Metal Interfaces with a Buffer Layer

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Tunable Schottky Barrier at MoSe/Metal Interfaces with a Buffer Layer Le Huang, Bo Li, MianZeng Zhong, Zhongming Wei, and Jingbo Li J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 19 Apr 2017 Downloaded from http://pubs.acs.org on April 21, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Tunable Schottky Barrier at MoSe2/metal Interfaces with a Buffer Layer Le Huang,† Bo Li,‡ Mianzeng Zhong,‡ Zhongming Wei,∗,‡ and Jingbo Li∗,† School of Materials and Energy, Guangdong University of Technology, Guangzhou, Guangdong 510006, and State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100083, China E-mail: [email protected]; [email protected]

Abstract Transition metal dichalcogenide monolayers have gained significant attention because of their excellent physical properties and promising applications as a channel material in the next-generation transistors. In this work, we focus on contacts at the surface of various metals and single-layer MoSe2 . Partial Fermi level pinning is demonstrated by the first-principle calculations, which indicates modulation of the electron Schottky barrier. Upon inserting a VS2 layer between MoSe2 layer and metal electrodes, all the n-type contacts at MoSe2/metal interfaces turn into p-type and the hole Schottky barrier can be tuned effectively by varying metal electrodes. The high work function of the VS2 layer exerts significant influence on the band realignment of MoSe2 , making all the n-type contacts at MoSe2 /metal interfaces become p-type contacts at MoSe2 /VS2 -metal interfaces. Variation of the Schottky barriers ∗ To

whom correspondence should be addressed of Materials and Energy, Guangdong University of Technology, Guangzhou, Guangdong 510006 ‡ State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100083, China † School

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and band alignments with the work function of metal electrodes demonstrated a partial Fermi level pinning at the interfaces of MoSe2 /metal and MoSe2 /VS2 -metal. The partial Fermi level pinning results from the low density of interfacial states, which can be reflected partly by the interaction between MoSe2 layer and metal electrodes. Our results would provide guidelines for designing novel 2D nanoelectronic devices with good performance.

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Among various two-dimensional (2D) materials, monolayer transition-metal dichalcogenides (TMDs) with intrinsic direct band gaps are considered promising condidates as channel materials in next generation transistors. 1–7 Field effect transistors based on TMDs exhibit outstanding performances such as high on/off current ratio of about 108 1,2,8,9 and high field effect mobility for both electrons and holes. 8,10,11 While the performance of the electronics based on TMDs has been severely limited by the high contact resistance at metal-TMDs junctions. 12 This high resistance is dominated by a large Schottky barrier height (SBH), which cannot be effectively lowered due to the Fermi level pinning (FLP). 13,14 Besides, a finite SBH also reduces the carrier injection efficiency. 15–18 The phenomena of Fermi level pinning are normally induced by defect states or interfacial states at the surface or interface due to the presence of dangling bonds in semiconductors. Unlike conventional semiconductors including Si and GaAs, 2D TMDs have no dangling bonds at 2


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their surfaces. When 2D TMDs make van der Waals contacts with other materials, charge carriers transferring is normally impeded by the existing tunnel barrier at the contacting interfaces. This mechanism of charge transfer also shows difference from co-planar junctions. 19 The interaction between TMDs and metals surfaces has been demonstrated relatively weak, making a low density of midgap interfacial states. Partial rather than total Fermi level pinning usually can be observed. Indeed, many recent works demonstrated a partial Fermi level pinning at metal/TMDs interfaces, 13,20,21 indicating that the Schottky barrier height can be tuned effectively by varying metal electrodes. A metal with sufficiently low work function is still demanded to obtain a zero SBH. Quasi-Ohmic contact has been achieved in MoS2 field effect transistors by using scandium as electrodes. 6 Theoretical calculations have predicted that Cu(111) is a good candidate to form Ohmic contact to monolayer phosphorene. 22 It is generally recognized that the Fermi level can be unpinned by inserting a layer of 2D material between the metal surface and TMDs to break the direct metal-TMDs interaction and destroy the interface states. 23–26 For example, Mojtaba Farmanbar et al. demonstrated that inserting a boron nitride monolayer between the metal and MoS2 is a practical method to obtain vanishing SBH. 23 A relatively weak Fermi level pinning compared to MoS2 /metal systems also can be realized by using a graphene oxide as an efficient hole injection layer between MoS2 and metals. 24 Besides, molybdenum trioxide is proved an efficient hole injection layer to MoS2 and WSe2 . 25,26 More recently, Y. Liu et al. demonstrated that Fermi level is completely unpinned and enables effective lowering of SBH in van der Waals metel-TMD junctions. 27 Tunability of the electron/hole SBH is of great significance for design of nanoelectronic devices with great performance. Compared with those about MoS2 , the studies of the nature of the metal-MoSe2 contact are more limited. Additionally, MoSe2 can be n-type and p-type conductor when contacting with different metal electrodes. When an elemental metal contacted with MoSe2 monolayer, whether it is n-type or p-type is not clear by now. What’s more, the tunability of the electron/hole SBH by varying metal electrodes needs to be determined. As a p-type 2D semiconductor, how to lower the hole SBH at MoSe2 -metal interfaces is of great significance for

