Abstract
Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries. However, despite strong scientific and practical importance, conclusive experimental evidence of confined acoustic phonon polarization branches in individual free-standing nanostructures is lacking. Here we report results of Brillouin—Mandelstam light scattering spectroscopy, which reveal multiple (up to ten) confined acoustic phonon polarization branches in GaAs nanowires with a diameter as large as 128 nm, at a length scale that exceeds the grey phonon mean-free path in this material by almost an order-of-magnitude. The dispersion modification and energy scaling with diameter in individual nanowires are in excellent agreement with theory. The phonon confinement effects result in a decrease in the phonon group velocity along the nanowire axis and changes in the phonon density of states. The obtained results can lead to more efficient nanoscale control of acoustic phonons, with benefits for nanoelectronic, thermoelectric and spintronic devices.
Highlights
Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries
The discovery of the large contribution of the long-mean free path (MFP) phonons to thermal conductivity makes the task of determining the length scale of the onset of confinement effects even more important
The important attributes of the samples were diameter uniformity within each batch, large distance, H, between the NWs, and large length, L, of NWs
Summary
The phonon states in semiconductors can undergo changes induced by external boundaries. Another point of view[2,10,15,18] considers that phonon spatial confinement effects start to take place when D is on the order of the average phonon mean free path (MFP), defined in kinetic theory as the grey MFP: LG 1⁄4 3K/(CVus), which is about 20 nm for bulk GaAs at RT (here, K and CV are the thermal conductivity and specific heat capacity, respectively) This is considered of importance because energy differences between the phonon subbands in the middle of the Brillouin zone (BZ) can result in changes in the electron—phonon scattering, and, correspondingly, electron relaxation rates, at low temperature[2,12,23]. The discovery of the large contribution of the long-MFP phonons to thermal conductivity makes the task of determining the length scale of the onset of confinement effects even more important
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