The Effect of Interface Diffusion on Raman Spectra of Wurtzite Short-Period GaN/AlN Superlattices

Carrier Recombination in Nitride-Based Light-Emitting Devices: Multiphonon Processes, Excited Defects, and Disordered Heterointerfaces

We study recombination processes in nitride LEDs emitting from 270 to 540 nm with EQE ranging from 4% to 70%. We found a significant correlation between the LEDs' electro-optical properties and the degree of nanomaterial disorder (DND) in quantum wells (QWs) and heterointerfaces. DND depends on the nanoarrangement of domain structure, random alloy fluctuations, and the presence of local regions with disrupted alloy stoichiometry. The decrease in EQE values is attributed to increased DND and excited defect (ED) concentrations, which can exceed those of Shockley-Read-Hall defects. We identify two mechanisms of interaction between EDs and charge carriers that lead to a narrowing or broadening of electroluminescence spectra and increase or decrease EQE, respectively. Both mechanisms involve multiphonon carrier capture and ionization, impacting EQE reduction and efficiency droop. The losses caused by these mechanisms directly affect EQE dependencies on current density and the maximum EQE values for LEDs, regardless of the emission wavelength. Another manifestation of these mechanisms is the reversibility of LED degradation. Recombination processes vary depending on whether QWs are within or outside the space charge region of the p-n junction.

Achieving atomically ordered GaN/AlN quantum heterostructures: The role of surface polarity

Interface engineering in heterostructures at the atomic scale has been a central research focus of nanoscale and quantum material science. Despite its paramount importance, the achievement of atomically ordered heterointerfaces has been severely limited by the strong diffusive feature of interfacial atoms in heterostructures. In this work, we first report a strong dependence of interfacial diffusion on the surface polarity. Near-perfect quantum interfaces can be readily synthesized on the semipolar plane instead of the conventional c-plane of GaN/AlN heterostructures. The chemical bonding configurations on the semipolar plane can significantly suppress the cation substitution process as evidenced by first-principles calculations, which leads to an atomically sharp interface. Moreover, the surface polarity of GaN/AlN can be readily controlled by varying the strain relaxation process in core-shell nanostructures. The obtained extremely confined, interdiffusion-free ultrathin GaN quantum wells exhibit a high internal quantum efficiency of ~75%. Deep ultraviolet light-emitting diodes are fabricated utilizing a scalable and robust method and the electroluminescence emission is nearly free of the quantum-confined Stark effect, which is significant for ultrastable device operation. The presented work shows a vital path for achieving atomically ordered quantum heterostructures for III-nitrides as well as other polar materials such as III-arsenides, perovskites, etc.

Excitation and detection of acoustic phonons in nanoscale systems

Phonons play a key role in the physical properties of materials, and have long been a topic of study in physics. While the effects of phonons had historically been considered to be a hindrance, modern research has shown that phonons can be exploited due to their ability to couple to other excitations and consequently affect the thermal, dielectric, and electronic properties of solid state systems, greatly motivating the engineering of phononic structures. Advances in nanofabrication have allowed for structuring and phonon confinement even down to the nanoscale, drastically changing material properties. Despite developments in fabricating such nanoscale devices, the proper manipulation and characterization of phonons continues to be challenging. However, a fundamental understanding of these processes could enable the realization of key applications in diverse fields such as topological phononics, information technologies, sensing, and quantum electrodynamics, especially when integrated with existing electronic and photonic devices. Here, we highlight seven of the available methods for the excitation and detection of acoustic phonons and vibrations in solid materials, as well as advantages, disadvantages, and additional considerations related to their application. We then provide perspectives towards open challenges in nanophononics and how the additional understanding granted by these techniques could serve to enable the next generation of phononic technological applications.