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By means of a joint experimental and theoretical approach we provide the first single molecule imaging and characterization of CO2 on Ni(110), chemisorbed with high charge transfer from the substrate in an activated state that plays a crucial role in the hydrogenation process. Low temper- ature scanning tunneling microscopy images of single CO2 molecules are combined with ab-initio density functional theory calculations, where dispersive forces and zero point energy estimates have been explicitly accounted for. We obtain a detailed characterization of the adsorption geometries and an estimate of the energies corresponding to the different adsorbed states. A consistent pic- ture of CO2 chemisorption on Ni(110) is provided on the basis of the newly available information, yielding a deeper insight into the previously existing spectroscopic and theoretical data.

We compute within the effective-mass theory and without adjustable parameters the Stark effect for shallow P donors in Si with anisotropic band structure. Valley-orbit coupling is taken into account in a nonperturbative way and scattering effects of the impurity core are included to properly describe low-lying impurity states. The ground-state energy slightly decreases with increasing electric field up to a critical value ${E}_{\mathit{cr}}\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{cm}$, at which the donor can be ionized by tunneling due to a field-induced mixing of the ``$1s$-like'' singlet ground state with a ``$2{p}_{0}$-like'' excited state in zero field. The resulting ground-state wave function at high field extends significantly outside the potential barrier surrounding the impurity. Calculations of the hyperfine splitting and of the $A$-shell superhyperfine coupling constants as a function of the electric field complete the work.

Ab initio calculations combined with a recently developed macroscopic-averaging technique have been used to examine the anisotropy of the aluminum work function. Our results indicate that the Al(111) work function is substantially lower than the Al(100) work function, and slightly below the Al(110) work function. This is in contrast to other fcc metals, for which a $(110)\ensuremath{\rightarrow}(100)\ensuremath{\rightarrow}(111)$ increasing trend is observed experimentally and predicted by Smoluchowski's rule. We show that the aluminum work-function anomaly, which is in agreement with experiment, is due to the increased $p$-atomic-like character of the density of states at the Fermi energy, as compared to most other fcc metals.

We investigated the energetics as well as the structural and electronic properties of chemically abrupt $\mathrm{A}\mathrm{l}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}(100)$ and engineered $\mathrm{A}\mathrm{l}/\mathrm{S}\mathrm{i}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}(100)$ Schottky diodes with 2 monolayers of Si using the ab initio pseudopotential approach. Results for the atomic relaxation at the interface, its influence on the Schottky barrier, and the dependence of the formation energy on the As and Ga chemical potentials are presented. We show that a reversal in the relative stability of the As- and Ga-terminated junctions occurs within the experimentally accessible range of the As and Ga chemical potentials, for both $\mathrm{A}\mathrm{l}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ and $\mathrm{A}\mathrm{l}/\mathrm{S}\mathrm{i}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ interfaces. This reversal could explain the role played by the excess cation and anion flux in the fabrication of tunable $\mathrm{A}\mathrm{l}/\mathrm{S}\mathrm{i}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}(100)$ Schottky diodes.

Al/ZnSe(100) Schottky barriers fabricated on $c(2\ifmmode\times\else\texttimes\fi{}2),$ $2\ifmmode\times\else\texttimes\fi{}1,$ and $1\ifmmode\times\else\texttimes\fi{}1$ reconstructed surfaces were studied by means of photoemission spectroscopy and first-principles calculations. Relatively similar values of the Schottky barriers were found for interfaces fabricated on Zn-stabilized $c(2\ifmmode\times\else\texttimes\fi{}2)$ and Se-dimerized $2\ifmmode\times\else\texttimes\fi{}1$surfaces, while substantially lower values of the $p$-type barriers were predicted theoretically and observed experimentally for junctions grown on the Se-rich $1\ifmmode\times\else\texttimes\fi{}1$ surface.

We present empirical tight-binding and ab initio pseudopotential calculations of the electronic states in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/(\mathrm{AlAs})}_{n}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ (100) structures with $n=1$ and 2. For $n=1$, we find that a resonant state occurs in the valence-band at the GaAs edge. The existence of this state, which cannot be accounted for in an effective-mass picture, is due to the nonlocal angular-momentum-dependent potential describing the single-atom substitution ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}:\mathrm{A}\mathrm{l}}_{\mathrm{Ga}}.$ The $p$ component of this potential is repulsive for the GaAs electrons, consistently with the local effective-mass description. However, its $d$ component is attractive and is responsible for the occurrence of the resonant state at the GaAs valence-band edge. For $n=2$ the resonant state occurs at lower energy than in the $n=1$ case (the tight-binding model predicts the resonance 26 meV below the GaAs valence-band edge) and its coupling with the continuum is stronger. The variation of the energy position of this resonance with the thickness of ${(\mathrm{AlAs})}_{n}$ layers could be responsible for the large differences recently observed in the luminescence spectra of $n=1$ and 2 structures.

