Phonons In Nanowires Research Articles

Effect of spin–orbit coupling on the hot-electron energy relaxation in nanowires

The energy relaxation of hot electrons is proposed based on the spin–orbit (SO) interaction of both Rashba and Dresselhaus types with the effect of hot phonons. A continuum theory of optical phonons in nanowires taking into account the influence of confinement is used to study the hot-electron energy relaxation. The energy relaxation due to both confined (CO) and interface (IO) optical phonon emission on nanowire radius, electrical field strength, parameters of SO couplings and electron temperature is calculated. For considered values of the nanowire radius as well as other system parameters, scattering by IO phonons prevails over scattering by CO phonons. The presence of an electric field leads to the decrease of power loss in transitions between states with the same spin quantum numbers. With the increase of the electric field strength, the influence of the Dresselhaus SO interaction on the energy relaxation rate decreases. The effect of SO interaction does not change the previously obtained increasing dependence of power loss on electron temperature. The sensitivity of energy relaxation to the electric field also through the Rashba parameter allows controlling the rate of energy by electric field.

Acoustic phonons in nanowires probed by ultrafast pump-probe spectroscopy

Abstract The fascinating relationship between structure and property in nanowires has enabled a wealth of applications in photonics and electronics. The behavior of phonons in nanowires is also modified compared to their bulk counterparts. In this review, we provide an overview of the recent efforts to investigate the properties of acoustic phonons in nanowires using ultrafast optical methods. In particular, we focus on the calculation of the modified phonon dispersion in nanowires and how to address them optically. We then discuss experimental investigations in arrays of nanowires and a single nanowire. The analysis of phonon behavior reveals the possibility to perform advanced mechanical characterization and to vary the thermal properties of nanowires. The review concludes with a brief perspective on future research directions, from phonon-induced control over properties to three-dimensional (3D) acoustic nano-imaging.

Insight into phonon scattering in Si nanowires through high-field hole transport: Impacts of boundary condition and comparison with bulk phonon approximation

The impact of how to model phonon scattering on hole transport in Si nanowires was studied based on Boltzmann’s transport equation. Boundary conditions for atomistic description of phonons in nanowires and approximation by bulk acoustic and optical phonons were analyzed in terms of their impacts on high-field hole transport. The boundary conditions for phonons influence the drift velocity and momentum relaxation time, especially at low electric field, but the energy relaxation time hardly depends on the boundary conditions. The impacts by the change of boundary conditions can be approximated by the change of the strength of acoustic phonon scattering in bulk phonon picture, though the behavior of energy relaxation and distribution function of holes can not be reproduced by bulk phonon approximation.

Effect of cross-sectional geometry on thermal conductivity of Si1−xGex nanowires

By incorporating the direction-dependent phonon-boundary scattering from the surface of the nanowires with different cross-sectional shapes into the linearized phonon Boltzmann transport equation, we theoretically investigate the effect of cross-sectional geometry on the thermal conductivity of [Formula: see text] nanowires. It is demonstrated that the surface-to-volume ratio (SVR) is a universal gauge for both pure silicon nanowires (SiNWs) and silicon–germanium nanowires (SiGe NWs), and the thermal conductivity of nanowires decreases monotonically with the increase of SVR. We also find that the thermal conductivity of high-frequency phonons in nanowires is more strongly SVR dependent than that of low-frequency phonons, and the thermal conductivity of high-frequency phonons is severely suppressed by alloy scattering, therefore the SVR dependence on thermal conductivity of [Formula: see text] NWs decreases with the increase of Ge atom concentration [Formula: see text] [Formula: see text]. These findings are useful for understanding and tuning the thermal conductivity of nanowires by geometry.

Acoustic Phonons in Nanowires with Embedded Heterostructures

Acoustic phonons are studied in finite nanowires based on the elastic continuum model. The phonon dispersion relations are compared for different types of approximations both for pure nanowires and nanowire heterostructures including cubic and wurtzite materials. We find that the isotropic approximation highly underestimates the frequency of torsional modes. The impact of heterostructures on the lowest modes is rather weak and can be described by an average of the different structure parameters.

Renormalization Plus Convolution Method for Atomic-Scale Modeling of Electrical and Thermal Transport in Nanowires

Based on the Kubo-Greenwood formula, the transport of electrons and phonons in nanowires is studied by means of a real-space renormalization plus convolution method. This method has the advantage of being efficient, without introducing additional approximations and capable to analyze nanowires of a wide range of lengths even with defects. The Born and tight-binding models are used to investigate the lattice thermal and electrical conductivities, respectively. The results show a quantized electrical dc conductance, which is attenuated when an oscillating electric field is applied. Effects of single and multiple planar defects, such as a quasi-periodic modulation, on the conductance of nanowires are also investigated. For the low temperature region, the lattice thermal conductance reveals a power-law temperature dependence, in agreement with experimental data.

Phonon modes in Si [111] nanowires

We solve a long standing question concerning the boundary conditions for optical phonons in nanowires. The controversy between the so-called clamped and free conditions is well documented in standard literature. We answer this question of the boundary conditions by presenting a spring-and-mass model of the Stillinger and Weber type to calculate optical and acoustic phonons in Si [111] nanowires. Our results show that this model, in parallel to the bulk case, is sufficient to describe important features of the phonon spectrum. Furthermore, our results predict that optical phonons in nanowires are subject to clamped boundary conditions.