Randall Feenstra

The research activities of my group deal with structural and electronic properties of semiconductor materials and devices. A major tool used in the studies is the scanning tunneling microscope, which allows one to image the atomic structure of a surface and to perform spectroscopic measurements of the electronic energy levels. Many of the studies deal with semiconductor heterostructures consisting of multiple layers of different types of material, with the goal of understanding how the structure of the device (including imperfections and defects) determines its electronic properties. Growth of semiconductor heterostructures has been performed in my laboratory using molecular beam epitaxy, for GaN in particular (a semiconductor with a relatively large band gap, used for blue light-emitting devices and for microwave transistor applications).
Most recently we have focused on the study of two-dimensional (2D) materials, including graphene and hexagonal boron nitride (h-BN). We prepare these materials by growth at high temperatures, and we characterize them using both scanning tunneling microscopy and low-energy electron microscopy. The latter permits both diffraction and imaging of the surfaces, with nm-scale resolution. Additionally, spectroscopic observation of energy levels above the vacuum level is performed, which is particularly useful for these 2D materials. Heterostructures consisting of alternating layers of graphene and h-BN are being studied, because of the unique current-voltage characteristic for tunneling in such structures.