12. Nanowire Growth Activities:
Contact person:
Lucia Sorba
The main activity of the group is the growth of III-V semiconductor nanowires (NWs) with the chemical beam epitaxy (CBE) technique. CBE combines the advantages of MBE (fine control of layer thickness, sharp interfaces between different materials, in situ monitoring and characterization via RHEED) with the ease of use and the flexibility given by metalorganic precursor sources. Available group-V materials are arsenic (TBAs), phosphorous (TBP), and antimony (TDMASb and TMSb); for group III there are indium (TMIn), gallium (TEGa), and aluminium (TMAl) precursors; for n-doping we use selenium (TBSe). This allows us to grow InAs, GaAs, AlAs, InP, GaP, InSb, GaSb, and their alloys and heterostructures.
InAs NWs are the focus of the group: the small band gap, small electron effective mass, large Landè g-factor, and the ease to form good Ohmic contacts make them ideal to study low temperature transport phenomena and for the fabrication of quantum electronic and photonic devices. We can use two different approaches for the growth of InAs NWs: the Au-assisted method, in which Au nanoparticles are employed to drive the one-dimentional crystal growth, or the so called catalyst-free growth, without the use of any metal nanoparticle.
One of the most peculiar features of semiconductor NWs is their nanometer-sized cross-section, which gives the possibility to combine dissimilar materials, also with a high lattice mismatch, without the formation of defects at the interface, thanks to the strain relaxation along the NW sidewalls. As a consequence, defect-free axial heterostructures non achievable in bulk can be obtained in NWs. Some examples are reported in Figure 1: InAs/InSb axial NWheterostructures (a) and InAs/GaAsaxial NWheterostructures (b). Even more complex axial heterostructurescan be realized by inserting thin InPsegments as a barrier material within InAsNWs. These allow for instance the formation ofInAs quantum dotsobtained by confining a short (~20 nm) InAs segment within two thin (~5 nm) InP barriers (see Fig. 1(c)). This kind of nanostructures is currently being used at the NEST laboratory for the investigation of highly-controllable single-electron devices where both the energy spectrum and the spin configuration can be controlled electrostatically.
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Fig 1: Axial NW heterostructures: InAs/InSb (a), InAs/GaAs (b) and InAs/InP (c).
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Radial heterostructures, where the chemical composition of the material is modulated perpendicularly to the wire axis (the so called core-shell configuration) can also be grown. In this case, the controlled growth of one or more shells can passivate existing surface states, enable new interface properties and introduce unique electronic functions. As an example, InAs/GaSb core/shell NWs obtained by catalyst-free growth are shown in Fig. 2. The InAs/GaSb heterojunction has a broken-gap band alignment which can lead to the coexistence of mobile electrons and holes in different regions of the samples and can be adopted to realize inter-band tunnel devices and to study induced topological states and quantum transport phenomena.
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Fig 2: Radial InAs/GaSb NW heterostructures obtained with the catalyst-free growth approach and schematic view of the peculiar boken-gap heterojunction of this system.
V. Zannier, F. Rossi, V.G. Dubrovskii, D. Ercolani, S. Battiato and L. Sorba, Nanoparticle Stability in Axial InAs-InP Nanowire Heterostructures with Atomically Sharp Interfaces, Nano Lett, 18, 167-174 (2018).
V. Zannier, D. Ercolani, U. P. Gomes, J. David, M. Gemmi, V. G. Dubrovskii and L. Sorba, Catalyst Composition Tuning: The Key for the Growth of Straight Axial Nanowire Heterostructures with Group III Interchange, Nano Lett, 16, 7183–7190 (2016).
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Growth dynamics studies and NW characterization via scanning electron microscopy are performed in-house within the group, while transmission electron microscopy, electrical transport, optical characterization, and device fabrication are carried out through well-established internal and external collaborations.
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