Ultralow Schottky Barrier Height Achieved by Using Molybdenum

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Functional Inorganic Materials and Devices

Ultralow Schottky Barrier Height Achieved by Using Molybdenum Disulfide/Dielectric Stack for Source/Drain Contact Seung-Hwan Kim, Kyu-Hyun Han, Euyjin Park, Seung-Geun Kim, and Hyun-Yong Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10746 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

Ultralow Schottky Barrier Height Achieved by Using Molybdenum Disulfide/Dielectric Stack for Source/Drain Contact Seung-Hwan Kim,1 Kyu Hyun Han,1 Euyjin Park,1 Seung-Geun Kim,2 and Hyun-Yong Yu1,2,* 1School

of Electrical Engineering, Korea University, Seoul 02841, Korea

2Department

of Semiconductor Systems Engineering, Korea University, Seoul 02841, Korea

*corresponding author: [email protected]

KEYWORDS: Schottky barrier height, Fermi-level pinning, molybdenum disulfide, metalinduced gap state, III–V semiconductor, germanium, source/drain contact.

ACS Paragon Plus Environment

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ABSTRACT

Energy barrier formed at a metal/semiconductor interface is a critical factor determining the performance of nanoelectronic devices. Although diverse methods for reducing the Schottky barrier height (SBH) via interface engineering have been developed, it is still difficult to achieve both an ultralow SBH and a low dependence on the contact metals. In this study, a novel structure, namely a metal/transition metal dichalcogenide (TMD) interlayer (IL)/dielectric IL/semiconductor (MTDS) structure, was developed to overcome these issues. Molybdenum disulfide (MoS2) is a promising TMD IL material owing to its interface characteristics, which yields a low SBH and reduces the reliance on contact metals. Moreover, an ultralow SBH is achieved via the insertion of an ultrathin ZnO layer between MoS2 and a semiconductor, thereby inducing an n-type doping effect on the MoS2 IL and forming an interface dipole in a favorable direction at the ZnO IL/semiconductor interfaces. Consequently, the lowest SBH (0.07 eV) and a remarkable improvement in the reverse current density (by a factor of approximately 5,400) are achieved, with a wide room for contact metal dependence. This study experimentally and theoretically validates the effect of the proposed MTDS structure, which can be a key technique for next-generation nanoelectronics.


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1. INTRODUCTION Engineering of the Schottky barrier height (SBH) at a metal/semiconductor (MS) interface is at the centre of recent nanoelectronics contact structures because of its critical role in device performance.1,2 Conventional source/drain (S/D) contact techniques exhibit numerous disadvantages for modern nanoscale devices, such as the formation of a large S/D contact region, the non-uniformity of the contact property, and especially a large SBH.3,4 In particular, regarding S/D contact techniques, the SBH is a major hurdle for improving the device performance. A new contact scheme that has an ultralow SBH with non-alloyed contact is essential for overcoming these problems. To effectively reduce the SBH, an in-depth study on interface properties, such as van der Waals (vdW) interactions, metal-induced gap states (MIGS), and interface dipole formation, is needed.2,5–8 To overcome these contact issues for next-generation nanoelectronic technologies, a comprehensive strategy should be designed employing various methods for modulating the SBH of semiconductors. In addition to conventional semiconductors, such as Si, Ge, and III-V compound semiconductors, two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have a significant effect on the Fermi-level (FL) pinning at the surface, giving rise to a large SBH when the MS contact is formed.9–11 Non-ideal physics of these interface characteristics have been widely described in the literature, and numerous studies on inducing FL unpinning, e.g., via surface passivation, interface dipoles, and metal/interlayer (IL)/semiconductor (MIS) structures, have been reported.12–14 The FLs of metals are strongly attached to near charge neutrality levels (CNLs) of the semiconductors when the metals are directly deposited on the semiconductor surface.15,16 Given that the CNLs of the semiconductors are mostly far from their conduction band minimum (CBM), a large SBH is induced. As a result, it becomes difficult to modulate the


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SBH regardless of the contact metal work-function values.17 This strong FL pinning can be alleviated by the formation of an MIS structure that can reduce the main cause of FL pinning, i.e., the MIGS, at the semiconductor surface.15–17 Although a MIS structure can achieve SBH reduction with an ultrathin dielectric IL, it has a poor selection of contact metals, owing to the unpinned metal/IL interface.18 Only small work-function contact metals can form a low SBH for an n-type semiconductor within an MIS structure.18,19 Recently, to solve these problems related to the MIS structure, a metal/TMD IL/semiconductor (MTS) structure was developed.18,20 This MTS structure can achieve better electrical characteristics and widen the room for contact metal adoption owing to the properties of TMD materials.18 However, the efficiency of the MIGS alleviation of the TMD IL is too low owing to its narrow band gap, resulting in an increase in its thickness for achieving the optimum SBH. Therefore, the development of a new contact structure that can overcome the problems of both MIS and MTS contact structures for attaining better device performance is necessary. In this study, we developed a novel structure, i.e., a metal/TMD IL/dielectric IL/semiconductor (MTDS) contact, on GaAs substrates. The structure exhibits a low SBH and excellent electrical properties with an ultrathin double layer composed of MoS2/ZnO and MoS2/TiO2 stacks. The effect of the interface dipole formed by inserting an ultrathin dielectric IL between the MoS2 IL and GaAs on the electrical properties of MTDS contacts has been investigated. The main advantage of MTDS contacts, i.e., the low contact metal dependence, has been confirmed by using various contact metals.


