Characterization of degradation and failure in long-wave quantum cascade lasers

Critical to the continued evolution of quantum cascade lasers (QCLs) is a thorough understanding of their degradation and failure modes. In this work, electron microscopy techniques are used to characterize long-term failure in a series of longwave buried heterostructure (BH) QCLs. Understanding the long-term QCL degradation process is critical for the improvement of QCL reliability, so this work is focused on characterizing damage propagation in a series of long-wave QCLs that experienced varying degrees of performance degradation following a long-term burn in test. For this burn-in test, a batch of long wave InGaAs/AlInAs/InP-based QCLs were run in continuous-wave (CW) operation for up to ~600 hours. The tested lasers saw a wide range of outcomes, from total device failure to no degradation at all. The damage in these chips was then characterized with techniques such as Focused Ion Beam (FIB) milling and Transmission Electron Microscope (TEM) imaging. These techniques are used to probe the diffusion of both large- and atomic-scale damage features throughout the laser cavity. FIB milling unearths damaged regions of the chip that were hidden from SEM, enabling the mapping of the propagation of large-scale damage regions that began at the facet and propagated further into the substrate and the laser core. TEM on cross-sections of the laser core and its surrounding cladding was then employed to analyze localized dislocation networks in the vicinity of the laser core. This revealed distinct crystalline defects that varied in species and scale between failed and live devices. These characterization techniques contribute to a multidimensional understanding of defect diffusion during long-term testing, which is a key step in improving the reliability of future QCL designs.