I09-Surface and Interface Structural Analysis
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Luke
Thomas
,
Theo D. C.
Hobson
,
Laurie J.
Phillips
,
Kieran J.
Cheetham
,
Neil
Tarbuck
,
Leanne A. H.
Jones
,
Matthew J.
Smiles
,
Chris H.
Don
,
Pardeep K.
Thakur
,
Mark
Isaacs
,
Huw
Shiel
,
Stephen
Campbell
,
Vincent
Barrioz
,
Vin
Dhanak
,
Tim
Veal
,
Jonathan D.
Major
,
Ken
Durose
Diamond Proposal Number(s):
[28268]
Open Access
Abstract: This paper is motivated by the potential advantages of higher doping and lower contact barriers in CdTe photovoltaic devices that may be realized by using n- type rather than the conventional p-type solar absorber layers. We present post-growth doping trials for indium in thin polycrystalline CdTe films using diffusion of indium metal and with indium chloride. Chemical concentrations of indium up to 1019 cm-3 were achieved and the films were verified as n-type by hard x-ray photoemission. Post growth chlorine treatment (or InCl3) was found to compensate the n-doping. Trial structures comprising CdS/CdTe:In verified that the doped absorber structures performed as expected both before and after chloride treatment, but it is recognized that this is not an optimum combination. Hence in order to identify how the advantages of n-type absorbers might be fully realized in future work, we also report simulations of a range of p-n junction combinations with n-CdTe, a number of which have the potential for high Voc.
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Jun 2022
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I09-Surface and Interface Structural Analysis
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Matthew J.
Smiles
,
Thomas
Shalvey
,
Luke
Thomas
,
Theodore D. C.
Hobson
,
Leanne A. H.
Jones
,
Laurie
Phillips
,
Christopher
Don
,
Thomas
Beesley
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Ken
Durose
,
Jonathan D.
Major
,
Tim
Veal
Diamond Proposal Number(s):
[31170]
Open Access
Abstract: Germanium selenide (GeSe) bulk crystals, thin films and solar cells are investigated with a focus on acceptor-doping with silver (Ag) and the use of an Sb2Se3 interfacial layer. The Ag-doping of GeSe occurred by a stoichiometric melt growth technique that created Ag-doped GeSe bulk crystals. A combination of capacitance voltage measurements, synchrotron radiation photoemission spectroscopy and surface space-charge calculations indicate Ag-doping increases the hole density from 5.2×1015 cm-3 to 1.9×1016 cm-3. The melt-grown material is used as the source for thermally evaporated GeSe films within solar cells. The cell structure with the highest efficiency of 0.260% is FTO/CdS/Sb2Se3/undoped-GeSe/Au compared with solar cells without the Sb2Se3 interfacial layer or with the Ag-doped GeSe.
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Apr 2022
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I09-Surface and Interface Structural Analysis
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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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Diamond Proposal Number(s):
[21995, 26285]
Abstract: Ni-rich lithium nickel manganese cobalt (NMC) oxide cathode materials promise Li-ion batteries with increased energy density and lower cost. However, higher Ni content is accompanied by accelerated degradation and thus poor cycle lifetime, with the underlying mechanisms and their relative contributions still poorly understood. Here, we combine electrochemical analysis with surface-sensitive X-ray photoelectron and absorption spectroscopies to observe the interfacial degradation occurring in LiNi0.8Mn0.1Co0.1O2–graphite full cells over hundreds of cycles between fixed cell voltages (2.5–4.2 V). Capacity losses during the first ∼200 cycles are primarily attributable to a loss of active lithium through electrolyte reduction on the graphite anode, seen as thickening of the solid-electrolyte interphase (SEI). As a result, the cathode reaches ever-higher potentials at the end of charge, and with further cycling, a regime is entered where losses in accessible NMC capacity begin to limit cycle life. This is accompanied by accelerated transition-metal reduction at the NMC surface, thickening of the cathode electrolyte interphase, decomposition of residual lithium carbonate, and increased cell impedance. Transition-metal dissolution is also detected through increased incorporation into and thickening of the SEI, with Mn found to be initially most prevalent, while the proportion of Ni increases with cycling. The observed evolution of anode and cathode surface layers improves our understanding of the interconnected nature of the degradation occurring at each electrode and the impact on capacity retention, informing efforts to achieve a longer cycle lifetime in Ni-rich NMCs.
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Feb 2022
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[25807]
Abstract: The key charge transfer processes in energy storage devices occur at the electrode-electrolyte interface, which is typically buried making it challenging to access the interfacial chemistry. In the case of Li-ion batteries, metallic Li electrodes hold promise for increasing energy and power densities, and when used in conjunction with solid electrolytes adverse safety implications associated with dendrite formation in organic liquid electrolytes can potentially be overcome. To better understand the stability of solid electrolytes when in contact with alkali metals and the reactions that occur, here we consider the deposition of thin (~10 nm) alkali metal films onto solid electrolyte surfaces, that are thin enough that X-ray photoelectron spectroscopy can probe the buried electrode-electrolyte interface. We highlight the importance of in situ alkali metal deposition, by assessing the contaminant species that are present after glovebox handling and the use of ‘inert’ transfer devices. Consequently, we compare and contrast three available methods for in situ alkali-metal deposition; Li sputter deposition, Li evaporation, and Li plating induced by e− flood-gun irradiation. Studies on both a sulphide solid electrolyte (Li6PS5Cl), and a single-layer graphene probe surface reveal that the more energetic Li deposition methods such as sputtering can induce surface damage and interfacial mixing that is not seen with thermal evaporation. This indicates that appropriate selection of the Li deposition method for in situ studies is required to observe representative behaviour, and the results of previous studies involving energetic deposition may warrant further evaluation.
