  Chalcogenide phase-change materials (PCMs) are widely applied in electronic
and photonic applications, such as non-volatile memory and neuro-inspired
computing. Doped Sb$_2$Te alloys are now gaining increasing attention for
on-chip photonic applications, due to their growth-driven crystallization
features. However, it remains unknown whether Sb$_2$Te also forms a metastable
crystalline phase upon nanoseconds crystallization in devices, similar to the
case of nucleation-driven Ge-Sb-Te alloys. Here, we carry out ab initio
simulations to understand the changes in optical properties of amorphous
Sb$_2$Te upon crystallization and post annealing. During the continuous
transformation process, changes in the dielectric function are highly
wavelength-dependent from the visible-light range towards the telecommunication
band. Our finite-difference time-domain simulations based on the ab initio
input reveal key differences in device output for color display and photonic
memory applications upon tellurium ordering. Our work serves as an example of
how multiscale simulations of materials can guide practical photonic
phase-change applications.
