Fig. 1. (Color online) Atomic structure of representative 2D semiconductor materials and phase engineered contacts. (a, b) Top and side view of 2H stacked structure of 2D semiconductor. (c) Device model of 2D SCs based device. (d–f) Different types of metal–semiconductor junction with contact interface.

Fig. 2. (Color online) (a, b) 3D top contact and 3D edge contact configuration of metal-2D SCs contact. (c, d) 2D top contact and 2D edge contact of metallic 2D-SCs contact geometries.

Fig. 3. (Color online) Schematic diagram of a conventional Si-based NMOS FET.

Fig. 4. (Color online) (a) Schematic diagram of various metal–MoS₂ band alignments along with work function of metal (Ф_M). The Fermi level of metal is lined up close to conduction band of MoS₂. The left-hand part in Fig. 4(a) is the actual Ф_SB extracted from experiment, which is different from the theoretical value calculated by the difference of Ф_M (right-hand part in Fig. 4(a)) and electron affinity (χ) of MoS₂. For example, theoretical values of Ф_SB for Sc–MoS₂ and Pt–MoS₂ contact are about –0.7 and 1.4 eV, respectively. However, the actual Ф_SB obtained from experiment is only 30 and 230 meV, indicating that there is a strong Fermi level pinning effect at the interface of metal and 2D SCs. (b) Energy band diagrams of MoS₂ with and without 2D metallic materials.

Fig. 5. (Color online) Metal contact investigation of MoS₂ device. (a) Schematic diagram of back gated MoS₂ transistor. (b, c) SEM and AFM images of device with MoS₂ thickness of 3 nm. (d) The expected position of the metal Fermi level across the band of MoS₂, where the Ф_SB was decided by the Schottky–Mott rule. (e) Transfer characteristics of MoS₂ device with different contact metal. The inset is the actual position of Fermi level of from experimental date. (f) Temperature-dependent transfer curves with Ni contact, where the contribution of thermionic emission and thermally assisted tunneling are marked. (g) Arrhenius-type plot from (a). (h) Extracted actual Ф_SB for Ni-contacted MoS₂ device. (i) The relationship of Ф_SB with metal work function^[86]. Copyright 2013, American Chemical Society.

Fig. 6. (Color online) In contacted MoS₂ transistors. (a) Device mode of MoS₂ based back-gated device. The electrodes consist of 10 nm In layer capped with 100 nm Au. (b) Cross-sectional annular dark field (ADF) scanning transmission electron microscopy (STEM) of interface of In-MoS₂. (c, d) Extracted contact resistance using transmission line method for MoS₂ with thickness of 0.7 and 8.1 nm, respectively. (e, f) Comparison of contact resistance versus carrier concentration with different contact materials from literatures. (g, h) Transfer characteristics of device with monolayer MoS₂ and temperature-dependent transfer curves for 8 nm MoS₂ device. (i) Output curves with linear relationship indicates ohmic contact. (j) Extracted Ф_SB with value of 110 meV, indicates the ideal contact between the interface of In and MoS₂, which is nearly the same as the value decided by the Schottky–Mott rule. The inset is the energy band diagram of MoS₂ and In^[87]. Copyright 2019, Nature Publishing Group.

Fig. 7. (Color online) Transferred metal electrode contacted MoS₂ FET. (a–c) Cross-sectional atomic mode and optical images of MoS₂ device with transferred Au and the transferred Au mechanically released. (d–f) Cross-sectional schematics and optical images of MoS₂ device with traditional electron-beam-deposited Au and the deposited Au mechanically released. (g, h) Cross-sectional schematics and TEM images of transferred and evaporated Au. (i, j) Transfer curves of MoS₂ devices with different transferred and deposited metals. (k) Experimentally extracted Ф_SB for different transferred and evaporated metals^[60]. Copyright 2018, Nature Publishing Group.

