Internal quantum efficiency of AlGaN/AlN quantum dot superlattices for electron-pumped ultraviolet sources

Abstract

In this paper, we describe the growth and characterization of  530-nm-thick superlattices (100 periods) of AlxGa1-xN/AlN (0 ≤ x ≤ 0.1) Stranski-Krastanov quantum dots for application as the active region of electron-beam pumped ultraviolet lamps.Highly dense (>10 11 cm -2 ) quantum dot layers are deposited by molecular beam epitaxy, and we explore the effect of the III/V ratio during the growth process on their optical performance. The study considers structures emitting in the 244-335 nm range at room temperature, with a relative linewidth in the 6-11% range, mainly due to the QD diameter dispersion inherent in self-assembled growth. Under electron pumping, the emission efficiency remains constant for acceleration voltages below  9 kV. The correlation of this threshold with the total thickness of the superlattice and the penetration depth of the electron beam confirms the homogeneity of the nanostructures along the growth axis.Below the threshold, the emission intensity scales linearly with the injected current. The internal quantum efficiency is characterized at low injection, which reveals the material properties in terms of non-radiative processes, and high injection, which emulates carrier 2 injection in operation conditions. In quantum dots synthesized with III/V ratio < 0.75, the internal quantum efficiency remains around 50% from low injection to pumping power densities as high as 200 kW/cm 2 , being the first kind of nanostructures that present such stable behaviour.

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

“…The addition of Φ Al implies an increase of the metal/N flux ratio, (Φ Al + Φ Ga )/Φ N , during the QD deposition. In ref, we demonstrated that it is possible to grow Al x Ga 1– x N QDs ( x = 0, 0.1) with high IQE (around 50%) when keeping the metal/N ratio below 0.75. Under these conditions, the growth kinetics are dominated by Ga, whose diffusion length is larger than that of Al at the QD growth temperature.…”

“…An active region comprising 100 periods of QDs has a total thickness of approximately 550 nm. This thickness implies that the maximum acceleration power that can be applied for electron beam excitation is around 9 kV, as determined through Monte Carlo simulations of the electron beam penetration depth . All the characterization presented in this paper was conducted using acceleration voltages ranging from 5 to 6 kV.…”

“…More recent studies showed that even smaller (2.5–3 ML) Al x Ga 1– x N/AlN QDs ( x ≤ 0.1) could attain IQEs close to 50% if the metal/N ratio during the growth remained around 0.4, dropping for smaller metal/N values . Furthermore, their efficiencies remain stable as a function of the optical pumping power up to 200 kW/cm 2 , proving that such QDs can be used for application in devices with various pumping requirements. Overall, the incorporation of AlGaN QD layers in UV has garnered considerable attention, showcasing promising outcomes for the integration of QDs into devices that emit in the first disinfection band, around 270 nm. – However, extending this technology to the far UV-C range presents additional challenges due to the higher Al contents needed in the QDs, such as (i) the impact of the reduced lattice mismatch on the QD morphology since the QD formation is not driven by the accumulated elastic energy in the layer but by the low surface energy with respect to the basal (0001) plane, (ii) the effect of the reduced band offset on the carrier confinement, (iii) the enhanced density of point defects in the Al-containing alloy (vacancies and pollutants), and (iv) the structural defects generated by the necessity to grow at lower temperatures than ideal for AlN due to the significant difference in binding energy between Al–N and Ga–N.…”

“…Further studies demonstrated that the IQE could be higher than 50% in the 276–296 nm spectral range in Al x Ga 1– x N/AlN QDs generated from the deposition of 4–5 monolayers (ML) of Al x Ga 1– x N (always with x ≤ 0.1) . More recent studies showed that even smaller (2.5–3 ML) Al x Ga 1– x N/AlN QDs ( x ≤ 0.1) could attain IQEs close to 50% if the metal/N ratio during the growth remained around 0.4, dropping for smaller metal/N values . Furthermore, their efficiencies remain stable as a function of the optical pumping power up to 200 kW/cm 2 , proving that such QDs can be used for application in devices with various pumping requirements.…”

AlGaN/GaN asymmetric graded-index separate confinement heterostructures designed for electron-beam pumped UV lasers

“…This technology has allowed the fabrication of ZnSebased pulsed lasers that emit up to 600 W at 535 nm [12]. There are some studies of electron beam pumped AlGaN for the implementation of UV lamps [13][14][15][16][17][18][19][20]. Wunderer et al demonstrated UV lasing from an AlGaN/GaN heterostructure containing InGaN quantum wells (QWs) under pulsed electron beam excitation at 77 K [21].…”