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Wongil Shin, Gyuho Myeong, Kyunghwan Sung, Seungho Kim, Hongsik Lim, Boram Kim, Taehyeok Jin, Jihoon Park, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Fei Liu, Sungjae Cho

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[Steep-slope Schottky diode with cold metal source](https://mdr.nims.go.jp/datasets/ad61bd53-92bc-4f50-90d6-4ceb28d22bc7)

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Steep-slope Schottky diode with cold metal sourceAppl. Phys. Lett. 120, 243506 (2022); https://doi.org/10.1063/5.0097408 120, 243506© 2022 Author(s).Steep-slope Schottky diode with cold metalsourceCite as: Appl. Phys. Lett. 120, 243506 (2022); https://doi.org/10.1063/5.0097408Submitted: 28 April 2022 • Accepted: 06 June 2022 • Published Online: 15 June 2022 Wongil Shin, Gyuho Myeong, Kyunghwan Sung, et al.https://images.scitation.org/redirect.spark?MID=176720&plid=1691476&setID=378288&channelID=0&CID=617080&banID=520579169&PID=0&textadID=0&tc=1&type=tclick&mt=1&hc=f1f4e980f6fdb758ad150527206ab82c09d61755&location=https://doi.org/10.1063/5.0097408https://doi.org/10.1063/5.0097408https://orcid.org/0000-0001-5089-4215https://aip.scitation.org/author/Shin%2C+Wongilhttps://aip.scitation.org/author/Myeong%2C+Gyuhohttps://aip.scitation.org/author/Sung%2C+Kyunghwanhttps://doi.org/10.1063/5.0097408https://aip.scitation.org/action/showCitFormats?type=show&doi=10.1063/5.0097408http://crossmark.crossref.org/dialog/?doi=10.1063%2F5.0097408&domain=aip.scitation.org&date_stamp=2022-06-15Steep-slope Schottky diode with coldmetal sourceCite as: Appl. Phys. Lett. 120, 243506 (2022); doi: 10.1063/5.0097408Submitted: 28 April 2022 . Accepted: 6 June 2022 .Published Online: 15 June 2022Wongil Shin,1 Gyuho Myeong,1 Kyunghwan Sung,1 Seungho Kim,1 Hongsik Lim,1 Boram Kim,1 Taehyeok Jin,1Jihoon Park,1 Kenji Watanabe,2 Takashi Taniguchi,2 Fei Liu,3,a) and Sungjae Cho1,a)AFFILIATIONS1Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea2National Institute for Materials Science, Namiki, Tsukuba, Ibaraki 305-0044, Japan3School of Integrated Circuits, Peking University, Beijing 100871, Chinaa)Authors to whom correspondence should be addressed: feiliu@pku.edu.cn and sungjae.cho@kaist.ac.krABSTRACTToday’s circuit technology requires low-power transistors and diodes to extend Moore’s law. While research has been focused on reducingpower consumption of transistors, low-power diodes have not been widely studied. Here, we report a low-power, thus steep-slope Schottkydiode, with a “cold metal” source. The Schottky barrier between metal electrode and bulk MoS2 enabled the diode behavior, and the steep-slope diode IV curve originated from the change in the density of states of a graphite (cold metal) source with a bias voltage. The MoS2Schottky diode with a cold metal exhibits an ideality factor (g)< 1 for more than four decades of drain current with a sizable rectifying ratio(108). The realization of a steep-slope Schottky diode paves the way to the improvement in low-power circuit technology.VC 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0097408Two-dimensional (2D) van der Waals (vdW) materials areknown as potential candidates for use as next-generation devicesbecause of their unique electronic and optoelectronic properties.1–8Monolayer (ML) transition metal dichalcogenides (TMDC), such asWS2, WSe2, MoSe2, and MoS2, are being widely used to develop elec-tronic devices, including field-effect transistors (FETs), diodes, andsensors, since they have suitable bandgap and high carrier mobility.9–17Among TMDCs, the monolayer MoS2 is most frequently used indeveloping electronic devices owing to its high carrier mobility(>200 cm2/V s).17–20 However, the formation of defects and impuritiesduring the metal deposition process at the metal–semiconductor inter-face is a bottleneck for improving device performance.21–23Because electronic miniaturization and Moore’s law areapproaching their limit, reducing the energy consumption of elec-tronic devices has drawn significant attention.24,25 To reduce energyconsumption, the thermionic limit of the transistor with subthresholdswing (SS)¼ 60mV/dec has been overcome by proposing alternatetransistors, such as the tunnel field-effect transistor (TFET), the nega-tive capacitance field-effect transistor (NC-FET), and the Dirac-sourcefield-effect transistor (DS-FET).26–41 However, the steep-slope diodehas not been studied widely although diodes are key electronic compo-nents along with transistors.In this work, we report a steep-slope MoS2 Schottky diode witha sub-unity