Abnormal Radiative Interband Transitions in High-Al-Content AlGaN Quantum Wells Induced by Polarized Orbitals
AlGaN has attracted considerable interest as a wide (direct)-band-gap semiconductor with high thermal and mechanical stability. Thus, it can be used to develop optoelectronic devices operating within the ultraviolet region at high power and under harsh environmental conditions. Despite their recognized prospective applications, Al-rich AlGaN optical devices suffer from low external quantum efficiency. To trace the origin of the said problem, a cathodoluminescence system combined with two scanning probes was set up to investigate the cross-section luminescence of the sample related to application bias. The luminescence from the quantum wells in a deep ultraviolet light-emitting device was identified by layer-resolved spectroscopy. Results show that the primary radiative emission at the band edge exhibits an abnormal behavior, which is different from the other emission that is dependent on external electric fields. First-principles simulations demonstrate that the dispersive crystal field split-off hole (CH) band caused by hole deconfinement is responsible for the abnormal radiative emissions. Analysis of the constituent orbitals of the hole bands reveals a strong head-over-head lobe structure in the barrier along the  direction in the pz orbitals, contributing mainly to the CH band. Meanwhile, a weak side-by-side (0001) in-plane lobe structure is present in the px and py orbitals, contributing to the heavy and light hole bands. This study may serve as a basis for further investigations on quantum efficiency improvement in high-Al-content AlGaN optoelectronic devices.