S. Nazir

High charge carrier density at the NaTaO3/SrTiO3 hetero-interface

The formation of a (quasi) two-dimensional electron gas between the band insulators NaTaO3 and SrTiO3 is studied by means of the full-potential linearized augmented plane-wave method of density functional theory. Optimization of the atomic positions points to only small changes in the chemical bonding at the interface. Both the p-type (NaO)−/(TiO2)0 and n-type (TaO2)+/(SrO)0 interfaces are found to be metallic with high charge carrier densities. The effects of O vacancies are discussed. Spin-polarized calculations point to the formation of isolated O 2p magnetic moments, located in the metallic region of the p-type interface.

Molecular distortion and charge transfer effects in ZnPc/Cu(111)

The adsorption geometry and electronic properties of a zinc-phthalocyanine molecule on a Cu(111) substrate are studied by density functional theory. In agreement with experiment, we find remarkable distortions of the molecule, mainly as the central Zn atom tends towards the substrate to minimize the Zn-Cu distance. As a consequence, the Zn-N chemical bonding and energy levels of the molecule are significantly modified. However, charge transfer induces metallic states on the molecule and therefore is more important for the ZnPc/Cu(111) system than the structural distortions.

Vacancy induced metallicity at the CaHfO3/SrTiO3 interface

Density functional theory is used to study the electronic properties of the oxide heterointerface CaHfO3/SrTiO3. Structural relaxation is carried out with and without O vacancies. As compared to related interfaces, strongly reduced octahedral distortions are found. Stoichiometric interfaces between the wide band gap insulators CaHfO3 and SrTiO3 turn out to exhibit an insulating state. However, interface metallicity is introduced by O vacancies, in agreement with experiment. The reduced octahedral distortions and necessity of O deficiency indicate a less complicated mechanism for the creation of the interfacial electron gas.

Band structure engineering and vacancy induced metallicity at the GaAs-AlAs interface

We study the epitaxial GaAs-AlAs interface of wide gap materials by full-potential density functional theory. AlAs thin films on a GaAs substrate and GaAs thin films on an AlAs substrate show different trends for the electronic band gap with increasing film thickness. In both cases, we find an insulating state at the interface and a negligible charge transfer even after relaxation. Differences in the valence and conduction band edges suggest that the energy band discontinuities depend on the growth sequence. Introduction of As vacancies near the interface induces metallicity, which opens great potential for GaAs-AlAs heterostructures in modern electronics.

The metallic interface between the two band insulators LaGaO3 and SrTiO3

The formation of metallic interface states between the two band insulators LaGaO3 and SrTiO3 is studied by the full-potential linearized augmented plane-wave method based on density functional theory. Structural optimization of the atomic positions points to only small changes of the chemical bonding at the interface. The n-type (LaO/TiO2) and p-type (GaO2/SrO) interfaces turn out to be metallic. Reduction of the O content increases the conductivity of the n-type interface, while the p-type interface can be turned gradually from a hole doped into an electron doped state.

High mobility of the strongly confined hole gas in AgTaO3/SrTiO3

A theoretical study of the two-dimensional hole gas at the (AgO)−/(TiO2)0 p-type interface in the AgTaO3/SrTiO3 (001) heterostructure is presented. The Ag 4d states strongly hybridize with the O 2p states and contribute to the hole gas. It is demonstrated that the holes are confined to an ultra thin layer (∼4.9A) with a considerable carrier density of ∼1014cm-2. We estimate a hole mobility of 18.6 cm2 V−1 s−1, which is high enough to enable device applications.

Suppression of the two-dimensional electron gas in LaGaO3/SrTiO3 by cation intermixing.

Cation intermixing at the n-type polar LaGaO3/SrTiO3 (001) interface is investigated by first principles calculations. Ti⇔Ga, Sr⇔La, and SrTi⇔LaGa intermixing are studied in comparison to each other, with a focus on the interface stability. We demonstrate in which cases intermixing is energetically favorable as compared to a clean interface. A depopulation of the Ti 3dxy orbitals under cation intermixing is found, reflecting a complete suppression of the two-dimensional electron gas present at the clean interface.

Role of the electronegativity for the interface properties of non-polar heterostructures

Density functional theory is used to investigate the interfaces in the non-polar ATiO3/SrTiO3 (A=Pb, Ca, Ba) heterostructures. All TiO2-terminated interfaces show an insulating behavior. By reduction of the O content in the AO, SrO, and TiO2 layers, metallic interface states develop, due to the occupation of the Ti 3d orbitals. For PbTiO3/SrTiO3, the Pb 6p states cross the Fermi energy. O vacancy formation energies depend strictly on the electronegativity and the effective volume of the A ion, while the main characteristics of the interface electronic states are maintained.

The interface of the ferromagnetic metal CoS2 and the nonmagnetic semiconductor FeS2

The electronic and magnetic properties of the cubic pyrite CoS2/FeS2 interface are studied using the all-electron full-potential linearized augmented plane wave method. We find that this contact between a ferromagnetic metal and a nonmagnetic semiconductor shows a metallic character. The CoS2 stays close to half-metallicity at the interface, while the FeS2 becomes metallic. The magnetic moment of the Co atoms at the interface slightly decreases as compared to the bulk value and a small moment is induced on the Fe atoms. Furthermore, at the interface ferromagnetic ordering is found to be energetically favorable as compared to antiferromagnetic ordering.