  In analogy to III-V compounds, which have significantly broadened the scope
of group IV semiconductors, we propose IV-VI compounds as isoelectronic
counterparts to layered group V semiconductors. Using {\em ab initio} density
functional theory, we study yet unrealized structural phases of silicon
mono-sulfide (SiS). We find the black-phosphorus-like $\alpha$-SiS to be almost
equally stable as the blue-phosphorus-like $\beta$-SiS. Both $\alpha$-SiS and
$\beta$-SiS monolayers display a significant, indirect band gap that depends
sensitively on the in-layer strain. Unlike 2D semiconductors of group V
elements with the corresponding nonplanar structure, different SiS allotropes
show a strong polarization either within or normal to the layers. We find that
SiS may form both lateral and vertical heterostructures with phosphorene at a
very small energy penalty, offering an unprecedented tunability in structural
and electronic properties of SiS-P compounds.