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2. RESULTS AND DISCUSSION 2.1 Structures of MTDS Contacts. Figure 1a shows a three-dimensional (3D) schematic of MS, MTS, and MTDS contacts. Given that MoS2 flakes with random sizes were exfoliated on the substrate, the same contact regions were defined for precise design of comparative experiments. Therefore, the active regions were opened by a patterned IL dielectric (ILD) using silicon dioxide (SiO2) to match the sizes of the contact regions of all the contact structures. Additionally, an etching process was employed for controlling the thickness of the MoS2 IL because it was randomly formed via exfoliation. A flow chart of the fabrication and the method of the etching process are presented in Figure S1. A top-view optical micrograph (OM) image of an MTDS contact is shown in Figure 1b. The open active region and contacts on a MoS2 flake are clearly observed. Figure 1c shows a cross-sectional schematic of MS, MTS, and MTDS contacts. The electrical properties of the contact structures, such as the current density–voltage (J–V) characteristics and SBH, were evaluated by the vertical measurement method. Figure 1d presents a transmission electron microscopy (TEM) image of an Ti/MoS2/ZnO/n-GaAs structure, exhibiting a uniformly deposited ZnO IL and a well etched MoS2 IL on the GaAs substrate. 2.2 Electrical Characteristics of MTDS Contacts. Figure 2a and b show the J–V characteristics of all the implented MTDS contacts. The current densities of MS, MIS, and MTS contacts are also presented, for comparison. The current densities of Ti/MoS2/ZnO/n-GaAs contacts are depicted in Figure 2a. The MIS contacts with ZnO IL exhibit significantly higher current densities than the MS contacts, indicating that the ZnO IL effectively unpins the FL by reducing the MIGS at the GaAs surface. The highest current density of the MIS contacts is exhibited by the Ti/ZnO (2 nm)/n-GaAs contact and is ~386 times


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higher than that of the MS contact. For MTS contacts, the current density is obtained from a previous work, and its structure is Ti/MoS2 (20 nm)/n-GaAs.[18] When the MoS2 IL is inserted between the metal and the ZnO IL, the electrical properties are significantly enhanced, because the MoS2 IL causes the FL of the metal to be strongly pinned to the CBM of the MoS2 IL. Thus, all the MTDS contacts exhibit higher current densities than the MS and MIS contacts. The highest current density among the MTDS contacts with the ZnO IL is obtained with a 2 nm-thick ZnO IL and a 3 nm-thick MoS2 IL. The current density of the Ti/MoS2 (3 nm)/ZnO (2 nm)/nGaAs contact is ~5,400 and ~14 times higher than those of Ti/n-GaAs and Ti/ZnO (2 nm)/nGaAs, respectively. The current densities of the MIS and MTDS contacts with TiO2 ILs are shown in Figure 2b. Similar to the MIS contacts with ZnO ILs, these MIS contacts exhibit higher current densities than the MS contacts. Additionally, the electrical properties of the MTDS contacts with TiO2 ILs are superior to those of the MIS contacts. The highest current density is obtained with a 9 nm-thick MoS2 IL and a 2 nm-thick TiO2 IL. This current density is lower than that of the MTDS contact with the ZnO IL. The reason for the difference in the optimal current density between the MTDS contacts with the ZnO and TiO2 ILs was well elucidated by investigating the interface dipole formation. The reason for the difference in the optimal thickness of the MTDS contacts with ZnO and TiO2 IL is due to the difference of band gap properties between ZnO and TiO2. Given that wide band gap materials effectively unpin the FL compared to narrow band gap materials,[17] the ZnO IL (Eg = ~ 3.37 eV) reduces the MIGS at the GaAs surface better than the TiO2 IL (Eg = ~ 2.85 eV) with the same thickness. Therefore, a thicker MoS2 IL is needed for the MTDS contacts with TiO2 IL to alleviate FL pinning. Both types of MTDS contacts with an MoS2 IL beyond the optimum thickness exhibit slight degradation of electrical properties because the series resistance of the IL and the SBH is


