Technical: This project aims to advance the understanding of the epitaxial growth and doping of the Bi2X3 (X=Se, Te) topological insulators and their modulation-doped superlattices. The critical issue is to control impurity incorporation vs. intercalation during growth, with the goal to selectively designate the sites for desired properties. In-situ scanning tunneling microscopy/spectroscopy is carried out to study the surface morphology, atomic structure, and carrier scattering by steps and magnetic impurities, and to determine how the defining properties of topological insulators are affected by magnetic impurity doping, gap opening, and warping of the Dirac cone. Scanning tunneling microscopy is also used to investigate the transport properties of topological insulator films and heterostructures in situ. Element-specific electronic and magnetic properties of dopants are determined by x-ray absorption spectroscopy and magnetic circular dichroism, while structural characterization is carried out using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy, and magnetic and transport properties by temperature- and field-dependent magnetization measurements and Hall effects. The experimental efforts are complemented by density functional theory calculations of the energetics of impurity adsorption and doping, the changes in the electronic band structure, the x-ray absorption spectroscopy and magnetic circular dichroism spectra, and scanning tunneling microscopy images, which helps to identify the most promising heterostructures to be synthesized and their experimental signatures.
The project addresses basic research issues in a topical area of materials science with high technological relevance. With precise control of composition and doping, molecular beam epitaxy facilitates the synthesis of topological insulator heterostructures congruent with existing semiconductor device fabrication infrastructure, which can potentially bring a new paradigm for the next generation electronics. Graduate students are trained in an interdisciplinary environment for materials growth, characterization, and modeling. The PIs' active involvement in an on-going Research Experience for Teachers outreach program continues to bring cutting-edge research on materials sciences to high school students to impact their perception of science and technology, and to inspire them to seek college degrees and careers in science and engineering.