Promising Approach for High-Performance MoS2 Nanodevice: Doping

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A Promising Approach for High Performance of MoS2 Nanodevice: Doping the BN Buffer Layer to Eliminate the Schottky Barriers Jie Su, Li-ping Feng, Xiaoqi Zheng, Chenlu Hu, Hongcheng Lu, and Zhengtang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10967 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Applied Materials & Interfaces

A Promising Approach for High Performance of MoS2 Nanodevice: Doping the BN Buffer Layer to Eliminate the Schottky Barriers Jie Sua, Li-ping Feng*a, Xiaoqi Zhenga, Chenlu Hua, Hongcheng Lub, Zhengtang Liua

a

State Key Lab of Solidification Processing, College of Materials Science and

Engineering, Northwestern Polytechnical University, Xi’an, Shaanxi, 710072, P. R. China b

Department of Physics, Duke University, Durham, North Carolina 27708, United

States E-mail: [email protected]

KEYWORDS: doping, BN buffer layer, MoS2, Schottky barrier, density functional theory

ABSTRACT: Reducing the Schottky barrier height (SBH) of metal-MoS2 interface with no deteriorating the intrinsic properties of MoS2 channel layer is crucial to realize the high-performance MoS2 nanodevice. To realize this expectation, a promising approach is present in this study by doping the boron nitride (BN) buffer layer between metal electrode and MoS2 channel layer. Results demonstrate that, no matter the types of concentrations of dopants, the intrinsic electronic structure and low electron effective mass of MoS2 channel layer, as well as the weak Fermi level pinning effects of metal/BN-MoS2 interfaces are preserved and not deteriorated. More

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importantly, the n-type and p-type SBHs of metal/BN-MoS2 interfaces are significantly reduced by the electron-poor and electron-rich dopants, respectively, when the doped BN buffer layer spreads all over the nanodevice, which is in contrast to the traditional doping rule. Moreover, both the n-type and p-type SBHs are further decreased and even eliminated when the concentrations of dopants increase. The n-type SBH of doped Au/BXN-MoS2 interface and the p-type SBH of doped Pt/BNX-MoS2 interface can be reduced to -0.21 and -0.61 eV by doping with high concentration of Li and O, respectively. This theoretical work provides an effective and promising method to realize high-performance MoS2 nanodevices with negligible SBHs.

1. INTRODUCTION As a transition metal dichalcogenide semiconductor, monolayer MoS2 has received considerable attentions due to its extraordinary electronic, optical, mechanical, and thermal properties, etc.1-6 Its sizable direct band gap of ~ 1.78 eV and fascinating physical behavior near the band edges enable monolayer MoS2 to be suitable for kinds of nanodevices, including electronic, optoelectronic, photovoltaic, photochemical, and spintronic nanodevices.7-13 More importantly, the ultra-thin thickness (~7 Å) and dangling-bond-free provide excellent gate electrostatic controls to suppress short channel effects of field-effect transistors (FETs).7-8 Moreover, the fabricated monolayer MoS2 FETs have demonstrated some excellent performances, such as high on/off ratio of 108,8 high photoresponsivity,12 and strong


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spin-polarization13. Hence, MoS2 FETs have been regarded as potential nanodevices in further. However, these MoS2 FETs are Schottky barrier transistors with large Schottky barrier heights (SBHs), which greatly limit the carrier injection efficiency and the performance of MoS2 FETs.14-16 Thus, reducing the SBHs is crucial to realize the high-performance MoS2 nanodevice. Several works have been conducted to reduce the SBHs, while the SBHs cannot be effectively reduced owing to the Fermi level pinning effects at metal-MoS2 interfaces.17-27 The Fermi level pinning effect at metal-MoS2 interface is mainly induced by the interfacial states of MoS2 layer, which are caused by the interlayer interaction between metal electrode and MoS2 layer.24-28 Increasing the interlayer separation between metal electrode and MoS2 layer can weaken the interfacial states and Fermi level pinning effects.27-28 Recent studies reported that inserting a thin buffer layer29-39, especially the BN buffer layer, between the metal electrode and MoS2 layer to form a metal/insertion-MoS2 interface (as displayed in Fig. 1) is an effective way to improve both the interlayer separation and Fermi level pinning effect, and then tune the SBHs. For example, our previous studies showed that the Fermi level pinning effect of metal-MoS2 interface is reduced from 0.71 to 0.43 by a monolayer BN buffer layer. 34 Moreover, the SBHs of all metal-MoS2 interfaces are declined upon forming the metal/BN-MoS2 interfaces.32,

