55 research outputs found
Hard X-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction
The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, nondestructive, in-situ measurement ideal for this system. We have thus applied hard X-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few kilo-electron-volt energy range to probe the structure of an epitaxially grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO(001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 ± 4 Å, with the minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and X-ray reflectivity measurements. Through the STEM measurements, we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 Å. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few-angstrom precision
Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO/SrTiO Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
LaNiO (LNO) is an intriguing member of the rare-earth nickelates in
exhibiting a metal-insulator transition for a critical film thickness of about
4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such
thin films also show a transition to a metallic state in superlattices with
SrTiO (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to
better understand this transition, we have studied a strained LNO/STO
superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an
(LaAlO)(SrAlTaO) substrate using soft x-ray
standing-wave-excited angle-resolved photoemission (SWARPES), together with
soft- and hard- x-ray photoemission measurements of core levels and
densities-of-states valence spectra. The experimental results are compared with
state-of-the-art density functional theory (DFT) calculations of band
structures and densities of states. Using core-level rocking curves and x-ray
optical modeling to assess the position of the standing wave, SWARPES
measurements are carried out for various incidence angles and used to determine
interface-specific changes in momentum-resolved electronic structure. We
further show that the momentum-resolved behavior of the Ni 3d eg and t2g states
near the Fermi level, as well as those at the bottom of the valence bands, is
very similar to recently published SWARPES results for a related
LaSrMnO/SrTiO superlattice that was studied using the
same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which
further validates this experimental approach and our conclusions. Our
conclusions are also supported in several ways by comparison to DFT
calculations for the parent materials and the superlattice, including
layer-resolved density-of-states results
Simultaneous ambient pressure X-ray photoelectron spectroscopy and grazing incidence X-ray scattering in gas environments
We have developed an experimental system to simultaneously observe surface
structure, morphology, composition, chemical state, and chemical activity for
samples in gas phase environments. This is accomplished by simultaneously
measuring X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray
scattering (GIXS) in gas pressures as high as the multi-Torr regime, while also
recording mass spectrometry. Scattering patterns reflect near-surface sample
structures from the nano- to the meso-scale. The grazing incidence geometry
provides tunable depth sensitivity while scattered X-rays are detected across a
broad range of angles using a newly designed pivoting-UHV-manipulator for
detector positioning. At the same time, XPS and mass spectrometry can be
measured, all from the same sample spot and in ambient conditions. To
demonstrate the capabilities of this system, we measured the chemical state,
composition, and structure of Ag-behenate on a Si(001) wafer in vacuum and in
O atmosphere at various temperatures. These simultaneous structural,
chemical, and gas phase product probes enable detailed insights into the
interplay between structure and chemical state for samples in gas phase
environments. The compact size of our pivoting-UHV-manipulator makes it
possible to retrofit this technique into existing spectroscopic instruments
installed at synchrotron beamlines. Because many synchrotron facilities are
planning or undergoing upgrades to diffraction limited storage rings with
transversely coherent beams, a newly emerging set of coherent X-ray scattering
experiments can greatly benefit from the concepts we present here.Comment: 21 pages, 4 figure
Bulk Electronic Structure of Lanthanum Hexaboride (LaB6) by Hard X-ray Angle-Resolved Photoelectron Spectroscopy
In the last decade rare-earth hexaborides have been investigated for their
fundamental importance in condensed matter physics, and for their applications
in advanced technological fields. Among these compounds, LaB has a special
place, being a traditional d-band metal without additional f- bands. In this
paper we investigate the bulk electronic structure of LaB using hard x-ray
photoemission spectroscopy, measuring both core-level and angle-resolved
valence-band spectra. By comparing La 3d core level spectra to cluster model
calculations, we identify well-screened peak residing at a lower binding energy
compared to the main poorly-screened peak; the relative intensity between these
peaks depends on how strong the hybridization is between La and B atoms. We
show that the recoil effect, negligible in the soft x-ray regime, becomes
prominent at higher kinetic energies for lighter elements, such as boron, but
is still negligible for heavy elements, such as lanthanum. In addition, we
report the bulk-like band structure of LaB determined by hard x-ray
angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES
experimental results by the free-electron final-state calculations and by the
more precise one-step photoemission theory including matrix element and phonon
excitation effects. In addition, we consider the nature and the magnitude of
phonon excitations in HARPES experimental data measured at different
temperatures and excitation energies. We demonstrate that one step theory of
photoemission and HARPES experiments provide, at present, the only approach
capable of probing true bulk-like electronic band structure of rare-earth
hexaborides and strongly correlated materials.Comment: Total 26 pages, Total 11 figure
Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO embedded in GdTiO
For certain conditions of layer thickness, the interface between GdTiO
(GTO) and SrTiO (STO) in multilayer samples has been found to form a
two-dimensional electron gas (2DEG) with very interesting properties including
high mobilities and ferromagnetism. We have here studied two trilayer samples
of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001)
(LaAlO)(SrAlTaO) (LSAT), with the STO layer
thicknesses being at what has been suggested is the critical thickness for 2DEG
formation. We have studied these with Ti-resonant angle-resolved (ARPES) and
angle-integrated photoemission and find that the spectral feature in the
spectra associated with the 2DEG is present in the 1.5 unit cell sample, but
not in the 1.0 unit cell sample. We also observe through core-level spectra
additional states in Ti and Sr, with the strength of a low-binding-energy state
for Sr being associated with the appearance of the 2DEG, and we suggest it to
have an origin in final-state core-hole screening.Comment: 12 pages, 4 figure
Characterization of free standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy
Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a
(Si/Mo) multilayer mirror substrate are characterized by hard x-ray
photoemission spectroscopy (HXPS), and by standing-wave HXPS (SW-HXPS).
