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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.


Related in: MedlinePlus

(a) Transfer characteristics of a representative transistor with a 30-nm-thick MoS2 film on a linear scale (left y-axis) and a log scale (right y-axis). The threshold voltage, Vt, is determined by the intercept on the x-axis with the regression fitted line to the linear scale characteristics. The insets represent the energy band diagrams corresponding to the applied Vg in three distinct regions: (1) at flat band, (2) below threshold, and (3) above threshold. ϕbn and ϕbp indicate electron and hole SBH, respectively. (b) Dependence of Ion and Ioff on MoS2 flake thickness. The grey dashed line serves as a guide to the eye. (c) Thickness-dependence of Ion/Ioff. The red dashed line serves as a guide to the eye. Inset is the zoom in for the first 100 nm. The red solid line in the inset is a linear fit on the semi-log scale.
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f3: (a) Transfer characteristics of a representative transistor with a 30-nm-thick MoS2 film on a linear scale (left y-axis) and a log scale (right y-axis). The threshold voltage, Vt, is determined by the intercept on the x-axis with the regression fitted line to the linear scale characteristics. The insets represent the energy band diagrams corresponding to the applied Vg in three distinct regions: (1) at flat band, (2) below threshold, and (3) above threshold. ϕbn and ϕbp indicate electron and hole SBH, respectively. (b) Dependence of Ion and Ioff on MoS2 flake thickness. The grey dashed line serves as a guide to the eye. (c) Thickness-dependence of Ion/Ioff. The red dashed line serves as a guide to the eye. Inset is the zoom in for the first 100 nm. The red solid line in the inset is a linear fit on the semi-log scale.

Mentions: The transfer characteristics of a multilayer-MoS2 FET shows a typical n-type unipolar carrier transport behavior (Fig. 3a). This confirms a rather small (<0.1 eV), if not negligible, Schottky barrier height (SBH) for electrons at the Au/Ti-MoS2 contacts (Supplementary Fig. S1). The SBH for holes is, thus, high (>1.2 eV) since the sum of electron SBH and hole SBH should approximately be equal to the energy bandgap (1.3 eV) of MoS2 (inset in Fig. 3a). Tunneling through the Schottky barrier at the metal/MoS2 contacts not only limits the charge injection in the device at its on-state but also plays a critical role when evaluating the device off-state in the subthreshold region of the transistor. The small electron SBH facilitates the injection of accumulated electrons at positive gate voltage, Vg, while the large hole SBH suppresses the injection of inverted holes at negative Vg. This results in the n-type unipolar behavior with a high Ion/Ioff and is in stark contrast to the ambipolar conduction behavior in graphene FETs with a low Ion/Ioff32. The FET with a 30-nm-thick multilayer MoS2 in Fig. 3a operates as a depletion-mode FET with a large negative threshold voltage, Vt, and a high Ion/Ioff of 105. All the FETs fabricated in this work exhibit the same n-type characteristics, regardless of the thickness of the MoS2 film in channel (Supplementary Fig. S2).


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) Transfer characteristics of a representative transistor with a 30-nm-thick MoS2 film on a linear scale (left y-axis) and a log scale (right y-axis). The threshold voltage, Vt, is determined by the intercept on the x-axis with the regression fitted line to the linear scale characteristics. The insets represent the energy band diagrams corresponding to the applied Vg in three distinct regions: (1) at flat band, (2) below threshold, and (3) above threshold. ϕbn and ϕbp indicate electron and hole SBH, respectively. (b) Dependence of Ion and Ioff on MoS2 flake thickness. The grey dashed line serves as a guide to the eye. (c) Thickness-dependence of Ion/Ioff. The red dashed line serves as a guide to the eye. Inset is the zoom in for the first 100 nm. The red solid line in the inset is a linear fit on the semi-log scale.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: (a) Transfer characteristics of a representative transistor with a 30-nm-thick MoS2 film on a linear scale (left y-axis) and a log scale (right y-axis). The threshold voltage, Vt, is determined by the intercept on the x-axis with the regression fitted line to the linear scale characteristics. The insets represent the energy band diagrams corresponding to the applied Vg in three distinct regions: (1) at flat band, (2) below threshold, and (3) above threshold. ϕbn and ϕbp indicate electron and hole SBH, respectively. (b) Dependence of Ion and Ioff on MoS2 flake thickness. The grey dashed line serves as a guide to the eye. (c) Thickness-dependence of Ion/Ioff. The red dashed line serves as a guide to the eye. Inset is the zoom in for the first 100 nm. The red solid line in the inset is a linear fit on the semi-log scale.
Mentions: The transfer characteristics of a multilayer-MoS2 FET shows a typical n-type unipolar carrier transport behavior (Fig. 3a). This confirms a rather small (<0.1 eV), if not negligible, Schottky barrier height (SBH) for electrons at the Au/Ti-MoS2 contacts (Supplementary Fig. S1). The SBH for holes is, thus, high (>1.2 eV) since the sum of electron SBH and hole SBH should approximately be equal to the energy bandgap (1.3 eV) of MoS2 (inset in Fig. 3a). Tunneling through the Schottky barrier at the metal/MoS2 contacts not only limits the charge injection in the device at its on-state but also plays a critical role when evaluating the device off-state in the subthreshold region of the transistor. The small electron SBH facilitates the injection of accumulated electrons at positive gate voltage, Vg, while the large hole SBH suppresses the injection of inverted holes at negative Vg. This results in the n-type unipolar behavior with a high Ion/Ioff and is in stark contrast to the ambipolar conduction behavior in graphene FETs with a low Ion/Ioff32. The FET with a 30-nm-thick multilayer MoS2 in Fig. 3a operates as a depletion-mode FET with a large negative threshold voltage, Vt, and a high Ion/Ioff of 105. All the FETs fabricated in this work exhibit the same n-type characteristics, regardless of the thickness of the MoS2 film in channel (Supplementary Fig. S2).

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.


Related in: MedlinePlus