34,

38-39

However, some reduced SBHs of

metal/BN-MoS2 interfaces are still large and far from satisfactory.34, 38 For example, the reduced n-type SBH (SBHn) of Au/BN-MoS2 interface (~0.14 eV) and p-type SBH (SBHp) of Pt/BN-MoS2 interface (~0.52 eV) are still large, although they are much



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lower than the SBHn of Au-MoS2 interface (~0.62 eV) and the SBHp of Pt-MoS2 interface (~0.80 eV).34-35 Further improving the SBHs of metal/BN-MoS2 interfaces is still vital to realize high performance nanodevices. Generally speaking, doping is a useful approach to improve the contact resistances and SBHs of metal-semiconductor interfaces.22-23, 40-41 Moreover, electron-rich and electron-poor dopants are beneficial to reduce the SBHn and SBHp, respectively. For instance, when the MoS2 layer is doped by niobium, the SBHp of Pd-MoS2 interface is reduced from 0.38 to 0 eV40; when the MoS2 layer doped by chloride, the contact resistance of Ni-MoS2 interface is reduced from 5 to 0.5 KΩ·µm.23 Nevertheless, these traditional doping methods can significantly deteriorate the band gap and carrier mobility of MoS2 layer,42-43 and then hamper to achieve higher performance MoS2 nanodevices. To overcome this issue, a new doping method, doping the BN buffer layer, is designed to reduce the SBHs of metal/BN-MoS2 interfaces and preserve the excellent intrinsic properties of MoS2 layer in this study. To date, none attentions have been paid to preliminarily judge whether dopants in the BN buffer layer facilitate to realize high performance MoS2 nanodevices. Therefore, we comprehensively study the electronic properties of doped metal/BN-MoS2 interfaces with different dopants in the BN buffer layer. Note that the SBH is not only related to the electronic properties of interface, but also connected with the electronic structures of channel material (see Fig. 1c). In the type-I FET, the channel material is the isolated monolayer MoS2 as shown in Fig. 1a, while in the type-II FET, the channel material may be the BN-MoS2 heterostructure as exhibited in Fig. 1b. Thus, the electronic properties of doped


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BN-MoS2 heterostructure in the type-II FET are also investigated by density functional theory. 2. COMPUTATIONAL METHOD The geometry optimizations and electronic calculations are performed using the CASTEP code with an ultrasoft pseudopotential method44 and VASP code with a projector augmented wave (PAW) method45-46, respectively. The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional47 and van der Waals correction proposed by the Grimme potential (D2)48 are employed. Such method has widely been employed to study the contact properties of metal-MoS2 interfaces.21,

24, 34-37

Dipoles corrections are used to eliminate the spurious interaction

between periodic slabs. All structures are relaxed until the force on each atom is less than 0.01 eV·Å-1. The energy cutoff is 400 eV. The Monkhorst-Pack k-point mesh is sampled with a separation of 0.05 and 0.015 Å-1 in the Brillouin zone during the relaxation and electronic calculation periods, respectively. The metal/BN-MoS2 interfaces are modeled in a supercell periodic in the z direction with six metal atomic layers in the most stable surface orientation (Au(111), and Pt(111)), a 5×5 monolayer h-BN supercell, a 4×4 monolayer MoS2 supercell, and a vacuum region of 15 Å. The detailed models have been fully described and displayed in our previous work.34 For the doped metal/BN-MoS2 interface, various dopants (Li, Be, B, C, N, O, F) are selected to substitute the boron or nitride atoms. Fig. 1d, as an example, demonstrates the most stable positions of dopants in the BN buffer layer.



