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. Heterobilayer crystals have been shown to have a longer exciton lifetime and valley depolarization time. Yet most reported valley polarization was low; the valley selection rules and mechanisms of valley depolarization remain controversial. Here, we show that nearly pure valley polarizations are achievable in high-quality heterostructures with precise momentum-valley alignment. Together with time-resolved and temperature dependence measurements, we furthermore identify both singlet and brightened triplet interlayer excitons with opposite valley polarizations, which originate from direct band-gap transitions and are localized at the Hhh atomic registry - corresponding to the band minima of the heterostructure. Our results also reveal ultrafast interlayer electron transfer and strongly suppressed interlayer exciton valley depolarization. Our results pave the way for using semiconductor heterobilayers to control valley selection rules for valleytronic applications.
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