Intrinsic nanotwin effect on thermal boundary conductance in bulk and single-nanowire twinning superlattices

Abstract

Coherent twin boundaries form periodic lamellar twinning in a wide variety of semiconductor nanowires, and are often viewed as near-perfect interfaces with reduced phonon and electron scattering behaviors. Such unique characteristics are of practical interest for high-performance thermoelectrics and optoelectronics; however, insufficient understanding of twin-size effects on thermal boundary resistance poses significant limitations for potential applications. Here, using atomistic simulations and ab-initio calculations, we report direct computational observations showing a crossover from diffuse interface scattering to superlattice-like behavior for thermal transport across nanoscale twin boundaries present in prototypical bulk and nanowire Si examples.Intrinsic interface scattering is identified for twin periods ≥ 22.6 nm, but also vanishes below this size to be replaced by ultrahigh Kapitza thermal conductances. Detailed analysis of vibrational modes shows that modeling twin boundaries as atomically-thin 6H-Si layers, rather than phonon scattering interfaces, provides an accurate description of effective cross-plane and in-plane thermal conductivities in twinning superlattices, as a function of the twin period thickness.3

Low thermal conductivity in Si/Ge hetero-twinned superlattices

“…The previous simulated and experimental results indicated that the thermal transport properties in SLs can be adjusted by regulating the lattice period 6,7 and the structures of interfaces. 8,9 According to the structures of the interfaces, SLs can be divided into two kinds: (1) heterostructure SLs (SLs consist of two components 10 ), and (2) homo-coherent twinned SLs (SLs consist of nanotwinned layers from one component 9 ). It has been demonstrated that the cross-plane thermal conductivity of crystalline heterostructure SLs is much lower than that of bulk materials with a single component, and, sometimes even lower than those of their corresponding alloys.…”

“…It can be seen that compared with single crystal Si, Grüneisen parameters of Si/Ge hetero-twinned SLs, the conventional Si/Ge SLs and Si/Ge alloy are increased, while this parameter for the twinned Si is only slightly reduced. Aer calculation of phase velocity and Grüneisen parameters, according to eqn (9), A is the only parameter needed to be determined. To this end, the average values of group velocity, phase velocity and Grüneisen parameters of single crystal Si in Table 1 are used, and the value of A is determined as 9.1 Â 10 À5 .…”

Phonon coherence and its effect on thermal conductivity of nanostructures

“…The impact of twinning on heat conduction in NWs has attracted a plenty of research interest [156][157][158][159]. Using atomistic simulations, Porter et al reported a crossover from diffusive interface scattering to SL-liked behavior for phonon transport across twin boundaries in SiNW [156]. By nonequilibrium molecular dynamics simulations, Xiong et al found that the thermal conductivity of the Si metamaterial NWs decreases firstly with twinning SL period increases, reaching a minimum value and then progressively increases [157].…”

Phonon scattering by nanoscale twin boundaries

“…The Fourier transform taken across the boundary between two domains are shown in the inset of Fig. 2e, which can be indexed as [1 10] according to the 43 F m cubic structure of InSb. It is the superposition of two sets of reflection spots from neighboring domains (marked separately by white and yellow circles).…”

“…Theoretical investigations about phonon scattering by twins were reported recently, however the results were controversial [39][40][41][42][43][44]. Several reports predicted noticeable reduction of thermal conductivity (κ) due to the presence of twins [39,[41][42][43]. Relationship between κ and the spacing of twins, angle between the neighboring twins, and number of twins perpendicular to the direction of heat flow were reported [39,41].…”