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The formation energy of doped BN layer is calculated using the formula: ௗ௘௙௘௖௧

௣௘௥௙௘௖௧

‫ܧ‬௙௢௥௠ = ‫ܧ‬௧௢௧௔௟ − ‫ܧ‬௧௢௧௔௟ ௗ௘௙௘௖௧

where ‫ܧ‬௧௢௧௔௟

௣௘௥௙௘௖௧

and ‫ܧ‬௧௢௧௔௟

+ ݊௑ ሺߤ஻ − ߤ௑ ሻ + ݊௒ ሺߤே − ߤ௒ ሻ

are total energies of BN layers with and without

dopants, respectively. ߤ஻ , ߤே , ߤ௑ , and ߤ௒ are the atomic chemical potentials of B, N atoms, and X, Y dopants, respectively. ݊௑ and ݊௒ are the numbers of X and Y dopants, respectively. 3. RESULTS AND DISCUSSION 3.1 Formation energy Fig. 2 illustrates the formation energies of doped BN layers. It can be found that the formation energies of electron-poor dopants substituting B atoms (viz. BLi and BBe) are lower than those of electron-poor dopants substituting N atoms (viz. NLi and NBe). Such phenomenon is inversed when the electron-rich dopants dope the BN layer (viz. BO, BF, NO and NF). It suggests that Li and Be dopants prefer to substitute the B atoms, while O and F dopants tend to substitute the N atoms. In addition, the formation energies of C dopants substituting B and N atoms are about 0.06 and 0.11 eV, respectively, consistent with previous reports.49 Such comparable formation energies indicate that both B and N atoms are viably substituted by C dopants. Although the formation energies of antisite defects (viz. BN and NB) are higher than those of doped-free BN layer, such antisite defects have been observed in the BN sheet in experiment.50 Thus, the Li, Be, C, N dopants substituting B atoms, and B, C, O, F dopants substituting N atoms are studied in this study. 3.2 Electronic structures of doped BN-MoS2 heterostructure


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Fig. 3 displays the electronic structures of doped-free and doped BN-MoS2 heterostructures (viz. the channel materials in the type-II FETs). For comparison, the electronic structures of isolated monolayer MoS2 (viz. the channel material in the type-I FET) and BN are analyzed firstly. It can be seen from Fig. 3a that the band gaps of isolated monolayer MoS2 and BN are 1.78 and 4.57 eV, respectively. Albeit these band gaps are lower than the experimental values,1, 51 the electronic structures around the Fermi level are consistent with previous experimental and theoretical results.52-53 For instance, the conduction band minimum (CBM) of isolated monolayer MoS2 is characterized by Mo dz2 orbitals coupling with weak S p orbitals, the valence band maximum (VBM) is mainly contributed by Mo dxy+dx2-y2 orbitals. For the doped-free BN-MoS2 heterostructure in Fig. 3b, the VBM and CBM of MoS2 layer appear in the energy gap of BN layer, resulting in doped-free BN-MoS2 a type-I heterostructure. As a result, the electronic properties of doped-free BN-MoS2 heterostructure are dominated by the MoS2 layer. In other words, the channel material in the type-II FET is dominated by the MoS2 layer in the channel region (viz. MoS2 channel layer). In addition, the electronic features of MoS2 channel layer in the doped-free BN-MoS2 heterostructure are similar to those of isolated monolayer MoS2. For example, the CBM and VBM of MoS2 channel layer in the doped-free BN-MoS2 heterostructure are also dominated by Mo dz2 and (dxy+dx2-y2) orbitals, respectively. Furthermore, the electron effective masses of MoS2 channel layer in the doped-free BN-MoS2 heterostructure of about 0.47m0 is close to that of isolated monolayer MoS2 (~0.48m0).54-55 Such phenomena are also found in other 2D-MoS2 van der Waals



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heterostructures.56 These results indicate that the BN buffer layer has negligible influence on the intrinsic properties of MoS2 channel layer in the type-II FETs. Compared to the doped-free BN-MoS2 heterostructure, similar characteristics of electronic structures (including the electronic structure distributions and electron effective masses) of MoS2 channel layers are also observed in the doped BN-MoS2 heterostructures although some gap states appear in the band gap of BN layer, such as the MoS2 channel layers in doped BNC-MoS2 and BCN-MoS2 heterostructures in Fig. 3(c-d). Moreover, these electronic features and electron effective masses of MoS2 channel layer are preserved even if increasing the concentration of C dopants, as illustrated in Fig. 3(e-f). Similar phenomena are occurred in other doped BN-MoS2 heterostructures, no matter the types and concentrations of dopants. These characters suggest that introducing dopants into the BN buffer layer have little influence on the electron mobility of MoS2 channel layer in the type-II FET, which are different to traditional doping methods.42-43 To further study the electronic properties of MoS2 channel layer in the doped BN-MoS2 heterostructure, the positions of band edges of MoS2 channel layer are illustrated in Fig. 4. The VBM and CBM of isolated monolayer MoS2 are -4.20 and -5.98 eV, respectively; while the VBM and CBM of MoS2 channel layer in the doped-free BN-MoS2 heterostructure are changed to -4.27 and -5.95 eV, respectively. That is because weak charge transfers between BN and MoS2 layers of BN-MoS2 heterostructure (see Fig. S1) are sufficient to give rise to dipoles between BN and MoS2 layers,57 which can push the CBM and VBM of MoS2 channel layer move


