Jerry L. Whitten

Dissociative chemisorption of CH4 on Ni(111)

The dissociative chemisorption of methane at an atop‐atom site on a (111) surface of nickel is treated using a many‐electron embedding theory to describe bonding, modeling the lattice as a 41‐atom, three layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. Ni 3d orbitals are explicitly included on seven nickel atoms on the surface. The calculated activation energy of CH4 adsorbed at an atop Ni site to produce CH3 and H coadsorbed at separated threefold sites is 17 kcal/mol. The dissociation of CH4 to CH3(ads)+H (ads) is predicted to be 2.8 kcal/mol exothermic. The Ni 3d orbitals contribute to the bonding by directly mixing with methane C–H orbitals during the dissociation process and through a direct interaction of 3d9 and 3d10 configurations at the transition state. The dissociation pathway and the bonding properties of adsorbed CH4 and coadsorbed CH3 and H are disc...

Chemisorption of atomic H and CHx fragments on Ni(111)

Abstract The adsorption of atomic hydrogen, methylidyne, methylene and methyl on the (111) surface of nickel is treated using a many-electron embedding theory to describe bonding, modelling the lattice as a 28-atom, three-layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. All adsorbed species are found to bind strongly to the Ni(111) surface at three-fold and bridge sites with adsorption energies ranging from 1.5 to 3.1 eV. Atop Ni adsorption sites have significantly higher energy. Bond lengths, angles and vibrational frequencies for adsorbates on the surface are calculated. The geometries of the adsorbed species and the nature of the bonding to the surface are discussed.

Generation of Highly n-Type Titanium Oxide Using Plasma Fluorine Insertion

True n-type doping of titanium oxide without formation of midgap states would expand the use of metal oxides for charge-based devices. We demonstrate that plasma-assisted fluorine insertion passivates defect states and that fluorine acts as an n-type donor in titanium oxide. This enabled us to modify the Fermi level and transport properties of titanium oxide outside the limits of O vacancy doping. The origin of the electronic structure modification is explained by ab initio calculation.

Electronic structure of noncrystalline transition metal silicate and aluminate alloys

A localized molecular orbital description ~LMO! for the electronic states of transition metal ~TM! noncrystalline silicate and aluminate alloys establishes that the lowest conduction band states are derived from d states of TM atoms. The relative energies of these states are in agreement with the LMO approach, and have been measured by x-ray absorption spectroscopy for ZrO2 ‐ SiO2 alloys, and deduced from an interpretation of capacitance‐voltage and current‐voltage data for capacitors with Al2O3 ‐T a 2O5 alloy dielectrics. The LMO model yields a scaling relationship for band offset energies providing a guideline for selection of gate dielectrics for advanced Si devices.

Conduction band-edge States associated with the removal of d-state degeneracies by the Jahn-Teller effect

X-ray absorption spectroscopy (XAS) is used to study band edge electronic structure of high-/spl kappa/ transition metal (TM) and trivalent lanthanide rare earth (RE) oxide gate dielectrics. The lowest conduction band d/sup */-states in TiO/sub 2/, ZrO/sub 2/ and HfO/sub 2/ are correlated with: 1) features in the O K/sub 1/ edge, and 2) transitions from occupied Ti 2p, Zr 3p and Hf 4p states to empty Ti 3d-, Zr 4d-, and Hf 5d-states, respectively. The relative energies of d-state features indicate that the respective optical bandgaps, E/sub opt/ (or equivalently, E/sub g/), and conduction band offset energy with respect to Si, E/sub B/, scale monotonically with the d-state energies of the TM/RE atoms. The multiplicity of d-state features in the Ti L/sub 2,3/ spectrum of TiO/sub 2/, and in the derivative of the O K/sub 1/ spectra for ZrO/sub 2/ and HfO/sub 2/ indicate a removal of d-state degeneracies that results from a static Jahn-Teller effect in these nanocrystalline thin film oxides. Similar removals of d-state degeneracies are demonstrated for complex TM/RE oxides including Zr and Hf titanates, and La, Gd and Dy scandates. Analysis of XAS and band edge spectra indicate an additional band edge state that is assigned Jahn-Teller distortions at internal grain boundaries. These band edges defect states are electronically active in photoconductivity (PC), internal photoemission (IPE), and act as bulk traps in metal oxide semiconductor (MOS) devices, contributing to asymmetries in tunneling and Frenkel-Poole transport that have important consequences for performance and reliability in advanced Si devices.

