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Register a new userDirect Observation of Interface and Nanoscale Compositional Modulation in Ternary III-As Heterostructure Nanowires
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Added by
Peter Krogstrup Ph.D
Publication date
2013
fields
Physics
and research's language is
English
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Straight, axial InAs nanowire with multiple segments of GaInAs were grown. High resolution X-ray energy-dispersive spectroscopy (EDS) mapping reveal the distribution of group III atoms at the axial interfaces and at the sidewalls. Significant Ga enrichment, accompanied by a structural change is observed at the GaInAs/InAs interfaces and a higher Ga concentration for the early grown GaInAs segments. The elemental map and EDS line profile infer Ga enrichment at the facet junctions between the sidewalls. The relative chemical potentials of ternary alloys and the thermodynamic driving force for liquid to solid transition explains the growth mechanisms behind the enrichment.
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Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries. Modification of acoustic phonon spectrum in structures with periodically modulated elastic constant or mass density - referred to as phononic crystals - has been proven experimentally and utilized in practical applications. A possibility of modifying acoustic phonon spectrum in individual nanostructures via spatial confinement would bring tremendous benefits for controlling phonon-electron interaction and thermal conduction at nanoscale. However, despite strong scientific and practical importance, conclusive experimental evidence of acoustic phonon confinement in individual free-standing nanostructures, e.g. nanowires, is still missing. The length scale, at which phonon dispersion undergoes changes and a possibility of the phonon group velocity reduction, are debated. Here, we utilize specially designed high-quality GaAs nanowires (NWs) with different diameters, D, and large inter-nanowire distances to directly demonstrate acoustic phonon confinement. The measurements conducted with Brillouin - Mandelstam spectroscopy reveal confined phonon polarization branches with frequencies from 4 GHz to 40 GHz in NWs with D as large as ~128 nm, i.e. at length scale, which exceeds the grey phonon mean-free path in GaAs by an almost an order of magnitude. The phonon dispersion modification and phonon energy scaling with D in individual nanowires are in excellent agreement with theory. The obtained results can lead to more efficient nanoscale control of acoustic phonons, with benefits for nanoelectronics, thermoelectric energy conversion, thermal management, and novel spintronic technologies.
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