On the degradation of InGaAsP/InP-based bulk lasers

Degradation of InGaAsP-InP-based buried-heterostructure bulk (BH-bulk) lasers has been studied by means of electroluminescence (EL), photoluminescence (PL), electron-beam-induced current (EBIC) and transmission electron microscopy (TEM). Lasers with p/n as well as semi-insulating (SI) current-blocking layers were studied. The results show that moderate increases in the threshold current correlate well with formation of dark defects (DD's) (i.e., dark-line defects (DLD's) or dark-spot defects (DSD's), which cannot be distinguished in our case due to the narrowness of the laser stripe.) The DD's were found to be caused by dislocation loops. The dependence of threshold current increase on the number of DD's is explained in terms of a model which includes effects due to the DD's, as well as changes in the regions outside the DD's. The latter is found to be responsible for the major part of the threshold current increase. Values for the ratio between the carrier lifetimes inside and outside the DD's are presented, for the first time. In our lasers, strong degradation differs from moderate degradation in that DD's do not form during aging. The presence of dislocation loops only at the sidewalls of the active stripe in lasers with p/n current-blocking layers points to the sidewalls as being critical. The near absence of dislocation loops and the smaller increase in threshold current in SI lasers which have degraded strongly, compared to the strongly degrading p/n lasers, suggest that strong degradation is a synergistic combination of damage in the sidewalls and Zn indiffusion from the current-blocking layers.