  Capping layers are essential for protecting phase change materials (PCMs)
used in non-volatile photonics technologies. This work demonstrates how
$(ZnS)_{0.8}-(SiO_2)_{0.2}$ caps radically influence the performance of
$Sb_{2}S_{3}$ and Ag-doped $Sb_{2}S_{3}$ integrated photonic devices. We found
that at least 30 nm of capping material is necessary to protect the material
from Sulfur loss. However, adding this cap affects the crystallization
temperatures of the two PCMs in different ways. The crystallization temperature
of $Sb_{2}S_{3}$ and Ag-doped $Sb_{2}S_{3}$ increased and decreased
respectively, which is attributed to interfacial energy differences. Capped and
uncapped Ag-doped $Sb_{2}S_{3}$ microring resonator (MRR) devices were
fabricated and measured to understand how the cap affects the device
performance. Surprisingly, the resonant frequency of the MRR exhibited a larger
red-shift upon crystallization for the capped PCMs. This effect was due to the
cap increasing the modal overlap with the PCM layer. Caps can, therefore, be
used to provide a greater optical phase shift per unit length, thus reducing
the overall footprint of these programmable devices. Overall, we conclude that
caps on PCMs are not just useful for stabilizing the PCM layer, but can also be
used to tune the PCM crystallization temperature and reduce device footprint.
Moreover, the capping layer can be exploited to enhance light-matter
interactions with the PCM element.