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upward and downward, respectively58. Compared to the doped-free BN-MoS2 heterostructure, the VBM and CBM of MoS2 channel layer in the electron-rich (BC, BN, NO, and NF) doped BN-MoS2 heterostructures are shifted up; while the VBM and CBM of MoS2 channel layer in the electron-poor (BLi, BBe, NB, and NC) doped BN-MoS2 heterostructures are shifted down. Similar characters have been observed in the defective AlN-MoS2 heterostructures.59 Since electron-rich (or electron-poor) dopants in the BN layer induce some occupied (or unoccupied) gap states in the band gap of BN layer, which are close to the band edges of MoS2 layer of the doped BN-MoS2 heterostructure, as demonstrated in Fig. 3. Furthermore, charges in the occupied and unoccupied gap states can be easily excited to the conduction and valence band of MoS2 layer of BN-MoS2 heterostructure, and then move down and up the band edges of MoS2 channel layer.58-59 In addition, these occupied and unoccupied gap states slightly enhance and weaken the charge transfers between BN and MoS2 layers (see Fig. S1), which can improve and decrease the dipoles of BN-MoS2 heterostructrues, respectively, and then further decline and raise the band edges of MoS2 channel layer. Moreover, with the increasing doping concentration, more gap states and charge transfers are observed, as exhibited in Fig. 3 and Fig. S1. Thus, as the doping concentration increases, the band edges of MoS2 channel layers in the electron-rich and electron-poor doped BN-MoS2 heterostructures are further increased and decreased, respectively. Nevertheless, it should be noted that the extent of variation of VBM and CBM is independent of the electron difference between the dopant and the replaced atom. In addition, irrespective of the kinds and concentrations



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of dopants, BN-MoS2 heterostructures show direct band gaps of about 1.66~1.68 eV, which are slightly smaller than the band gap of isolated monolayer MoS2. Although dopants in the BN layer have negligible effects on the weak charge transfers between BN and MoS2 layers, which are sufficient to slightly tune the band gap of MoS2 channel layer.60 3.3 Electronic structures of doped metal/BN-MoS2 interfaces Fig. 5 illustrates the electronic structures of doped metal/BN-MoS2 interfaces. It should be noted that although the GGA function underestimates the band gap of MoS2 layer, this underestimation has negligible substantial effect on investigating the interface hybridization and contact properties due to the following three reasons. First, the GGA and more accurate GW calculations give a same direction of charge transfer.26 Second, the band alignment differs little between GGA and GW calculations except for the gap size. Third, compared to the GW result, the band gap calculated by GGA is closer to the transport band gap of monolayer MoS2 FET because the strong Columbic screening by metal surface significantly minimizes the exciton binding energies and many body effects of monolayer MoS2 in FET.26 For the isolated monolayer MoS2, its band edges consist mainly of the Mo 4d and S 3p states. Upon forming a metal-MoS2 interface, such as the Au-MoS2 interface, these states of MoS2 layer spread into the band gap to form interfacial gap states and fill in the band gap, because the strong charge transfers between Au surface and MoS2 layer break the charge distributions of Mo-S bondings (see Fig. 6a and Fig. S2), and then change the energies of Mo 4d states of MoS2 layer.27 As a result, the MoS2 layer


