Strongly Confined Excitons in GaN/AlN Nanostructures with Atomically Thin GaN Layers for Efficient Light Emission in Deep-Ultraviolet.

Fascinating optical properties governed by extremely confined excitons have been so far observed in 2D crystals like monolayers of transition metal dichalcogenides. These materials, however, are limited for production by epitaxial methods. Besides, they are not suitable for the development of optoelectronics for the challenging deep-ultraviolet spectral range. Here, we present a single monolayer of GaN in AlN as a heterostructure fabricated by molecular beam epitaxy, which provides extreme 2D confinement of excitons, being ideally suited for light generation in the deep-ultraviolet. Optical studies in the samples, supplemented by a group-theory analysis and first-principle calculations, make evident a giant enhancement of the splitting between the dark and bright excitons due to short-range electron-hole exchange interaction that is a fingerprint of the strongly confined excitons. The practical significance of our results is in the observation of the internal quantum yield of the room-temperature excitonic emission as high as ∼75% at 235 nm.

Formation of three-dimensional islands in subcritical layer deposition in Stranski-Krastanow growth.

A new method for the formation of three-dimensional (3D) strained islands in lattice-mismatched (B on A) heteroepitaxy is proposed. Once B forms a wetting layer of a subcritical thickness, material C is deposited, which is lattice matched to A and does not wet B. Then B and C phase separate forming local B-rich and C-rich domains on the surface. The thickness of B-rich domains thus exceeds locally that of the initial film of B, and 3D islands may form as it is demonstrated by modeled phase diagrams of the C/B/A system. We show that the growth of the subcritical InAs/GaAs(100) film followed by the deposition of AlAs results (i) in the formation of Al-rich and In-rich domains in the wetting layer, confirmed by chemically sensitive scanning transmission electron microscopy, and (ii) in the stimulated onset of 3D islands, as evidenced both by high resolution transmission electron microscopy and by a significant redshift of the photoluminescence spectrum, which is in agreement with the proposed model.