Low-threshold alxGa1-xAs visible and IR-light-emitting diode lasers

The detailed properties of "close-confinement" LPE (AlGa)As junction lasers have been studied in the Al composition range where the alloy has a direct bandgap transition. It has been shown that the laser efficiency is essentially independent of wavelength at 300°K between 9000 and 8000 Å. This is important in applications where photocathodes are used to detect the laser radiation or where optical pumping of Nd:YAG laser is desired. Continuously operating red-emitting lasers at 77°K (6635 Å) have also been fabricated. At room temperature, the lowest threshold current density, 8300 A/cm²for low Al content, ( ) is only slightly higher than in GaAs, which is indicative of the excellent metallurgical properties possible in this alloy system. An external differential quantum efficiency of 43 percent at 300°K ( ) nearly equals the best GaAs laser value. Furthermore, the relatively low series resistance in these devices makes possible power conversion efficiencies of ∼10 percent at 300°K and ∼50 percent at 77°K. The variation of the laser threshold currents and efficiency with increasing Al content is very close to the theoretically predicted variation based on the change of the internal quantum efficiency, assuming a direct-to-indirect bandgap energy transition at 1.92 eV. This suggests that the best devices perform very nearly at their theoretically predicted maximum in relation to present similarly constructed GaAs lasers.