  Two-dimensional (2D) ferrovalley semiconductor (FVSC) with spontaneous valley
polarization offers an exciting material platform for probing Berry phase
physics. How FVSC can be incorporated in valleytronic device applications,
however, remain an open question. Here we generalize the concept of
metal/semiconductor (MS) contact into the realm of valleytronics. We propose a
half-valley Ohmic contact based on FVSC/graphene heterostructure where the two
valleys of FVSC separately forms Ohmic and Schottky contacts with those of
graphene, thus allowing current to be valley-selectively injected through the
`Ohmic' valley while being blocked in the `Schottky' valley. We develop a
theory of contact-limited valley-contrasting current injection and demonstrate
that such transport mechanism can produce gate-tunable valley-polarized
injection current. Using RuCl$_2$/graphene heterostructure as an example, we
illustrate a device concept of valleytronic barristor where high valley
polarization efficiency and sizable current on/off ratio, can be achieved under
experimentally feasible electrostatic gating conditions. These findings uncover
contact-limited valley-contrasting current injection as an efficient mechanism
for valley polarization manipulation, and reveals the potential of valleytronic
MS contact as a functional building block of valleytronic device technology.