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increased.21 The reverse current densities and the SBH of the MIS, MTS, and MTDS contacts at –1 V for different IL thicknesses are shown in Figure S2 and S3, respectively. 2.3 Band Diagram of the MS, MIS, MTS, and MTDS Structure. The band diagrams of the Ti/n-GaAs, Ti/ZnO/n-GaAs, Ti/MoS2/n-GaAs, and Ti/MoS2/ZnO/nGaAs structures are depicted in Figure 3a–d, respectively. It is well known that a large SBH is formed for n-type GaAs, while the MS contact is formed, because the electron wave function easily penetrates into GaAs, thereby generating MIGS at the GaAs surface.22 These MIGS severely induce an unwanted dipole at the Ti/n-GaAs interface, causing strong FL pinning as shown in Figure 3a. Thus, a large SBH is formed for metal/n-GaAs contacts with contact metals. To eliminate the MIGS generation at the semiconductor surface, which is one of the main causes of strong FL pinning, numerous IL insertion methods have been reported.9,12–17 Figure 3b shows conventional MIS contacts using thin dielectric materials, such as Al2O3, ZnO, and TiO2. Among the dielectric materials, the most promising candidates are ZnO and TiO2 because of their high electron affinity, which is similar to that of GaAs, giving rise to a negative conduction band offset (CBO) (-0.23 eV for ZnO/GaAs) with respect to GaAs.7,8 However, although the wide band gap properties of these materials effectively block the electron wave function penetration, the electrical properties of MIS contacts strongly depend on their thicknesses, owing to the tradeoff between the MIGS reduction and the series resistance of ILs as a function of the IL thickness. Furthermore, a wide band gap limits the selection of contact metals. Only a few contact metals featuring work-function values near 4.0 eV can be applied for n-type S/D contacts. An MTS contact is another S/D contact technique, which has been recently developed to overcome the problems of MIS contacts, as shown in Figure 3c.18 The advantageous interface characteristics of MoS2, such as great SBH controllability, a near-zero barrier at the metal/MoS2 interface, and


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low contact metal dependence, make the MTS contact a substitute for the MIS contact. However, it still has critical issues to overcome. For instance, the MoS2 IL is too thick to be applied to various nanoelectronic devices. The MoS2 IL must be thick because the electron wave function from the metal cannot drastically decay, owing to its narrow band gap (1.2 eV). Therefore, a new technique that can exploit the strengths and compensate for the weaknesses of MIS and MTS contacts is needed. To reduce the MoS2 IL thickness and obtain additional interface effects for reducing the SBH, an MTDS contact has been newly proposed. As shown in Figure 3d, the SBH of GaAs is reduced, and the band of MoS2 is lowered. Especially, the thickness of the MoS2 IL is significantly reduced. These results are due to the following effects: i) dielectric materials with a positive fixed charge, inducing band bending of the MoS2;23–25 ii) dielectric materials with an oxygen areal density (OAD) lower than those of Ga2O3 and As2O3, inducing further SBH reduction;8,26,27 and iii) the wide band gap of the dielectric materials effectively blocking the rest of the electron wave function from penetrating the MoS2 IL, which reduces the MoS2 IL thickness. Those effects are well described in Sections 2.4 and 2.5. 2.4 Effect of n-type Doping Using MoS2 IL/Dielectric IL Stack. Figure 4a presents a cross-sectional schematic of an MTDS contact, indicating the induced charge at the MoS2/ZnO or MoS2/TiO2 interface, and the band diagram of an MTDS contact with and without n-type doping. When the dielectric material, which has a positive fixed charge, directly contacts the MoS2 surface, the surface potential of MoS2 is pulled down, as depicted in Figure 4a.23 Therefore, the conduction band of MoS2 is bent down near the FL of the MTDS contact, which indicates an increase of tunneling probability and conductivity of the MoS2 IL. Both ZnO and TiO2 have native point defects, corresponding to oxygen vacancies, and they typically act as positive charge.[28–31] Thus, the positively charged ZnO and TiO2 ILs can bend


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down the band at the MoS2/ZnO or MoS2/TiO2 interface. Figure 4b presents the Raman spectra to verify the n-type doping effect of ZnO and TiO2. For this Raman spectroscopy analysis, MoS2/n-GaAs, MoS2/ZnO/n-GaAs, and MoS2/TiO2/n-GaAs samples were prepared. Two Raman peaks corresponding to in-plane (E12g peak) and out-of-plane (A1g peak) are observed at approximately 387 and 412 cm-1, respectively, for the MoS2/n-GaAs substrate (reference sample). Additionally, it is shown that both the E12g and the A1g peaks of the MoS2/ZnO/n-GaAs and MoS2/TiO2/n-GaAs samples were red-shifted (negative shift) compared with the reference sample. This red shift indicates that the MoS2 ILs were n-doped, which is consistent with the results of various MoS2 doping studies.24,25,32 Figure 4c and d show the X-ray photoelectron spectroscopy (XPS) spectra of the Mo 3d and S 2p peaks, which support the results of the Raman spectroscopy analysis. For both the ZnO- and TiO2-inserted samples, the two peaks are positively shifted (from 232.68 to 232.78 eV for Mo 3d3/2, from 229.58 to 229.68 eV for Mo 3d5/2, from 163.58 to 163.78 eV for S 2p1/2, and from 162.38 to 162.58 eV for S2p3/2). This positive shifting phenomenon indicates that the FL of MoS2 is close to the CBM of MoS2, corresponding to an MoS2 IL that is more n-doped than that of an MTS contact.25,33,34 These results confirm that the MoS2 IL is n-doped well using ultrathin ZnO and TiO2 ILs. 2.5 Effect of Interface Dipole Formation at Dielectric IL/GaAs Interface. The causes of the formation of an interface dipole between two different dielectric materials are still not known with certainty. However, several leading theories explaining the dipole phenomenon have been reported.27,35–37 One of the main parameters determining the dipole direction is the OAD, which is based on the diffusion of oxygen ions.27,35 Oxygen ions in an oxide layer with a higher OAD are diffused into another oxide layer with a lower OAD, resulting in negative dipole formation at the lower OAD layer. Figure 5a presents the OADs of ZnO and


