arXiv subjects

Shiyou Chen

Publications and source records attributed to Shiyou Chen.

At least 19 recordsLinked to original sources

arXiv2026

Carrier Capture at Defects from Finite-Temperature Lattice Dynamics

Defect-assisted carrier capture is commonly described within nonradiative multiphonon (NMP) theory using normal modes of the equilibrium defect structure. This description becomes inadequate when finite-temperature lattice fluctuations explore configurations that cannot be represented by a fixed normal-mode basis. Here, we present a trajectory-based method for calculating carrier capture rate from first principles, which allows lattice relaxation and electron--lattice coupling matrix element to be reconstructed directly from correlation functions of finite-temperature lattice dynamics. For hole capture at C$_\mathrm{N}$ in GaN within harmonic regime, the method reproduces static NMP results including mode mixing and agrees with experiment. For oxygen vacancy in SiO$_2$, by contrast, the thermally sampled potential energy surface is substantially softer than the zero-temperature normal-mode harmonic expansion, strongly modifying the lattice relaxation and electron--lattice coupling, and producing pronounced changes in both the capture coefficient and its temperature dependence. These results establish the finite-temperature configurational ensemble, rather than phonon occupations alone, as an essential ingredient of defect-assisted carrier capture.

cond-mat.mtrl-sci↗

arXiv2026

Predicting large-supercell defect formation energies from machine-learning charge density models trained on small supercells

First-principles defect calculations are often limited by the cost of the large supercells required to suppress image interactions. Machine-learning interatomic potentials (MLIPs) provide another alternative, but training defect MLIPs typically requires thousands of structures and weeks of data generation. Since charge density is the key to density-functional-theory (DFT), we propose a machine-learning charge density (MLCD) route for predicting defect formation energies with higher data efficiency. We optimize the training set by integrating small supercells of varying sizes for better extrapolation, allocating their proportions based on spatial charge-density analysis. With only 96 supercells containing 16--96 atoms as the dataset, MLCD accurately predicts the formation energies of four intrinsic defects in 360-atom supercells, with defect-wise mean absolute error below 0.05 eV. In contrast, MLIPs trained on the same dataset can err by more than 1 eV. These results show that charge-density learning enables more robust cross-size transfer than direct energy-force fitting and that mixed-size data design can substantially reduce the cost of defect prediction.

cond-mat.mtrl-sci↗

arXiv2026

Nonradiative Multiphonon Model of Deep-Level Transient Spectroscopy: Beyond Henry-Lang Model

Deep-level transient spectroscopy (DLTS) is a key experimental method for defect characterization, yet its analysis remains controversial, and the two widely used models developed by Henry and Lang are conflicting. We show that the Henry-Lang model is valid only under the Condon approximation, as well as high-temperature and strong electron-phonon coupling approximations, which cause incorrect temperature dependence of carrier emission and capture. Here we develop a rigorous nonradiative multiphonon (NMP) model, and demonstrate that the temperature dependence is governed predominantly by effective phonons with large phonon wavefunction overlap and high thermal occupation. The effective phonons are strongly correlated with lattice relaxation.The neglect of this correlation in existing DLTS models introduces substantial errors when they are used to fit DLTS-measured emission rates. Our comparison for 21 different defects in 12 semiconductors, including Si, SiC and Ga$_2$O$_3$, shows that the Henry-Lang model gives a completely different temperature dependence of carrier capture cross section from that obtained using the rigorous NMP model, with errors reaching up to six orders of magnitude at room temperature. Our study highlights the necessity of revisiting previous DLTS analysis studies using the rigorous NMP model.

cond-mat.mtrl-sci↗

arXiv2026

A wrong ground-state structure of HfO$_2$ predicted by machine-learning interatomic potentials based on the PBE functional

Machine-learning interatomic potentials (MLIPs) have become powerful tools for material simulations. Many MLIPs are trained based on density functional theory (DFT) datasets generated with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Using a PBE-based MLIP for HfO2, we identify a previously unreported low-energy I41/amd structure, which is predicted to be more stable than the well-known ground-state structure, the monoclinic P21/c structure. Since experiments show clearly that HfO2 takes the P21/c structure as the ground state, this is obviously a wrong prediction. Unfortunately, the same prediction is also made by widely used PBE-based foundation models such as NequIP-OAM-L and MatterSim-v1-5M. Comparisons among different DFT functionals show that this error originates from the PBE functional, which overstabilizes low-density structures containing sixfold Hf-O octahedral units, such as the I41/amd and Pbcn phases. The error also affects the calculated energy landscapes and barrier heights along ferroelectric HfO2 polarization switching paths when there are large lattice relaxations. Fortunately, the error can be largely suppressed by other functionals such as PBEsol and local density approximation. Our study serves as a warning about the impact of errors in exchange-correlation functional approximations on the reliability of MLIP simulations of crystal structures and phase transitions.

