  In this work, the structural, electrical, and optical properties of bilayer
SiX (X= N, P, As, and Sb) are studied using density functional theory (DFT).
Five different stacking orders are considered for every compound and their
structural properties are presented. The band structure of these materials
demonstrates that they are indirect semiconductors. The out-of-plane strain has
been applied to tune the bandgap and its electrical properties. The bandgap
increases with tensile strain, whereas, compressive strain leads to
semiconductor-to-metal transition. The sensitivity of the bandgap to the
pressure is investigated and bilayer SiSb demonstrates the highest bandgap
sensitivity to the pressure. These structures exhibit Mexican hat-like valence
band dispersion that can be approved by a singularity in the density of states.
The Mexican-hat coefficient can be tuned by out-of-plane strain. Optical
absorption of these compounds shows that the second and lower valence bands due
to the high density of states display a higher contribution to optical
transitions.