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TiO2, as well as the average value for Ga2O3 and As2O3 based on the OAD model, which is expressed as follows:8

( )

𝑚 𝜎= 𝑁∙𝛿

―

2 3

,

(1)

where m is the molecular weight, N is the number of oxygen atoms in an oxide molecule, and δ is the density of the molecules. A comparison of OAD values reveals that the ZnO IL forms a favorable interface dipole direction at the GaAs surface because the OAD of the GaAs is higher than that of the ZnO IL, causing oxygen ion diffusion from GaAs to the ZnO IL. However, the TiO2 IL induces the opposite interface dipole direction, increasing the SBH of GaAs. Figure 5b illustrates the band diagrams of the ZnO and TiO2/n-GaAs structures with and without an interface dipole based on the difference of the OAD values between the ILs and the native oxide of the GaAs. Although the native oxides of GaAs are removed by a cleaning process, they are thinly formed by an oxide material deposition process; thus, the interface dipole is formed at the ILs/n-GaAs interface. Because the oxygen ions of the native oxides of GaAs are diffused into the ZnO IL, a positive potential is induced at the GaAs surface. Therefore, the energy band at the GaAs side bends down, resulting in SBH lowering, whereas the energy band at the ZnO side bends up, as shown in Figure 5b. Note that the effective SBH increases for the TiO2/n-GaAs structure because the oxygen ions are diffused into the GaAs surface from TiO2. Thus, the SBH of the MTDS contact with TiO2 IL is relatively larger than that with ZnO IL. To verify the interface dipole formation, XPS analysis for Ga 3d and As 3d was conducted. As shown in Figure 5c and e, the binding energy difference between the GaAs bulk peak and the Ga-O bond peak (BEGa-O – BEGa-As) is reduced from 1 eV (20.5 eV – 19.5 eV) to 0.93 eV (20.4 eV – 19.47 eV). Figure 5d and f show similar results; i.e., the difference in the binding energy between the


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GaAs bulk peaks and the As-O bond peak (BEAs-O – BEAs-Ga) is reduced from 3.28 eV (44.5 eV – 41.22 eV) to 3.18 eV (44.4 eV – 41.22 eV). Thus, Δ(BEGa-O – BEGa-As) is –0.07 eV (0.93 eV – 1 eV) and Δ(BEAs-O – BEAs-Ga) is –0.1 eV (3.18 eV – 3.28 eV), indicating that a more positive potential is induced at the GaAs surface for the MoS2/ZnO/n-GaAs structure than for the MoS2/n-GaAs structure.8,26 In contrast, because the TiO2 IL has a higher OAD than the GaAs, a negative potential is induced at the GaAs surface, yielding an unwanted interface dipole direction and an increase in the SBH. As a result, ZnO is a suitable material for an IL inserted between MoS2 and GaAs, which can pull down the energy band of both materials, thereby facilitating the electron transport. 2.6 Verification of Contact Metal Dependency. To confirm the contact metal dependence of an MTDS contact, four different contact metals (Ti, 4.33 eV; Cu, 4.7 eV; Au, 5.15 eV; Pt, 5.75 eV) are used for such a MTDS contact with a 3 nm-thick MoS2 IL and a 2 nm-thick ZnO IL. Figure 6a shows the reverse current densities of the MIS and MTDS contacts at –1 V with four different contact metals. The current densities of MIS contacts differed significantly for different contact metals. Given that the ZnO IL has a wide band gap, the FLs of the contact metals are unpinned at the metal/ZnO IL interface.17 However, it is well known that the MoS2 IL has severe FL pinning to its CBM.38,39 Therefore, little contact metal dependence of the SBH is observed at the MTDS contacts. The reverse current densities of the MTDS contacts at –1 V with four different contact metals are presented in Figure S4. The FL alignment of the four different contact metals with respect to the energy band of GaAs for all the considered contact structures is depicted in Figure 6b. In the ideal case, the FLs of the contact metals are attached to the GaAs surface, according to the work-function value of the contact metals. However, in the case of a real MS contact, the FLs are strongly pinned near the


