Fermi Level Pinning at Electrical Metal Contacts. of Monolayer Molybdenum Dichalcogenides

Transcription

1 Supporting information Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides Changsik Kim 1,, Inyong Moon 1,, Daeyeong Lee 1, Min Sup Choi 1, Faisal Ahmed 1,2, Seunggeol Nam 3, Yeonchoo Cho 3, Hyeon-Jin Shin 3, Seongjun Park 3, and Won Jong Yoo 1, * 1 Samsung-SKKU Graphene Center (SSGC), SKKU Advanced Institute of Nano-Technology (SAINT), 2 School of Mechanical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Korea, 3 Device & System Research Center, Samsung Advanced Institute of Technology (SAIT), 130 Samsung-ro, Yeongtong-gu, Suwon, Gyeonggi-do 16676, Korea These authors contributed equally to this work. *Corresponding author, yoowj@skku.edu 1

2 Supporting Information 1. Pinning factor and charge neutrality level for hole SBH. According to the Schottky-Mott rule, p-type SBH (ϕ Bp ) decreases with increasing metal work function (ϕ m ). φ Bp = χ+ g φ m E (1) Here, χ is electron affinity. E g is band gap of semiconductor. The sum of electron affinity and band gap is valence band maximum. If p-type semiconductor wasn t pinned, the slope of p-type SBH vs. metal work function (S = dϕ Bp /dϕ m ) would be 1. In order to adopt the pinning factor and charge neutrality level (CNL, ϕ CNL ) for p-type SBH, we used following equations: φ S ( φ φ ) + ( χ + E g φ = Sφ + b (2) Bp = m CNL CNL ) m For S = 1, p-type SBH can be determined by the Schottky-Mott rule: ϕ Bp = E g + χ ϕ m. For S = 0, p-type SBH can be obtained from ϕ Bp = E g + χ ϕ CNL. Based on this equation, we calculated the pinning factor and the CNL of MoTe 2. 2

3 Supporting Information 2. Fabrication process Figure S1. (a) Monolayer MoS 2 or MoTe 2 exfoliated onto a 285 nm thick SiO 2 layer-covered Si wafer. (b) 2 µm wide channel defined by plasma etching of SF 6 /O 2. (c) Metal electrodes formed for the TLM measurement. (d) Back- to- back Schottky junction consisting of a reverse Schottky diode as source, a simple resistor as the channel, and a forward Schottky diode as the drain. 3

4 Supporting Information 3. Material details MoS 2 : Synthetic Molybdenum Disulfide obtained from 2D Semiconductors ( Bulk MoS 2 60 Intensity (a.u.) Raman shift (cm -1 ) Figure S2. (a) Raman spectroscopy: The bulk MoS 2 flakes show two prominent Raman peaks at 384cm -1 (E 2g - in plane-) and 408cm -1 (A 1g out-of-plane) and the FWHM (full-widthat-half-maximum) is less than 4-5cm -1 showing highly crystalline nature. PL intensity (a.u.) Photon energy (ev) Mono MoS2 Figure S2. (b) Photoluminescence (PL): For the monolayer of MoS 2, molybdenum disulfide possess direct band-gap is observed at 1.85eV. 4

5 MoTe 2 : MoTe 2 (Molybdenum Ditelluride) obtained from HQgraphene ( Figure S2. (c) Raman spectroscopy: The A 1g mode peak is at 171 cm -1 and E 1 2g mode peak is at 234 cm -1, for bulk MoTe 2 flakes. 5

6 Supporting Information 4. Extraction of Schottky barrier heights Derivation of 2D and 3D thermionic emission equations Thermionic emission: Carrier concentration: Fermi-Dirac distribution function: 3D density of states & thermionic emission: = 2 exp = E > E C, E C E f >>kt = 4 2 h =2 2 h exp = exp exp, 2D density of states & thermionic emission: = 4 h = 4 h = 4 h exp = exp exp, = 8 Based on electron confinement of 2D structure, thermionic emission equation can be modified by the energy independent density of states. The current I defined by 2D thermionic emission equation employs the reduced power law from T 2 to T 3/2. The SBH for 2D transport contact is extracted from the slope of a linear fit to ln (I/T 3/2 ) as a function of 1/T. The SBH difference between 3D and 2D thermionic emission is ~ 0.01eV. h 6

