I09-Surface and Interface Structural Analysis
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Niels
Kubitza
,
Isabel
Huck
,
Hanna
Pazniak
,
Curran
Kalha
,
David
Koch
,
Bo
Zhao
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Aysha A.
Riaz
,
Wolfgang
Donner
,
Hongbin
Zhang
,
Benjamin
Moss
,
Ulf
Wiedwald
,
Anna
Regoutz
,
Christina S.
Birkel
Diamond Proposal Number(s):
[29451]
Abstract: MAX phases are almost exclusively known as carbides, while nitrides and carbonitrides form a significantly underrepresented subgroup even though they have been shown to possess enhanced properties in comparison to their carbide counterparts. One example is the nitride phase Cr2GaN which exhibits a spin density wave magnetic state below T = 170 K, while the metallic carbide phase Cr2GaC follows the MAX phase-typical Pauli-paramagnetic behavior. To investigate the influence on the materials/functional properties of mixing carbon and nitrogen on the X-site, this study aims to synthesize and comprehensively characterize the hitherto unknown carbonitride phase Cr2GaC1-xNx and compare it to the parent phases. Due to the challenging synthesis of (carbo)nitrides in general, a sol-gel-assisted approach is applied which was recently developed by our group. This process was further improved by using time-efficient microwave heating, leading to a highly phase pure product. STEM-EDX analyses reveal a C/N ratio of roughly 2:1. Temperature-dependent XRD measurements confirm the literature-known magnetic phase transition of the parent nitride phase Cr2GaN, while the incorporation of carbon suppresses the latter. Nonetheless, magnetic characterization of the phases reveals that the magnetic behavior can be specifically influenced by changing the composition of the X site, resulting in an increase of the susceptibility by increasing the nitrogen amount. Overall, these findings further substantiate the big potential in nitrogen-containing MAX phases, which will also serve as starting materials for future doping studies, i.e. on the M- and A-site, and as precursors for novel 2D MXenes.
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Apr 2024
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I09-Surface and Interface Structural Analysis
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Curran
Kalha
,
Laura E.
Ratcliff
,
Giorgio
Colombi
,
Christoph
Schlueter
,
Bernard
Dam
,
Andrei
Gloskovskii
,
Tien-Lin
Lee
,
Pardeep K.
Thakur
,
Prajna
Bhatt
,
Yujiang
Zhu
,
Jürg
Osterwalder
,
Francesco
Offi
,
Giancarlo
Panaccione
,
Anna
Regoutz
Diamond Proposal Number(s):
[29451]
Open Access
Abstract: Metal hydrides are potential candidates for applications in hydrogen-related technologies, such as energy storage, hydrogen compression, and hydrogen sensing, to name just a few. However, understanding the electronic structure and chemical environment of hydrogen within them remains a key challenge. This work presents a new analytical pathway to explore these aspects in technologically relevant systems using hard x-ray photoelectron spectroscopy (HAXPES) on thin films of two prototypical metal dihydrides:
YH
2
−
δ
and
Ti
H
2
−
δ
. By taking advantage of the tunability of synchrotron radiation, a nondestructive depth profile of the chemical states is obtained using core-level spectra. Combining experimental valence-band (VB) spectra collected at varying photon energies with theoretical insights from density functional theory (DFT) calculations, a description of the bonding nature and the role of
d
versus
s
p
contributions to states near the Fermi energy are provided. Moreover, a reliable determination of the enthalpy of formation is proposed by using experimental values of the energy position of metal
s
-band features close to the Fermi energy in the HAXPES VB spectra.
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Nov 2023
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I09-Surface and Interface Structural Analysis
|
Rose M.
Snyder
,
Mikkel
Juelsholt
,
Curran
Kalha
,
Jason
Holm
,
Elisabeth
Mansfield
,
Tien-Lin
Lee
,
Pardeep K.
Thakur
,
Aysha A.
Riaz
,
Benjamin
Moss
,
Anna
Regoutz
,
Christina S.
