  Low carrier mobility and high electrical contact resistance are two major
obstacles prohibiting explorations of quantum transport in TMDCs. Here, we
demonstrate an effective method to establish low-temperature Ohmic contacts in
boron nitride encapsulated TMDC devices based on selective etching and
conventional electron-beam evaporation of metal electrodes. This method works
for most extensively studied TMDCs in recent years, including MoS2, MoSe2,
WSe2, WS2, and 2H-MoTe2. Low electrical contact resistance is achieved at 2 K.
All of the few-layer TMDC devices studied show excellent performance with
remarkably improved field-effect mobilities ranging from 2300 cm2/V s to 16000
cm2/V s, as verified by the high carrier mobilities extracted from Hall effect
measurements. Moreover, both high-mobility n-type and p-type TMDC channels can
be realized by simply using appropriate contact metals. Prominent Shubnikov-de
Haas oscillations have been observed and investigated in these high-quality
TMDC devices.
