AlGaN/AlN Stranski–Krastanov Quantum Dots for Highly Efficient Electron Beam-Pumped Emitters: The Role of Miniaturization and Composition to Attain Far UV-C Emission

Abstract

Conventional ultraviolet lamps for disinfection emit radiation in the 255−270 nm range, which poses a high risk of causing cancer and cataracts. To address these concerns, solid-state UV-C sources emitting below 240 nm are attractive as a safe and sustainable disinfection solution for occupied spaces. This article delves into the extension of Al x Ga 1−x N/AlN quantum dot (QD) technology toward the far UV-C range. The structural and optical impact of increasing the Al content in the QDs through the increase of the Al flux and eventual correction of the Ga flux, and the effect of extreme miniaturization of the QDs achieved by reducing their growth time, are explored. The internal quantum efficiency of self-assembled QDs, grown with a metal/N ratio ranging from 0.5 to 0.8, remains around 50% regardless of the Al content (up to 65%) or emission wavelength (300−230 nm). However, QDs emitting below 270 nm display bimodal luminescence due to inhomogeneous in-plane emission caused by fluctuations in the QD shape linked to extended defects. Reducing the QD size exacerbates this bimodality without altering the emission wavelength. Power efficiencies under electron beam pumping range from 0.4 to 1%, with the potential for improvement through surface treatments enhancing light extraction efficiency.

Effect of Extended Defects on AlGaN Quantum Dots for Electron-Pumped Ultraviolet Emitters

“…By integrating the full CL intensities at room temperature and 5 K and comparing their values, the IQE is estimated to be 46% . This value remains consistently high (46 ± 6%) throughout the sample and aligns with our previous report of IQE around 50% for QDs emitting between 320 and 230 nm . The variation of the CL intensity with temperature normalized to its low temperature value is depicted in Figure c.…”

“…14 This value remains consistently high (46 ± 6%) throughout the sample and aligns with our previous report of IQE around 50% for QDs emitting between 320 and 230 nm. 13 The variation of the CL intensity with temperature normalized to its low temperature value is depicted in Figure 2c. From 5 K to around 100 K, the CL intensity shows only a very slight increase with temperature, which is attributed to the enhanced carrier diffusion from the barriers toward the QDs.…”

“…44−46 However, the above-described bimodality of the QD emission becomes more important in samples emitting at wavelengths shorter than 270 nm. 13 To extend our investigation to shorter wavelengths, we have incorporated sample S2 into our study, which contains 65% Al in the QDs, with a main CL emission at 223 nm at 5 K and 233 nm at room temperature, as shown in Figure 7a. While the article provides a thorough analysis of S1, the introduction of S2 serves as an example that demonstrates that the optical behavior and defect structure observed in S1 persist in samples with higher Al content.…”

“…However, the luminescence spectra display a clearly resolved broad secondary emission at longer wavelengths, centered around 260 nm, both at room temperature and 5 K. This sample has an estimated IQE ≈ 38%, which is promising for the development of far UV-C emitters. 13 Figure 7b displays a top-view SEM image of S2, unveiling a morphology akin to S1, characterized by shallow pits but with a higher density (about 4 × 10 8 cm −2 ). When we compare Figure 7b with the λ-filtered CL map depicted in Figure 7c, it becomes clear that the emission at longer wavelengths aligns with the shallow pits, suggesting a phenomenon similar to that observed in S1.…”

“…The three-dimensional (3D) carrier confinement within QDs prevents carrier diffusion toward nonradiative centers, leading to improved radiative efficiency even at high temperatures . Previous studies have demonstrated Al x Ga 1– x N/AlN QDs with an internal quantum efficiency (IQE) of around 50% in the UV-C range, maintaining stable efficiency even when exposed to optical pumping power levels reaching up to 200 kW/cm 2 . , However, samples emitting at wavelengths shorter than 270 nm present broad or bimodal spectra. It is crucial to minimize bimodal emission or emission line width to achieve highly efficient devices for disinfection.…”