Mn-doping reveals a thermal gap and natural p-type conductivity in Bi2O2Se

Abstract

Bi₂O₂Se is a semiconductor that is being intensively studied due to its many extraordinary properties. Since about 2010, research on polycrystals has focused on thermoelectric materials. For the last 10 years, single crystal research has been driven by its quasi 2D structure with unexpectedly high permittivity (ε_r ≈ 500), which promotes high electron mobility. Bi₂O₂Se thus outperforms other 2D materials in many parameters. However, the high permittivity is also responsible for the extremely low critical concentration of the metal–insulator transition (n ≈ 10¹⁵ cm⁻³). Thus, Bi₂O₂Se is so far only available as an n-type semiconductor largely with metal-like properties, although the electron concentration can range over 6 orders of magnitude (n ≈ 10¹⁵–10²¹ cm⁻³), reportedly due to the very high concentration of selenium vacancies or selenium antisites on the Bi site. In this paper, we consider Mn doping in Bi₂O₂Se, Bi_2−xMn_xO₂Se. The Mn doping leads to a decrease in the electron concentration and, for the first time, to a transition of the material to p-type conductivity. A thermal gap (≈ 0.9 eV) can be deduced from the temperature dependence of the electrical conductivity. The p-type transition is related to the interaction of Mn with the defect structure of Bi₂O₂Se. Our experiments suggest that the most abundant defects, besides the Se vacancies V_Se, are the substitutional defect Se atom at the Bi site, Se_Bi and the O atom at the Se site. From high resolution XRD analysis, we conclude that Mn reduces its concentration and brings the structure to the p-type state. From DFT calculations and magnetic data we infer the substitution of Bi by Mn (Mn_Bi, in a high spin state, μ ≅ 5μ_B), although all experiments indicate a very low solubility n_Mn = 2.67 × 10¹⁸ cm⁻³ based on magnetic data.