I09-Surface and Interface Structural Analysis
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Jiaye
Zhang
,
Zhenni
Yang
,
Siliang
Kuang
,
Ziqi
Zhang
,
Shenglong
Wei
,
Joe
Willis
,
Tien-Lin
Lee
,
Piero
Mazzolini
,
Oliver
Bierwagen
,
Shanquan
Chen
,
Zuhuang
Chen
,
Duanyang
Chen
,
Hongji
Qi
,
David
Scanlon
,
Kelvin H. L.
Zhang
Diamond Proposal Number(s):
[31069, 31681]
Abstract: The bulk and surface electronic structures of Sn-doped 𝛽−Ga2O3 thin films have been studied by soft and hard x-ray photoemission spectroscopy (soft PES at 1486.6 eV and HAXPES at 5920 eV). The experimental spectra are compared with density functional theory calculated density of states in the valence band and conduction band. Excellent agreement was found between experimental spectra and calculated density of states by taking into account the photoionization cross section of different orbitals involved in the valence and conduction bands. The electronic states derived from Ga 4𝑠 character are selectively enhanced by HAXPES. This allows us to infer that the states at the conduction band and bottom of the valence band contain pronounced Ga 4𝑠 character. The occupation of the lower conduction band in degenerately Sn-doped Ga2O3 is clearly observed by HAXPES, which allows for direct measurement of Burstein-Moss shift and band-gap renormalization as a function of Sn doping. A comparison of the valence band spectra of Sn-doped Ga2O3 films with Si-doped samples suggests that Sn doping has different effects on the electronic structure than Si doping. An in-gap electronic state is observed for Sn-doped Ga2O3, which is attributed to self-compensating Sn2+ related defects. Furthermore, a larger band-gap renormalization is found in Sn-doped samples, because the Sn 5𝑠 dopant orbital mixes strongly with the host Ga 4𝑠 derived conduction band. Finally, a comparison of the valence band and core-level spectra excited with soft and hard x rays allows us to identify an upward band bending at the surface region of Sn-doped Ga2O3 films.
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Sep 2024
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[24219]
Abstract: Developing transparent p-type oxide semiconductor has been the long-standing subject of interest for optoelectronic devices, but hindered by the strongly localized valence band (VB) structure intrinsic to metal oxides. Sn2+ oxides represented by SnO are proposed as promising p-type semiconductors since the Sn 5s2 state could help to alleviate the carrier localization at the VB. In this work, using a combination of X-ray spectroscopies and density functional theory calculations, we explore the electronic structures of Sn2+ based Sn2Nb2O7 and Sn2Ta2O7 pyrochlores as wide bandgap p-type oxide semiconductors. Our results show that Sn2Nb2O7 and Sn2Ta2O7 have large optical bandgaps of 2.8 eV and 3.4 eV respectively, and better chemical stability over SnO. Both the experiment and theoretical calculations verified the presence of Sn 5s2 states at the top of VB of Sn2Nb2O7 and Sn2Ta2O7, and the Sn 5s2 states increase the VB dispersion and result in lower hole effective masses of 2.09 me and 2.23 me for Sn2Nb2O7 and Sn2Ta2O7 respectively but work less effectively than that for SnO. The different VB features originate from the varied Sn-O interactions influenced by crystal structures. The lattice distortions in SnO allow the hybridization between Sn 5p orbitals with occupied (Sn 5s-O 2p)* states, forming asymmetrically distributed electronic states with enhanced dispersions. However, in Sn2Nb2O7 and Sn2Ta2O7, these interactions are forbidden by their cubic symmetry and lead to the less dispersive electronic states. Increasing lattices distortions in Sn2Nb2O7 and Sn2Ta2O7 would be necessary to achieve higher hole mobilities. Our findings elucidate the microscopic origins of the opto-electronic properties in Tin (II) pyrochlore oxides, highlighting the significant role of synergistic valence band modulation and crystal structural design in advancing high performance p-type oxide semiconductors.
