  In this paper, the electrical and spin properties of mono- and bilayer HfSSe
in the presence of a vertical electric field are studied. The density
functional theory is used to investigate their properties. Fifteen different
stacking orders of bilayer HfSSe are considered. The mono- and bilayer
demonstrate an indirect bandgap, whereas the bandgap of bilayer can be
effectively controlled by electric field. While the bandgap of bilayer closes
at large electric fields and a semiconductor to metal transition occurs, the
effect of a normal electric field on the bandgap of the monolayer HfSSe is
quite weak. Spin-orbit coupling causes band splitting in the valence band and
Rashba spin splitting in the conduction band of both mono- and bilayer
structures. The band splitting in the valence band of the bilayer is smaller
than a monolayer, however, the vertical electric field increases the band
splitting in bilayer one. The stacking configurations without mirror symmetry
exhibit Rashba spin splitting which is enhanced with the electric field.
