Using a first principles theoretical approach, we show that vacancies give anomalously strong suppression of the lattice thermal conductivity, κ, of cubic Boron arsenide (BAs), which has recently been predicted to have an exceptionally high κ. This effect is tied to the unusually large phonon lifetimes in BAs and results in a stronger reduction in the BAs κ than occurs in diamond. The large bond distortions around vacancies cannot be accurately captured using standard perturbative methods and are instead treated here using an ab initio Green function approach. As and B vacancies are found to have similar effects on κ. In contrast, we show that commonly used mass disorder models for vacancies fail for large mass ratio compounds such as BAs, incorrectly predicting much stronger (weaker) phonon scattering when the vacancy is on the heavy (light) atom site. The quantitative treatment given here contributes to fundamental understanding of the effect of point defects on thermal transport in solids and provides guidance to synthesis efforts to grow high quality BAs.

Silicon carbide (SiC) is a wide band gap semiconductor with a variety of industrial applications. Among its many useful properties is its high thermal conductivity, which makes it advantageous for thermal management applications. In this paper we present ab initio calculations of the in-plane and cross-plane thermal conductivities, κ in and κ out , of three common hexagonal polytypes of SiC: 2H, 4H and 6H. The phonon Boltzmann transport equation is solved iteratively using as input interatomic force constants determined from density functional theory. Both κ in and κ out decrease with increasing n in nH SiC because of additional low-lying optic phonon branches. These optic branches are characterized by low phonon group velocities, and they increase the phase space for phonon-phonon scattering of acoustic modes. Also, for all n, κ in is found to be larger than κ out in the temperature range considered. At electron concentrations present in experimental samples, scattering of phonons by electrons is shown to be negligible except well below room temperature where it can lead to a significant reduction of the lattice thermal conductivity. This work highlights the power of ab initio approaches in giving quantitative, predictive descriptions of thermal transport in materials. It helps explain the qualitative disagreement that exists among different sets of measured thermal conductivity data and provides information of the relative quality of samples from which measured data was obtained.

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The promise enabled by boron arsenide’s (BAs) high thermal conductivity (κ) in power electronics cannot be assessed without taking into account the reduction incurred when doping the material. Using first principles calculations, we determine the κ reduction induced by different group IV impurities in BAs as a function of concentration and charge state. We unveil a general trend, where neutral impurities scatter phonons more strongly than the charged ones. CB and GeAs impurities show by far the weakest phonon scattering and retain BAs κ values of over ~1000 W⋅K−1⋅m−1 even at high densities. Both Si and Ge achieve large hole concentrations while maintaining high κ. Furthermore, going beyond the doping compensation threshold associated to Fermi level pinning triggers observable changes in the thermal conductivity. This informs design considerations on the doping of BAs, and it also suggests a direct way to determine the onset of compensation doping in experimental samples.

We carry out novel ab-initio calculations of fully coupled electron and phonon transport and show that mutual drag causes the thermopower to be dominated by transport of phonons, rather than electrons, at room temperature in the case of n-doped 3C-SiC. The thermopower is insensitive to impurity scattering. Phonon drag also strongly boosts the intrinsic electron mobility, thermal conductivity and the Lorenz number. This work establishes the roles of microscopic scattering mechanisms in the emergence of strong drag effects in transport of the interacting electron-phonon gas.