I09-Surface and Interface Structural Analysis
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M.
Stübinger
,
J.
Gabel
,
Philipp
Scheiderer
,
M.
Zapf
,
M.
Schmitt
,
P.
Schütz
,
B.
Leikert
,
J.
Küspert
,
M.
Kamp
,
P. K.
Thakur
,
T.-L.
Lee
,
P.
Potapov
,
A.
Lubk
,
B.
Büchner
,
M.
Sing
,
R.
Claessen
Diamond Proposal Number(s):
[17499, 23737]
Abstract: A heterostructure consisting of the Mott insulator LaVO3 and the band insulator SrTiO3 is considered
a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of
LaVO3 match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the
nonpolar/polar interface when integrated with SrTiO3, also offer remarkable advantages over conventional solar
cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition
are scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of
soft and hard x-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the
electrical properties, band bending, and band alignment of LaVO3/SrTiO3 heterostructures. We find a critical
LaVO3 thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3d states at the
interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as
the driving mechanism for the formation of the metallic interface in LaVO3/SrTiO3.
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Jun 2021
|
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I09-Surface and Interface Structural Analysis
|
Huw
Shiel
,
Theodore D. C.
Hobson
,
Oliver S.
Hutter
,
Laurie J.
Phillips
,
Matthew J.
Smiles
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Jack E. N.
Swallow
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Jonathan D.
Major
,
Ken
Durose
,
Tim D.
Veal
Diamond Proposal Number(s):
[23160]
Open Access
Abstract: Antimony selenide (Sb2
2
Se3
3
) possesses great potential in the field of photovoltaics (PV) due to its suitable properties for use as a solar absorber and good prospects for scalability. Previous studies have reported the growth of a native antimony oxide (Sb2
2
O3
3
) layer at the surface of Sb2
2
Se3
3
thin films during deposition and exposure to air, which can affect the contact between Sb2
2
Se3
3
and subsequent layers. In this study, photoemission techniques were utilized on both Sb2
2
Se3
3
bulk crystals and thin films to investigate the band alignment between Sb2
2
Se3
3
and the Sb2
2
O3
3
layer. By subtracting the valence band spectrum of an in situ cleaved Sb2
2
Se3
3
bulk crystal from that of the atmospherically contaminated bulk crystal, a valence band offset (VBO) of −1.72
−
1.72
eV is measured between Sb2
2
Se3
3
and Sb2
2
O3
3
. This result is supported by a −1.90
−
1.90
eV VBO measured between Sb2
2
O3
3
and Sb2
2
Se3
3
thin films via the Kraut method. Both results indicate a straddling alignment that would oppose carrier extraction through the back contact of superstrate PV devices. This work yields greater insight into the band alignment of Sb2
2
O3
3
at the surface of Sb2
2
Se3
3
films, which is crucial for improving the performance of these PV devices.
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Jun 2021
|
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I09-Surface and Interface Structural Analysis
|
C.
Kalha
,
S.
Bichelmaier
,
N. K.
Fernando
,
J. V.
Berens
,
P. K.
Thakur
,
T.-L.
Lee
,
J. J.
Gutiérrez Moreno
,
S.
Mohr
,
L. E.
Ratcliff
,
M.
Reisinger
,
J.
Zechner
,
M.
Nelhiebel
,
A.
Regoutz
Diamond Proposal Number(s):
[19885]
Open Access
Abstract: The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallization scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high-temperature events or its behavior when exposed to air. Here, a combined soft and hard x-ray photoelectron spectroscopy (XPS) characterization approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behavior of a 300 nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales, and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the bulk titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft x-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO2, WO3, and a WO3 oxide coordinated with a titanium environment. This combinatorial characterization approach provides valuable insights into the thermal and oxidation stability of TiW alloys from two depth perspectives, aiding the development of future device technologies.
|
May 2021
|
|
I09-Surface and Interface Structural Analysis
|
Huw
Shiel
,
Oliver S.
Hutter
,
Laurie J.
Phillips
,
Jack E. N.
Swallow
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Vinod R.
Dhanak
,
Jonathan D.
Major
,
Tim D.
