N. Barrett

HfO2/SiO2 interface chemistry studied by synchrotron radiation x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy using synchrotron radiation has been used to investigate the HfO2/SiO2 interface chemistry of high-quality 0.6 and 2.5 nm HfO2/0.6 nm SiO2/Si structures. The high energy resolution (0.15 eV) along with the high brightness level allows us to separate, unambiguously, on both Hf 4f and Si 2p core-level spectra, interfacial Hf–silicate bonds from bulk HfO2 and SiO2 contributions, thus making possible subsequent quantitative treatments and modeling of the interfacial layer structure. Careful assessment of the energy shift of the interfacial components shows that Si-rich Hf silicates are present. The underlying assumption that initial-state contribution dominates the observed Si 2p shift is briefly discussed.

Unraveling the conduction mechanism of Al-doped ZnO films by valence band soft x-ray photoemission spectroscopy

We report on the correlation between the electrical behavior and valence band spectra of undoped and Al-doped ZnO films, obtained by using x-ray photoelectron spectroscopy. Although Al-doping can induce a conductivity increase of two orders of magnitude, we show that the gap persists and there is no semiconductor–metal transition upon doping. For the 3% Al-doped ZnO film, we measure a reduction in the band gap of ∼150meV with respect to the undoped and the 1% doped films. Our results suggest that the band conduction mechanism proposed for undoped ZnO at room temperature still dominates the conduction process in doped films.

High-resolution photoelectron spectroscopy of Ge-based HfO2 gate stacks

High-resolution photoelectron spectroscopy with synchrotron radiation (energy resolution of 50meV) is used to investigate interfacial properties of Ge∕GeOx (1nm)∕HfO2 (1nm) gate stacks. With soft x rays, a reliable Ge3d core-level study is possible thanks to the much lower cross section of the Hf5p core level than that using AlKα radiation. It is clearly shown that Hf-germanate bonding states are formed at the GeOx∕HfO2 interface, with an additional Ge3d spectral component shifted to lower binding energy relative to GeO2.

Thermal stability of the HfO2∕SiO2 interface for sub-0.1μm complementary metal-oxide-semiconductor gate oxide stacks: A valence band and quantitative core-level study by soft x-ray photoelectron spectroscopy

Synchrotron-radiation photoelectron spectroscopy is used to study the valence-band structure and the core-level photoemission spectra of HfO2 ultrathin films grown onto SiO2∕Si substrates by atomic layer deposition (ALD). We determine the band offsets (valence and conduction) of HfO2 to Si as a function of postdeposition annealing treatments (under an inert N2 atmosphere or in situ in ultrahigh vacuum) and find a significant evolution, the conduction-band offset remaining larger than 1.5eV. The Si2p and the Hf4f core-level spectra give detailed information on the composition and the spatial extent of the interfacial Hf silicate layer formed between the SiO2 bottom oxide and the HfO2 ALD thin film. By a quantitative treatment of the Si2p core-level intensities, we examine the thermal stability of the interface silicate after postdeposition annealing under N2 and in situ annealing in ultrahigh vacuum (UHV), both at 800°C. The as-deposited layer gives rise to a HfO2∕Hf0.35Si0.65O2∕SiO2 stack with correspondi...

Thickness-dependent polarization of strained BiFeO3 films with constant tetragonality.

We measure the ferroelectric polarization of BiFeO3 films down to 3.6 nm using low energy electron and photoelectron emission microscopy. The measured polarization decays strongly below a critical thickness of 5-7 nm predicted by continuous medium theory whereas the tetragonal distortion does not change. We resolve this apparent contradiction using first-principles-based effective Hamiltonian calculations. In ultrathin films, the energetics of near open circuit electrical boundary conditions, i.e., an unscreened depolarizing field, drive the system through a phase transition from single out-of-plane polarization to nanoscale stripe domains. It gives rise to an average polarization close to zero as measured by the electron microscopy while maintaining the relatively large tetragonal distortion imposed by the nonzero polarization state of each individual domain.

Direct quantification of gold along a single Si nanowire.

The presence of gold on the sidewall of a tapered, single silicon nanowire is directly quantified from core-level nanospectra using energy-filtered photoelectron emission microscopy. The uniform island-type partial coverage of gold determined as 0.42+/-0.06 (approximately 1.8 ML) is in quantitative agreement with the diameter reduction of the gold catalyst observed by scanning electron microscopy and is confirmed by a splitting of the photothresholds collected from the sidewall, from which characteristic local work functions are extracted using a model of the full secondary electron distributions.

Extrinsic screening of ferroelectric domains in Pb(Zr0.48Ti0.52)O3

The variation in the surface potential as a function of the ferroelectric polarization of micron scale domains in a thin epitaxial film of Pb(Zr0.48Ti0.52)O3 is measured using mirror electron microscopy. Domains were written using piezoforce microscopy. The surface potential for each polarization was deduced from the mirror to low energy electron microscopy transition in the local reflectivity curve. The effect of extrinsic screening of the fixed polarization charge at the ferroelectric surface is demonstrated. The results are compared with density functional theory calculations.

Screening of ferroelectric domains on BaTiO3(001) surface by ultraviolet photo-induced charge and dissociative water adsorption

The surface potential contrast between oppositely polarized ferroelectric domains of a BaTiO3(001) single crystal under ultraviolet (UV) illumination before and after the dissociative adsorption of water is measured using the transition from mirror electron microscopy to low-energy electron microscopy. Both photo-generated free charge carriers and dissociative adsorption of water are effective screening mechanisms of the surface polarization charge. The screening rate is dominated by drift, whereas the relaxation in the absence of UV light is driven by thermal diffusion. Surface chemistry plays an important role in the surface charge dynamics.

Orientation-dependent work function of in situ annealed strontium titanate.

We have used energy-filtered x-ray photoelectron emission microscopy (XPEEM) and synchrotron radiation to measure the grain orientation dependence of the work function of a sintered niobium-doped strontium titanate ceramic. A significant spread in work function values is found. Grain orientation and surface reducing/oxidizing conditions are the main factors in determining the work function. Energy-filtered XPEEM looks ideally suited for analysis of other technologically interesting polycrystalline samples.

Interface electronic structure in a metal/ferroelectric heterostructure under applied bias

The effective barrier height between an electrode and a ferroelectric (FE) depends on both macroscopic electrical properties and microscopic chemical and electronic structure. The behavior of a prototypical electrode/FE/electrode structure, Pt/BaTiO