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CNL of GaAs. Therefore, a large SBH is formed regardless of the contact metal work-function. To alleviate the effect of the FL pinning, an MIS contact was introduced, and the FL was successfully unpinned by reducing the MIGS at the GaAs surface. However, specific contact metals must be used for the MIS contact, because metals with a large work-function such as Au and Pt induce a large SBH for an n-type semiconductor. The extracted SBHs of the metal/ZnO (2 nm)/n-GaAs contacts are 0.23, 0.49, 0.58, and 0.66 eV with Ti, Cu, Au, and Pt, respectively. Regarding the MTS contact, an ultralow contact metal dependence on SBH modulation with a low SBH value is observed in a previous study.18 The SBH was significantly reduced by inserting a 20 nm-thick MoS2 IL between the metal and GaAs, and the strong FL pinning at the MoS2 surface widens the room for contact metal adoption. For MTDS contacts, an ultralow SBH (~ 0.07 eV) is formed by using an MoS2 (3 nm)/ZnO (2 nm) stack inducing several interface techniques, such as the n-type doping effect and proper interface dipole formation. Given that the FLs of the contact metals are strongly pinned near the CBM of the MoS2 IL, the MTDS contacts have ultralow contact metal dependence, resulting in similar SBH values with four different contact metals (0.07, 0.075, 0.073, and 0.08 eV for Ti, Cu, Au, and Pt, respectively). These SBH values are lower than those of previously reported contact structures for GaAs.8,18,19,40 The SBH extraction methods are presented in Figure S5.


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3. CONCLUSIONS A novel MTDS contact structure for an S/D contact by inserting a thin MoS2 layer as a TMD IL and ZnO as a dielectric IL was developed. To improve the electrical characteristics of MIS and MTS contacts, we introduced several interface engineering techniques, such as n-type doping of the MoS2 IL and proper interface dipole formation using a MoS2 IL/ZnO IL stack. ZnO is a promising dielectric IL material because of its wide band gap and low CBO to GaAs. Furthermore, it can induce an n-type doping effect on the MoS2 IL because of its positive fixed charge and can also induce a favorable interface dipole direction at the ZnO IL/GaAs interface, causing further SBH lowering. Consequently, the MTDS contact with a MoS2 (3 nm)/ZnO (2 nm) stack exhibits the lowest SBH value (0.07 eV) and a reverse current density ~5,400 times higher than that of the MS contact. The ultralow contact metal dependence of the MTDS contact was investigated using four different contact metals, namely Ti, Cu, Au, and Pt. The proposed MTDS contact structure is a great candidate for next-generation nanoelectronic applications owing to its various advantages.


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4. METHODS 4.1. Device Fabrication. The Si-doped n-type GaAs (Nd = 4 × 1017 cm–3) was sequentially cleaned with acetone, isopropanol, and deionized water. Subsequently, the native oxides of the GaAs substrate were removed by using a 20% HCl solution for 1 min. Ultrathin dielectric materials, i.e., ZnO and TiO2, were deposited via atomic layer deposition (ALD). A ZnO IL was formed by using a diethyl zinc (DEZ) precursor with an H2O gas reactant at 150 °C. A TiO2 IL was formed by using a titanium tetraisopropoxide (TTIP) precursor with an H2O gas reactant at 250 °C. To form MTDS or MTS contacts, MoS2 flakes were transferred onto GaAs, ZnO/GaAs, or TiO2/GaAs surfaces by using polydimethylsiloxane (PDMS) as a carrier. The SiO2 ILD was patterned to define the active region for matching the S/D contact size. Plasma etching process was conducted using an inductively coupled plasma reactive ion etcher (ICP-RIE). SF6 plasma was used for the MoS2 IL etching process, with the following parameters: a source power of 50 W, a bias power of 15 W, a chamber pressure of 50 mTorr, and a gas flow of 20 sccm. After completing the IL stacks, i.e., bare GaAs, ZnO/GaAs, TiO2/GaAs, MoS2/GaAs, MoS2/ZnO/GaAs, and MoS2/TiO2/GaAs, the top contact metal stack, i.e., Au/Ti, was deposited using an electronbeam (e-beam) evaporator. To investigate the dependence of the electrical properties on the contact metal work-function, Cu, Au, and Pt were also deposited using the same e-beam evaporator. 4.2. Electrical Measurement. The electrical properties of the fabricated MS, MIS, MTS and MTDS contacts were measured using electrical measurement equipment (Keithley 4200-SCS with direct-current source unit module: 4200-SMU). 4.3. Spectroscopic and Structure Analyses. XPS analysis was conducted with an aluminum Kα line X-ray of 1486.6 eV to investigate the effects of the interface dipole and the MoS2 doping


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by using XPS equipment (K-alpha plus, Thermo). Raman spectra were obtained using a Raman microscopic system (ARAMIS IR2, Horiba) with a 532-nm laser for analyzing the MoS2 doping effect. The cross-sectional structure of the Ti/MoS2/ZnO/n-GaAs was scanned using spherical aberration-corrected TEM (Cs-TEM) (JEM-ARM200F, JEOL Ltd.).

ASSOCIATED CONTENT Supporting Information Fabrication process flow and etching method of the MTDS contact; reverse current density of the discussed contacts; SBH of the discussed contacts; reverse current density of the MTDS contacts with four different contact metals; calculation of the SBH of the Ti/MoS2 (3 nm)/ZnO (2 nm)/n-GaAs contact using temperature-dependent forward current characterization.

