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Alexandra-Madalina Siladie

AlxGa1-xN and AlN nanowires for the realization of efficient UV-C emitting diodes

Published on 8 November 2019
Thesis presented November 08, 2019

Solid-state deep UV Light emitting diodes (LEDs) based on AlxGa1-xN material are nowadays gaining particular attention due to their potential for replacing mercury lamps, currently used for sterilization and water disinfection applications. However, the realization of planar efficient emitting devices is limited by a high density of extended defects and difficult efficient dopant incorporation affecting both optical and electrical properties. As a strategy to alleviate this difficulty, I have focused on the study of nanowire based heterostructure devices, due to their advantage of elastically relaxing the strain during growth, coupled with a higher dopant solubility limit and an eased light extraction coming from their particular morphology.
First, correlated experiments of Atom Probe Tomography (APT), Energy Dispersive X-ray Spectroscopy (EDX) or Raman spectroscopy performed on GaN pn junctions grown by plasma assisted molecular beam epitaxy (PA-MBE) have shown that both n-type and p-type dopants, namely Si and Mg, respectively, exhibit an inhomogeneous radial distribution, with dopant incorporation upper limits attaining 1021 atoms/cm3 at the periphery, higher than in 2D layers. The study of Mg incorporation by APT concluded on the understanding of the incorporation mechanism, preferential on the m-plane side-wall and assisted by H due to the high stability of the Mg-H complex in N rich conditions.
The second part of the work is dedicated to the study of the more challenging Mg dopant incorporation and activation in AlN alloy, successfully attained in this PhD by In-Mg co-doping. The efficient incorporation of this dopant in AlN nanowires assisted by small concentration of In has been assessed by a series of techniques (EDX, Raman). Ab-initio theoretical calculations have shown that the efficient incorporation of Mg in an Al-substitutional site is due to a process involving the nitrogen vacancies forming an In-VN complex. The formation of AlN NW p-n junction has been concomitantly assessed by electron beam induced current (EBIC) experiments putting in evidence the electrical field associated with the junction. An extensive study of the electrical activation of acceptor impurities has been further achieved by electron beam irradiation of the samples and characterized in EBIC experiments.

AlGaN, AlN, LED, nanowires, molecular beam epitaxy, Electron Beam Induced Current

On-line thesis.