ZnMgSSe, forming a type I heterostructure with Zn(Cd)Se, fully lattice-matched to GaAs with an energy gap tunable up to similar to 4.5 eV, has made possible continuous-wave (CW) operation of both a green laser diode (LD) and a blue LD at room temperature. The device characteristics of the II-VI wide-gap LD's are becoming as good as those of established III-V LD's, except for device lifetime. Remaining key issues are p-doping in the wide-gap ZnMgSSe and reliability of II-VI wide-gap LD's, Valance-band engineering via superlattice (SL) use is proposed, based on the amphoteric defect model, for removing the doping limit in the p-type ZnMgSSe. This will lead to CW operation of a blue-emitting laser diode with a wavelength of 450 similar to 460 nm. For reliability, employing a GaAs-buffer layer has made possible room-temperature (RT) CW operation with a lifetime of 1 hour. Analysis of the degradation process shows that no catastrophically fast degradation occurs when II-VI LD's degrade. The reliability of the ZnMgSSe-based LD's would soon be established, and the II-VI wide-gap LD's will likely blossom colorfully in the near future.