Information on the chemical composition and on the chemical states of the
elements within the nanoribbons was obtained by HXPS and on the quantitative
depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves
to x-ray optical calculations, the chemical depth profile of the InAs(QM) and
its interfaces were quantitatively derived with angstrom precision. We
determined that: i) the exposure to air induced the formation of an InAsO
layer on top of the stoichiometric InAs(QM); ii) the top interface between the
air-side InAsO and the InAs(QM) is not sharp, indicating that
interdiffusion occurs between these two layers; iii) the bottom interface
between the InAs(QM) and the native oxide SiO on top of the (Si/Mo)
substrate is abrupt. In addition, the valence band offset (VBO) between the
InAs(QM) and the SiO/(Si/Mo) substrate was determined by HXPS. The value of
eV is in good agreement with literature results obtained
by electrical characterization, giving a clear indication of the formation of a
well-defined and abrupt InAs/SiO heterojunction. We have demonstrated that
HXPS and SW-HXPS are non-destructive, powerful methods for characterizing
interfaces and for providing chemical depth profiles of nanostructures, quantum
membranes, and 2D layered materials.Comment: three figure
Recommended from our members
Interface properties and built-in potential profile of a LaCr O3/SrTi O3 superlattice determined by standing-wave excited photoemission spectroscopy
LaCrO3(LCO)/SrTiO3(STO) heterojunctions are intriguing due to a polar discontinuity along [001], exhibiting two distinct and controllable charged interface structures [(LaO)+/(TiO2)0 and (SrO)0/(CrO2)-] with induced polarization, and a resulting depth-dependent potential. In this study, we have used soft- and hard-x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to quantitatively determine the elemental depth profile, interface properties, and depth distribution of the polarization-induced built-in potentials. We observe an alternating charged interface configuration: a positively charged (LaO)+/(TiO2)0 intermediate layer at the LCOtop/STObottom interface and a negatively charged (SrO)0/(CrO2)- intermediate layer at the STOtop/LCObottom interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy-loss spectroscopy mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO and STO contributions to the valence-band (VB) spectra. Using a two-step analytical approach involving first SW-induced core-level binding-energy shifts and then VB modeling, the variation in potential across the complete superlattice is determined in detail. This potential is in excellent agreement with density functional theory models, confirming this method as a generally useful tool for interface studies
Interface properties and built-in potential profile of a LaCrO/SrTiO superlattice determined by standing-wave excited photoemission spectroscopy
LaCrO (LCO) / SrTiO (STO) heterojunctions are intriguing due to a
polar discontinuity along (001), two distinct and controllable interface
structures [(LaO)/(TiO) and (SrO)/(CrO)], and
interface-induced polarization. In this study, we have used soft- and hard
x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to generate a
quantitative determination of the elemental depth profiles and interface
properties, band alignments, and the depth distribution of the
interface-induced built-in potentials in the two constituent oxides. We observe
an alternating charged interface configuration: a positively charged
(LaO)/(TiO) intermediate layer at the
LCO/STO interface and a negatively charged
(SrO)/(CrO) intermediate layer at the
STO/LCO interface. Using core-level SW data,
we have determined the depth distribution of species, including through the
interfaces, and these results are in excellent agreement with scanning
transmission electron microscopy and electron energy loss spectroscopy
(STEM-EELS) mapping of local structure and composition. SW-XPS also enabled
deconvolution of the LCO-contributed and STO- contributed
matrix-element-weighted density of states (MEWDOSs) from the valence band (VB)
spectra for the LCO/STO superlattice (SL). Monitoring the VB edges of the
deconvoluted MEWDOS shifts with a change in probing profile, the alternating
charge- induced built-in potentials are observed in both constituent oxides.
Finally, using a two-step simulation approach involving first core-level
binding energy shifts and then valence-band modeling, the built-in potential
gradients across the SL are resolved in detail and represented by the depth
distribution of VB edges.Comment: Main text: 29 pages, 5 figures; Supplementary Information: 20 pages,
10 figure
Energetic, spatial and momentum character of a buried interface: the two-dimensional electron gas between two metal oxides
The interfaces between two condensed phases often exhibit emergent physical
properties that can lead to new physics and novel device applications, and are
the subject of intense study in many disciplines. We here apply novel
experimental and theoretical techniques to the characterization of one such
interesting interface system: the two-dimensional electron gas (2DEG) formed in
multilayers consisting of SrTiO (STO) and GdTiO (GTO). This system has
been the subject of multiple studies recently and shown to exhibit very high
carrier charge densities and ferromagnetic effects, among other intriguing
properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells
STO/3 unit cells of GTO] using a unique array of photoemission
techniques including soft and hard x-ray excitation, soft x-ray angle-resolved
photoemission, core-level spectroscopy, resonant excitation, and standing-wave
effects, as well as theoretical calculations of the electronic structure at
several levels and of the actual photoemission process. Standing-wave
measurements below and above a strong resonance have been introduced as a
powerful method for studying the 2DEG depth distribution. We have thus
characterized the spatial and momentum properties of this 2DEG with
unprecedented detail, determining via depth-distribution measurements that it
is spread throughout the 6 u.c. layer of STO, and measuring the momentum
dispersion of its states. The experimental results are supported in several
ways by theory, leading to a much more complete picture of the nature of this
2DEG, and suggesting that oxygen vacancies are not the origin of it. Similar
multi-technique photoemission studies of such states at buried interfaces,
combined with comparable theory, will be a very fruitful future approach for
exploring and modifying the fascinating world of buried-interface physics and
chemistry.Comment: 34 pages, 10 figure
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