  We demonstrate that encapsulation of atomically thin black phosphorus (BP) by
hexagonal boron nitride (h-BN) sheets is very effective for minimizing the
interface impurities induced during fabrication of BP channel material for
quantum transport nanodevices. Highly stable BP nanodevices with ultrahigh
mobility and controllable types are realized through depositing appropriate
metal electrodes after conducting a selective etching to the BP encapsulation
structure. Chromium and titanium are suitable metal electrodes for BP channels
to control the transition from a p-type unipolar property to ambipolar
characteristic because of different work functions. Record-high mobilities of
6000 $cm^2V^{-1}s^{-1}$ and 8400 $cm^2V^{-1}s^{-1}$ are respectively obtained
for electrons and holes at cryogenic temperatures. High-mobility BP devices
enable the investigation of quantum oscillations with an indistinguishable
Zeeman effect in laboratory magnetic field.
