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Development of UV-C laser diodes on AlN substrate

UV-C laser diodes (LDs) have not been realized for many years owing to the problems of crystal quality and p-type conductivity control. In our group, AlGaN-based LD structures with low dislocation density were fabricated using AlN single-crystal substrates, and a p-type cladding layer with sufficient hole concentration was realized without impurities doped by distributed polarization doping (DPD). As a result, we have demonstrated pulsed current injection UV-C LDs at room temperature. We have also developed an on-wafer process technology to solve the problems of LDs manufactured by the conventional cleavage method. The key points of this method are the flatness of mirror facets, their angle to the cavity, and the coating of distributed Bragg reflector (DBR) on the mirror facets formed perpendicular to the wafer. The method is a combination of dry etching and TMAH wet etching to selectively expose the m-plane, and the DBR fabrication by atomic layer deposition (ALD) which provides good coverage. The LDs fabricated by the on-wafer method were observed to lase in the UV-C region when a pulsed current was injected similarly to the LDs fabricated by the cleavage method. These results indicate the potential of not only the realization of UV-C LDs but also for the fabrication of devices using high-Al-composition AlGaN with p-type conductivity and optical integrated circuits.