Structural, elastic and electronic properties of 2H- and 4H-SiC

Physical review. B, Condensed matter, 1994

A systematic study of ground-state properties of cubic and hexagonal silicon carbide polytypes (3C-, 6H , 4H-, and-2H-SiC) is reported using well converged density-functional calculations within the local-density approximation and norm-conserving, fully separable, soft, ab initio pseudopotentials. Equilibrium results are obtained for the lattice parameters, atomic positions, bond lengths and angles, cohesive energies, and the bulk modulus. The internal degrees of freedom, i.e. , atomic relaxations, are fully taken into account. The results are discussed in comparison with experimental data. We derive trends with the hexagonality for the molecule volume and the energetic ordering of the polytypes. Driving forces for the polytypism and the atomic relaxations are discussed. I. INTRODUCTION Silicon carbide (SiC) is an interesting semiconductor for various electronic, optoelectronic, optical, thermal, and mechanical applications in high-power and hightemperature devices. In addition, SiC is one of the few compounds which form many stable and long-range ordered modifications, the so-called polytypes. Currently, considerable effort is being made to prepare bulk SiC crystals as well as thin layers of different polytypes of good quality. On the other hand, many researchers have refocused their attention to the electrical, optical, elastic, and thermal properties of the material. All these properties depend directly or indirectly via the electronic band structure or the lattice vibrations on the atomic structure of the polytype. There also seems to be an internal relationship between the stability of SiC polytypes and the exact atomic positions, which should be displaced with respect to that of the ideal tetrahedral structure, which is realized, e. g., in the cubic zinc-blende 3t-SiC. These atomic relaxations differ for the various polytypes. They can therefore tell us about the driving forces of the polytypism. More than a hundred different SiC polytypes are known. z However, only for the wurtzite 2H-SiC (Refs. 3 and 4) and another hexagonal modification, 6H-SiC, have careful x-ray measurements of the nonideal bond lengths and angles been done. Si and C NMR studies reported the occurrence of different kinds of sites in common polytypes, e. g., 3C, 4H, 6H, and 15R. Ab initio pseudopotential calculations are mainly directed to the ground-state properties of the simplest polytypes, the zinc-blende and the wurtzite structure. Only in a few cases have polytypes with larger unit cells as 4H, 6H, and 15R been attacked.

2022

SiC polytypes have been studied for decades, both experimentally and with atomistic simulations, yet no consensus has been reached on the factors that determine their stability and growth. Proposed governing factors are temperature-dependent differences in the bulk energy, biaxial strain induced through point defects, and surface properties. In this work, we investigate the thermodynamic stability of the 3C, 2H, 4H, and 6H polytypes with density functional theory (DFT) calculations. The small differences of the bulk energies between the polytypes can lead to intricate changes in their energetic ordering depending on the computational method. Therefore, we employ and compare various DFT-codes: VASP, CP2K, and FHI-aims; exchange-correlation functionals: LDA, PBE, PBEsol, PW91, HSE06, SCAN, and RTPSS; and ten different van der Waals (vdW) corrections. At T=0 K, 4H-SiC is marginally more stable than 3C-SiC, and the stability further increases with temperature by including entropic effec...

Physica B: Condensed Matter, 1993

To study the structural and electronic properties of Sic, we have performed self-consistent pseudopotential calculations for three SIC structures, namely 6H, 8H and a supercell containing thick slabs of the wurtzite and cubic forms. The supercell calculations were performed with and without including the structural relaxation of the wurtzite structure. Our main findings are: (i) the dipole set up at the stacking boundary is not centred on the central bond but on the adjacent C atom, and the disturbance to the charge density ranges about 3.5 8, on either side; (ii) the spontaneous polarization in Sic polytypes can be interpreted as a superposition of localized dipoles due to each of the stacking boundaries, and it is mainly due to the charge density redistribution; (iii) the valence-band offset between the cubic and wurtzite forms, determined using the same supercell, is 0.13 eV, with the valence band edge of the wurtzite structure being higher in energy; (iv) from (i), one can easily explain the MAS-NMR data concerning the inequivalent Si and C sites and the structural relaxation of Sic polytypes; (v) the effect of the macroscopic electric fields on the relative stability of Sic polytypes is discussed and found to be negligible.

Physical Review B, 1998

The total energy differences between various SiC polytypes (3C, 6H, 4H, 2H, 15R, and 9R) were calculated using the full-potential linear muffin-tin orbital method using the Perdew-Wang generalized gradient approximation ͓I. P. Perdew, in Electronic Structure of Solids '91, edited by P. Ziesche and H. Eschrich ͑Akademie-Verlag, Berlin, 1991͒, p. 11͔ to the exchange-correlation functional in the density-functional method. Numerical convergence versus k-point sampling and basis-set completeness are demonstrated to be better than 0.5 meV/atom. The parameters of several generalized anisotropic next-nearest-neighbor Ising models are extracted and their significance and consequences for epitaxial growth are discussed. ͓S0163-1829͑98͒01019-4͔

