Tunable Ohmic, p-Type Quasi-Ohmic, and n-Type Schottky Contacts of

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Tunable Ohmic, p-type Quasi-Ohmic and n-type Schottky Contacts of Monolayer SnSe with Metals Cheng He, Ming Cheng, Tong Tong Li, and Wenxue Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00276 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Nano Materials

Tunable Ohmic, p-type Quasi-Ohmic and n-type Schottky Contacts of Monolayer SnSe with Metals Cheng He,† Ming Cheng,† Tongtong Li,† Wenxue Zhang‡ †State

Key Laboratory for Mechanical Behavior of Materials, School of Materials Science

and Engineering, Xi’an Jiaotong University, Xi’an 710049, China ‡School

of Materials Science and Engineering, Chang’an University, Xi’an 710064, China

Abstract Monolayer (ML) SnSe, a p-type IV-VI semiconductor, has drawn tremendous attention due to its chemical stability, high electrostatic gating efficiency and carrier mobility, and it has been synthesized through different ways. We have comprehensively investigated the properties of the interfaces between ML SnSe and some regular metals by using first principle calculations. The metallization of ML SnSe appears in ML SnSe-Ag, Al, Au, Cu and Cr systems. Lateral n-type Schottky contacts with the electron Schottky barrier height of 0.42 eV and 0.32 eV are formed, respectively, when ML SnSe contacts with metal Ag and Al. And a lateral p-type quasi-Ohmic contact with the hole Schottky barrier height of 0.02 eV is formed when ML SnSe contacts with metal Au. Surprisingly, Ohmic contacts are formed when ML SnSe contacts with metal Cr and Cu. Our research not only has a deep understanding of the characteristics of the interfaces between ML SnSe and the metals, but also offers a reference in the electrode selection for ML SnSe devices. Keywords: DFT; ML SnSe; Metal contact; Interface; Schottky barrier

Corresponding

Authors: W. X. Zhang ([email protected]) 1

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1. Introduction Because of their excellent performance and great potential in nano-electronics applications, two-dimensional (2D) materials, such as graphene1-2, phosphorene3 and transition metals dichalcogenides (TMDS) 4-6, have attracted great attentions. However, 2D materials for field effect transistors (FETs) need to satisfy three basic requirements: reasonable band gaps, relatively high mobility and relatively low mismatchs with metal surfaces. Although graphene has a superior carrier mobility (8390 cm2 V-1 s-1), the absence of an electronic band gap limits graphene applications with a high on-off ratio7-8. TMDs possess proper band gaps (1.0~2.0 eV), but their carrier mobilities (about 200 cm2 V-1 s-1) are relatively low9-10. Phosphorene with a high carrier mobility (1000 cm2 V-1 s-1) and a medium band gap (0.3~1.0 eV) is unstable above ambient temperature11-12. Therefore, it is urgent to get some 2D materials, which can satisfy the three basic requirements mentioned above. Fortunately, monolayer (ML) SnSe, an important p-type IV-VI semiconductor, has Pm21n symmetry, an appropriate band gap (0.89~1.63 eV)13-15 and a high carrier mobility of the order of 103 cm2 V-1 s-116. At the same time, it is thermally stable at room temperature13, 17-18 and environmentally friendly. Thus, SnSe has a great potential for light emitting transistor and solar cell device, etc17,

19-20.

In addition, there are some other

advantages such as earth-abundance21, high photoresponsivity22 and high electrostatic gating efficiency. 2D SnSe flakes have been synthesized through vapor transport deposition, wet chemistry23, Li-intercalation exfoliation24, one-pot synthetic method14 and sonication liquid exfoliation15. By performing the ultraviolet photoelectron spectroscopy spectrums of the SnSe nanofilms, the work function of SnSe is gained to be 4.19 eV25. Moreover, there are many theoretical researches on the properties of ML SnSe. Wang et al.’s research