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nanoelectronics designing. Here, a systematic density functional theory study of the interfaces between single layer MoSe2 and variety of metals is performed to investigate the Fermi level pinning (FLP) behavior. A partial FLP is observed due to weak interaction between MoSe2 layer and metals and the electron/hole SBH can be tuned effectively by varying metal electrodes. We also demonstrated that inserting a VS2 layer indeed weakens the interaction at the contacting interfaces and a similar partial FLP is observed. Furthermore, the transition from n-type contacts to p-type contacts can be brought about by inserting a VS2 layer. The inserted VS2 layer with high work function may act as an efficient hole injection layer to MoSe2 and makes the modulation of hole SBH by varying metal electrodes more practical. Our results would provide guidelines to develop approaches to form Ohmic contact to p-type conducting MoSe2 .

RESULTS AND DISCUSSION Figure 1 depicts the equilibrium geometry of MoSe2 /metal and MoSe2 /VS2 -metal systems. The optimized lattice constants of monolayer MoSe2 are a = b = 3.32 Å. Various metals including selectron metals (Ag, Al, Au, Cd, Er, Mg) and d-electron metals (Hf, Ir, Pd, Pt, Sc, Ti, Zr) whose inplane lattices are almost commensurable to MoSe2 are employed to make contacts with single-layer √ √ MoSe2 in this work. For example, putting a 3 × 3R30◦ MoSe2 lattice on top of a 2 × 2 Au(111) lattice, 13,28 requires increasing the metal lattice parameter by 0.8%. Mg(111) lattice is compressed by 4.3% to overcome the mismatch with MoSe2 unitcell. The overall induced mismatch between metals and single-layer MoSe2 is smaller than 5.0%, which is considered to exert little influence on our results. To characterize the contact interaction strength, the binding energy Eb between metals and MoSe2 layer is calculated as Eb = (Etotal − Emetal − EMoSe2 )/NMo , where Etotal , Emetal and EMoSe2 are the total energy of the combined systems, isolated metal electrodes and isolated MoSe2 monolayer, respectively. NMo is the number of Mo atom in corresponding systems. Figure 1e depicts

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Figure 1: (color online) (a-b) The lattice structures √ of single-layer MoSe2 on top of 2 × 2 Au(111) √ ◦ and 1 × 1 Mg. Two black dashed frames present 3 × 3R30 and unitcell MoSe2 . (c-d) The lattice structures of MoSe2 /Au and MoSe2 /VS2 -metal from side view (left panels). The integrated charge density difference is shown in the right panels. The red dashed line plots the integrated charge density difference of the MoSe2 /Pt interface to make comparison with that of MoSe2 /Au. (e) Binding energy Eb as a function of the separation d between MoSe2 and Au(111) and Pt(111) surface.

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the evolution of the binding energy of Au/MoSe2 and Pt/MoSe2 systems as functions of interlayer distances. It is clear that the binding of MoSe2 to Au is considerably weaker than that of MoSe2 to Pt. The interlayer distances as well as the binding energies between metals and MoSe2 at equilibrium state are plotted in Figure 3a-b. Our results indicate that d-electron metals contact closer to MoSe2 than s-electron metals, which is consistent with previous results. 13 As a result, stronger binding occurs in d-electron metals/MoSe2 than that in s-electron metals/MoSe2 contacts. Further insight into the interaction between MoSe2 layer and metals can be obtained from ∫

∫

charge difference analysis, 29–31 which is calculated as ∆ρ (z) = ρmetal/MoSe2 dxdy− ρmetal dxdy−

