Defect investigation and temperature analysis of high-power AlGaInP laser diodes during catastrophic optical damage

Abstract

A variety of applications, such as photodynamic therapy, require efficient red-emitting semiconductor lasers with high output power and continuous wave life times >1,000 h. AlGaInP lasers have shown to be the best candidates in this spectral range. However, compared to infrared emitters, high-power performance is still limited by major degradation effects, especially by catastrophic optical damage (COD). COD in AlGaInP lasers has hardly been researched in the past. We employed an innovative combination of concepts, namely microphotoluminescence (μPL) mapping and focused ion beam (FIB) microscopy to investigate the COD induced defects in AlGaInP broad-area lasers. High resolution μPL demonstrated that those defects are composed of highly nonradiative complex dislocation networks, which start from the output facet and propagate in form of branches inside the cavity. Moreover, FIB analysis showed that those dark line defects are confined to the active region, containing the quantum wells and the waveguide. In addition, facet temperature changes during COD were analyzed by means of micro-Raman spectroscopy. Although no visible damage at the coated output facet could be observed, an extreme temperature increase in the immediate vicinity of the COD starting point at the laser output facet was detected. This was followed by a sudden temperature decrease afterward. After analyzing several lasers with emission wavelengths between 635 nm and 650 nm, we concluded that in AlGaInP lasers, absorption of stimulated photons at the laser facet is the major source of facet heating, and that a critical facet temperature must be reached in order for COD to occur. This understanding of nature and behavior of COD is a key element for further improvement in efficiency of high-power diode lasers.