Capping layers are essential for protecting phase change materials (PCMs) used in nonvolatile photonics technologies. This work demonstrates how (ZnS)_0.8-(SiO₂)_0.2 caps radically influence the performance of Sb₂S₃ and Ag-doped Sb₂S₃ 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 Sb₂S₃ and Ag-doped Sb₂S₃ increased and decreased, respectively, which is attributed to interfacial energy differences. Capped and uncapped Ag-doped Sb₂S₃ microring resonator (MRR) devices were fabricated and measured to understand how the cap affects the device performance. Surprisingly, the resonance 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 concluded 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 the light–matter interactions with the PCM element.