Physical Review B, 2005

We present electronic structure and total energy calculations for SiC in a variety of polytype structures using the NRL nonorthogonal tight-binding method. We develop one set of parameters optimized for a combination of electronic and energetic properties using a sp basis, and one optimized for electronic properties using a spd basis. We compute the energies of polytypes with up to 62 atoms per unit cell, and find that the hexagonal wurtzite structure is highest in energy, the 4H structure is lowest in energy, and the cubic zinc-blende structure is in between, in agreement with our linear augmented plane-wave and other calculations. For the sp model we find that the electronic structure of the cubic and hexagonal structures are in good agreement with densityfunctional theory calculations only for the occupied bands. The spd parametrization optimized for the electronic structure of the zinc-blende and wurtzite structures at the equilibrium volume reproduces nearly perfectly both the valence and conduction bands. The sp tight-binding model also yields elastic constants, phonon frequencies, stacking fault energies, and vacancy formation energies for the cubic structure in good agreement with available experimental and theoretical calculations. Using molecular dynamics simulations we compute the finite-temperature thermal expansion coefficient and atomic mean-square displacements in good agreement with available first-principles calculations.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2006

The electronic structure, bonding, and optical properties of six polymorphs of SiC: 3C, 2H, 4H, 6H, 15R, and 21R were studied by the density functional-based first-principles OLCAO method. The results were compared with other existing calculations as well as experimental data. It is shown that the different stacking sequences of the Si-C bi-layers in these polymorphs result in minute but recognizable differences in the partial density of states, Mulliken effective charges, and bond order values, indicating the importance of the intermediate range ordering in these crystals. The optical properties calculation for these polymorphs also shows some marked differences among them and can be explained, at least partially, by the LDA-based electronic band structures. Also presented is the calculated X-ray absorption near edge spectroscopy (XANES) of the Si-K, Si-L 3 and N-K edges in 3C-SiC.

Physical review. B, Condensed matter, 1994

Ab initio total-energy studies are used to determine the lattice constants and the atomic positions within the unit cells for 3C-, 6H-, 4H-, and 2H-SiC. The electronic structures are calculated for the atomic geometries obtained theoretically within the density-functional theory (DFT) and the localdensity approximation (LDA). We state more precisely the ordering of the conduction-band minima and derive effective masses. By adding quasiparticle corrections to the DFT-I DA band structures we 6nd indirect fundamental energy gaps in agreement with the experiment. A physical explanation of the empirical Choyke-Hamilton-Patrick relation is given. Band discontinuities, bandwidths, crystalfield splittings, and ionic gaps are discussed versus hexagonality.

Physical Review B, 1994

The plane-wave pseudopotential approach to density-functional theory (DFT) in the local-density approximation has been applied to investigate a variety of ground-state properties of the 3C, 2H, and 4H polytypes of silicon carbide. The linear-response theory within DFT has been used to obtain lattice-dynamical properties of cubic Sic such as the phonon-dispersion curves, phonon eigenvectors, elastic and Griineisen constants, as well as the thermal expansion coefFicient and specific heat within the quasiharmonic approximation. Finally, we present some results for phonon-dispersion curves in the hexagonal 2K (wurtzite) and 4K structure. These results are analyzed and discussed in view of further applications to temperature-dependent properties.

Journal of Nano- and Electronic Physics

Molecular dynamics simulations using the Tersoff bond-order potential are employed to study the effects of temperature and grain size on mechanical properties of nanocrystalline silicon carbide. In this work, the simulated nanocrystalline SiC samples have a mean grain size varying from 2.5 to 5 nm and contain about 10 5 atoms in the model system. Tension tests with periodic boundary conditions and engineering strain rate of 10 -4 ps -1 are simulated, which result in the stress-strain curves of the single-and nanocrystalline SiC in terms of the average virial stress and true strain. The elastic moduli of the single-and nanocrystalline silicon carbide are determined from fitting the stress-strain curves. In this work, the Young's modulus of nanocrystalline SiC is compared with those of the monocrystalline SiC for different temperatures in the range from 300 K to 3000 K. The numerical results show that the temperature has an obvious effect on Young's modulus, which is attributed to the large volume fraction of grain boundaries in nanocrystalline samples. With increasing temperature, the nanocrystalline SiC shows a brittle-to-ductile transition at temperatures above 600 K. In addition, the reduction in Young's modulus of the nanocrystalline SiC with increasing temperature exhibits a nonlinear trend. It is found that the plasticity of the nanocrystalline SiC samples sharply increases at temperatures above 2000 K. This effect was explained by a decrease in the melting point of the nanocrystalline materials in comparison to monocrystalline solids. The grain size dependence of elastic modulus of nanocrystalline SiC only becomes distinct at high temperatures and at a grain size greater than about 3 nm, while at room temperature elastic properties are almost invariant with the change of grain size. We expect that the quantifications of temperature and grain size dependence of mechanical properties will have implications in the development of nanocrystalline silicon carbide nanostructured materials for high performance structural applications.

2021