A group of researchers recently published a paper in the journal ACS Nano that demonstrated the feasibility of using vertically stacked graphene-tungsten disulfide (WS2)-graphene (GWG) van der Waals (vdW) heterostructure-based field-effect tunneling transistors (FETTs) to achieve highly tunable carrier tunneling.
Study: Highly Tunable Carrier Tunneling in Vertical Graphene–WS2–Graphene van der Waals Heterostructures. Image Credit: Kateryna Kon/Shutterstock.com
Limitations of Existing Two-dimensional (2D) Material-based Transistors
2D materials such as transition metal dichalcogenides (TMDCs) and graphene have demonstrated significant potential for semiconductor devices such as photodetectors and field-effect transistors (FETs) due to their exceptional electronic properties and atomically thin thickness.
Graphene transistors display a large on-state current and ultrahigh-carrier mobility. However, the zero-band-gap property of graphene limits the off-state current of graphene FETs and leads to a low ON/OFF ratio, which restricts the use of graphene FETs in logic circuit applications.
Although transistors based on 2D TMDCs with appropriate bandgap demonstrate a high ON/OFF ratio, the on-state currents of such FETs are often limited by the Schottky barrier that forms between the TMDC channels and metallic contacts as a result of the Fermi level pinning effect and mismatch in their work functions.
Additionally, the sub-threshold swing of 2D FETs at room temperature has a fundamental limit of 60 millivolts per decade. Thus, for next-generation FETs, both 2D TMDC and graphene transistors cannot meet all performance requirements, such as small subthreshold swing, low operation voltage, and a high ON/OFF ratio.
Significance of vdW Heterostructures to Develop Tunneling Devices
vdW heterostructures with vertically arranged 2D crystals have demonstrated their potential for different optoelectronic and electronic applications such as flexible optoelectronic devices and ultrathin transistors. vdW heterostructure properties can be controlled precisely by adjusting the 2D component material type, band alignment, and the number of layers.
For instance, vdW heterostructures combined with hexagonal boron nitride (hBN) and graphene were used to fabricate photovoltaic devices and resonant tunneling diodes successfully. Quantum tunneling can occur in such graphene/hBN heterostructures due to the atomically thin thickness of the insulating hBN layer sandwiched between two graphene layers.
Based on graphene/hBN heterostructures, the application of similar graphene-2D material-graphene heterostructures was investigated in magnetic tunnel junctions and FETTs. However, the low ON/OFF ratio and small on-state current of the graphene/hBN vdW heterostructures due to the small changes in graphene Fermi level compared to the large barrier height adversely impacted the device performance of FETTs.
Although molybdenum ditelluride (MoTe2)/graphene vdW heterostructures-based FETTs demonstrate a significantly higher on-state current, these tunneling devices have a low ON/OFF ratio and large off-state current, which necessitates the identification of other 2D materials with a proper bandgap size to improve the device characteristics of FETTs.
Novel Way to Fabricate Next-generation FETs
Among the semiconducting 2D materials, WS2 is more suitable than other 2D materials for improving the FETT device characteristics due to its robust photoluminescence, valley-dependent optical selection rules, large spin-orbit splitting and excitonic binding energy, high carrier mobility, layer-dependent bandgap. Additionally, WS2 displays strong light-matter interaction, which indicates its potential in different optoelectronic device applications.
For vertical vdW heterostructure-based tunneling devices, WS2 can act as an effective tunnel barrier owing to its proper bandgap size. In the bulk form and monolayer limit, WS2 shows an indirect bandgap of 1.4 electron volt and a direct bandgap of 2.1 electron volt, respectively.
The effective barrier height in the graphene-WS2-graphene (GWG) heterostructures is appropriate for efficient carrier tunneling. Moreover, the change in graphene Fermi level comparable to the effective barrier height allows the development of high-performance GWG FETTs with a higher ON/OFF ratio compared to graphene/hBN/graphene and graphene/MoTe2/graphene FETTs.
Fabrication and Evaluation of GWG vdW Heterostructure-based FETTs
In this study, researchers fabricated high-performance GWG heterostructure FETTs using dry transfer vdW stacking methods and evaluated their overall performance. The WS2 tunneling barrier, graphene electrodes and back gate, and hBN substrate were exfoliated mechanically from WS2, graphite, and hBN bulk single crystals.
The graphene back gate layer was exfoliated on the silicon dioxide/silicon substrate, while the other 2D crystals were exfoliated on various polydimethylsiloxane (PDMS) membranes. Subsequently, the vertically arranged graphene-hBN-graphene-WS2-graphene vdW heterostructures were assembled layer by layer by the dry transfer method using a home-built transfer stage. The bottom and top graphene layers, WS2 tunnel barrier, and back gate graphene were contacted separately by electrodes using an e-beam evaporation process and standard e-beam lithography.
A confocal Raman spectrometer was used to measure the Raman spectra of the WS2 and graphene layers under ambient conditions at room temperature, while the WS2 flake thickness was determined using tapping-mode atomic force microscopy. The synthesized FETT devices' tunneling current-voltage (I-V) characteristics were determined using a low-temperature probe station with lock-in amplifiers and source meters.
Significance of the Study
Graphene-hBN-graphene-WS2-graphene vdW heterostructure-based highly tunable FETTs were fabricated successfully. The graphene back gate and hBN substrate substantially improved the tunneling and electronic performances of the synthesized devices. WS2 acted as a suitable vertical tunneling barrier for the device and the barrier height was controlled effectively by the external gate voltage.
The carrier transport in the GWG heterostructures was modulated effectively using external electric fields. A low off-state current of one picoampere, a subthreshold swing of 0.45 volts per decade, and a high ON/OFF ratio that exceeded the value of 106 at room temperature were observed in the synthesized devices.
By increasing the bias voltage, the charge polarity of the device was successfully tuned from an n-type tunneling device under a small bias to a bipolar complementary metal-oxide-semiconductor (CMOS)-like tunneling device under a large bias due to the transition from direct tunneling to Fowler−Nordheim tunneling.
The I-V characteristics were dependent on the temperature, which indicated that both thermionic emission and tunneling contributed to the operation current that significantly enhanced the on-state current of the FETTs. The thermionic emission current mechanism substantially increased the on-state current limit that often exists in tunneling transistors.
To summarize, the findings of this study demonstrated that the GWG heterostructure-based FETTs could fully meet the performance requirements of next-generation nanoelectronic devices.
Luo, F., Luo, Q., Li, M. et al. (2022.) Highly Tunable Carrier Tunneling in Vertical Graphene−WS2−Graphene van der Waals Heterostructures. ACS Nano https://pubs.acs.org/doi/10.1021/acsnano.2c00536