We present a comparison between the predictions of two theoretical models and experimental results on ultrathin GaAs layers with a thickness in the range from 1 to 8 ML embedded in bulk (AlxGa1-x)As, 0.30 less than or equal to x less than or equal to 0.34. The theoretical predictions, obtained from an empirical tight-binding Green's-function approach and from the usual effective-mass approximation, are compared to photoluminescence and photoluminescence excitation data of a set of multiple quantum-well samples grown by metal-organic vapor-phase epitaxy on substrates with different misorientations. We find that the optical transitions are narrower in samples with slightly misoriented substrates, although their spectral position remains unchanged. This suggests that the substrate misorientation favors a good quality of the ternary alloy. The observed optical transitions of our thin layers compare well with the predictions of both models. However, the lack of a reliable exciton theory for such ultrathin layers becomes apparent.

We calculate the quasiparticle band structure of NiO and ${\mathrm{CaCuO}}_{2}$ by using a model self-energy correction which approximates closely the GW method of Hedin. We obtain energy gaps and magnetic moments in agreement with experiment. For NiO, our results agree with integrated and angle-resolved photoemission experiments in the low binding energy region. The spectral distributions of O 2p and Ni 3d states are discussed, in relationship with the available experimental data. Our results demonstrate that the addition of self-energy corrections to local-spin-density band calculations provides a meaningful description of many aspects of the electronic states of transition-metal oxides. As expected, the satellite structures observed in photoemission experiments are not found in our calculations.

The AlN/SiC band discontinuities in zinc-blende ~110!, ~111!, and wurtzite ~0001! heterostructures were examined using the ab initio pseudopotential approach. At the nonpolar AlN/SiC~110! junction, we find a valence-band offset of 1.7 eV. At the polar heterojunctions the band alignment depends on the interface composition, and valence-band offsets as high as 2.5 eV are obtained for neutral interfaces with an Al/Si mixed plane and as low as 1.3 eV with a N/C mixed plane. Atomic relaxation plays a major role in determining the offset. The structural change from cubic ~111! to hexagonal ~0001! heterojunctions affects predominantly the conduction-band offset, and has only a minor influence on the valence discontinuity. @S0163-1829~97!50612-6# The group-III nitrides and SiC are of considerable interest for device applications in blue-violet optoelectronics and high-temperature electronics. The ideal lattice match of SiC and AlN, and the relatively low misfit with GaN, make SiC a suitable substrate and AlN an adequate buffer layer for GaNbased light-emitting devices. 1 The main parameters which determine carrier injection and confinement in the resulting heterostructures are the valence- and conduction-band offsets. Presently, very few experimental 2 or theoretical results 3‐5 are available on the band offsets of heterovalent nitride heterojunctions, and little is known of their dependence on orientation, heterovalency, and polytype. Here we present a first-principles study of these properties for the prototype lattice-matched AlN/SiC system. Heterovalent quaternary systems such as AlN/SiC give rise to complex interface configurations. Two types of abrupt AlN/SiC interfaces may be distinguished: those containing only cation-anion bonds ~Si-N,C-Al! and those involving cation-cation and/or anion-anion bonds ~C-N,Si-Al!. Previous theoretical studies 3,4 showed that the former are more stable than the latter, consistent with a recent transmissionelectron microscopy analysis of the 2H-AlN/6H-SiC~0001!

The dependence on hydrostatic pressure and alloy composition of the Schottky-barrier height at ideal, defect-free, $\mathrm{Al}/{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}(100)$ junctions is investigated by means of an ab initio pseudopotential approach. The results reproduce closely the experimental data and, in contrast to recent proposals, demonstrate that the barrier heights can be explained without invoking interface defects. The alloy-composition dependence is understood by extending to metal/semiconductor contacts the linear-response-theory approach currently used for semiconductor heterojunctions. The pressure variation of the barriers can be obtained from the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ band-edge deformation potentials.

