paper

Chemical Migration and Dipole Formation at van der Waals Interfaces between Magnetic Transition Metal Chalcogenides and Topological Insulators

arXiv:2004.05748 · doi:10.1103/PhysRevMaterials.4.054001

Abstract

Metal and magnetic overlayers alter the surface of the topological insulator (TI) bismuth selenide (Bi $_{2}$ Se $_{3}$ ) through proximity effects but also by changing the composition and chemical structure of the Bi $_{2}$ Se $_{3}$ sub-surface. The interface between Bi $_{2}$ Se $_{3}$ and Mn metal or manganese selenide was explored using x-ray photoelectron spectroscopy (XPS) revealing chemical and electronic changes at the interface. Depositing Mn metal on Bi $_{2}$ Se $_{3}$ without an external source of Se shows unexpected bonding within the Mn layer due to Mn-Se bonding as Se diffuses out of the Bi $_{2}$ Se $_{3}$ layer into the growing Mn film. The Se out-diffusion is further evidenced by changes in Bi core levels within the Bi $_{2}$ Se $_{3}$ layers indicating primarily Bi-Bi bonding over Bi-Se bonding. No out-diffusion of Se occurred when excess Se is supplied with Mn, indicating the importance of supplying enough chalcogen atoms with deposited metals. However, Bi $_{2}$ Se $_{3}$ core level photoelectrons exhibited a rigid chemical shift toward higher binding energy after depositing a monolayer of MnSe $_{2 - x}$ , indicating a dipole within the overlayer. Stoichiometry calculations indicated that the monolayer forms MnSe preferentially over the transition metal dichalcogenide (TMD) phase MnSe $_{2}$ , providing a consistent picture of the dipole formation in which a plane of Se anions sits above Mn cations. This study shows that chemical diffusion and dipole formation are important for Mn-Bi $_{2}$ Se $_{3}$ and MnSe $_{2 - x}$ -Bi $_{2}$ Se $_{3}$ and should be considered carefully for TMD/TI interfaces more generally.

12 pages, 6 figures + 1 supplemental figure