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in Au-MoS2 interface is metallic, as exhibited in Fig. 5a. In addition, these interfacial gap states mean strong Fermi level pinning effects at the Au-MoS2 interface.27, 61 However, for the doped-free Au/BN-MoS2 interface in Fig. 5b, the electronic structures of MoS2 layer are similar to those of isolated monolayer MoS2. That is because the MoS2 layer in the doped-free Au/BN-MoS2 interface undergoes negligible charge transfers (just accept 0.12e, as seen in Fig. 6b), which are insufficient to break the charge distributions of Mo-S bondings (see Fig. S2). Moreover, none interfacial gap states are observed for the MoS2 layer, which means weak Fermi level pinning effects at the doped-free Au/BN-MoS2 interface, in contrast to that of metal-MoS2 interface. In addition, compared to the Au-MoS2 interface, the Fermi level of doped-free Au/BN-MoS2 interface is closer to the CBM of MoS2 layer. It indicates that the SBH of Au-MoS2 interface can be tuned by the BN buffer layer. Compared to the doped-free Au/BN-MoS2 interface, the general electronic features of MoS2 layer are well preserved in the doped Au/BN-MoS2 interfaces no matter the kinds of dopants because dopants in the BN layer have negligible influences on the charge transfers between BN and MoS2 layers (see Fig. 6c) and the symmetrical charge distributions of Mo-S bondings (see Fig. S2). For example, the band edges of MoS2 layers in both the doped Au/BNC-MoS2 and Au/BCN-MoS2 interfaces consist dominantly of the Mo 4d and S 3p states, in Fig. 5(c-d), consistent with those of doped-free Au/BN-MoS2 interface. Moreover, obvious band gaps without any interfacial gap states are observed for these MoS2 layers. It indicates that the dopants in the BN buffer layer have negligible effect on the weak Fermi level



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pinning effects. However, different to the doped-free Au/BN-MoS2 interface, the Fermi levels of electron-rich doped Au/BN-MoS2 interfaces (e.g. Au/BCN-MoS2 interface in Fig. 5c) and electron-poor doped Au/BN-MoS2 interfaces (e.g. Au/BNC-MoS2 interface in Fig. 5d) further shift toward the CBM and VBM of MoS2 layer, respectively. These suggest that the SBH of Au/BN-MoS2 interface can be further tuned by both electron-rich and electron-poor dopants in the BN buffer layer. When the concentrations of dopants increase, these general electronic properties of MoS2 layer are still preserved, as shown in Fig. 5(e-f). Since the doping concentration has little effect on the charge transfers between BN and MoS2 layers (see Fig. 6d) and the charge distributions of Mo-S bondings (see Fig. S2). However, the positions of Fermi levels continue to be shifted with the enlarging concentrations of dopants. As a result, the SBH of Au/BN-MoS2 interface can be further tuned by the concentration of dopants in the BN buffer layer. These phenomena are also observed in other metal/BN-MoS2 interfaces. 3.4 Schottky barriers of metal/BN-MoS2 interfaces 3.4.1 SBH in the type-I FET According to the Fig. 1, the SBHs in the type-I FET are extracted by the difference between the Fermi level of metal/BN-MoS2 interface and the band edges of isolated monolayer MoS2, and shown in Fig. 7. In the type-I FET, the SBHn of doped-free Au/BN-MoS2 interface and the SBHp of doped-free Pt/BN-MoS2 interface are 0.11 and 0.48 eV, respectively. These values are smaller than the SBHs of Au-MoS2 (~0.69 eV) and Pt-MoS2 (~0.80 eV) interfaces.21, 27, 61 Moreover, the SBHn


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of Au/BN-MoS2 interfaces are reduced just when the BN buffer layers are doped by electron-rich atoms (except for F atom); the SBHp of Pt/BN-MoS2 interfaces are decreased just when the BN buffer layers are doped by electron-poor atoms (except for Li atom). The lowest SBHn of doped Au/BNO-MoS2 interface and the lowest SBHp of doped Pt/BBeN-MoS2 interface are reduced to -0.01 and 0.33 eV, respectively. Since the electron-rich (or electron-poor) dopants in the BN buffer layer can enhance (or weaken) the charge transfer from BN layer to metal surface, and then decrease (or increase) the work function of metal/BN-MoS2 interface. For instance, when the BN buffer layer doped by NO dopant, the charge accumulation at the Au surface and the work function of doped Au/BN-MoS2 interface are enlarged and reduce from 0.85e to 1.17e, and 4.25 to 4.19 eV, respectively, as illustrated in Fig. 6 and Fig. S3. These variation tendencies of SBHs in the type-I FET are consistent with the traditional doping rule.22-23, 40-41 Nevertheless, it should be noted that the variation of SBH in the type-I FET is independent of the electron difference between the dopant and doped atom. For example, the SBHn of doped Au/BLiN-MoS2 interface of about 0.09 eV is lower rather than larger than that of doped Au/BBeN-MoS2 interface (0.12 eV), albeit the electron difference between Li and B is larger than that between Be and B elements. Since the radius difference between Li and B atoms is significant larger than that between other dopants and B atom, causing the atoms around the Li dopant in the doped Au/BLiN-MoS2 interface undergo more significantly relaxation. As a result, the directions of interface dipoles of doped Au/BLiN-MoS2 interface are disarranged more significantly.38 Consequently, although the larger electron difference



