Thermal conductivity of Si/SiGe superlattice films

Abstract

We have evaluated the thermal conductivity of Si/SiGe superlattice films by theoretical analysis and experiment. In experiments, the ultrahigh vacuum chemical vapor deposition is employed to form the Si/ Si 0.71 Ge 0.29 and Si/ Si 0.8 Ge 0.2 superlattice films. The cross-plane thermal conductivities of these superlattice films are measured based on the 3 method. In the theoretical analysis, the phonon transport in Si/ Si 1−x Ge x superlattice film is explored by solving the phonon Boltzmann transport equation. The dependence of the thermal conductivity of the Si/ Si 1−x Ge x superlattice films on the superlattice period, the ratio of layer thicknesses, and the interface roughness is of interest. The calculations show that when the layer thickness is on the order of one percentage of the mean free path or even thinner, the phonons encounter few intrinsic scatterings and consequently concentrate in the directions having high transmissivities. Nonlinear temperature distributions are observed near the interfaces, arising from the size confinement effect and resulting in a slight increase in the film thermal resistances. The interface resistance due to the interface scattering/ roughness, which is nearly independent of the film thickness, nonetheless dominates the effective thermal conductivity, especially when the superlattice period is small. Finally the experimental measurements agree with the theoretical predictions if the specular fraction associated with the interface is properly taken.

Micron-scale ballistic thermal conduction and suppressed thermal conductivity in heterogeneously interfaced nanowires

“…Controlled experiments on Si nanowires indeed display the expected diffusive transport behavior, confirming the validity of our experiments (see the Appendix). The results of Si-Ge core-shell nanowires not only largely deviate from the indirect estimates that l < 100 nm for SiGe alloys [14,[28][29][30][31], but they are also much longer than l ∼ 1 μm proposed for the best thermal conductors like diamond and graphene [1]. The finding also highlights the impor- tance of our direct, rigorous, thermal transport measurements on l.…”

Tailoring thermal conductivity by engineering compositional gradients in Si1−x Ge x superlattices

“…The conductivity values measured at 300 K for the three samples considered in this work are shown in Fig. 3(b), accompanied by a comparison with other undoped SLs [9][10][11][12][13][14]. This figure illustrates the dependence of the thermal conductivity on the SL period.…”

“…2015, 8(9): 2833-2841 increases this value to 0.95 [7]. Additionally, the incorporation of a high interface density in epitaxial heterostructures has been recognized as an efficient approach to enhance boundary phonon scattering at low frequencies with a corresponding decrease in thermal conductivity [8][9][10][11][12][13][14][15][16][17]. SiGe planar superlattices (SLs) have thus emerged as potential candidates for both TE cooling of microelectronic devices and medium-temperature TE power generation [18,19].…”

The thermoelectric properties of Ge/SiGe modulation doped superlattices

“…7, see Table I for comparisons of literature values). Whilst there have been a significant number of publications investigating the thermal conductivity of Si/Ge and Si/SiGe superlattices, [18][19][20][21] there are far fewer publications investigating doped superlattices with sufficient results to quote ZT, power factors, or the output from a cooler. [22][23][24] To date these reported results for Si/Ge and Si/SiGe superlattices are only for n-type material (Ref.…”