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shows that ML SnSe is a prospective thermoelectric material, which could be used for nanoscale thermoelectric devices17. The In-doped ML SnSe displays half-metal property, and Ga-doped ML SnSe has a promising application in optoelectronic because of the appearance of red shift21. Therefore, it is quite promising that ML SnSe is applied as the channel material of FET. In a FET with 2D semiconductor as the channel material, a contact with metal electrodes is absolutely necessary, where Schottky barrier (SB) is usually formed. The SB can induce an extra contact resistance and thus decrease the carrier transfer efficiency between the 2D semiconductor and the metal electrodes, which often significantly reduces the performance of FET26. Therefore, the formation of low contact resistance can improve the performance of FET . Liu et al. have experimentally achieved the transistors with a two-terminal electron mobility of 260 cm2 V-1 s-1 at room temperature and a hole mobility of 175 cm2 V-1 s-1 by transferring metal films with different work functions29. Moreover, Pan et al.’s research shows that a n-type Schottky contact forms in

ML phosphorene-Au

system with the smallest electron Schottky barrier height (SBH), and a p-type Schottky contact forms in ML phosphorene-Pd system with the smallest hole SBH3. For all we know, the interfacial properties between ML SnSe and metals have not been systematically investigated. In this paper, we have provided a systematic study of the electrical contacts between ML SnSe and the some regular metals (Ag, Al, Au, Cu and Cr) with a top contact by using first principle electronic structure calculations. The pristine structure of ML SnSe is almost well preserved on Ag, Al, Cu and Au electrodes, while it is seriously destroyed on Cr electrode. When contacting with the metal electrodes, ML SnSe undergoes a metallization

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due to the heavy hybridization of its band structure. In addition, the lateral n-type Schottky contacts are formed in ML SnSe-Ag and Al systems, and a lateral quasi-Ohmic contact is formed in ML SnSe-Au system. Surprisingly, Ohmic contacts are formed in ML SnSe-Cr and Cu systems. Our results provide a deep understanding of insight into the characteristics of the interfaces between ML SnSe and metal electrodes and a reference in the electrode selection for ML SnSe devices.

2. Computational methods All calculations, including geometry optimizations and electronic structure calculations,

were

performed

with

the

projector-augmented

wave

(PAW)

pseudopotential30-31 implemented in the Vienna ab initio simulation package (VASP) code32. The cutoff energy was set to 400 eV, 1 × 10−5 eV was chosen for the energy break criterion, and the maximum residual force was set to 0.01 eV/Å. To consider Van der Waals (vdW) interactions between the layers, a DFT-D dispersion-correction scheme of Grimme was taken 33-34. A dipole correction was taken to avoid the forged interaction between the dipole moments in the direction perpendicular to the interface28, 35. During the optimization, the lattice constants of the interfacial systems and the four layers metal atoms at the bottom were fixed. The vacuum buffer space of more than 15 Å was added to avoid the interlayer interactions caused by periodicity. The total electron density distribution was calculated by using the ultrasoft pseudopotential and plane-wave basis set with a cutoff energy of 400 eV implemented in CASTEP code. Forcite code was used to perform the classical molecular dynamics (MD) simulations in order to study the thermodynamic stability of ML SnSe in the canonical (NVT) ensemble with COMPASS forcefield (condensed-phase optimized molecular potentials for atomistic

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simulation studies)36. Volume V and temperature T are constants, and N is the atom number. T is imposed by the Andersen algorithm37. The constant-temperature MD simulations start from a low temperature (300 K)38. The initial configuration for any given T was considered to be the final one from the previous T. The temperature was increased with a step of 100~200 K in 104 ps with a time step of 1 fs until the structure of ML SnSe is seriously destroyed.

