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Abstract: This work reports a systematic investigation of the defect states and carrier compensation mechanisms in Si- and Sn-doped β-Ga2O3 using hybrid density functional theory calculations combined with high-resolution photoemission spectroscopy. The calculations show that Si introduces a shallower donor level ε(0/+1) ≈ 0.17 eV than does Sn (≈0.23 eV). The deeper donor level in Sn results from the stabilization of neutral SnGa via Sn 4d–O 2p hybridization. At higher doping levels, gallium vacancies (VGa) act as the dominant acceptor-type defects. Notably, Sn exhibits a strong tendency to form split-vacancy complexes (Snic-2VGa), with formation energies as low as −1.52 eV under O-rich conditions, indicating their spontaneous formation and self-compensation, whereas the analogous Si-related complexes are significantly less favorable. Electronic structure and orbital analysis reveal that Sn generates localized 5s-derived in-gap states, whereas Si preserves delocalized 3p–O 2p bonding, consistent with the presence of in-gap states in Sn-doped but not Si-doped films. These findings identify dopant–vacancy coupling as the key mechanism governing compensation and provide a mechanistic basis for dopant selection and defect engineering in high-performance β-Ga2O3 devices.
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Apr 2026
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I06-Nanoscience (XPEEM)
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Dong
Li
,
Ying
Zhou
,
Kai
Jiang
,
Tiesheng
Wang
,
Chao
Yun
,
Yongli
Yu
,
Xuegang
Chen
,
Sixu
Wang
,
Shiqing
Deng
,
Yajing
Liu
,
Dazhi
Wang
,
Rui
Wu
,
Yuhao
Qiu
,
Shenghao
Cai
,
Erwen
Zhang
,
Maosheng
Liu
,
Xiaozhi
Zhan
,
Linglong
Li
,
Qian
Li
,
Tao
Zhu
,
Kelvin H. L.
Zhang
,
Shuai
Dong
,
Weiwei
Li
Open Access
Abstract: Materials with room-temperature magnetic ordering and switchable polarization are essential for spintronic devices. Although 3 d transition metal oxides exhibit potential, their Curie temperature (TC) remains unsatisfactory, and coexistence of magnetic and polar order has not been realized in 4 d/5 d oxides. Here, through epitaxial strain and 3d−4d cation ordering engineering, a ferrimagnetic insulating state (TC ~ 623 K) is achieved in La2CoRuO6 films, coexisting with switchable short-range polar nanodomains. Atomic-scale investigations and density functional theory calculations reveal that compressive strain enhances lattice distortions. These distortions, combined with high-spin state of Co2+ ions and ordered B-site cations, significantly enhance Co-O-Ru antiferromagnetic superexchange, inducing the ferrimagnetic insulating state. Concurrently, the gradient BO6 octahedral rotations with inhomogeneous evolution trigger B-site ions’ displacements, driving the formation of polar nanodomains. Our work fills the experimental gap in realizing magnetic and polar order coexistence in 4 d/5 d oxides and opens new avenues for designing high-TC multiferroics.
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Mar 2026
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I09-Surface and Interface Structural Analysis
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Yue
Yu
,
Xu
Zhang
,
Wenjing
Xu
,
Rongkun
Chen
,
Cien
Liu
,
Yiru
Zhao
,
Yushuo
Hu
,
Zhilai
Fang
,
Ning
Jia
,
Xiangyu
Xu
,
Kelvin H. L.
Zhang
Diamond Proposal Number(s):
[37428]
Abstract: GaN-based deep ultraviolet (DUV) optoelectronic devices have garnered considerable attention for applications in sterilization, biological detection, and optical communications. However, the performance of current DUV optoelectronic devices is limited by the insufficient DUV transparency of conventional electrodes. In this work, the epitaxial growth of degenerately Si-doped Ga2O3 films on GaN as a promising DUV transparent electrode is reported. The 0.5% Si doped Ga2O3 (n+-Ga2O3) films exhibit DUV transparency exceeding 85% in the spectral range from 280 to 400 nm wavelength. Such a high DUV transparency is attributed to the ultrawide bandgap of ≈5.0 eV of the n+-Ga2O3 film induced by the Burstein–Moss effect due to degenerate doping. Moreover, the n+-Ga2O3 film exhibits a very low specific contact resistance of 1.96 × 10−4 Ω cm2 to GaN. High-resolution X-ray photoemission spectroscopic (XPS) study reveals that n+-Ga2O3 forms a type-II staggered band alignment with GaN with a low interface barrier of 0.15 eV and a narrow band bending thickness of a few nm. The small barrier, together with the degenerately doped Ga2O3 film, enables excellent electrical contact at the n+-Ga2O3/GaN interface and low contact resistance. This work demonstrates n+-Ga2O3 as a promising alternative for DUV transparent electrode for GaN-based DUV devices.
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Jan 2026
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I09-Surface and Interface Structural Analysis
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Wenjing
Xu
,
Hailing
Guo
,
Zhenni
Yang
,
Yihong
Chen
,
Xiangyu
Xu
,
Tien-Lin
Lee
,
Duanyang
Chen
,
Xinxin
Yu
,
Yuzheng
Guo
,
Zhaofu
Zhang
,
Hongji
Qi
,
Kelvin H. I.
