Tailoring Quantum Tunneling in a Vanadium‐Doped WSe2/SnSe2 Heterostructure

Abstract 2D van der Waals layered heterostructures allow for a variety of energy band offsets, which help in developing valuable multifunctional devices. However, p–n diodes, which are typical and versatile, are still limited by the material choice due to the fixed band structures. Here, the vanadium dopant concentration is modulated in monolayer WSe2 via chemical vapor deposition to demonstrate tunable multifunctional quantum tunneling diodes by vertically stacking SnSe2 layers at room temperature. This is implemented by substituting tungsten atoms with vanadium atoms in WSe2 to provoke the p‐type doping effect in order to efficiently modulate the Fermi level. The precise control of the vanadium doping concentration is the key to achieving the desired quantum tunneling diode behaviors by tuning the proper band alignment for charge transfer across the heterostructure. By constructing a p–n diode for p‐type V‐doped WSe2 and heavily degenerate n‐type SnSe2, the type‐II band alignment at low V‐doping concentration is clearly shown, which evolves into the type‐III broken‐gap alignment at heavy V‐doping concentration to reveal a variety of diode behaviors such as forward diode, backward diode, negative differential resistance, and ohmic resistance.

realize accessible integration. Chemical vapor deposition (CVD) technique is a promising approach in a wafer-scale synthesis for industrial applications. Moreover, introducing the dopant during CVD growth is an effective way to change the carrier density in the material and thus modulate the position of the Fermi level. [13] Here, we report a reliable and repeatable method to synthesize vanadium (V) substituted WSe 2 monolayer by CVD and demonstrate multifunctional p-n diode behaviors in V-doped WSe 2 /SnSe 2 heterostructures. A liquid precursor with tungsten host and vanadium dopant atoms is adopted in our approach with only two zones for the precise control of V-doping concentration in WSe 2 . The coverage of the grown-WSe 2 flakes is increased up to 90% at a high liquid precursor concentration ( Figure S1, Supporting Information). In addition to the p-type V-doped WSe 2 monolayer, multilayer SnSe 2 as an n-type material is introduced to construct the p-n junction. We observe the diverse p-n diode behaviors in V-WSe 2 /SnSe 2 devices with various V-doping concentrations at room temperature, including quantum tunneling p-n diodes for the forward rectification, backward rectification, negative differential resistance (NDR), and ohmic resistance. Figure 1a,b is the schematics for synthesizing V-doped WSe 2 monolayer with the CVD approach. Liquid precursor containing solutions of ammonium metatungstate (AMT: W-precursor) and ammonium metavanadate (AMV: V-precursor) together with a promoter of alkali metal (NaOH) and iodixanol is spin-casted on SiO 2 /Si substrate and then introduced into a two-zone furnace CVD for the selenization 2D van der Waals layered heterostructures allow for a variety of energy band offsets, which help in developing valuable multifunctional devices. However, p-n diodes, which are typical and versatile, are still limited by the material choice due to the fixed band structures. Here, the vanadium dopant concentration is modulated in monolayer WSe 2 via chemical vapor deposition to demonstrate tunable multifunctional quantum tunneling diodes by vertically stacking SnSe 2 layers at room temperature. This is implemented by substituting tungsten atoms with vanadium atoms in WSe 2 to provoke the p-type doping effect in order to efficiently modulate the Fermi level. The precise control of the vanadium doping concentration is the key to achieving the desired quantum tunneling diode behaviors by tuning the proper band alignment for charge transfer across the heterostructure. By constructing a p-n diode for p-type V-doped WSe 2 and heavily degenerate n-type SnSe 2 , the type-II band alignment at low V-doping concentration is clearly shown, which evolves into the type-III broken-gap alignment at heavy V-doping concentration to reveal a variety of diode behaviors such as forward diode, backward diode, negative differential resistance, and ohmic resistance.
process. The molar ratio of AMT to AMV in the liquid precursor is precisely controlled using a micropipette.
