UCLA

Over 50 years, Moore’s law has successfully predicted the progress of the silicon electronics industry. However, Moore’s law is approaching to the end recently, and new material and novel device type will be needed for next-generation devices. Two-dimensional (2D) layered material is one of the promising candidates due to its intrinsic desirable features, such as diverse electronic and magnetic properties, carrier mobility protection with decreasing body thickness, and flexibility for wearable applications. Furthermore, hetero-structure comprising of 2D crystals is of growing interest since various combinations are possible for multiple purposes as more and more van der Waals materials have been discovered. In a hetero-structure, electrons can propagate not only within 2D in-plane direction but also in the vertical out-of-plane direction. However, our understanding of the vertical carrier transport is greatly less than that on the lateral. Thus, using lateral electron energy band diagrams are still the main vehicle in 2D vertical hetero-structure device analysis, which may not be correct.

In this dissertation, silicon-based tunneling devices were fabricated and used to investigate electron transport properties when electrons go into or go across a 2D materials with the measurement of electron tunneling spectroscopy. We firstly examine the role of 2D sheet when electrons propagate perpendicularly across it. Here graphene is used as a platform since it is the earliest discovered one, and it has the most mature development including understanding and growth techniques. Graphene together with its neighboring van de Waals gaps serves as a tunnel barrier and barely has interaction with the vertically tunneling electrons. However, since graphene can still trap a fraction of carriers, we can take advantage of it and control the carrier flux via the adjustment of graphene electrical potential.

In addition to vertically propagating across a graphene sheet, electrons can go into graphene lateral band structure and transport within graphene. In chapter 3, we introduce a new model of the interfacial oscillation states at graphene-silicon hetero-junction, which is found and confirmed for the first time. Because of the presence of this discrete interfacial quantum state, Fano-Feshbach resonance is induced by its interaction with graphene’s continuum lateral energy diagram. This study provided a further elucidation of the interfacial effect in a low-dimension materials based system.

The capability of our silicon-based tunneling device along with electron tunneling spectroscopy is not limited to the graphene-silicon interface but also able to investigate electron in-plane transport behavior within 2D hetero-structure. Since large-size devices and their macroscopic characteristics would be needed for our everyday applications in the future, the strength of our tunneling device over the conventional scanning tunneling spectroscopy with a sharp tip is its scalable detecting area. Here, a study on graphene/hexagonal boron nitride hetero-stack prepared by chemical vapor deposition and large-area wet transfer techniques shows multiple secondary Dirac points and the preferred relative rotation angle of ~4� and ~7�. The theoretical calculation was also implemented to support our experimental observation. Raman spectroscopy and scanning tunneling microscope were carried out to confirm the Moiré pattern formation. This study provides a useful way to macroscopically conduct research on the electronic behavior of a van der Waals material, and our findings may be used when graphene/hexagonal boron nitride hetero-structure is pushed to practical applications. Undoubtedly, further careful study is needed for more detailed verification.