Interface Recombination in Ga- and N-Polar GaN/(Al,Ga)N Quantum Wells Grown by Molecular Beam Epitaxy

We explore and systematically compare the morphological, structural, and optical properties of $\mathrm{Ga}\mathrm{N}$/($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PA MBE) on freestanding $\mathrm{Ga}\mathrm{N}$$(0001)$ and $\mathrm{Ga}\mathrm{N}$$(000\overline{1})$ substrates. Samples of different polarity are found to be comparable in terms of their morphological and structural perfection and exhibit essentially identical quantum well widths and $\mathrm{Al}$ content. Regardless of the crystal orientation, the exciton decay in the MQWs at $10$ K is dominantly radiative and the photoluminescence (PL) energy follows the quantum confined Stark effect for different quantum well widths. A prominent free-to-bound transition involving interface shallow donors is, however, visible for the N-polar MQWs. At room temperature, in contrast, the exciton decay in all samples is dominated by nonradiative recombination taking place at point defects, presumably $\mathrm{Ca}$ or ${V}_{\mathrm{N}}$ located at the bottom QW interface. Remarkably, the N-polar MQWs exhibit a higher PL intensity and longer decay times than the $\mathrm{Ga}$-polar MQWs at room temperature. This improved internal quantum efficiency is attributed to the beneficial orientation of the internal electric field that effectively reduces the capture rate of minority carriers by interface trap states.