Acoustic Phonons in Nanowires with Embedded Heterostructures (original) (raw)
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Research Article Acoustic Phonons in Nanowires with Embedded Heterostructures
2016
Link to publication Citation for published version (APA): Ayedh, H., & Wacker, A. (2011). Acoustic phonons in nanowires with embedded heterostructures. Journal of Nanomaterials, 2011, [743846]. 10.1155/2011/743846 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investiga...
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.
Acoustic phonon modes of rectangular quantum wires
Journal of Physics: Condensed Matter, 1997
Acoustic phonon modes of a free-standing rectangular quantum wire of cubic crystals are theoretically investigated using an algorithm developed to analyze data from resonant ultrasound spectroscopy. The normal phonon modes are classified by their spatial symmetries into a compressional mode termed the dilatational mode and non-compressional modes referred to as the flexural, torsional and shear modes. The formalism we present is quite general and can be applied to wires of any cubic material. As an example, the dispersion relations are obtained for square and rectangular wires of GaAs, taking into account anisotropic elasticity of the material. The dispersion curves for a rectangular wire are compared with those of the approximate hybrid modes referred to as the thickness and width modes, and the validity of the modes is discussed. The existence of edge modes is confirmed by examining the spatial distribution of displacement vectors.
Robust modeling of acoustic phonon transmission in nanomechanical structures
Applied Physics Letters, 2019
The transmission of acoustic phonons is an important element in the design and performance of nanomechanical devices operating in the mesoscopic limit. Analytic expressions for the power transmission coefficient, T , exist only in the low-frequency (quasi-static) limit described by thin-plate elastic theory, and for well-defined elastic wave-guiding geometries. We compare two numerical techniques based on finite-element computations to determine the frequency dependence of T for arbitrary phonon scattering structures. Both methods take into account acoustic mode conversion to acoustic and optical modes. In one case, phase and amplitude of complex-valued reflected waves are determined and related to transmission through a Fresnel equation, while in the other the magnitude of the transmitted mechanical power is directly calculated. The numerical robustness of these methods is demonstrated by considering the transmission across an abrupt junction in a rectangular elastic beam, a well-known problem of considerable importance in mesoscopic device physics. The simulations presented extend the standard results for acoustic phonon transmission at an abrupt junction, and are in good agreement with analytic predictions from thin-plate elastic theory in the long-wavelength limit. More generally, the numerical methods developed provide an effective tool for calculating acoustic mode energy loss in nano-mechanical resonators through mode conversion and heat transfer in arbitrary mesoscopic structures.
Shear Acoustic Phonons in Multilayer Arsenide Semiconductor Nanostructures
Journal of Nano- and Electronic Physics, 2019
Using the elastic continuum model, exact analytical solutions for the equations of motion for the elastic medium of a multilayer resonant tunneling nanosystem describing the shear modes of acoustic phonons are obtained. The expressions describing the components of the stress tensor arising in the studied nanostructure and boundary conditions for the components of the elastic displacement vector and the components of the stress tensor are obtained. Using the obtained equations of motion for the elastic medium and boundary conditions, the theory of the spectrum and phonon modes for shear acoustic phonons is developed in the proposed work for a plane arsenide semiconductor nanostructure. It is shown that the spectrum of the displaced acoustic phonons of the studied nanosystem is obtained from the dispersion equation following from the boundary conditions using transfer-matrix method. Using the orthonormality condition, the normalized modes of shear acoustic phonons are obtained. For the parameters of the three-barrier nanostructure-the active zone of a quantum cascade detector-the calculation of the spectrum of acoustic phonons and its dependencies on the wave vector and the geometric parameters of the nanostructure has been performed. It is shown that the calculated dependences of the spectrum of acoustic phonons on the wave vector form three groups with boundary values equal to the corresponding energies of acoustic phonons in massive crystals. Also it is obtained that an increase in the thickness of the internal barrier at constant other geometrical parameters of the nanosystem leads to a steady decrease in the values of the phonon energy levels energies. The proposed theory can be used to study the scattering of electron fluxes on acoustic phonons in multilayer resonant-tunneling structures.
Phonon Dispersion and Lattice Properties
2016
Phonon dispersions in 100 silicon nanowires (SiNW) are modeled using a Modified Valence Force Field (MVFF) method based on atomistic force constants. The model replicates the bulk Si phonon dispersion very well. In SiNWs, apart from four acoustic like branches, a lot of flat branches appear indicating strong phonon confinement in these nanowires and strongly affecting their lattice properties. The sound velocity (V snd) and the lattice thermal conductance (κ l) decrease as the wire cross-section size is reduced whereas the specific heat (Cv) increases due to increased phonon confinement and surface-to-volume ratio (SVR).
Generation of coherent acoustic phonons in piezoelectric semiconductor heterostructures
2003
We review some experimental and theoretical aspects of coherent acoustic phonon generation in piezoelectric semiconductor multiple quantum wells. In order to model more advanced and complicated nano-acoustic devices, a macroscopic continuum theory for the generation and propagation of coherent acoustic phonons in piezoelectric semiconductor heterostructures is presented. The macroscopic approach is applicable in the coherent regime, and can be easily utilized to study coherent acoustic devices based on piezoelectric semiconductor heterosutructures. For each phonon mode, the corresponding coherent acoustic field obeys a loaded string equation. The driven force has contributions from the piezoelectric and deformation potential couplings. We applied the theory to model the generation of coherent longitudinal acoustic phonons in (0001)-oriented InGaN/GaN multiple quantum wells. The numerical results are in good agreement with the experimental ones. By using the macroscopic theory, we also investigated the crystal-orientation effects on the generation of coherent acoustic phonons in wurtzite multiple quantum wells. It was found that coherent transverse acoustic phonons dominate the generation for certain orientation angles.
Acoustic phonon dispersion in single-crystal
Journal of Physics: Condensed Matter, 1999
The longitudinal elastic constants of C 60 single crystals were measured in the 100 and 111 directions near T c = 260 K by an ultrasonic CW resonance technique in the MHz region. The comparison with previous low-frequency (f = 1 Hz) elastic measurements yields a huge acoustic phonon dispersion near T c. We show that the dispersion is due to the crossover from isothermal to adiabatic behaviour.