Reduced thermal conductivity of Si/Ge random layer nanowires: A comparative study against superlattice counterparts

Abstract

Si/Ge nanowires are considered to be promising candidates as efficient thermoelectric materials due to their remarkable thermal insulating performance over bulk counterparts. In this study, thermal insulating performance of Si/Ge nanowires of randomly organized layer thickness, called random layer nanowires (RLNWs), is systematically investigated and compared against superlattice nanowires (SLNWs).The thermal conductivity (TC) of these structures is evaluated via non-equilibrium molecular dynamic simulations, and more informative insight is gained by normal mode decomposition and lattice dynamics calculations. It is demonstrated that the modes in random layer structures, in general, exhibit similar characteristics except the degree of localization to the corresponding superlattice counterparts by comparing the mode spectral energy densities, relaxation times, density of states, and participation ratios. For all physical and geometrical conditions investigated here, RLNWs show improved thermal insulating performance over corresponding SLNWs. More importantly, a RLNW of low mean layer thickness attains even lower TC than the corresponding Si/Ge alloy nanowire indicating the effectiveness of the random layer arrangements. An anomalous trend in TC of RLNWs (larger than the bulk counterpart) is observed at higher cross-sectional widths, and it is explained as a competing effect of phonon localization and wall scattering. Moreover, it is illustrated that the effectiveness of thermal insulating performance of RLNW depends on the fraction of coherent phonons that exist and how effectively those phonons are subject to localization under different cases.

Thermal Transport in Silicon-Germanium Superlattices at Low Temperatures

“…At this stage, we should count the energy variations of each cell to calculate thermal flux. In simulations, the energy list of temperatures is calculated from equation (6) in advance. Then, the temperature of each cell can be decided by its energy from this list by the bisection method.…”

“…Owing to different lattice constants between two materials, their phonon dispersions are not the same and the phenomenon of lattice mismatch exists, which causes the special thermal properties and electronic characters [1]. Interfacial thermal resistances in superlattices [2][3][4][5][6][7][8][9][10], such as AlAs/GaAs, Si/Ge, Si/SiGe, and Si 1−x Ge x /Si 1−y Ge y , have attracted keen interests of many researchers. Rezgui et al [2] presented an enhanced ballistic-diffusive equation to solve the heat transport across germanium-silicon interface and emphasized the importance of phonon-boundary interactions and surface roughness effect.…”

Effect of morphology on the phonon thermal conductivity of Si, Ge, and Si/Ge core/shell nanowires

“…However, due to the high thermal conductivity values, the conventional bulk Si and Ge (130−150 W/(m • K) [5] and 50−60 W/(m • K) [6], respectively, at 300 K) have a low thermoelectric efficiency. Special attention at present is paid to low-dimensional semiconductor materials, such as semiconductor nanowires (1D), thanks to their low thermal conductivity [7][8][9] and rather a high mobility of charge carriers [10,11]. The Si nanowires feature considerable anisotropy of thermal conductivity depending on structure growth direction, while the lowest thermal conductivity is typical of the 111 orientation as compared to the 100 and 110 orientations [12].…”

Effect of morphology on the phonon thermal conductivity in Si/Ge superlattice nanowires

“…9) Among such low-dimensional materials, nanowires (NWs) also attract attention, and in particular, Si and Ge NWs, which are characterized by sufficiently high mobility of charge carriers. 10,11) Si and Ge NWs have been studied extensively, [10][11][12][13][14][15][16][17][18][19][20][21] since Si and Ge are common materials in microelectronics. Numerical estimates of κ L of homogeneous Si and Ge nanowires are in the range of ∼10−25 W/(m•K) and ∼6−14 W/(m•K), respectively, 15,16,19) and these values are an order of magnitude less than the one for Si and Ge bulks (∼140 W/(m•K) 22) and ∼55 W/(m•K), 23) respectively).…”

“…25,26) However, these defects led not only to a decrease in the total thermal conductivity but also to a decrease in the power factor (S 2 σ) and ZT values. In addition, results of theoretical 13,19,20) and experimental [24][25][26] studies show that κ L of NWs increases with diameter pointing out variation in the diameter of a NW to be a feasible way to change κ L . It is also theoretically demonstrated that the interface can provide an additional reduction in κ L for Si/Ge superlattice NWs (with alternating Si and Ge segments along the NW axis) compared with homogeneous NWs 13,15,16,19,21) mainly considering 〈100〉-oriented NWs with square or round cross-sections.…”