Molecular Doping of Multilayer MoS2 Field-effect Transistors: Reduction in Sheet and Contact Resistances

Abstract: For the first time, polyethyleneimine (PEI) doping on multilayer MoS2 field-effect transistors are investigated. A 2.6 times reduction in sheet resistance, and 1.2 times reduction in contact resistance have been achieved. The enhanced electrical characteristics are also reflected in a 70% improvement in ON current, and 50% improvement in extrinsic field-effect mobility. The threshold voltage also confirms a negative shift upon the molecular doping. All studies demonstrate the feasibility of PEI molecular doping in MoS2 transistors, and its potential applications in layer-structured semiconducting 2D crystals.

• The paper demonstrates that PEI doping reduces sheet resistance by 2.6× and contact resistance by 1.2× in MoS2 FETs.
• It reports a 70% increase in ON-current and a 50% enhancement in field-effect mobility after doping.
• The study validates molecular doping as a potent technique for optimizing the electrical properties of 2D semiconductor devices.

Molecular Doping of Multilayer MoS Field-Effect Transistors

The paper presents a detailed examination of molecular doping on multilayer molybdenum disulfide (MoS$_2$) field-effect transistors (FETs), emphasizing the application of polyethyleneimine (PEI) as an electron-donating dopant. MoS$_2$, a transition metal dichalcogenide (TMD), has emerged as a focal point in electronic device research due to its advantageous properties, such as a substantial bandgap and moderate carrier mobility, promising for various applications including sensors and logic devices.

Key Findings

The research demonstrates that employing PEI on MoS$_2$ FETs effects significant reductions in both sheet and contact resistances, registering decreases by factors of 2.6 and 1.2, respectively. These improvements in resistances contribute to a notable increase in ON-current by 70% and an enhancement in extrinsic field-effect mobility by 50%. Furthermore, PEI doping results in a negative shift of the threshold voltage, corroborating its strong n-type doping effect. Such modifications underscore the potential of molecular doping strategies in optimizing the electrical performance of layer-structured 2D semiconductors.

Experimental Approach

MoS$_2$ flakes were mechanically exfoliated and transferred onto a silicon substrate with a SiO$_2$ overlayer. The authors employed electron-beam lithography to pattern contacts in a transmission line method (TLM) configuration and subsequently deposited PEI. This step involved soaking the MoS$_2$ FETs in a PEI solution, followed by rinsing and drying. Electrical characterizations were performed in ambient conditions, revealing the extent of PEI's impact on device performance.

Results and Discussion

The investigation provided quantitative analyses showing reduced effective Schottky barrier height and improved carrier concentrations, attributed to electron transfer from PEI to MoS$_2$ flakes. The back-gate bias experiments revealed that both sheet and contact resistances were substantially lower post-PEI doping at specific conditions, ensuring effective reduction in resistivity across the examined TLM configurations.

Although the ON/OFF current ratio experienced a reduction post-doping, affected primarily by the diminished electrostatic control due to top-layer charge transfer from PEI, the overall device performance including ON-current was substantially improved. Additionally, the paper discusses the stability and reversibility of the doping effects, suggesting potential strategies for restoring device characteristics through re-doping procedures.

Implications and Future Directions

This research indicates the feasibility of using molecular doping to enhance the electrical properties of MoS$_2$ FETs, which has broader implications for developing high-performance 2D semiconductors in future CMOS technologies. The doping methodology discussed could extend to other TMDs, potentially enabling widespread use in applications requiring energy-efficient and flexible electronic devices.

Future research could focus on refining doping techniques to achieve precise control over doping levels and uniformity across device arrays. Moreover, integrating such doped-MoS$_2$ transistors into complex circuitry may illuminate further interactions between different material interfaces, paving the way for innovative semiconductor applications in nanoelectronics.