Dissociative adsorption of H2 on Ni(111)

Ab initio configuration interaction calculations are performed to study the dissociative adsorption of H2 on a Ni(111) surface. The lattice is modeled as an embedded three‐layer 41‐atom cluster. Ni 3d orbitals are explicitly included on seven Ni atoms on the surface. H is preferentially chemisorbed at a threefold site on Ni(111) and the calculated binding energy of 62 kcal/mol, H–Ni distance of 1.86 A, and H vibrational frequency of 1176 cm−1 are in excellent agreement with experimental data. H adsorbed at bridge and on‐top Ni sites is 2.5 and 8.1 kcal/mol less stable, respectively. The heat of reaction H2 (gas)→2 H (ads) is calculated to be 22.0 kcal/mol exothermic. When two H atoms are adsorbed as nearest neighbors to the same Ni atom, threefold sites are preferred with H atoms adsorbed at fcc–fcc, hcp–hcp, or across atom fcc–hcp sites. These structures are consistent with the observed (2×2)−2H low energy electron diffraction pattern. The average adsorption energy per H is calculated to be 62 kcal/mol f...

Electronic structure of high-k transition metal oxides and their silicate and aluminate alloys

This article addresses differences between the electronic structure of: (i) alternative high-k transition metal (TM) rare earth dielectrics and (ii) SiO2 and Si oxynitride alloys by presenting a systematic x-ray absorption spectroscopy study of transitions between TM n p-core states and TM metal n+1−d⋆ and n+2 s⋆ antibonding/conduction band states (n=2, 3, and 4) that is complemented by studies of O atom K1 edge absorption spectra. Ab initio calculations based on small clusters establish the localization of the n+1 d⋆ states on the TM metals. Ab initio electronic structure calculations are also used to interpret other aspects of the optical, ultraviolet, x-ray, and electron spectroscopies, and also provide a basis for interpretation of electrical results, thereby narrowing the field of possible replacement dielectrics for advanced semiconductor devices.

Theoretical studies of H2 desorption from Si(100)–2×1H

Theoretical studies of H2 desorption from a cluster model of the Si(100)–2×1H surface show that the desorption pathway is symmetrical and has a desorption energy barrier of 3.75 eV and a corresponding adsorption energy barrier of 1.15 eV. The proper treatment of electron correlation lowers the desorption energy barrier considerably. The present results suggest that the desorption of two hydrogen atoms from different Si atoms of a surface dimer is not the desorption pathway observed experimentally at activation energies in the range 2.0–2.9 eV.

The adsorption of benzene on Ni(111)

Abstract Using a many-electron embedding theory, the present work treats the adsorption of benzene on Ni(111) by modelling the lattice as a 28-atom, three-layer cluster. Ab initio valence orbital CI calculations carried out on a local surface region permit an accurate description of bonding at the surface. Benzene is found to be adsorbed molecularly, parallel to the surface, at a threefold site, bonding primarily through its π electron system. The equilibrium distance is 2.2 A from the surface. The calculations show no distortion of the benzene ring, other than a 2% expansion. CH bonds tilt away from the surface 8.5°. The adsorption energy is calculated to be 1.2 eV. Adsorption at the threefold hollow site gives a slightly higher energy than the hep site, but the difference may be within the uncertainty of the calculation. The energy of the bridge adsorption site is higher than that of the hcp site, by 0.8 eV. Atop atom sites were not investigated. Due to the limited number of geometries investigated, particularly those involving Kekule and buckling distortions, some aspects of the benzene geometry and energetics of different sites remain uncertain.

Theoretical studies of surface reactions: embedded cluster theory

Abstract A theoretical approach to the description of the electronic structure of molecules adsorbed on solid surfaces and surface reactions is described. The objective is a quantitative, molecular level, understanding of surface processes, including chemisorption energetics, adsorbate structure and reaction mechanisms. The electronic structure problem is formulated as an embedded cluster of atoms in which a localization transformation is used to define the electronic subspace that interacts strongly with the adsorbate and surface region. This permits ab initio SCF and CI calculations of molecular quality to be performed on a portion of the lattice—adsorbate system. Assumptions of the theory are discussed and applications to chemisorption on transition metal substrates are referenced.