The hexagonal shape of flakes is preserved in V-doped WSe 2 samples (up to V-10%), similar to the pristine WSe 2 flakes ( Figure 1c; Figure S2, Supporting Information). The V-10% sample shows a uniform contrast in the entire flake area with a height profile of 0.8 nm, confirmed by atomic force microscope (AFM). The monolayer is maintained up to V-10% doping ( Figure S2, Supporting Information), at which multilayers are partially spotted. The highly crystalline atomic structure of V-doped WSe 2 monolayer (V-2%) is observed without noticeable defect sites from the high-resolution transmission electron microscopy (HRTEM) image with selective-area electrondiffraction pattern (Figure 1d). The interlayer distances of the (100) and (110) planes are identified to be 0.28 and 0.16 nm, respectively, in consistent with the reported values in WSe 2 . [14] To explore V-incorporation in WSe 2 , we further performed energy-dispersive X-ray (EDX) spectroscopy and annular dark field scanning TEM (ADF-STEM). The chemical elements of V atom in addition to W and Se atoms were clearly discernible in EDX spectrum ( Figure 1e) and EDX mapping ( Figure S3, Supporting Information). Unlike the HRTEM image in Figure 1d, numerous dark spots are clearly distinguished in ADF-STEM image (Figure 1f). The intensity of STEM is proportional to the atomic number (AN) by a* (AN)x, where a is constant and x ranges from 1.5 to 2.5. [15] Therefore, the lowest atomic number of V (AN = 23) atoms are seen as the dark spots compared with brightest W (AN = 74) and Se (AN = 34) atoms. After Gaussian filtering (Figure 1g), V atoms are clearly seen by substituting W atoms. The average intensities of W (155), 2Se (97), and V (28) atoms are well fitted to a*(AN)x (a = 0.29, x = 1.55), indicating that the dark spots are indeed V atoms (Figure 1h). Se-vacancies are also presented in Figure S4 of the Supporting Information with a vacancy density of 1.7 × 10 13 cm −2 . Figure 2a demonstrates the Raman spectra of V-doped WSe 2 with different V-doping concentrations ( Figure S5, Supporting Information). The Raman shift near 250 cm −1 is ascribed to the mixed modes of E 1 2g (strain) with A 1g (charge transfer) and is red-shifted to 3 cm −1 at 4% V-doping concentration. Such a red-shift is dominantly ascribed to the A 1g mode by the p-doping effect of V atoms on WSe 2 . [16] The 2LA(M) mode appears around 260 cm −1 and is gradually developed as V-doping concentration increases, reflecting the emergent crystalline disorders due to the randomly distributed V atoms. [17] The photoluminescence (PL) is further conducted to study the p-doping effect of V-substitution to WSe 2 ( Figure 2b). The asymmetric PL emission near 1.66 eV is reminiscent of the positive trions in intrinsic p-doped WSe 2 (trion binding energy = ≈30 meV). [18] As V-doping concentration increases, the PL intensity is reduced due to charge (hole) screening and the position is red-shifted by the developed positive trions. [19] To investigate V-doping effect on the electrical performance, we fabricated field effect transistors (FETs) using V-doped WSe 2 with Pd contact electrodes for efficient hole transport ( Figure 2c). The pristine WSe 2 -FET shows ambipolar transfer characteristics at a drain bias of 0.5 V. This prevails the p-type behavior as V-doping concentration increases. The threshold voltage in the hole region is shifted to the positive Adv. Sci. 2020, 7, 1902751 Figure 1. Synthesis and atomic structure of monolayer V-doped WSe 2 . a) Schematic for preparation process of water-based precursor. The precursor containing sodium metal (promoter), ammonium tungstate (W-precursor), and ammonium vanadate (V-precursor) are spin-casted onto SiO 2 /Si wafer. b) Two-zone furnace CVD process for growing V-doped WSe 2 . The precursor-coated substrate is introduced into the CVD chamber and is followed by selenization. c) Optical images of pure and V-doped WSe 2 . The monolayer (0.8 nm) was retained up to V-10% doping. The scale bar is 150 µm. d,e) High-resolution TEM image (d) and EDS spectrum (e) of V-2% WSe 2 . The scale bar is 1 nm. f,g) Unfiltered (f) and Gaussian-filtered (g) ADF-STEM images of V-2% WSe 2 . After the filtering process, the substituted V atoms in W sites are clealy seen. The scale bar is 2 nm. h) The number of W, Se, and V atoms with the STEM intensity. The scale bar is 1 nm.