Veal
Diamond Proposal Number(s):
[23160]
Abstract: Sb2Se3 is a promising material for use in photovoltaics, but the optimum device structure has not yet been identified. This study provides band alignment measurements between Sb2Se3, identical to that used in high-efficiency photovoltaic devices, and its two most commonly used window layers, namely, CdS and TiO2. Band alignments are measured via two different approaches: Anderson’s rule was used to predict an interface band alignment from measured natural band alignments, and the Kraut method was used in conjunction with hard X-ray photoemission spectroscopy to directly measure the band offsets at the interface. This allows examination of the effect of interface formation on the band alignments. The conduction band minimum (CBM) of TiO2 is found by the Kraut method to lie 0.82 eV below that of Sb2Se3, whereas the CdS CBM is only 0.01 eV below that of Sb2Se3. Furthermore, a significant difference is observed between the natural alignment- and Kraut method-determined offsets for TiO2/Sb2Se3, whereas there is little difference for CdS/Sb2Se3. Finally, these results are related to device performance, taking into consideration how these results may guide the future development of Sb2Se3 solar cells and providing a methodology that can be used to assess band alignments in device-relevant systems.
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Dec 2020
|
|
|
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Abstract: High resolution X-ray diffraction (HRXRD) pattern of highly c-oriented (002) thin film provide a direct evidence of the formation of R-3c orthorhombic ZnCO3 structural phase. Room temperature ferromagnetism (RTFM) is measured using bulk vibrating sample magnetometery (VSM). A strong orbital anisotropy has been observed by X-ray linear dichroism (XLD) experiment, which also supports that ZnCO3 phase is structurally non-distortive after carbon-implantation. The absence of X-ray magnetic circular dichroism (XMCD) signal at O K- and C K-edges suggest that bulk VSM ferromagnetism is not related to the magnetic polarization of O 2p/C 2p orbitals hybridized with Zn-states. These experiments clearly indicate that the complex C–ZnO/ZnCO3 phase should be ferromagnetic having out-of-plane/in-plane orbital reconstruction.
|
Nov 2020
|
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I09-Surface and Interface Structural Analysis
|
Jack E. N.
Swallow
,
Christian
Vorwerk
,
Piero
Mazzolini
,
Patrick
Vogt
,
Oliver
Bierwagen
,
Alexander
Karg
,
Martin
Eickhoff
,
Jörg
Schörmann
,
Markus R.
Wagner
,
Joseph William
Roberts
,
Paul R.
Chalker
,
Matthew J.
Smiles
,
Philip
Murgatroyd
,
Sara
Mohamed
,
Zachary W.
Lebens-Higgins
,
Louis F. J.
Piper
,
Leanne A. H.
Jones
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Joel B.
Varley
,
Juergen
Furthmüller
,
Claudia
Draxl
,
Tim D.
Veal
,
Anna
Regoutz
Diamond Proposal Number(s):
[21430, 24670]
Abstract: The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy effcient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.
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Sep 2020
|
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I09-Surface and Interface Structural Analysis
|
Christopher H.
Don
,
Huw
Shiel
,
Theodore D. C.
Hobson
,
Christopher N.
Savory
,
Jack E. N.
Swallow
,
Matthew J.
Smiles
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Ken
Durose
,
Jonathan D.
Major
,
Vinod R.
Dhanak
,
David O.
Scanlon
,
Tim D.
Veal
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The presence of a lone pair of 5s electrons at the valence band maximum (VBM) of Sb2Se3 and the resulting band alignments are investigated using soft and hard X-ray photoemission spectroscopy in parallel with density functional theory (DFT) calculations. Vacuum-cleaved and exfoliated bulk crystals of Sb2Se3 are analysed using laboratory and synchrotron X-ray sources to acquire high resolution valence band spectra with both soft and hard X-rays. Utilising the photon-energy dependence of different orbital cross-sections and corresponding DFT calculations, the various orbital contributions to the valence band could be identified, including the 5s orbital's presence at the VBM. The ionization potential is also determined and places the VBM at 5.13 eV below the vacuum level, similar to other materials with 5s2 lone pairs, but far above those of related materials without lone pairs of electrons.
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Aug 2020
|
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I09-Surface and Interface Structural Analysis
|
Leanne A. H.
Jones
,
Wojciech M.
Linhart
,
Nicole
Fleck
,
Jack E. N.
Swallow
,
Philip A. E.
Murgatroyd
,
Huw
Shiel
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Laurence J.
Hardwick
,
Jonathan
Alaria
,
Frank
Jaeckel
,
Robert
Kudrawiec
,
Lee A.
