Hole doping problem in Bi$_{2}$Se$_{3}$ thin films and its solution: Role of interfacial vs bulk defects

Bi$_{2}$Se$_{3}$, one of the most widely studied topological insulators, is naturally electron (n-type) doped. In bulk crystals and thick films, n-type Bi$_{2}$Se$_{3}$ can be converted into p-type through compensation doping. However, such a compensation doping scheme has so far failed to achieve p-type Bi$_{2}$Se$_{3}$ thin films. Here, we show that there exists a thickness-dependent p-doping problem in Bi$_{2}$Se$_{3}$ thin films, which originates from the high density of interfacial defects compared with that of the bulk, and provide a solution to this long standing puzzle. Reducing the interfacial defects by employing an effective buffer layer on the bottom and a capping layer on the top, we facilitate the compensation doping scheme and achieved dominant p-type carriers in Bi$_{2}$Se$_{3}$ thin films down to the thinnest topological regime. Availability of p-type Bi$_{2}$Se$_{3}$ thin films will open new opportunities in this active field of topological materials.