∫

ρMoSe2 dxdy, where ρmetal/MoSe2 , ρmetal and ρMoSe2 are the charge density at the (x, y, z) point in

combined system, isolated metal and isolated MoSe2 layer. As can be seen in Figure 1c-d, charge transferring occurs between metals and MoSe2 . Because there are no dangling bonds at the surface of 2D MoSe2 , the interaction with metal surfaces should be rather weak, which reflects partly a rather low density of interfacial states. Therefore, surface charge transferring causes significant change in metal work function and partial FLP. To make comparison, we also plot the integrated charge density difference at the MoSe2 /Pt interface by red dashed line in Fig. 1c. It is clear that much more charge transfers between MoSe2 layer and Pt than that between MoSe2 layer and Au, which are in good consistence with our binding energy calculations. Comparing the charge transferring in MoSe2 /s-electron metal (Au is taken as an example) with that in MoSe2 /d-electron metal (Pt is taken as an example), much more charge transferring between d-electron metal and MoSe2 than that between s-electron metal and MoSe2 owing to the smaller interlayer distance between them. Figure 2a-f shows the projected band structures of several metal/MoSe2 systems. As can be seen, the general features of MoSe2 monolayer are well perserved. The electronic structure of MoSe2 layer is not destroyed when contacting with elemental metals. The d-electron metals exert much more significant influence on the electronic properties of MoSe2 because of strong coupling between d-orbital of Mo and metal atoms. It is worth noting that the Schottky barrier heights of electrons (Φe ) and holes (Φh ) in different metal/MoSe2 contacts vary with the work function of

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Figure 2: (color online) (a-f) Projected band structures of single-layer MoSe2 contacting with several elemental metals. The Fermi level is set as zero. The bands dominated by MoSe2 layer and metal atoms are plotted by red and gray dots, respectively. (g) Evolution of the Schottky barrier height of electrons (Φe ) in MoSe2 /metal contacts as a function of the work function of MoSe2 /metal. The red squares and blue triangles present results of s-electron metals and d-electron metals, respectively.

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metals. Φe and Φh are determined by the energy defference between the Fermi level and band edges of MoSe2 layer in the contacts, Φe = ECBM − EF and Φh = EF − EV BM . Figure 2g shows the evolution of Φe as a function of the work function of the MoSe2 /metal systems employed here. It is found that Φe (Φh ) exhibits linear variation with the work function with a slope of 0.67 (FLP coefficient), suggesting a partial FLP in MoSe2 /metal systems. Interestingly, s-electron metals and d-electron metals share almost the same FLP coefficients, regardless of their different interaction strength with MoSe2 . The partial Fermi level pinning can be attributed to the low density of interfacial states which can be reflected partly rather than totally by the interaction between MoSe2 layer and metal electrodes. Though monolayer MoSe2 binds with d-electron metals more strongly than with s-electron metals, the Fermi level pinning is not enhanced. Because interactions between MoSe2 and metals are van der Waals force, which is so weak that it exerts little influence on the density of interfacial states. So s- and d- electron metals share similar FLP coefficients when contacting MoSe2 layer. 4.0

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Figure 3: (color online) Summary of (a) the separation d and (b) the binding energy in various MoSe2 /metal interfaces. The red squares and blue triangles present results of s-electron metals and d-electron metals, respectively. (c-d) Band realignment of MoSe2 layer when contacting with various metals. The Fermi level is set as zero. Schottky barrier heights of electrons and holes are summarized in Figure 3c-d. Upon making contacts with metals, the MoSe2 /metal interface becomes metallic. Significant energy level realignment can be observed in MoSe2 /metal complexes. Take MoSe2 /Al as an example, the Fermi 8


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level of the combined system shifts upwards, approaching the conduction band minimum (CBM) of MoSe2 . Consequently, electrons are injected into the MoSe2 layer, forming n-type contact. In contrast, the Fermi level of MoSe2 /Pt moves towards the valence band maximum (VBM) of MoSe2 , leading to hole injection into MoSe2 layer and the formation of p-type contact. According to our results, most of the studied metals have lower work functions than monolayer MoSe2 . Significant energy level realignment of metal/MoSe2 interfaces results in the formation of the n-type contacts. Apart from platinum with the high work function of about 5.75 eV, all the other metals tends to form n-type contact with MoSe2 layer. Our results show negative Φe when MoSe2 contacts with erbium (Er) and Scandium (Sc), indicating Ohmic contacts between MoSe2 layer and corresponding metals. Au