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Jun 2024
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I09-Surface and Interface Structural Analysis
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Abstract: Copper antimony sulfide (CuSbS2) is a p-type semiconductor that has an appropriate band gap of 1.5 eV and a large optical absorption coefficient (>105 cm−1), rendering it an emerging candidate for photoelectrochemical (PEC) water-splitting to produce green H2. However, the current understanding of the essential electronic structure of CuSbS2 and its correlation with PEC activity are limited, but it is very important to devise strategies for further PEC property improvements. Here, we report on the synthesis of CuSbS2 thin films with high quality and achieve a record-high photocurrent density of 6.3 mA cm−2 at 0.0 V vs. reversible hydrogen electrode with an F-doped tin oxide/CuSbS2/CdS/Pt photocathode. More importantly, a synergistic combination of X-ray photoemission spectroscopy and optical spectroscopy was used to unravel the electronic structure of CuSbS2. Our results show that the valence band of CuSbS2 consists of strongly hybridized states of S 3p and Cu 3d, to a lesser extent, affected by Sb 5p/5s. The implication of the electronic structure on the PEC activity and strategies for further improvement by using n-type CdS to construct a built-in electric field to facilitate photogenerated carrier transportation, are discussed.
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Sep 2023
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I06-Nanoscience (XPEEM)
I09-Surface and Interface Structural Analysis
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Dong
Li
,
Hongguang
Wang
,
Kaifeng
Li
,
Bonan
Zhu
,
Kai
Jiang
,
Dirk
Backes
,
Larissa S. I.
Veiga
,
Jueli
Shi
,
Pinku
Roy
,
Ming
Xiao
,
Aiping
Chen
,
Quanxi
Jia
,
Tien-Lin
Lee
,
Sarnjeet S.
Dhesi
,
David O.
Scanlon
,
Judith L.
Macmanus-Driscoll
,
Peter A.
Van Aken
,
Kelvin H. L.
Zhang
,
Weiwei
Li
Diamond Proposal Number(s):
[25425, 26901, 29616, 31069]
Open Access
Abstract: Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO3 with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO3 films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO6 octahedral rotations throughout LaCoO3 films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO3 films while suggesting potential applications toward low-power spintronic devices.
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Jun 2023
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[24219, 31681]
Abstract: Wide bandgap oxide semiconductors have gained significant attention in the fields from flat panel displays to solar cells, but their uses have been limited by the lack of high mobility p-type oxide semiconductors. Recently, β-phase TeO2 has been identified as a promising p-type oxide semiconductor with exceptional device performance. In this Letter, we report on the electronic structure of β-TeO2 studied by a combination of high-resolution x-ray spectroscopy and hybrid density functional theory calculations. The bulk bandgap of β-TeO2 is determined to be 3.7 eV. Direct comparisons between experimental and computational results demonstrate that the top of a valence band (VB) of β-TeO2 is composed of the hybridized Te 5s, Te 5p, and O 2p states, whereas a conduction band (CB) is dominated by unoccupied Te 5p states. The hybridization between spatially dispersive Te 5s2 states and O 2p orbitals helps us to alleviate the strong localization in the VB, leading to small hole effective mass and high hole mobility in β-TeO2. The Te 5p states provide stabilizing effect to the hybridized Te 5s-O 2p states, which is enabled by structural distortions of a β-TeO2 lattice. The multiple advantages of large bandgap, high hole mobility, two-dimensional structure, and excellent stability make β-TeO2 a highly competitive material for next-generation opto-electronic devices.
