We developed novel rare-earth doped phosphors for mercury-free narrowband ultraviolet light sources. The narrowband ultraviolet emission is a typical feature of the intra-orbital electron transition in the rare-earth Gd~(3+) ion. Al_(1-x)Gd_xN epitaxial films were grown by using an ultra-pure reactive sputtering technique. A narrowband luminescence line from Gd~(3+) ions appears at 318 nm. We have elucidated the energy transfer mechanism from the host AlN crystal to Gd~(3+) ion and developed light-emitting devices using the Al_(1-x)Gd_xN thin film. Furthermore, recently, we developed Gd~(3+)-doped oxide phosphor crystal and demonstrated medical treatment effects for skin disease.
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