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 [1,2].
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 therefore to obtain a more detailed understanding of the structure of 2D semiconductor – dielectric interfaces. 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 as such the 2D semiconductor behavior by means of physical and electrical characterization methods. We will focus both on amorphous high-k dielectric layers (e.g. HfO2, Al2O3, ZrO2, TiO2 ...) and/or 2D dielectric layers (e.g. hexagonal boron nitride, Ti(1-)O2 ...). The combination of 2D semiconductors with 2D dielectric layers is of particular interest: as all interfaces are 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.
 M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh and H. Zhang, Nature Chemistry, 5, 263 (2013).
 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nature nanotechnology, 6, 147 (2011).
 Li, S.-L.; Tsukagoshi, K.; Orgiu, E.; Samorì, P. Chem. Soc. Rev., 45, 118 (2015).
Required background: material engineering, material science, nanotechnology, chemistry, physics
Type of work: 10% literature, 90% experimental work
Supervisor: Annelies Delabie
Daily advisor: Dennis Lin
The reference code for this PhD position is STS1712-46. Mention this reference code on your application form.