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Robert Köster

Self-assembled growth of catalyst-free GaN wires by metal-organic vapour phase epitaxy

Published on 28 June 2010


Thesis presented June 28, 2010

Abstract:
A new catalyst-free method has been developed to grow self-assembled GaN wires on c-plane sapphire substrates by metal-organic vapor phase epitaxy. This approach, based on in situ deposition of thin SiNx layer (~2 nm), enables epitaxial growth of c-oriented wires with 200-1500 nm diameters and a large length/diameter ratio (>100) on c-plane sapphire substrate. The detailed study of the growth mechanisms shows that a combination of key parameters is necessary to obtain vertical growth. In particular, the duration of SiNx deposition prior to the wire growth is critical for controlling the epitaxy with the substrate. The GaN seed nucleation time determines the mean-size diameter and structural quality and a high Si-dopant concentration promotes the vertical growth. Such GaN wires exhibit UV-light emission centered at about 350 nm and a weak yellow band (~550 nm) at low temperature. This approach may be viewed as a fast and reproducible technique to grow GaN wires by MOVPE. Compared to other techniques, it allows studying quite systematically the influence of the growth parameters without being dependent on time consuming ex situ surface preparations like surface patterning used in selective area growth (SAG). The growth of heterostructures, as longitudinal n-u and n-p wires (using Si and Mg dopants) as well as a core-shell InGaN/GaN MQW using the wires as templates has been demonstrated. The growth occurs on the non-polar m-plane facets of the wire and not on c-plane as it is the case on 2D materials with the same sapphire substrate. It gives different piezoelectric contributions to the wires optical properties, which have been studied by cathodo- and photo-luminescence. The fundamental building blocks of wire-based blue LED were demonstrated during this thesis. The realization of an efficient device still requires a deeper understanding and optimization of the parameters controlling the material growth and an optimization of the electrical contacts.

Keywords:
nanowires, GaN, MOVPE, growth, optics, heterostructures, quantum wells

On-line thesis.