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results in the stronger charge transfer at the interface region, the interface dipoles and work function of doped Au/BLiN-MoS2 interface are lower than those of doped Au/BBeN-MoS2 interface (see Fig. S3), and leading to the lower SBHn for doped Au/BLiN-MoS2 interface. Similar phenomena are also observed in other Li and F doped metal/BN-MoS2 interfaces, as shown in Fig. 7(a-b). When the doping concentrations increase, the variations of SBHs in the type-I FET are shown in Fig. 7c. Except for the doped metal/BNF-MoS2 and metal/BLiN-MoS2 interfaces, the SBHn of doped Au/BN-MoS2 interfaces and the SBHp of doped Pt/BN-MoS2 interfaces are further reduced with the enlarging concentrations of electron-rich and electron-poor dopants, respectively. Taking the doped Au/BNnO-MoS2 interface as an example, the SBHn of doped Au/BNO-MoS2, Au/BN2O-MoS2, and Au/BN3O-MoS2 interfaces are -0.01, -0.03, and -0.06 eV, respectively, which are lower than the SBHn of doped-free Au/BN-MoS2 interface. Since more and more electrons are accumulated at the Au surface with the increasing concentration of O dopant (see Fig. 6), resulting in the decreasing work function of doped Au/BNnO-MoS2 interface. Similar mechanisms are also suitable to other doped metal/BN-MoS2 interfaces. In addition, for the doped metal/BNnF-MoS2 and metal/BnLiN-MoS2 interfaces, their SBHs vary irregularly with the doping concentrations. Because more significant interfacial structure deformations and disarranged directions of interfacial dipoles are occurred, albeit the charge transfers are increased as the doping concentration increases, which lead to the irregular variation of work function of doped metal/BN-MoS2 interface.38


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3.4.2 SBH in the type-II FET Fig. 7 also illustrates the SBHs of metal/BN-MoS2 interfaces in the type-II FET. It can be seen that the SBHn of doped-free Au/BN-MoS2 interface and the SBHp of doped-free Pt/BN-MoS2 interface in the type-II FET are about 0.04 and 0.41 eV, respectively. These values are not only smaller than the SBHs of metal-MoS2 interfaces, but also lower than the SBHs of doped-free metal/BN-MoS2 interfaces in the type-I FET. That is because the CBM and VBM of MoS2 channel layer in the type-II FET are larger and smaller than those in the type-I FET, respectively, as demonstrated in Fig. 4. Moreover, the SBHn of Au/BN-MoS2 interfaces and the SBHp of Pt/BN-MoS2 interfaces in the type-II FETs are decreased when the BN buffer layers are doped by electron-poor and electron-rich dopants, respectively, as shown in Fig. 7(a-b). These phenomena are in contrast to the variation of SBHs of doped metal/BN-MoS2 interfaces in the type-I FET and the traditional doping rules.22-23, 40-41 One main reason is that, the work function of interface and the band edges of channel MoS2 layer in the type-II FET are simultaneously raised or downed by dopants in the BN buffer layer, but the variation of work function of doped metal/BN-MoS2 interface is slower than the variation of band edge of channel MoS2 layer in the type-II FET, as shown in Fig. 4 and Fig. S3. Since two-dimensional channel materials do not have the bulk materials’ reservoir of a large amount of electrons, particularly compared to the electron sea in metal/BN-MoS2 interface.27 A slight charge transfer more significantly change the orbital energy of two-dimensional channel material than that of interface, and then more obviously change the band edges of MoS2 channel layer than the work



