Mo-Based Transition-Metal-Dichalcogenide Junctionless Field-Effect-Transistors
Authors: Mirabelli Gioele , Duffy Ray , Hurley P. K. , Monaghan Scott , Cherkaoui Karim , Schmidt Michael , Sheehan Brendan , Povey Ian M , McCarthy Melissa , Nagle Roger , Bell Alan 
Published: February 2020

One of the most promising candidates as substitution for silicon in modern electronics devices are Transition-Metal-Dichalcogenides (TMDs). These are semiconductors in the form of MX2, where M is a transition metal (Mo, W, Hf), and X is a chalcogen (S, Se, Te). The weak Van-Der-Waals force between each layer of which they are formed, allow them to be easily exfoliated by scotch tape technique from their bulk form, as first was shown for graphene.[i] However one of the main limitations of graphene is its zero bandgap and resulting low on/off current ratio, which limits its usefulness for field-effect-transistor (FET) applications. On the other hand, TMDs show a finite direct bandgap between 1-2 eV, predicted by density-functional-theory (DFT) calculations,[ii]which make them suitable for logic devices. Their ultra-thin nature would allow a better electrostatic control for device applications, with respect to the state-of-the-art devices, and better immunity to short-channel effects, earning the attention of the electronics and optoelectronics community.

In this work, we report the electrical measurements of a back-gated MoS2  flake doped with Nb, and back-gated undoped MoTe2 mechanical exfoliated from crystals. The flakes were then put on 85 nm thick Si oxide on a highly doped Si handle wafer. Ti/Au (5/45 nm) deposited on top allowed the realization of a back-gate structure, which was analyzed through Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The device was then characterized by voltage and temperature dependence of the current.

For the MoS2, the temperature dependency of the device shows semiconductor behavior and, the doping effect by Nb atoms introduces acceptors in the structure, with a concentration 7.44x1019 cm-3 measured by Hall effect. The p-type doping is confirmed by all the electrical measurements, making the structure a junctionless transistor. In addition, other parameters regarding the contact resistance between the metal and MoS2 are extracted thanks to a simple Transfer Length Method structure, showing a promising contact resistivity of 1.07x10-7 Ohm.cm2 and a sheet resistance of  2.36x102 Ω/sq. 

For the MoTe2 the crystal was nominally undoped, making it easier to electrically fully-deplete, but consequently the on-current, contact resistivity and sheet resistance were worse than in the highly-doped MoS2.

In Fig. 1 is a representative SEM image of the contacted MoS2 flake device, with the contact tracks numbered 1 to 4. 

In Fig. 2. is a representative TEM image of the overall structure of the flake. It is possible to note the undulations of the 10 nm MoS2 flake along the surface of the substrate.

Electrical measurements were carried out to confirm the electrical conductivity of the device. Fig 3 shows Ids-Vbgcurve between the first and second pad confirming a p-type field-effect-transistor. In summary, Mo-based TMD flakes were mechanically exfoliated and contacted. The p-type behavior of the MoS2 flake, expected since it is doped with Nb atoms, was confirmed by electrical measurements. The low contact resistance and resistivity data reported, even if extracted with a simple TLM structure, can be related to the high p-type doping of the flake, which, together with other processes widely used to improve metal contacts and obtain clean TMDs surfaces, can be helpful in reaching a better TMD/metal interface. Moreover, the high doping can be the key to the introduction of TMD-based junctionless transistors, in which the good characteristics of TMDs and the uncomplicated process required by this kind of architecture may be combined.

[i]  K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric Field Effect in Atomically Thin Carbon Films”, Science 306,  666-669 (2004).

[ii]  O. V. Yazyev and A. Kis, “MoS2 and semicondutors in the flatland”, Materials Today  Vol.  18, Number 1  January/February 2015.

Figure 1