Exceptionally Strong Phonon Scattering by B Substitution in Cubic SiC

Abstract

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...

Combined treatment of phonon scattering by electrons and point defects explains the thermal conductivity reduction in highly-doped Si

“…It is important to note how the present case is very different from our previous work on SiC. 34,35 In the present work, we observe a significant contribution of both EPS and PDPS to κ of a highly-doped system; whereas in our previous work we observed that PDPS was sufficient on its own to correctly predict the κ of B-doped cubic SiC owing to the resonant phonon scattering that boron causes. 34,35 The resonant scattering was at least one to two orders of magnitude higher than that caused by other defects and resulted in a drastic reduction (approximately two orders of magnitude at room temperature) in the thermal conductivity even at relatively mod- est defect concentrations (≈ 10 20 cm −3 ).…”

“…Neither EPS nor PDPS can be captured by conventional parameterized models. Together with our previous works on doping diamond, SiC, GaN and GaAs, 34,[36][37][38] we show that, at temperatures above 300 K, PDPS is the most dominant scattering mechanism in highly B-and P-doped Si and cannot be neglected.…”

Analytical Models of Phonon–Point-Defect Scattering

“…The scattering due to local, static strain (η) depends on the anharmonicity of the distorted bonds, as captured in the Grüneisen parameter (γ). Notably, both the ∆K and ∆R effects are captured in the changes to the DFT-calculated interatomic force constants, which change locally in response to both structural relaxation and an altered chemical environment 13 . Additionally, the strain scattering strength is scaled by the coefficient Q, which approximates the number of distorted nearest neighbor bonds around a point defect.…”

“…These simulations have shown very good quantitative agreement with experiments for a range of materials and have provided useful insights regarding the mechanisms of phonon-defect scattering 10,11 . However, multiple calculations are required to compute defect structures, evaluate scattering strengths, and solve the Boltzmann transport equation for the thermal conductivity [12][13][14][15] . Often, these techniques are too expensive and system dependent for routine modelling used to determine the dominant scattering mechanisms in a system 16 .…”

Mie scattering of phonons by point defects in IV-VI semiconductors PbTe and GeTe

“…Since the wavelengths of thermal phonons are usually many times of the lattice spacing, the scattering of thermal phonons by a point defect is often considered as Rayleigh scattering and exhibits very weak scattering strength for low frequency phonons.The Rayleigh scattering is well captured by models suggested by Klemens [14] and Tamura [15], which assume that the change in mass and interatomic force constants (IFCs) by a point defect is limited to a single lattice site. The recently developed ab initio Green's function approach [16,17] also shows the Rayleigh scattering behavior for strongly bonded materials, such as diamond and BAs [18][19][20][21][22][23][24].In this report, we show that the point defects in IV-VI semiconductors (PbTe and GeTe), unlike the previously studied strongly bonded materials, cause Mie scattering of thermal phonons. The IV-VI semiconductors are widely used for the applications of thermoelectrics and phase change materials where the thermal conductivity is a key parameter for the device figure-of-merit.…”