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End-Bonded Metal Contacts on WSe2 FieldEffect Transistors Chun-Hao Chu,† Ho-Chun Lin,‡,§ Chao-Hui Yeh,† Zheng-Yong Liang,† Mei-Yin Chou,‡,§ and Po-Wen Chiu*,†,‡,∥ †
Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan § Department of Physics, National Taiwan University, Taipei 10617, Taiwan ∥ Frontier Research Center on Fundamental and Applied Science of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan
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‡
S Supporting Information *
ABSTRACT: Contact engineering has been the central issue in the context of high-performance field-effect transistors (FETs) made of atomic thin transition metal dichalcogenides (TMDs). Conventional metal contacts on TMDs have been made on top via a lithography process, forming a top-bonded contact scheme with an appreciable contact barrier. To provide a more efficient pathway for charge injection, an end-bonded contact scheme has been proposed, in which covalent bonds are formed between the contact metal and channel edges. Yet, little efforts have been made to realize this contact configuration. Here, we bridge this gap and demonstrate seeded growth of end-bonded contact with different TMDs by means of chemical vapor deposition (CVD). Monolayer WSe2 FETs with a CVD-grown channel and end contacts exhibit improved performance metrics, including an on-current density of 30 μA/μm, a hole mobility of 90 cm2/ V·s, and a subthreshold swing of 94 mV/dec, an order of magnitude superior than those of top-contact FET counterparts that share the same channel material. A fundamental NOT logic gate constructed using top-gated and end-bonded WSe2 and MoS2 FETs is also demonstrated. Calculations using density functional theory indicate that the superior device performance stems mainly from the stronger metal−TMD hybridization and substantial gap states in the end-contact configuration. KEYWORDS: TMD, high mobility, field-effect transistors, end contact, top contact, Schottky barrier
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devices are often obscured by a large parasitic contact resistance.2,4−7 As is the case with carbon nanotubes, a key obstacle to the ultrascaled TMD transistors is forming lowresistance and scalable contacts.8,9 With the nature of atomic thickness in the vertical direction, metal contacts on TMDs are generally applied on top, forming so-called top-bonded contacts. With this contact scheme, the contact resistance strongly depends on the contact area, indicating a weak metal− TMD coupling. The contact resistance increases rapidly as the contact area shrinks. Despite the weak metal−TMD coupling, interface dipoles develop as a result of charge redistribution in
ransition metal dichalcogenides (TMDs) constitute a class of semiconductor materials that can be used as an alternative to silicon for the continuous scaling of devices down to the atomic scale.1 These layered semiconductors possess a variety of fascinating properties for transistor applications, including pristine interfaces without out-of-plane dangling bonds, which ensures a uniform electrostatic potential landscape for high mobility motion of charge carriers, atomic-scale thinness (99.999%) at a
CONCLUSIONS In conclusion, we demonstrate end-bonded contact geometry on WSe2 FETs through direct growth of the channel material from the edge of predefined metal contact. The resulting topcontact FETs show device performance superior to that of conventional top-contact geometry. The experimental findings reflect the fact that the top contact forms a contact barrier, through which the charge injection efficiency is low, rationally attributed to the weak orbital overlap predicted by the density functional theory. The finite thickness of the WO3−x bottom seed layer is translated into multilayer WSe2 at the contact side. This multilayer termination at the contact can be regarded as another advantage in addition to the stronger hybridization between metal and WSe2 electronic states that facilitates charge injection. The main challenge to form an end-bonded contact with TMD lies in the high temperature process, at which most of thin metal films become highly volatile, in particular, when selenium or sulfur is dissolved into the metal. Considering the compliance of the metal work function with the TMD, the choice of metal types is limited. As such, an effective way to cope with the volatility of the predefined 8152
DOI: 10.1021/acsnano.9b03250 ACS Nano 2019, 13, 8146−8154
Article
ACS Nano pressure of 3 kg/cm2. After overnight exposure to oxygen, a highquality ultrathin AlOx was formed, which acts as a gate dielectric. Then, a 300 nm thick Al top gate was deposited using thermal evaporation. Having formed a T-shape Al top gate, thermal evaporation was applied for the self-aligned drain/source metallization with Pd/Au (12 nm/28 nm). The gate capacitance of the AlOx thin film was measured to be ∼930 nF/cm2. For the ionic liquid gate, a small droplet of the EMIM-TFSI IL (Sigma-Aldrich) was applied onto the devices using a micropipette in a glovebox, covering the WSe2 active channel and the source, drain, and gate electrodes. The gate capacitance of the ionic liquid was ∼7 μF/cm2. Measurements and Characterizations. X-ray photoemission spectra of CVD-grown WSe2 were obtained using a VG Scientific ESCALAB 250 system. The X-ray source was generated from the Al target (1486.8 eV) with a pass energy of 20 eV, and the takeoff angle for the collection of photoelectrons was 90° from the surface normal. For Raman spectra, a high-resolution micro-Raman spectrometer (DXR, Thermo Scientific) equipped with a motorized sample stage was used to acquire the spectra. The excitation wavelength was 532 nm (2.33 eV) with a power below 0.1 mW to avoid laser-induced heating and damage. The laser spot size at the focus was around 500 nm in diameter under a 100× objective lens. For photoluminescence spectra, a high-resolution micro-PL spectrometer (HORIBA, iHR550) equipped with a motorized sample stage was used to acquire the spectra. Computational Methods. First-principles calculations were performed using the Vienna ab initio simulation package (VASP)42−44 with the projector-augmented wave method.45,46 A plane-wave energy cutoff of 400 eV was employed. A 6 × 6 × 1 and 5 × 1 × 1 k-mesh was used for the top-contact and end-contact configurations, respectively. The spin−orbit interaction was not included due to the size of the supercell. We used the local density approximation47 for structural optimization with a supercell containing an 18 Å vacuum in order to simulate the 2D system. The lattice constant of monolayer WSe2 used in the simulation was 3.28 Å.
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b03250. Details of WSe2 characterization using SEM, TEM, and XPS; device characterizations for gate capacitance; atomic model for band structure calculations (PDF)
AUTHOR INFORMATION Corresponding Author
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[email protected]. ORCID
Zheng-Yong Liang: 0000-0003-3970-3849 Po-Wen Chiu: 0000-0003-4909-0310 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We thank the Ministry of Science and Technology (MOST) Taiwan Grants MOST 107-2119-M-007-011-MY2, MOST 106-2119-M-007-008-MY3, and MOST 106-2628-M-007-003MY3 as well as Academia Sinica (AS) Grant AS-TP-106-A07. REFERENCES (1) Liu, L.; Kumar, S. B.; Ouyang, Y.; Guo, J. Performance Limits of Monolayer Transition Metal Dichalcogenide Transistors. IEEE Trans. Electron Devices 2011, 58, 3042−3074. 8153
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DOI: 10.1021/acsnano.9b03250 ACS Nano 2019, 13, 8146−8154