side, while the on/off current ratio reaches to ≈10 5 at V-4% doping (Figure 2d). At a high V-doping concentration of 10%, the FET shows a heavily degenerate behavior with a negligible current modulation within the entire range of gate voltages (−50 to 50 V). Moreover, the hole mobility and carrier density increase as a function of V-doping concentration (Figure 2e; for the detail calculation, see Note S1 and S2, and Figure S6, Supporting Information).
We next construct two-terminal p-n diodes by vertically stacking the grown V-doped WSe 2 as a p-type and the exfoliated SnSe 2 as an n-type, as shown schematically (Figure 3a). The source and drain electrodes of Pd are deposited on the V-WSe 2 /SnSe 2 heterostructure (Figure 3b; for the detail fabrication process, see Figure S7, Supporting Information). [20,21] The thickness of SnSe 2 flakes in all diodes were 11-17 nm, as confirmed by AFM for the controlled experiments ( Figure S8, Supporting Information). The Fermi level of the V-doped WSe 2 is modulated with V-doping concentration. Meanwhile, the multilayer SnSe 2 FET device reveals a heavily degenerate n-type behavior by the high n-type current of ≈100 µA µm −1 ( Figure S8, Supporting Information). As V-doping concentration increases, the Fermi level is shifted toward the valence band edge up to V-4% and further into the valence band at V-10%, implying a near broken-gap band alignment with n-type SnSe 2 (Figure 3c). [22] We now demonstrate a variety of diode behaviors in V-WSe 2 /SnSe 2 devices by varying the V-doping concentration at room temperature. The diode device at V-1% ( Figure 3d) as well as the pristine WSe 2 device ( Figure S9, Supporting Information) shows a typical forward rectifying diode behavior. The Fermi level of V-1% or pristine WSe 2 sample locates near the middle of the bandgap, allowing for a large current flow from WSe 2 to SnSe 2 driven by the forward bias while suppressing the reverse current. At V-2% doping, the Fermi level downshifts closer to the valence band. An asymmetric ambipolar transistor is manifested, preferring a backward rectifying diode behavior (Figure 3e). A more prominent device is constructed at V-4% doping. NDR behavior is clearly observed in the forward bias region from 2.8 to 5.0 V with a peak-to-valley ratio of 1.6 ( Figure 3f). When the Fermi level of n-SnSe 2 is located above the valence band edge of V-doped WSe 2 , the current drops gradually with the reduction in electron bandto-band tunneling (BTBT). At a high V-doping concentration (V-10%), the V-doped WSe 2 turns into the degenerate state with the Fermi level getting inside the valence band, which is confirmed by the subtle gating effect in the entire range of gate bias ( Figure S10, Supporting Information). In such a case, the diode behaves as an ohmic resistance due to the presence of both degenerate n-SnSe 2 and degenerate p-WSe 2 (Figure 3g). The contact issues of resistance are excluded  here owing to the near-ohmic contact in V-doped WSe 2 /Pd junctions ( Figure S11, Supporting Information). [10] The band bending of alignments in heterostructures occurs in planar V-doped WSe 2 rather than the vertical V-WSe 2 /SnSe 2 junction due to the thin thickness of monolayer V-WSe 2 [9,23] and low carrier density while the V-doping concentration is low. In view of the SnSe 2 , which is degenerate and appears in metal, the band bending can be negligible. According to Poisson's equation, band alignments in the V-WSe 2 /SnSe 2 heterojunctions are calculated upon different V-doping concentrations in V-doped WSe 2 (insets in Figure 3d-g; the details are provided in Note S3 and Figure S12, Supporting Information). The band bending (depletion region width) in the V-WSe 2 at low V-doping concentration are much longer than the atomic thickness of monolayer V-WSe 2 , inducing the bands bend along the planar direction in V-WSe 2 . The initial type II band alignment is preserved at low V-concentration (up to V-4%). Further increase V-concentration to V-10%, the width of the depletion layer decreases, eventually obtains a type III brokengap alignment, implying the evolution of the carrier transport mechanism.
