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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) Simulated transfer curves of FETs with various MoS2 thicknesses. (b) Simulated thickness-dependence of Ion/Ioff.
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f4: (a) Simulated transfer curves of FETs with various MoS2 thicknesses. (b) Simulated thickness-dependence of Ion/Ioff.

Mentions: It is well known that in the monolayer 2D materials, the charge carriers are confined in the 2D planes. This confinement can result in some unique characteristics not common in 3D materials, e.g. Si. When the layer thickness is increased, the carriers can hop freely between neighboring layers and move in the whole 2D layered material37. As a result, the carriers distribute fairly uniformly in the 2D material. In this aspect, multilayer-MoS2 films can be approximated as a homogeneous semiconductor and simulated with traditional device simulators. We have therefore used a commercial simulation tool SILVACO TCAD38 to numerically solve the coupled Poisson and continuity equations for the multilayer-MoS2 FETs. Our focus here is on charge and current distributions in MoS2. For simplicity, the electron SBH is set to 0.1 eV and the unintentional n-doping concentration, Nd, in MoS2 is assumed to be 3.5 × 1017 cm−3, in order to attain identical Vt between the simulation and experiments. The doping concentration39 in MoS2 is found to vary from 1016 to 1019 cm−3. The other material parameters used in the simulation are shown in Supplementary Table S1113940. The simulated transfer characteristics of multilayer-MoS2 FETs for various channel thicknesses (Fig. 4a) and the variation of Ion/Ioff with t (Fig. 4b) are in good agreement with the experimental results. In particular, the simulated Ion/Ioff shows a steep decrease with t around ~50 nm, matching very well with the data in Fig. 3c. This critical thickness is strongly correlated with Wmax that is related to Nd by the following formula36:


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) Simulated transfer curves of FETs with various MoS2 thicknesses. (b) Simulated thickness-dependence of Ion/Ioff.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: (a) Simulated transfer curves of FETs with various MoS2 thicknesses. (b) Simulated thickness-dependence of Ion/Ioff.
Mentions: It is well known that in the monolayer 2D materials, the charge carriers are confined in the 2D planes. This confinement can result in some unique characteristics not common in 3D materials, e.g. Si. When the layer thickness is increased, the carriers can hop freely between neighboring layers and move in the whole 2D layered material37. As a result, the carriers distribute fairly uniformly in the 2D material. In this aspect, multilayer-MoS2 films can be approximated as a homogeneous semiconductor and simulated with traditional device simulators. We have therefore used a commercial simulation tool SILVACO TCAD38 to numerically solve the coupled Poisson and continuity equations for the multilayer-MoS2 FETs. Our focus here is on charge and current distributions in MoS2. For simplicity, the electron SBH is set to 0.1 eV and the unintentional n-doping concentration, Nd, in MoS2 is assumed to be 3.5 × 1017 cm−3, in order to attain identical Vt between the simulation and experiments. The doping concentration39 in MoS2 is found to vary from 1016 to 1019 cm−3. The other material parameters used in the simulation are shown in Supplementary Table S1113940. The simulated transfer characteristics of multilayer-MoS2 FETs for various channel thicknesses (Fig. 4a) and the variation of Ion/Ioff with t (Fig. 4b) are in good agreement with the experimental results. In particular, the simulated Ion/Ioff shows a steep decrease with t around ~50 nm, matching very well with the data in Fig. 3c. This critical thickness is strongly correlated with Wmax that is related to Nd by the following formula36:

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.