arXiv subjects

Junlei Zhao

Publications and source records attributed to Junlei Zhao.

At least 19 recordsLinked to original sources

arXiv2026

Machine-learning-guided molecular dynamics simulations of point defect evolution in beta-Ga2O3 during ion implantation and annealing

In beta-gallium oxide (beta-Ga2O3), Ga-ion implantation and annealing induce abundant point defects. To overcome conventional Wigner-Seitz (WS) defect analysis limitations, a defect identification algorithm based on similarity matching and DBSCAN clustering is developed for beta-Ga2O3. It distinguishes lattice atoms from defects at high concentrations and identifies eight Ga interstitial configurations (Gaia to Gaih). Comparing SRIM and MD data highlights electronic stopping effects: neglecting them overestimates ion range and defect concentration. Across five fluences (1 to 5 x 10^14 cm-2), 1373 K is the optimal recovery temperature. Multiscale analyses using hydrostatic stress, PRDF, and defect concentration reveal defect evolution. Ga interstitials (Gai) occupy tetrahedral and octahedral sites, driving a defect-mediated phase transition from beta- to gamma-Ga2O3. Increasing fluences reduce beta-phase recovery and increase gamma-phase transformation irreversibly. Oxygen interstitial (Oi) migration is sensitive to annealing temperature, which enhances O-sublattice recrystallization.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic Core-Shell Swift Heavy Ion Tracks in beta-Ga2O3

Swift heavy ion (SHI) irradiation generates nanoscale ion tracks through intense electronic excitation, yet the microscopic mechanisms governing their morphology and phase stability in low symmetry oxides remain poorly understood. Here, a multiscale atomistic simulation framework is used to investigate the formation and recovery of SHI-induced tracks in monoclinic $\beta$-Ga2O3 over a wide range of electronic energy losses (Se) and crystallographic orientations. A sequence of distinct structural responses is identified with increasing Se: (i) complete lattice recovery at low Se; (ii) recrystallization into a metastable $\gamma$-Ga2O3 phase at intermediate Se; and (iii) the formation of core-shell ion tracks at high Se, consisting of an amorphous core surrounded by a recrystallized $\gamma$-phase shell. Despite the essentially isotropic initial energy deposition, the final ion-track morphology exhibits pronounced crystallographic anisotropy, governed by orientation-dependent recovery dynamics. The superior recrystallization along the [010] direction is attributed to its exceptionally high elastic stiffness. Notably, SHI irradiation perpendicular to the (100) plane induces a more severe structural response at low Se ($\le$ 10 keV/nm), however, at higher Se, it yields a smaller residual ion track compared to the other orientations. The simulated ion-track sizes show excellent quantitative agreement with the available experimental measurements over a wide range of Se values. These findings establish a unified atomic-scale picture of core-shell track formation and anisotropic recovery in $\beta$-Ga2O3.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic Kinetics of Ion-Irradiation-Induced Phase Transition in Gallium Oxide

Radiation-tolerant semiconductors have traditionally been engineered by the principle of suppressing defect accumulation and amorphization, based on the assumption that radiation damage is inherently stochastic. Here we show that, in monoclinic $\beta$-\ce{Ga2O3}, a promising ultrawide-bandgap semiconductor, surface crystallographic orientation deterministically governs radiation tolerance through highly anisotropic kinetics of the $\beta$-to-$\gamma$ phase transition. Using machine-learning molecular dynamics coupled with a local configurational-entropy descriptor, we quantitatively map anisotropic $\beta$-to-$\gamma$ transition kinetics, showing that the critical dose, transition-layer depth, and kinetic stability of the $\gamma$-phase are fundamentally governed by surface orientation. Under ion irradiation, non-channeling surfaces such as (100), (001), and (-201) undergo severe surface amorphization, whereas the strongly channeling (010) surface resists damage accumulation and promotes subsurface $\gamma$-phase nucleation. During thermal annealing recovery process, these initial states follow two distinct recovery pathways: the channeling (010) surface reverts directly from $\gamma$-to-$\beta$, whereas non-channeling surfaces follow a sequential amorphous-to-$\gamma$-to-$\beta$ transition pathway. This work establishes surface orientation as a fundamental design principle for achieving radiation tolerance through controlled polymorphic transitions, providing a universal framework for engineering functional materials capable of withstanding extreme irradiation environments.