The aforementioned appealing device characteristics modulated by V-doping concentration are further improved for high device efficiency via electrostatic doping. The I DS -V DS output curves are measured at various gate biases from −30 to −110 V (Figure 4). In addition to the p-type V-doping in WSe 2 , the hole doping by the negative gate bias is applied to boost the efficiency without altering the device characteristics. [24,25] The Fermi level in heavily n-doped SnSe 2 is strongly pinned. In the forward rectifying diode at 1% V-WSe 2 (Figure 4a), a high current is achieved at a large negative gate bias; however, it yields a high rectification ratio (I 3V /I −3 V ) of 135 at V G = −30 V, benefiting from the suppression of minority carrier drift in the off-state compared to that at V G = −110 V. For the backward rectifying diode, the rectification ratio (I −3 V /I 3V ) increases up to 2.8 at V G = −110 V (Figure 4b). Meanwhile, the peak-to-valley ratio in NDR diode rises up to 2.3 at V G = −110 V (Figure 4c). Similar device characteristics are obtained from other four devices with the corresponding V-doping concentration ( Figure S13, Supporting Information). Figure 4d summarizes the improved device performances in terms of the rectification ratio (forward and backward) and peak-to-valley ratio with the gate bias modulation. The reproducibility of our devices is ensured with reasonable error bars. As the negative gate bias increases, the rectification ratio degrades in the forward direction but is improved in the backward diode. The peak-to-valley ratio is inversely proportional to the gate bias with a maximum value of 2.3, especially with 4% V-WSe 2 , higher than the value of 1.8 obtained from the WSe 2 /SnSe 2 without h-BN insulating layer. [26] This is ascribed to the increase in available energy states for tunneling, modulated by the high V-doping concentration. Diverse diode behaviors indicate different carrier transport mechanisms. In the pristine WSe 2 or V-1% device, there is a depletion region in WSe 2 nearby the heterointerfaces ( Figure S12a, Supporting Information). Under a positive drain bias (Figure 5a), the energy bands of WSe 2 shift down, thinning the barrier width. The majority electrons residing in the SnSe 2 conduction band, therefore, overcome the triangular barrier and migrate to the other side, generating a forward current. This process is dominated by the thermionic Fowler-Nordheim (FN) tunneling or FN tunneling, which can be modeled as [27] where A is the effective contact area, q is the unit charge, h is the Planck's constant, m and m* are the electron free mass and effective mass, respectively, and W and ϕ are the barrier width and height, respectively. Based on the derivation, 1/V follows ln(I/V 2 ) in the relationship of negative linear correlation. The ln(I/V 2 )-1/V plot ( Figure S14, Supporting Information) is calculated from Figure 3d, confirming FN tunneling mechanism with the negative slop at a high drain bias (near zero). [28] When V DS < 0, the minority carrier drift starts to dominate the current flowing from SnSe 2 to WSe 2 . Since the intrinsic concentration of minority carriers is limited and the diffusion length stays identical, the reverse current is small and tends to saturate, showing a forward rectifying behavior.
