Semi-empirical Hartree-Fock Calculations Research Articles

Advances in understanding the defects contributing to the tail states in pure amorphous silicon

The tight-binding molecular dynamics simulations and reverse Monte Carlo structural modeling method were applied in order to investigate the existence of small bond angles (like those in triangles and squares) in amorphous silicon networks. The influence of small bond angles on the electronic density of states was analyzed. The presence of a number of smaller bond angles is necessary for a proper reproduction of the neutron diffraction data of amorphous silicon. Semiempirical Hartree–Fock calculations show that these arrangements provide higher energy levels in electronic density of states which are localized on these local structures. Accepting this result we must reconsider the electronic density of states of amorphous semiconductors. The localized mobility gap has structure i.e. two characteristic peaks can be found inside the tail. First larger peak belongs to the squares while the peak at larger energy is formed by triangles.

What is the origin of tail states in amorphous semiconductors?

The tight-binding molecular dynamics simulations and reverse Monte Carlo structural modeling method were applied in order to investigate the existence of small bond angles (like those in triangles and squares) in amorphous silicon networks. The influence of small bond angles on the electronic density of states was analyzed. The presence of a number of smaller bond angles is necessary for a proper reproduction of the neutron diffraction data of amorphous silicon. Semi-empirical Hartree–Fock calculations show that these arrangements provide higher energy states in electronic density of states which are localized on these local structures. We consider that smaller bond angles are a new type of defects. These defect states are highly energetic and they are the origin of tail in amorphous semiconductors.

Direct observation of the ^{2}D_{3∕2}-^{2}D_{5∕2} ground-state splitting in Xe^{9+}

We have used an electron beam ion trap to observe a visible line at 598.30(13) nm that corresponds to the 4d9 2D3/2–4d9 2D5/2 magnetic dipole transition within the ground state configuration of Xe9+. We have found no evidence to support the claim by others that a line near this position originates from Xe31+. The comparison of the measured wavelength with previous indirect experimental values, and the dependence of the line intensity on the electron beam energy, confirms the validity of the present identification. Our measured wavelength is in agreement with semiempirical Hartree-Fock calculations but shows a discrepancy with ab initio multiconfiguration Dirac-Fock calculations. We propose that the line may be useful as a diagnostic for extreme ultraviolet lithography light sources.

Structure, Energetics, and Thermal Stability of Nitrogen-Vacancy-Related Defects in Nitrogen Doped Silicon

The electronic structure, formation energy, and thermal stability of nitrogen-vacancy related complexes in silicon have been investigated using density functional theory and semi-empirical Hartree-Fock calculations. The calculated energies of formation in the ground state showed that was not stable, whereas when formed from was the most stable, followed by and formed from a divacancy. The calculated free energy changes of the considered chemical reactions confirmed the low stability of compared to The latter can form during crystal growth from whereas reactions between and divacancy can also occur upon wafer heating. At low nitrogen concentration cm−3), only about 10% of vacancy concentration was converted into while at a high nitrogen concentration cm−3) about 75% of vacancies are trapped by nitrogen. appeared to create a potential well of −2.4 eV for oxygen and about −0.3 eV for vacancies, suggesting that the stable is a nucleus for oxygen precipitation while it is a weak trapping center for vacancies. © 2003 The Electrochemical Society. All rights reserved.

Manifestation of Polaronic States in Ferroelectric Relaxor PMN

Small polaron states in the model ferroelectric relaxor PMN were confirmed in this work by detection of the characteristic IR-absorption accompanied by light-induced polaron transport. The mechanism of an influence of mobile polaronic clusters on the polar cluster reorientations and, as a result, on the co-operative phenomena in relaxor is developed. Here mobile polaronic clusters can move on the interfaces of polar clusters with carrying of their polarizations. The latter are moving self-consistently with polaronic clusters, and are involved to the reorientation-like motion. The polar cluster-polar cluster interaction between such type reorienting polarizations of different clusters can be a possible origin of phase transitions in PMN-like relaxors. Semi-empirical Hartree-Fock calculations of electronic polaron states in some model ABO 3 -systems performed are in a good agreement with the experiment.

