Stress‐Induced Dark Line Defect Formation in GaAlAs : Si LED's

Abstract

Homojunction, graded bandgap LED's have been demonstrated to be highly reliable. During accelerated aging (30 mA forward bias, 250°C), these LED's generally degrade slowly without the formation of dark lines or dark spots. It has recently been found, however, that some LED's show a rapid reduction of light output even when aged without current bias at 200°C. <110> oriented dark lines appeared in the electroluminescence image of heavily degraded devices. To determine the source of the DLD's, the material quality of degraded LED's and unaged LED's from the same wafer was assessed using etch pitting and transmission cathodoluminescence. Examination of the devices revealed the presence of small pyramids, composed mainly of silicon, on the p‐surface of some of the LED's; no unusual features were found on the remaining LED's. A dislocation network surrounding each pyramid was found to be initially present. Due to bonding and thermal stresses applied to the pyramids, these networks enlarged with aging along the {111} planes and appeared as <110> DLD's in the electroluminescence image upon reaching the p‐n junction. In degraded LED's from pyramid‐free regions of the wafer with pyramids, <110> DLD's initiate at dicing damage and regions of the p‐surface subject to high bonding stress, i.e. edges and corners. The growth of these DLD's is identical to those initiating at pyramids. A similar study of LED's from a wafer that is totally free of pyramids shows that DLD's are not formed in devices that degrade slowly.