  The double carrier transport has been observed in thin film black phosphorus
(BP) field effect transistor (FET) devices in highly electron doped region. BP
thin films with typical thickness of 15 nm were encapsulated by hexagonal boron
nitride (h-BN) thin films to avoid degradation by air exposure. Their Hall
mobility has reached 5300 cm2/Vs and 5400 cm2/Vs at 4.2 K in the hole and
electron doped regions, respectively. The gate voltage dependence of
conductivity exhibits an anomalous shoulder structure in 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 the second
carriers, which originate from the second subband with localized band edge.