We calculate ab initio the electronic states, spontaneous polarization P, and Born dynamical charge ${\mathit{Z}}^{\mathrm{*}}$ of ZnO, using the local-density approximation (LDA), the Hartree-Fock (HF), and a model GW scheme. Upon going from HF to GW and to LDA, the d bands raise substantially in energy, the model GW providing the best overall agreement with experiment. By contrast, the three schemes give the concordant values P=-0.047 C/${\mathrm{m}}^{2}$ and ${\mathit{Z}}^{\mathrm{*}}$=\ifmmode\pm\else\textpm\fi{}2.1, in agreement with available experimental data. While the value of ${\mathit{Z}}^{\mathrm{*}}$ suggests a rigid-ion-like behavior of ZnO, its band-by-band decomposition reveals anomalous contributions from O 2s, Zn 3d, and O 2p bands, indicating a substantial interaction between the corresponding occupied atomic orbitals.

In ferroelectric perovskites, the strong spontaneous polarization is due to giant values of the Born effective charges, which are much larger than predicted on the basis of the nominal ionicity: we investigate the physical origin of such charges, which by definition measure the current traversing the sample during polarization reversal. Our first-principles calculations for ${\mathrm{KNbO}}_{3}$ identify unambiguously the dominant mechanism with the presence of covalent bonding. The giant charges are reduced to their nominal values in a computational experiment performed on a fake material, where covalence has been artificially suppressed.

We present a scheme for calculating the (spin-unrestricted) Hartree-Fock (HF) band structure of periodic solids that uses the accurate linearized-augmented-plane-wave basis set. In contrast with linear-combination-of-atomic-orbitals-like schemes, the convergence of the variational HF results can be easily monitored, and the cumbersome evaluation of multicenter integrals is avoided. Potentials and charge densities are evaluated without any shape approximation, and the singularity due to the long-range nature of the Coulomb potential is handled in reciprocal space. All the elements of the Periodic Table can be equally treated, and the relativistic effects for heavy elements are included as in the standard local-density-approximation case. The method is tested on silicon and diamond, where recent HF calculations are available for comparison. The diagonal Coulomb-hole-plus-screened-exchange approximation can also be implemented with little additional computational effort.

Ground-state properties of ${\mathrm{CaCuO}}_{2}$, a parent compound of the high-${\mathit{T}}_{\mathit{c}}$ superconductors, are calcula- ted ab initio using a LAPW spin-unrestricted Hartree-Fock (UHF) scheme. While ${\mathrm{CaCuO}}_{2}$ is a metal within the local-spin density approximation, UHF retains the essential nonlocality of exchange, and predicts a wide gap insulator with an antiferromagnetic moment \ensuremath{\mu}\ensuremath{\approxeq}0.8${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. Results for hypothetical ferromagnetic ${\mathrm{CaCuO}}_{2}$ indicate that the insulating character does not depend on the magnetic ordering. The shortcomings of UHF are corrected in part with a simple diagonal-screening model.

The atomic and electronic structures of the $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ (001) surface are studied using the density-functional theory approach. From calculated surface energies for different surface terminations, we identified a mixed dense Al-O (001) layer, containing both octahedral aluminum and oxygen atoms, as the most stable $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ (001) surface. We found that environmental ${\text{O}}_{2}$ gas does not affect the surface stoichiometry. Comparison of the electronic structure of the surface and the bulk $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ shows that the band gap at the surface is slightly smaller than that of the bulk $\ensuremath{\gamma}$ alumina due to the contributions of the surface $\text{O}\text{ }2p$ states.

Les calculs des niveaux d'energie accepteurs dans GaAs montrent que les masses effectives des trous couramment utilisees sont en desaccord avec les donnees spectroscopiques concernant les accepteurs peu profonds. Ces donnees spectroscopiques ainsi que la plupart des donnees de la spectroscopie des puits quantiques et de la resonance cyclotron sont expliquees par l'ensemble des parametres γ 1 =7,10±0,15; γ 2 =2,02±0,15 et γ 3 =2,91×0,10; qui correspond a une anisotropie de la bande de valence superieure a celle rapportee dans les etudes magneto-optiques interbandes. On deduit egalement les energies d'ionisation des niveaux accepteurs pour C, Be, Mg, Zn, Si, et Cd

On considere les trois principales orientations cristallographiques ((001), (110) et (111)) de ces heterojonctions. Analyse des resultats selon une approche de reponse lineaire dans laquelle l'interface est consideree comme une perturbation dans un cristal periodique moyenne. Description des effets des contraintes interfaciales et des autoenergies induites par le desordre et la correlation electronique

The dynamical Born charge tensor of brookite $\mathrm{Ti}{\mathrm{O}}_{2}$ has been calculated and analyzed in terms of the Wannier functions associated with the O $2s∕2p$ valence states. It is shown that the fundamental $\mathrm{O}{\mathrm{Ti}}_{3}$ triangular complex, whose opposite distortion in rutile and anatase explains the anisotropy of the Born charge in these polymorphs, is also the relevant building block for understanding this quantity in brookite. This compound consists of an equal number of distorted rutilelike and anataselike $\mathrm{O}{\mathrm{Ti}}_{3}$ structural units, which are nonplanar due to the low ${C}_{1}$ site symmetry of oxygen atoms. Born charge tensors associated with the two nonequivalent O sites correspond in terms of values and anisotropies to those for rutile and anatase, and, consistent with the acoustic sum rule, the Ti charge tensor is an ``average'' of those for the two O types. The intermediate character of brookite, between rutile and anatase, should be expected also for other physical properties.