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Figure 4: (color online) (a) Band structures of Au(111), VS2 monolayer and VS2 -Au complex. The vacuum level is set as zero and is taken as reference (b) Summary of the separation d between MoSe2 and VS2 -metal in MoSe2 /VS2 -metal systems To tune the SBH more effectively, an inert layer can be inserted between metal electrodes and TMDs to break the interaction between metal and TMDs. 23,25,26 Here, a vanadium disulfide (VS2 ) layer is introduced between metal and MoSe2 due to its metallic conductivity and commensurable lattice to MoSe2 . To keep unity with other results in this work, the Hubbard U correction was not 9



taken into consideration to explore the specific electronic structure of monolayer VS2 . Additionally, its specific electronic structure does not play an important role in the contacts. The calculated work function of single-layer VS2 is 5.7 eV. The Au electrode shows a higher work function the isolated VS2 monolayer. We can assume that electrons will transfer from Au to VS2 layer upon binding together. As a result, the Fermi level of Au declines and that of monolayer VS2 rises. Both the Fermi levels of Au and VS2 layer approach each other to an unified Fermi level in VS2-Au system, as shown in the right panel of Fig. 4a. Our results indicate that adsorption of a VS2 layer has a dramatic effect on the VS2 -metal work function. To determine the interaction between MoSe2 layer and VS2 -metal, the interlayer distance between MoSe2 and VS2 -metal is calculated in Figure 4b. As expected, the interlayer distance exhibits slight difference in all the studied MoSe2 /VS2 metal complexes, indicating that the MoSe2 layer is subjected to similar interactions. Further insight comes from the projected band structures of MoSe2 /VS2 -metal complexes, as shown in Figure 5a-f. As can be seen, the general features of MoSe2 layer are well preserved no matter the type of elemental metals. The CBM and VBM of MoSe2 layer remain at the same position with the corresponding structures without VS2 inserted. Its calculated band gap obtained from the projected band structure also shows little difference. In Figure 5g, we plot the evolution of Φe as a function of the work function of VS2 -metal. It is clear that Φe shows linear variation with the work function of VS2 -metal electrodes with a slope of 0.65. These results indicate that a partial FLP also exists in MoSe2 /VS2 -metal systems. Additionally, s-electron metal-VS2 /MoSe2 and d-electron metal-VS2 /MoSe2 share the same FLP coefficient because all the VS2 -metal electrodes exert almost the same influence on the electronic structure of MoSe2 layer. As a VS2 layer is inserted between elemental metal and MoSe2 , the FLP behavior is dominated by the interaction between MoSe2 and VS2 , which is independent of the type of elemental metal. What makes the difference is that the work function of VS2 -metal shows obvious difference from that of corresponding elemental metals. Overall, the FLP coefficient is determined by the interaction between the MoSe2 layer and metal (VS2 -metal) electrodes. The weak van der Waals interaction which the MoSe2 layer endured results in similar partial Fermi level pinning.

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Figure 5: (color online) (a-f) Projected band structures of single-layer MoSe2 contacting with VS2 metal electrodes. The Fermi level is set as zero. The bands dominated by MoSe2 layer and VS2 metal are plotted by red and gray dots, respectively. (b) Evolution of the Schottky barrier height of electrons (Φe ) in MoSe2 /VS2 -metal contacts as a function of the work function of MoSe2 /VS2 metal.

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Figure 6a-b show the band realignments of MoSe2 when contacting with different s-electron metals and d-electron metals with a VS2 inserted. Compared with MoSe2 /metal contacts, we can find that inserting a VS2 layer exerts significant influence on Φe and Φh . For example, the Φe of MoSe2/Au(111) interface increases by 0.78 eV when a VS2 layer is inserted and the Φh drops by 0.56 eV in MoSe2 /VS2 -Pt(111) contacts. Ohmic cantact disappears and a Φe about 0.72 eV emerges when a VS2 layer is inserted between Sc(111) and MoSe2 layer. Furthermore, the increased Φe and reduced Φh induced by inserting VS2 layer lead to the transition from n-type contacts at MoSe2 /metal interfaces to p-type contacts at MoSe2 /VS2 -metal interfaces.

