Yoon’s group gave a presentation at the Materials Research Society conference. The work is conducted in close collaborations with Dr. Kyu-Bum Han, Dr. Jeffery Aguiar, Dr. Brian van Devener, Prof. Vikram Deshpade, and Prof. Taylor Spark.
ABSTRACT: Bi2-xSbxTe3-ySey (BSTS) is an interesting new topological insulator with applications ranging from THz optoelectronics to quantum computing. Recent theoretical calculations have suggested that BSTS has superior topological insulating (TI) properties. The large area crystallinity and stoichiometry of synthesized BSTS, however, presents a challenge to measure unique TI properties. In this work, we present a hybrid growth method of large BSTS single crystals (> 5 mm) and 3D characterization techniques of the materials.
The BSTS single crystals are tailored to achieve structural and stoichiometric uniformity throughout the material. These single crystals were synthesized in a vacuum ampule that started with randomly oriented polycrystalline starting BSTS materials. An optimized Bridgman and zone melting technique was then applied to the starting batch materials to grow high purity crystals at 850 °C with a translation rate of 4 mm/hour. BSTS single crystals were extracted from the different sections of the ampule to study in detail. In order to investigate the chemical and structural properties of the BSTS TIs from surface to bulk interior, we used two complementary electron beam based techniques including: 1) a series of energy dispersive X-ray (EDS) measurements on bulk crystals in a scanning electron microscope (SEM), and (2) cross-sectional EDS on BSTS lamella in a scanning transmission electron microscope (STEM). In the EDS/SEM technique, the electron beam was injected directly into the surface of the samples. As the electron beam energy increases from 5 kV to 30 kV, the probing bulb size and sampling volume also increases from 70 nm to 1.5 μm in BSTS. The EDS measurements were then compared to Monte Carlo electron beam simulations, and the results showed a significant difference in the composition of selenium (Se). In the high-resolution EDS/STEM of the cross-sectional lamella, we also measured a higher Se concentration at the surface (<50 nm) than deeper into the interior. The similarity between the EDS/SEM bulk measurements and the EDS/STEM cross-sectional measurements indicate that the BSTS material does indeed have a Se-rich surface. In parallel, we collected selected area electron beam diffraction and high-resolution atomic scale lattice images. The obtained data suggested that the surface of BSTS crystal is highly ordered. Aligning both EDS and atomic resolution imaging furthermore resolved Se and Te alternation and subsequent build-up at the interface between the surface layer and the underlying single crystals. Overall, our results clearly delineate the necessary structural and chemical insights to further tailor and innovate topological insulators for a variety of properties, including emerging growth strategies to realize class leading 2D materials.