  Silicon telluride (Si2Te3) is a silicon-based 2D chalcogenide with potential
applications in optoelectronics. It has a unique crystal structure where Si
atoms form Si-Si dimers to occupy the "metal" sites. In this paper, we report
an ab initio computational study of its optical dielectric properties using the
GW approximation and the Bethe-Salpeter equation (BSE). Strong optical
anisotropy is discovered. The imaginary part of the dielectric constant in the
direction parallel to the Si-Si dimers is found to be much lower than that
perpendicular to the dimers. We show this effect originates from the particular
compositions of the wavefunctions in the valence and conduction bands. BSE
calculations reduce GW quasiparticle band gap by 0.3 eV in bulk and 0.6 eV in
monolayer, indicating a large excitonic effect in Si2Te3. Furthermore,
including electron-hole interaction in bulk calculations significantly reduces
the imaginary part of the dielectric constant in the out-of-plane direction,
suggesting strong interlayer exciton effect in Si2Te3 multilayers.
