Reduction of bulk carrier concentration in Bridgman-grown Bi2Se3 topological insulator by crystallization with Se excess and Ca doping

• Doping with Ca and crystallization with Se excess is applied to reduce carrier concentration in Bi2Se3.
• Reduction of the electron concentration down to 1017 cm−3 is achieved.
• Hole concentration up to 1018 cm−3 is achieved.
• Large single-crystalline grains up to about 1 cm2×10 cm are obtained.
• It is necessary to compromise between a low carrier concentration and a good crystal morphology.

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 non-stoichiometric growth allowed to reduce the room-temperature electron concentration from ~1019 cm−3 to ~1017 cm−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 1×1018 cm−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 2×1018 cm−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.