3. Results and discussions 3.1 Interface modeling and stability The accuracy of our calculation procedure is tested using pristine ML SnSe. As shown in Figures 1(a) and (b), the optimized ML SnSe has Pm21n symmetry of an orthogonal lattice, which is lower than that of its bulk counterpart due to the absence of inversion symmetry. In respect to ML SnSe, the optimized in-plane lattice constants and buckling distance are a = 4.35 Å, b = 4.24 Å and dS = 2.71 Å, respectively, which are in good agreement with the values in the previous studies39-41. Figure 1(c) presents the electronic band structure of the ML SnSe with a 1 × 1 unit cell. The ML SnSe has a quasi-direct band gap of 0.91 eV based on the PBE functional, which is in line with the previous results17, 39. As shown in Figure 1(c), both the valence band maximum (VBM) and the secondary conduction band minimum (CBM) stay at the same point located on the X-Г path in the Brillouin zone (BZ). To examine the thermodynamic stability of ML SnSe, the potential energy fluctuations with respect to time of ML SnSe at different temperatures are calculated. A 5 × 5 × 1 supercell with 100 atoms is built to simulate the 2D sheet for minimizing the constraint caused by periodicity. Figure 2 shows potential energy fluctuation with respect 5



to time of ML SnSe at 500 K, and its insets show the atomic configuration of ML SnSe at the end of MD simulation. We all know that most electronic devices operate at temperatures below 500 K, and thus from the perspective of thermal stability, ML SnSe can be used in most electronic devices. The potential energy fluctuations with respect to time of the ML SnSe at 300, 700, 900 and 1000 K are also calculated and given in Figure S1. We found that the atomic structures of ML SnSe are almost invariant after heating for 104 ps at 300, 500, 700 and 900 K; however, its structure begins to bend obviously when heated at 1000 K. Therefore, it is concluded that ML SnSe is thermally stable in a wide temperature range from 300 K to 900 K. Comprehensively considering the physical properties (melting point, lattice constants and work function) as well as the chemical properties (stability and toxicity) of all metals, we selected Ag, Al, Au, Cr, and Cu metals as substrates of ML SnSe FETs. In terms of the convergence tests for the metal layers in the previous researches26-27, we choose six metal layers to construct the metal surfaces and the interfacial systems with ML SnSe, which is shown in Figure 1(d). In the real experiment that a 2D material is deposited on a metal substrate, the lattice parameters of 2D material are easier to change rather than those of bulk metals. Therefore, the lattice constants of metals are fixed, and the lattice constants of ML SnSe are adjusted to accommodate to those of the metals. The 2 × 1 unit cell of ML SnSe matches the 3 × 1 unit cell of Ag, Al, Au and Cr (110) faces, and the 3 × 1 unit cell of ML SnSe matches the 5 ×

3 unit cell of Cu (111) face. The corresponding average

lattice mismatches range from 1.68% to 3.25%, as given in Table 1. The stacking configurations with the lowest total energy are selected from all the possible stacking configurations with high symmetry and as initial configurations during optimization to

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obtain the most stable structures. The optimized configurations of the ML SnSe-metal systems are shown in Figure 3. It is shown that the pristine structure of ML SnSe changes slightly and is almost well preserved on Ag, Al, Cu and Au electrodes, while it is strongly and seriously destroyed on Cr electrode. The definition of the minimum interatomic distance (dmin) is the minimum distance between metal and Sn (Se) atoms at the interface. The definition of the interfacial distance (dS-M) at the equilibrium state is the average distance from the interfacial metal layer to the interfacial SnSe layer in the direction perpendicular to the interface. The main data of the ML SnSe-metal systems are calculated and summarized in Table 1. The binding energy (Eb) of the ML SnSe-metal system per interfacial Sn or Se atom is defined as: Eb = (ESnSe + EM ― ESnSe - M) N

