Gyu Chull Han
Theoretical Investigation of Molybdenum Disulfide (MoS2) Field-Effect Transistors for Electronic and Optoelectronic Applications
After the first demonstration of graphene, 2D materials have attracted tremendous attentions in the electronic devices community. Graphene was regarded as a promising material for electronics due to its extremely high carrier mobility. However, graphene is semi-metallic that doesn’t have bandgap in its inherent form, which makes it inappropriate for switching devices in digital logic circuits. On the other hand, there exist another class of new 2D materials, transition metal dichalcogenide (TMDCs). They are atomically thin materials with structure of a transition metal atoms (Mo, W) sandwiched by two chalcogenide atoms (S, Te, Se). Due to its thinness and an appropriate bandgap, many research groups have investigated TMDCs field-effect transistors and photonic devices. Among many contenders of TMDCs, molybdenum disulfide (MoS2) is one of the most viable candidates for future digital applications, showing promising characteristics for electronic and optoelectronic switches, including large on/off current ratio, small sub-threshold swing, and high field-effect mobility.
In this thesis, performance of MoS2 field effects transistors (FETs) is predicted and investigated by modeling and simulations so that the results can provide guidelines to experiments for fabrications and optimizations. Transport properties are simulated using the non-equilibrium Green’s function (NEGF) formalism, which solves Schrödinger equation under non-equilibrium and open boundary conditions. Simulations of these nano-scale electron devices are performed self-consistently between the electrostatic potential and the charges inside the device. Hamiltonian for describing kinetic and potential energy of electrons in the materials is described in effective-mass or tight-binding method.
Even though many experiments demonstrated promising device characteristics of MoS2 FETs, the investigation on contact-dependent behaviors of them is still in its infancy. In fact, different types of contacts and their quality can significantly affect the performance of such nanoscale devices. Therefore, in this chapter, using the self-consistent quantum transport simulations, the performance variability of MoS2 FETs based on different types of contacts is investigated. Varying the Schottky barrier in MoS2 FETs affects the output characteristics more significantly than the transfer characteristics. If doped contacts are realized, the performance variation due to non-ideal contacts becomes negligible; otherwise, channel doping can effectively suppress the performance variability in metal-contact devices. A scaling study also reveals that, for sub-10-nm channels, doped-contact devices can be more robust in terms of switching, while metal-contact MoS2 FETs can undergo the smaller penalty in output conductance.
MoS2 also gives intriguing optoelectronic properties that offer practical feasibility of MoS2 thin-film transistors (TFTs) for photodetector applications. TFTs using single layer or multilayer MoS2 are showing higher responsivity than that of graphene TFTs in phototransistors. Recently, responsivity of MoS2 has been drastically increased after various efforts are made to boost it. While multilayer MoS2 can be more advantageous than single layer MoS2 for optoelectronic devices in terms of higher density-of-states and wider spectral response, the responsivity of multilayer MoS2 phototransistors has remained much lower than that of single layer MoS2 photodevices mainly due to the indirect bandgap of multilayer MoS2. Here, an alternative approach is introduced and analyzed to obtain high responsivity in MoS2 phototransistors. Conducted simulations indicate that the gate underlaps play a key role for the enhancement of photoresponsivity. The comprehensive investigations suggest that high responsivity can be achieved in indirect-bandgap multilayer MoS2 phototransistors by optimizing optoelectronic design. The results further demonstrate the particular potential of multilayer MoS2 for optoelectronic applications such as touch screen panels, image sensors, solar cells, and communication devices.
Hamiltonian matrix based on effective mass only describes near the conduction band minima for n-type conduction. In principle, tight-binding (TB) method can provide more precise electronic band description of the material, which is utilized in the later part of this thesis to investigate device characteristics of single layer TMDC FETs, especially for MoS2. As result of the simulations based on the third-nearest-neighbor (TNN) TB parameters, negative differential resistance (NDR) behavior can be observed, which is defined as a phenomenon that current is decreased with increasing applied bias. The significant effects of the negative output characteristics on the device applications are investigated in detail by varying the transverse modes of the band structure.