2D‐GaN/AlNMonolayer‐Thick Quantum Wells

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.

Recent Progress on Aluminum Gallium Nitride Deep Ultraviolet Lasers by Molecular Beam Epitaxy

“…A significant amount of efforts have been devoted to the MBE growth of the DUV emitting structures based on ultrathin GaN QWs, with the first work dated back to 2009. [76,[96][97][98][114][115][116][117][118][119][120][121][122] This is mainly motivated by two advantages of using ultrathin GaN QWs for DUV emission: 1) the strong localization of electron and hole wave functions toward the center of the ultrathin GaN QWs, and 2) a large energy separation between upper heavy hole and lower crystal-field split-off subbands, which could benefit the development of DUV LEDs and lasers in a number of ways such as mitigating the QCSE and enhancing the optical gain of the TE-polarized emission compared with conventional AlGaN QWs. [76,[96][97][98]114,[116][117][118][119][120][121][123][124][125][126][127][128][129][130] In addition, the strong confinement-induced large exciton binding energy (up to 230 meV) can prevent the thermal dissociation of excitons at RT, [114] and the excitonic effects in ultrathin GaN/AlN QWs have been recently demonstrated experimentally.…”