Birkel
Diamond Proposal Number(s):
[29451]
Abstract: MAX phases with the general formula Mn+1AXn are layered carbides, nitrides, and carbonitrides with varying stacking sequence of layers of M6X octahedra and the A element depending on n. While “211” MAXphases (n = 1) are very common, MAX phases with higher n, especially n ≥ 3, have hardly been prepared. This work addresses open questions regarding the synthesis conditions, structure, and chemical composition of the “514” MAX phase. In contrast to literature reports, no oxide is needed to form the MAX phase, yet multiple heating steps at 1,600 °C are required. Using high-resolution X-ray diffraction, the structure of (Mo1-xVx)5AlC4 is thoroughly investigated, and Rietveld refinement suggests P-6c2 as the most fitting space group. SEM/EDS and XPS show that the chemical composition of the MAX phase is (Mo0.75V0.25)5AlC4. It was also exfoliated into its MXene sibling (Mo0.75V0.25)5C4 using two different techniques (using HF and an HF/HCl mixture) that lead to different surface terminations as shown by XPS/HAXPES measurements. Initial investigations of the electrocatalytic properties of both MXene versions show that, depending on the etchant, (Mo0.75V0.25)5C4 can reduce hydrogen at 10 mA cm–2 with an overpotential of 166 mV (HF only) or 425 mV (HF/HCl) after cycling the samples, which makes them a potential candidate as an HER catalyst.
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Jun 2023
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[29451]
Open Access
Abstract: Interdiffusion phenomena between adjacent materials are highly prevalent in semiconductor device architectures and can present a major reliability challenge for the industry. To fully capture these phenomena, experimental approaches must go beyond static and post-mortem studies to include in situ and in-operando setups. Here, soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) is used to monitor diffusion in real-time across a proxy device. The device consists of a Si/SiO2/TixW1−x(300 nm)/Cu(25 nm) thin film material stack, with the TixW1−x film (x = 0.054, 0.115, 0.148) acting as a diffusion barrier between Si and Cu. The interdiffusion is monitored through the continuous collection of spectra whilst in situ annealing to 673 K. Ti within the TiW is found to be highly mobile during annealing, diffusing out of the barrier and accumulating at the Cu surface. Increasing the Ti concentration within the TixW1−x film increases the quantity of accumulated Ti, and Ti is first detected at the Cu surface at temperatures as low as 550 K. Surprisingly, at low Ti concentrations (x = 0.054), W is also mobile and diffuses alongside Ti. By monitoring the Ti 1s core level with HAXPES, the surface-accumulated Ti was observed to undergo oxidation even under ultra-high vacuum conditions, highlighting the reactivity of Ti in this system. These results provide crucial evidence for the importance of diffusion barrier composition on their efficacy during device application, delivering insights into the mechanisms underlying their effectiveness and limitations.
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May 2023
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I09-Surface and Interface Structural Analysis
|
Maria
Basso
,
Elena
Colusso
,
Chiara
Carraro
,
Curran
Kalha
,
Aysha A.
Riaz
,
Giada
Bombardelli
,
Enrico
Napolitani
,
Yu
Chen
,
Jacek
Jasieniak
,
Laura E.
Ratcliff
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Anna
Regoutz
,
Alessandro
Martucci
Diamond Proposal Number(s):
[29451]
Open Access
Abstract: The thermochromic properties of vanadium dioxide (VO2) offer great advantages for energy-saving smart windows, memory devices, and transistors. However, the crystallization of solution-based thin films at temperatures lower than 400°C remains a challenge. Photonic annealing has recently been exploited to crystallize metal oxides, with minimal thermal damage to the substrate and reduced manufacturing time. Here, VO2 thin films, obtained via a green sol-gel process, were crystallized by pulsed excimer laser annealing. The influence of increasing laser fluence and pulse number on the film properties was systematically studied through optical, structural, morphological, and chemical characterizations. From temperature profile simulations, the temperature rise was confirmed to be confined within the film during the laser pulses, with negligible substrate heating. Threshold laser parameters to induce VO2 crystallization without surface melting were found. With respect to furnace annealing, both the crystallization temperature and the annealing time were substantially reduced, with VO2 crystallization being achieved within only 60 s of laser exposure. The laser processing was performed at room temperature in air, without the need of a controlled atmosphere. The thermochromic properties of the lasered thin films were comparable with the reference furnace-treated samples.
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May 2023
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I09-Surface and Interface Structural Analysis
|
Laura E.
