2D Materials: Theoretical Study of Magnetic and Contact Properties

The integrated circuit is without a doubt one of the most influential inventions in all of human history. While every technological revolution has had massive impacts across human societies, modern electronic circuits have increased the rate of change by orders of magnitude and this process shows no signs of stopping. As society has become accustomed to the rapid pace of technological development, the expectations for further improvements are more and more demanding. The silicon transistor was the ideal vehicle for such a rapid development, as transistors typically become more powerful and less costly to make when their size is decreased. With the added bonus of being able to cram more transistors into the same chip, the electronics revolution started, and a snowball effect of increasingly complexity and performance was unleashed onto the market, leading to the highly interconnected society we live in today. However, the benefits of decreasing transistor dimensions cannot last forever. There are certain extremely fundamental limits, at the nanometer scale, to how far one can go in making smaller and smaller devices. At some point, transistors begin to suffer from all sorts of performance-degrading issues, such as short-channel effects, increased leakage, fabrication difficulties, etc. Even more fundamental issues arise once the device dimensions go down to only a few nanometers, where quantum effects can seriously degrade traditional silicon-based transistors. It is with these scaling limitations in mind that researchers started looking very seriously at a relatively new class of materials: two-dimensional (2D) materials. 2D materials are atomically thin materials, consisting of a single layer not bound covalently in the out-of-plane direction. The 2D nature of these materials is of course in stark contrast with more “normal” materials, such as silicon or iron, which have covalent bonds in three dimensions. It turns out that due to the special structure of 2D materials, the physical properties are also extremely interesting, and worth investigating seriously. At present, various classes of 2D materials have been found, and many 2D materials have corresponding stacked layered versions with their own special properties. Add in, for example, the fact that one can dope these materials of make heterostructures out of several different kinds, then one can start to appreciate the vast parameter space that can be explored in the search for interesting applications. In this work, I focus on the applications of 2D materials in logic and memory devices. More specifically, I discuss the studies done by myself and my collaborators on the magnetic properties of layered WSe2 and PtSe2, and the calculation of the contact resistance between a metal and a 2D semiconductor. In the first part of the thesis, I share our investigation on the nature and stability of magnetic phases of doped intercalated WSe2 and PtSe2. We showed that, depending on the dopant, the stable magnetic phase at low temperature can be drastically different in both stability and type (ferro- or antiferromagnetic). We further showed that the presence of W or Pt vacancies in the lattice can be used to control the thermodynamic stability of the intercalated structures. Finally, we investigated the effect of the Pt vacancies on the magnetism in intercalated PtSe2. We showed that even though the spin polarization around the Pt atoms is very small, the Pt electronic cloud mediates longer magnetic interactions. Therefore, the presence or absence of Pt vacancies has a strong impact on the magnetic phases in the intercalated PtSe2. In the second part of this thesis, transport properties at a metal-2D semiconductor contact are the main topic. More specifically, I, along with my collaborators, have created a flexible model that can be used to efficiently simulate metal-2D semiconductor contacts and extract key parameters, such as the contact resistance. We studied the effects of device parameters, such as backgate bias, but also simulation parameters, such as the size of the simulation domain used to solve the Poisson equation. Crucially, we found that the contact resistance can be underestimated by over an order of magnitude when the Poisson domain is too small. In the final chapter, I provide an overview of the main achievements of the thesis and discuss potential avenues for future research.