  Atomically thin two-dimensional (2D) layered semiconductors such as
transition metal dichalcogenides (TMDs) have attracted considerable attention
due to their tunable band gap, intriguing spin-valley physics, piezoelectric
effects and potential device applications. Here we study the electronic
properties of a single layer WS$_{1.4}$Se$_{0.6}$ alloys. The electronic
structure of this alloy, explored using angle resolved photoemission
spectroscopy, shows a clear valence band structure anisotropy characterized by
two paraboloids shifted in one direction of the k-space by a constant in-plane
vector. This band splitting is a signature of a unidirectional Rashba spin
splitting with a related giant Rashba parameter of 2.8 0.7 eV . The combination
of angle resolved photoemission spectroscopy with piezo force microscopy
highlights the link between this giant unidirectional Rashba spin splitting and
an in-plane polarization present in the alloy. These peculiar anisotropic
properties of the WS$_{1.4}$Se$_{0.6}$ alloy can be related to local atomic
orders induced during the growth process due the different size and
electronegativity between S and Se atoms. This distorted crystal structure
combined to the observed macroscopic tensile strain, as evidenced by
photoluminescence, displays electric dipoles with a strong in-plane component,
as shown by piezoelectric microscopy. The interplay between semiconducting
properties, in-plane spontaneous polarization and giant out-of-plane Rashba
spin-splitting in this two-dimensional material has potential for a wide range
of applications in next-generation electronics, piezotronics and spintronics
devices.
