Dipole Contact Engineering for Field-Effect Transistors Based on Two-Dimensional Materials

Abstract

In the next several years and decades, the expanded use of artificial intelligence and edge computing will demand more powerful and energy-efficient electronics. Two-dimensional (2D) semiconductors, and in particular transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂), are promising candidates for future field-effect transistors. TMDs can enable aggressive lateral and vertical device scaling, and they can add computing power density and new memory and sensing capabilities via 3D integration. However, several key challenges remain before 2D-channel transistors become commercially viable, including large contact resistances at the source and drain due to the van der Waals surface of 2D materials and the Fermi level pinning effect. A variety of methods have been explored to make ohmic contacts to MoS₂, the most promising of which so far is to use semimetals such as Bi and Sb, however these materials suffer from thermal instability. This thesis addresses these challenges by (1) exploring the ultimate limit of contact metal workfunction scaling to better understand the metal-MoS₂ interface, and (2) introducing a new method of reducing contact resistance to 2D materials by inserting dipole layers at the contact interface. Initial work on ultralow-workfunction (ULWF) metal deposition on MoS₂ and subsequent device fabrication is presented, though further study is required to mitigate effects from deposition equipment and the reactive nature of these metals. In parallel, the Janus TMD MoSSe is explored as an example system for dipole contacts, with extensive material characterization of the Janus TMD MoSSe being performed, and the effect of a dipole layer on the contact properties of FETs being established. Together, these results are a significant step towards solving one of the major hurdles for the commercial introduction of 2D-channel transistors.