ideality factor. The ideality factor (g) represents the steep-ness of the drain current increase with the bias voltage. The idealityfactor g can be obtained from the Schottky diode equation,ID ¼ IS 1� eqVD=gkBTð Þ; (1)where ID and IS are the drain and saturation current, respectively. Byusing cold metal (graphite) as a source electrode in our MoS2 Schottkydiode, we achieved a sub-unity ideality factor for more than four deca-des of drain current with a sizable rectifying ratio (IonIoff > 108). In con-trast to the constant density of states in a regular metallic material,graphite has an abrupt change in density of states near charge neutral-ity points, which results in a super-exponentially decaying carrier den-sity with energy (see supplementary material S1).Figures 1(a)–1(c) show the optical and schematic image of ourMoS2 diode consisting of four components: (i) graphite contact on amonolayer MoS2 with a monolayer hexagonal boron nitride (hBN)spacer between them, (ii) monolayer MoS2 channel, (iii) bulk MoS2drain with direct metal (chromium/gold, Cr/Au) contact, and (iv) top,control (chromium/gold, Cr/Au), and back (platinum, Pt) gate electro-des. To avoid air exposure and oxidation of the MoS2 monolayer,Appl. Phys. Lett. 120, 243506 (2022); doi: 10.1063/5.0097408 120, 243506-1VC Author(s) 2022Applied Physics Letters ARTICLE scitation.org/journal/aplhttps://doi.org/10.1063/5.0097408https://doi.org/10.1063/5.0097408https://www.scitation.org/action/showCitFormats?type=show&doi=10.1063/5.0097408http://crossmark.crossref.org/dialog/?doi=10.1063/5.0097408&domain=pdf&date_stamp=2022-06-15https://orcid.org/0000-0001-5089-4215https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0003-2547-095Xmailto:feiliu@pku.edu.cnmailto:sungjae.cho@kaist.ac.krhttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/https://doi.org/10.1063/5.0097408https://www.scitation.org/doi/suppl/10.1063/5.0097408https://scitation.org/journal/aplwe mechanically exfoliate the MoS2, stack the vdW heterostructures viathe dry transfer method,28,42,43 and encapsulate them using hBN insidean N2-filled glovebox (see supplementary material S2). We used Ramanspectra measurements to confirm the number of layers in MoS2 flakes(see supplementary material S3).44,45 We also used a monolayer MoS2connected naturally with bulk MoS2. Since direct metal deposition maycause damage and change the electronic property of monolayer MoS2,we contacted the monolayer MoS2 using monolayer hBN/graphite atthe source side and Cr/Au metal deposition on the bulk MoS2 (naturallyconnected to the monolayer MoS2) at the drain side. Figure 1(c) showsthe device schematic, and there are three kinds of gate electrodes—platinum backgate (BG), control gate (CG), and topgate (TG). The roleof each gate is little bit different. By controlling the backgate, the Fermilevel of monolayer MoS2 and that of part of graphite contact are raisedor lowered. The control gate can change the Fermi level of graphitecontact. In this experiment, we make the graphite contact p-type or n-type by controlling this control gate. At last, the topgate controls theFermi level of the channel MoS2. We used monolayer hBN to formOhmic contact between graphite and monolayer MoS2. We fabricatedadditional devices to investigate the properties of our electrodes (seesupplementary material S4). Figure 1(d) shows the IV curves of a MoS2monolayer in contact with monolayer hBN/graphite electrodes, whileFig. 1(e) shows IV curves of bulk MoS2 in contact with the Cr/Au metal.The MoS2 device in contact with the monolayer MoS2/graphite showslinear IV curves at all gate voltages, indicating Ohmic contact formation.On the other hand, bulk MoS2 in contact with the Cr/Au metal showsnonlinear IV curves, which prove Schottky contact (see supplementarymaterial S5 and S6).21,46–54Figure 2(a) presents the drain current (ID) vs bias voltage (Vbias)curves of the MoS2 Schottky diode at VBG¼�1.0V, VTG¼þ0.5V,FIG. 1. (a) Optical image of fabricated cold metal source device. (b) Red, light green, and dark green dashed lines indicate graphite, monolayer MoS2, and bulk MoS2, respec-tively. To prevent contamination with chemicals and oxygens, we fully encapsulate the heterostructure with �10 nm thickness of top and bottom hBN. Yellow dashed line indi-cates platinum backgate. We used graphite contact with monolayer hBN as a source and metal contacts on bulk MoS2 as a drain. (c) Schematic of the cold metal sourcedevice. There are three kinds of gate. The topgate is placed on the channel MoS2, and it controls the Fermi level of the channel. The control gate is placed on the graphite con-tact, and it can control the Fermi level of graphite contact. At last, the backgate is placed under the monolayer MoS2 region, and it controls the Fermi level of part of graphitecontact as well as the monolayer MoS2 channel. (d) Ohmic contact behavior of two graphite contacts with monolayer hBN on monolayer MoS2. According to any gate voltages,the characteristic ID-Vbias curves show Ohmic behavior. (e) Non-Ohmic contact behavior of two metal contact on bulk MoS2. All the characteristic ID–Vbias curves show non-Ohmic contact with a full range of VBG. Insets of Figs. 1(d) and 1(e) show the measured device (see supplementary material S4).Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 120, 243506 (2022); doi: 10.1063/5.0097408 120, 243506-2VC Author(s) 2022https://www.scitation.org/doi/suppl/10.1063/5.0097408https://www.scitation.org/doi/suppl/10.1063/5.0097408https://www.scitation.org/doi/suppl/10.1063/5.0097408https://www.scitation.org/doi/suppl/10.1063/5.0097408https://www.scitation.org/doi/suppl/10.1063/5.0097408https://www.scitation.org/doi/suppl/10.1063/5.0097408https://scitation.org/journal/apland VCG¼�0.5V, where the graphite electrode is p-type. Figures2(b) and 2(c) show the corresponding band diagrams at the forwardand reverse bias voltage (on- and off-state). Note that the density ofstates in graphite changes significantly near the charge neutrality point.As the drain bias voltage applied to the graphite decreases negativelyfrom the off-state, a forward bias is applied with on-current flowingthrough the channel, which indicates that the Schottky barrier at theinterface between the metal contact and n-type MoS2 dominates thediode operation. When a negative bias voltage Vbias < 0 is applied tographite, the electrochemical potential of the graphite electrode isincreased by eVbias with respect to that of the metal contact. It is note-worthy that the electrons in the graphite source contributing to the cur-rent injection should have energy between EF in graphite and the greendotted line in the band diagram, which is determined by the bulk MoS2conduction band edge. As the Vbias< 0 increases in the positive direc-tion from the on- to off-state, the electrochemical potential of graphitedecreases with the energy crossing the band edge of graphite movingcloser to the charge neutrality point (CNP). In this case, due to thereduced graphite density of state (DOS) from the on- to off-state(thereby super-exponential decrease in carrier density of the graphitesource), the IV curves from the on- to off-state become steeper than theconventional Schottky diode, and the ideality factor becomes less thanunity at room temperature. Figures 3(a) and 3(b) show the controlgate-dependent diode characteristic curve and ideality factor gave-1dec.The ideality factor gave-1dec becomes less than unity only when graphiteis p-type. When graphite is n-type, the ideality factor gave-1dec becomesgreater than unity as in conventional Schottky diode.Monolayer graphene has a linear density of states to filter highenergy electrons and, thus, has been applied to realize steep slope DSFETs and diodes. Can graphite be applied as a cold electron injectionsource to realize steep switching similarly? To address this questiontheoretically, we compared transmissions of monolayer graphene andgraphite as shown in Fig. 4(a), which are calculated by atomistic quan-tum transport simulations based on the density functional theory cou-pled with nonequilibrium Green’s function (NEGF) theoryimplemented in the Nanodcal package.55 Intrinsic monolayer gra-phene has massless Dirac electrons and a linear energy dispersionaround the Fermi level. Therefore, the transmission of monolayer gra-phene linearly depends on energy. Electrons in graphite are no longermassless and have finite states at the Fermi level. Even though graphitedoes not have a linear energy dispersion, transmission monotonicallyincreases with the energy difference with respect to the Fermi level,which is similar to that of graphene as shown in Fig. 4(a). Switchingproperties of the graphite-MoS2 diode as shown in Fig. 4(b) can beestimated by Landauer–B€uttiker formula,56I ¼ 2ehð1EtopT Eð Þ fL E � EFLð Þ � fR E � EFRð Þð ÞdE; (2)where Etop is the top of channel barrier, fL=R is the Fermi distributionfunction of left/right lead, and EFL and EFR are the Fermi levels of leftand right leads, respectively. Figure 4(c) shows ID vs VD of the graph-ite-MoS2 diode with a p-type graphite. It is found that the ideality fac-tor is smaller than 1 as �0.52V < VD <�0.32V, and the minimumvalue