An ultraviolet semiconductor light emitting device includes a semiconductor stack, a trench, a filling insulator, and first and second electrodes. The semiconductor stack includes first and second conductivity-type semiconductor layers and an active layer therebetween that includes an AlGaN semiconductor material. The trench extends through the second conductivity-type semiconductor layer and the active layer to the first conductivity-type semiconductor layer and has a first width. The filling insulator fills the trench such that the filling insulator extends at least through the active layer in the trench and includes of an insulating material having a particular refractive index. The first electrode is connected to the first conductivity-type semiconductor layer, and the second electrode is connected to the second conductivity-type semiconductor layer.