E‐beam pumped mid‐UV sources based on MBE‐grown AlGaN MQW

Abstract

Phone: þ7 812 292 7124, Fax: þ7 812 297 1017 A 60 mW output power has been achieved in mid-UV (l ¼ 270 nm) spontaneous sources with electron-beam pulsescanning pumping, fabricated from AlGaN MQW heterostructures grown by PA MBE on c-Al 2 O 3 substrates. Under the CW pumping at much lower excitation power density the mid-UV sources demonstrate a 4.7 mW output power. In that regime the power efficiency of the structures is about 0.24%, while their internal quantum efficiency is estimated to be as high as 50%.

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. Standard approaches to improve the EQE do not work in this case due to a rather strong intrinsic nonresonant absorption associated with the Urbach tail in the AlN buffer layer, which increases for the MQW structures with the shorter emission wavelength (thinner GaN QWs), giving rise the observed reduction of the radiation yield even with a single‐pass output (see Figure d).…”

“…The dependence of output power on e‐beam current under continuous wave (CW) and pulse scan (PS) pumping modes for the devices is shown in Figure a,b. The output power measured by the PC was approximately 70% of the real value . Under PS excitation, the corresponding light output powers at RT increase almost linearly with the e‐beam current and raise up to maximum values of 24.8, 122.5, and 178.8 mW for the MQW structures with the GaN QW nominal thicknesses of one, two, and three MLs, respectively, at the e‐beam current of 1.2–1.25 mA and the electron energy E e of 20 keV.…”

“…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.…”

High‐Output‐Power Ultraviolet Light Source from Quasi‐2D GaN Quantum Structure

“…To overcome some of the aforementioned issues which limit the output power, we propose, here, a novel active region, i.e., quasi‐2D GaN layers inserted in AlGaN matrix. In comparison with conventional AlGaN QWs, such quasi‐2D GaN layers are formed by sub‐monolayer digital alloying (SDA) technique, and show the following advantages: (1) the carrier localization is greatly promoted both in vertical and lateral directions; (2) the UV light emission along c ‐axis (TE, E ⊥ c ) is predicted to be dominant, and (3) the crystalline quality should be good since the quasi‐2D GaN layers are coherently grown on the AlGaN layers and hence free of misfit dislocations . Moreover, to avoid the difficulty in p‐type doping of high‐Al‐content AlGaN and UV light absorption by p‐type GaN, an electron‐beam (e‐beam) pumping method is used instead of conventional electrical injection as reported previously .…”