John E. Jaffe

Thermodynamic stability of high-K dielectric metal oxides ZrO2 and HfO2 in contact with Si and SiO2

We present theoretical and experimental results regarding the thermodynamic stability of the high-k dielectrics ZrO2 and HfO2 in contact with Si and SiO2. The HfO2/Si interface is found to be stable with respect to formation of silicides whereas the ZrO2/Si interface is not. The metal–oxide/SiO2 interface is marginally unstable with respect to formation of silicates. Cross-sectional transmission electron micrographs expose formation of nodules, identified as silicides, across the polycrystalline silicon/ZrO2/Si interfaces but not for the interfaces with HfO2. For both ZrO2 and HfO2, the x-ray photoemission spectra illustrate formation of silicate-like compounds in the MO2/SiO2 interface.

Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia

Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia-doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.

Oxygen vacancies and ferromagnetism in CoxTi1−xO2−x−y

Cobalt-doped titanium dioxide, or CTO, has emerged in the past 2 years as a semiconducting, transparent, room-temperature ferromagnet. Very recently it has been shown that the magnetism in CTO often originates in surface nanoparticles or Co-rich regions that have a much-enhanced substitutional Co content up to 40% of Ti sites, so that magnetic CTO is not a true dilute magnetic semiconductor (DMS), but rather a fairly high-density spin system. In this work we describe a computational study of Co-rich CTO using the generalized gradient approximation to the density functional theory within the supercell model. Our total energy calculations show a strong tendency for Co-atom clustering or segregation on Ti sites. There is also a strong tendency for the oxygen vacancies to form complexes with the Co atoms. In addition, we find that the oxygen stoichiometry plays an essential role in determining the system’s magnetic order. The largest ordered moments require at least enough oxygen vacancies to put all of the C...

Ab initio study of high pressure phase transition in GaN

Abstract The total energy of GaN as a function of unit cell volume has been calculated for the wurtzite, zinc-blende, and rocksalt phases by the ab initio all-electron periodic Hartree-Fock method. The gallium 3 d levels were treated as fully relaxed band states, and the internal coordinates c / a and u in the wurtzite phase were optimized. The calculated transition pressure between the wurtzite rocksalt phases comes out to be about 52 GPa at the Hartree-Fock level and about 35 GPa at the correlated level. The calculated electronic structure shows strong hybridization of Ga 3 d and N 2 s states with the ordering as Ga 3 d s p in all the phases. The band gap is direct at Γ in the wurtzite and zinc-blende phases and is indirect in the high pressure rocksalt phase where the valence band maximum is shifted away from the Γ point.

Gaussian basis density functional theory for systems periodic in two or three dimensions: Energy and forces

We describe a formulation of electronic density functional theory using localized Gaussian basis functions for systems periodic in three dimensions (bulk crystals) or two dimensions (crystal slabs terminated by surfaces). Our approach generalizes many features of molecular density functional methods to periodic systems, including the use of an auxiliary Gaussian basis set to represent the charge density, and analytic gradients with respect to nuclear coordinates. Existing quantum chemistry routines for analytic and numerical integration over basis functions can be adapted to our scheme with only slight modifications, as can existing extended Gaussian basis sets. Such basis sets permit accurate calculations with far fewer basis functions (and hence much smaller matrices to diagonalize) than plane‐wave based methods, especially in surface calculations, where in our approach the slab does not have to repeat periodically normal to the surface. Realistic treatment of molecule–surface interactions is facilitate...

Electronic structure of high pressure phase of AlN

Results of ab initio Hartree–Fock calculations for the electronic structure of aluminum nitride in the (high-pressure) rocksalt phase are reported. In the rocksalt phase, the calculated lattice constant is 3.982 A with the bulk modulus of 329 GPa. The band structure is predicted to be indirect at the X point with a gap of 8.9 eV. In this phase, the bonding is shown to be essentially ionic between Al and N. The direct gap shows a stronger linear dependence on pressure with a pressure derivative of 68 meV/GPa compared to that of the indirect gap with a pressure derivative of 31.7 meV/GPa.