cond-mat.mtrl-sci↗

arXiv2026

HotLoop Optimization of Petawatt Laser Focal Spot via a Twin-Focus Scheme

Achieving diffraction-limited focusing of high-power laser pulses to generate ultra-high intensities is crucial for developing compact laser-driven particle accelerators and exploring strong-field quantum electrodynamics. However, accurately diagnosing and optimizing the focal spots of petawatt (PW) laser pulses remains a significant challenge. In this work, we present an experimental methodology utilizing a twin-focus scheme to precisely characterize the intensity distribution and wavefront of focused PW femtosecond laser pulses, and employ it to elucidate their power-dependent evolution. Furthermore, we optimized the focal spots at full power via our in-situ wavefront correction method termed ``HotLoop', achieving a Strehl ratio of 0.80 for 1 PW laser pulses. Consequently, the cutoff proton energies in laser proton acceleration experiments were significantly enhanced. The success of this approach underscores the necessity of in-situ high-energy wavefront correction for ultra-high intensity laser-matter interactions.

physics.optics↗

arXiv2026

Substitutional platinum as an efficient nonradiative recombination center in silicon

Platinum (Pt) is widely used for carrier-lifetime control in silicon power devices, yet the microscopic nonradiative recombination mechanism of the substitutional platinum ($\text{Pt}_\text{Si}$) dopant remains debated. Using first-principles calculations combined with nonradiative multiphonon theory, we systematically investigate the electronic structures and carrier capture dynamics of $\text{Pt}_\text{Si}$. Our results show that both the donor ($+/0$) and acceptor ($0/-$) levels of $\text{Pt}_\text{Si}$ exhibit large capture cross sections for electron and hole carriers, thereby making $\text{Pt}_\text{Si}$ an effective recombination center. Notably, the calculated capture cross sections are sensitive to the symmetry-equivalent defect configurations with different Jahn-Teller distortions. By accounting for two different $D_{2d}$ configurations of neutral $\text{Pt}_\text{Si}$ during transitions properly, our calculated carrier capture cross sections align well with experimental values. This work provides a microscopic picture of the carrier capture processes induced by $\text{Pt}_\text{Si}$ and emphasizes the importance of symmetry-equivalent configurations in defect-assisted nonradiative recombination.

cond-mat.mtrl-sci↗

arXiv2026

Machine learning Hamiltonian enables scalable and accurate defect calculations: The case of oxygen vacancies in amorphous SiO$_2$

Point defects critically influence the properties of materials and devices, yet density functional theory (DFT) remains computationally demanding for defect supercell calculations. Machine learning interatomic potentials (MLIPs) offer high efficiency but require extensive datasets. MLIPs trained only on defect configurations in small supercells exhibit systematic energy errors in larger supercells, demonstrating limited transferability. Here, we present a machine learning Hamiltonian (MLH) model-based method for calculating total energies and atomic forces in defect supercells with linear-scaling computational cost, enabling efficient structural relaxation and accurate formation energy predictions. We take oxygen vacancies in amorphous SiO$_2$ as an example and train the MLH model on defect configurations in 95-atom supercells, with the training data derived from 120 self-consistent field calculations and 12 structural relaxations. The MLH model enables efficient structural relaxations for host (defect-free) and defect systems in larger supercells, avoiding the systematic energy errors observed in MLIPs. The cancellation of energy errors between host and defect systems yields accurate formation energy predictions, with deviations from DFT below 50 meV. The proposed method holds significant potential for defect simulations in complex materials.

cond-mat.mtrl-sci↗

arXiv2026

RASP: Reliability ab initio simulation package of MOSFETs based on all-state model

