Large Thermoelectric Figure-of-Merits from SiGe Nanowires by Simultaneously Measuring Electrical and Thermal Transport Properties

Abstract

The strongly correlated thermoelectric properties have been a major hurdle for high-performance thermoelectric energy conversion. One possible approach to avoid such correlation is to suppress phonon transport by scattering at the surface of confined nanowire structures. However, phonon characteristic lengths are broad in crystalline solids, which makes nanowires insufficient to fully suppress heat transport. Here, we employed Si-Ge alloy as well as nanowire structures to maximize the depletion of heat-carrying phonons. This results in a thermal conductivity as low as ∼1.2 W/m-K at 450 K, showing a large thermoelectric figure-of-merit (ZT) of ∼0.46 compared with those of SiGe bulks and even ZT over 2 at 800 K theoretically. All thermoelectric properties were "simultaneously" measured from the same nanowires to facilitate accurate ZT measurements. The surface-boundary scattering is prominent when the nanowire diameter is over ∼100 nm, whereas alloying plays a more important role in suppressing phonon transport for smaller ones.

Phonon Scatterings in the Lattice Thermal Conductivity of Si_(1-x) Ge_x Alloy Nanowires: Theoretical Study

“…However, in bulk semiconductors for temperatures in the region of T=0.1θ K, the dominant phonon scattering mechanism is U-processes [33,53,54], but in this NW its influence becomes apparent above 260 K. These results suggest that alloy scattering plays a key role and is important in depressing and lowering the LTC with the coexistence of an anharmonicity contribution at intermediate and high temperatures, which is in good agreement with the experimental explanations at [21][22][23]. To justifying the approach mentioned above, LTC was recalculated for an Si 0.14 Ge 0.86 alloy NW of diameter 161 nm [22], and compared with Si 0.91 Ge 0.09 alloy NW of diameter 160 nm [21]; the result is shown in figure 4A. For an Si 0.14 Ge 0.86 NW of diameter 161 nm, l is equal to 6.2 micrometers [22], and it was found that C L = 50 nm, θ = 178 K, P = 0.4 and γ = 0.77 for a best fit with the experimental data, and the decrease in C L is due to high Ge concentration ( 86% ) [22] which relates to the phononfrequency dependence, as mentioned before.…”

“…None of the NW samples treated in this work are intentionally doped, they are relatively defect free and pure [21][22][23], so the electronic contribution is neglected and only the lattice contribution is taking into account. Steigmeier and Abeles' model [27] of the lattice thermal conductivity (LTC) of bulk alloys, based on the Callaway's model [28], was used in this work.…”

“…Therefore, the effects of boundary scattering and phonon confinement should be considered together when calculating LTC [51]. In alloy NWs, thermal conductivity is more sensitive to surface effect than size effect [21,22]. However, it is reported that the LTC of Si 1-x Ge x alloy NWs of small diameter is proportional to nanowire diameter, whereas this dependence becomes less as the diameter increases [34].…”

“…To justifying the approach mentioned above, LTC was recalculated for an Si 0.14 Ge 0.86 alloy NW of diameter 161 nm [22], and compared with Si 0.91 Ge 0.09 alloy NW of diameter 160 nm [21]; the result is shown in figure 4A. For an Si 0.14 Ge 0.86 NW of diameter 161 nm, l is equal to 6.2 micrometers [22], and it was found that C L = 50 nm, θ = 178 K, P = 0.4 and γ = 0.77 for a best fit with the experimental data, and the decrease in C L is due to high Ge concentration ( 86% ) [22] which relates to the phononfrequency dependence, as mentioned before. [21][22][23].…”

Interface bond relaxation on the thermal conductivity of Si/Ge core-shell nanowires

“…In comparison to the individual silicon nanowires, Si/Ge CSNWs have evident advantages because the covering layer offers the possibility of a partial and internal charge separation, an efficient passivation of the surface trap states and so on. 7 Up to date, significant progresses for Si/Ge CSNWs both experimentally [8][9][10][11][12][13][14] and theoretically [15][16][17][18][19][20][21][22][23][24][25][26][27][28] have demonstrated that the thermal transport properties of Si/Ge CSNWs can be modified at room temperature compared to that of the bare Si or Ge nanowires. Classically, the standard macroscopic approach describing heat transport in semiconductors is the well-known Fourier's law, i.e., J = −κ∇T, where J and ∇T are the heat flux density in the material and the temperature gradient, respectively.…”