LDA and GGA calculations of alkali metal adsorption at the (001) surface of MgO

The adsorption geometry, binding energy and electronic structure of alkali metal overlayers on the MgO (001) surface have been studied by means of density functional theory, using Gaussian-type orbitals to expand the wave functions and electronic charge density. A two-dimensionally periodic slab of MgO with alkali metal adsorbed at one surface was used to model the semi-infinite system. Li, Na, and K were considered at both half- and quarter-monolayer coverage. Results were compared for the local density approximation and for two different forms of the generalized gradient approximation. In all cases Li was found to interact with the surface approximately twice as strongly as Na and three times as strongly as K. The epitaxial binding energies were, however, always less than or close to the bulk cohesive energies of the respective alkali metals, suggesting an instability of the adsorbed film toward the formation of two- or three-dimensional islands, in agreement with experiment. Spin polarized and unpolari...

Abinitio study of a CO monolayer adsorbed on the (101̄0) surface of ZnO

Periodic Hartree–Fock total energy calculations on two‐dimensionally periodic slabs have been used to predict the equilibrium geometry of a monolayer of carbon monoxide molecules adsorbed on the nonpolar (1010) surface of ZnO. Two physisorbed (or weakly chemisorbed) minimum energy configurations are found. In one the CO molecules adsorb with their oxygen atoms coordinated to surface Zn atoms, while in the other the carbon atoms are coordinated to surface Zn atoms. The two calculated minima are very close in energy. In the second geometry, the C–Zn ‘‘bond’’ and the C–O bond make angles of 32.5° and 39.5° with the surface normal, and the intramolecular bond shortens slightly from its free value in reasonable agreement with experimental results. No binding of CO to the surface oxygen atoms is predicted. Surface‐related changes in the vibrational frequencies for the adsorbed molecules agree reasonably well with infrared spectroscopic data, and the ‘‘carbon‐down’’ binding energy of the molecule with the surfa...

First-principles study of noncommutative band offsets at [alpha]-Cr2O3/[alpha]-Fe2O3(0001) interfaces

Using first-principles density functional theory, we have modeled the atomic, electronic and magnetic structure of epitaxial interfaces between alpha-hematite and alpha-chromia (corundum structure) in the hexagonal (0001) basal plane. Our model was a superlattice with a period of about 27.5A, corresponding to the shortest-period superlattice considered in a recent series of experiments (Chambers et al., Phys. Rev. B 61, 13223 (2000)). Two different epitaxial interface structures were studied: (i) an oxygen plane separating an Fe double layer from a Cr double layer, or (ii) a metal double layer split between Fe and Cr. We found that these two structures are close in total energy but have distinct spin structure and different valence band offsets (chromia above hematite by 0.4 and 0.6 eV for (i) and (ii) respectively), possibly explaining the experimental non-commutative band offset seen in this system (0.3±0.1 eV for hematite grown atop chromia, and 0.7±0.1 eV for the reverse).

Defect-induced Nonpolar-to-polar Transition at the Surface of CuInSe2

Abstract In contrast to zinc-blende semiconductors, where the nonpolar (110) surface has the lowest energy, our first-principles calculations on the chalcopyrite semiconductor CuInSe 2 reveal that facets terminated by the (112)-cation and (112)-Se polar surfaces are lower in energy than the unfaceted (110) plane, despite the resulting increased surface area. This explains the hitherto puzzling existence of polar microfacets on nominally nonpolar (110) chalcopyrite surfaces. The extraordinary stability of these polar facets originates from the effective neutralization of surface charge by low-energy ordered Cu In antisite or Cu vacancy surface defects, while the relaxed but defect-free (112) surface is metallic and much higher in energy. We explain the low carrier density of the observed faceted surface in terms of autocompensation between opposite-polarity facets.