Functionality enhancement of two-dimensional transition metal dichalcogenide-based transistors

Atomically thin molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂), members of the transition metal dichalcogenide family, have emerged as prototypical two-dimensional semiconductors with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors, the mechanically flexible and transparent nature of 2D MoS₂ and WSe₂ make them even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of these materials can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to their contact, doping and mobility engineering must be overcome. This dissertation reviews the important technologically relevant properties of semiconducting 2D TMDs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS₂ and WSe₂-based devices. Finally, this dissertation provides a comprehensive insight into two novel and promising engineering solutions that can be employed to address the all-important issues of contact resistance, controllable and area-selective doping, and charge carrier mobility enhancement (electrons in MoS₂ and holes in WSe₂) in these devices. Specifically, this work sheds light upon the interfacial-oxygen-vacancy mediated n-doping of MoS₂ by high-κ dielectrics, such as HfO₂, Al₂O₃ and TiO₂, using detailed experimental characterizations and theoretical calculations. This n-doping effect on MoS₂ by high-κ dielectrics is proposed as a mechanism responsible for the performance enhancement observed in MoS₂ devices upon encapsulation in high-κ dielectric environments. This work also sheds light upon the band structure engineering and p-doping of layered WSe₂ using a simple and facile one-step chemical functionalization technique utilizing ammonium sulfide solution. Detailed experimental and theoretical studies once again reveal the underlying mechanism responsible for the p-doping in WSe₂ after chemical treatment. Results show that the doping techniques presented in this dissertation can easily be adapted to obtain high-performance FETs based on 2D MoS₂ and WSe₂. Finally, some future research directions, based on the work presented in this dissertation, are highlighted.Electrical and Computer Engineerin

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides. Here, the phenomenon of oxygen impurities trapping photo-excited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for the carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in transition metal dichalcogenides, and facilitates defect engineering in relevant material and device applications