Double carrier transport in electron-doped region in black phosphorus FET

Double carrier transport has been observed in thin film black phosphorus (BP) field effect transistor devices in the highly electron-doped region. BP thin films with a typical thickness of 15 nm were encapsulated by hexagonal boron nitride thin films to avoid degradation by air exposure. Their Hall mobility reached 5300 cm2/V s and 5400 cm2/V s at 4.2 K in the hole- and electron-doped regions, respectively. The gate voltage dependence of conductivity exhibits an anomalous shoulder structure in the electron-doped region. In addition, at gate voltages above the shoulder, the magnetoresistance changes to positive, and there appears an additional slow Shubnikov-de Haas oscillation. These results strongly suggest the appearance of second carriers, which originate from the second subband with a localized band edge.