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Mar 2023
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[24219, 31069]
Abstract: Ga
2
O
3
is emerging as a promising wide band-gap semiconductor for high-power electronics and deep ultraviolet optoelectronics. It is highly desirable to dope it with controllable carrier concentrations for different device applications. This work reports a combined photoemission spectroscopy and theoretical calculation study on the electronic structure of Si doped
Ga
2
O
3
films with carrier concentration varying from
4.6
×
10
18
c
m
−
3
to
2.6
×
10
20
c
m
−
3
. Hard x-ray photoelectron spectroscopy was used to directly measure the widening of the band gap as a result of occupation of conduction band and band-gap renormalization associated with many-body interactions. A large band-gap renormalization of 0.3 eV was directly observed in heavily doped
Ga
2
O
3
. Supplemented with hybrid density functional theory calculations, we demonstrated that the band-gap renormalization results from the decrease in energy of the conduction band edge driven by the mutual electrostatic interaction between added electrons. Moreover, our work reveals that Si is a superior dopant over Ge and Sn, because
Si
3
s
forms a resonant donor state above the conduction band minimum, leaving the host conduction band mostly unperturbed and a high mobility is maintained though the doping level is high. Insights of the present work have significant implications in doping optimization of
Ga
2
O
3
and realization of optoelectronic devices.
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Nov 2022
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I09-Surface and Interface Structural Analysis
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Hongxia
Wang
,
Meiyan
Cui
,
Gaoliang
Fu
,
Jiaye
Zhang
,
Xingyu
Ding
,
Irene
Azaceta
,
Matthew
Bugnet
,
Demie M.
Kepaptsoglou
,
Vlado K.
Lazarov
,
Víctor A.
De La Pena O'Shea
,
Freddy E.
Oropeza
,
Kelvin H. L.
Zhang
Abstract: The design of heterostructured transition metal-based electrocatalysts with controlled composition and interfaces is key to increasing the efficiency of the water electrolysis and the elucidation of reaction mechanisms. In this work, we report the synthesis of well-controlled vertically aligned Ni/NiO nanocomposites consisting of Ni nanoclusters embedded in NiO, which result in highly efficient electrocatalysts for overall water splitting. We show that such a high catalytic efficiency toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) originates from a synergetic effect at Ni/NiO interfaces that significantly reduces the energy barrier for water dissociation, and favours the formation of reactive H* intermediates on the Ni side of the interface, and OHads on the NiO side of the interface. A study of water chemisorption based on near-ambient pressure photoelectron spectroscopy indicates that the abundant hetero-interfaces in Ni/NiO nanocomposite promote the dissociation of water with a three-fold increase in the surface concentration of OHads compared with pure NiO. Density functional theory calculations indicate that Ni/NiO interface leads to the reduction of the water dissociation energy barrier due to a high concentration of oxygen vacancies at NiO side of the interface, whereas the formation of highly active metallic Ni sites with an optimal value of Gibbs free energy of H* (ΔGH* = −0.16 eV) owes to a favourable adjustment of the electron energetics at the interface, thus accelerating the overall electrochemical water splitting.
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Sep 2022
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I06-Nanoscience (XPEEM)
|
Dong
Li
,
Bonan
Zhu
,
Dirk
Backes
,
Larissa S. I.
Veiga
,
Tien-Lin
Lee
,
Hongguang
Wang
,
Qian
He
,
Pinku
Roy
,
Jiaye
Zhang
,
Jueli
Shi
,
Aiping
Chen
,
Peter A.
Van Aken
,
Quanxi
Jia
,
Sarnjeet S.
Dhesi
,
David O.
Scanlon
,
Kelvin H. L.
Zhang
,
Weiwei
Li
Diamond Proposal Number(s):
[25425, 26901, 29616]
Abstract: Strain engineering of epitaxial transition metal oxide heterostructures offers an intriguing opportunity to control electronic structures by modifying the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, we demonstrate that the electronic structure, magnetic and transport properties of
La
0.9
Ba
0.1
MnO
3
thin films can be effectively controlled by epitaxial strain. Spectroscopic studies and first-principles calculations reveal that the orbital occupancy in Mn
e
g
orbitals can be switched from the
d
3
z
2
−
r
2
orbital to the
d
x
2
−
y
2
orbital by varying the strain from compressive to tensile. The change of orbital occupancy associated with Mn
3
d
-O
2
p
hybridization leads to dramatic modulation of the magnetic and electronic properties of strained
La
0.9
Ba
0.1
MnO
3
thin films. Under moderate tensile strain, an emergent ferromagnetic insulating state with an enhanced ferromagnetic Curie temperature of 215 K is achieved. These findings not only deepen our understanding of electronic structures, magnetic and transport properties in the
La
0.9
Ba
0.1
MnO
3
system, but also demonstrate the use of epitaxial strain as an effective knob to tune the electronic structures and related physical properties for potential spintronic device applications.
