Ferecrystal Thin Films Annealed in Selenium Vapor

Bismuth selenide and niobium diselenide layers make up a ferecrystal thin film with 30 singly alternating repeat layers. Amorphous precursors were deposited via Modulated Elemental Reactants (MER) physical vapor deposition, then annealed on a hot plate in a nitrogen environment to form turbostratically disordered layers. In this study, the ferecrystals were annealed individually in a furnace at various temperatures and durations while in a sealed, evacuated tube with powdered tin diselenide creating a selenium vapor pressure. The experiment attempts to understand how resistivity properties change as a result of a post-annealing treatment on these films.

X-ray diffraction showed structural changes in the film. Partial or full phase separation occurred in films post-annealed at temperatures between 225° C and 400° C. Islands of trigonal Bi₂Se₃ formed at the surface of the film, as seen in cross-sectional images obtained using Transmission Electron Microscopy techniques. In-plane diffraction scans were indexed for hk0 reflections, and showed that trigonal Bi₂Se₃, orthorhombic BiSe, and trigonal NbSe₂ were in all post-annealed samples annealed at temperatures 225° C and greater, with the exception of the fully segregated sample which contained only Bi₂Se₃ and NbSe_2. Out-of-plane x-ray diffraction revealed 00l reflections changed in intensity and shifted slightly to higher diffraction angles as the temperature at which the film was post-annealed was increased. Partially phase separated samples contained Bi₂Se₃, BiSe, and NbSe_{2 ,} while the completely phase separated sample had only Bi₂Se₃ and NbSe₂. Films with more Bi₂Se₃ had more temperature dependent resistivity trends.

Complete phase separation occurred in a film that was vapor annealed for excess time, and included a surface phase of only Bi₂Se₃ and a second layer of NbSe₂. Selenium content generally increased in films as time vapor-annealed increased. Film thickness was calculated using a modified version of Bragg’s Law. Thickness generally increased with time in the partially phase separated samples, however the completely phase separated sample had a significant reduction in film thickness because there were two crystalline layers instead of 30 individual 2D layers. Films with more Bi₂Se₃ had greater resistivity trends likely due to the structural defects affecting electron transport in the film. This vapor annealing technique facilitated opportunity for structural changes and defects to occur due to the nature of the vapor diffusion through the surface of the film, rather than filling holes in the original annealed film. Future experiments could involve conditions that limit the loss of selenium and restrict phase separation.