Design and Optimization of Two-dimensional Molybdenum Disulfide Field-effect Transistor: Density Functional Theory Study and Characterization
Creators: Gao, Junsen
Publisher: name: University of Alberta Library
Year: 2022

<p>The IC industry is faced with challenges of following the Moore&#39;s law. During the scaling process of ICs, issues such as gate leakage have become the major obstacle. Showing great stacking ability, flexibility and functionality, 2D materials have become promising candidates for solving the existing issues of the IC scaling. Among the 2D materials, MoS2 and TMDC members are considered as suitable 2D channel materials for their tunable non-zero bandgap energy. Thus, designing high-performance flexible MoS2 electronic devices such as 2D MoS2 field-effect transistors is the focus of this work.</p>

<p>2D FET design includes the contact and channel design. To design high-performance MoS2 FETs, it is crucial to find good Schottky/Ohmic contacts. In this work, a comprehensive computational study based on the density functional theory has been performed. The projected local density of states analysis is employed to extract the Schottky barrier height of optimized Au-MoS2, W-MoS2 and Mo-MoS2 contacts. The simulation results demonstrate that Mo tends to form the best Schottky contact with low Schottky barrier height. As compared to Mo, Au forms a high-resistance Ohmic contact with the ML MoS2. It is observed that metal contacts introduce metallization to the intrinsic ML MoS2. This effect affects the carrier mobility and designed functionality of the devices, which also impacts the controllability of the device fabrication.</p>

<p>To overcome the metallization issues of the metal contacts, Van der Waals contact TiS2 has been investigated using DFT techniques. It is observed that TiS2 contact adds p-type doping to the ML MoS2 while the graphene contact tends to introduce n-type doping. TiS2 can be utilized as Ohmic contact material if the ML MoS2 is p-type doped. While the ML MoS2 is n-type doped, the Schottky barrier height of TiS2 contact ranges from 0.3 to 1.35 eV, which is based on the doping concentration of the ML MoS2. It is demonstrated that, TiS2 contact can preserve the bandgap of the ML MoS2 after forming the contact. For this reason, 2D semi-metallic materials can be applied in the 2D electronics as pristine contacts in the future.</p>

<p>Au-MoS2 contacts are designed and fabricated to verify our computational works. By using microscopy-assisted mechanical exfoliation technique, MoS2 flakes with the thickness of 4L, 89L, and 55L are deposited onto the Au electrodes with optimized comb patterns. The annealing process greatly reduces the series resistance and improves the contact quality. The I-V characterization results prove that the well-defined thermionic model and series-resistance model fail to extract correct I-V parameters of the fabricated back-to-back Au-MoS2 contacts. Using the novel image-force model, the two contacts in the Au-(4L)MoS2 back-to-back Schottky diodes show Schottky barrier height of 0.134 eV and 0.137 eV respectively. The I-V characterization demonstrates that the reverse-biased contact in the back-to-back Schottky diodes determine the I-V characteristics. The extracted experimental Schottky barrier height is within the prediction of our simulation.</p>

<p>2D material properties change with monolayer stacking. It is essential to study the stacking-dependent electronic properties of MoS2 as it is difficult to deposit MoS2 samples with uniform orientation angle. DFT is employed to study the effective mass variation of 2-10L AA and AB stacking MoS2. The simulation reveals that AA stacking MoS2 shows the smallest effective mass while the AB stacking MoS2 shows the largest effective mass. The Mulliken population, electron density and electrostatic differential potential simulations demonstrate that the MoS2 lattice with AB stacking orientation shows stronger in-plane scattering effect on the carriers than AA stacking MoS2, leading to larger carrier effective mass in AB stacking MoS2.</p>

<p>Finally, to explore the channel structure beyond MoS2, DFT simulations have been performed to investigate the emergent properties of ML TMDC heterojunctions. The band shifting and bandgap modification of the constituent layers of the TMDC heterojunctions show a strong correlation with the interface electron orbital coupling, revealed by ED and projected band structure simulation. Type I and Type II heterojunctions can be formed by ML semiconducting TMDC materials, while the ML WTe2 is able to form n-type or p-type Schottky contacts with other ML TMDCs. These heterostructure-introduced emergent properties lead to novel designs and applications.</p>