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Anjali Harikumar

Development of AlGaN nanostructures for the fabrication of electron-pumped UV emitters

Published on 14 June 2022
Thesis presented June 14, 2022

This project is a contribution to the development of high-brightness, mercury-free, 100% recyclable and high-gloss ultraviolet (UV) lamps for disinfection at 270 nm. The performance of AlGaN-based UV LEDs remains limited by carrier injection problems, due to the high activation energy of dopants in this material and the difficulties to implement ohmic contacts. To circumvent these problems, we propose to pump an active region based on AlGaN nanostructures with an electron beam. For investigating the active media for electron-pumped UV lamps for disinfection, two types of heterostructures are considered, namely AlGaN/AlN quantum dots (QDs) embedded in GaN nanowires (NWs) synthesized on Si(111) substrates and AlGaN/AlN QDs grown by the Stranski-Krastanov (SK) method on AlN-on-sapphire templates. These choices were based on the high internal quantum efficiency (IQE) that can be achieved in QDs thanks to the three dimensional confinement of carriers. The growth of all the structures was performed by plasma-assisted molecular beam epitaxy.
The first stage of the work involved establishing the feasibility of QDs on NWs for this application. Structural and optical studies demonstrated that the QD dimensions were quite homogeneous along the 400-nm-long superlattice (SL), giving a single emission line tunable in the range from 340 to 258 nm. We demonstrated IQE levels higher than 60% at 340 nm at room temperature, decreasing when reducing the emission wavelength. At 270 nm, the IQE was around 30%. These values were obtained under low injection, but remain stable for pumping densities up to 200 kW/cm2. Under electron beam pumping, channeling effects due to the NW geometry can be compensated by increasing the active region length by 60% in comparison to planar layers. The external quantum efficiency under optical pumping provides promising results (3.42±0.55%).
The second stage of the work involved studying AlGaN/AlN SK-QD SLs. It is demonstrated that by modifying the III/V ratio and/or the composition of the QDs, the peak emission wavelength can be tuned from 225 nm to 335 nm while keeping the IQE around 50% in the 250-335 nm range, and higher than 35% in the 225-250 nm range. The IQE of these nanostructures remain stable from low to high injection power densities up to 200 kW/cm2. An average power efficiency of 0.38% is measured for as-grown structures, and that without any treatment to improve the light extraction efficiency or thermal management. These results are very promising for the development of an efficient, environment friendly UV technology for disinfection.

AlGaN, Quantum dots, Nanowires, Molecular beam epitaxy, electron pumping, ultraviolet lamp

On-line thesis.