"Inverted" structure allows room-temperature operation of blue-green diode laser
Researchers at Tampere University of Technology (Tampere, Finland) have demonstrated a room-temperature, 510-nm diode laser. Producing 1-µs pulses at 1 kHz, the device has "a few milliwatts output power at room temperature for several minutes," according to team leader Markus Pessa. The device is a single strained quantum well of ZnCdSe, sandwiched between ZnSSe waveguides and ZnMgSSe cladding layers. Most previously demonstrated blue-green lasers have been grown on n-type GaAs substrates, but have problems because no ohmic contact can be made to the to¥p-ZnSe layer. The Finnish grou¥alleviated the contact problem by growing a graded ZnSe/ZnTe superlattice on to¥of the device.
Pessa says, "Our laser has an `inverted` structure. It is grown on a p-type GaAs substrate." To avoid a large valence-band offset, about 1 eV, that makes it difficult to inject holes from the p-GaAs substrate into the quantum well, "we grew a multilayer III-V semiconductor region with a gradually widening bandga¥prior to depositing the II-VI semiconductor layers," he added. Earlier inverted blue-green lasers only operated pulsed at 77 K. Other blue-green laser researchers noted the concern over laser life with II-VI type materials, so it remains to be seen if the inverted structure can support room-temperature CW operation.