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Contact effect of ReS/metal interface Jae Young Park, Hang-Eun Joe, Hyong Seo Yoon, SangHyuk Yoo, Taekyeong Kim, Keonwook Kang, Byung-Kwon Min, and Seong Chan Jun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06432 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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ACS Applied Materials & Interfaces

Contact effect of ReS2/metal interface Jae Young Park1,‡, Hang-Eun Joe1,‡, Hyong Seo Yoon1,‡, SangHyuk Yoo1, Taekyeong kim2, Keonwook Kang1, Byung-Kwon Min1, and Seong Chan Jun1,* 1

Department of Mechanical Engineering, Yonsei University, Seoul, Korea

2

Department of Physics, Hankuk University of Foreign Studies, Yongin 449-791, Republic of Korea

*Correspondence and requests for materials should be addressed to S.C.J. (email: [email protected]) ‡

These authors contributed equally to this article.

Keyword: ReS2, Rhenium Disulfide, TMDc, contact resistance, Field-effect transistor, Schottky diode, Kelvin Probe Force Microscopy, Density Functional Theory

Abstract Rhenium disulfide (ReS2) has attracted immense interest as a promising two-dimensional material for optoelectronic devices owing to its outstanding photonic response based on its energy band gap’s insensitivity to the layer thickness. Here, we theoretically calculated the electrical band structure of mono-, bi-, and tri-layer ReS2 and experimentally found the work function to be 4.8 eV, which was shown to be independent of the layer thickness. We also

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evaluated the contact resistance of a ReS2 field-effect transistor (FET) using a Y-function method with various metal electrodes, including graphene. The ReS2 channel is a strong ntype semiconductor; thus, lower work function than those of metals tends to lead to a lower contact resistance. Moreover, the graphene electrodes, which were not chemically or physically bonded to ReS2, showed the lowest contact resistance, regardless of the work function, which suggests a significant Fermi-level pinning effect at the ReS2/metal interface. In addition, an asymmetric Schottky diode device was demonstrated using Ti or graphene for ohmic contacts and Pt or Pd for Schottky contacts. The ReS2-based transistor used in this study on the work function of ReS2 achieves the possibility of designing next-generation nanologic devices.

Introduction In the last decade, two-dimensional (2D) materials have emerged as alternatives to silicon for nanodevices.1-3 Firstly, graphene is considerably attractive because of its high carrier mobility and cut-off frequency.4 However, it does not exhibit a band gap; thus, to overcome this limitation, transition metal dichalcogenides (TMDcs) have been extensively explored due to their sufficient band gap, which is essential for achieving a high on/off current ratio for logic devices.5 Moreover, the layer-dependent electronic properties of semiconducting TMDcs exhibit unique controllable characteristics.6 An indirect band structure was observed from a few layers of various TMDcs such as MoS2,7 MoSe2,8 WS2,9 and WSe29, even though their corresponding monolayer flakes provided a direct band gap. This high tunability can be very advantageous, and their properties can be precisely controlled. However, controlling the layer thickness with a tolerance of less than few nanometers is extremely difficult, resulting in unreliable and uncontrollable device


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performance. Recently, ReS2 has become a potential candidate for optoelectronic devices due to its direct band gap—regardless of the number of layers—enabling efficient photon absorption/emission and carrier transport.10-12 However, no fundamental investigation on electronic devices using ReS2 has been sufficiently conducted. In particular, the work function of ReS2, which plays an important role in the electrical characteristics of a device, must be studied to optimize the contact resistance.13 In this study, we investigated field-effect transistors (FET) composed of ReS2 and various metallic materials. The work functions of ReS2 with various layer thicknesses were measured using scanning Kelvin probe force microscopy (KPFM). The effect of metal work function on the FET device was also evaluated using various contacts, including metals and graphene.

