Reduction in threshold current density of 355 nm UV laser diodes

We have investigated the characteristics of 355 nm UV laser diodes. Two laser diode structures were prepared on a grooved AlGaN template as well as on a flat AlGaN template. The threading dislocation densities on the grooved and flat AlGaN were 2.5×108 cm-2 and 6.5×109 cm-2, respectively. The threshold current density of the laser diode on the grooved AlGaN was 6 kA/cm2, while no lasing was observed from the laser diode on the flat AlGaN. We also estimated the internal quantum efficiency as well as the injection efficiency in the UV laser diode structures by considering internal quantum efficiency of both optical and electrical excitation. At a carrier density of 1.2×1019 cm-3, the internal quantum efficiency of the laser diode structure on the grooved AlGaN was about 50%. This is twice that of the laser diode on the flat AlGaN. At the same time, the internal quantum efficiency estimated from the optical excitation also reached about 50%, even at a carrier density of 3×1018 cm-3. This implies that the injection efficiency in our UV laser diode structure was only 25%. Improving the injection efficiency should be effective for significantly reducing the threshold current density. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)