An imaging device may include single-photon avalanche diodes (SPADs). To improve the sensitivity and signal-to-noise ratio of the SPADs, light scattering structures may be formed in the semiconductor substrate to increase the path length of incident light through the semiconductor substrate. The light scattering structures may include a low-index material formed in trenches in the semiconductor substrate. The light scattering structures may have different sizes and/or a layout with a non-uniform number of structures per unit area. SPAD devices may also include isolation structures in a ring around the SPADs to prevent crosstalk. The isolation structures may include metal-filled deep trench isolation structures. The metal filler may include tungsten.