AUTHOR INFORMATION Corresponding Author Hyun-Yong Yu School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, 02841, Korea (E-mail: [email protected], Tel: +82-2-3290-4830, Fax: +82-2-921-0544). Funding Sources This work was supported in part by the Technology Innovation Program within the Ministry of Trade, Industry and Energy, Korea, under Grant 10052804, in part by the Basic Science


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Research Program within the Ministry of Science, ICT, and Future Planning through the National Research Foundation of Korea under Grant 2017R1A2B4006460, and in part by Samsung Electronics.


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REFERENCES (1) Liu, Y.; Guo, J.; Zhu, E.; Liao, L.; Lee, S.-J.; Ding, M.; Shakir, I.; Gambin, V.; Huang, Y.; Duan, X. Approaching the Schottky-Mott in van der Waals Metal-Semiconductor Junctions. Nature 2018, 557, 696–700. (2) Liu, Y.; Stradins, P.; Wei, S.-H. Van der Waals metal-semiconductor junction: Weak Fermi Level Pinning Enables Effective Tuning of Schottky Barrier. Sci. Adv. 2016, 2, 1–7. (3) Gong, R.; Wang, J.; Liu, S.; Dong, Z.; Yu, M.; Wen, C. P.; Cai, Y.; Zhang, B.; Analysis of Surface Roughness in Ti/Al/Ni/Au Ohmic Contact to AlGaN/GaN High Electron Mobility Transistors. Appl. Phys. Lett. 2010, 97, 062115. (4) Kim, S.-H.; Kim, G.-S.; Park, J.; Lee, C.; Kim, H.; Kim, J.; Shim, J. H.; Yu, H.-Y. Novel Conductive Filament Metal-Interlayer-Semiconductor Contact Structure for Ultralow Contact Resistance Achievement. ACS Appl. Mater. Interfaces 2018, 10, 26378–26386. (5) Zhou, W.; Guo, Y.; Liu, J.; Wang, F. Q.; Li, X.; Wang, Q. 2D SnSe-based vdW Heterojunctions: Tuning the Schottky Barrier by Reducing Fermi Level Pinning. Nanoscale 2018, 10, 13767–13772. (6) Padilha, J. E.; Fazzio, A.; Silva, A. J. R. 3D Band Diagram and Photoexcitation of 2D–3D Semiconductor Heterojunctions. Phys. Rev. Lett. 2015, 114, 066803. (7) Kim, S.-H.; Kim, G.-S.; Kim, J.-K.; Park, J.-H.; Shin, C.; Choi, C.; Yu, H.-Y. Fermi-Level Unpinning Using a Ge-Passivated Metal-Interlayer-Semiconductor Structure for Non-Alloyed Ohmic Contact of High-Electron-Mobility Transistors. IEEE Electron Device Lett. 2015, 36, 884–886.


17


Page 18 of 29

(8) Kim, S.-W.; Kim, S.-H.; Kim, G.-S.; Choi, C.; Choi. R.; Yu, H.-Y. The Effect of Interfacial Dipoles on the Metal-Double Interlayers-Semiconductor Structure and Their Application in Contact Resistivity Reduction. ACS Appl. Mater. Interfaces 2016, 8, 35614–35620. (9) Monch, W. On the Physics of Metal-Semiconductor Interfaces. Rep. Prog. Phys. 1990, 53, 221–278. (10) Schulman, D. S.; Arnold, A. J.; Das, S. Contact Engineering for 2D Materials and Devices. Chem. Soc. Rev. 2018, 47, 3037–3058. (11) Allain, A.; Kang, J.; Banerjee, K.; Kis, A. Electrical Contacts to Two-Dimensional Semiconductors. Nat. Mater. 2015, 14, 1196–1205. (12) Kim, S.-H.; Kim, G.-S.; Kim, S.-W.; Kim, J.-K.; Choi, C.; Park, J.-H.; Choi, R.; Yu, H.-Y. Non-Alloyed Ohmic Contacts on GaAs Using Metal-Interlayer-Semiconductor Structure with SF6 Plasma Treatment. IEEE Electron Device Lett. 2016, 37, 373–376. (13) Zheng, S.; Lu, H.; Liu, H.; Liu, D.; Robertson, J. Insertion of an Ultrathin Al2O3 Interfacial Layer for Schottky Barrier Height Reduction in WS2 Field-Effect Transistors. Nanoscale 2019, 11, 4811–4821. (14) Kim, G.-S.; Kim, S.-H.; Lee, T. I.; Cho, B. J.; Choi, C.; Shin, C.; Shim, J. H.; Kim, J.; Yu, H.-Y. Fermi-Level Unpinning Technique with Excellent Thermal Stability for n-Type Germanium. ACS Appl. Mater. Interfaces 2017, 9, 35988–35997. (15) Agrawal, N.; Lin, J.; Barth, M.; White, R.; Zheng, B.; Chopra, S.; Gupta, S.; Wang, K.; Gelatos, J.; Mohney, S. E.; Datta, S. Fermi Level Depinning and Contact Resistivity Reduction


