Defect engineering of ultra-thin layered molybdenum disulfide towards novel memristive and optoelectronic applications

Abstract

In the ever-evolving field of nanoelectronics, new paradigms are constantly sought-after to improve the performance of next-generation circuit components. In the last decade, two-dimensional (2D) semiconductors have emerged as promising candidates to usher in a new age of low-power devices. The family of transition metal dichalcogenides, such as molybdenum disulfide (MoS2), offers a unique versatility of physical properties (flexibility, transparency, charge carrier confinement) and integrates easily with existing silicon-based fabrication processes. To a large extent, the implementation of potential applications is governed by the properties of the material systems. The ability to manipulate these properties on the nanoscale is a central theme of this dissertation, with a particular focus on modulating the electrical properties of devices based on 2D MoS2 by the purposeful introduction of defects. Two main avenues were pursued in this thesis to study the modulation of the electronic and optoelectronic properties of ultra-thin MoS2 - plasma treatment and helium ion beam irradiation. Both techniques were established as viable methods for activating novel functionalities in low-dimensional materials; despite residing on the opposite ends of the spectra of fabrication scalability, cost, time efficiency, site-specificity and dose control. Plasma processing allows for wafer-scale, fast and cost-efficient surface doping of on-substrate MoS2 samples. Time-controlled exposures to a mixture of O2:Ar (1:3) plasma resulted in a dose-dependentrecovery of electron mobility in MoS2 field effect transistors - studied as a function of material thickness from 1 to 10 layers. Detailed spectroscopic and microscopic characterisation revealed the role played by a sub-stoichiometric phase of surface-bound MoOx in electrostatically screening the underlying MoS2 transistor channel; thus boosting the carrier mobility in highlydefective MoS2 devices. Robust simulations confirmed the beneficial role of the highly conductive oxide phase on the transistor channel conductance. This study of the dose and layer number dependence was extended to low-temperature (80 K) electrical characterisation of bilayer MoS2 as a function of increasing plasma processing time.The Efros-Shklovskii hopping regime of the untreated sample was seen to transition to a mixed thermally-activated regime at high temperatures upon plasma treatment. Another crossover was observed in the defective system, below 181 K, to a strongly short-range Arrhenius regime due to a critical defect density achieved by the continuing plasma treatment. Investigation of the temperature-dependent emission current revealed a significant lowering of the effective Schottky barrier height for the bilayer MoS2 channel with plasma treatment. Meanwhile, it was demonstrated that the photoresponsivity of synthetic MoS2 monolayer phototransistors can be enhanced ten-fold by the introduction of these surface-bound molybdenum oxides by rapid plasma exposures (2 seconds). The effect of the mobility and photoresponsivity enhancement was found to depend on the laser power and was more prominent at powers exceeding several microWatts. Helium ion beam irradiation is a site-specific method for the dose-controlled sputtering of material down to nanometre-scale regions in layered semiconductors. Previous work has demonstrated that it is a practical method for the introduction of sulfur vacancies in MoS2. This also leads to a dose-mediated resistivity tuning in the material by several orders of magnitude. In this thesis, it was demonstrated that by tuning the irradiation strategy and localising the damage to specific sites, the electronic characteristics of on-dielectric MoS2 field effect transistors can be well-controlled. The effects of irradiating a monolayer MoS2 device with the helium ion beam as a function of increasing exposure area were examined. Increased irradiation areas resulted in poorer on/off ratios for the transistor, a severe deterioration of carrier mobility and an increased hole-branch conduction due to the saturation of vacancy sites by atmospheric p-type dopants. Building on this knowledge, a hybrid MoS2 device known as the memtransistor was fabricated by confining MoS2 channel-bisecting irradiations to quasi-one dimensional line scans. These highly site-specific irradiations create charged defects in the MoS2 lattice. The reversible drift of these locally-seeded defects in the applied electric field modulates the resistance of the semiconducting channel, enabling versatile memristive functionality on the nanoscale. These devices can reliably retain their resistance ratios and set/reset biases for up to 1180 switching cycles, while a range of optimal voltage ramping speeds was also investigated. Long-term potentiation and depression with sharp habituation that is demanded by future neuromorphic architectures was demonstrated in these devices. The fabricated memtransistor devices were characterised through spectroscopic and microscopic methods to determine the nature of the resistive switching. The property of device asymmetry was found to play a key role in the emergence of bipolar memristive behaviour exhibited by the devices. The proposed physical mechanism for the switching relies on the cyclical shrinkage and extension of the asymmetric sulfur vacancy distribution introduced into the MoS2 channel by the He+ ion beam. The individual device resistance states were probed and characterised by means of cryogenic charge transport down to 1.5 K. These experiments uncovered the impact of sulfur vacancy drift on the device operation; parametrised by large differences in the gate threshold voltage, electron mobility scaling with temperature, the electron localisation lengths, the amplitude of 1/f noise and the current density saturation between the two resistance states.Moreover, an early onset metal-insulator transition was observed in the low resistance sate, as mediated by the reversible doping brought about by the bi-directional drift of sulfur vacancies. This method of inducing resistive switching in monolayer MoS2 thus also demonstrates the utility of field-controllable dopant drift in facilitating exotic physical phenomena in 2D materials. The work described herein establishes the feasibility of ion irradiation for the controllable fabrication of 2D memristive devices, with promising key performance parameters such as low power consumption. The applicability of these devices for synaptic emulation may thus address the demands of future neuromorphic hardware architectures.