High-efficiency electron-beam-pumped sub-240-nm ultraviolet emitters based on ultra-thin GaN/AlN multiple quantum wells grown by plasma-assisted molecular-beam epitaxy on c-Al₂O₃

Abstract

We report the internal structures and emission properties of GaN/AlN single- and multiple-quantum-well (QW) heterostructures with well widths of dw = 1–4 monolayers (MLs), grown by plasma-assisted molecular-beam epitaxy on c-sapphire at metal-rich conditions and low temperatures (∼700 °C). The formation of plane QWs with abrupt symmetrical interfaces is confirmed by both scanning transmission electron microscopy and X-ray diffraction analysis. Pulse-scanning and continuous-wave output powers of 150 and 28 mW, respectively, at a peak emission wavelength of 235 nm were achieved at 300 K in an electron-beam-pumped deep-ultraviolet (1.5 ML-GaN/5.5 nm-AlN)360 multiple-QW emitter with a maximum efficiency of 0.75%.

Deep Ultraviolet Light Source from Ultrathin GaN/AlN MQW Structures with Output Power Over 2 Watt

“…Unfortunately, power efficiencies of these e‐beam‐pumped light sources are not well satisfactory yet, being less than 1%, as shown in Figure c, and comparable with that recently reported for molecular beam epitaxy (MBE) grown GaN/AlN MQW . However, keeping in mind the total e‐beam pumping efficiency of about 25%, the external quantum efficiency (EQE) is about 3.2%, which is ≈70% of the theoretically estimated output from the flat plate for a single‐pass emission (4.7%), giving a lower estimate for the IQE of the three MLs MQW sample.…”

“…This provides a unique advantage over conventional LEDs at UVC range, since the p‐type doping for high‐Al‐content AlGaN is extremely difficult. In fact, e‐beam‐pumped UVC light sources make rapid progress in recent years and their output power increases up to 100–230 mW at ≈240 nm, which is higher than 150–180 mW reported for the best UVC LEDs emitting at a longer wavelength of 280 nm …”

“…A promising approach that dramatically improves the light output power is electron‐beam (e‐beam) pumping, especially for the short‐wavelength UVC spectral range . This approach allows one to bypass the need for p‐type or n‐type injection layers and, thus, can largely increase the carrier injection efficiency.…”

“…First, we designed the structure of ultrathin GaN/AlN MQWs, which is shown in Figure a for the fabrication of e‐beam‐pumped DUV light sources. We grew 100 and even more periods of MQWs for the active regions which are thick enough to match the large e‐beam penetration depth at the applied e‐beam acceleration voltage of ≈20 kV . AlN barriers were used instead of commonly used AlGaN, which are beneficial for realizing the shortest emission wavelength, the maximum compressive strain in the GaN wells, and the maximum quantum confinement.…”

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

“…For example, the substrate temperature for growing AlGaN thin films by MBE is generally below 800 C, [88][89][90][91][92] whereas by MOCVD it requires 1000-1200 C. [24,34,[93][94][95] Lastly, the MBE growth chamber is equipped with RHEED, which allows an in situ monitoring of the growth and an instantaneous adjustment on the growth parameters, beneficial to the growth of ultrathin layers, e.g., the monolayer (ML)-thick GaN QWs. [96][97][98] In the next two subsections, we will show that benefited from these merits, high-quality AlGaN alloys including both thin films and nanowires can be grown by MBE, which provides a promising path to the electrically injected AlGaN DUV lasers by MBE.…”