cond-mat.mtrl-sci↗

arXiv2025

A General-Purpose and Efficient Machine-Learned Potential for SiC from Ambient to Extreme Environments

Silicon carbide (SiC) polymorphs are widely employed as nuclear materials, mechanical components, and wide-bandgap semiconductors. The rapid advancement of SiC-based applications has been complemented by computational modeling studies, including both ab initio and classical atomistic approaches. In this work, we develop a computationally efficient, general-purpose machine-learned interatomic potential (ML-IAP) capable of multimillion-atom molecular dynamics simulations over microsecond timescales. Using the ML-IAP, we map a broad pressure-temperature phase diagram and the threshold displacement energy distributions for the 2H and 3C polymorphs. Across a benchmark covering conditions from ambient to extreme, including high-pressure/high-temperature states and high-energy cascade damage, tabGAP provides a favorable balance of accuracy, robustness, transferability, and computational cost among the tested empirical and ML-IAPs.

cond-mat.mtrl-sci↗

arXiv2025

Ions leaving no tracks

The paths of swift heavy ions are typically traceable in solids, because of confined electronic interactions along the paths, inducing what is known in literature as 'ion tracks', i.e. nano-sized in cross-section cylindrical zones of modified material extending for microns in length. Such tracks readily form in materials exhibiting low thermal conductivities, in particular insulators or semiconductors, altering the homogeneity of materials. In this work, using recently discovered gamma/beta-Ga2O3 polymorph heterostructures we show that, in contrast to the trends in many other materials, including that in beta-Ga2O3, swift heavy ions leave no tracks in gamma-Ga2O3. We explained this trend in terms of amazingly fast disorder recovery, occurring because of multiple configurations in the gamma-Ga2O3 lattice itself, so that the disorder formed by ion impacts gets rapidly erased, giving a perception of ions leaving no tracks. As such, gamma-Ga2O3, readily integrated with beta-Ga2O3 in polymorph heterostructures, may become a promising semiconductor platform for devices capable to operate in extremely harsh radiation environments.

cond-mat.mtrl-sci↗

arXiv2025

Edge-Dependent Step-Flow Growth Mechanism in $\beta$-Ga$_{2}$O$_{3}$ (100) Facet at the Atomic Level

Homoepitaxial step-flow growth of high-quality $\beta$-Ga$_{2}$O$_{3}$ thin films is essential for the advancement of high-performance Ga$_{2}$O$_{3}$-based devices. In this work, the step-flow growth mechanism of $\beta$-Ga$_{2}$O$_{3}$ (100) facet is explored by machine-learning molecular dynamics simulations and density functional theory calculations. Our results reveal that Ga adatoms and Ga-O adatom pairs, with their high mobility, are the primary atomic species responsible for efficient surface migration on the (100) facet. The asymmetric monoclinic structure of $\beta$-Ga$_{2}$O$_{3}$ induces a distinct two-stage Ehrlich-Schwoebel barrier for Ga adatoms at the [00$\overline{1}$] step edge, contributing to the suppression of double-step and hillock formation. Furthermore, a miscut towards [00$\overline{1}$] does not induce the nucleation of stable twin boundaries, whereas a miscut towards [001] leads to the spontaneous formation of twin boundaries. This research provides meaningful insights not only for high-quality $\beta$-Ga$_{2}$O$_{3}$ homoepitaxy but also the step-flow growth mechanism of other similar systems.