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Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Using a Reduced Titania Interlayer in n-Silicon Metal-Insulator-Semiconductor Ohmic Contacts. Appl. Phys. Lett. 2014, 104, 112101. (16) Gupta, S.; Manik, P. P.; Mishra, R. K.; Nainani, A.; Abraham, M. C.; Lodha, S. Contact Resistivity Reduction through Interfacial Layer Doping in Metal-Interfacial LayerSemiconductor Contacts. J. Appl. Phys. 2013, 113, 234505. (17) Agrawal, A.; Shukla, N.; Ahmed, K.; Datta, S. A Unified Model for Insulator Selection to Form Ultra-Low Resistivity Metal-Insulator-Semiconductor Contacts to n-Si, n-Ge, n-InGaAs. Appl. Phys. Lett. 2012, 101, 042108. (18) Kim, S.-H.; Han, K. H.; Kim, G.-S.; Kim, S.-G.; Kim, J.; Yu, H.-Y. Schottky Barrier Height Modulation Using Interface Characteristics of MoS2 Interlayer for Contact Structure. ACS Appl. Mater. Interfaces 2019, 11, 6230–6237. (19) Hu, J.; Saraswat, K. C.; Wong, H.-S. P. Metal/III-V Schohttky Barrier Height Tuning for the Design of Nonalloyed III-V Field-Effect Transistor Source/Drain Contacts. J. Appl. Phys. 2010, 107, 063712. (20) Nam, S.-G.; Cho, Y.; Lee, M.-H.; Shin, K. W.; Kim, C.; Yang, K.; Jeong, M.; Shin, H.-J.; Park, S. Barrier Height Control in Metal/Silicon Contacts with Atomically Thin MoS2 and WS2 Interfacial Layers. 2D Materials 2018, 5, 041004. (21) Kim, J.-K.; Kim, S.-H.; Kim, T.; Yu, H.-Y. Universal Metal-Interlayer-Semiconductor Contact Modeling Considering Interface-State Effect on Contact Resistivity Degradation. IEEE Trans. Electron Devices 2018, 65, 4982–4987.


19


Page 20 of 29

(22) Hu, J.; Nainani, A.; Sun, Y.; Saraswat, K. C.; Wong, H.-S. P. Impact of Fixed Charge on Metal-Insulator-Semiconductor Barrier Height Reduction. Appl. Phys. Lett. 2011, 99, 252104. (23) Na, J.; Joo, M.-K.; Shin, M.; Huh, J.; Kim, J.-S.; Pia, M.; Jin, J.-E.; Jang, H.-K.; Choi, H. J.; Shim, J. H.; Kim, G.-T. Low-Frequency Noise in Multilayer MoS2 Field-Effect Transistors: the Effect of High-k Passivation. Nanoscale 2014, 6, 433–441. (24) Kang, D.-H.; Kim, M.-S.; Shim, J.; Jeon, J.; Park, H.-Y.; Jung, W.-S.; Yu, H.-Y.; Pang, C.H.; Lee, S.; Park, J.-H. High-Performance Transition Metal Dichalcogenide Photodetectors Enhanced by Self-Assembled Monolayer Doping. Adv. Funct. Mater. 2015, 25, 4219–4227. (25) Heo, K.; Jo, S.-H.; Shim, J.; Kang, D.-H.; Kim, J.-H.; Park, J.-H. Stable and Reversible Triphenylphosphine-Based n-Type Doping Technique for Molybdenum Disulfide (MoS2). ACS Appl. Mater. Interfaces 2018, 10, 32765–32772. (26) Kim, S.-H.; Kim, G.-S.; Kim, S.-W.; Yu, H.-Y. Effective Schottky Barrier Height Lowering Technique for InGaAs Contact Scheme: DMIGS and Dit Reduction and Interfacial Dipole Formation. Appl. Surf. Sci. 2018, 453, 48–55. (27) Kita, K.; Toriumi, A. Origin of Electric Dipoles Formed at High-k/SiO2 Interface. Appl. Phys. Lett. 2009, 94, 132902. (28) Janotti, A.; Van de Walle, C. G. Oxygen Vacancies in ZnO. Appl. Phys. Lett. 2005, 87, 122102.


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Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Ondo-Ndong, R.; Essone-Obame, H.; Moussambi, Z. H.; Koumba, N. Capacitive Properties of Zinc Oxide Thin Films by Radiofrequency Magnetron Sputtering. J. Theor. Appl. Phys. 2018, 12, 309–317. (30) Pan, X.; Yang, M.-Q.; Fu, X.; Zhang, N.; Xu, Y.-J. Defective TiO2 with Oxygen Vacancies: Synthesis, Properties and Photocatalytic Application. Nanoscale 2013, 5, 3601–3614. (31) Bera, M. K.; Mahata, C.; Maiti, C. K. Reliability of Ultra-Thin Titanium Dioxide (TiO2) Films on Strained-Si. Thin Solid Films 2008, 517, 27–30. (32) Park, H.-Y.; Dugasani, S. R.; Kang, D.-H.; Jeon, J.; Jang, S. K.; Lee, S.; Roh, Y.; Park, S. H.; Park, J.-H. n- and p-Type Doping Phenomenon by Artificial DNA and M-DNA on TwoDimensional Transition Metal Dichalcogenides. ACS Nano 2014, 8, 11603–11613. (33) Mahn, B.; Roth, F.; Knupfer, M. Absence of Photoemission from the Fermi Level in Potassium Intercalcated Picene and Coronene Films: Structure, Polaron, or Correlation Physics? J. Chem. Phys. 2012, 136, 134503. (34) Fang, H.; Tosun, M.; Seol, G.; Chang, T. C. Degenertate n-Doping of Few-Layer Transition Metal Dichalcogenides by Potassium. Nano Lett. 2013, 13, 1991–1995. (35) Kirsch, P. D.; Sivasubramani, P.; Huang, J.; Young, C. D.; Quevedo-Lopez, M. A.; Wen, H. C.; Alshareef, H.; Choi, K.; Park, C. S.; Freeman, K.; Hussain, M. M.; Bersuker, G.; Harris, H. R.; Majhi, P.; Choi, R.; Lysaght, P.; Lee, B. H.; Tseng, H.-H.; Jammy, R.; Boscke, T. S.; Lichtenwalner, D. J.; Jur, J. S.; Kingon, A. I. Dipole Model Explaining High-k/Metal Gate Field Effect Transistor Threshold Voltage Tuning. Appl. Phys. Lett. 2008, 92, 092901.


