Impact of interface interdiffusion on band structure and near-infrared absorption in ultra-thin ScAlN/GaN quantum wells

Intersubband (ISB) absorption was investigated in ultra-thin, lattice-matched Sc0.14Al0.86N/GaN multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy. MQWs with nominal GaN well widths of 1–2 nm exhibit ISB absorption peaks spanning the energy range of 623–752 meV. Metal-modulated epitaxy of the ScAlN layers was employed to improve interface abruptness and resulted in consistently higher absorption energies than measured for samples containing ScAlN grown under nitrogen-rich conditions. Structural analysis with scanning transmission electron microscopy (STEM) showed that compositionally graded interfaces caused by Sc–Ga–Al intermixing are the limiting factor for achieving the transition energies predicted by theoretical calculations for ultra-thin quantum wells. Interface interdiffusion reduces the quantum confinement, increases the effective well widths, and leads to lower ISB energies. Calculations incorporating the STEM-estimated composition profiles and assuming a spontaneous polarization difference of −0.068 C/m2 at the Sc0.14Al0.86N/GaN interface accurately reproduce the experimental results. A simulation model using simplified interdiffusion layers is proposed to establish the quantitative relationship between interface broadening and ISB energies. The model elucidates the dependence of the measured transition energies on growth conditions such as temperature and growth mode. It also provides a practical framework for the optimization of nitride intersubband photonic devices.