Burton
,
Aron
Walsh
,
Jonathan M.
Skelton
,
Tim D.
Veal
,
Vin R.
Dhanak
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The effects of Sn
5
s
lone pairs in the different phases of Sn sulphides are investigated with photoreflectance, hard x-ray photoemission spectroscopy (HAXPES), and density functional theory. Due to the photon energy-dependence of the photoionization cross sections, at high photon energy, the Sn
5
s
orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn
5
s
state contribution at the top of the valence band in the different Sn-sulphides, SnS,
Sn
2
S
3
, and
SnS
2
, to be clearly identified. SnS and
Sn
2
S
3
contain Sn(II) cations and the corresponding Sn
5
s
lone pairs are at the valence band maximum (VBM), leading to
∼
1.0
–1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast,
SnS
2
only contains Sn(IV) cations, no filled lone pairs, and therefore has a
∼
2.3
eV room-temperature band gap and much lower VBM compared with SnS and
Sn
2
S
3
. The direct band gaps of these materials at 20 K are found using photoreflectance to be 1.36, 1.08, and 2.47 eV for SnS,
Sn
2
S
3
, and
SnS
2
, respectively, which further highlights the effect of having the lone-pair states at the VBM. As well as elucidating the role of the Sn
5
s
lone pairs in determining the band gaps and band alignments of the family of Sn-sulphide compounds, this also highlights how HAXPES is an ideal method for probing the lone-pair contribution to the density of states of the emerging class of materials with
n
s
2
configuration.
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Jul 2020
|
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I09-Surface and Interface Structural Analysis
|
Minh Hai
Tran
,
Ali M.
Malik
,
Michael
Dürrschnabel
,
Anna
Regoutz
,
Pardeep
Thakur
,
Tien-Lin
Lee
,
Delwin
Perera
,
Leopoldo
Molina-Luna
,
Karsten
Albe
,
Jochen
Rohrer
,
Christina S.
Birkel
Diamond Proposal Number(s):
[24670]
Abstract: Two-dimensional carbides/nitrides, typically called MXenes, are an emerging member of the ever-growing family of two-dimensional materials. The prediction of a ferromagnetic groundstate in chromium-containing MXenes has triggered growing interest in their chemical exfoliation from Cr-based MAX phases. However, the exfoliation poses serious difficulties using standard etching agents such as hydrofluoric acid (HF). Here, we investigate the exfoliability of Cr2GaC particles by chemical etching with aqueous HF both experimentally and theoretically. Structural and microstructural analyses show that the Cr2GaC particles decompose into chromium carbide and oxide without the formation of a Cr-based MXene. A thermodynamic analysis based on ab initio electronic structure calculations reveals that the exfoliation of Cr-based MXene from Cr2GaC by HF-etching is inhibited by more favorable competing reactions. This result confirms the experimental finding and suggests that HF is an unsuitable etching agent for a successful exfoliation of Cr2GaC.
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Jul 2020
|
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I09-Surface and Interface Structural Analysis
|
Judith Veronika
Berens
,
Sebastian
Bichelmaier
,
Nathalie K.
Fernando
,
Pardeep Kumar
Thakur
,
Tien-Lin
Lee
,
Manfred
Mascheck
,
Tomas
Wiell
,
Susanna K.
Eriksson
,
J. Matthias
Kahk
,
Johannes
Lischner
,
Manesh
Mistry
,
Thomas
Aichinger
,
Gregor
Pobegen
,
Anna
Regoutz
Diamond Proposal Number(s):
[19885]
Open Access
Abstract: SiC is set to enable a new era in power electronics impacting a wide range of energy technologies, from electric vehicles to renewable energy. Its physical characteristics outperform silicon in many aspects, including band gap, breakdown field, and thermal conductivity. The main challenge for further development of SiC-based power semiconductor devices is the quality of the interface between SiC and its native dielectric SiO2. High temperature nitridation processes can improve the interface quality and ultimately the device performance immensely, but the underlying chemical processes are still poorly understood. Here, we present an energy-dependent hard X-ray photoelectron spectroscopy (HAXPES) study probing non-destructively SiC and SiO2 and their interface in device stacks treated in varying atmospheres. We successfully combine laboratory- and synchrotron-based HAXPES to provide unique insights into the chemistry of interface defects and their passivation through nitridation processes.
|
Apr 2020
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