Results and discussion To analyze the metal/ReS2 interface, we fabricated a ReS2 FET device with various metals and measured the current using a three-probe system, as shown in Figure 1(a). Figure 1(b) shows a scanning electron microscope (SEM) image of the Schottky junction. It is necessary to understand the band structure of ReS2 before analyzing the electrical characteristics. In Figure 1(c), ReS2 has a distorted 1T phase, which combines buckled sulfide layers and zigzagged Re chains along lattice vectors in the plane.14 Even though ReS2 is also a member of TMDcs, such a unique lattice structure due to the bonding between Re significantly affects the formation of this distorted structure, as known in the case of ReS2 and ReSe2.14-18 Based on this atomic structure, density functional theory was used for the calculation of the band structures of mono-, bi-, and tri-layer ReS2, as shown in Figure 1 (d). Exact methods for the simulations are explained in Figure S2 of the supporting information. Hereafter, ReS2 is



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denoted by the number of layers, e.g., 1L, 2L, and 3L, for 1, 2, and 3 layers, respectively. The band gap of monolayer ReS2 is 1.42 eV, which is similar to values in previous reports.14,19,20 In addition, its electron affinity is 4.32 eV, which is an important factor in determining the ideal Schottky barrier height. Regardless of the number of layers, the conduction band minimum (CBM) and valence band minimum coincide at the Г point, resulting in direct band characteristics. Interestingly, the electron affinity value is negligibly changed with increasing layers while the band gap decreases significantly from 1.42 (1L) to 1.26 eV (3L). However, the gap change from 2L to 3L (0.05 eV) is much smaller than that between 1L and 2L (0.1 eV), which suggests the saturation of the band gap.

To completely understand the electrical characteristics, it is essential to obtain an accurate value for the work function. The potential difference between ReS2 and the goldcoated tip was measured using KPFM to estimate the work function of ReS2. A specimen composed of ReS2 flakes on a gold substrate was used because the gold substrate provides a stable contrast at the back surface field of the surface potential (SP) map. KPFM is a variant of atomic force microscopy (AFM) that employs the two-pass lift mode using a cantilever tip with metallic coatings.21 During KPFM measurement, as illustrated in Figure 3(a), a SP map is obtained by forward and backward scanning of an oscillating cantilever tip. As the cantilever tip approaches the sample surface, the potential difference between the tip and sample generates an electrostatic force. The electrostatic force is expressed as follows:22

Force=

(

)

2 q s q t 1 dC Vapplied -Vcontact 2 + 4πεz 2 dz ,

(1)

where dC/dz is the derivative of the capacitance between the sample and the tip, qs is the surface charge, and qt is the charge induced on the tip. Alternating and direct current (AC and


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DC, respectively) voltages are applied between the tip and the sample. The AC bias is applied to the tip to induce vertical vibration at a constant resonant frequency, and the DC bias is applied to eliminate variables such as the electrostatic force and frequency term; the DC bias corresponds to Vcontact and indicates the SP. The measured SP indicates the work function difference between the AFM tip and the surface region;23 that is, SPsubstrate = (φtip -φsubstrate ) / e for the substrate side and SPReS2 = (φtip -φReS2 ) / e for the ReS2 side.

φsubstrate , φReS2 ,

and

φ tip are the work functions of the electrode, ReS2, and AFM tip, respectively. The SP difference, ∆SP = SPsubstrate -SPReS2 = (φReS2 - φsubstrate ) / e , corresponds to the band bending in the vacuum level (Evac) at thermal equilibrium and is equal to the work function difference between the gold substrate and ReS2.24 Figure 2(b) and (c) show the topological and SP maps of a cropped ReS2 flake on the gold substrate, respectively. The ReS2 flake has three regions with different layer thicknesses: 3– 4L, 7–10L, and 21–26L. The number of layers was estimated by comparing their thicknesses. The SP map shows a uniform color indicating ReS2, which indicates that the SP difference was independent of the number of layers, in contrast with MoS2.22 This is strong evidence that the Fermi level of the ReS2 flakes is negligibly influenced by the number of layers due to the weak van der Waals force between each layer.14 The work function of ReS2 was estimated using the work function of the gold tip (4.95 eV), which was calibrated from highly oriented pyrolytic graphite (Figure S3 of Supporting Information). The SP difference between ReS2 and the gold substrate was 0.12 V, and the work function of ReS2 was 4.83 eV, which is 0.12 eV lower than that of the gold tip. This is sufficient data on the band structure to understand the device performance of ReS2. Table 1 presents detailed values of the energy band gap, work function, and electron affinity of 1L, 2L, and 3L ReS2.