Adsorption and decomposition of methane on gallium oxide films

Surface reactions are investigated by a sensitive mass spectrometer. Ga 2O 3 films are exposed to square pulses of methane. From the experimental data the desorption energies and the frequency factors are calculated for H 2O and CO 2. Semiempirical Hartree–Fock calculations with the Modified Neglect of Diatomic Overlap — Parametric Method 3 (MNDO-PM3) method give information about the adsorption process. The energy gain of physisorption amounts to 0.27 eV. The Heat of Formation (HoF) of the chemisorbed CH 3 group and the H atom is 4.05 eV lower in comparison to the value of the physisorbed methane. After the CO latt. group formation the energy gain is 7.33 eV. The binding energy of the formed CO latt. is calculated to 2.07 eV. The charge transfer is estimated using the Mulliken charges. A reaction scheme for the catalytic oxidation of methane by lattice oxygen is proposed.

Semi-empirical hartree-fock calculations for interacting Li impurities in KTaO3

Equilibrium off-center displacements of two Li impurities substituting neighboring K sites in KTaO3, the relaxation pattern of O ions around the impurities and the Li-Li interaction energies in fixed and relaxed O surrounding have been calculated using the semi-empirical INDO method. The Li off-center displacement is 0.6 – 0.7 Å in all cases: the displacements of O nearest neighbors to Li are up to 0.06 Å, and the displacement of more distant atoms negligibly small, within the calculational accuracy. The Li-Li interaction energy is positive, revealing the deviations from the dipole-dipole interaction in a polarized crystal, and strongly enhanced due to the O relaxation.

Self-Ordered Second-Component Nb Clusters in KNbxTa1?xO3 Solid Solutions and Their Physical Properties

Semi-empirical Hartree-Fock calculations using the intermediate neglect of the differential overlap (INDO) method, for self-ordered cubic symmetry clusters of seven Nb ions in KTaO 3 are performed with the aim of verifying the cluster model [1] of second component-induced phase transitions in ferroelectric perovskite matrices. It is shown that a seven-particle cluster in KTaO 3 :Nb has two types of states with different nature. Namely, a state with a dilatation combined with an off-center displacement of the central Nb ion in [111] directions in the cluster, and a state with a full-symmetric compression without any off-center effect. The consequences of such cluster structures on the multi-well potential are discussed. In particular, the resonance tunneling between cluster excited states characterized by low frequency on-center vibrations, on the one hand, and excited states of off-center cluster vibrations, on the other, gives rise to low lying tunnel states. The latter states interact with soft TO modes and form the displacive type ferroelectric phase transition induced by impurities. This new type of displacive-like ordering co-exists with order-disorder effects with development of the central peak. All these results are in agreement with experimental data on KTaO 3 with Nb impurities. The results of the semi-empirical INDO calculations for seven Nb ions forming an impurity cluster in KTaO 3 confirm the key assumptions of the cluster model of induced phase transitions.

Semi-empirical Hartree—Fock calculations on the mechanism of enniatin B mediated transport of sodium ions

The passage of sodium ions through enniatin B is studied by the CNDO method. Electronic dissociation energy of the enniatin BNa + complex in water is 0.496 au for the open form (0.397 au for the closed form). The open complex is found to be more stable than the closed one in water. The dissociation energy changes when counterions representing charged heads of phospho-lipids are placed around the ionophore. Variation of counterion charges leads to different possibilities for the transport of Na + across a lipid membrane. These are (i) no transport, (ii) transport through channels and (iii) transport of the ion—ionophore complex. A pH dependence that relies on the nature of counterion charges is predicted for the transport.

Beam-foil lifetime study of Al-like titanium, iron and nickel

The lifetimes of the resonance levels in the Al-like ions Ti X, Fe XIV and Ni XVI have been obtained from beam-foil measurements using a variety of analytical techniques. The spectral analysis and identification of cascade transitions necessary for application of the ANDC procedure have been aided by recent observations of laser-produced plasmas. The lifetime values obtained from ANDC analyses for the 3s 3p2 doublet levels agree well with predictions based on semi-empirical Hartree-Fock calculations. However, the ANDC results for the 3s2 3d levels fall between the values obtained using multi-exponential curve-fitting and from the calculations.