cond-mat.mtrl-sci↗

arXiv2024

Optical library of Ga2O3 polymorphs

Gallium oxide is an emerging material of interest due to its unique combination of functional properties and the existence of multiple polymorphs - {\alpha}, {\beta}, {\gamma}, {\delta}, and {\kappa} - each exhibiting distinct characteristics arising from their different lattice symmetries. Optical properties are particularly important, as they determine potential device applications and enable phase identification. However, direct comparison of optical signatures, including key parameters such as bandgaps, is hindered by inconsistent, sparse, or even missing data in the literature. To address this issue, in the present work we systematically cross-correlate optical emission and absorption features of {\alpha}, {\beta}, {\gamma}, {\delta}, and {\kappa} thin films, as well as differently oriented {\beta}-phase bulk crystals and {\gamma}/{\beta} double polymorph structures. We demonstrate that optical bandgaps and emission features scale consistently across the polymorphs when methodological uncertainties are minimized by applying identical experimental conditions and unified analysis procedures to a structurally similar set of thin film samples. In addition, we extend conventional far field optical phase identification to the nanoscale by reporting near field optical signatures of Ga2O3 polymorphs via nano FTIR. Overall, the present dataset provides a comprehensive reference of near- and far-field optical polymorph signatures to support ongoing multidisciplinary research on Ga2O3.

cond-mat.mtrl-sci↗

arXiv2024

Band Offsets at \beta/{\gamma}-$\mathrm{Ga}_{2}\mathrm{O}_{3}$ Interface

Ultrawide bandgap semiconductor gallium oxide (Ga2O3) and its polymorphs have recently attracted increasing attention across physics, materials science, and electronics communities. In particular, the self-organized formation of the beta/gamma-Ga2O3 double polymorph structures was demonstrated recently [A. Azarov et al., Nat. Commun. 14, 4855 (2023)], paving the way for prospective applications of such structures in electronics. Consequently, determining the conduction band offset in such structures is crucial since it dictates the behavior of conduction electrons at the interface and, consequently, the potential functionality of such interfaces. Thus, in this work, we calculate the band offsets at the beta/gamma-Ga2O3 interface using density functional theory in correlation with the data provided by the experimental atomistic interface analysis. Specifically, to unravel the strain state of the beta/gamma-Ga2O3 interface, nanoscale strain maps were recorded using high-resolution transmission electron microscopy. In its turn, theoretically, lineup potential and vacuum alignment methods were used to analyze the band offsets, with and without strain, at the beta/gamma-Ga2O3 interface. Altogether, the collected results suggest that the band offsets between the beta and gamma phases are likely not exceeding a few hundred meV, remaining highly sensitive to the strain state at the interface. At this end, we conclude that even though the formation of a two-dimensional electron gas (2DEG) at the beta/gamma interface is theoretically possible, the gradual strain relaxation--if it occurs as a function of the distance from the interface--poses a significant challenge, as it may shift the 2DEG localization or even reduce the overall probability of its formation.

cond-mat.mtrl-sci↗

arXiv2024

Phase glides and self-organization of atomically abrupt interfaces out of stochastic disorder in $\alpha$-Ga$_{2}$O$_{3}$

Disorder-induced ordering and unprecedentedly high radiation tolerance in $\gamma$-phase of gallium oxide is a recent spectacular discovery at the intersection of the fundamental physics and electronic applications. Importantly, by far, these data were collected with initial samples in form of the thermodynamically stable $\beta$-phase of this material. Here, we investigate these phenomena starting instead from already metastable $\alpha$-phase and explain radically new trend occurring in the system. We argue that in contrast to that in $\beta$-to-$\gamma$ disorder-induced transitions, the O sublattice in $\alpha$-phase exhibits hexagonal close-packed structure, so that to activate $\alpha$-to-$\gamma$ transformation significant structural rearrangements are required in both Ga and O sublattices. Moreover, consistently with theoretical predictions, $\alpha$-to-$\gamma$ phase transformation requires accumulation of the substantial tensile strain to initiate otherwise impossible lattice glides. Thus, we explain the experimentally observed trends in term of the combination of disorder and strain governing the process. Finally, and perhaps most amazingly, we demonstrate atomically abrupt $\alpha$/$\gamma$ interfaces paradoxically self-organized out of the stochastic disorder.

cond-mat.mtrl-sci↗

arXiv2024

Orientation-dependent surface radiation damage in $\beta$-Ga2O3 explored by multiscale atomic simulations