As transistors continue to scale down, device reliability has become a critical concern. In order to accurately simulate defect-induced reliability degradation in MOSFET based logic, memory and power devices, we develop RASP (Reliability Ab initio Simulation Package), which implements the all-state model for reliability simulation. Unlike conventional two-state and four-state models that consider only two and four defect configurations respectively, the all-state model systematically considers all possible defect configurations in amorphous gate dielectrics and all nonradiative multiphonon (NMP) and thermal transition pathways among them. With defect parameters obtained from ab initio calculations as input, RASP enables accurate simulation of threshold voltage shifts caused by defects. Using RASP to simulate oxygen vacancies in a-SiO$_2$, we find that they are a non-negligible source of negative bias temperature instability (NBTI).

cond-mat.mtrl-sci↗

arXiv2026

Thermal Casimir Force Imaging of Nonequilibrium Hot Electrons

The thermal Casimir effect, arising from fluctuating electromagnetic fields of thermally agitated charges, induces thermosensitive forces and presents a novel approach to detecting nanoscale hot electrons, elusive yet ubiquitous in modern miniaturized transistors. However, detecting thermal Casimir forces at the nanoscale remains extremely challenging due to background forces such as electrostatic force and quantum Casimir force. In this study, we present the first non-contact force measurement of hot electrons based on the thermal Casimir effect. Using an atomic force microscope (AFM) with a dual-resonant tip, we achieve thermosensitive force detection of nonequilibrium hot electrons while effectively suppressing background thermo-insensitive forces, thereby distinguishing them from cold electrons. In silicon nanoconstriction devices, the measured thermal Casimir pressure reaches approximately 3 bar at a separation of 5 nm at an electron temperature of about 10^3 K. Our work introduces a novel methodology for hot electron nanothermometry and provides critical insights into the thermo-mechanical properties of post-Moore nanoelectronics.

cond-mat.mes-hall↗

arXiv2026

Symmetry Adapted Analysis of Screw Dislocation: Electronic Structure and Carrier Recombination Mechanisms in GaN

As fundamental one-dimensional defects, screw dislocations profoundly reshape the energy landscape and carrier dynamics of crystalline materials. By restoring the exact algebra of the screw dislocation group, we unveil the latent symmetry constraints that govern the electronic structure, providing a more rigorous physical picture than the conventional treatments. When applied to GaN, the method yields a band-connectivity constraint and rigorous dipole selection rules for polarization-resolved transitions. Combined with computed Hamiltonian matrix, the approach gives symmetry-filtered radiative and dielectric calculations and reveals a piezoelectrical effect at the dislocation core that strongly suppresses radiative recombination. The pronounced dominance of non-radiative capture over radiative recombination highlights the detrimental impact of screw dislocations on the luminous efficiency of GaN, providing a theoretical foundation for optimizing dislocation-limited optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Electroluminescence of NV Color Centers in Diamond p-i-n Diodes mediated by Charge-state Dynamics

As the electroluminescence (EL) of NV color centers in diamond has been realized in p-i-n diodes,the underlying mechanism remains a puzzle for longer than a decade. In this study,using first-principles approaches,the electronic configurations and the possible transitions are comprehensively investigated. Based on the calculated carrier cross sections and transition rates,the mechanism of the EL of NV centers and the charge-state dynamics are revealed. The continuous EL is maintained by the cycle of NV0 ground (GNV0),NV+ metastable (MNV+) and NV0 excited state (ENV0). The weaker EL intensity compared to photoluminescence (PL) is explained by the bottleneck transition from MNV+ to ENV0 and another non-luminescent transition cycle. Additionally,our results also explain the disappearance of the luminescence of NV- as a result of unbalanced transitions between NV- and NV0. This study not only reveal the mechanism of electroluminescence of NV centers and explain experimental observations,but also provide first-principles insights to understand the charge-dynamics of other color centers under electric and optical field.

cond-mat.mtrl-sci↗

arXiv2025

"One defect, one potential" strategy for accurate machine learning prediction of defect phonons

