Electrical Characterization of 2D Transition Metal Dichalcogenides via SPM

In a conventional planar silicon field effect transistor (FET), when its lateral dimension becomes smaller than the transistor thickness, the gate controllability becomes weaker. This leads to adverse short-channel effects, including saturation of the carrier mobility in the channel, leakage current, time-dependent dielectric breakdown, and channel hot-carrier degradation.

To ensure efficient electrostatic control from the gate, the transistor body thickness must be reduced. Theoretical studies have demonstrated that particularly 2D transition metal dichalcogenides (TMDs) can:

  • Have excellent electrostatic gate control
  • Decrease off-state power consumption
  • Outperform Si as the channel material
  • Enable the atomic-level scaling
  • Further extend Moore’s Law [1-6] 

This is because of the atomic thickness and dangling bond-free surface of two-dimensional (2D) materials. 

Suitable methods to characterize the intrinsic electrical and physical properties of as-deposited 2D materials are a key link between the performance of 2D materials-based electronic devices and the quality of as-deposited 2D materials. 

The performance of 2D materials-based devices can be better understood, controlled, and improved thanks to this link. However, there are limited methods for analyzing the intrinsic electrical properties of as-deposited 2D materials on the nanoscale without any transfer and patterning process. 

To investigate the intrinsic electrical properties of as-deposited 2D TMDs, scanning probe microscopy (SPM) is utilized in this article. Conductive atomic force microscope (C-AFM) is carried out directly on the surface of as-grown 2D materials without any patterning. 

C-AFM enables the correlation of the electrical conductivity of as-grown 2D materials to their topography, linking the electrical properties of 2D materials to their physical properties such as layer thickness, etc. 

C-AFM supplies comprehensive information of as-deposited 2D materials and helps the user assess the effect of these intrinsic properties on 2D materials-based nanoelectronics. 

Experimental Details

Pt/Ir coated Si probes (spring constant k~3 N/m, resonance frequency f0~ 75 kHz, PPP-EFM) on a Park NX-Hivac AFM under high vacuum (~10-5 Torr) are utilized to assess the intrinsic electrical properties of the as-grown MoS2 and WS2 layers on sapphire are evaluated by C-AFM. 

The water layer which always exists on the sample can be reduced by the high vacuum environment.[4,6] The bias of the C-AFM measurement is applied to the sample chuck and the resulting current is measured via a linear current amplifier.  

The applied bias to gather all the C-AFM current maps is all at 1 V. Silver paint is applied to the top and side of the samples to ensure electrical contact. 

Results and Discussion

As seen in Figure 1b, the as-deposited MoS2 layer on on-axis cut sapphire shows a non-uniform conductance over the whole surface in the C-AFM current maps, even though the topography maps in Figure 1a exhibit a fully coalesced monolayer MoS2 with ~37% superficial crystals on top of it (named as 1.3 ML)...

(a, c) C-AFM topography of 1.3 ML MoS2 grown on on-axis and off-axis 1º cut sapphire, respectively. (b, d) The simultaneously acquired C-AFM current maps of (a, c), respectively. Non-homogenous and poorly conductive regions in the first single layer of MoS2 are highlighted in pink by current thresholding (~ 0.3 µA).

Figure 1. (a, c) C-AFM topography of 1.3 ML MoS2 grown on on-axis and off-axis 1º cut sapphire, respectively. (b, d) The simultaneously acquired C-AFM current maps of (a, c), respectively. Non-homogenous and poorly conductive regions in the first single layer of MoS2 are highlighted in pink by current thresholding (~ 0.3 μA). Image Credit: Images are reproduced with permission.[7] Copyright 2021, American Chemical Society. 

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This information has been sourced, reviewed and adapted from materials provided by Park Systems Europe.

For more information on this source, please visit Park Systems Europe.

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