7 Figure S3. (a) Back-to-back Schottky junction consisting of a reverse Schottky diode at the source, a simple resistor at the channel, and a forward Schottky diode at the drain. (b) Schematic plot of the thermionic emission properties, illustrating the dependence of SBH on gate voltage and temperature. Inset diagrams describe carrier transport mechanisms depending on temperature and VG. When applied VD is high, drain side tilted down. The end point of linearity is the true SBH at VFB. Under VFB, effective SBH increases linearly due to VG control. Note that unlike conventional silicon-based devices, in which conventional Schottky diodes are controlled by the Schottky barrier on only one side of the device, semiconductor devices fabricated using 2D materials cannot act as efficient Schottky diodes because both the source and drain metal electrodes create Schottky contacts, as illustrated in schematic diagram of figure S3a. Figure S3b plots the SBH and electron transport induced by applying a gate voltage (V G ) bias, assuming the presence of thermal energy. V G and the temperature affected carrier transport, which proceeded mainly through tunneling or thermionic emission mechanisms at 7

8 the interface between the metal and the 2D material. The thermal effects were highlighted by measuring the carrier transport in the high-temperature range K, under which conditions thermionic emission occurs more actively than at low temperatures of K. 8

9 Supporting Information 5-1. SBH and energy band values of MoS 2 Metal [ev] e-sbh [ev] #1 #2 #3 Average Ti Cr Au Pd Pinning factor 0.11 Table S1. SBH of MoS 2 CB [ev] 4.28 S 0.11 E G [ev] 1.88 b VB [ev] 6.16 CNL [ev] Table S2. Energy levels, pinning factor, y-intercept, and CNL of MoS 2 We fabricated 3 devices with different metal contacts and used average value for analyzing Fermi level pinning, as shown in table S1. 9

10 Supporting Information 5-2 SBH and energy band values of MoTe 2 Metal Low T range High T range h-sbh [ev] e-sbh [ev] h-sbh [ev] e-sbh [ev] Ti Cr Au Pd Pinning factor Table S3. SBH of MoTe 2 CB [ev] 3.71 S E G [ev] 1.28 b VB [ev] 4.99 CNL [ev] Table S4. Energy levels, pinning factor, y-intercept, and CNL of MoTe 2 Unlike MoS 2 devices, MoTe 2 devices show ambipolar property with strong p-type characteristics and different temperature dependence on carrier type. The linearity of SBH is also harder to define than MoS 2. This is because thermionic currents of both carriers are overlapped by tunneling current of each carrier. Specially, in MoTe 2 /Au, no end point of linearity is observed at high temperature range for hole SBH due to V G limitation. In Table S4-2, we gathered hole and electron SBH for different temperature range. In order to describe hole Fermi level pinning, we used hole SBH at low temperature range. This indicates that temperature dependent transfer curves seem to have limitation to obtain the SBH for ambipolar materials. 10

11 Supporting Information 6. Detailed references Method Ref # Thickness Metal work function SBH Pinning factor Reference Transistor temperature dependence Diode Temperature Transistor Vd intercept bulk Sc bulk Ti bulk Ni S. Das et al Nano Letters 13(1) bulk Pt bulk Au bulk Pd N. Kaushik et al Applied Physics Letters 105(11) ~7 Ni ~7 Pd ~7 Ni ~7 Pd S. Bhattacharjee et al IEEE Electron Device Letters 37(1) bulk Mo G. Yoo et al IEEE Electron Device Letters 36(11) layer Ti layer Pt S. Lee et al Nano Letters 16(1) layer Ti nm Co ferro nm Co ferro A. Dankert et al ACS Nano 8(1) nm Gr J. Yoon et al Small 9(19) nm bulk bulk mono Ti Cr Au Ti nm bulk bulk mono Ti Cr Gr/AU Ti S. Kim et al Applied Physics A 117(2) K. Sano et al Japanese Journal of Applied Physics 55(3) D. Qiu et al Scientific Reports W. Liu et al ACS Nano 9(8) ~15 Ti ~15 Ni ~15 Au N. Kaushik et al ACS applied materials & interfaces 8(1) ~15 Pd Gr Ti 0.4 J. Kwak et al Nano Letters 14(8) mono Ti B. Liu et al ACS Nano 8(5) tri Py ferro mono Py ferro 0.08 W. Wang et al Scientific Reports mono Py ferro CAFM Pt tip Pt F. Giannazzo et al Physical Review B 92(8) Table S5. Experimentally obtained SBH for bulk and monolayer MoS 2 In this Table, the test method, thickness, device structure, and obtained SBH and pinning factor are shown in detail. Referring to this table, in this work we gathered bulk SBH by using the same method that measures drain current at flat band voltage for various temperatures. 11