(1),

where ESnSe, EM and ESnSe-M represent the total energies for the freestanding ML SnSe, the freestanding metal surface and the interfacial system, respectively, and N represents the number of interfacial Sn or Se atom. In terms of dmin, dS-M and Eb, the adsorption of ML SnSe on different metal surfaces could be categorized into three categories, namely weak, medium and strong bonding. The first category is ML SnSe adsorbed on Ag and Al metals with weak bonding, which is characterized by a large dmin of 2.60~2.68 Å and dS-M of 2.38~2.49 Å, and small Eb of 0.57~0.66 eV. The second category is ML SnSe adsorbed on Au and Cu metals with medium bonding, which shows a large dmin of 2.63~2.67 Å and dS-M of 2.32~2.49 Å, and medium Eb of 0.72~0.74 eV. The third category is ML SnSe adsorbed on Cr metal with strong bonding, which features a small dmin of 2.36 Å and dS-M of 2.23 Å, and large Eb of 1.44 eV. The results are similar with the binding energies of ML InSe and Ag, Cu, Au and Cr metals, which are 0.72, 0.86, 0.77 and 1.70 eV, respectively42. However,

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the binding energy of graphene/graphdiyne and Ag, Al, Au and Cu metals ranges from 0.027~0.043 and 0.09~0.12 eV43-44. Therefore, ML SnSe is more active than graphene and graphdiyne. To further understand the bond strength between ML SnSe and the metals, the Mulliken population analyses are performed for the contacted systems, and the results are listed in Table 1. The transferred charges per Sn (Se) atom from metals to ML SnSe are 0.15, 0.06, 0.20, 0.35 and 0.31 electrons for Ag, Al, Au, Cr and Cu-ML SnSe systems, respectively, which is accordance with the results of Eb. It is shown that the more the charge transfers, the larger Eb is. The difference of the bond strength can also be interpreted by the difference in the electronic configurations of Ag, Al, Au, Cr and Cu atoms. The outermost orbits of freestanding Ag (4d105s1), Au (5d106s1), Cu (3d104s1) and Al (3s23p1) atoms have only one unpaired electron, respectively, indicating that only one covalent bond is formed. The outer orbits of freestanding Cr (3d54s1) atom have six unpaired electrons, indicating that six unpaired electrons are provided to form covalent bonds. Therefore, the bonds between ML SnSe and Cr metal are stronger than those between ML SnSe and Ag, Al, Au and Cu metals.

3.2 Electronic properties To analyze the electrical properties of the ML SnSe-metal systems, the band structures of the freestanding ML SnSe with 3 × 1 unit cell and the interfacial systems are determined and displayed in Figure 4. The 3 × 1 ML SnSe has a quasi-direct band gap of 0.86 eV at the PBE level, which is 0.05 eV lower than that of ML SnSe with a 1 × 1 unit cell. Compared with the band structure of the freestanding ML SnSe, the conduction bands of ML SnSe in ML SnSe-Ag, Al, Au and Cu systems move obviously down, and the valence

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bands move slightly down, which induce that the EF always crosses ML SnSe derivedbands, indicating the metallization of ML SnSe in these systems. In addition, the band structures of ML SnSe in the interfacial systems are seriously destroyed, which indicates that the chemical or covalent bonds are formed at the interface between ML SnSe and these metal electrodes. According to the degree of band hybridization, the interfaces can be divided into three types: strong, medium and weak hybridization. There is strong hybridization between ML SnSe and Cr metal. The conduction and valence bands of ML SnSe are obviously mixed around 0.5 eV for ML SnSe-Au/Cu systems, which indicates medium hybridization between ML SnSe and Au, Cu metals. The band gap of ML SnSe in ML SnSe-Ag/Al systems still exists, which shows weak hybridization between ML SnSe and Ag, Al metals. The band hybridizations of ML SnSe with metals are accordance with their bonding level and the results of dmin, dS-M and Eb. In order to further understand the hybridization between ML SnSe and the metals, the partial density of states (PDOS) of the freestanding ML SnSe with 3 × 1 unit cell and ML SnSe in the interfacial systems are calculated and shown in Figure 5. Comparing the PDOS of ML SnSe in the interfacial systems and that of the freestanding ML SnSe, we can see that the original band gap of ML SnSe vanishes due to the appearance of s and p states around EF in all the absorption systems, which is a typical behavior of metallization. In addition, the pz orbital of SnSe dominates the states around EF for ML SnSe contacting with Ag, Al, Au and Cu metals, while the s, px, py and pz orbits of SnSe almost equally contribute to the states around EF for ML SnSe-Cr system. It is shown that orbital hybridization is stronger in ML SnSe-Cr system than ML SnSe-Ag, Al, Au and Cu systems. Namely, ML SnSe has more intense bonding with Cr metal than Ag, Al, Au and Cu metals,

