Two-dimensional layered semiconductors have recently captured the imagination of scientists and engineers because of their unusual physical properties and potential for many applications. For example, transistors fabricated from these materials could be used in consumer electronics; photovoltaics derived from the materials could offer greater energy independence; and molecular sensing using these materials could aid homeland security. To realize the potential of these new materials and use them efficiently in a variety of devices, it is critical to understand and develop electrical contacts that allow current to flow in and out of the devices without significant losses. A graduate student and undergraduate students are receiving direct training in science and engineering research methods as they address this scientific problem. The principal investigator is also gaining experience that informs upper-level material science and engineering courses, as well as outreach activities for underrepresented middle- and high-school students.
This project addresses the need for low-resistance Ohmic contacts for two-dimensional layered dichalcogenide semiconductors, including molybdenum disulfide and tantalum diselenide. Investigators are using thermodynamic calculations and phase diagrams to select metal contacts that are likely to exhibit different types of reactions upon annealing. The scenarios for reaction include diffusion of the metal into the semiconductor, substitution of other transition metals for molybdenum or tungsten, and formation of a different layered semiconductor with a different band gap. Such reactions offer the potential to dope the semiconductor or alter the Schottky barrier height at the metal/semiconductor interface, and they could therefore reduce the resistance of the contacts. The reactions are under study using transmission electron microscopy (including aberration-corrected scanning/transmission electron microscopy), which is well suited to detecting changes in the arrangement of atoms at or near the metal/semiconductor interface. Researchers also employ complementary approaches to measure specific contact resistance and gain greater insight into current transport through the contacts. The project contributes to a deeper understanding of contacts to two-dimensional layered dichalcogenide semiconductors and more generally to the science of processing layered materials.
