Chemical changes accompanying facet degradation of AlGaAs quantum well lasers

Abstract

The temperature of single quantum well semiconductor laser facets increases during operation, eventually reaching a critical temperature, thermal runaway, and catastrophic optical damage. A study of changes in composition of the near-surface region of facets which accompany heating has been carried out for continuously operated, uncoated AlGaAs-GaAs-AlGaAs graded index separately confined heterostructure single quantum well lasers. High resolution depth profiles by scanning Auger microscopy show that the laser facets can be quite variable in initial composition, and undergo pronounced stoichiometry changes even during the first few minutes of operation. At longer times a continuing out-migration of the group III elements is observed. Unlike the double heterojunction lasers, facet oxidation is not pronounced and is not responsible for diffusion of Ga and Al. There are indications, however, that a slow leakage of oxygen into the crystal may occur. Spatially resolved analyses provide evidence that carrier-mediated elemental redistribution is an important factor in facet degradation. The progressive accumulation of defects which may act as non-radiative recombination centers provides a simple means of facet heating. Analyses of lasers which have suffered catastrophic damage indicate that the facets are not always melted, and that there is no typical chemical state which distinguishes them from facets of lasers which are fully operational. These results are compared to studies of facet degradation in double heterojunction lasers. Implications of the data for models of catastrophic optical damage are discussed.