MgZnCdSe-based II-VI compounds have great potential for use in visible-wavelength laser diodes. Until recently, however, researchers had been unable to achieve lasing with these semiconductors because there were no suitable P-type cladding layers. Now a team at Sophia University in Tokyo has demonstrated MgSe/BeZnTe superlattices as a P-type cladding for yellow-green laser diodes, which should enable reliability and longer lifetimes.Researchers at Sophia University in Tokyo have achieved 565-nm emission from a II-VI laser diode. The structure of the device is shown. Courtesy of Katsumi Kishino.III-V semiconductors such as AlGaInP and InGaN do not readily emit in the yellow-green, and extending their output wavelength to the middle of the visible range compromises their lasing performance.Although researchers have successfully demonstrated blue-green laser diodes based on ZnCdSe/MgZnSSe, these devices have been stymied by lifetimes of only 400 hours -- well below the practical level of 10,000. ZnCdSSe-based lasers have had similar problems. Grown on GaAs substrates, they experience rapid degradation caused by a compressive strain introduced into the ZnCdSe or ZnCdSSe active regions.In response, scientists have suggested MgZnCdSe. Sophia researcher Katsumi Kishino explained that using these materials enables the growth of II-VI on InP and maintains lattice-matching between the active layers and the cladding, thus yielding long lifetimes. Moreover, using beryllium chalcogenides suppresses the generation and propagation of defects all the more.The researchers employed molecular beam epitaxy to grow wafers on [100]-oriented, S-doped InP with Si-doped InGaAs buffer layers and fabricated gain-guiding laser diodes with gold electrodes. They tested the 565-nm devices under pulsed current injection at 77 K using a HeCd laser with a pulse repetition rate of 1 kHz and a pulse width of 500 ns."No degradation of the [laser diode] with the ZnCdSe/BeZnTe superlattice active layer was observed over 3500 hours under DC-current conditions," Kishino said. "This suggests that the degradation mechanism of our materials on InP may differ from that of II-VI on GaAs previously investigated."Road to applicationThere are other means to generate yellow-green light -- notably, second-harmonic generation with near-IR emitters. But fabricating devices that employ this process requires precise optical beam alignment, severely limiting the practicality of mass-producing such diode-based devices. Thus, the II-VI laser diodes may open up practical applications that were previously thought to be unattainable.The current research marks only the first step toward commercialization, however. Several problems must be addressed before the devices can be put on the market. "Lasing was observed only at a low temperature, perhaps due to the high resistances of the [laser diodes] and a higher threshold current density, 2.5 kA/cm2," Kishino said. "These two problems are not essential problems and could be overcome by optimizing the structure."