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Figure 6: (color online) Band realignment of MoSe2 layer when contacting with various VS2 -metal electrodes. The Fermi level is set as zero. It is worth noting that the FLP coefficient in MoSe2 /VS2 -metal is a little larger than the cases without a VS2 buffer layer. Inserting a VS2 layer seems not an effective method to weaken the FLP in MoSe2 /metal contacts. The weak vdW interaction between MoSe2 and VS2 -metal electrodes exerts little influence on the FLP. The weak contacting results in the unchanged density of midgap interfacial states and similar FLP to MoSe2 /metal contacts though inserting a VS2 layer induces larger interlayer distance and less transferring charge. Low density of interfacial states in all the contacts gives rise to the partial FLP when making contacts with metals. Practically, defects and 12


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impurities in MoSe2 layer will enhance the density of midgap interface states and lead to strong FLP when it contacts with metals. In this case, efficiency of inserting an inert layer to unpin the Fermi level may be improved significantly. What’s more, inserting an inert layer will make tuning the electron and hole SBH by varying metal electrodes feasible. Additionally, all the results are based on the contacts between monolayer MoSe2 and metal electrodes. It is known that the thickness of MoSe2 will exert influence on the band alignment of MoSe2 . It has been demonstrated that the band gap of MoSe2 decreases with the number of layer. Usually, the work function of 2D materials also shows a declining trend with their thickness. 32 It can be assumed that the Schottky barrier height should show difference when TMDs with different thickness make contacts with metal electrodes. Partial Fermi level pinning with various coefficient can be observed in these systems. So it is reasonable to assume that our results based on the contacts between monolayer MoSe2 and metal electrodes are suitable to similar contacts TMDs of different thickness and metals. Our results will present guidelines to design nanoelectronic devices based on TMDs with high performance. Our conclusions on FLP in MoSe2 /metal are applicable to the contacts between other 2D materials and elemental metals.

CONCLUSION Our density functional theory calculation results show that contacting MoSe2 with various elemental metals leads to partial Fermi level pinning and the electron/hole SBH can be modulated effectively by varying metal electrodes. This behavior is attributed to the rather low density of interfacial states at the interfaces between MoSe2 layer and metal electrodes, which can be reflected by the weak interaction between them. Most MoSe2 /metal contacts are n-type and Ohmic contact can be observed in MoSe2 /Sc contact. Inserting a VS2 layer between the metal and MoSe2 weakens the interaction between MoSe2 layer and VS2 -metal, leading to a weaker FLP. VS2 buffer layer with a high work function exerts significant influence on the band realignments of MoSe2 , resulting in the transition from n-type contacts at MoSe2 /metal interfaces to p-type contacts at

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MoSe2 /VS2 -metal interfaces. The inserted VS2 may act as an efficient hole injection layer to MoSe2 and enables tuning of the hole SBH by varying metal electrodes more practical. Our results would provide guidelines to develop approaches to design nanoelectronic devices based on TMDs with high performance.

COMPUTATIONAL DETAILS The density functional theory (DFT) 33 calculations are implemented in the VASP code within the projector-augmented plane-wave method. 34–36 The general gradient approximation (GGA) 37 of Perdew, Burke and Ernzerhof (PBE) is used to describe the exchange correlation functional with the partial core correction included. 38 The plane-wave cutoff energy is set as 450 eV and a vacuum larger than 12 Å is used to eliminate the interaction between adjacent slabs. The (111) surfaces of metals are strained to match the optimized lattice constant of MoSe2 monolayer. MoSe2 unitcell √ √ and 3 × 3R30◦ cell are untilized to minimize the lattice mismatch with different metals, as shown in Figure 1. The first Brilloin zone is sampled with 9 × 9 × 1 and 5 × 5 × 1 Monkhorst-Pack √ √ grid for relaxation of the unitcell MoSe2 /metal and 3 × 3R30◦ MoSe2 /metal systems employed here, respectively. 39 To fully consider the interaction between MoSe2 layer and metals, six layer of metals are adopted in all the employed systems. The non-local correlation vdW-DF method is utilized for the dispersion correction in the van der Waals interaction between MoSe2 and VS2 layers. 40,41 All the structures are fully relaxed with a force tolerance of 0.02 eV/Å on each atom. The electronic optimization stops when the total energies of neighboring optimization loops differ below 10−4 eV.

AUTHOR INFORMATION Corresponding Author ∗ E-mail:

[email protected]; [email protected]

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFB0700700), the National Natural Science Foundation of China (grant no. 61622406, 11674310, 61571415, 51502283), "Hundred Talents Program" of Chinese Academy of Sciences (CAS), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

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Φe


Metal Interfaces with a Buffer Layer

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