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which is accordance with the results of band structures. The total electron distributions of ML SnSe-Ag, Au, and Cr systems are computed to interpret the interactions between ML SnSe and the metal electrodes, which are shown in Figure S2. There is no significant electron accumulation in ML SnSe-Ag interface, indicating the formation of a weak bonding. A small quantity of electrons are accumulated in ML SnSe-Au interface, in agreement with the formation of a medium bonding. Electrons are apparently accumulated in ML SnSe-Cr interface, in line with the formation of a strong bonding. To further assess the interactions between ML SnSe and the metal surfaces, it is necessary to understand the mechanism of the charge distribution and charge transfer between ML SnSe and the metal surfaces, which can be visualized by CDD, as shown in Figure 6. The CDD is calculated as follows: ∆ρ(z) = ρSnSe/M(z) ― ρSnSe(z) ― ρM(z)

(2),

where ρSnSe/M, ρSnSe and ρM are the charge densities of the SnSe/metal system, the SnSe and metal, respectively. It can be observed from Figure 6 that the accumulation occurs at the center of the interface, and the reduction occurs at the side of the interface for ML SnSeAg, Al, Cu and Cr systems. Namely, the charge accumulates at the interfacial center at the equilibrium state for all the systems except for ML SnSe-Au system. In terms of charge transfer at the interface, SnSe/Cr system is obviously more than the other systems, which shows that the interaction between ML SnSe and Cr is stronger than that of the other ML SnSe-metal systems. The result is accordance with the binding energy. Moreover, one can observe that charges at interfacial center are mainly from SnSe layer for ML SnSe-Ag, Al, Au and Cu systems, while that are mainly from Cr layer for ML SnSe-Cr system. It can be

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interpreted by the fact that Cr among all the metals is the most active, namely the easiest to lose electrons.

3.3 Schottky barrier and tunneling barrier The Schottky barrier plays an important role in a FET. A low SB will decrease the scattering of electron, increase the electron transport, and improve the performance of a FET. The schematic diagram of a ML SnSe FET is shown in Figure 7. There are two different SBs in a FET: (1) vertical SB at the interface B between the metal and ML SnSe under metal, (2) lateral SB at the interface D between the interfacial system and the channel SnSe45. According to the band structures and PDOS of the contacted systems, ML SnSe is metallized when contacting with Ag, Al, Au, Cr and Cu metals, which results in the absence of vertical SB at interface B and the possible formation of lateral SB at interface D. Lateral SBH is determined by the energy difference between EF of interfacial systems, and the CBM (electron SBH) and VBM (hole SBH) of channel ML SnSe. In a FET configuration, DFT-GGA method is more appropriate than the hybrid functional and quasi-particle methods for the calculations of the transport gap and the CBM and VBM of a semiconductor due to the semiconductor doped with a metal electrode or a gate3,

46.

Therefore, the work function, the energies at CBM (EC) at VBM (EV), and EF in the following section are calculated by DFT-GGA method. In addition, quantum transport method is another better way to estimate SBH, because it takes into account the interaction between the interface system and the channel. However, due to the constraints of our own conditions, the quantum transport calculations can not be performed right now, and thus the accurate pinning factor can not be gotten. The work functions of the interfacial systems and EC, EF and EV of the freestanding