Ultrawide bandgap semiconductor $\beta$-Ga2O3 holds extensive potential for applications in high-radiation environments. One of the primary challenges in its practical application is unveiling the mechanisms of surface irradiation damage under extreme conditions. In this study, we investigate the orientation-dependent mechanisms of radiation damage on four experimentally relevant $\beta$-Ga2O3 surface facets, namely, (100), (010), (001), and (-201), at various temperatures. We employ a multiscale atomic simulation approach, combining machine-learning-driven molecular dynamics (ML-MD) simulations and density functional theory (DFT) calculations. The results reveal that Ga vacancies and O interstitials are the predominant defects across all four surfaces, with the formation of many antisite defects Ga_O and few O_Ga observed. Among the two Ga sites and three O sites, the vacancy found in the O2 site is dominant, while the interstitials at the Ga1 and O1 sites are more significant. Interestingly, the (010) surface exhibits the lowest defect density, owing to its more profound channeling effect leading to a broader spread of defects. The influence of temperature on surface irradiation damage of $\beta$-Ga2O3 should be evaluated based on the unique crystal surface characteristics. Moreover, the formation energy and defect concentration calculated by DFT corroborate the results of the MD simulations. Comprehending surface radiation damage at the atomic level is crucial for assessing the radiation tolerance and predicting the performance changes of $\beta$-Ga2O3-based device in high-radiation environments.

cond-mat.mtrl-sci↗

arXiv2024

Crater-shaped Enrichment of $\mathrm{V}_\mathrm{Si}$ Color Centers in $4H$-SiC using Single-Pulse Near-Infrared Femtosecond Laser Processing

Currently, Si vacancy ($\mathrm{V}_\mathrm{Si}$) color centers in SiC are of significant interest due to their potential applications in quantum sensing and quantum communication. Meanwhile, the qualities of laser-induced color centers are well guaranteed. Femtosecond laser processing suffices for increasing the yield of $\mathrm{V}_\mathrm{Si}$ color centers in bulk materials and forms crater-shaped enriched regions on the surface. However, there is a notable absence of existing simulation methods to explain the mechanisms behind laser-assisted $\mathrm{V}_\mathrm{Si}$ color center generation. In this work, we design a three-dimensional molecular dynamics (3D-MD) model using an integral hemi-ellipsoidal shell mathematical model to simulate the interaction of Gaussian laser beams with bulk materials. Furthermore, we calculate the transmittance, absorption coefficient, refractive index, and reflectivity of $4H$-SiC. Then, the absorptance of a 1030 nm laser in 350 {\mu}m-thick $4H$-SiC material is abtained to simulate the energy loss during the actual processing. Finally, the study analyzes the movement trajectories of $\mathrm{V}_\mathrm{Si}$ color centers and explains the source of $\mathrm{V}_\mathrm{Si}$ on the surface. This analysis explains the reasons for the enrichment of color centers in the crater-shaped regions formed after laser deposition. Our work provides an effective 3D-MD modeling approach to study the processing mechanisms of laser interaction with semiconductor materials, offering insights into efficient $\mathrm{V}_\mathrm{Si}$ color center creation processes.

physics.optics↗

arXiv2024

Self-assembling of multilayered polymorphs with ion beams

Polymorphism contributes to the diversity of nature, so that even materials having identical chemical compositions exhibit variations in properties because of different lattice symmetries. Thus, if stacked together into multilayers, polymorphs may work as an alternative approach to the sequential deposition of layers with different chemical compositions. However, selective polymorph crystallization during conventional thin film synthesis is not trivial; e.g. opting for step-like changes of temperature and/or pressure correlated with switching from one polymorph to another during synthesis is tricky, since it may cause degradation of the structural quality. In the present work, applying the disorder-induced ordering approach we fabricated such multilayered polymorph structures using ion beams. We show that during ion irradiation of gallium oxide, the dynamic annealing of disorder may be tuned towards self-assembling of several polymorph interfaces, consistently with theoretical modelling. Specifically, we demonstrated multilayers with two polymorph interface repetitions obtained in one ion beam assisted fabrication step. Importantly, single crystal structure of the polymorphs was maintained in between interfaces exhibiting repeatable crystallographic relationships, correlating with optical cross-sectional maps. This data paves the way for enhancing functionalities in materials with not previously thought capabilities of ion beam technology.

