  Vertical ferroelectricity where a net dipole moment appears as a result of
in-plane ionic displacements has gained enormous attention following its
discovery in transition metal dichalcogenides. Based on first-principles
calculations, we report on the evidence of robust vertical ferroelectricity
upon interlayer sliding in layered semiconducting $\beta$-ZrI$_{2}$, a sister
material of polar semimetals MoTe$_{2}$ and WTe$_{2}$. The microscopic origin
of ferroelectricity in ZrI$_{2}$ is attributed to asymmetric shifts of
electronic charges within a trilayer, revealing a subtle interplay of rigid
sliding displacements and charge redistribution down to ultrathin thicknesses.
We further investigate the variety of ferroelectric domain boundaries and
predict a stable charged domain wall with a quasi-two-dimensional electron gas
and a high built-in electric field that can increase electron mobility and
electromechanical response in multifunctional devices. Semiconducting behaviour
and a small switching barrier of ZrI$_{2}$ hold promise for novel ferroelectric
applications, and our results provide important insights for further
development of slidetronics ferroelectricity.
