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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.

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II Vapour phase technologies for self-assembly of quasi-1D semiconductor nanostructures 2.1 Self-assembly of quasi-1D nanostructures Semiconductor nanodevices based on quasi-1D nanostructures call for new methods of nano-scale manufacturing, as “top-down” processes 1,2,3,4 , which are dominant in the current ULSI technologies of semiconductors, cannot be easily (sophisticated equipment or intrinsic limitations in spatial resolution) or economically (high production costs) applied to the new class of devices. Self-assembly of semiconductor nanostructures is an alternative “bottom- up” approach that has many potentialities. Nanowires with designed dimensions and properties can be fabricated more easily by self-assembling methods because in such methods individual building blocks (atoms or molecules) interact each- others in pre-defined ways, resulting in the spontaneous self-organization of size- selected nanostructures 5 . Any self-assembly nanotechnology process has the following advantages 6 1) it is a parallel process, a feature very important at the nano-scale to guarantee high productivity; 2) it can make structures with sub-nanometer precision (typically in the 1-100 nm range); 3) it is able to generate three-dimensional nano-scale architectures; 4) the process can be easily altered by external forces and geometrical constraints to dynamically re-assemble/re-configure on-demand any nano-scale object. A classical example of a self-assembly process is the quantum dots (QDs) formation on semiconductor surfaces by metalorganic vapour phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) using the Stranski-Krastanov growth mode 7 . In this process a very thin layer of the desired material (a few nanometers) is grown epitaxially on a substrate with a different lattice constant and the strain in

Tesi di Dottorato

Dipartimento: Dipartimento di Ingegneria dell'Innovazione

Autore: Pasquale Paiano Contatta »

Composta da 130 pagine.


Questa tesi ha raggiunto 105 click dal 23/08/2011.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.