Monolayer-Thick GaN/AlN Multilayer Heterostructures for Deep-Ultraviolet Optoelectronics

Recent progress in the development of monolayer (ML)-thick GaN/AlN multilayer heterostructures for deep-ultraviolet (UV) optoelectronics is reviewed. Analysis of both plasma-assisted molecular beam epitaxy and metal-organic vapor phase epitaxy shows that extreme interface sharpness and sub-ML accuracy in setting the layer thickness are attractive features of the former, whereas the lowest density of threading dislocations and wide possibilities for the implementation of various 2D growth mechanisms are the advantages of the latter. The structural properties of ML GaN/AlN heterostructures are evaluated not only by standard X-ray diffraction and scanning transmission electron microscopy, but also by Raman spectroscopy. Theoretical and experimental studies of the optical properties of ML-thick GaN/AlN quantum wells (QWs) reveal that quenching of the quantum-confined Stark effect, suppression of transverse electric transverse magnetic polarization switching, as well as the excitonic nature of UV-radiative recombination in ultrathin (1-2 ML) QWs ensure in such structures a high internal quantum yield of 75% for UV radiation at 235 nm at room temperature. The possibilities of using ML-GaN/AlN heterostructures to fabricate UVC emitters of spontaneous and stimulated emissions in a wide range of output powers with various pumping techniques are considered, and the most important problems are formulated.