  Quantum wells constitute one of the most important classes of devices in the
study of 2D systems. In a double layer QW, the additional "which-layer" degree
of freedom gives rise to celebrated phenomena such as Coulomb drag, Hall drag
and exciton condensation. Here we demonstrate facile formation of wide QWs in
few-layer black phosphorus devices that host double layers of charge carriers.
In contrast to tradition QWs, each 2D layer is ambipolar, and can be tuned into
n-doped, p-doped or intrinsic regimes. Fully spin-polarized quantum Hall states
are observed on each layer, with enhanced Lande g-factor that is attributed to
exchange interactions. Our work opens the door for using 2D semiconductors as
ambipolar single, double or wide QWs with unusual properties such as high
anisotropy.
