted line in Fig. 2(b). The thickness of the bottom BP, MoS2, and top BP flakes were ~11, ~5.6, and ~5.3 nm, respectively. The thickness of flakes in this study was less than 40 nm, and the thickness of the bottom and upper BP flakes was different to investigate the difference in transport properties of these asymmetric junctions. Raman spectra of the BP/MoS2/BP heterostructure are shown in Fig. 2(d). Peaks observed at ~361, ~439, and ~466 cm−1 correspond to the A , B , and A phonon modes of BP, respectively, while the peaks observed at ~383 and ~408 cm−1 correspond to the E 4 ACS Paragon Plus Environment
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A
phonon modes of MoS2, respectively.37-39 Raman modes from the overlapped region of
the stacked BP/MoS2/BP layers well correspond with the peaks for each flake, which indicates that a strain-free heterojunction with van der Waals interactions was formed.33 Analysis of a cross-section TEM image from another BP/MoS2/BP heterojunction sample confirmed that the dry-transfer technique created well-formed double heterojunctions. The TEM image shows that each of the BP/MoS2/BP flakes was vertically stacked and arranged without any wrinkles (Fig. 2(e)). An ultra-thin amorphous region of less than 4 nm was also observed, which is assumed to be a native oxide layer on the surface of the BP flake, as the surface of BP is readily converted to the native oxide P2O5 in ambient air.27,40,41 However, this oxide layer can be ignored because it is thin enough to allow charge carrier transfer via tunneling.42 The energy-band structure of the pnp heterojunction can be represented as shown in Fig. 2(f). Charge transfer due to the diffusion of majority carriers occurs with the formation of the BP/MoS2 p-n heterojunction, resulting in a depletion region at the BP/MoS2 interface. In this case, an abrupt p-n junction is formed, as has been experimentally and theoretically demonstrated in previous studies.32,33,43 The energy-band structure of the pnp heterojunction in this study (Fig. 2(f)) is considered to be symmetric because BP flakes with more than 5 layers have same energy bandgap as a bulk BP.44 However, the operation of practical double heterojunction can be asymmetric due to the unequal resistances of the two heterojunctions that result from the different thicknesses of the top and bottom BP flakes and their orientations.45 Figures 3(a) and 3(b) show the electrical properties of the p-BP(top)/n-MoS2 and the nMoS2/p-BP(bottom) p-n junctions, respectively from the BP/MoS2/BP pnp heterojunctions. Typical rectifying behaviors were observed in each p-n junction, and their rectification ratios (⏐I1V/I−1V⏐) were 45 and 25, respectively. The ideality factor was calculated by the following equation: I=
−1
where I is the current through the diode, V is the voltage across the diode, I0 is the dark saturation current, n is the ideality factor, k is the Boltzmann constant, and T is the temperature.14 The resulting ideality factors were 1.98 and 2.18 for the p-BP (top)/n-MoS2 and n-MoS2/p-BP (bottom) p-n junctions, respectively. The ideality factors are higher than that of an ideal p-n junction diode but are similar to those found in previous studies.32,36 The 5 ACS Paragon Plus Environment
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formation of an ideal junction between BP and MoS2 seems to be hindered by the native oxide of BP and residue from the dry-transfer process. Figure 3(c) shows the I-V characteristic of the pnp double heterojunction measured as a function of the applied voltage at different current injection between bottom BP and MoS2. The p-n heterojunction between the p-BP (top) and n-MoS2 layer is forward-biased, with the bottom p-BP connected to the ground. The device operation was assumed that a steep barrier for the holes and a small barrier for the electrons are formed at the BP/MoS2 junction, allowing the injected electrons, which drift in the direction of the applied voltage, to be significant contribution to the measured current. It is expected that bandgap tuning through fine control of BP and MoS2 thickness, modulation of band alignment with doping, and use of other 2DMs will enable improvements in junction operation and device performance. This can also affect the device operation when recording the I-V between p-BP(top)/n-MoS2 junction as a function of In with the n-MoS2 grounded, as shown in Fig. 3(d). In this study, the 2DM-based pnp double heterojunctions function in the region where pnp heterojunction bipolar transistors would operate in the common-base mode, but the output current was slightly amplified by inserting a positive base current (In) between the p-BP (bottom) and the n-MoS2. The current gain (α) obtained from fig. 3(d) was approximately 2.75 for an In of 50 nA. This would correspond to a common-base operation mode for a heterojunction bipolar transistor.14,46 The electrical properties of 2DMs such as BP and MoS2 can be modulated readily with electrostatic gating. In particular, the effects of gating on BP are more efficient than on other 2DMs as a result of its narrow bandgap and unpinned Fermi-level characteristics.33 A backgated pnp double heterojunction was fabricated to analyze the effects of electrostatic gating on the device performances (Fig. 4(a)). Figure 4(b) shows the rectification ratio (|I /I
|)