Ratcliff
,
Takayoshi
Oshima
,
Felix
Nippert
,
Benjamin M.
Janzen
,
Elias
Kluth
,
Rüdiger
Goldhahn
,
Martin
Feneberg
,
Piero
Mazzolini
,
Oliver
Bierwagen
,
Charlotte
Wouters
,
Musbah
Nofal
,
Martin
Albrecht
,
Jack E. N.
Swallow
,
Leanne A. H.
Jones
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Curran
Kalha
,
Christoph
Schlueter
,
Tim D.
Veal
,
Joel B.
Varley
,
Markus R.
Wagner
,
Anna
Regoutz
Diamond Proposal Number(s):
[21430, 24670]
Open Access
Abstract: Ga2O3 and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga2O3 offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ-Ga2O3 presents a particular challenge across synthesis, characterisation, and theory due to its inherent disorder and resulting complex structure – electronic structure relationship. Here, density functional theory is used in combination with a machine learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ-Ga2O3. The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by PLE spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step towards exploring how their electronic structure can be understood in terms of local coordination and overall structure.
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Jul 2022
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[24248]
Open Access
Abstract: Power semiconductor device architectures require the inclusion of a diffusion barrier to suppress or at best prevent the interdiffusion between the copper metallization interconnects and the surrounding silicon substructure. The binary pseudo-alloy of titanium–tungsten (TiW), with >70 at. % W, is a well-established copper diffusion barrier but is prone to degradation via the out-diffusion of titanium when exposed to high temperatures (≥400 ∘C). Here, the thermal stability of physical vapor deposited TiW/Cu bilayer thin films in Si/SiO2(50 nm)/TiW(300 nm)/Cu(25 nm) stacks were characterized in response to annealing at 400 ∘C for 0.5 h and 5 h, using a combination of soft and hard x-ray photoelectron spectroscopy and transmission electron microscopy. Results show that annealing promoted the segregation of titanium out of the TiW and interdiffusion into the copper metallization. Titanium was shown to be driven toward the free copper surface, accumulating there and forming a titanium oxide overlayer upon exposure to air. Annealing for longer timescales promoted a greater out-diffusion of titanium and a thicker oxide layer to grow on the copper surface. However, interface measurements suggest that the diffusion is not significant enough to compromise the barrier integrity, and the TiW/Cu interface remains stable even after 5 h of annealing.
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Apr 2022
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I09-Surface and Interface Structural Analysis
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C.
Kalha
,
L. E.
Ratcliff
,
J. J. Gutiérrez
Moreno
,
S.
Mohr
,
M.
Mantsinen
,
N. K.
Fernando
,
P. K.
Thakur
,
T.-L.
Lee
,
H.-H.
Tseng
,
T. S.
Nunney
,
J. M.
Kahk
,
J.
Lischner
,
A.
Regoutz
Diamond Proposal Number(s):
[27164]
Abstract: Tungsten (W) is an important and versatile transition metal and has a firm place at the heart of many technologies. A popular experimental technique for the characterization of tungsten and tungsten-based compounds is x-ray photoelectron spectroscopy (XPS), which enables the assessment of chemical states and electronic structure through the collection of core level and valence band spectra. However, in the case of tungsten metal, open questions remain regarding the origin, nature, and position of satellite features that are prominent in the photoelectron spectrum. These satellites are a fingerprint of the electronic structure of the material and have not been thoroughly investigated, at times leading to their misinterpretation. The present work combines high-resolution soft and hard x-ray photoelectron spectroscopy (SXPS and HAXPES) with reflected electron energy loss spectroscopy (REELS) and a multitiered ab initio theoretical approach, including density functional theory (DFT) and many-body perturbation theory (G0W0 and
GW
+
C
), to disentangle the complex set of experimentally observed satellite features attributed to the generation of plasmons and interband transitions. This combined experiment-theory strategy is able to uncover previously undocumented satellite features, improving our understanding of their direct relationship to tungsten's electronic structure. Furthermore, it lays the groundwork for future studies into tungsten-based mixed-metal systems and holds promise for the reassessment of the photoelectron spectra of other transition and post-transition metals, where similar questions regarding satellite features remain.
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Jan 2022
|
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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.
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May 2021
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