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Gabriel Tourbot

Structural and optical properties of MBE-grown InGaN/GaN nanowire heterostructures for LEDs

Published on 11 June 2012


Thesis presented June 11, 2012

Abstract:
This work reports on the molecular-beam-epitaxial growth of InGaN/GaN nanowires on Si(111) substrates. The deposition of InGaN in nitrogen-rich conditions on preexisting GaN nanowires allows us to maintain the columnar structure. Wire morphology varies strongly with the indium concentration in the fluxes. At low nominal In flux, it concentrates in the wire core, resulting in a spontaneous InGaN-GaN core-shell structure. In spite of the high indium content in the core, strain relaxation is purely elastic in these structures. On the other hand, using higher nominal In fluxes lead to plastic relaxation and no phase separation is observed. Luminescence is dominated by carrier localization phenomena, allowing for a low quenching of the emission up to room temperature. Studying InGaN insertions confirms that in spite of the small diameter of the wires, growth is dominated by strain relaxation effects, and InGaN nucleates as facetted islands. The incorporation of indium occurs preferentially at the top of the islands, resulting in a radial composition gradient which leads to the spontaneous growth of a core-shell structure. Growth in metal-rich conditions results in a very strong lateral growth, far superior for InGaN than for GaN: excess In has a surfactant effect limiting the axial growth rate and promoting lateral growth.

Keywords:
LEDs, Quantum dots, Semiconductors, Nanowires, MBE, Nitrides

On-line thesis.