Oleg Pankratov

Ab initio study of graphene on SiC.

Employing density-functional calculations we study single and double graphene layers on Si- and C-terminated 1x1-6H-SiC surfaces. We show that, in contrast with earlier assumptions, the first carbon layer is covalently bonded to the substrate and cannot be responsible for the graphene-type electronic spectrum observed experimentally. The characteristic spectrum of freestanding graphene appears with the second carbon layer, which exhibits a weak van der Waals bonding to the underlying structure. For Si-terminated substrate, the interface is metallic, whereas on C face it is semiconducting or semimetallic for single or double graphene coverage, respectively.

First-principles elastic constants and electronic structure of α-Pt2Si and PtSi

We have carried out a first-principles study of the elastic properties and electronic structure for two roomtemperature stable Pt silicide phases, tetragonal a-Pt2Si, and orthorhombic PtSi. We have calculated all of the equilibrium structural parameters for both phases: the a and c lattice constants for a-Pt2Si and the a, b, and c lattice constants and four internal structural parameters for PtSi. These results agree closely with experimental data. We have also calculated the zero-pressure elastic constants, confirming prior results for pure Pt and Si and predicting values for the six ~nine! independent, nonzero elastic constants of a-Pt2Si ~PtSi!. These calculations include a full treatment of all relevant internal displacements induced by the elastic strains, including an explicit determination of the dimensionless internal displacement parameters for the three strains in a-Pt2Si for which they are nonzero. We have analyzed the trends in the calculated elastic constants, both within each material as well as among the two silicides and the pure Pt and Si phases. The calculated electronic structure confirms that the two silicides are poor metals with a low density of states at the Fermi level, and consequently we expect that the Drude component of the optical absorption will be much smaller than in good metals such as pure Pt. This observation, combined with the topology found in the first-principles spin-orbit split band structure, suggests that it may be important to include the interband contribution to the optical absorption, even in the infrared region.

Ab initiostudy of the migration of intrinsic defects in3C−SiC

The diffusion of intrinsic defects in 3C-SiC is studied using an ab initio method based on density functional theory. The vacancies are shown to migrate on their own sublattice. The carbon split-interstitials and the two relevant silicon interstitials. namely the tetrahedrally carbon-coordinated interstitial and the (110)-oriented split interstitial, are found to be by far more mobile than the vacancies. The metastability of the silicon vacancy, which transforms into a vacancy-antisite complex in p-type and compensated material, kinetically suppresses its contribution to diffusion processes. The role of interstitials and vacancies in the self-diffusion is analyzed. Consequences for the dopant diffusion are qualitatively discussed. Our analysis emphasizes the relevance of mechanisms based on silicon and carbon interstitials.

Solubility of nitrogen and phosphorus in 4H-SiC: A theoretical study

The n-type dopants phosphorus and nitrogen, and their complexes with intrinsic point defects are investigated in 4H-SiC by first-principles theory. The solubility and electrical activation of the dopants in thermodynamic equilibrium are calculated. For nitrogen, a saturation of the electrical activation above a certain critical concentration is found that is driven by a preferential incorporation of nitrogen into electrically passive nitrogen-vacancy complexes. This explains the observations of recent experiments. An almost complete phosphorus activation is found up to the solubility limit. We suggest that the low phosphorus doping achieved by sublimation growth is related to the growth kinetics.

Structure and vibrational spectra of carbon clusters in SiC

high-frequency LVM’s up to 250meV. The isotope shifts resulting from a 13 C enrichment are analyzed. In the light of these results, the photoluminescence centers DII and P U are discussed. The dicarbon antisite is identified as a plausible key ingredient of the DII-center, whereas the carbon split-interstitial is a likely origin of the P T centers. The comparison of the calculated and observed high-frequency modes suggests that the U-center is also a carbon-antisite based defect.

Many-Body Effects in the Excitation Spectrum of a Defect in SiC

We show that electron correlations control the photophysics of defects in SiC through both renormalization of the quasiparticle band structure and excitonic effects. We consider the carbon vacancy with two possible excitation channels that involve conduction and valence bands. Corrections to the Kohn-Sham ionization levels strongly depend on the defect charge state. Excitonic effects introduce a redshift of 0.23 eV. The analysis reassigns excitation mechanism at the thresholds in photoinduced paramagnetic resonance measurements [J. Dashdorj, J. Appl. Phys. 104, 113707 (2008)].

Carbon antisite clusters in SiC: A possible pathway to the D II center

The photoluminescence center D I I is a persistent intrinsic defect which is common in all SiC polytypes. Its fingerprints are the characteristic phonon replicas in luminescence spectra. We perform ab initio calculations of vibrational spectra for various defect complexes and find that carbon antisite clusters exhibit vibrational modes in the frequency range of the D I I spectrum. The clusters possess very high binding energies which guarantee their thermal stability-a known feature of the D I I center. The dicarbon antisite (C 2 ) S i (two carbon atoms sharing a silicon site) is an important building block of these clusters.

Hydrodynamic theory of an electron gas

The generalized hydrodynamic theory, which does not require assumption of a local equilibrium, is derived in the long-wave limit of a kinetic equation. In contrast to the Bloch hydrodynamics, the theory is applicable to a collisionless electron gas and correctly describes the plasmon dispersion. In a low-frequency (collision dominated) limit, Navier-Stokes hydrodynamics is recovered. In the linear approximation, the generalized hydrodynamics becomes equivalent to phenomenological theory of highly viscous fluids.

Electron spectrum of epitaxial graphene monolayers

Epitaxial graphene on SiC possesses, quite remarkably, an electron spectrum similar to that of freestanding samples. Yet, the coupling to the substrate, albeit small, affects the quasiparticle properties. Combining \emph{ab initio} calculations with symmetry analysis, we derive a modified Dirac-Weyl Hamiltonian for graphene epilayers. While for the epilayer on the C-face the Dirac cone remains almost intact, for epilayers on the Si-face the band splitting is about 30\,meV. At certain energies, the Dirac bands are significantly distorted by the resonant interaction with interface states, which should lead to mobility suppression, especially on the Si-face.

Generalized Kohn-Sham system in one-matrix functional theory

A system of electrons in a local or nonlocal external potential can be described with one-matrix functional theory (1MFT), which is similar to density-functional theory (DFT) but takes the one-particle reduced density matrix (one-matrix) instead of the density as its basic variable. Within 1MFT, Gilbert derived [Phys. Rev. B 12, 2111 (1975)] effective single-particle equations analogous to the Kohn-Sham (KS) equations in DFT. The self-consistent solution of these 1MFT-KS equations reproduces not only the density of the original electron system but also its one-matrix. While in DFT it is usually possible to reproduce the density using KS orbitals with integer (0 or 1) occupancy, in 1MFT reproducing the one-matrix requires in general fractional occupancies. The variational principle implies that the KS eigenvalues of all fractionally occupied orbitals must collapse at self-consistency to a single level. We show that as a consequence of the degeneracy, the iteration of the KS equations is intrinsically divergent. Fortunately, the level-shifting method, commonly introduced in Hartree-Fock calculations, is always able to force convergence. We introduce an alternative derivation of the 1MFT-KS equations that allows control of the eigenvalue collapse by constraining the occupancies. As an explicit example, we apply the 1MFT-KS scheme to calculate the ground state one-matrix of an exactly solvable two-site Hubbard model.