Atomic vibrations play a critical role in phonon-assisted electron transitions at defects in solids. However, accurate phonon calculations in defect systems are often hindered by the high computational cost of large-supercell first-principles calculations. Recently, foundation models, such as universal machine learning interatomic potentials (MLIPs), emerge as a promising alternative for rapid phonon calculations, but the quantitatively low accuracy restricts its fundamental applicability for high-level defect phonon calculations, such as nonradiative carrier capture rates. In this paper, we propose a "one defect, one potential" strategy in which an MLIP is trained on a limited set of perturbed supercells. We demonstrate that this strategy yields phonons with accuracy comparable to density functional theory (DFT), regardless of the supercell size. The predicted accuracy of defect phonons is validated by phonon frequencies, Huang-Rhys factors, and phonon dispersions. Further calculations of photoluminescence (PL) spectra and nonradiative capture rates based on this defect-specific model also show good agreements with DFT results, meanwhile reducing the computational expenses by more than an order of magnitude. Our approach provides a practical pathway for studying defect phonons in 10$^4$-atom large supercell with high accuracy and efficiency.

cond-mat.mtrl-sci↗

arXiv2025

Carrier Emission and Capture Competition mediated A(n)BC Recombination Model in Semiconductors with Multi-Level Defects

The ABC model has been widely used to describe the carrier recombination rate, in which the rate of non-radiative recombination assisted by deep-level defects is assumed to depend linearly on excess carrier density $\Delta n$, leading to a constant recombination coefficient A. However, for multi-level defects that are prevalent in semiconductors, we demonstrate here that the rate should depend nonlinearly on $\Delta n$. When $\Delta n$ varies, the carrier capture and emission of defects can change the defect density distribution in different charge states, which can further change the carrier capture and emission rates of the defects and thus make the recombination rate depend non-linearly on $\Delta n$, leading to an $A(n)$ function. However, in many recent calculation studies on carrier recombination rate of multi-level defects, only carrier capture was considered while carrier emission from defect levels was neglected, causing incorrect charge-state distribution and misleading linear dependence of the rate on $\Delta n$. For $\text{V}_{\text{Ga}}$-$\text{O}_{\text{N}}$ in GaN and $\text{Pb}_\text{I}$ in CsPbI$_3$, our calculations showed that neglecting the carrier emission can cause the recombination rate underestimation by more than 8 orders of magnitude when $\Delta n$ is $10^{15}$ cm$^{-3}$. Our findings suggest that the recent studies on carrier recombination assisted by multi-level defects should be revisited with carrier emission considered, and the widely-used $ABC$ model should be reformed into the $A(n)BC$ model.

cond-mat.mtrl-sci↗

arXiv2025

Defect Phonon Renormalization during Nonradiative Multiphonon Transitions in Semiconductors

As a typical nonradiative multiphonon transition in semiconductors, carrier capture at defects is critical to the performance of semiconductor devices. Its transition rate is usually calculated using the equal-mode approximation, which assumes that phonon modes and frequencies remain unchanged before and after the transition. Using the carbon substitutional defect ($\text{C}_\text{N}$) in GaN as a benchmark, here we demonstrate that the phonon renormalization can be significant during defect relaxation, which causes errors as large as orders of magnitude in the approximation. To address this issue, we consider (i) Duschinsky matrix connecting the initial-state and final-state phonons, which accounts for the changes in phonon modes and frequencies; and (ii) the off-diagonal contributions in total transition matrix element, which incorporates the cross terms of electron-phonon interactions between different modes. With this improvement, the calculated transition rates show agreements with experimental results within an order of magnitude. We believe the present method makes one step forward for the accurate calculation of multiphonon transition rate, especially in cases with large defect relaxations.

cond-mat.mtrl-sci↗

arXiv2024

Si/SiO$_\text{2}$ MOSFET Reliability Physics: From Four-State Model to All-State Model

As implemented in the commercialized device modeling software, the four-state nonradiative multi-phonon model has attracted intensive attention in the past decade for describing the physics in negative bias temperature instability (NBTI) and other reliability issues of Si/SiO$_\text{2}$ MOSFET devices. It was proposed initially based on the assumption that the oxygen vacancy defects (V$_\text{O}$) in SiO$_\text{2}$ dielectric layer are bistable in the Si-dimer and back-projected structures during carrier capture and emission. Through high-throughput first-principles structural search, we found V$_\text{O}$ on non-equivalent O sites in amorphous SiO$_\text{2}$ can take 4 types of structural configurations in neutral state and 7 types of configurations in +1 charged state after capturing holes, which produce a wide range of charge-state transition levels for trapping holes. The finding contrasts the structural-bistability assumption and makes the four-state model invalid for most of O sites. To describe the reliability physics accurately, we propose an all-state model to consider all these structural configurations as well as all the carrier capture/emission transitions and thermal transitions between them. With the all-state model, we show that the V$_\text{O}$ defects play important roles in causing NBTI, which challenges the recent studies that discarded V$_\text{O}$ as a possible hole trap in NBTI. Our systematical calculations on the diversified V$_\text{O}$ properties and the all-state model provide the microscopic foundation for describing the reliability physics of MOSFETs and other transistors accurately.

