Interface Chemistry of Contact Metals and Ferromagnets on the Topological Insulator Bi2Se3

The interface between the topological insulator (TI) Bi2Se3 and deposited metal films is investigated using X-ray photoelectron spectroscopy including conventional contact metals (Au, Pd, Cr, and Ir) and magnetic materials (Co, Fe, Ni, Co0.8Fe0.2, and Ni0.8Fe0.2). Au is the only metal to show little or no interaction with the Bi2Se3, with no interfacial layer between the metal and the surface of the TI. The other metals show a range of reaction behaviors with the relative strength of reaction (obtained from the amount of Bi2Se3 consumed during reaction) ordered as Au < Pd < Ir < Co <= CoFe < Ni < Cr < NiFe < Fe, in approximate agreement with the behavior expected from the Gibbs free energies of formation for the alloys formed. Post metallization anneals at 300 degrees C in vacuum were also performed for each interface. Several of the metal films were not stable upon anneal and desorbed from the surface (Au, Pd, Ni, and Ni0.8Fe0.2), while Cr, Fe, Co, and Co0.8Fe0.2 showed accelerated reactions with the underlying Bi2Se3, including interdiffusion between the metal and Se. Ir was the only metal to remain stable following anneal, showing no significant increase in reaction with the Bi2Se3. This study reveals the nature of the metal Bi2Se3 interface for a range of metals. The reactions observed must be considered when designing Bi2Se3-based devices.