No abstract

No abstract

The unique structural and optical properties of two‐dimensional (2D)‐GaN multiple quantum wells (MQWs) with a nominal thickness of 1.5–2 monolayers (MLs) in AlN barriers allow one to consider them as highly efficient electron‐beam pumped ultraviolet‐C (UVC) emitters. These structures grown by both plasma‐assisted molecular beam epitaxy (PA MBE) and metalorganic vapor‐phase epitaxy on standardc‐sapphire substrates demonstrate an output optical pulse power of 1 and 2.2 W at a wavelength of 240 and 260 nm, respectively. This article focuses on the PA MBE of 2D‐GaN/AlN MQW structures using metal‐rich conditions and relatively low substrate temperatures (∼700 °C), which ensure the step‐flow growth of all layers with precise control of their thickness and abrupt interfaces. The revealed high internal quantum efficiency up to 75% in the (1–2)ML‐thick GaN/AlN QWs formed under these conditions is explained not only by the suppression of quantum‐confined Stark effect and the absence of TE/TM polarization switching in binary few‐ML‐thick QWs, but also by an enhanced rate of the excitonic transitions that mainly determine radiative recombination in such ultra‐thin QWs. In addition, the unusual quasi‐stress‐free growth of 2D‐GaN/AlN MQW structures allowed to increase the number of QWs up to 360 with an active region thickness of ∼2 μm.

We realize and investigate a nonlinear metasurface taking advantage of intersubband transitions in ultranarrow GaN/AlN multi-quantum well heterostructures. Owing to huge band offsets, the structures offer resonant transitions in the telecom window around 1.55 µm. These heterostructures are functionalized with an array of plasmonic antennas featuring crosspolarized resonances at these near-infrared wavelengths and their second harmonic. This kind of nonlinear metasurface allows for substantial second-harmonic generation at normal incidence which is completely absent for an antenna array without the multi-quantum well structure underneath. While the second harmonic is originally radiated only into the plane of the quantum wells, a proper geometrical arrangement of the plasmonic elements permits to redirect the second-harmonic light to free-space radiation, which is emitted perpendicular to the surface.

The results of joint theoretical and experimental studies aimed at revealing features in the Raman spectra, which can be used for evaluation of the interface quality between GaN and AlN layers in short-period GaN/AlN superlattices (SLs) are presented. The Raman spectra for SLs with sharp interface and with different degree of interface diffusion are simulated by ab initio calculations and within the framework of random-element isodisplacement model, respectively. The comparison of the results of theoretical calculations and experimental data obtained on PA MBE and MOVPE grown SLs, leads to conclusion that the spectral region of the A1(LO) confined phonons is very sensitive to the degree of interface sharpness. As a result of comprehensive studies, the correlations between the parameters of the A1(LO) confined phonons and the structure of SLs are obtained. The results of the complex studies can be used to optimize the parameters of the growth process in order to form structurally perfect short-period GaN/AlN SLs.