  Recently, it has been known that the hidden Rashba (R-2) effect in
two-dimensional materials gives rise to a novel physical phenomenon called
spin-layer locking (SLL). However, not only has its underlying fundamental
mechanism been unclear, but also there are only a few materials exhibiting weak
SLL. Here, through the first-principles density functional theory and model
Hamiltonian calculation, we reveal that the R-2 SLL can be determined by the
competition between the sublayer-sublayer interaction and the spin-orbit
coupling (SOC), which is related to the Rashba strength. In addition, the
orbital angular momentum distribution is another crucial point to realize the
strong R-2 SLL. We propose that a novel 2D material Si$_2$Bi$_2$ possesses an
ideal condition for the strong R-2 SLL, whose Rashba strength is evaluated to
be 2.16 eV{\AA}, which is the greatest value ever observed in 2D R-2 materials
to the best of our knowledge. Furthermore, we reveal that the interlayer
interaction in a bilayer structure ensures R-2 states spatially farther apart,
implying a potential application in spintronics.
