We use ab-initio calculations to predict the thermal conductivity of cubic SiC with different types of defects. An excellent quantitative agreement with previous experimental measurements is found. The results unveil that BC substitution has a much stronger effect than any of the other defect types in 3C-SiC, including vacancies. This finding contradicts the prediction of the classical massdifference model of impurity scattering, according to which the effects of BC and NC would be similar and much smaller than that of the C vacancy. The strikingly different behavior of the BC defect arises from a unique pattern of resonant phonon scattering caused by the broken structural symmetry around the B impurity.Silicon carbide (SiC) plays a fundamental role in many emerging technologies, ranging from biomedical sensors to optoelectronics, power electronics and photovoltaics [1][2][3][4][5][6][7][8][9]. Most notably, this material has been termed the "linchpin to green energy" that may replace Si-based technology in power electronics [1], owing partly to its large lattice thermal conductivity (κ). From the many stable polytypes of SiC [10], two of the hexagonal ones, 6H-SiC and 4H-SiC, have been extensively studied and widely used [10][11][12]. In contrast, the structurally less complex cubic polytype of SiC with zinc-blende structure (3C-SiC) is much less well understood, despite presumably having the best electronic properties [13], and, as we will see, possibly a higher κ than the other polytypes. This is partly due to the difficulty in synthesizing high quality crystals, although recent improvements in 3C-SiC growth techniques have prompted a renewed interest in it [13].Surprisingly, the reference measurements of κ on pure undoped 3C-SiC are over 20 years old and little detail is known about the quality of the samples [10,14]. The reference value of κ for 3C phase is perplexingly lower than that for the structurally more complex 6H phase, raising doubts about whether this is truly an intrinsic property or just a consequence of the defective, polycrystalline quality of the 3C-SiC samples. It is then clear that to understand the conduction properties of 3C-SiC, and to harness its full potential, one must first comprehend the way defects affect it. As we show here, by comparing predictive ab-initio calculations with experiments on defective samples, a richer physical picture emerges, unveiling the striking differences in the way different dopants affect κ. This also indirectly suggests that the intrinsic κ of defect-free 3C-SiC should be much higher than previously reported and surpass that of the 6H phase.In this paper, we compare our results to the κ(T ) curves for doped samples of 3C-SiC [15]. We use an abinitio approach to quantify the phonon scattering rates of N C substitutional defects. The predicted κ is in excellent agreement with the experimental results. This then allows us to explain the effect of codoping with N and B, and shows that B impurities scatter phonons two orders of magnitude more strongly overall...
Several ternary “Janus” metal dichalcogenides such as {Mo,Zr,Pt}-SSe have emerged as candidates with significant potential for optoelectronic, piezoelectric, and thermoelectric applications. SnSSe, a natural option to explore as a thermoelectric given that its “parent” structures are SnS 2 and SnSe 2 has, however, only recently been shown to be mechanically stable. Here, we calculate the lattice thermal conductivities of the Janus SnSSe monolayer along with those of its parent dicalchogenides. The phonon frequencies of SnSSe are intermediate between those of SnSe 2 and SnS 2 ; however, its thermal conductivity is the lowest of the three and even lower than that of a random Sn[S 0.5 Se 0.5 ] 2 alloy. This can be attributed to the breakdown of inversion symmetry and manifests as a subtle effect beyond the reach of the relaxation-time approximation. Together with its low favorable power factor, its thermal conductivity confirms SnSSe as a good candidate for thermoelectric applications.
An energetic and dynamical stability analysis of five candidate structures—hexagonal, buckled hexagonal, litharge, inverted litharge, and distorted-NaCl—of the SnS monolayer is performed using density functional theory. The most stable is found to be a highly distorted-NaCl-type structure. The thermoelectric properties of this monolayer are then calculated using the density functional theory and the Boltzmann transport equation. In terms of phonon scattering, there is a sharp contrast between this monolayer and bulk materials, where normal processes are more important. The calculations reveal that the SnS monolayer has enhanced electrical performance as compared to the bulk phase. As a consequence, high figures of merit ZT∼5 and ZT∼1.36 are predicted at 600 and 300 K, respectively, for the monolayer, ∼33 times higher than the ZT of its bulk analog. Therefore, this structure is an interesting candidate for room-temperature thermoelectric applications. A comparison between the fully ab initio results and simpler models based on relaxation times for electrons and phonons highlights the efficiency of computationally inexpensive models. However, ab initio calculations are found to be very important for the prediction of thermal transport properties.
The origins of the dramatic effect of some dopants on the thermal conductivity of semiconductors are studied. By analyzing the cases of B-doped 3C-SiC, B-doped diamond, N-doped diamond and a simple linear chain model, small symmetry breaking structural distortions and a high density of states are identified as the key ingredients in resonant phonon scattering.
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