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An atomistic model of substoichiometric germanium oxide is generated through ab initio molecular dynamics. The resulting structure shows a predominance of threefold coordinated Ge and O atoms. We also generate substoichiometric models through bond-switching Monte-Carlo simulations, which preserve the fourfold Ge and the twofold O coordinations. These differing structures are energetically competitive. Alignment of their electron densities of states to that of GeO"2 reveals that the band-gap reduction is similar for both structures, mainly occurring through a shift of the valence band edge.

Models of amorphous (HfO"2)"x (SiO"2)"1"-"x for varying hafnium content x are generated by ab initio molecular dynamics. The structural properties are analyzed in terms of pair distribution functions from which typical bond lengths and coordination numbers are extracted. Electronic band gaps are calculated with a hybrid density functional and are found to decrease in a nonlinear way with increasing x, in accord with experimental observations. Static dielectric constants are evaluated through the application of a finite electric field. The calculated values show a linear dependence on x, supporting recent experimental data.

Experimental characterization is facing severe difficulties in elucidating the nature of defects at interfaces between silicon and amorphous high-@k oxides. This motivates recourse to first principle simulation approaches based on density functional theory. A major issue for these approaches is the choice of the model structure. Indeed, the most trivial choice of parent crystalline structures may fail in describing the amorphous nature of the oxide or the specific bonding arrangements occurring at the interface. A second critical issue relates to the determination of energy levels with respect to band edges of the semiconductor and the oxide, which often represent the only contact with experimental data. An improved description of the electronic structure is required going beyond standard density functional approaches that severely underestimate electronic band gaps. Here, these issues are illustrated by a comparative study of the oxygen vacancy in monoclinic HfO"2, in amorphous HfO"2, and at the Si-HfO"2 interface.

Using density functional calculations, we studied substoichiometric Ge oxides through a series of crystalline models representing the various oxidation states. We evaluated deviations with respect to a bond-energy description in terms of penalty energies for intermediate oxidation states of Ge. Corresponding bond length variations and band gaps are also given. The calculated penalty energies are significantly lower than for Si indicating higher stability of the substoichiometric phase at Ge/GeO2 interfaces compared to Si/SiO2 interfaces. (C) 2009 Elsevier B.V. All rights reserved.

An atomistic model of the Ge-GeO2 interface has been generated through first-principle methods based on density functional theory. The interface model consists of amorphous GeO2 connected to crystalline Ge through a substoichiometric oxide region showing regular structural parameters. Structural and electronic properties are compared to available experimental data and studied as they evolve across the Ge-GeO2 interface. (C) 2009 Elsevier B.V. All rights reserved.