Comparative analysis of the emission properties of polar GaN/AlN and AlGaN/AlN multi-quantum wells

The emission properties of GaN/AlN and Al_0.08Ga_0.92N/AlN polar multi-quantum wells (MQWs) have been investigated by photoluminescence (PL) and time-resolved PL (TRPL) measurements as a function of temperature and pressure. The results were correlated with ab initio calculations. The samples were grown by plasma-assisted molecular-beam epitaxy. Their structural/chemical features were characterised by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and secondary ion mass spectrometry (SIMS). The well/barrier equal widths in the series of samples varied from 1.5 to 5 nm for GaN/AlN and from 2 to 4 nm for AlGaN/AlN MQWs, respectively. Due to the strong internal electric fields, both types of MQWs revealed some similarities in their optical properties: a redshift of the PL with increasing well width, a strong increase of the PL decay times, and a decrease of the pressure coefficients for wider wells. However, some differences were also observed: the emission energies of AlGaN/AlN structures were higher due to the presence of aluminium in the wells, activation energies were lower due to lower band offset, and internal quantum efficiencies and pressure coefficients were higher because of smaller lattice mismatch in AlGaN/AlN (weaker piezoelectric effects). The PL energies, oscillator strengths, and PL pressure dependencies were determined by density functional theory calculations, obtaining reasonable agreement with experimental measurements. Our analysis demonstrated a significant influence of piezoelectric effects on the emission properties of nitride-based QWs, and revealed that nonlinear processes are more important in the AlGaN/AlN system than in GaN/AlN, pointing to the presence of an additional mechanism of optical emission in AlGaN alloys.