A photonic device includes a silicon layer, wherein the silicon layer extends from a waveguide region of the photonic device to a device region of the photonic device, and the silicon layer includes a waveguide portion in the waveguide region. The photonic device further includes a cladding layer over the waveguide portion, wherein the device region is free of the cladding layer. The photonic device further includes a low refractive index layer in direct contact with the cladding layer, wherein the low refractive index layer comprises silicon oxide, silicon carbide, silicon oxynitride, silicon carbon oxynitride, aluminum oxide or hafnium oxide. The photonic device further includes an interconnect structure over the low refractive index layer.