Design and Validation of a Custom Zone Leveling Furnace for Bi_1-xSb_x Crystal Growth

Bi_1-xSb_x alloys (0.07 < x < 0.22) are well-known narrow-bandgap semiconductors that are promising for low-temperature thermoelectrics. In recent years, inversion of the conduction and valence bands at the L-point was demonstrated for x ∼ 0.1, leading to their identification as strong topological insulators with non-trivial weak indices.Thus, BiSb alloys are ideal candidates for achieving topologically-protected conduction channels along dislocations. However, due to the large difference in Bi and Sb melting temperatures, and large liquid-solid separation, growth of Bi_1-xSb_x crystals is difficult to achieve using Czochralski and Bridgman methods. Therefore, we are building a horizontal zone refining instrument [1] based upon a Lathe with gearing modified for <1 mm/hour movement. To avoid phase separation during solidification, optical heating elements are mounted on a linear translation stage that facilitates generation and confinement of the molten zone. Components are fabricated via additive manufacturing using a wide range of polymer-based materials that meet the high-heat and minimal-vibration requirements. In addition to describing the zone leveling furnace design, we will present movies of Bi-Sb crystal growth and subsequent characterization of the Bi-Sb ingot crystallinity and transport properties. We will also describe progress towards generating dislocations appropriate for hosting topologically-protected states in Bi-Sb alloys.