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To characterize the Schottky characteristics of the interface between ReS2 and the metal contacts, ReS2 FET devices were prepared using Pt, Pd, Ti, and Al contacts. The devices had an equally thick ReS2 layer to eliminate the layer-dependence effect. The work functions of Al25, Ti26, Pd27, and Pt28 have been reported to be 4.06−4.2625, 4.326, 5.30−5.6027, and 5.71 eV

28

, respectively. The band structure of ReS2 and the work functions of metals are

compared in the energy band diagram presented in Figure 3(a), wherein the current–voltage characteristics of ReS2 FET devices employing Pt, Pd, Au/Ti, and Al metal contacts are compared. Figure 4(b) shows the drain current (Id) as a function of the source–drain voltage (Vds). The current transport of the Al contact was ten times higher than that of the Pt contact. Figure 3(c) shows Id as a function of the back-gate voltage (Vg) on a logarithmic scale. All curves were obtained under conditions of Vds = 1 V. The Id−Vg curves show n-type unipolar behavior with various metals. This observation indicates that the Fermi-level pinning of ReS2 close to the conduction band affected the metal–ReS2 interface. A higher threshold voltage (VT) was observed for the metal contacts with a higher work function. By comparing the subthreshold slopes, it was observed that the relative speed of switching, which is the gate voltage range from OFF to ON, was lower for the metal contacts with the higher work functions. Interestingly, the ON currents for the ReS2 FET devices were dependent on Vg. As the Schottky barrier height is higher due to the band-edge pinning of ReS2 to the work function of the metal, the barrier effect remains in strong inversion, which suppresses the current saturation. The electrical effect of ReS2–metal interfaces on the work function of the metals was also compared by measuring the contact resistance of the Schottky interface using the YFM.29 The Y-function is expressed in general terms as follows:


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Y=

Id gm

=

Id dId / dVg

(2) ,

where gm is the device transconductance, which is a derivative of Id with respect to Vg. The drain current term in the Y-function is expressed as follows:29

Id =

W Qµ eff × Vds L ,

(3)

where W/L is the ratio of the channel length to the width, Q is the channel charge per unit area, and µeff is the effective mobility. Q and µeff can be expressed using the difference between the gate voltage and threshold voltage (Vg − VT) as follows: 







Q ≈ Cox  Vg −VT  and µ eff =

µ0 1 + θ ( Vg − VT )

(4) ,

where Cox is the gate capacitance (between the gate and the channel layer) per unit area, µ0 is the intrinsic mobility, and θ is the mobility attenuation factor. Thus, Id can be determined from the linear region of the transfer characteristics and expressed using the attenuation term as follows:30

Id =

µ0 W Cox ( Vg − VT ) × Vds L 1 + θ ( Vg − VT )

(5) ,

As COX between the gate and channel used in this study was constant at 1.2116 × 10−14 Fm−2, the equation was solved using the relation:

C ox = (ε 0 ε r )/d .

The permittivity of free space is

ε 0 = 8.854 ×10−12 Fm−1 , the relative permittivity of SiO2 is ε r = 3.9, and the thickness of the SiO2 layer was d = 285 nm. By considering the transfer characteristics, the Y-function in equation (2) reduces to



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Y=

µ 0 C ox Vds

W L

1 − θ ′ ( Vg − VT )

2

(V

g

− VT )

(6) ,

where θ' is the derivative of θ with respect to Vg. By assuming that θ is independent of Vg (θ' = 0) 12, the Y-function can be simplified as follows:

Y= µ0COX Vds

W ( Vg -VT ) L ,

(7)

where COX, Vds, and W/L are constant. Figure 3(d) shows the Y-function with respect to Vg for the respective devices employing Pt, Pd, Ti/Au, and Al contacts. VT and µ0 were estimated by linear fitting the Y-function in the strong inversion region. The estimated values of VT were 8.3, 4.34, −27.2, and −43.2V for Pt, Pd, Ti/Au, and Al, respectively. The highest threshold voltage was observed for the ReS2 FET device employing the Pt contact because Pt has the highest work function. The mobility attenuation factor (θ) is expressed as follows:

