  Covalent-polar semiconductors that show intrinsic two-dimensional (2D)
vertical polarization present new device opportunities. These materials differ
from ordinary ferroelectrics in that they are able to maintain polarization
normal to a surface even with an unscreened depolarization field. Identifying
phases that exhibit intrinsic 2D vertical polarization is an ongoing challenge.
Here we report via computational material design the discovery of a new
promising phase, specifically 2D LiAlTe$_2$. The design idea is developed from
the physical understanding of three-dimensional hyperferroelectric covalent
polar semiconductors. We used the structure determination method combining
swarm intelligence algorithm and first-principles calculations to identify
energetically stable structures. In addition to the expected layered version of
bulk LiAlTe$_2$, $\beta$-LiAlTe$_2$, we find a novel 2D structure,
$\gamma$-LiAlTe$_2$. In this phase, the vertical dipole can be switched between
0.07 and -0.11 e$\cdot$angstrom. This switching is triggered by the movement of
Li atom between two local energy minima. The associated asymmetric double-well
energy profile can be continuously tuned by the applied electric field as well
as strain. There is, therefore, a reversible transition between two polar
states. This discovered off-plane switchability provides an opportunity for the
2D $\gamma$-LiAlTe$_2$ based interfacial phase change memory device for example
by growing $\gamma$-LiAlTe$_2$/GeTe heterostructures.
