Reduction of bulk carrier concentration in Bridgman-grown Bi sub(2)Se sub(3) topological insulator by crystallization with Se excess and Ca doping

Bismuth selenide topological insulator with a lowered bulk carrier concentration was grown by the vertical Bridgman method from stoichiometric or non-stoichiometric melt, both with and without calcium doping. Limits for the reduction of the carrier concentration were investigated. The nonstoichiometric growth allowed to reduce the room-temperature electron concentration from ~10 super(19) cm super(-3) to ~10 super(17)cm super(-3). The increase of the selenium-to-bismuth ratio led to a phase separation and formation of metallic selenium precipitates within the van der Waals gaps. Crystallization from the selenium-rich melt with calcium acceptor added allowed for a further donor compensation and obtaining of p-type material. The lowest hole concentration achieved was 1 x 10 super(18) cm super(-3). It was thus shown that it is necessary to compromise between a low carrier concentration and a good crystal morphology-the electron concentration of 2 x 10 super(8) cm super(-3) constitutes a limit below which precipitation of Se occurs in the growth from Se-rich melt. The case of p-type doping requires both adding the Ca acceptor and performing crystallization from non-stoichiometric melt, which is associated with the occurrence of precipitates of foreign phases.