11



ML SnSe are displayed in Figure 8. Lateral n-type Schottky contacts are formed for ML SnSe-Ag and Al contacts with lateral electron SBH of 0.42 and 0.32 eV, respectively. Lateral p-type quasi-Ohmic contact is formed for ML SnSe-Au contact with a lateral hole SBH of 0.02 eV, which is close to that (0.09 eV) of the GeSe-Au contact35. However, the SBH of ML SnSe-Au system is much lower than that of phosphorene-Au (0.20 eV)3, ML InSe-Au (0.26 eV)42, MoS2-Au (0.76 eV)26. Therefore, the performance of ML SnSe-Au system is superior to that of other semiconductor-Au systems, and ML SnSe-Au system has great application potential in electronic equipment. Surprisingly, lateral p-type Ohmic contacts are formed for ML SnSe-Cr and Cu contacts, since EF of the interfacial systems are lower than EV of the freestanding ML SnSe. Ohmic contact is also found in ML InSeCu system. Due to the formation of a low contact resistance, Ohmic contact is conductive to a high on-current and high-frequency operation in transistors47. Therefore, ML SnSe-Cr and Cu systems will be possibly used for FETs with high on-current and high-frequency operation. The electrostatic potentials of the ML SnSe-metal systems are shown in Figure 3. The electrostatic potential between ML SnSe and metal above EF can be considered as tunneling barrier, which is marked by blue rectangle. The tunneling barrier is also a significant characteristic to assess a semiconductor-metal contact, and its height (ΔV) and width (WB) are defined as the height and half width of the rectangle, respectively. The barrier height is the lowest barrier to be overcome by the electrons at EF from metal to ML SnSe6, 48. From the viewpoint of physics, the definition in our paper is more meaningful than that in other metal contact studies27, 49. The tunneling probability (TB) from metals to the SnSe is defined as46:

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(

TB = exp -2

2m∆V ħ

× WB

)

(3),

where m is the mass of a free electron, and ħ is reduced the Plank’s constant. According to the definition, it is concluded that the larger TB is, the higher the charge injection efficiency is. In addition, a high charge injection efficiency suggests the formation of a low contact resistance. As is shown in Table 1, the TB of ML SnSe-Cu, Ag, Au and Al interfaces are 73.00%, 85.50%, 86.93% and 95.21%, respectively, which are much larger than those of other semiconductor-metal interfaces26, 46. In particular, the TB of ML SnSe-Cr interface is 100%, which shows that the interface has a perfect tunneling transmission. Therefore, ML SnSe-Cr interface has a low contact resistance and thus can enhance the device performance. Previous studies3, 46 show that the TB of the semiconductor-metal interface is relation to the bonding at the interface. ML SnSe-Cr interface with strong bonding does not have tunneling barrier, and ML SnSe-Ag, Al (weak bonding) and Au, Cu (medium bonding) interfaces have certain tunneling barrier. This is consistent with the conclusion published in the previous studies3, 46.

4. Conclusions In conclusion, we have comprehensively investigated the properties of the interfaces between ML SnSe and Ag, Al, Au, Cr and Cu electrodes by using first principle electronic structure calculations. The metallization of ML SnSe appears in ML SnSe-Ag, Al, Au, Cu and Cr systems. The lateral n-type Schottky contacts are formed in ML SnSe-Ag and Al systems with electron SBH of 0.42 and 0.32 eV, respectively, and a p-type quasi-Ohmic contact is formed in ML SnSe-Au system with a hole SBH of 0.02 eV. Surprisingly, Ohmic contacts are formed in ML SnSe-Cr and Cu systems. In addition, the TB of SnSe-Cu, Ag, Au and Al interfaces are 73.00%, 85.50%, 86.93% and 95.21%, respectively, which shows 13



that lower contact resistances are formed at the ML SnSe-metal interfaces. In particularly, SnSe-Cr system with TB of 100% has a perfect tunneling transmission. Our study reveals the interfacial properties between ML SnSe and the metals and offers a reference for the choice of an appropriate metal electrode in ML SnSe devices.

Supporting information Figures about the thermal stability of ML SnSe and the total electron density distributions of the interface systems.

