  Two-dimensional semiconductors feature valleytronics phenomena due to locking
of the spin and momentum valley of the electrons. However, the valley
polarization is intrinsically limited in monolayer crystals by the fast
intervalley electron-hole exchange. Hetero-bilayer crystals have been shown to
have a longer exciton lifetime and valley depolarization time. But the reported
valley polarization was low; the valley selection rules and mechanisms of
valley depolarization remains controversial. Here, we report singlet and
brightened triplet interlayer excitons both with over 80% valley polarizations,
cross- and co-polarized with the pump laser, respectively. This is achieved in
WSe2/MoSe2 hetero-bilayers with precise momentum valley alignment and narrow
emission linewidth. The high valley polarizations allow us to identify the band
minima in a hetero-structure and con_rm unambiguously the direct band-gap
exciton transition, ultrafast charge separation, strongly suppressed valley
depolarization. Our results pave the way for using semiconductor heterobilayers
to control valley selection rules for valleytronic applications.
