Au-catalized self-assembly of III-V semiconductor nanowires by metalorganic vapour phase epitaxy

The work presented in this research thesis has dealt with the Au-catalyzed MOVPE growth and properties of self-assembled quasi-1D nanocrystals of III-V compounds. The topic represents a field of intense activities carried out by numerous research laboratories throughout world.
In this thesis, emphasis was given on the study of the VLS growth of III-V compound nanowire structures, and in particular of GaAs and AlGaAs nanowires, as well as of GaAs/AlGaAs core/shell nanowires. With the exclusion of GaAs, such quasi-1D nanostructures were not sufficiently studied in the relevant literature previous to this work, despite their potential applications to nano-electronics and nano-photonics. MOVPE growth experiments were performed by the VLS self-assembly method, using Au-catalyst NPs obtained either from direct self-assembly of thin Au films on substrates, or deposited from colloidal solutions.
As-grown nanowire structures were characterised in terms of morphology, size, structure and vibrational/radiative properties; their properties were connected to growth conditions for a first optimization round. The morphology, structure, and size of Au-catalyst NPs synthesised/deposited on the surface of various crystalline semiconductors were also studied in details, as part of this work.
Au-catalyzed MOVPE growth of well-aligned (kink-free) GaAs nanowires on (111)B GaAs substrates has been demonstrated at temperatures down to 400°C for the first time in the present work using TBAs and TMGa precursors in H2 ambient. Colloidal Au NPs were used as catalysts.
Structural analyses of GaAs nanowires allowed to evidence the occurrence of NPs at the nanowire tips; these NPs are composed of crystalline fcc Au, with a preferred (111) orientation. In addition, the occurrence of the GaAu2 alloy (γ phase), likely at the nanowire/Au-nanoparticle interface, has been demonstrated, suggesting that the catalyst NP at the tip of the nanowires is liquid at the growth temperature, as expected for a VLS process.
Au-catalyzed MOVPE growth of AlGaAs nanowires on (111)B GaAs substrates were also demonstrated for the first time in this thesis. Preliminary EDS analyses on the samples confirmed the presence of Al in the nanowires, whereas no diffusion of Au inside the nanowires occur. AlGaAs nanowires with relatively narrow base diameter distributions grew along the [111] direction; however, extensive nanowire kinking was observed, likely a result of Al-induced growth instabilities at the catalyst/nanowire interface. AlGaAs nanowires grown with a lower Al fraction in the vapour showed better structure (almost kink-free) and morphology, confirming that TMAl flow is a critical parameter for the VLS growth of AlGaAs nanowires. The dependence of AlGaAs nanowire growth rates on temperature is similar to that observed for GaAs nanowires, but different activation energies were obtained for their axial and lateral growth.
Finally, GaAs/AlGaAs core/shell nanowires were demonstrated utilising a two step growth process. Cylindrically-shaped GaAs core nanowires with average diameters in the 60 – 70 nm range were grown at 400°C by Au-catalyzed MOVPE and covered with Al0.34Ga0.66As shells grown by conventional MOVPE at 650°C. As-grown nanostructures were kink-free and well-aligned along the [111]B direction.
Further improvements in the radiative and structural properties of GaAs-based nanowires will require a more systematic study and correlation of their properties with VLS growth conditions. Besides, intentional n- and p-type doping of the nanowires need to be studied. This will allow the necessary materials performances and process optimization for GaAs-AlGaAs quasi-1D nanostructures to find applications to the realization of novel nano-electronic and nano-photonic devices.