Disparities between the predicted and the measured effective work functions (EWFs) in advanced metal oxide semiconductor devices, or Fermi-level pinning (FLP), have gained significant attention when high-$k$ dielectrics began to emerge. Using a systematic approach for EWF extraction, combined with a comparison to unpinned SiO${}_{2}$ references, it was found that no intrinsic FLP exists in the model dielectric Al${}_{2}$O${}_{3}$. Extrinsic FLP was found with one of the metals investigated Ta, where a 0.4 eV increase in the EWF was observed in the electrical characteristics and confirmed by backside spectroscopy. The physical origins of the band offsets related to the EWF increase have been analyzed in detail. A 2-nm interfacial layer at the Ta-Al${}_{2}$O${}_{3}$ interface has been found and is suggested as the source of extrinsic FLP. Interfacial dipoles originating from Ta-O chemical bonds are considered as the mechanism responsible for the band offset. The results of this model system are then used to explain some of the peculiarities occurring at complex devices which are used in technological applications.

Band offsets and Fermi level pinning at metal-Al 2 O 3 interfaces

Ab-initio calculations based on density functional theory have been employed to study the structural, electronic, and optical properties of yttrium (Y), scandium (Sc), zirconium (Zr), and niobium (Nb) doped α-Al2O3 with corundum structure. Exchange and correlation effects between electrons have been treated by generalized gradient approximation within the Perdew-Burk-Ezrenhof parameterization and by recently developed Tran-Blaha modified Becke-Johnson approach. Most attention in the work has been paid to the impurity d states, whose energy splitting has been analyzed in terms of the crystal field theory and whose influence on the gap size and the offset of the bands around it has been carefully evaluated. The influence of these states on modification of the optical absorption edge and the static dielectric constant of the doped systems has been also studied. It is concluded that only the Y doped α-Al2O3 (1) preserves the size of the band gap of the pure alumina, (2) does not change significantly the band ...

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An ab-initio study of electronic and optical properties of corundum Al2O3 doped with Sc, Y, Zr, and Nb

The present study reports on the origins of room temperature ferromagnetism in zinc oxide (ZnO)-Al nanoparticles using a combination of X-ray absorption near edge structure (XANES) experiments and density functional theory (DFT) simulations. Our findings reveal that the spontaneous magnetization observed in these systems originates from the adsorption of Al on surfaces of ZnO nanoparticles. Our DFT simulations have identified unique configurations for Al adsorption on ZnO surfaces that lead to a spin-polarized charge transfer to O 2p states in surface and subsurface layers. XANES spectra of the magnetic ZnO/Al nanoparticles provide the necessary experimental evidence for the charge transfer to ZnO surfaces and confirm the origin of ferromagnetic behavior. Our results illustrate a complex interplay between the atomic level interfacial structure and the resulting ferromagnetic ordering in metal-coated semiconductor oxide nanostructures.

Surface charge transfer induced ferromagnetism in nanostructured ZnO/Al

A systematic alteration in the optical properties of W:Al2O3 nanocomposite films is demonstrated by precisely varying the W cycle percentage (W%) from 0 to 100% in Al2O3 during atomic layer deposition. The direct and indirect band energies of the nanocomposite materials decrease from 5.2 to 4.2 eV and from 3.3 to 1.8 eV, respectively, by increasing the W% from 10 to 40. X-ray absorption spectroscopy reveals that, for W% < 50, W is present in both metallic and suboxide states, whereas, for W% ≥ 50, only metallic W is seen. This transition from dielectric to metallic character at W% ∼ 50 is accompanied by an increase in the electrical and thermal conductivity and the disappearance of a clear band gap in the absorption spectrum. The density of the films increases monotonically from 3.1 g/cm3 for pure Al2O3 to 17.1 g/cm3 for pure W, whereas the surface roughness is greatest for the W% = 50 films. The W:Al2O3 nanocomposite films are thermally stable and show little change in optical properties upon annealing i...

W:Al2O3 Nanocomposite Thin Films with Tunable Optical Properties Prepared by Atomic Layer Deposition

Atomic structures and physical properties of Gd-doped alumina clusters—namely, GdAl2n–1O3n and Gd2Al2n–2O3n with n = 1–10—have been studied within the framework of spin-polarized density functional theory and the projector augmented wave pseudopotential method. We find that the atomic structures of the host clusters (Al2O3)n are not changed significantly by the substitutional doping of a Gd atom on Al sites. Our results show that in the size range of the clusters we studied, a Gd atom prefers a maximum 4-fold-coordinated Al-site, except for n = 8, in which a 5-fold-coordinated Al site is favored. The substitution of Al with Gd atom is energetically favorable. This is in contrast to the substitutional doping of Gd in the bulk alumina corundum structure that is endothermic. There is a net magnetic moment of 7 μB per Gd atom, which is mostly localized on the Gd atom. Further substitution of an Al atom with Gd in GdAl2n–1O3n clusters results in the lowest-energy configuration's being either ferromagnetic or a...

First Principles Calculations for Structural, Electronic, and Magnetic Properties of Gadolinium-Doped Alumina Clusters

We have investigated the electronic structure of transition-metal (TM=Sc, Y, Zr, and Nb)-doped Al2O3 by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) In valence bands of these TM-doped Al2O3 measured by XPS, the highest occupied levels and the shapes of valence bands are almost unchanged from the pure alumina On the other hand, XAS spectra obtained at the oxygen K-edge show that Nb- and Zr-doped Al2O3 show localized d states in the bandgap below the conduction band minimum, while Y- and Sc-doped Al2O3 have d states inside the conduction band of the original Al2O3 This implies that Y- and Sc-doped Al2O3 will show little bandgap degradation and maintain the same band offset as silica at the Si interface, and can serve as promising candidates for an alternative gate oxide

Jong-Cheol Lee

Papers

Bandgap states in transition-metal (Sc, Y, Zr, and Nb)-doped Al2O3