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function of interface in the type-II FET, as demonstrated in Fig. S4. Note that, all the reduced SBHs of doped metal/BN-MoS2 interfaces in the type-II FET are eliminated. Moreover, the lowest SBHs, especial the SBHp of Pt/BN-MoS2 interfaces, in the type-II FET are significant lower than those in the type-I FET, as comparatively shown in Fig. 7(a-b). For examples, the lowest SBHn of doped Au/BN-MoS2 interface and the lowest SBHp of doped Pt/BN-MoS2 interface in the type-II FET are -0.13 and -0.19 eV, respectively, which are far lower than the lowest SBHn of about -0.01 eV and SBHp of about 0.37 eV in the type-I FET. It indicates that doping the BN buffer layer by electron-poor (or electron-rich) dopants in the type-II FET is an more effective way to reducing the SBHn (or SBHp) of metal/BN-MoS2 interfaces, compared to the reduction SBHn (or SBHp) in the type-I FET by doping electron-rich (or electron-poor) dopants. Since the variations of band edges of doped BN-MoS2 heterostructures are significant larger than the variations of work function of doped metal/BN-MoS2 interface, such as the variation of CBM of doped BN-MoS2 heterostructure of about 0.15~0.68 eV is several times larger than that of work function of doped Au/BN-MoS2 interface (about 0.01~0.14 eV), as demonstrated in Fig. S3. As a result, the variation of SBH of doped Au/BN-MoS2 interfaces in the type-II FET is significant larger than that of work function of doped Au/BN-MoS2 interface. Note that the variation of SBH in the type-I FET is directly determined by that of work function according to the definition of SBH. Thus, the reduced SBHs of doped metal/BN-MoS2 interfaces in the type-II FET are lower than those reduced SBHs in the type-I FET. However, like the variation of SBH in the type-I FET, the


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variation of SBH in the type-II FET is also independent of the electron difference between the dopant and doped atoms. For example, the SBHp of doped Pt/BNO-MoS2 interface of about -0.19 eV is lower instead of larger than that of doped Pt/BNF-MoS2 interface (-0.11 eV) albeit the electron difference between O and N atom is smaller than that between F and N atom. In addition, all the SBHn of doped Au/BN-MoS2 interface and the SBHp of doped Pt/BN-MoS2 interface in the type-II FET are further reduced and even eliminated with the increasing concentrations of electron-poor and electron-rich dopants, respectively, as illustrated in Fig. 7d. The lowest SBHn of doped Au/BnLiN-MoS2 and the lowest SBHp of doped Pt/BNnO-MoS2 interface can be reduced to -0.21 and -0.61 eV, respectively, which are not only far lower than the SBHn of Au/BN-MoS2 interface and the SBHp of Pt/BN-MoS2 interface, but also lower than the lowest SBHs in the type-I FET, as comparatively shown in Fig. 7(c-d). It further indicates that doping the BN buffer layer in the type-II FET instead of the type-I FET is an more effective way to reducing the SBH of metal/BN-MoS2 interfaces. CONCLUSION Effects of dopants in the BN buffer layer on the electronic properties of metal/BN-MoS2 interfaces are comprehensively studied through density functional theory. Note that, by doping the BN buffer layer, the SBHn and SBHp of metal/BN-MoS2 interfaces in the type-II FET are reduced by electron-poor and electron-rich dopants, respectively, which are in contrast to the traditional doping rule and the variation of SBHs in the type-I FET. Moreover, the reductions of SBHs in the



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type-II FET are more significant than those in the type-I FET. That is because the variation of work function of doped metal/BN-MoS2 interface is slower than that of the band edge of channel MoS2 layer. Importantly, both the SBHn and SBHp are further decreased and even eliminated with the increasing doping concentrations. The SBHn of doped Au/BXN-MoS2 interface and the SBHp of doped Pt/BNX-MoS2 interface are reduced to -0.21 and -0.61 eV by high concentrations doping of Li and O dopants, respectively. In addition, the intrinsic electronic structure and low electron effective mass of MoS2 channel layer, as well as the weak Fermi level pinning effects of metal/BN-MoS2 interfaces are not deteriorated by the doping of BN buffer layer, irrespective of the types and concentrations of dopants. This work provides an effective and promising way to reduce the SBHs of metal-MoS2 interface without deteriorating the intrinsic properties of MoS2 channel layer in high-performance MoS2 nanodevice.

