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Thickness Considerations of Two-Dimensional Layered Semiconductors for Transistor Applications.

Zhang Y, Li H, Wang H, Xie H, Liu R, Zhang SL, Qiu ZJ - Sci Rep (2016)

Bottom Line: The decrease in Ion/Ioff is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm.This excellent agreement confirms that multilayer-MoS2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate Ion/Ioff.Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of ASIC and System, School of Information Science and Technology, Fudan University, Shanghai 200433, China.

ABSTRACT
Layered two-dimensional semiconductors have attracted tremendous attention owing to their demonstrated excellent transistor switching characteristics with a large ratio of on-state to off-state current, Ion/Ioff. However, the depletion-mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given Ion/Ioff as an acceptable specification. Identifying the optimum thickness range is of significance for material synthesis and device fabrication. Here, we systematically investigate the thickness-dependent switching behavior of transistors with a wide thickness range of multilayer-MoS2 films. A difference in Ion/Ioff by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in Ion/Ioff is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm. For t larger than 100 nm, Ion/Ioff approaches unity. Simulation using technical computer-aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of Ion/Ioff with t. This excellent agreement confirms that multilayer-MoS2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate Ion/Ioff. Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.

No MeSH data available.


(a) Schematic representation and (b) Optical image of the field-effect transistor based on multilayer MoS2.
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f1: (a) Schematic representation and (b) Optical image of the field-effect transistor based on multilayer MoS2.

Mentions: A schematic representation of a back-gate multilayer-MoS2 transistor used in our work is shown in Fig. 1a, whereas a typical top view photomicrograph of a fabricated device is given in Fig. 1b. Isolated MoS2 flakes on the SiO2/Si substrate were exfoliated from a bulk MoS2 crystal using a conventional mechanical exfoliation technique32. The sample preparation and device fabrication are detailed in Methods. The thickness of different flakes, t, was measured by means of atomic force microscopy (AFM), as illustrated in Fig. 2. Only one channel length of 10 μm is used for all devices and it is defined by the spacing of a Cu grid shadow mask. However, the channel width that is determined by the width of the flakes varies in the range of 2–60 μm as a result of the stochastic nature of the MoS2 exfoliation process. The source and drain metal used in our devices is 50 nm Au with a 5 nm thick Ti adhesion layer. The Ti adhesion layer that is in intimate contact with MoS2 has a work function of ∼4.3 eV, which is very close to the conduction band edge of thin-layer MoS2933. Furthermore, Ti is a transition metal with its d-electron orbitals mixing favorably with the 4d states of Mo and resulting in an increase in the density of states at the Fermi level and a strong Fermi level pinning at the contact93435. Therefore, this favorable interface geometry is expected to facilitate a good chemical bonding and allow for a maximized electron injection at the source/drain contacts with an increased overlap between the states at the interface.


Thickness Considerations of Two-Dimensional Layered Semiconductors for Transistor Applications.

Zhang Y, Li H, Wang H, Xie H, Liu R, Zhang SL, Qiu ZJ - Sci Rep (2016)

(a) Schematic representation and (b) Optical image of the field-effect transistor based on multilayer MoS2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4940740&req=5

f1: (a) Schematic representation and (b) Optical image of the field-effect transistor based on multilayer MoS2.
Mentions: A schematic representation of a back-gate multilayer-MoS2 transistor used in our work is shown in Fig. 1a, whereas a typical top view photomicrograph of a fabricated device is given in Fig. 1b. Isolated MoS2 flakes on the SiO2/Si substrate were exfoliated from a bulk MoS2 crystal using a conventional mechanical exfoliation technique32. The sample preparation and device fabrication are detailed in Methods. The thickness of different flakes, t, was measured by means of atomic force microscopy (AFM), as illustrated in Fig. 2. Only one channel length of 10 μm is used for all devices and it is defined by the spacing of a Cu grid shadow mask. However, the channel width that is determined by the width of the flakes varies in the range of 2–60 μm as a result of the stochastic nature of the MoS2 exfoliation process. The source and drain metal used in our devices is 50 nm Au with a 5 nm thick Ti adhesion layer. The Ti adhesion layer that is in intimate contact with MoS2 has a work function of ∼4.3 eV, which is very close to the conduction band edge of thin-layer MoS2933. Furthermore, Ti is a transition metal with its d-electron orbitals mixing favorably with the 4d states of Mo and resulting in an increase in the density of states at the Fermi level and a strong Fermi level pinning at the contact93435. Therefore, this favorable interface geometry is expected to facilitate a good chemical bonding and allow for a maximized electron injection at the source/drain contacts with an increased overlap between the states at the interface.

Bottom Line: The decrease in Ion/Ioff is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm.This excellent agreement confirms that multilayer-MoS2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate Ion/Ioff.Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of ASIC and System, School of Information Science and Technology, Fudan University, Shanghai 200433, China.

ABSTRACT
Layered two-dimensional semiconductors have attracted tremendous attention owing to their demonstrated excellent transistor switching characteristics with a large ratio of on-state to off-state current, Ion/Ioff. However, the depletion-mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given Ion/Ioff as an acceptable specification. Identifying the optimum thickness range is of significance for material synthesis and device fabrication. Here, we systematically investigate the thickness-dependent switching behavior of transistors with a wide thickness range of multilayer-MoS2 films. A difference in Ion/Ioff by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in Ion/Ioff is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm. For t larger than 100 nm, Ion/Ioff approaches unity. Simulation using technical computer-aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of Ion/Ioff with t. This excellent agreement confirms that multilayer-MoS2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate Ion/Ioff. Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.

No MeSH data available.