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Apr 2022
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I09-Surface and Interface Structural Analysis
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Jiaye
Zhang
,
Joe
Willis
,
Zhenni
Yang
,
Xu
Lian
,
Wei
Chen
,
Lai-Sen
Wang
,
Xiangyu
Xu
,
Tien-Lin
Lee
,
Lang
Chen
,
David O.
Scanlon
,
Kelvin H. I.
Zhang
Diamond Proposal Number(s):
[24219]
Open Access
Abstract: Deep UV transparent thin films have recently attracted considerable attention owing to their potential in UV and organic-based optoelectronics. Here, we report the achievement of a deep UV transparent and highly conductive thin film based on Si-doped Ga2O3 (SGO) with high conductivity of 2500 S/cm. The SGO thin films exhibit high transparency over a wide spectrum ranging from visible light to deep UV wavelength and, meanwhile, have a very low work-function of approximately 3.2 eV. A combination of photoemission spectroscopy and theoretical studies reveals that the delocalized conduction band derived from Ga 4s orbitals is responsible for the Ga2O3 films’ high conductivity. Furthermore, Si is shown to act as an efficient shallow donor, yielding high mobility up to approximately 60 cm2/Vs. The superior optoelectronic properties of SGO films make it a promising material for use as electrodes in high-power electronics and deep UV and organic-based optoelectronic devices.
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Mar 2022
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I09-Surface and Interface Structural Analysis
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Jueli
Shi
,
Ethan A.
Rubinstein
,
Weiwei
Li
,
Jiaye
Zhang
,
Ye
Yang
,
Tien-Lin
Lee
,
Changdong
Qin
,
Pengfei
Yan
,
Judith L.
Macmanus-Driscoll
,
David O.
Scanlon
,
Kelvin H.l.
Zhang
Diamond Proposal Number(s):
[24219]
Open Access
Abstract: Oxide semiconductors are key materials in many technologies from flat-panel displays，solar cells to transparent electronics. However, many potential applications are hindered by the lack of high mobility p-type oxide semiconductors due to the localized O-2p derived valence band (VB) structure. In this work, the VB structure modulation is reported for perovskite Ba2BiMO6 (M = Bi, Nb, Ta) via the Bi 6s2 lone pair state to achieve p-type oxide semiconductors with high hole mobility up to 21 cm2 V−1 s−1, and optical bandgaps widely varying from 1.5 to 3.2 eV. Pulsed laser deposition is used to grow high quality epitaxial thin films. Synergistic combination of hard x-ray photoemission, x-ray absorption spectroscopies, and density functional theory calculations are used to gain insight into the electronic structure of Ba2BiMO6. The high mobility is attributed to the highly dispersive VB edges contributed from the strong coupling of Bi 6s with O 2p at the top of VB that lead to low hole effective masses (0.4–0.7 me). Large variation in bandgaps results from the change in the energy positions of unoccupied Bi 6s orbital or Nb/Ta d orbitals that form the bottom of conduction band. P–N junction diode constructed with p-type Ba2BiTaO6 and n-type Nb doped SrTiO3 exhibits high rectifying ratio of 1.3 × 104 at ±3 V, showing great potential in fabricating high-quality devices. This work provides deep insight into the electronic structure of Bi3+ based perovskites and guides the development of new p-type oxide semiconductors.
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Jan 2022
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