Zhang
Diamond Proposal Number(s):
[37428]
Abstract: In this work, we investigate the electronic structure and interfacial band alignment of β-(Al𝑥Ga1−𝑥)2O3/Ga2O3 heterojunctions using a combination of synchrotron-based hard x-ray photoemission spectroscopy (HAXPES) and first-principles hybrid density functional theory calculations. β-(Al𝑥Ga1−𝑥)2O3 films with Al compositions of x = 0.12, 0.19, and 0.29 were grown on Fe-doped β-Ga2O3 (010) substrates via pulsed laser deposition. The band gap of β-(Al𝑥Ga1−𝑥)2O3 increases from (4.83 ± 0.05) eV (x = 0) to (5.37 ± 0.08) eV (x = 0.29), primarily driven by an upward shift of the conduction band edge due to hybridization between Al 3s and Ga 4s states, while the valence band edge exhibits a slight downward shift. Both experimental HAXPES data and theoretical calculations confirmed the formation of a “type I” (straddling) band alignment in the β-(Al𝑥Ga1−𝑥)2O3/Ga2O3 heterojunctions. For instance, at x = 0.29, the conduction band offset and valence band offset are approximately 0.33 and 0.21 eV, respectively. These findings provide valuable insights for designing modulation-doped β-(Al𝑥Ga1−𝑥)2O3/Ga2O3 heterostructures, enabling the realization of a two-dimensional electron gas and its application in high-frequency electronic devices.
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Oct 2025
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I09-Surface and Interface Structural Analysis
|
Ziqi
Zhang
,
Yan
Wang
,
Zhenni
Yang
,
Wenjing
Xu
,
Yan
Sun
,
Yihong
Chen
,
Xiangyu
Xu
,
Ye
Yang
,
Duanyang
Chen
,
Xin
Dong
,
Hao
Long
,
Hongji
Qi
,
Kelvin H. L.
Zhang
Diamond Proposal Number(s):
[37428]
Open Access
Abstract: In this work, homoepitaxial growth and in-depth study on the electronic structure of Si doped β-Ga2O3 thin films on (100) oriented Fe doped Ga2O3 substrates are reported. Carrier concentrations ranging from 3.7 × 1018 cm−3 to 1.6 × 1020 cm−3 were achieved with Si doping level from 0.01% to 1%. The highest conductivity achieved is 468.8 S/cm, making the films promising ohmic contact electrodes. Synchrotron based hard x-ray photoemission spectroscopy (HAXPES) were used to gain insights into the evolution of electronic structure of the degenerately doped films. The results show that the optical bandgaps of the films increase with Si doping, because of the progressive occupation of the bottom of the conduction band by the electrons, i.e., Burstein-Moss shift (∆BM). On the other hand, doping induced bandgap renormalization (∆BGR) leads to narrowing of the fundamental bandgap. The ∆BGR value is slightly higher than those of (010) oriented films at a similar carrier concentration. The anisotropic nature of β-Ga2O3, including the crystal field splitting and the dielectric constant, was found to contribute to the observed crystal orientation-dependent ∆BGR behavior. By understanding the Burstein-Moss and bandgap renormalization effects, the electronic and optical properties of β-Ga2O3 materials can be optimized for optoelectronic device applications.
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Oct 2025
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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
|
Siliang
Kuang
,
Zhenni
Yang
,
Ziqi
Zhang
,
Ziqian
Sheng
,
Shenglong
Wei
,
Yihong
Chen
,
Wenjing
Xu
,
Ye
Yang
,
Duanyang
Chen
,
Hongji
Qi
,
Kelvin H. L.
Zhang
Diamond Proposal Number(s):
[37428]
Abstract: In this work, we report the transport, defect state and electronic structure properties of unintentionally doped (UID) and Sn doped β-Ga2O3 homo-epitaxial thin films grown by molecular beam epitaxy (MBE) with electron density ranging from 2.1×1016 to 2.8×1019 cm-3. The UID film with an electron density of 2.1×1016 cm-3 exhibits a notable RT mobility of 129 cm2/Vs and a peak mobility of 900 cm2/Vs at 80 K, achieving the state-of-the-art level for MBE-grown Ga2O3 films. Temperature dependent Hall measurement reveal that Sn dopants have an activation energy of 56.7 meV. Synchrotron-based photoemission spectroscopy were further used to study insights into the evolution of electronic properties induced by Sn doping. An in-gap defect state was observed at the 1.5 eV above the valence band maximum for the Sn-doped Ga2O3 film. The in-gap state acts as self-compensating centers affecting the overall doping efficiency and mobility. Furthermore, photoemission spectroscopic study also reveals an upward surface band bending existing at the surface region of Sn doped Ga2O3 films. The identification of the in-gap state and surface upward band bending have significant implications for understanding the doping mechanisms in Ga2O3 and its electronic device applications.
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Sep 2024
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|
I09-Surface and Interface Structural Analysis
|
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
|
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
|
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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