Heat source distribution, vertical structure, and coating influences on the temperature of operating 0.98 μm laser diodes: Photothermal reflectance measurements

Abstract

In the present work single-quantum-well laser diodes operating at 0.98 μm are investigated by photothermal reflectance microscopy. Temperature maps were obtained for the output facet of all devices studied. Furthermore, the temperature distribution was determined along the cavity (on the ridge) of lasers soldered with the junction side up. Near the facets, the measured temperature was found to be about seven times the bulk’s temperature, indicating the presence of an important surface heat source. The signal phase distribution of the laser facet shows the important role of the vertical structure on the heat confinement. Comparison between experiments and calculations shows that the confinement layers (GaAlAs and GaInP) thermal parameters are the principal responsible for the heat propagation in these structures near the active region. The same calculations show the role of the coating (Al2O3) in the heat propagation, and give a quantitative ratio between surface and bulk heat sources. Measurements made on the facet and on the ridge as a function of injection current were found to present a quite similar behavior, leading to the conclusion that thermal effects are strongly dominant in these measurements, masking any carrier or electroreflectance effects. Finally, measurements made under different light output power conditions and under the same injection current conditions showed that the surface heat source is caused by laser light absorption at the facets.