Discrete electron fluctuations in van der Waals heterojunction

Abstract

Two-dimensional (2D) materials, including transition metal dichalcogenides (TMDs) and graphene have emerged as promising candidates for next-generation nanoelectronic devices due to their atomically thin geometry. This low dimensional geometry allows interesting applications which is difficult to achieve using bulk 3D materials. The absence of dangling bonds on their surfaces reduces interface scattering and allows for better control of carrier transport. The layered nature of these materials also allows for the design of multilayer channel field-effect transistors (FETs), and the current-carrying capacity of the device can be significantly enhanced while still preserving a strong gate electrostatics. The heterojunctions made by stacking several of these materials also introduces the possibility of a fast transfer of charge carriers between two layers. Understanding the underlying physics is essential for utilizing these properties for real-life applications. In this thesis we study the electrical behaviour of such heterojunction system and current fluctuation caused by the coulombic interaction of high effective electron charge density because of strong vertical quantum confinement coming from layered nature and artificially generated horizontal confinement (through a gate voltage). In the first part we study the behaviour of current flowing through different stacks of multilayer vertical heterojunction. The study shows, by coupling a layer of Black Phosphorus, having a very high mobility, to the van der Waal structure beneath it, pulls the current though a very small and confined region via tunnelling. This behaviour is observed as saturation of current with an increase in the gate voltage. We perform an intricate optimization study of the heterojunction design. When there is strong confinement of charge near or in the path of current, device gains the ability to sense the discrete charge fluctuation. In the next part of the thesis we study these fluctuations caused by electrons trapped in electrostatically created confinement. The electrons moves into or out of the confined region in a random fashion, which manifests as discrete, time dependent random fluctuation in the current through the device. The randomness of events is analysed to demonstrate the nature of these fluctuations. We verify the random nature of the generated sequences using NIST tests.