Crystal Phase Selective Growth in GaAs/InAs Core–Shell Nanowires

We present a novel type of core–shell nanowires in which only certain parts of the core are covered by the shell. This is achieved by the crystal phase selective growth of the InAs shell on zinc blende GaAs nanowires with controlled wurtzite inclusions. The shell grows on the zinc blende phase, but its growth is hindered on the wurtzite crystal phase. Nucleation of InAs occurs exclusively on the zinc blende GaAs regions. The wurtzite segments are placed inside self-catalyzed GaAs nanowires by partially consuming and refilling the Ga droplet. The crystal phase selective growth of InAs on the side facets of the GaAs nanowires is explained by the local environment of each new In atom. Because of unbalanced neighbors on the wurtzite side facets, the growth of a highly lattice mismatched material is hindered. This happens not only on the wurtzite segments, but also on regions being characterized by a high density of twins

Molecular Beam Epitaxy Growth of GaAs/InAs Core–Shell Nanowires and Fabrication of InAs Nanotubes

We present results about the growth of GaAs/InAs core–shell nanowires (NWs) using molecular beam epitaxy. The core is grown via the Ga droplet-assisted growth mechanism. For a homogeneous growth of the InAs shell, the As₄ flux and substrate temperature are critical. The shell growth starts with InAs islands along the NW core, which increase in time and merge giving finally a continuous and smooth layer. At the top of the NWs, a small part of the core is free of InAs indicating a crystal phase selective growth. This allows a precise measurement of the shell thickness and the fabrication of InAs nanotubes by selective etching. The strain relaxation in the shell occurs mainly via the formation of misfit dislocations and saturates at ∼80%. Additionally, other types of defects are observed, namely stacking faults transferred from the core or formed in the shell, and threading dislocations

Amphoteric Nature of Sn in CdS Nanowires

High-quality CdS nanowires with uniform Sn doping were synthesized using a Sn-catalyzed chemical vapor deposition method. X-ray diffraction and transmission electron microscopy demonstrate the single crystalline wurtzite structure of the CdS/Sn nanowires. Both donor and acceptor levels, which originate from the amphoteric nature of Sn in II–VI semiconductors, are identified using low-temperature microphotoluminescence. This self-compensation effect was cross examined by gate modulation and temperature-dependent electrical transport measurement. They show an overall n-type behavior with relatively low carrier concentration and low carrier mobilities. Moreover, two different donor levels due to intrinsic and extrinsic doping could be distinguished. They agree well with both the electrical and optical data