Phase-engineered low-resistance contacts for ultrathin ​MoS2 transistors

Journal name:
Nature Materials
Volume:
13,
Pages:
1128–1134
Year published:
DOI:
doi:10.1038/nmat4080
Received
Accepted
Published online
Corrected online

Abstract

Ultrathin ​molybdenum disulphide (​MoS2) has emerged as an interesting layered semiconductor because of its finite energy bandgap and the absence of dangling bonds. However, metals deposited on the semiconducting 2H phase usually form high-resistance (0.7 kΩ μm–10 kΩ μm) contacts, leading to Schottky-limited transport. In this study, we demonstrate that the metallic 1T phase of ​MoS2 can be locally induced on semiconducting 2H phase nanosheets, thus decreasing contact resistances to 200–300 Ω μm at zero gate bias. Field-effect transistors (FETs) with 1T phase electrodes fabricated and tested in air exhibit mobility values of ~50 cm2 V−1 s−1, subthreshold swing values below 100 mV per decade, on/off ratios of >107, drive currents approaching ~100 μA μm−1, and excellent current saturation. The deposition of different metals has limited influence on the FET performance, suggesting that the 1T/2H interface controls carrier injection into the channel. An increased reproducibility of the electrical characteristics is also obtained with our strategy based on phase engineering of ​MoS2.

At a glance

Figures

  1. 1T and 2H phases of MoS2.
    Figure 1: 1T and 2H phases of ​MoS2.

    a,b, Crystal structures of the 2H and 1T phases, respectively. In the upper diagram, trigonal prismatic (a) and octahedral (b) coordinations are shown. The lower panel shows the c-axis view of single-layer TMD with trigonal prismatic (a) and octahedral (b) coordinations. Atom colour code: purple, metal; yellow, chalcogen. c, High-resolution transmission electron microscope image of an atomically thin phase boundary (indicated by the arrows) between the 1T and 2H phases in a monolayered ​MoS2 nanosheet. Scale bar, 5 nm. d, Photoluminescence map of a triangular monolayered ​MoS2 nanosheet. The left side of the triangle is the 2H phase, whereas the right side was converted to the 1T phase. The 2H phase shows noticeably brighter PL than the 1T phase. e, Electrostatic force microscopy phase image of a monolayered ​MoS2 nanosheet showing the difference between locally patterned 2H (bright colour) and 1T (dark colour) phases. Scale bars in d,e are 1 μm. f, XPS spectra showing the Mo3d and S2s peaks of the 1T and 2H phases of ​MoS2. Typical experimentally measured spectra are shown in black and fits are shown in red (for the 2H phase component) and green (for the 1T phase component). The lower curve is 100% 2H phase, whereas the top curve can be fitted with both 1T and 2H phase components.

  2. Contact resistance of 1T and 2H phases.
    Figure 2: Contact resistance of 1T and 2H phases.

    ad, Resistance versus 2H channel lengths for ​Au deposited directly on the 2H phase (a,b) and on the 1T phase (c,d). Extrapolation of the red lines yields contact resistances (Rc) of 1.1 kΩ μm for 2H (b) and 0.2 kΩ μm for 1T (d) contacts at zero gate bias. Inset in d shows the percentage decrease in contact resistance with gate bias. Optical microscope images and device schematics are also shown. Scale bars in device photos are 5 μm. The channel in 1T electrode devices is shorter than in 2H devices owing to the diffusion of ​butyl lithium into the masked region. The error bars in a,c result from averaging of least five measurements on at least three devices. e,f, Drain current (Id) characteristics of back-gated FETs for 0–1 V drain–source voltages (Vds) and gate–source voltages Vgs ranging from −30 V to 30 V, showing Schottky behaviour for ​gold directly onto the 2H phase (e) and linear behaviour for the 1T electrodes (f).

  3. Properties of field-effect transistors with 1T and 2H contacts.
    Figure 3: Properties of field-effect transistors with 1T and 2H contacts.

    a,b, Transfer characteristics of bottom- and top-gated devices, respectively, measured at Vds = 1 V. (Logarithmic scale on the left and linear scale on the right.) Blue curves represent devices with 1T phase electrodes and the black curves are with ​Au on the 2H phase. The red curve in a is the Id 1T channel device, showing an absence of gate modulation owing to the metallic character of the 1T phase. The device width was 1.4 μm and device length was 1.2 μm. The linear fits used to extract the subthreshold swing values are also shown. c,d, Output characteristics of devices up to Vds = 5 V and Vgs ranging from −10 V to 30 V, showing reasonable saturation for ​Au (c) and 1T electrodes (d).

  4. Influence of the metal electrode work function on FET properties.
    Figure 4: Influence of the metal electrode work function on FET properties.

    a,b, Transfer (with Vds ranging from 0.5 V to 2.0 V) and output (with Vgs ranging from −30 V to 30 V) characteristics, respectively, for devices with ​Pd deposited on top of 1T electrodes. c,d, Transfer and output characteristics, respectively, for devices with ​Ca deposited on top of 1T electrodes. The device properties are not substantially influenced by the work function of the metals used.

Change history

Corrected online 12 September 2014
In the version of this Article originally published online, for Fig. 1c, the size given for the scale bar was incorrect; it should have been '5 nm'. This error has now been corrected in all versions of the Article.

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Author information

Affiliations

  1. Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, USA

    • Rajesh Kappera,
    • Damien Voiry &
    • Manish Chhowalla
  2. MPA-11 Materials Synthesis and Integrated Devices, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

    • Sibel Ebru Yalcin,
    • Brittany Branch,
    • Gautam Gupta &
    • Aditya D. Mohite

Contributions

M.C. conceived the idea, designed the experiments, analysed the data and wrote the manuscript. R.K., D.V. and A.D.M. conceived the idea and designed the experiments with M.C., synthesized the materials, fabricated the devices, made the measurements and analysed the data. S.E.Y. performed the AFM, EFM and PL mapping measurements. B.B. performed the gate dielectric depositions. G.G. helped to analyse the data and assisted in optimizing the 1T phase transformation chemistry. All the authors have read the manuscript and agree with its content.

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