θ = θ0 +µ0CoxRc W L

,

(8)

where θ0 is the intrinsic attenuation factor and Rc is the contact resistance. In this study, the intrinsic attenuation factor related to remote phonon scattering and the surface roughness is negligible (θ0 = 0). Figure 3(e) shows the mobility attenuation factor (θ) as a function of Vg for each device. The mobility attenuation factor was estimated from equation (5) in the strong inversion region of the transfer curve, where the gate voltage is higher than the threshold voltage. Figure 3(f) shows the intrinsic mobility (µ0) and the contact resistance (Rc) for each ReS2 device according to the work function of the metal. The values of µ0 for Pt, Pd, Ti/Au, and Al were estimated to be in the range of 8.75 − 10.11 cm2V−1s−1 using equation (7). The


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differences in the intrinsic mobility were small enough to be negligible. This indicates that the effects of contact resistance were eliminated for the ReS2 FET devices. The values of Rc were estimated using equation (8). As the work function of the metal increased, a higher contact resistance was observed for the metal contacts with a higher work function. In fact, this value is the upper limit of the contact resistance and showed a tendency toward an increase in the resistance according to the work function. Figure 4(a) shows a schematic of the asymmetric ReS2 FET with Ti and Pt contacts and the energy diagram for the electrical measurements. The Pt contact was a drain contact. Figure 4(b) shows the Id−Vds curve, which represents the rectifying characteristics. This curve shows a drastic increase in the range of positive bias (Vds > 0). In the range of negative bias (Vds < 0), no current flow was observed. This indicates that the contact resistance between Pt and ReS2 induces a Schottky barrier. The rectifying ratio was estimated to be ~104.5. This value is much higher than that of the MoS2 asymmetric FET device reported in a previous study.22 Accordingly, the feasibility of ReS2 asymmetric FET devices for current rectification was confirmed. Figure 4(c) and (d) show the current rectifying characteristics, which were measured by modulating Vg and Vds, respectively. In Figure 4(c), the Id−Vds characteristics of the few-layer ReS2 devices were measured in the Vg range of −50 to 50 V. A higher current was observed with a positive increase in the gate voltage. In the range of Vds > 0.5 V, a high forward-current flow was observed at a high Vg in the Id−Vds curves. Similarly, a drastic increase in the current occurred at Vds = 0.5, 0.7, and 1.0 V in the Id−Vg curves, as shown in Figure 5(d). This demonstrates the shorter range of Vg in reaching the strong inversion region compared to that of the symmetric device shown in Figure 4(c), which indicates a rapid increase in the current from OFF to ON. This provides evidence that ReS2 can be used in rapidly expanding switching device applications. Graphene contacts were also evaluated to examine the feasibility of a completely two-



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dimensional device. We fabricated a ReS2 FET device with graphene contacts, as shown in Figure 5(a). The work function of few-layer graphene (>4 layers) was saturated at ~4.6 eV.31 The few-layer graphene electrode was expanded using bulk Pd because the interface of Pd and graphene has a lower contact resistance.32 Figure 5(b) shows the output characteristics of the device, wherein Id increased linearly in response to Vds, which means that the ReS2/graphene composite has an ohmic contact. This slope became steeper with a positive Vg. The transfer characteristics were also analyzed to estimate the contact resistance of graphene with ReS2, as shown in Figure 5(c). The contact resistance of graphene, which was also extracted using the YFM, was lower than those of Pt, Pd, Au/Ti, and Al. For the TMDc materials, Fermi-level pinning near the edge of the conduction band, which is generated by the location of the S-vacancy defect level and the charge neutral level, causes high contact resistance at their interface with the bulk metals.33,34 However, the pinning effect of ReS2 to graphene was reduced due to a van der Waals gap (vdW gap), as shown in Figure 5(d). Instead, the work function of graphene is tuned by electrostatic doping from ReS2, and its Fermi level shifts to the CBM of ReS2, similar to that of a MoS2/graphene junction observed in a previous report.35 Thus, the ReS2/graphene junction generates low contact resistance, and if the electrode can be fabricated using only graphene, a much lower resistance can be achieved. An asymmetric FET was also fabricated using graphene and Pd contacts. The Pd contact was a drain electrode. The device had a high rectifying current (ratio: ~102), as shown in Figure 5(e). Since the graphene/ReS2 junction exhibits a much lower contact resistance than the Pd/ReS2 junction, the rectifying current flowed in the direction of the positive Vds. These data clearly show that the van der Waals junction between 2D metallic and semiconducting materials is beneficial for an ohmic contact device. In future studies, the Pd contact can be substituted using other 2D materials with high work functions.