reaches 0.73. In the bias voltage region, current is increased byfour orders of magnitude. The reason for such steep switching is thathigh energy electrons are partly filtered as the top of channel barrieraround the intrinsic Fermi level of graphite as shown in Fig. 4(d). Asthe diode is switched from the off- to on-state as shown in Fig. 4(d),the injected electron density of states around the top of channel barrieris increased simultaneously, which is different from conventionalmetal with constant DOS and makes the diode switch much fasterthan the conventional Schottky diode.FIG. 2. (a) Characteristic ID–Vbias curve at VBG¼�1.0, VCG¼�0.5, and VTG¼þ0.5 V. Ideality factor below unity keeps for 4 decades, and ideality factor for 1 decade is0.7532. (b) Band diagram of the diode off-state. (c) Band diagram of the diode-on state with cold carrier injection from graphite contact.Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 120, 243506 (2022); doi: 10.1063/5.0097408 120, 243506-3VC Author(s) 2022https://scitation.org/journal/aplIn summary, we develop a steep-slope Schottky diode with coldcarrier injection consisting of monolayer MoS2 naturally connected tobulk MoS2 and graphite cold metal contact. The results demonstratedthat using cold metal sources increases the Schottky diode switchingslope. As the cold carrier injected from the cold metal source(graphite), the Schottky diode exhibits an unconventional ideality fac-tor of (g)< 1 for more than three decades of drain current with a size-able rectifying ratio (�108) (see supplementary material S9). Ourdemonstration of the steep-slope Schottky diode paves the way toachieving low-power circuit technology requiring rectifiers.FIG. 3. (a) Characteristic ID vs Vbias curves at different control gate voltage. Under VCG < 0 V, characteristic curves show steep-slope behavior. (b) VCG vs ideality factor (aver-age over one decade). For VCG�þ0.0 V, the ideality factor remained below unity for one decade.FIG. 4. (a) Transmissions of graphite andmonolayer graphene obtained by first-principles quantum transport simulations.The transmission of graphite is normalizedto the monolayer. (b) Simulation model forthe steep slope diode with the cold metalsource of graphite. Vbias is applied tographite contact, and the graphite contactis on the monolayer MoS2 channel. (c)ID–Vbias curve and ideality factor of thegraphite-MoS2 diode with p-type graphiteby theoretical calculations. (d) Band dia-gram for the graphite-MoS2 diode with p-type graphite at on- and off-states.Applied Physics Letters ARTICLE scitation.org/journal/aplAppl. Phys. Lett. 120, 243506 (2022); doi: 10.1063/5.0097408 120, 243506-4VC Author(s) 2022https://www.scitation.org/doi/suppl/10.1063/5.0097408https://scitation.org/journal/aplSee the supplementary material for more details regarding themanuscript.S.C. acknowledges support from National ResearchFoundation of Korea (Grant Nos. 2020M3F3A2A01081899 and2020R1A2C2100258). F.L. acknowledges support from NationalScience Foundation of China (Grant No. 61974003) and the 111Project (Grant No. B18001).AUTHOR DECLARATIONSConflict of InterestThe authors have no conflicts to disclose.Author ContributionsWongil Shin: Data curation (equal); Investigation (equal);Methodology (equal); Writing – original draft (equal); Writing –review and editing (equal). Gyuho Myeong: Data curation (equal);Investigation (equal); Methodology (equal); Writing – original draft(equal). Kyunghwan Sung: Data curation (supporting); Investigation(supporting); Methodology (supporting). Seungho Kim: Data cura-tion (supporting); Investigation (supporting); Methodology (support-ing). Hongsik Lim: Data curation (supporting). Boram Kim: Datacuration (supporting). Taehyeok Jin: Data curation (supporting).Jihoon Park: Data curation (supporting). Kenji Watanabe: Resources(equal). Takashi Taniguchi: Resources (equal). Fei Liu: Formal analy-sis (equal); Writing – original draft (equal). Sungjae Cho:Conceptualization (equal); Formal analysis (equal); Funding acquisi-tion (equal); Project administration (equal); Supervision (equal);Writing – original draft (equal); Writing – review and editing (equal).DATA AVAILABILITYThe data that support the findings of this study are availablewithin the article and its supplementary material.REFERENCES1K. S. Novoselov, V. I. Fal’Ko, L. Colombo, P. R. Gellert, M. G. Schwab, and K.Kim, Nature 490, 192 (2012).2N. O. Weiss, H. Zhou, L. Liao, Y. Liu, S. Jiang, Y. Huang, and X. Duan, Adv.Mater. 24, 5782 (2012).3Q. H. Wang, K. Kalantar-Zadeh, A. 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