Electron microscopy study of thermal stress-induced damage in long-wave quantum cascade lasers

Abstract

This work presents an electron microscopy study of damage propagation in long-wave buried heterostructure quantum cascade lasers (QCLs) subjected to varying degrees of thermal stressing through long-term continuous wave (CW) burn-in testing. After over 500 h of burn-in, two lasers failed suddenly due to facet-level damage, which was preceded by a minor degradation in optical performance. A third laser survived over 600 h of burn-in without any optical degradation. Select subjects of this test, along with an unstressed QCL, were characterized through a combination of scanning electron microscopy (SEM), focused-ion-beam (FIB), and transmission electron microscopy (TEM) techniques. SEM and FIB analysis of both live and failed stressed devices suggests the facet is the most likely origin of failure. TEM analysis of identically packaged QCLs at different stages of their operational life cycle, from unstressed to failed, reveals insights into how defects near the laser core diffuse during operational stressing. This study identifies pre-existing defects concentrated around the interface of the iron-doped InP region in unstressed QCLs. TEM of live stressed devices reveals that these defects diffuse during the thermal stress relaxation process that occurs during burn-in, forming a dislocation network near the active region. Finally, TEM of failed devices suggests that this dislocation network can diffuse enough to degrade the laser and ultimately lead to the onset of catastrophic optical damage at the facet.