Transient epitaxial dynamics and cascade continuity in long-duration terahertz quantum cascade laser growth

Reproducibility in terahertz-frequency quantum cascade laser (THz QCL) growth by molecular beam epitaxy is commonly discussed in terms of controlling growth parameters, including growth rate, alloy composition, and doping level. Although control of these parameters is essential, the role of long-term dynamical stability in preserving the structural continuity of the cascade remains less well understood. Here, we provide experimental support for a physical picture in which discrete transient growth events can partition the active region into portions with different effective periodicities, thereby disrupting cascade continuity in a manner distinct from monotonic drift. Using in situ pyrometry phase analysis combined with high-resolution x-ray diffraction (HRXRD) and device characterization, we show that nominally identical GaAs/AlGaAs THz QCL structures can exhibit markedly different dynamical evolution during growth. These differences are reflected in pyrometry phase departure, satellite peak splitting, and device behavior consistent with a narrowed common transport window. Representative post-operation images of Ga-source components show liquid-Ga accumulation and morphologies consistent with redistribution near the evaporation aperture after extended operation. These observations, together with the pyrometry and HRXRD results, are consistent with a possible Ga-source contribution to transient growth instability during long-duration molecular beam epitaxy growth. The work also highlights the sensitivity of THz QCL structures as probes of transient epitaxial dynamics.