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Page 22 of 29

(36) Yamamoto, Y.; Kita, K.; Kyuno, K.; Toriumi, A. Study of La-Induced Flat Band Voltage Shift in Metal/HfLaOx/SiO2/Si Capacitors. Jpn. J. Appl. Phys. 2007, 46, 7251–7255. (37) Kamimuta, Y.; Iwamoto, K.; Nunoshige, Y.; Hirano, A.; Mizubayashi, W.; Watanabe, Y.; Migita, S.; Ogawa, A.; Ota, H.; Nabatame, T.; Toriumi, A. Comprehensive Study of VFB Shift in High-k CMOS – Dipole Formation, Fermi-level Pinning and Oxygen Vacancy Effect -. IEEE Int. Electron Devices Meet. 2007, 341–344. (38) Kim, G.-S.; Kim, S.-H.; Park, J.; Han, K. H.; Kim, J.; Yu, H.-Y. Schottky Barrier Height Engineering for Electrical Contacts of Multilayered MoS2 Transistors with Reduction of MetalInduced Gap States. ACS Nano 2018, 12, 6292–6300. (39) Kim, C.; Moon, I.; Lee, D.; Choi, M. S.; Ahmed, F.; Nam, S.; Cho, Y.; Shin, H.-J.; Park, S.; Yoo, W. J. Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides. ACS Nano 2017, 11, 1588–1596. (40) Lee, C. W.; Kim, Y. T. High Temperature Thermal Stability of Plasma-Deposited Tungsten Nitride Schottky Contacts to GaAs. Solid-State Electron. 1995, 38, 679–682.


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Figure 1. (a) 3D schematic of MS, MTS, and MTDS contacts. (b) Top-view OM image of an Au/Ti/MoS2/ZnO/n-GaAs contact. (c) Cross-sectional schematics of MS, MTS, and MTDS contacts. (d) Cross-sectional TEM image of an Ti/MoS2/ZnO/n-GaAs contact.


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Figure 2. J–V characteristics of the MS, MIS, MTS, and MTDS contacts with the (a) ZnO IL and (b) TiO2 IL, respectively.


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Figure 3. Band diagrams of the (a) Ti/n-GaAs, (b) Ti/ZnO/n-GaAs, (c) Ti/MoS2/n-GaAs, and (d) Ti/MoS2/ZnO/n-GaAs contacts.


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Figure 4. (a) Cross-sectional schematic and band diagram of the MTDS contact for investigating the interfacial doping mechanism at the MoS2/ZnO interface. (b) Raman spectra of the MoS2/nGaAs, MoS2/ZnO/n-GaAs, and MoS2/TiO2/n-GaAs contacts. (c) XPS spectra of the Mo 3d peaks for the MoS2/n-GaAs, MoS2/ZnO/n-GaAs, and MoS2/TiO2/n-GaAs contacts. (d) XPS spectra of the S 2p peaks for the MoS2/n-GaAs, MoS2/ZnO/n-GaAs, and MoS2/TiO2/n-GaAs contacts.


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Figure 5. (a) OAD (σ) values for four types of oxides normalized with respect to the average value for Ga2O3 and As2O3. (b) Band diagrams of the MoS2/ZnO/n-GaAs and MoS2/TiO2/nGaAs contacts. (c) and (e) XPS spectra of the Ga 3d peaks for the MoS2/n-GaAs and MoS2/ZnO/n-GaAs contacts, respectively. (d) and (f) XPS spectra of the As 3d peaks for the MoS2/n-GaAs and MoS2/ZnO/n-GaAs contacts, respectively.


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Figure 6. (a) Reverse current densities of the MIS and MTDS contacts at -1 V with Ti (4.33 eV), Cu (4.7 eV), Au (5.15 eV), and Pt (5.75 eV). (b) FL alignment of the four different contact metals with respect to the energy band of GaAs for the discussed contact structures.


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Table of Contents Graphic


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Ultralow Schottky Barrier Height Achieved by Using Molybdenum

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