The bonding Wannier functions (WF's) associated with the O 2s/2p valence states of TiO 2 have been calculated for both the futile and the anatase forms. They are similar in the two polymorphs, and correspond to three distorted O sp 2 -like hybrids in the plane of the Ti 3 coordination triangle, and one p-like orbital perpendicular to this plane. For both structures, the WF's show that the OTi 3 triangular complex is the relevant structural unit for understanding TiO 2 electronic properties. WF's differences between the two phases are quantitatively measured with a multipole expansion of the static charge density of the fourfold WF complex. These differences reflect the opposite distortion from equilateral shape that the isosceles Ti 3 triangle has in the two structures, and correlate quantitatively with the opposite anisotropy of the dynamical Bom charge tensors calculated for the two polytypes.

The electronic charge densities and work functions at sharp metallic facet edges are determined from ab initio calculations, combined with macroscopic averaging techniques. In particular, we examine how two different work functions coexist at close range near edges between inequivalent facets. The surface ionic relaxation at facet edges is shown to influence appreciably the local electrostatic potential in the vacuum. Various edges between Al(100) and Al(111) facets are studied, as well as between Na(110) facets. We also develop a model of electronic surface dipoles, which accounts for the surface charge transfer between inequivalent facets, and which allows us to predict the influence of the shape and size of a macroscopic crystal on its work functions.

Using a combined experimental and theoretical approach, we address C 1s core-level shifts in amorphous carbon. Experimental results are obtained by x-ray photoelectron spectroscopy (XPS) and electron-energy-loss spectroscopy (EELS) on thin-film samples of different atomic density, obtained by a pulsed-laser deposition growth process. The XPS spectra are deconvoluted into two contributions, which are attributed to sp(2)- and sp(3)-hybridized atoms, respectively, separated by 0.9 eV, independent or atomic density, The sp(3) hybridization content extracted from XPS is consistent with the atomic density derived from the plasmon energy in the EELS spectrum. In our theoretical study, we generate several periodic model structures of amorphous carbon of different densities applying two schemes of increasing accuracy in sequence. We first use a molecular-dynamics approach, based on an environmental-dependent tight-binding Hamiltonian to quench the systems from the liquid phase. The final model structures are then obtained by further atomic relaxation using a first-principles pseudopotential plane-wave approach within density-functional theory. Within the latter framework. we also calculate carbon 1s core-level shifts for our disordered model structures. We find that the shifts associated to threefold- and fourfold- coordinated carbon atoms give rise to two distinct peaks separated by about 1.0 eV. independent of density, in close agreement with experimental observations. This provides strong support for decomposing the XPS spectra into two peaks resulting from sp(2)- and sp(3)-hybridized atoms. Core-hole relaxations effects account for about 30% of the calculated shifts.

From first principles we have examined the band offsets of selected zinc-blende, wurtzite, and mixed zinc-blende/wurtzite GaN/AlN, GaN/SiC, and AlN/SiC heterostructures, and their dependence on various structural and chemical properties of the interfaces. Contrary to the case of the conventional, small-gap, semiconductor heterojunctions, local atomic interfacial relaxation has a major influence on the offsets of the polar heterovalent nitride systems. However, provided this effect is taken into account, the band-offset dependence on interface orientation, strain, and heterovalency can still be qualitatively explained using linear-response-theory schemes. We also show that a change in the band-gap discontinuity resulting from a cubic (111) to hexagonal (0001) polytype transformation in nitride heterostructures will be selectively found in the conduction-band offset.

Zn/ZnSe(001) interfaces fabricated by metal deposition at room temperature onto ZnSe(001) c(2{times}2), 2{times}1, and 1{times}1 surfaces were studied by means of x-ray photoemission spectroscopy and current-voltage and capacitance-voltage measurements. All junctions exhibited an ideal unreactive behavior and an identical Schottky barrier height of 1.85 eV (for p-type conduction). Ab initio pseudopotential calculations for model interface configurations provide a microscopic explanation of the different behavior of Zn/ZnSe junctions as compared to Al/ZnSe and Au/ZnSe junctions.