Using plasma-induced molecular beam epitaxy, a team at Technische Universität München in Garching, Germany, has grown N-type AlN on P-type diamond to form a heterojunction diode. The device, which was described in the Jan. 13 issue of Applied Physics Letters, demonstrates the feasibility of growing III-nitride materials on diamond, which may enable the development of improved UV optoelectronic devices.The researchers deposited the AlN on a 2 x 2 x 0.5-mm boron-doped diamond substrate at a growth rate of 0.18 µm/h. They evaporated 150-µm-diameter Ti/Al ohmic contacts on the AlN side of the structure and 300-µm-diameter ones on the back side of the diamond. The device produced 442-nm emission at a forward bias voltage, with a weaker peak in the UV.