(Invited) 2D MoS2 Film Logic Devices: Challenges and Solutions Through Molecular Functionalization
Authors: Lockhart de la Rosa César Javier 
Published: February 2020

2D materials have demonstrated enormous potential for a great number of applications such as sensors, spintronic, superconductors and (photo)electronic devices. From these 2D materials, semiconducting transition metal dichalcogenides (MX2) are of special interest for electronic logic devices such as Field Effect Transistors (FET) given their interesting properties such as ultra-thin bodies and high electronic band gap that could enable lower passive power dissipation and further boost the performance of the devices. Molybdenum disulfide (MoS2), a member of the semiconducting MX2 family, is normally used to represent the family due to its robustness toward environment and its natural or synthetic availability. Nevertheless, several challenges need to be addressed before implementation of MX2 based logic devices such as the understanding and proper characterization of such devices, the reduction of the high contact resistance and the controllable doping of such devices.
The focus here will be the understanding and improvement of the Metal/MoS2 contact resistance and the controllable doping of MoS2 based devices. To achieve this first, the results of careful experiments comparing different characterization techniques are introduced and the most reliable way for parameters extraction are stablished (figure a and b). From the results of these characterization it is observed that the contact resistance is dominated by a Schottky barrier (SB) in the Metal/MoS2 leading to a high contact resistance of the devices. This contact resistance is observed to overtake the channel resistance for channels smaller than 100nm. Thus, for further scaling of the channel length (to enhance the performance of the FET) the reduction of the contact resistance is of primordial interest. The observations from these experiments are then used to carefully model the behavior of MoS2 FET using a semi-classical model. Further insight on the nature of the high contact resistance is gained with this model and two mayor trajectories for the electron injection from the metal to the MoS2 film are identified. First, a vertical trajectory where the electrons are injected from the metal to the MoS2 region under the contact and then are driven toward the channel through the MoS2 film (dashed line fig. c). Secondly, a lateral trajectory where the electrons are injected directly from the border of the metal to the channel MoS2 film in the channel region (solid line fig. c).
More over, from the model it is demonstrated that one effective way to reduce the contact resistance is to dope the region of the MoS2 film immediately after the contact to enhance the lateral trajectory that is always the most relevant. For the controllable doping two approaches are presented to demonstrate effective and controllable surface doping without degrading the carrier mobility: self-assembled physisorbed molecules and spin-coating of polymers. The advantage of the self-assembly relies on controlling the periodicity of the potential landscape through the self-assembly of the molecules, while in the case of the polymer, the application technique is more industry friendly and the technique is more robust. For the first approach oleylamine (OA) is used. The self-assembly and the doping are effectively demonstrated (fig. d and e) together with the reduction of the contact resistance after doping. Equally, for the more processing-friendly polymer functionalization (polyvinyl-alcohol) doping is demonstrated (fig. f). Finally, the relation between the thickness of the MoS2 film and the surface doping approach are explored and it was concluded that surface doping is optimal for thicknesses below 5.2nm.

Figure 1