Two-dimensional electronics and optoelectronics: From materials syntheses to device applications

The current research on semiconductor device has pushed the scaling of the devices into sub-10 nanometers (nm) regime. While most of the current devices are made on silicon germanium, and III-V materials, people are looking for new materials for use in novel semiconductor devices: either for use in extremely scaled device in sub-10 or even sub-5 nm devices, or for use in other situations such as flexible electronics or low power and lower cost IoT (Internet of Things) applications. Two-dimensional (2D) materials have attracted extensive research interests in their physical, chemical and mechanical properties.Since the discovery of graphene, which a single layer carbon atoms obtained by exfoliating from graphite by scotch tape, the research activities on 2D materials have increased exponentially during the past few years. The high mobility and ultra-thin body makes graphene interesting for electronics applications. However, the lack of a bandgap of graphene led to study of other 2D materials.Two of them have attracted a lot of interests recently, one is called molybdenum disulfide (MoS2), and the other is black phosphorus. Most of my research is based on these two materials, I tried to study from the synthesis of the materials, and then study the electronics applications of these materials. In the first part of the thesis, an introduction of the background of the current research on 2D materials for electronics applications will be given. Also, the basic background of the materials I studied will be given. In the second part of the paper, I will discuss about the electronic device applications of these materials. A detailed study on heterostructure device based on van der Waals interactions will be discussed, which is new concept devices based on the unique characteristics of 2D materials. In the third part, the optoelectronic applications of the materials will be discussed. The effect of device structure will be discussed. The plasmonic structure is added to achieve better device performance, while simulations were performed to get a in-depth understanding. In the fourth part, the stability of these materials will be discussed. Unlike the traditional semiconductor materials, which has already been studied for years to make them stable and reliable for semiconductor device applications, these novel nano materials are still suffering from some stability issues. In this chapter, a detailed study of the stability of these materials are described, some of the phenomenon are quite helpful for understanding the device characteristics, while some are useful for making these device more stable

Towards High-Performance Two-Dimensional Black Phosphorus Optoelectronic Devices: the Role of Metal Contacts

The metal contacts on 2D black phosphorus field-effect transistor and photodetectors are studied. The metal work functions can significantly impact the Schottky barrier at the metal-semiconductor contact in black phosphorus devices. Higher metal work functions lead to larger output hole currents in p-type transistors, while ambipolar characteristics can be observed with lower work function metals. Photodetectors with record high photoresponsivity (223 mA/W) are demonstrated on black phosphorus through contact-engineering.Comment: 4 Page

Switching Mechanism in Single-Layer Molybdenum Disulfide Transistors: an Insight into Current Flow across Schottky Barriers

In this article, we study the properties of metal contacts to single-layer molybdenum disulfide (MoS2) crystals, revealing the nature of switching mechanism in MoS2 transistors. On investigating transistor behavior as contact length changes, we find that the contact resistivity for metal/MoS2 junctions is defined by contact area instead of contact width. The minimum gate dependent transfer length is ~0.63 {\mu}m in the on-state for metal (Ti) contacted single-layer MoS2. These results reveal that MoS2 transistors are Schottky barrier transistors, where the on/off states are switched by the tuning the Schottky barriers at contacts. The effective barrier heights for source and drain barriers are primarily controlled by gate and drain biases, respectively. We discuss the drain induced barrier narrowing effect for short channel devices, which may reduce the influence of large contact resistance for MoS2 Schottky barrier transistors at the channel length scaling limit.Comment: ACS Nano, ASAP (2013

SARS-Cov-2 trajectory predictions and scenario simulations from a global perspective: a modelling study

The coronavirus SARS-CoV-2 emerging from Wuhan, China has developed into a global epidemic. Here, we combine both human mobility and non-pharmaceutical interventions (social-distancing and suspected-cases isolation) into SEIR transmission model to understand how coronavirus transmits in a global environment. Dynamic trends of region-specific time-variant reproduction number, social-distancing rate, work-resumption rate, and suspected-cases isolation rate have been estimated and plotted for each region by fitting stochastic transmission processes to the real total confirmed cases reported of each region. We find after shutdown in Wuhan, the reproduction number in Wuhan greatly declined from 6·982 (95% CI 2·558–14·668) on January 23rd, 2020 to 1.130 (95% CI 0.289–3.279) on February 7th, 2020, and there was a higher intervention level in terms of social-distancing and suspected-cases isolation in Wuhan than the Chinese average and Western average, for the period from the shutdown in Wuhan to mid-March. Future epidemic trajectories of Western countries up to October 10th, 2020, have been predicted with 95% confidence intervals. Through the scenario simulation, we discover the benefits of earlier international travel ban and rigorous intervention strategies, and the significance of non-pharmaceutical interventions. From a global perspective, it is vital for each country to control the risks of imported cases, and execute rigorous non-pharmaceutical interventions before successful vaccination development

Black Phosphorus–Monolayer MoS₂ van der Waals Heterojunction p–n Diode

Phosphorene, a elemental 2D material, which is the monolayer of black phosphorus, has been mechanically exfoliated recently. In its bulk form, black phosphorus shows high carrier mobility (∼10 000 cm²/V·s) and a ∼0.3 eV direct band gap. Well-behaved p-type field-effect transistors with mobilities of up to 1000 cm²/V·s, as well as phototransistors, have been demonstrated on few-layer black phosphorus, showing its promise for electronics and optoelectronics applications due to its high hole mobility and thickness-dependent direct band gap. However, p–n junctions, the basic building blocks of modern electronic and optoelectronic devices, have not yet been realized based on black phosphorus. In this paper, we demonstrate a gate-tunable p–n diode based on a p-type black phosphorus/n-type monolayer MoS₂ van der Waals p–n heterojunction. Upon illumination, these ultrathin p–n diodes show a maximum photodetection responsivity of 418 mA/W at the wavelength of 633 nm and photovoltaic energy conversion with an external quantum efficiency of 0.3%. These p–n diodes show promise for broad-band photodetection and solar energy harvesting