Structures for a thermo-optic phase shifter and methods of forming such structures. The structure comprises a waveguide structure including a waveguide core. The structure further comprises a silicide layer, a first dielectric layer arranged in a lateral direction between the silicide layer and the waveguide core, and a second dielectric layer positioned over the waveguide core, the silicide layer, and the first dielectric layer. The first dielectric layer comprises a first material having a first thermal conductivity, and the second dielectric layer comprises a second material having a second thermal conductivity that is less than the first thermal conductivity.