cond-mat.mtrl-sci↗

arXiv2024

Large-scale atomistic study of plasticity in amorphous gallium oxide with a machine-learning potential

Compared to the widely investigated crystalline polymorphs of gallium oxide (Ga2O3), knowledge about its amorphous state is still limited. With the help of a machine-learning interatomic potential, we conducted large-scale atomistic simulations to investigate the glass transition and mechanical behavior of amorphous Ga2O3 (a-Ga2O3). During the quenching simulations, amorphization of gallium oxide melt is observed at ultrahigh cooling rates, including a distinct glass transition. The final densities at room temperature have up to 4% variance compared to experiments. The glass transition temperature is evaluated to range from 1234 K to 1348 K at different cooling rates. Structural analysis of the amorphous structure shows evident similarities in structural properties between a-Ga2O3 and amorphous alumina (a-Al2O3), such as radial distribution function, coordination distribution, and bond angle distribution. An amorphous gallium oxide structure that contains approximately one million atoms is prepared for the tension simulation. A highly plastic behavior is observed at room temperature in the tension simulations, comparable to amorphous alumina. With quantitative characterization methods, we show that a-Ga2O3 can possibly has a higher nucleation rate of localized plastic strain events compared to a-Al2O3, which can increase the material's resistance to shear banding formation during deformation.

cond-mat.mtrl-sci↗

arXiv2024

Ultrahigh Stability of O-Sublattice in $\beta$-Ga$_2$O$_3$

Recently reported remarkably high radiation tolerance of $\gamma$/$\beta$-Ga$_2$O$_3$ double-polymorphic structure brings this ultrawide bandgap semiconductor to the frontiers of power electronics applications that are able to operate in challenging environments. Understanding the mechanism of radiation tolerance is crucial for further material modification and tailoring of the desired properties. In this study, we employ machine-learning-enhanced atomistic simulations to assess the stability of both the gallium (Ga) and oxygen (O) sublattices under various levels of damage. Our study uncovers the remarkable resilience and stability of the O-sublattice, attributing this property to the strong tendency of recovery of the O defects, especially within the stronger disordered regions. Interestingly, we observe the opposite behavior of the Ga defects that display enhanced stability in the same regions of increased disorder. Moreover, we observe that highly defective $\beta$-Ga$_2$O$_3$ is able to transform into $\gamma$-Ga$_2$O$_3$ upon annealing due to preserved lattice organization of the O-sublattice. This result clearly manifests that the ultrahigh stability of the O-sublattice provides the backbone for the exceptional radiation tolerance of the $\gamma$/$\beta$ double-polymorphic structure. These computational insights closely align with experimental observations, opening avenues for further exploration of polymorphism in Ga$_2$O$_3$ and potentially in analogous polymorphic families spanning a broad range of diverse materials of complex polymorphic nature.

cond-mat.mtrl-sci↗

arXiv2024

Crystallization Instead of Amorphization in Collision Cascades in Gallium Oxide

Disordering of solids typically leads to amorphization, but polymorph transitions, facilitated by favorable atomic rearrangements, may temporarily help to maintain long-range periodicity in the solid state. In far-from-equilibrium situations, such as atomic collision cascades, these rearrangements may not necessarily follow a thermodynamically gainful path, but may be kinetically limited. In this Letter, we focused on such crystallization instead of amorphization in collision cascades in gallium oxide (\ce{Ga2O3}). We determined the disorder threshold for irreversible $β$-to-$γ$ polymorph transition and explained why it results in elevating energy to that of the $γ$-polymorph, which exhibits the highest polymorph energy in the system below the amorphous state. Specifically, we demonstrate that upon reaching the disorder transition threshold, the \ce{Ga}-sublattice kinetically favors transitioning to the $γ$-like configuration, requiring significantly less migration for \ce{Ga} atoms to reach the lattice sites during post-cascade processes. As such, our data provide a consistent explanation of this remarkable phenomenon and can serve as a toolbox for predictive multi-polymorph fabrication.

cond-mat.mtrl-sci↗

arXiv2024

Generalized Algorithm for Recognition of Complex Point Defects in Large-Scale β-$\rm {Ga_2O_3}$

