A. Baldereschi

Band engineering at interfaces: theory and numerical experiments

Understanding the mechanisms which determine the band offsets and Schottky barriers at semiconductor contacts and engineering them for specific device applications are important theoretical and technological challenges. In this review, we present a theoretical approach to the band-line-up problem and discuss its application to prototypical systems. The emphasis is on ab initio computations and on theoretical models derived from first-principles numerical experiments. An approach based on linear-response-theory concepts allows a general description of the band alignment for various classes of semiconductor contacts and predicts the effects of various bulk and interfacial perturbations on the band discontinuities.

Deriving accurate work functions from thin-slab calculations

Quantum-size effects have been shown to influence significantly the determination of work functions from thin-slab calculations. We show here that a technique based on macroscopic averages can be used to reduce such effects and determine more precisely the work functions of metals from ab initio thin-film calculations. The technique combines the mean electrostatic potential step across the slab surface with the Fermi energy of a bulk crystal. The method is applied to Al(100) slabs containing 1-14 atomic layers.

Carbon Dioxide Hydrogenation on Ni(110)

We demonstrate that the key step for the reaction of CO 2 with hydrogen on Ni(110) is a change of the activated molecule coordination to the metal surface. At 90 K, CO 2 is negatively charged and chemically bonded via the carbon atom. When the temperature is increased and H approaches, the H-CO 2 complex flips and binds to the surface through the two oxygen atoms, while H binds to the carbon atom, thus yielding formate. We provide the atomic-level description of this process by means of conventional ultrahigh vacuum surface science techniques combined with density functional theory calculations and corroborated by high pressure reactivity tests. Knowledge about the details of the mechanisms involved in this reaction can yield a deeper comprehension of heterogeneous catalytic organic synthesis processes involving carbon dioxide as a reactant. We show why on Ni the CO 2 hydrogenation barrier is remarkably smaller than that on the common Cu metal-based catalyst. Our results provide a possible interpretation of the observed high catalytic activity of NiCu alloys.

Band structure and instability of Ge1−xSnx alloys

Abstract Self-consistent ab initio pseudopotential calculations for Ge1−xSnx alloys in the Virtual Crystal Approximation (VCA) and for zincblende (zb-) GeSn clearly show their instability against phase segregation at zero temperature. The GeSn bond is studied and compared to SiC and SiGe. The non-relativistic LDA band structures of VCAGe.5Sn.5 and zbGeSn are presented and show strong similarity. The resulting energy gaps for all χ are corrected empirically and prove the existence of a “direct gap window” for .26 ≤ χ ≤ .74, with a maximum energy gap of 0.61eV which gradually decreases to zero at χ = .74.

Band structure of semiconductor alloys beyond the virtual crystal approximation. effect of compositional disorder on the energy gaps in GaPxAs1−x

Abstract A general method to study the band structure of compositionally disordered semiconductors is proposed. The effects of chemical disorder are added to the virtual-crystal band structure using the pseudopotential method and second order perturbation theory. In GaPxAs1−x, chemical disorder is shown to give a significant contribution to the experimentally observed non-linear behaviour of the lowest energy gaps as function of composition.

STRUCTURAL MODELS OF AMORPHOUS CARBON SURFACES

Using tight-binding molecular dynamics, we have constructed structural models of amorphous carbon surfaces, by imposing tensile strain on computer generated networks containing 512 carbon atoms until fracture is produced and two surfaces are formed. Different tensile strains are applied along different directions, in order to mimic the effect of preparation conditions. The surfaces generated with different strains from networks with a given bulk atomic density, exhibit similar structures and formation energies differ by a few 0.01 eV/A2. Surface roughness increases with the amount of the graphitic component in the bulk sample. The calculated surface thicknesses are consistent with recent experimental data.

Schottky barrier heights at polar metal/semiconductor interfaces

Using a first-principle pseudopotential approach, we have investigated the Schottky barrier heights of abrupt Al/Ge, Al/GaAs, Al/AlAs, and Al/ZnSe (100) junctions, and their dependence on the semiconductor chemical composition and surface termination. A model based on linear-response theory is developed, which provides a simple, yet accurate description of the barrier-height variations with the chemical composition of the semiconductor. The larger barrier values found for the anion-terminated surface than for the cation-terminated surface are explained in terms of the screened charge of the polar semiconductor surface and its image charge at the metal surface. Atomic-scale computations show how the classical image charge concept, valid for charges placed at large distances from the metal, extends to distances shorter than the decay length of the metal-induced-gap states.

Schottky barrier tuning at Al/GaAs(100) junctions

Using an ab initio pseudopotential approach, we have investigated the electronic structure of ideal Al/GaAs(100) and Al/Ga1−xAlxAs(100) junctions, and the change of the corresponding Schottky barrier height versus the alloy composition x and in the presence of ultrathin group‐IV atom interlayers. We find large changes in the Schottky barrier height which agree well with the experimental data. In order to interpret the observed trends we have analyzed the charge density response to chemical substitutions near the junction. This allowed us to extend to metal/semiconductor interfaces a microscopic linear‐response theory approach previously employed to interpret band‐offset trends at semiconductor heterojunctions.

Can We Tune the Band Offset at Semiconductor Heterojunctions

The long-standing problem of determining which interface-specific properties affect the band offset at semiconductor heterojunctions is readdressed using a newly developed theoretical approach. The actual interface is considered as a perturbation with respect to a reference periodic system (virtual crystal). By comparison with state-of-the-art self-consistent calculations, we show that linear-response theory provides a very accurate description of the electronic structure of the actual interface in a variety of cases, and sheds light on the mechanisms responsible for the band offset. Results are presented for a number of lattice-matched junctions, both isovalent and heterovalent. It is shown that—within linear response theory—band offsets are genuine bulk properties for isovalent interfaces, whereas they do depend on the atomic structure of the junction for polar interfaces between heterovalent semiconductors. In the latter case, however, the interface-dependent contribution to the offset can be calculated—once the microscopic geometry of the junction is known—from such simple quantities as the lattice parameters and dielectric constants of the constituents. Perspectives for extending the theory to non-lattice-matched systems are also briefly discussed.

Maximally localized Wannier functions in antiferromagnetic MnO within the FLAPW formalism

We have calculated the maximally localized Wannier functions of MnO in its antiferromagnetic (AFM) rhombohedral unit cell, which contains two formula units. Electron Bloch functions are obtained with the linearized-augmented-plane-wave method within both the local-spin density (LSD) and the LSD+U schemes. The thirteen uppermost occupied spin-up bands correspond in a pure ionic scheme to the five Mn 3d orbitals at the Mn-1 (spin-up) site and the four O 2s/2p orbitals at each of the O-1 and O-2 sites. Maximal localization identifies uniquely four Wannier functions for each O, which are trigonally distorted sp(3)-like orbitals. They display a weak covalent bonding between O 2s/2p states and minority-spin d states of Mn-2, which is absent in a fully ionic picture. This bonding is the fingerprint of the interaction responsible for the AFM ordering, and its strength depends on the one-electron scheme being used. The five Mn Wannier functions are centered on the Mn-1 site, and are atomic orbitals modified by the crystal field. They are not uniquely defined by the criterion of maximal localization and we choose them as the linear combinations that diagonalize the r(2) operator, so that they display the D-3d symmetry of the Mn-1 site.