12 Method Ref # Thickness Metal work function SBH Pinning factor Reference mono Mo mono Ti mono In J. Kang Physical Review X 4(3) mono Au mono Pd mono Sc mono Mg mono Al mono Ti mono Cr mono Mo mono Ru Y. Guo et al ACS Applied Materials & Interfaces 7(46) mono Co mono Pd mono Ni mono Pt mono MoO mono Al mono Ag mono Au mono Pd C. Gong et al Nano Letters 14(4) mono Ir mono Pt mono Sc mono Sc mono Ti mono Ti mono Ag mono Ni mono Au mono Pt bi Sc H. Zhong et al Sceintific Reports bi Sc bi Ti bi Ti bi Ag bi Ni bi Au bi Pt Calculation Table S6. Theoretical SBH of MoS 2 Calculation 18 mono Sc mono Al mono Cr mono Ru mono Co mono Pd mono Pt Y. Guo et al ACS Applied Materials & Interfaces 7(46) Table S7. Theoretical SBH of MoTe 2 This table shows SBHs obtained from theoretical calculation. Interestingly, all the obtained pinning factors are around 0.3. From this table, SBH obtained only from monolayer 2D materials are used to compare with our data. In case of MoTe 2, we used hole SBH instead of electron SBH. 12

13 Supporting Information 7. The relation between SBH and R c Figure S4. The SBH and R c as function of V G. (a) MoS 2 /Ti contact, (b) MoS 2 /Cr contact, (c) MoS 2 /Au contact, (d) MoS2/Pd contact. The relation between R c and SBH is approximately linear for all the metals, as shown in figure S4. Only for the titanium, the trend is slightly deviating from the linear line, perhaps due to the formation of TiO 2 from the reaction of titanium and ambient oxygen. 13

14 Supporting Information 8. SBH and R c against V G before and after chemical doping Figure S5. The transfer curves and R c -V G, SBH-V G are plotted with before and after doping. (a), (c) are MoS 2 and (b), (d) are MoTe 2. Here, these chemical doping techniques, based on surface charge transfer dopants, can reduce R c and change carrier transport at the interface by affecting the area beneath metal contact. Their affected depth is around 1.5~3 nm which is deeper than the thickness of monolayer TMDCs. Figures S4 shows the R c against V G of monolayer MoS 2 and MoTe 2 FETs contacted with Cr. The R c and polarities are controlled well via chemical doping. The pristine MoS 2 (black color) was prepared first, and a 20 mm BV solution was then applied to induce degenerated n + doping (red color). A 20 mm AuCl 3 solution was used to induce a p-type doped state (blue color). We confirmed that a p-type transfer curve was clearly obtained, although the degree of doping was weaker than that obtained from BV due to the pristine 14

15 MoS 2 was n-type in nature. Figure S4b indicates that R c for the pristine MoS 2 was kωµm across the entire V G range. After BV doping, R c decreased significantly, to 10 kωµm at V G = 40. After AuCl 3 doping R c was measured to be 56 kωµm at V G = 40. MoS 2 doped with BV and AuCl 3 provided low values of R c, but both R c are still high due to thickness dependence. Furthermore, the SBH decreased, as shown in Table S8. The SBH of a pristine MoS 2 -Cr FET was found to be 0.1 ev (SBH for electrons), and this value decreased to 0.01 ev (SBH for holes) after AuCl 3 doping. The properties of a MoTe 2 FET in contact with Cr are shown in figure S4e. After BV doping, pristine p-type curve changed to n-type with high on/off ratio. The R c decreased to 3 4 kωµm after AuCl 3 doping. This large decrease was attributed to a change in the SBH of MoTe 2, which decreased significantly from 0.15 to below 0 ev after AuCl 3 doping. The negative SBH indicated that no physical barrier formed at the interface between MoTe 2 and Cr, thereby providing a low R c of 3 4 kωµm across the entire V G range. V G MoS 2 MoTe 2 Pristine n-doping p-doping Pristine n-doping p-doping Contact resistance [kω μm ] -40 V V SBH FB < 0 Table S8. A summary table listing the contact resistance (R c ) and SBH values of the devices. 15