Stability of low-carrier-density topological-insulator Bi$_2$Se$_3$ thin films and effect of capping layers

摘要

Although over the past number of years there have been many advances in thematerials aspects of topological insulators (TI), one of the ongoing challengeswith these materials is the protection of them against aging. In particular,the recent development of low-carrier-density bulk-insulating Bi$_2$Se$_3$ thinfilms and their sensitivity to air demands reliable capping layers to stabilizetheir electronic properties. Here, we study the stability of thelow-carrier-density Bi$_2$Se$_3$ thin films in air with and without variouscapping layers using DC and THz probes. Without any capping layers, the carrierdensity increases by ~150% over a week and by ~280% over 9 months. Insitu-deposited Se and ex situ-deposited Poly(methyl methacrylate) (PMMA)suppresses the aging effect to ~27% and ~88% respectively over 9 months. Thecombination of effective capping layers and low-carrier-density TI films willopen up new opportunities in topological insulators.