Thermoelectric properties of high-pressure silicon phases

The thermo emf in Czochralski-grown silicon single crystals (Cz-Si) was experimentally studied in a range of pressures up to 20 GPa. The pressure dependences revealed phase transitions in the metallic phase of silicon, which passed from tetragonal to orthorhombic and then to hexagonal lattice. The high-pressure silicon phases, as well as the metallic high-pressure phases in A_NB_8?N semiconductors, possess conductivity of the hole type. As the pressure decreases, the emf behavior reveals transitions to the metastable phases Si-XII and Si-III. Preliminary thermobaric treatment of the samples at a pressure of up to 1.5 GPa and a temperature of T=50–650°C influences the thermoelectric properties of Cz-Si at high pressures.     

Classification and structure of silicon carbide phases

Molecular-mechanical and semiempirical quantum-mechanical methods have been applied to simulate and calculate a geometrically optimized structure of clusters of polymorphic types of silicon carbide, and their structural parameters and some properties (densities, sublimation energies) have been determined. A classification of silicon carbide phases has been proposed, which shows the possible existence of twenty one SiC phases whose atoms are at crystallographically equivalent sites. The structures of seventeen proposed silicon carbide phases have been described and studied for silicon carbide for the first time.     

Electronic structure of silicon superlattices

Utilizing a new complex-band-structure technique, the electronic structure of model Si-Si_1?xGe_x and MOS superlattices have been obtained over a wide range of layer thickness d (11 ≤ d ≤ 110 A). For d ≥ 44 A, it is found that these systems exhibit a direct fundamental band gap. Further calculations of band-edge effective masses and impurity scattering rates suggest the possibility of a band-structure-driven enhancement in electron mobility over bulk silicon.     

Electronic structure of spiral magnetic manganite phases

An approach has been developed using the tight-binding and double-exchange Hamiltonian methods for calculating the E(k) spectrum of e _g electrons in noncollinear (spiral) magnetic structures of R _{1 − x} A _x MnO₃ (R = La, Pr, Nd, Sm; A = Ca, Sr, Ba) manganites. A magnetic structure in the form of a flat cycloid observed in TbMnO₃ below 28 K has been considered for undoped manganites (x = 0).     

Electronic structure of positive muon in silicon

The spin polarized electronic structure around a positive muon ⁺ at the tetrahedral interstitial site of silicon is calculated by use of the LCAO-Green's function method and the local spin density functional formalism. The bonding hyper deep impurity state below the valence band and the antibonding deep one in the gap caused by the strong sp hybridization in the majority spin band give the most large contribution to the hyperfine coupling constant of ⁺. The reduction of the hyperfine coupling constant,A, experienced by ⁺ at absolute zero is calculated to be 0.406 in satisfactory agreement with the experimental value of 0.405 ± 0.026 or 0.450 ± 0.020.     

Metadynamics simulations of the high-pressure phases of silicon employing a high-dimensional neural network potential

We study in a systematic way the complex sequence of the high-pressure phases of silicon obtained upon compression by combining an accurate high-dimensional neural network representation of the density-functional theory potential-energy surface with the metadynamics scheme. Starting from the thermodynamically stable diamond structure at ambient conditions we are able to identify all structural phase transitions up to the highest-pressure fcc phase at about 100 GPa. The results are in excellent agreement with experiment. The method developed promises to be of great value in the study of inorganic solids, including those having metallic phases.     

Electronic structure of defects in amorphous silicon

The effects of a variety of defect centres such as T⁰₃, T⁰₂, T⁰₁ and their interacting units on the electronic structure of the amorphous silicon has been studied in a cluster Bethe lattice method. The gap states arising from these defects may explain a number of experimental observations.     

Electronic structure of the isolated vacancy in silicon

The many-electron molecular orbital method predicts a ground state for the various charged vacancy centres in silicon which has a spin multiplicity in agreement with the electron spin resonance results. Symmetric relaxation and Jahn-Teller contributions have been included in first order. The quantitative features of the model are very different to those implied by the one-electron model.     

Predicted novel high-pressure phases of lithium

Under high pressure, "simple" lithium (Li) exhibits complex structural behavior, and even experiences an unusual metal-to-semiconductor transition, leading to topics of interest in the structural polymorphs of dense Li. We here report two unexpected orthorhombic high-pressure structures Aba2-40 (40 atoms/cell, stable at 60-80 GPa) and Cmca-56 (56 atoms/cell, stable at 185-269 GPa), by using a newly developed particle swarm optimization technique on crystal structure prediction. The Aba2-40 having complex 4- and 8-atom layers stacked along the b axis is a semiconductor with a pronounced band gap >0.8 eV at 70 GPa originating from the core expulsion and localization of valence electrons in the voids of a crystal. We predict that a local trigonal planar structural motif adopted by Cmca-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.     

Electronic structure calculations of high pressure phases of metal oxynitrides

Ab initio calculations are undertaken to compare properties of some phases of several metal oxides, namely titanium, tantalum, niobium and tungsten. The crystal structures considered follow that found in ZrO₂, namely monoclinic, cubic, tetragonal and orthorhombic. In general, it is found that high hydrostatic pressures are needed for synthesis of these materials, although each of them exhibit quite significant compressibility.     

Electronic structure of silicon dioxide (a review)

Silicon dioxide amorphous films are the key insulators in silicon integrated circuits. The physical properties of silicon dioxide are determined by the electronic structure of this material. The currently available information on the electronic structure of silicon dioxide has been systematized.     

Electronic structure model for n- and p-type silicon quantum dots

We present effective mass, single-particle calculations of the electronic structure of n- and p-type silicon quantum dots. The structures investigated approximate silicon quantum dots fabricated on 〈 001〉-oriented SIMOX wafers. The effects of possible built-in strain are investigated in the framework of deformation potential induced splitting of the six degenerate conduction band valleys and the splitting of the degeneracy at the top of the bulk valence band. We present the energy levels and their degeneracies as functions of the dimensions of simple tetragonal model quantum dots. Our results are relevant for silicon quantum dots that are sufficiently small such as to lead to a predominance of the confinement energy over the Coulomb energy.