Experimental Investigation of Acoustic Phonon Confinement Effects in Nanostructured Materials
- Author(s): Kargar, Fariborz
- Advisor(s): Balandin, Alexander A.
- et al.
This dissertation is focused on experimental investigation of confined acoustic phonons in nanostructures. 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 freestanding nanostructures is lacking. In addition, the length scale at which the phonon confinement effects start to appear is a point of debate. In this dissertation, I report results of Brillouin – Mandelstam light scattering spectroscopy for two distinct material systems: nanoporous alumina films and GaAs nanowire arrays. A combined investigation of thermal conductivity and acoustic phonon spectra was conducted with nanoporous alumina membranes with the pore diameter decreasing from D=180 nm to 25 nm. The Brillouin-Mandelstam spectroscopy measurements revealed bulk-like phonon spectrum in the samples with D=180-nm pores and spectral features, which were attributed to spatial confinement, in the samples with 25-nm and 40-nm pores. The velocity of the longitudinal acoustic phonons was reduced in the samples with smaller pores. Analysis of the experimental data and calculated phonon dispersion suggested that both phonon-boundary scattering and phonon spatial confinement affect heat conduction in membranes with the feature sizes D<40 nm. Results of the Brillouin – Mandelstam light scattering spectroscopy revealed multiple 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 have been found 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.