Influence of polytypism on thermal properties of silicon carbide

Electron-Phonon Interaction Contribution to the Total Energy of Group IV Semiconductor Polymorphs: Evaluation and Implications

In density functional theory (DFT)-based total energy studies, the van der Waals (vdW) and zero-point vibrational energy (ZPVE) correction terms are included to obtain energy differences between polymorphs. We propose and compute a new correction term to the total energy, due to electron-phonon interactions (EPI). We rely on Allen's general formalism, which goes beyond the quasi-harmonic approximation (QHA), to include the free energy contributions due to quasiparticle interactions. We show that, for semiconductors and insulators, the EPI contributions to the free energies of electrons and phonons are the corresponding zero-point energy contributions. Using an approximate version of Allen's formalism in combination with the Allen-Heine theory for EPI corrections, we calculate the zero-point EPI corrections to the total energy for cubic and hexagonal polytypes of carbon, silicon and silicon carbide. The EPI corrections alter the energy differences between polytypes. In SiC polytypes, the EPI correction term is more sensitive to crystal structure than the vdW and ZPVE terms and is thus essential in determining their energy differences. It clearly establishes that the cubic SiC-3C is metastable and hexagonal SiC-4H is the stable polytype. Our results are consistent with the experimental results of Kleykamp. Our study enables the inclusion of EPI corrections as a separate term in the free energy expression. This opens the way to go beyond the QHA by including the contribution of EPI on all thermodynamic properties.

Bright Silicon Carbide Single-Photon Emitting Diodes at Low Temperatures: Toward Quantum Photonics Applications

Color centers in silicon carbide have recently emerged as one of the most promising emitters for bright single-photon emitting diodes (SPEDs). It has been shown that, at room temperature, they can emit more than 10⁹ photons per second under electrical excitation. However, the spectral emission properties of color centers in SiC at room temperature are far from ideal. The spectral properties could be significantly improved by decreasing the operating temperature. However, the densities of free charge carriers in SiC rapidly decrease as temperature decreases, which reduces the efficiency of electrical excitation of color centers by many orders of magnitude. Here, we study for the first time the temperature characteristics of SPEDs based on color centers in 4H-SiC. Using a rigorous numerical approach, we demonstrate that although the single-photon electroluminescence rate does rapidly decrease as temperature decreases, it is possible to increase the SPED brightness to 10⁷ photons/s at 100 K using the recently predicted effect of hole superinjection in homojunction p-i-n diodes. This gives the possibility to achieve high brightness and good spectral properties at the same time, which paves the way toward novel quantum photonics applications of electrically driven color centers in silicon carbide.

Effects of Thermal Conductive Materials on the Freeze-Thaw Resistance of Concrete

To solve the problem of black ice, many studies are being carried out. The key in recent days is enhancing the thermal conductivity of concrete. In this study, to improve the thermal conductivity, silicon carbide was used to substitute 50% and 100% of the fine aggregate. In addition, steel fiber is not only for enhancing the mechanical properties but could enhance thermal conductive material. Hence, the arched-type steel fiber was used up to a 1% volume fraction in this study. Furthermore, graphite was used for 5% of the volume fraction for enhancing the thermal conductivity. However, thermal damage would occur due to the difference in thermal conductivity between materials. Therefore, the thermal durability must be verified first. The target application of the concrete in this study was its use as road paving material. To evaluate the thermal durability, freeze–thaw and rapid cyclic thermal attacks were performed. The thermal conductivity of the specimens was increased with the increase in thermal conductive materials. Graphite has already been reported to have a negative effect on mechanical properties, and the results showed that this was the case. However, the steel fiber compensated for the negative effect of graphite, and the silicon carbide provided a filler effect. Graphite also had a negative effect on the freeze–thaw and rapid cyclic thermal attack, but the steel fiber compensated for the reduction in thermal durability. The silicon carbide also helped to improve the thermal durability in the same way as steel fiber. Comprehensively, the steel fiber enhanced all of the properties of the tests. Using 100% silicon carbide was considered the acceptable range, but 50% of silicon carbide was the best. Graphite decreased all the properties except for the thermal conductivity. Therefore, the content of graphite or using other conductive materials used should be carefully considered in further studies.

Electronic, Mechanical and Elastic Anisotropy Properties of X-Diamondyne (X = Si, Ge)

The three-dimensional (3D) diamond-like semiconductor materials Si-diamondyne and Ge-diamondyne (also called SiC4 and GeC4) are studied utilizing density functional theory in this work, where the structural, elastic, electronic and mechanical anisotropy properties along with the minimum thermal conductivity are considered. SiC4 and GeC4 are semiconductor materials with direct band gaps and wide band gaps of 5.02 and 5.60 eV, respectively. The Debye temperatures of diamondyne, Si- and Ge-diamondyne are 422, 385 and 242 K, respectively, utilizing the empirical formula of the elastic modulus. Among these, Si-diamondyne has the largest mechanical anisotropy in the shear modulus and Young's modulus, and Diamond has the smallest mechanical anisotropy in the Young's modulus and shear modulus. The mechanical anisotropy in the Young's modulus and shear modulus of Si-diamondyne is more than three times that of diamond as determined by the characterization of the ratio of the maximum value to the minimum value. The minimum thermal conductivity values of Si- and Ge-diamondyne are 0.727 and 0.524 W cm-1 K-1, respectively, and thus, Si- and Ge-diamondyne may be used in the thermoelectric industry.

A comparative study of the mechanical and thermal properties of defective ZrC, TiC and SiC

ZrC and TiC have been proposed to be alternatives to SiC as fuel-cladding and structural materials in nuclear reactors due to their strong radiation tolerance and high thermal conductivity at high temperatures. To unravel how the presence of defects affects the thermo-physical properties under irradiation, first-principles calculations based on density function theory were carried out to investigate the mechanical and thermal properties of defective ZrC, TiC and SiC. As compared with the defective SiC, the ZrC and TiC always exhibit larger bulk modulus, smaller changes in the Young’s and shear moduli, as well as better ductility. The total thermal conductivity of ZrC and TiC are much larger than that of SiC, implying that under radiation environment the ZrC and TiC will exhibit superior heat conduction ability than the SiC. One disadvantage for ZrC and TiC is that their Debye temperatures are generally lower than that of SiC. These results suggest that further improving the Debye temperature of ZrC and TiC will be more beneficial for their applications as fuel-cladding and structural materials in nuclear reactors.

The mechanical and thermodynamic properties of β-Si1−xC

We report the enthalpy (a), free energy (b), entropy (c) and heat capacity (d) of β-Si_1–xC.

Band gap modulation of Si-C binary core/shell nanowires by composition and ratio

Core/shell nanowires (CSNWs) composed of Si, C, and SiC are promising systems for optoelectronic devices. Through computational investigations, we find that the band gaps (Eg) of these nanowires can be controlled not only by changing their composition, but also by adjusting the core/shell thickness ratio. For Si/SiC or SiC/C CSNWs with a fixed total number of layers, the dependence of Eg on the core/shell thickness ratio shows a bowing effect. Eg can be tuned from a few eV all the way to zero. These investigations provide direction for designing optoelectronic devices based on Earth-abundant elements.