Abstract

AlGaInN laser diodes are currently undergoing rapid development with a number of groups having now demonstrated room-temperature cw operation of InGaAlN laser diodes, and the commercialization of violet laser diodes has just recently begun. The reduction of the dislocation density in the GaN material has been shown to be an important factor to improve laser diode performance and lifetime. Although the benefits of low dislocation materials have been clearly demonstrated by the rapid progress in laser diode lifetime, there is still very little known on how other laser properties, i.e. the distributed loss or internal quantum efficiency, are effected by the dislocation density in the material. In this paper we will compare the performance characteristics of cw laser diodes grown by metal organic chemical vapor deposition on sapphire substrates with otherwise identical devices, but fabricated on laterally epitaxially overgrown GaN on sapphire (LEO) substrates. The InGaAlN films were processed into ridge-waveguide lasers with CAIBE etched mirrors and high reflective coatings. For improved thermal management the sapphire substrate was thinned and the devices were mounted p-side up on a heatsink. For devices grown on LEO substrates, room-temperature cw threshold current densities as low as 5.9 kA/cm² with emission wavelength near 400 nm have been observed. Under cw conditions, threshold currents were as low as 62 mA with threshold voltages of 7.5 V (as shown in Figure 1). CW laser operation was observed up to a heatsink temperature of 70°C. Significant improvements in light output vs. current were observed for devices grown on LEO substrates, with cw output powers greater than 20 mW and differential quantum efficiencies of 0.54 W/A. This improved performance can be attributed to the increased internal quantum efficiency and reduced distributed loss in the low dislocation density material obtained with LEO. The room-temperature cw operation lifetimes for the LEO grown devices was more than fifteen hours, measured at a constant output power of 2 mW. This is more than an order of magnitude better than results obtained for identical laser structures grown on sapphire substrates.

© 2000 IEEE