The electrical and optical properties of semiconductor materials are profoundly influenced by the atomic configurations and concentrations of intrinsic defects. This influence is particularly significant in the case of $β$-$\rm {Ga_2O_3}$, a vital ultrawide bandgap semiconductor characterized by highly complex intrinsic defect configurations. Despite its importance, there is a notable absence of an accurate method to recognize these defects in large-scale atomistic computational modeling. In this work, we present an effective algorithm designed explicitly for identifying various intrinsic point defects in the $β$-$\rm {Ga_2O_3}$ lattice. By integrating particle swarm optimization and hierarchical clustering methods, our algorithm attains a recognition accuracy exceeding 95% for discrete point defect configurations. Furthermore, we have developed an efficient technique for randomly generating diverse intrinsic defects in large-scale $β$-$\rm {Ga_2O_3}$ systems. This approach facilitates the construction of an extensive atomic database, crucially instrumental in validating the recognition algorithm through a substantial number of statistical analyses. Finally, the recognition algorithm is applied to a molecular dynamics simulation, accurately describing the evolution of the point defects during high-temperature annealing. Our work provides a useful tool for investigating the complex dynamical evolution of intrinsic point defects in $β$-$\rm {Ga_2O_3}$, and moreover, holds promise for understanding similar material systems, such as $\rm {Al_2O_3}$, $\rm {In_2O_3}$, and $\rm {Sb_2O_3}$.

cond-mat.mtrl-sci↗

arXiv2024

Threshold displacement energy map of Frenkel pair generation in $\rm Ga_2O_3$ from machine-learning-driven molecular dynamics simulations

$\beta$ phase gallium oxide ($\beta$-$\rm Ga_2O_3$) demonstrates tremendous potential for electronics applications and offers promising prospects for integration into future space systems with the necessity of high radiation resistance. Therefore, a comprehensive understanding of the threshold displacement energy (TDE) and the radiation-induced formation of Frenkel pairs (FPs) in this material is vital but has not yet been thoroughly studied. In this work, we performed over 5,000 molecular dynamics simulations using our machine-learning potentials to determine the TDE and investigate the formation of FPs. The average TDEs for the two Ga sites, Ga1 (tetrahedral site) and Ga2 (octahedral site), are 22.9 and 20.0 eV, respectively. While the average TDEs for the three O sites are nearly uniform, ranging from 17.0 to 17.4 eV. The generated TDE maps reveal significant differences in displacement behavior between these five atomic sites. Our developed defect identification methods successfully categorize various types of FPs in this material, with more than ten types of Ga FPs being produced during our simulations. O atoms are found to form two main types of FPs and the O split interstitial site on O1 site is most common. Finally, the recombination behavior and barriers of Ga and O FPs indicate that the O FP has a higher possibility of recovery upon annealing. Our findings provide important insights into the studies of radiation damage and defects in $\rm Ga_2O_3$ and can contribute to the design and development of $\rm Ga_2O_3$-based devices

cond-mat.mtrl-sci↗

arXiv2024

RESURF Ga$_{2}$O$_{3}$-on-SiC Field Effect Transistors for Enhanced Breakdown Voltage

Heterosubstrates have been extensively studied as a method to improve the heat dissipation of Ga$_{2}$O$_{3}$ devices. In this simulation work, we propose a novel role for $p$-type available heterosubstrates, as a component of a reduced surface field (RESURF) structure in Ga$_{2}$O$_{3}$ lateral field-effect transistors (FETs). The RESURF structure can eliminate the electric field crowding and contribute to higher breakdown voltage. Using SiC as an example, the designing strategy for doping concentration and dimensions of the $p$-type region is systematically studied using TCAD modeling. To mimic realistic devices, the impacts of interface charge and binding interlayer at the Ga$_{2}$O$_{3}$/SiC interface are also explored. Additionally, the feasibility of the RESURF structure for high-frequency switching operation is supported by the short time constant ($\sim$0.5 ns) of charging/discharging the $p$-SiC depletion region. This study demonstrates the great potential of utilizing the electrical properties of heat-dissipating heterosubstrates to achieve a uniform electric field distribution in Ga$_{2}$O$_{3}$ FETs.

physics.app-ph↗