153 research outputs found
Interface Engineering to Create a Strong Spin Filter Contact to Silicon
Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on
silicon is a perfect route to enrich silicon nanotechnology with spin filter
functionality.
To date, the inherent chemical reactivity between EuO and Si has prevented a
heteroepitaxial integration without significant contaminations of the interface
with Eu silicides and Si oxides.
We present a solution to this long-standing problem by applying two
complementary passivation techniques for the reactive EuO/Si interface:
() an hydrogen-Si passivation and () the
application of oxygen-protective Eu monolayers --- without using any additional
buffer layers.
By careful chemical depth profiling of the oxide-semiconductor interface via
hard x-ray photoemission spectroscopy, we show how to systematically minimize
both Eu silicide and Si oxide formation to the sub-monolayer regime --- and how
to ultimately interface-engineer chemically clean, heteroepitaxial and
ferromagnetic EuO/Si in order to create a strong spin filter contact to
silicon.Comment: 11 pages of scientific paper, 10 high-resolution color figures.
Supplemental information on the thermodynamic problem available (PDF).
High-resolution abstract graphic available (PNG). Original research (2016
Limitations of Near Edge X Ray Absorption Fine Structure as a tool for observing conduction bands in chalcopyrite solar cell heterojunctions
A non optimized interface band alignment in a heterojunctionbased solar cell can have negative eff ects on the current and voltage characteristics of the resulting device. To evaluate the use of Near Edge X ray Absorption Fine Structure spectroscopy NEXAFS as a means to measure the conduction band position, Cu In,Ga S2 chalcopyrite thin film surfaces were investigated as these form the absorber layer in solar cells with the structure ZnO Buffer Cu In,Ga S2 Mo Glass. The composition dependence of the structure of the conduction bands of CuInxGa1 xS2 has been revealed for x 0, 0.67 and 1 with both hard and soft NEXAFS and the resulting changes in conduction band off set at the junction with the bu ffer layer discussed. A comprehensive study of the positions of the absorption edges of all elements was carried out and the development of the conduction band with Ga content was observed, also with respect to calculated densities of state
A high resolution, hard x-ray photoemission investigation of La_(2-2x)Sr_(1+2x)Mn_2O_7 (0.30<x<0.50): on microscopic phase separation and the surface electronic structure of a bilayered CMR manganite
Photoemission data taken with hard x-ray radiation on cleaved single crystals
of the bilayered, colossal magnetoresistant manganite La_(2-2x)Sr_(1+2x)Mn_2O_7
(LSMO) with 0.30<x<0.50 are presented. Making use of the increased
bulk-sensitivity upon hard x-ray excitation it is shown that the core level
footprint of the electronic structure of the LSMO cleavage surface is identical
to that of the bulk. Furthermore, by comparing the core level shift of the
different elements as a function of doping level x, it is shown that
microscopic phase separation is unlikely to occur for this particular manganite
well above the Curie temperature.Comment: 7 pages, 5 figure
Electronic and magnetic structure of epitaxial NiO/FeO(001) heterostructures grown on MgO(001) and Nb-doped SrTiO(001)
We study the underlying chemical, electronic and magnetic properties of a
number of magnetite based thin films. The main focus is placed onto
NiO/FeO(001) bilayers grown on MgO(001) and Nb-SrTiO(001)
substrates. We compare the results with those obtained on pure FeO(001)
thin films. It is found that the magnetite layers are oxidized and Fe
dominates at the surfaces due to maghemite (-FeO) formation,
which decreases with increasing magnetite layer thickness. From a layer
thickness of around 20 nm on the cationic distribution is close to that of
stoichiometric FeO. At the interface between NiO and FeO we
find the Ni to be in a divalent valence state, with unambiguous spectral
features in the Ni 2p core level x-ray photoelectron spectra typical for NiO.
The formation of a significant NiFeO interlayer can be excluded by
means of XMCD. Magneto optical Kerr effect measurements reveal significant
higher coercive fields compared to magnetite thin films grown on MgO(001), and
a 45 rotated magnetic easy axis. We discuss the spin magnetic moments
of the magnetite layers and find that the moment increases with increasing thin
film thickness. At low thickness the NiO/FeO films grown on
Nb-SrTiO exhibits a significantly decreased spin magnetic moments. A
thickness of 20 nm or above leads to spin magnetic moments close to that of
bulk magnetite
K-edge X-ray absorption spectra in transition metal oxides beyond the single particle approximation: shake-up many body effects
The near edge structure (XANES) in K-edge X-ray absorption spectroscopy (XAS)
is a widely used tool for studying electronic and local structure in materials.
The precise interpretation of these spectra with the help of calculations is
hence of prime importance, especially for the study of correlated materials
which have a complicated electronic structure per se. The single particle
approach, for example, has generally limited itself to the dominant dipolar
cross-section. It has long been known however that effects beyond this approach
should be taken into account, both due to the inadequacy of such calculations
when compared to experiment and the presence of shake-up many-body satellites
in core-level photoemission spectra of correlated materials. This effect should
manifest itself in XANES spectra and the question is firstly how to account for
it theoretically and secondly how to verify it experimentally. By using
state-of-the-art first principles electronic structure calculations and 1s
photoemission measurements we demonstrate that shake-up many-body effects are
present in K-edge XAS dipolar spectra of NiO, CoO and CuO at all energy scales.
We show that shake-up effects can be included in K-edge XAS spectra in a simple
way by convoluting the single-particle first-principles calculations including
core-hole effects with the 1s photoemission spectra. We thus describe all
features appearing in the XAS dipolar cross-section of NiO and CoO and obtain a
dramatic improvement with respect to the single-particle calculation in CuO.
These materials being prototype correlated magnetic oxides, our work points to
the presence of shake-up effects in K-edge XANES of most correlated transition
metal compounds and shows how to account for them, paving the way to a precise
understanding of their electronic structure.Comment: 6 pages, 4 picture
Profiling the interface electron gas of LaAlO3/SrTiO3 heterostructures by hard X-ray photoelectron spectroscopy
The conducting interface of LaAlO/SrTiO heterostructures has been
studied by hard X-ray photoelectron spectroscopy. From the Ti~2 signal and
its angle-dependence we derive that the thickness of the electron gas is much
smaller than the probing depth of 4 nm and that the carrier densities vary with
increasing number of LaAlO overlayers. Our results point to an electronic
reconstruction in the LaAlO overlayer as the driving mechanism for the
conducting interface and corroborate the recent interpretation of the
superconducting ground state as being of the Berezinskii-Kosterlitz-Thouless
type.Comment: 4 pages, 4 figure
Investigation of Cu poor and Cu rich Cu In,Ga Se2 CdS interfaces using hard X ray photoelectron spectroscopy
Cu poor and Cu rich Cu In,Ga Se2 CIGSe absorbers were used as substrates for the chemical bath deposition of ultrathin CdS buffer layers in the thickness range of a few nanometers in order to make the CIGSe CdS interface accessible by hard X ray photo emission spectroscopy. The composition of both, the absorber and the buffer layer as well as the energetics of the interface was investigated at room temperature and after heating the samples to elevated temperatures 200 C, 300 C and 400 C . It was found that the amount of Cd after the heating treatment depends on the near surface composition of the CIGSe absorber. No Cd was detected on the Cu poor surface after the 400 C treatment due to its diffusion into the CIGSe layer. In contrast, Cd was still present on the Cu rich surface after the same treatment at 400
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