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Conclusion In the current study, we demonstrated the contact resistance and rectifying characteristics of ReS2 and investigated its work function using several metallic materials and graphene for FET devices. First, we investigated the intrinsic electrical properties of ReS2 using DFT calculations and KPFM measurements. The monolayer band gap was 1.42 eV and narrowed with an increasing number of layers. The electron affinity (4.30 eV) and work function of ReS2 (4.83 eV) were independent of the number of layers due to the weak van der Waals interactions between each layer. The intrinsic mobility and contact resistance were estimated using the YFM for various ReS2 FET devices with different contacts. Although the mobility was similar for all devices, the contact resistance from symmetric ReS2 junctions was determined using the work function difference at the junction interface. An asymmetric ReS2 FET device with Ti and Pt contacts exhibited a considerable rectifying ratio of up to 104.5, which leads to ultra-fast switching. The graphene–ReS2 interface showed a much lower contact resistance than our metal-based devices thanks to the van der Waals gap and the doping effect at the junction. An asymmetric ReS2 transistor using graphene and Pd presented high rectification. The control of the work function at the junction of ReS2 and metallic materials offers great potential in realizing various types of electrical devices.



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Corresponding Author *E-mail: [email protected]

Author Contributions J.Y.P, H.E.J, and H.S.Y contributed equally to this article.

Notes The authors declare no competing financial interests.

Acknowledgments This work was fully supported by the Korean Government (MSIP) (No. 2015R1A5A1037668) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST). In addition, this work was partially supported by the Priority Research Centers Program (2009-0093823), Korea Ministry of Environment as “Global Top Project (2016002130005),” Development of diagnostic system for mild cognitive impairment due to Alzheimer's disease (2015-11-1684).

Supporting information Fabrication method of ReS2 electrical device, the optical image and AFM profile of ReS2 flake, the method and results of DFT calculation, and schematic representation for measuring surface potential. This material is available free of charge via the Internet at http://pubs.acs.org.


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Figures

Figure. 1 (a) Schematic of the ReS2 FET. (b) SEM image of the ReS2–metal junction. Scale bar is 2μm (c) Top views of the distorted 1T phase structure of ReS2. (d) Electrical band structure of mono-, bi-, and tri-layer ReS2.



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Figure. 2 (a) Schematics of AFM and KPFM, (b) topology mapping, (c) potential mapping, and (d) the thickness and work function of ReS2 along the cropped line.


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Table. 1 Parameters of the electrical band structure (energy band gap, work function, and electron affinity) from 1L to 3L.



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Figure. 3 Device performances for devices with different metal contacts. (a) Energy band diagram of metal and ReS2 (b) I–V characteristics from −2 to 2 V, and (c) transfer characteristics. (d) Y-function graph extracted from Figure 2(b); the slope represents the mobility and gate voltage; the point where the Y value begins to increase is the threshold voltage. (e) Attenuation factor: the gate voltage where the attenuation reaches a maximum point is the threshold voltage; (f) intrinsic mobility of few-layer ReS2, where the contact resistance and the contact resistance at the Schottky junction interface is eliminated. The work functions follow the order: Pt < Pd < Au < Al.


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Figure. 4 (a) Schematic and energy diagram of ReS2 FETs with Ti and Pt contacts. (b) Rectifying characteristics from the output curve of the device on a logarithmic scale (the embedded graph shows a linear scale). (c) Output curve with applied gate bias and (d) transfer curve with applied source–drain voltage.



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Figure 5 (a) ReS2 FET with graphene electrodes, (b) variation in the drain current according to the source–drain voltage in the applied back-gate voltage range of −50 to 50 V, (c) transfer curve with increasing source–drain bias; the inset shows the contact resistance of graphene and various metals, (d) schematics of graphene and palladium contacts, (e) I–V curve of ReS2 asymmetric FET with graphene and Pd contacts.


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Graphical abstract


Metal Interface - ACS Applied Materials

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