A light emitting device includes a substrate, a buffer layer, a first active layer, and a plurality of mesa regions. A portion of the first active layer includes a first electrical polarity. The plurality of mesa regions includes at least a portion of the first active layer, a light emitting region on the portion of the first active layer, and a second active layer on the light emitting region. A portion of the second active layer includes a second electrical polarity. The light emitting region is configured to emit light which has a target wavelength between 200 nm to 300 nm. A thickness of the light emitting region is a multiple of the target wavelength, and a dimension of the light emitting region parallel to the substrate is smaller than 10 times the target wavelength, such that the emitted light is confined to fewer than 10 transverse modes.