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Vincent Grenier

GaN microwires with GaN/AlGaN core-shell heterostructures: From growth to single wire UV devices

Published on 17 February 2022
Thesis presented February 17, 2022

Abstract:
III-N based LEDs are excellent candidates for the increasing demand of efficient and environmentally friendly UV sources. Despite more than 20 years of research on planar UV LEDs leading to significant optimizations, improving the efficiency of UV sources remains a challenging objective. A promising approach to the development of UV devices is the use of nanostructures, such as nano/microwires. This PhD work concerns the growth and the optical/structural properties of non-polar GaN/AlGaN quantum wells, grown in core-shell geometry on GaN microwires sidewall. This type of original structure is not well documented in the literature, with only one demonstration of an LED device emitting at 318 nm. Thus, this work corresponds to pioneer research on core-shell microwires for UV emission, with a complete study ranging from the growth by vapour phase epitaxy of non-polar GaN/AlGaN heterostructures to the elaboration of UV electroluminescent devices from single wires.
We first studied the growth conditions of the AlGaN shell to control the development of non-polar GaN/AlGaN heterostructures. Secondly, structural characterizations were carried out to determine the crystal quality of the GaN/AlGaN quantum wells, the different growth rates of the epitaxial layers, as well as the 60% Al composition in the AlGaN alloy. The UV emission of the non-polar GaN/AlGaN wells was measured by cathodoluminescence and photoluminescence at low temperatures, showing the control of the UV emission in the 350-290 nm range by adjusting the quantum well thicknesses from 4.3 to 0.7 nm. On the other hand, optical characterizations showed localization phenomena for the thinnest wells that local cathodoluminescence measurements and TEM observations attribute to a quantum dot regime. Part of this work focused on stress relaxation through crack formation, which was observed by cathodoluminescence to have a negative impact on the quantum well emission. This study validated the existence of a relevant relaxation criterion for core-shell structures defined by an empirical threshold of elastic energy per unit area, which had already been estimated at 4 J.m-2 for planar structures. Thus, in order to respect this limit, a reduction in the thickness of the AlGaN barriers resulted in the complete suppression of cracks.
The last part of the PhD work was dedicated to preliminary studies of the electrical properties of GaN/Al0.6Ga0.4N heterostructure inserted in a core-shell p-n junction demonstrated by EBIC measurements. A first device was developed with 2.6 nm GaN wells to achieve UV-A electroluminescence at 340 nm. We also demonstrated single-wire UV-B electroluminescence at 310 nm for a quantum dot heterostructure, which reaches the state-of-the-art in terms of wavelength. Finally, a last UV-B LED device was realized with ultrathin AlGaN barriers, which allowed to circumvent the quantum dot regime and to obtain the growth of ultrathin wells in the monolayer regime. Thus, a heterostructure with Al0.6Ga0.4N barriers and GaN wells of only two monolayers thick (0.55 nm) was achieved, exhibiting an electroluminescence as low as 302 nm for core-shell structures on GaN microwires.

Keywords:
MOCVD growth, core-shell wires, UV, nanostructures, non-polar