cond-mat.mtrl-sci↗

arXiv2024

Electron acceleration and X-ray generation from near-critical-density carbon nanotube foams driven by moderately relativistic lasers

Direct laser acceleration of electrons in near-critical-density (NCD) carbon nanotube foams (CNFs) has its advantages in the high-efficiency generation of relativistic electrons and broadband X-rays. Here, we report the first simultaneous measurement on the spectra of laser-driven electrons and X-rays from CNFs at moderately relativistic intensities of around 5\times{10}^{19}\ W/cm^2.\ The density and thickness of the CNFs were scanned in the experiments, indicating the optimized electrons temperature of 5.5 MeV and X-ray critical energy of 5 keV. Two-dimensional (2D) particle-in-cell (PIC) simulations confirm that the electrons, with a temperature significantly higher than the pondermotive scale, are directly accelerated by the laser along the NCD plasma channel, while the bright X-rays are emitted by these electrons through betatron radiation or Thomson backscattering inside the channel. The simultaneously generated electrons and X-rays, automatically synchronized with the femtosecond laser driver, are suitable for applications such as bi-modal radiography.

physics.plasm-ph↗

arXiv2023

Metastability and anharmonicity enhance defect-assisted nonradiative recombination in low-symmetry semiconductors

Strong nonradiative recombination has been observed in quasi-one-dimensional antimony selenide, which runs counter to the simple intuition that claims high defect tolerance exists in semiconductors with antibonding state in the valence band and bonding state in the conduction band. Here we reveal such a defect intolerance actually stems from the richness of structural metastability and vibrational anharmonicity owing to the low-symmetry atomic structure. Taking the deep defect V$_{\rm Se}$ as a benchmark, we show the defect with its ground-state configuration alone does not act as a recombination center. Instead, we identify three different configurations with different formation energies, such richness of metastability offers a higher probability to accomplish a rapid recombination cycle. Another contributing factor is the anharmonicity in the potential energy surfaces that is caused by the large atomic relaxation, which elevates the total capture coefficient by 2-3 orders of magnitude compared with harmonic approximation. Therefore, the unique properties from both crystals and phonons in quasi-one-dimensional system enhance the nonradiative recombination, making the traditional intuition of defect tolerance invalid. These results highlight the importance of the correct identification of metastable defects and phonon anharmonicity in the nonradiative recombination in low-symmetry semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

Defect Regulation by Palladium Incorporation towards Grain Boundaries of Kesterite solar cells

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) solar cell has emerged as one of the most promising candidates for thin-film photovoltaics. However, severe charge losses occurring at the grain boundaries (GBs) of Kesterite polycrystalline absorbers has hindered the improvement of cell performance. Herein, we report a redox reaction strategy involving palladium (Pd) to eliminate atomic vacancy defects such as VSn and VSe in GBs of the Kesterite absorbers. We demonstrate that PdSex compounds could form during the selenization process and distribute at the GBs and the absorber surfaces; thereby aid in the suppression of Sn and Se volatilization loss and inhibiting the formation of VSn and VSe defects. Furthermore, Pd(II)/Pd(IV) serves as a redox shuttle, i.e., on one hand, Pd(II) captures Se vapor from the reaction environment to produce PdSe2, on the other hand, PdSe2 provides Se atoms to the Kesterite absorber by being reduced to PdSe, thus contributing to the elimination of pre-existing VSe defects within GBs. These effects collectively reduce defects and enhance the p-type characteristics of the Kesterite absorber, leading to a significant reduction in charge recombination loss within the cell. As a result, high-performance Kesterite solar cells with a total-area efficiency of 14.5% have been achieved. This remarkable efficiency increase benefited from the redox reaction strategy offers a promising avenue for the precise regulation of defects in Kesterite solar cells and holds generally significant implications for the exploration of various other photovoltaic devices.

cond-mat.mtrl-sci↗