Phase transitions, transfer and nanoscale growth of epitaxial Bi and Bi1-xSbx thin films
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Bismuth (Bi) and Bismuth-Antimony (Bi [subscript 1-x] Sb [subscript x]) alloys are considered very promising for emerging spintronic devices due to their large spin-orbit coupling, high mobility, and conductive, spin-split surface states, which are topologically non-trivial in Bi [subscript 1-x] Sb [subscript x]. Due to the long mean free path in Bi, quantum confinement effects become significant in relatively thick (~100 nm) films, resulting in the opening of a small, indirect band gap and enabling tuning of the electronic properties through the film thickness. Quantum confinement effects are expected to occur in Bi [subscript 1-x] Sb [subscript x] films at a similar length scale, which may enlarge the bulk band gap and extend the topologically insulating composition regime. When the film thickness of epitaxial Bi on Si(111) is reduced below a few nanometers, a puckered-layer allotropic structure similar to black phosphorus is stable. This puckered-layer structure is expected to exhibit unique properties, including a larger band gap and increased spin splitting, which may be useful for 2-D spintronics; however, the tendency of this structure to grow in small islands inhibits characterization. This dissertation explores the growth of both bulk-like and puckered-layer Bi and Bi [subscript 1-x] Sb [subscript x] on Si(111), and discusses how the unique properties of this system may be controlled through the growth parameters, film thickness, and composition. We find that while alloying bulk-like Bi with Sb in the quantum confinement thickness regime may increase the band gap, the crystalline orientation changes with increasing concentrations of Sb. This effect has not been observed in epitaxial Bi [subscript 1-x] Sb [subscript x] on other substrates, and significantly impacts the electronic properties of the films. In contrast, alloying Sb with nanoscale puckered-layer Bi improves the crystallinity and continuity, suggesting a promising route towards tuning the band structure of puckered-layer Bi and producing large-area films for electrical measurements. Finally, we demonstrate that epitaxial Bi and Bi [subscript 1-x] Sb [subscript x] films exhibit surprisingly weak adhesion to the Si(111) growth substrate, which may originate from the early allotropic transition. This weak adhesion enables the straightforward transfer of these films, opening a route toward the integration of epitaxial-quality Bi and Bi [subscript 1-x] Sb [subscript x] films with arbitrary substrates for novel heterostructures.