The Fermi level in V-2% WSe 2 locates closer to its valence band edge compared to that in pristine or V-1% sample because of the V-doping effect, and further approaches at a high negative gate bias. When V DS > 0, the SnSe 2 conduction band aligns with the forbidden band of WSe 2 , resulting in the forward FN tunneling current as in pristine WSe 2 or V-1% device (upper panel in Figure 5b). The depletion region in V-2% WSe 2 is much thinner but the built-in potential shows φ b < φ a (marked in upper panels in Figure 5b and Figure 5a, respectively) as indicated in the calculated band alignment (Figure S12b Supporting Information), leading to a smaller forward current. Meanwhile, the energy bands of V-WSe 2 move upward when V DS < 0, which allows for electrons from the WSe 2 valence band heading to the empty states in the SnSe 2 conduction band, generating a backward BTBT current. In this case, the thinner tunneling barrier promotes the efficient band-to-band tunneling in backward, whereas the lower built-in potential reduces the forward thermionic current, forming the backward rectifying behavior.
As V-4% WSe 2 -FET shows a strong p-type property, the original Fermi level is expected to locate near its valence band edge ( Figure S12c, Supporting Information). Gated by a high negative bias (Figure 5c), the electrostatic doping pushes the Fermi level into the valence band due to the strong accumulation of holes, leaving large amounts of empty states. Driven by a small positive drain bias (0 < V DS < V peak ), electrons from the SnSe 2 conduction band tunnel into the WSe 2 valence band owing to the overlap of equivalent energy states, and thus the forward tunneling current is generated. This current then reaches its peak when the SnSe 2 Fermi level matches the WSe 2 valence band edge, where the populations of equivalent states in two sides reach a maximum. A further increase in V DS induces the reduction in the tunneling current, namely, NDR behavior. The upper part of the SnSe 2 conduction band aligns with the forbidden band of WSe 2 , thereby prohibiting the BTBT of electrons, and the overlap region shrinks continuously until vanishes. Normally, I DS would pick up after the valley point and be dominated by the thermionic emission. However, this part is absent in some of our devices, which is probably because of the tunneling barrier or large hysteresis. Most of the drain bias drops at the wide interface barrier, which results in the appearance of peak current at a high V DS . In addition, the large hysteresis in the V-4% WSe 2 transistor (Figure S15a, Supporting Information) indicates the carrier trapping or scattering, which severely impedes the majority carrier diffusion. Apart from this, device with apparent I DS pick-up behavior is provided in Figure S15b of the Supporting Information with a peak-tovalley ratio of 1.4.
In conclusion, we have shown the tunable quantum tunneling in the V-doped WSe 2 /SnSe 2 heterostructure by varying the doping concentration at room temperature. A liquid precursor containing transition metal and vanadium dopant atoms is used in our approach to synthesize V-WSe 2 via CVD. The precise control of the doping concentration efficiently modulates the Fermi level position, achieving the p-type doping effect. This method induces the substitution of W by V, endowing a proper band bending for the desired band alignment, which determines the charge transfer across the heterostructure. We theoretically probed the variation in the band alignment upon to different V-doping concentration, where the intrinsic type-II band alignment is preserved at low concentration, and evolves into the type-III broken-gap alignment by heavily doping. Band alignments in various V-doped WSe 2 /SnSe 2 devices. a) In the pristine WSe 2 and V-1% devices, driven by a positive drain bias, the majority electrons thermally excited from the SnSe 2 conduction band dominate the carrier transport via FN tunneling, generating a forward rectifying current. φ a is the built-in potential. b) In the V-2% device, when V DS < 0, electrons tunnel from the WSe 2 valence band into the empty conduction band of SnSe 2 by the band-to-band tunneling, exhibiting a backward rectifying behavior. φ b is the built-in potential. c) In the V-4% WSe 2 , the Fermi level is expected to enter its valence band at a high negative gate due to the strong accumulation of holes. A tunneling channel is thereby established in the device for the band-to-band tunneling. Electrons are transferred across the tunneling barrier at the interface with an NDR behavior in the positive drain bias region.
Hence, the realization of diverse functions with the forward rectifying diode, backward rectifying diode, NDR diode, and ohmic resistance has been illustrated. They therefore suggest the promising opportunities to be applied for future integration in the scaled electronics.