for varying back-gate bias (Vg) from −60 to +60 V. The rectification ratio depends on the Vg because the number of charge carriers in both BP and MoS2 were affected by the Vg, where the maximum of the rectification ratio is reached at Vg=−20~−25 V. The electrical characteristics for varying Vg of the n-MoS2 /p-BP (bottom) and p-BP (top)/n-MoS2 p-n junctions are shown in Figs. 4(c) and 4(d), respectively. Less forward rectifying diode behaviors with decreasing the Vg were observed in Fig. 4(c) because n-MoS2 became less ntype around −50 V. Even backward rectifying diode characteristics were oberved, which can be explained by the tunneling phenomena at a Vg of −50 V in Fig. 4(c).17 The tendencies of the n-MoS2/p-BP (bottom) and p-BP (top)/n-MoS2 junctions were different, possibly because 6 ACS Paragon Plus Environment
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the back-gate field was screened by the underneath flakes. The governing resistance of the junction also affected the operations for the p-n junction owing to the different thickness of the top BP (~15 nm) and the bottom BP (~30 nm) flakes in relation to the MoS2 flake of the base (~10 nm). Therefore, we adjusted the level of the Vg applied on the respective pn junctions to optimize their rectification performances. The electrical properties of the pnp heterojunctions were measured as a function of the Vg (Fig. 5). Better performance was observed with Vg of −20 V, where the rectification ratio is optimal, compared with the zero Vg. As back-gate voltage was applied, the cut-off current at a bias of Vpn=−3 V on the p-BP (top)/n-MoS2 was reduced from −0.1 μA to −0.05 μA, and the output current (Ipn) became saturated because of the well-defined p-n junctions with high rectification ratios. The effect of the electrostatic gating improved the device performance of the pnp double heterojunctions, which correspond to a common-base operation mode for a pnp heterojunction bipolar transistor although further optimization is necessary in engineering the bandgap alignment.
Conclusion We demonstrated the fabrication and operation of pnp double heterojunctions with a BP/MoS2/BP structure. Nano-layer flakes were stacked vertically as p-BP, n-MoS2, and p-BP, respectively, using a dry-transfer technique. Two BP-MoS2 p-n heterojunctions in this structure showed the diode rectifying behaviors with the ideality factors of ~2. The effects of the electrostatic gating on the heterojunctions were also evaluated at varying Vg, where the rectification ratio increased to a maximum at a Vg of −20 V. The pnp double heterojunctions in this study modulated the output currents by controlling the input current. These results can provide a basis for advanced heterojunction devices composed of 2DMs.
Associated content
Supporting information The optical microscope images representing the fabrication process for pnp double heterojunctions of BP/MoS2/BP flakes 7 ACS Paragon Plus Environment
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Author information Corresponding Author *E-mail:
[email protected] (Jihyun Kim)
Notes The authors declare no competing financial interest
Acknowledgements This research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), the Ministry of Trade, Industry, and Energy (MOTIE) of Korea (No. 20172010104830),
and
the
Space
Core
Technology
Development
Program
(2017M1A3A3A02015033) through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning of Korea. The work at UF was partially supported by DTRA1-17-011
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Figure captions
Figure 1 Optical microscopic images ((a)-(d)) illustrating the fabrication process for a pnp double heterojunction structure using BP/MoS2/BP flakes. (d) Ti/Au electrodes for BP/MoS2/BP flakes was defined. Figure 2 (a) Schematic of a pnp BP/MoS2/BP double heterostructure on SiO2/p++-Si substrate with a back-gate electrode. (b) AFM image of the device represented in Figure 1. The green, blue, and red dotted lines indicate the bottom BP, MoS2, and top BP flakes. (c) AFM height profile of the layered BP/MoS2/BP along the black dotted line in (b). (d) Raman spectra of BP/MoS2/BP heterostructure (black line). The green, blue, and red line represent the Raman spectra of bottom BP, MoS2, and top BP, respectively. (e) Cross-sectional TEM image of the stacked BP/MoS2/BP. (f) Band structure of BP/MoS2/BP pnp heterostructure. Figure 3 Current-voltage (I-V) characteristics of (a) p-BP (top)/n-MoS2 and (b) n-MoS2/p-BP (bottom) heterojunctions (the insets show the I-Vs in log-scale). The electrical characteristics of the pnp heterostructure at various injection currents (In) measured with different bias conditions on (c) p-BP (bottom, grounded) and p-BP(top) and (d) n-MoS2 (grounded) and pBP (top). Figure 4 (a) Optical microscope image of the fabricated pnp heterostructure. (b) rectification ratios (|I /I
|) of n-MoS2/p-BP (bottom) and p-BP (top)/n-MoS2 heterojunctions with
varying Vg. I-V characteristics of (c) n-MoS2/p-BP (bottom) and (d) p-BP (top) /n-MoS2 junctions at different Vg. Figure 5 Output current characteristics of the stacked pnp heterostructure at varying In (forward-biased n-MoS2/p-BP(bottom) with n-MoS2 grounded) at (a) Vg=0 V and (b) Vg=−20 V
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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