Acknowledgments The authors acknowledge supports by National Natural Science Foundation of China (NSFC, Grant No. 51471124), Natural Science Foundation of Shaanxi province, China (2017JQ5045), the special fund for basic scientific research of central colleges of Chang’an University (No. 310831162002) and the Fundamental Research Funds for the Central Universities (xjj2016018).

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Table 1. Calculated interfacial properties of ML SnSe on metal surfaces: average lattice mismatch ε, minimum interatomic distance dmin, equilibrium distance dS-M (which refers to the average distance between the SnSe and metal surfaces), Mulliken charge ΔQT (which refers to the transferred charges per Se atom from metals to ML SnSe), binding energy Eb, work functions W (SnSe-metal contacts) and WM (freestanding metal surfaces). ΔV, WB, TB, and ФLB represents tunneling barrier height, tunneling barrier width, tunneling possibility and lateral SBH, respectively. Metal

Ag

Al

Au

Cr

Cu

ε (%)

1.68

2.77

1.68

2.85

3.25

dmin (Å)

2.68

2.60

2.63

2.36

2.67

dS-M (Å)

2.38

2.49

2.32

2.23

2.49

ΔQT (e)

-0.15

-0.06

-0.20

-0.35

-0.31

Eb (eV)

0.66

0.57

0.72

1.44

0.74

W (eV)

3.90

3.80

4.37

5.08

4.11

WM (eV)

4.17

3.92

4.90

4.77

4.60

ΔV (eV)

0.69

0.16

0.57

0

1.12

WB (Å)

0.18

0.12

0.18

0

0.29

TB (%)

85.50

95.21

86.93

100

73.00

ФLB (eV)

0.42n

0.32n

0.02p

0

0

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Captions Figure 1. (a) Side and (b) top views of freestanding ML SnSe. The rectangle indicates a SnSe unit cell. (c) Band structure of ML SnSe. The Fermi level is set to zero. (d) The typical configuration of SnSe on metal surface. The dark gray, orange and silvery balls represent Sn, Se and metal atoms, respectively. Figure 2. Potential energy fluctuation during MD simulation of ML SnSe at T= 500 K. The insets show atomic configuration of ML SnSe at the end of MD simulation. Figure 3. Side view of the optimized structures (left) and average electrostatic potential (right) along the direction normal to the interface of SnSe-Ag, Al, Au, Cr, and Cu systems, respectively. The Fermi level is set to zero. Figure 4. Projected band structures for the freestanding ML SnSe, ML SnSe-Ag, Al, Au, Cr, and Cu systems, respectively. The Fermi level is set to zero. Blue lines: band structures of the complete interface systems. Red dots: band structure of ML SnSe layer. The size of the dots is proportional to the contribution weight. The band structure of the freestanding ML SnSe is calculated in a 3×1 unit cell. Figure 5. Partial density of states (PDOS) of the freestanding ML SnSe and ML SnSe on Ag, Al, Au, Cr, and Cu surfaces, respectively. The Fermi level is set to zero. The PDOS of the freestanding ML SnSe is calculated in a 3×1 unit cell. Figure 6. The schematic view of charge density difference and planar-averaged electron density difference Δρ (z) for ML SnSe- Ag, Al, Au, Cr, and Cu systems. The red and blue color isosurfaces correspond to the accumulation and depletion of electrons (ρ = 1 × 10-3 e/A-3), respectively. Figure 7. Schematic cross-sectional view of a typical metal contact to ML SnSe. A, C, and

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E denote the three regions, while B and D are the two interfaces separating them. Red arrows show the pathway (A → B → C → D → E) of electron injection from contact metal (A) to ML SnSe channel (E). Inset figure shows schematic diagram of a ML SnSe FET. Figure 8. Line-up of the work functions of the interfacial systems and the EC, EF and EV of the freestanding ML SnSe. The blue dashed and the red solid lines present the work functions of the pure metals and the ML SnSe-metals systems, respectively.

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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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Figure 6.

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Figure 7.

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Figure 8.

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Table of Contents Graphic

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Tunable Ohmic, p-Type Quasi-Ohmic, and n-Type Schottky Contacts of

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