Gary G. Deleo

Optical absorption and electron paramagnetic resonance studies of chemically reduced congruent lithium niobate

Abstract An optical absorption and electron paramagnetic resonance (EPR) study has been performed on a series of chemically reduced congruent lithium niobate (LiNbO 3 ) samples. Chemical reduction enhances the high-temperature electrical conductivity and darkens an otherwise optically clear crystal. The resulting optical spectra show a sharp threshold absorption above 3.8 eV and the growth of a broad-band absorption featuring a peak at 2.4 eV. The strength of the broad-band absorption is directly correlated with the carriers responsible for the enhanced high-temperature electrical conductivity. Optical illumination (bleaching) at 87 K shifts the optical absorption peak from 2.4 to 1.6 eV and produces an enhanced Nb 4+ EPR signal. A quantitative analysis shows that the 1.6 eV optical absorption and the Nb EPR signal are both consistent with single electrons trapped on Nb atoms. The experimental results are discussed in terms of oxygen vacancy and bipolaron models.

Theory of hydrogen-impurity complexes in semiconductors

Abstract In this review of theory, the focus will be on a collection of hydrogen-containing complexes of current interest and controversy, with more general implications summarized where appropriate. In particular, I will include hydrogen complexes with single donors and acceptors in silicon and compound semiconductors, and with double donors and acceptors in silicon. Also, included is a brief discussion of the hydrogen-vacancy and hydrogen-molecule systems in silicon. The phenomena of interest include the equilibrium geometries, and the motion of the impurities as they vibrate, reorient, and dissociate within the solid. These characteristics are described as calculated using quantum mechanical methods; hence, electronic-structure methods are summarized as well.

Chapter 14 Computational Studies of Hydrogen-Containing Complexes in Semiconductors

Publisher Summary This chapter discusses the research of computational theorists on the microscopic properties of hydrogen-containing complexes in semiconductors. The theory of the ubiquitous class of hydrogen-containing complexes forms is described in the chapter. The goal of defect computations is normally to complement or guide corresponding experimental studies so that the defect is either properly identified or otherwise better understood. In addition, computational simulations can provide essential insight for understanding the classes of defects that are either undetectable by electron paramagnetic resonance or electrically inactive. The chapter reviews the theoretical framework and computational methods that have been developed for point defects in crystalline solids and applied to hydrogen-containing complexes in semiconductors. It also discusses (1) hydrogen interactions with silicon dangling bonds, (2) hydrogen-shallow donor and hydrogen-shallow acceptor complexes in silicon (their structure, migration, reorientation, and vibrational characteristics), (3) hydrogen donor/ acceptor pair complexes, (4) hydrogen–dopant complexes in compound semiconductors, and (5) hydrogen molecules in crystalline silicon.

Theory defects in silicon: Recent calculations using finite molecular clusters

Abstract Deep-level impurities in semiconductors are characterized by perturbations of the host potential which are relatively localized to the vicinity of the impurity atom. This localization suggests that the impurity and its environment may be adequately simulated by a finite cluster of atoms, where surface effects are handled by some suitable termination. We illustrate this technique for two classes of silicon impurities: (i) interstitial iron-group transition-metal atoms, and (ii) substitutional oxygen and nitrogen. The single particle electronic structures are generated by the ab - initio scattered-wave Xα method and by the semi-empirical modified neglect of diatomic overlap method (MNDO) where appropriate. We also go beyond the conventional single-particle picture by considering the following extensions: (i) electronic relaxation accompanying electronic transitions, (ii) many-electron effects, and (iii) spontaneous impurity displacement.

Simulation of Vacancy Pairs in GaN Using Tight-Binding Molecular Dynamics

The electronic structure, geometry and energetics of Ga vacancy pairs and N vacancy pairs in both wurtzite and zincblende GaN are investigated via molecular dynamics (MD) simulations using an empirical tight-binding (TB) model with total energy capabilities and supercells containing up to 216 atoms. Our calculations suggest that, by pairing, N vacancies, which in isolation act as shallow donors, can lower their collective formation energy by about 5 eV. In doing so, however, these N vacancies lose their shallow-donor character as the lattice relaxes in response to this aggregation. Contrasting with the N vacancies, the Ga vacancies are found to retain their isolated shallow acceptor behavior and do not gain significant energy upon aggregation. The possible implications for larger aggregate defects are discussed.

ULTRAVIOLET SPECTROSCOPIC ANALYSIS OF TRANSIENT MASS FLOW OUTBURST IN U CEPHEI

Spectra from the International Ultraviolet Explorer taken in 1989 September over one full orbital period of U Cephei (U Cep, HD 5796) are analyzed. The TLUSTY and SYNSPEC stellar atmospheric simulation programs are used to generate synthetic spectra to which U Cep continuum levels are normalized. Absorption lines attributed to the photosphere are divided out to isolate mass flow and accretion spectra. A radial velocity curve is constructed for conspicuous gas stream features, and shows evidence for a transient flow during secondary eclipse with outward velocities ranging between 200 and 350 km s{sup –1}, and a number density of (3 ± 2) × 10{sup 10} cm{sup –3}. The validity of C IV 1548 and 1550 and Si IV 1393 and 1402 lines are re-examined in the context of extreme rotational blending effects. A G-star to B-star mass transfer rate of (5 ± 4) × 10{sup –9} M{sub ☉} yr{sup –1} is calculated as an approximate upper limit, and a model system is presented.

A new tight binding total energy method for ferroelectric oxides

We have developed a tight-binding electronic structure framework, which includes explicit treatment of Coulombic interactions (with ion-size effects), charge transfer effects and an overlap matrix, and has total energy capabilities. This method is applied to the study of structural phase transitions in ferroelectric oxides, with emphasis on KNbO3, and, to a lesser degree, on KTaO3. In this framework the energy is composed of four parts; 1) a sum over the band energies, 2) ion-ion interaction, 3) an electron-electron double counting term, and 4) Coulombic interactions with ion size effects for near neighbors and point ion interactions for the rest. The self-energies are assumed to be different from the atomic term values due to charge transfer effects. The off site Hamiltonian matrix elements include interactions up to 3rd near-neighbors. The overlap matrix was taken to be proportional to the Hamiltonian matrix. The electrostatic interactions were calculated using the Ewald method, where for near-neighbors...

Tight-binding total energy study of phase transitions in KNBO3

Abstract We have developed a tight-binding electronic structure framework with total-energy capabilities which includes an explicit treatment of charge transfer effects and non-point-ion interactions. In this framework, the total energy is composed of four parts: i) A sum over the band energies, which includes charge transfer effects in the self energies and an overlap matrix proportional to the Hamiltonian matrix, ii) an electron-electron double counting term, iii) a core-core interaction term, and iv) a term that produces Coulombic interactions, where, for first near neighbors, we include a penetration factor that serves to appropriately decrease screening effects. Electronic self consistency is established by updating the self-energy contributions to the Hamiltonian according to the results of a population analysis. Based upon these calculations, we present a potential-energy surface of potassium niobate with respect to symmetry lowering displacements. These results are consistent with a system that sp...