ASSOCIATED CONTENT Supporting Information: Charge transfers of BN-MoS2 heterostructure systems. Charge densities of Au/BN-MoS2 interface systems. Variations of CBM of BN-MoS2 heterostructure systems and those of WF of Au/BN-MoS2 interface systems. The effect of an additional charge on the electrostatic potentials of BN-MoS2 heterostructure and Au/BN-MoS2 interface, respectively.


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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 61376091, and 11674265); Research Funds of the State Key Laboratory of Solidification Processing (Grant No. KP201614); Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (Grant No. CX201712); Excellent Doctorate Foundation of Northwestern Polytechnical University.

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(b)

(a) Metal

Metal

e

e

SBH

SBH MoS2

MoS2

(d)

(c)

Channel

B

N

S

Mo

Dopant

Fig. 1 Two typical monolayer MoS2 FETs with metal/BN-MoS2 interfaces are designed according to the size of BN layer: (a) the BN layer just under the metal surface in the type-I FET; (b) the BN layer spreads over the MoS2 layer in the type-II FET. (c) Band diagrams of the metal/BN-MoS2 interface. (d) The most stable positions of dopants in the BN layer of doped metal/BN-MoS2 interface.



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Fig. 2 Formation energies of doped BN layers dependents of the dopants.


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(b)

(a)

(c) 0.47m0

0.47m0

0.48m0

(e)

(d)

(f) 0.47m0

0.48m0

0.48m0

Fig. 3 Electronic structures (the left panel is band structure, the right panel is DOS) of BN-MoS2 heterostructures (viz. the channel materials in the type-II FETs): (b) doped-free, (c) NC doped, (d) BC doped, (e) N2C doped, (f) B2C doped. For comparison, the electronic structures of isolated monolayer MoS2 and BN are added into (a). The corresponding effective electron masses of MoS2 layers are marked in these figures for comparison.



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(a)

(b)

n=1

n=2

n=3

Fig. 4 Evolution of band edges of (a) doped BnXN-MoS2 and (b) BNnX-MoS2 heterostructures as functions of the concentrations of dopants. The magenta dot and black dash lines are the band edges of isolated monolayer MoS2 and doped-free BN-MoS2 heterostructure, respectively.


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(a)

(b)

(c)

(d)

(e)

(f)

Fig. 5 Density of states (DOS) of MoS2 layers in (a) Au-MoS2, (b) doped-free Au/BN-MoS2, (c) doped Au/BCN-MoS2, (d) doped Au/BNC-MoS2, (e) doped Au/B2CN-MoS2, and (f) doped Au/BN2C-MoS2 interfaces. The green and orange regions represent the valence and conduction bands of isolated monolayer MoS2 in the channel region, respectively.



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(a)

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(b) 0.12e 1.30e 0.85e

(c)

(d) 0.12e

0.13e

1.17e

1.53e

Fig. 6 Side view of the optimized structures and average electrostatic potentials in planes normal to the (a) Au-MoS2 interface, (b) doped-free Au/BN-MoS2 interface, (c) doped Au/BNO-MoS2 interface and (d) doped Au/BN2O-MoS2 interface, respectively. The amounts of accumulated electrons at the MoS2 layers and Au surfaces are obtained through Mulliken charge analysis and marked in the corresponding figures.


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(a)

(b)

(c)

(d)

Fig. 7 SBHs of doped metal/BNX-MoS2 (a) and metal/BXN-MoS2 (b) interfaces, respectively. The solid and dash lines in figure (a-b) represent the SBHs of doped meal/BN-MoS2 interfaces in the type-I and type-II FETs, respectively. SBHs of doped metal/BnXN-MoS2 and metal/BNnX-MoS2 interfaces in the type-I (c) and type-II (d) FETs vary with the concentrations of dopants, respectively. The light orange and light gray regions denote the doped Au/BN-MoS2 and Pt/BN-MoS2 interfaces, respectively.



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Title: A Promising Approach for High Performance of MoS2 Nanodevice: Doping the BN Buffer Layer to Eliminate the Schottky Barriers

Authors: Jie Su, Li-ping Feng, Xiaoqi Zheng, Chenlu Hu, Hongcheng Lu, Zhengtang Liu

Metal

SBH

MoS2 B

N

S

M

Dopant Electron-poor


Electron-rich

Promising Approach for High-Performance MoS2 Nanodevice: Doping

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