Fig. 8. (Color online) Edge contact investigation of MoS₂ device. (a) Schematic diagram of hBN encapsulated MoS₂ device with graphene edge contact. (b–d) Temperature dependent output curves, contact resistance and temperature dependent Hall mobility, respectively^[66]. Copyright 2016, Nature Publishing Group (e) Schematic of Al₂O₃ capped edge contacted MoS₂ device. (f–h) Output curves, transfer characteristic and mobility with different drain voltage for Al₂O₃ capped MoS₂ device. (i) Schematic of h-BN capped edge contacted MoS₂ device. (j–l) Output curves, transfer characteristic and mobility with different drain voltage for hBN capped MoS₂ device^[67]. Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 9. (Color online) Contact engineering of vertical 2D metallic-2H WSe₂ heterostructure. (a) Sequential growth scheme of vertical WSe₂ and WT₂ heterostructure. The inset is the optical images of WSe₂ and WTe₂. (b) Device mode of WSe₂ with and without contact. (c, d) Output curves and temperature-dependent conductivity of WSe₂ device with WTe₂ and Ti contact. (e) Built-in potential energy versus back-gated voltage for Ti and WTe₂ contacted WSe₂ devices. (f) Ф_SB height with the function of metal work function^[76]. Copyright 2013, American Chemical Society. (g, h) Atomic structure of van der Waals epitaxy growth of 2D metallic-SCs heterostructure and optical image of NbTe₂-WSe₂ heterostructure. (i, j) Schematic model of WSe₂ device with NbTe₂ and Ti contact. (k–n) Current–voltage and transfer characteristics of WSe₂ device with and without NbTe₂ contact. The inset is optical images of fabricated devices^[77]. Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 10. (Color online) Contact engineering of lateral graphene–MoS₂ and VS₂–MoS₂ heterostructures. (a) Schematic diagram of MoS₂–graphene lateral heterostructure. (b, c) Transfer and output curves of graphene contacted MoS₂ device. (d) Statistics of obtained mobility from graphene contacted MoS₂ device^[91]. Copyright 2016, Nature Publishing Group. (e) Schematic fabrication of the MoS₂–graphene lateral heterostructure. (f, g) Representative transfer and output curves of graphene contacted MoS₂ deice. (h) Total, contact and channel resistance of graphene contacted MoS₂ device^[92]. Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (i) Device schematic of VS ₂ contacted MoS₂. (j) I_ds–V_ds curves of MoS₂ device with VS₂ and Ni contact. (k) Extracted Ф_SB of VS₂ and Ni contacted MoS₂ as a function of V_bg. (l) Contact resistance of Ni and VS₂ contacted MoS₂ devices^[75]. Copyright 2018, American Chemical Society.

Fig. 11. (Color online) Electron donation, laser or Ar plasma induced metallic phase as contact. (a) Electrostatic force microscopy phase image of 1T–2H–1T lateral MoS₂ heterostructure. (b–d) Resistance, output and transfer curves of 2H phase MoS₂ with metallic 1T phase MoS₂ contact. The inset is the device model of MoS₂ device with Au and 1T metallic MoS₂ contact^[80]. Copyright 2014, Nature Publishing Group. Laser induced phase transition of metallic 1T’-MoTe₂. (e) Atomic structure and laser treated process. (f) Device schematic with metallic 1T’-MoTe₂ contact. (g) Output characteristic of MoTe₂ device with 1T’-MoTe₂ contact. (h) Field-effect mobility as a function of temperature of MoTe₂ device with Au and metallic MoTe₂ contact₂ contact^[80]. Copyright 2015, American Association for the Advancement of Science (AAAS). (i) Atomic structure and schematic representation of plasma-treated process. (j) Device models with Au or 1T/2H phase MoS₂ contact. (k, l) Output and transfer curves with different Au and 1T/2H phase MoS₂ contact^[81]. Copyright 2017, American Chemical Society.

Fig. 12. (Color online) Intercalation induced phase transition of SnS₂. (a) and (b) Atomic structures of pristine SnS₂, Cu and Co intercalated SnS₂. (c–f) Optical images of pure SnS₂, Cu, Co and Cu–Co intercalated SnS₂. (g) Temperature-dependent resistance of SnS₂, Cu–SnS₂, Co–SnS₂ and few layer graphene, where the Cu–SnS₂ presents semiconductor behavior while the Co–SnS₂ behaves as a metal like graphene. (h) Transfer curves of Cu–SnS₂ with p-type characteristic. (i) I–V curves of Co–SnS₂ contacted SnS₂ and Cu–SnS₂ devices. (j) SnS₂/Cu–SnS₂ junction with and without metallic Co–SnS₂ contact^[82]. Copyright 2018, Nature Publishing Group.

Fig. 13. (Color online) General scalable synthesis of precisely controlled nucleation position and growth process of 2D metallic-SCs heterostructures. (a) Schematic process of selectively patterned periodic defect arrays of 2D SCs (MoS₂, WSe₂ and WS₂) to grow 2D metallic materials (VSe₂, VS₂, CoTe₂, NiTe₂ and NbTe₂). (b) Schematic diagram of VSe₂ growth on patterned sites of WSe₂. (c–e) Optical images of periodical arrays of 2D metallic materials on WSe₂. (f) Optical image of VSe₂ contacted WSe₂ FET periodical arrays. (g–j) Electrical characterizations of the VSe₂ contacted WSe₂ FET^[101]. Copyright 2020, Nature Publishing Group.