  The combination of core/shell geometry and band gap engineering in nanowire
heterostructures can be employed to realize systems with novel transport and
optical properties. Here, we report on the growth of InAs/InP/GaAsSb
core-dual-shell nanowires by catalyst-free chemical beam epitaxy on Si(111)
substrates. Detailed morphological, structural, and compositional analyses of
the nanowires as a function of growth parameters were carried out by scanning
and transmission electron microscopy and by energy-dispersive X-ray
spectroscopy. Furthermore, by combining the scanning transmission electron
microscopy-Moire 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 all the crystallographic 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 InP shell thickness above 8
nm, some dislocations and roughening occur at the interfaces. This study
provides useful general guidelines for the fabrication of high-quality devices
based on these core-dual-shell nanowires.
