Dielectric Nanosheets for Scaled Two-Dimensional (2D) Material Devices

Two-dimensional (2D) materials, with graphene as the most studied representative, are an interesting class of materials with layered structures based on van der Waals interactions. There is a wide range of inorganic 2D materials available that, depending on their composition and structure, exhibit either insulating, metallic, semi-metallic or semiconducting properties. Especially the semiconducting 2D transition metal dichalcogenides (e.g. MoS2, WS2, WSe2 ...) are attracting interest for nano-electronic applications because of their atomic scale thickness, lack of dangling bonds (in their ideal form), large band gap values and structural stability. Because of their atomic scale thickness, the charge transport in 2D semiconductors depends to a large extend on the external surroundings. The interfaces and their stability during processing sequences will govern the electronic device functioning and performance. However, the interfaces in such structures are currently not sufficiently understood. Another related question is how to enable the downscaling the equivalent oxide thickness (EOT) of dielectrics on the 2D channel. Possible limitations could be posed by the nucleation behavior of the dielectric layer on a 2D material (often island-like for direct growth), by process induced surface residuals, by 2D channel density of states or by the van der Waals gap between the channel and the dielectrics. The objective of this PhD project is to obtain insight into the synthesis of dielectrics and the structure of the resulting 2D semiconductor – dielectric interface. We will investigate both the top and bottom dielectric interfaces and how different process steps like transfer, clean, functionalization and growth of dielectric layers affect the structure of the resulting interface and the device performance. We will explore amorphous high-k dielectric layers (e.g. HfO2, ZrO2, TiO2, Y2O3) as well as 2D dielectric nanosheets. As the relatively low dielectric constant of the hexagonal boron nitride may limit EOT scaling, we will investigate the synthesis and properties of alternative 2D dielectric nanosheets with high dielectric constants, such as Ti(1-d)O2 nanosheets and alternatives. 2D dielectric nanosheets are of particular interest as the 2D dielectric/2D semiconductor interface is based on van der Waals bonding. Such stacks represent ideal structures with fully passivated surfaces free of dangling bonds, for which performance degradation due to interface states or surface traps could be ruled out. Finally, the properties of the deposited dielectric layers, the resulting interface with the 2D semiconductor (e.g. interface state density) and the impact on the mobility of the 2D semiconductor will be characterized in electrical devices.