Self-catalyzed and catalyst-free III-V semiconductor nanowire grown by CBE

In this thesis, the growth dynamics and mechanisms of III-V semiconductor nanowires (NWs) and their heterostructures are studied. III-V NWs are realized by self-catalyzed and catalyst-free growth methods on Si (111) substrates by means of chemical beam epitaxy. The Au-free growth approach is particularly important for the integration of III-V semiconductors on silicon toward a CMOS-compatible electronics. The morphological and structural properties of the grown NWs are investigated by scanning (SEM) and transmission electron microscopy (TEM). These NWs exhibit very high aspect ratio and good material quality, which makes them useful to be employed for fundamental studies as well as for application in electronics and optoelectronics. The first part of the thesis is focused on the growth of InAs/InP/GaAsSb core-dual-shell (CDS) NWs. Detailed morphological, structural, and compositional analyses of the NWs as a function of growth parameters are carried out by SEM, TEM, and by energy-dispersive X-ray spectroscopy. Furthermore, by combining the scanning transmission electron microscopy-Moiré technique with geometric phase analysis, we studied the residual strain and the relaxation mechanisms in this system. We found that InP shell facets are well-developed along the crystallographic <110> and <112> directions only when the nominal thickness is above 1 nm, suggesting an island-growth mode. Moreover, the crystallographic analysis indicates that both InP and GaAsSb shells grow almost coherently to the InAs core along the ⟨112⟩ direction and elastically compressed along the ⟨110⟩ direction. For an InP shell thickness above 8 nm, some dislocations and roughening occur at the interface. This study provides useful general guidelines for the fabrication of high-quality devices based on these CDS NWs. Indeed, we investigated the tunnel coupling between the outer p-type GaAsSb shell and the n-type InAs core in InAs/InP/GaAsSb CDS NWs. Low-temperature (4.2 K) transport measurements in the shell-shell configuration in CDS NWs with 5 nm-thick InP barrier reveal a weak negative differential resistance. Differently, when the InP barrier thickness is increased to 10 nm, this negative differential resistance is fully quenched. The electrical resistance between the InAs core and the GaAsSb shell, measured in core-shell configuration, is significantly higher with respect to the resistance of the InAs core and of the GaAsSb shell. The field effect, applied via a back-gate, has an opposite impact on the electrical transport in the core and in the shell portions. Our results show that electron and hole free carriers populate the InAs and GaAsSb regions respectively and indicate InAs/InP/GaAsSb CDS NWs as an ideal system for the investigation of the physics of interacting electrons and holes at the nanoscale. The second part of this thesis is dedicated to the growth of self-catalyzed InAs/InSb axial heterostructures. The growth mechanisms of these heterostructures are thoroughly investigated as a function of the In and Sb line pressures, and growth time. Some interesting phenomena are observed and analysed. In particular, the presence of an In droplet on top of the InSb segment is shown to be essential to form axial heterostructures in the self-catalyzed vapor-liquid-solid mode. Axial versus radial growth rates of InSb segments are investigated under different growth conditions and described within a dedicated model containing no free parameters. It is shown that a widening of the InSb segment with respect to the InAs stem is caused by the vapor-solid growth on the nanowire sidewalls rather than by the droplet swelling. The In droplet can even shrink smaller than the nanowire facet under Sb-rich conditions. The third part of the thesis is focused on the realization of self-catalyzed InSb quantum dot (QD) embedded into InAs NW. A systematic study on the influence of the growth parameters on the morphology of such NWs is performed. Radial and axial growth rates are studied as a function of growth parameters in order to realize InSb QD NW with controlled morphology. In particular, we have explored different growth conditions to minimize the InAs shell around the InSb QD. We found that the shell thickness around the InSb QD decreases with increasing growth temperature while it increases with an increase of the As line pressure. Furthermore, from the high resolution-TEM analysis, we observed that InAs-stem and InAs-top segment have a wurtzite (WZ) crystal structure with several defects such as stacking faults and twins perpendicular to the growth direction. It is commonly observed that the InAs NWs grown by catalyst-free and self-catalyzed growth methods show highly defective (or mixed WZ/ZB) crystal structure. By contrast, here the InSb QD shows a defect-free zincblende (ZB) crystal structure without any stacking faults, consistently with the energetically preferred cubic structure of the InSb crystals generally attributed to the low ionicity of group III to Sb bonds. This study gives useful information for the realization of InSb QDs with controlled morphology and optimized quality embedded in InAs NWs in the self-catalyzed regime.