Scandium Diffusion-Driven Defect Formation and Thermal Stability in GaInN/GaN Multiple Quantum Wells

ScAlMgO4 (SAM) substrates offer lattice-matched templates for long-wavelength GaInN emitters. However, the thermal stability of Sc-containing interfaces during epitaxial processing remains a critical issue. In this work, we established an experimental model utilizing Sc-capped GaInN/GaN multiple quantum wells (MQWs) to isolate the intrinsic effects of Sc diffusion excluding Mg/O co-diffusion from SAM substrates. By systematically varying the capping structure and annealing temperature (600-900 °C), the diffusion behavior of Sc and its impact on structural properties and carrier recombination dynamics were investigated. Cross-sectional scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy demonstrated phase-selective incorporation, with Sc preferentially accumulating in the GaN barriers while the overall MQW periodicity was largely preserved. Optical measurements showed that moderate annealing at 600 °C reduced native defects and improved internal quantum efficiency (IQE). The most severe degradation occurred at 800 °C, accompanied by pronounced Sc redistribution and interface broadening. Time-resolved photoluminescence (TRPL) and temperature-dependent photoluminescence (TDPL) analyses indicated that the optical degradation was associated with the generation of nonradiative recombination centers (NRCs). First-principles calculations further suggest that Sc-VN complexes can introduce deep levels in the alloy model and may act as possible Sc-related NRCs. Furthermore, for the sample annealed at 900 °C, localized contrast variations were observed within the quantum wells, which may be associated with local alloy-composition inhomogeneity and enhanced carrier localization. This interpretation is consistent with the observed stronger exciton-phonon coupling, although the macroscopic optical efficiency remains low. These results clarify the physical mechanisms of Sc-induced optical degradation, providing essential insights into the impurity effects and solid-state stability limits of SAM-based nitride heterostructures.