  The two-dimensional (2D) IV-V semiconductors have attracted much attention
due to their fascinating electronic and optical properties. In this work, we
predicted three phases of silicon nitrides, denoted $\alpha$-Si$_{2}$N$_{2}$,
$\beta$-Si$_{2}$N$_{2}$, and $\gamma$-Si$_{4}$N$_{4}$, respectively. Both
$\alpha$-Si$_{2}$N$_{2}$ and $\beta$-Si$_{2}$N$_{2}$ consist of two buckled SiN
sheets, and $\gamma$-Si$_{4}$N$_{4}$ consists of two puckered SiN sheets. It is
challenging to transform between $\alpha$-Si$_{2}$N$_{2}$ and
$\beta$-Si$_{2}$N$_{2}$ because of the high energy barrier. The three
dynamically stable bilayers are semiconductors with fundamental indirect band
gaps from 0.25 eV to 2.92 eV. As expected, only the s and p orbitals contribute
to the electronic states, and the pz orbitals dominate near the Fermi level.
Furthermore, insulator-metal transitions occur in $\alpha$-Si$_{2}$N$_{2}$ and
$\beta$-Si$_{2}$N$_{2}$ under the biaxial strain of 16%. These materials
perhaps have potential applications in microelectronics and spintronics.