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Jan 2022
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I09-Surface and Interface Structural Analysis
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Caio
Silva
,
Daniela
Dombrowski
,
Nicolae
Atodiresei
,
Wouter
Jolie
,
Ferdinand
Farwick Zum Hagen
,
Jiaqi
Cai
,
Paul
Ryan
,
Pardeep Kumar
Thakur
,
Vasile
Caciuc
,
Stefan
Bluegel
,
David Andrew
Duncan
,
Thomas
Michely
,
Tien-Lin
Lee
,
Carsten
Busse
Diamond Proposal Number(s):
[14799, 16710, 19801]
Open Access
Abstract: The lattice mismatch between a monolayer of MoS2 and its Au(111) substrate induces a moiré superstructure. The local variation of the registry between sulfur and gold atoms at the interface leads to a periodic pattern of strongly and weakly interacting regions. In consequence, also the electronic bands show a spatial variation. We use scanning tunneling microscopy and spectroscopy (STM/STS), x-ray photoelectron spectroscopy (XPS) and x-ray standing wave (XSW) for a determination of the geometric and electronic structure. The experimental results are corroborated by density functional theory (DFT). We obtain the geometric structure of the supercell with high precision, identify the fraction of interfacial atoms that are strongly interacting with the substrate, and analyze the variation of the electronic structure in dependence of the location within the moiré unit cell and the nature of the band.
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Jan 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
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Caio C.
Silva
,
Daniela
Dombrowski
,
Abdus
Samad
,
Jiaqi
Cai
,
Wouter
Jolie
,
Joshua
Hall
,
Paul
Ryan
,
Pardeep K.
Thakur
,
David A.
Duncan
,
Tien-Lin
Lee
,
Udo
Schwingenschlögl
,
Carsten
Busse
Diamond Proposal Number(s):
[14799, 16710, 19801]
Abstract: We determined the structure of epitaxial
2H-TaS2
on Au(111) using the method of x-ray standing waves (XSW), supported by density functional theory (DFT) calculations and scanning tunneling microscopy (STM). The lattice mismatch between substrate and overlayer gives rise to a moiré superstructure, which modulates the structural and electronic properties. For a specific registry (S atoms directly above Au substrate atoms), local covalentlike bonds form, whereas globally weak van der Waals bonding prevails. Still, the
TaS2
layer remains rather flat. Significant charge transfer from Au(111) into the conduction band of the two-dimensional material is found.
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Nov 2021
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I09-Surface and Interface Structural Analysis
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Matthew J.
Smiles
,
Jonathan M.
Skelton
,
Huw
Shiel
,
Leanne A. H.
Jones
,
Jack E. N.
Swallow
,
Holly J.
Edwards
,
Thomas
Featherstone
,
Philip A. E.
Murgatroyd
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Vinod R.
Dhanak
,
Tim D.
Veal
Diamond Proposal Number(s):
[21431, 23160]
Open Access
Abstract: Germanium sulfide and germanium selenide bulk crystals were prepared using a melt growth technique. X-ray photoemission spectroscopy (XPS) was used to determine ionisation potentials of 5.74 and 5.48 eV for GeS and GeSe respectively. These values were used with the previously-measured band gaps to establish the natural band alignments with potential window layers for solar cells and to identify CdS and TiO2 as sensible choices. The ionisation potential of GeS is found to be smaller than in comparable materials. Using XPS and hard x-ray photoemission (HAXPES) measurements in conjunction with density-functional theory calculations, we demonstrate that stereochemically active Ge 4s lone pairs are present at the valence-band maxima. Our work thus provides direct evidence for active lone pairs in GeS and GeSe, with important implications for the applications of these and related materials, such as Ge-based perovskites.
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Sep 2021
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I09-Surface and Interface Structural Analysis
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Maximilian
Bauernfeind
,
Jonas
Erhardt
,
Philipp
Eck
,
Pardeep K.
Thakur
,
Judith
Gabel
,
Tien-Lin
Lee
,
Jörg
Schäfer
,
Simon
Moser
,
Domenico
Di Sante
,
Ralph
Claessen
,
Giorgio
Sangiovanni
Diamond Proposal Number(s):
[26419, 25151]
Open Access
Abstract: Large-gap quantum spin Hall insulators are promising materials for room-temperature applications based on Dirac fermions. Key to engineer the topologically non-trivial band ordering and sizable band gaps is strong spin-orbit interaction. Following Kane and Mele’s original suggestion, one approach is to synthesize monolayers of heavy atoms with honeycomb coordination accommodated on templates with hexagonal symmetry. Yet, in the majority of cases, this recipe leads to triangular lattices, typically hosting metals or trivial insulators. Here, we conceive and realize “indenene”, a triangular monolayer of indium on SiC exhibiting non-trivial valley physics driven by local spin-orbit coupling, which prevails over inversion-symmetry breaking terms. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice and unveil how a hidden honeycomb connectivity emerges from interference patterns in Bloch px ± ipy-derived wave functions.
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Sep 2021
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