arXiv subjects

Cheng-Wei Lee

Publications and source records attributed to Cheng-Wei Lee.

18 recordsLinked to original sources

arXiv2025

Ferroelectricity of Wurtzite Al$_{1-x}$Hf$_{x}$N Heterovalent Alloys

Thin films of aluminum hafnium nitride (Al$_{1-x}$Hf$_{x}$N) were synthesized via reactive magnetron sputtering for Hf contents up to $x$ = 0.13. X-ray diffraction showed a single $c$-axis oriented wurtzite phase for all films. Hard X-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs. electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient d$_{33,f,meas}$ measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent--and not just trivalent--alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices.

cond-mat.mtrl-sci↗

arXiv2025

Stability, growth, and doping of In$_{2}$(Si, Ge)$_{2}$O$_{7}$ as promising n-type wide-gap semiconductors

In this paper we investigate, computationally and experimentally, the phase stability, electronic structure properties, and the propensity for n-type doping of In$_{2}$X$_{2}$O$_{7}$ (X=Si, Ge) ternary oxides. This family of materials contains promising novel wide-gap semiconductors based on their estimated high $n$-type Baliga figures of merit and acceptable thermal conductivity for power electronics applications. Here, we find that both In$_{2}$Si$_{2}$O$_{7}$ and In$_{2}$Ge$_{2}$O$_{7}$ to be n-type dopable, with Zr providing between 10$^{16}$ and above 10$^{21}$ cm$^{-3}$ net donor concentrations under O-poor conditions, depending on the chemistry, structure (ground-state thorvetite or high-pressure pyrochlore) and synthesis temperature. Initial thin-film growth and annealing leads to polycrystalline In$_{2}$Ge$_{2}$O$_{7}$ thin films in thorvetite structure with band gap over 4 eV, and confirms Zr doping predictions by achieving electron concentrations at 10$^{14}$-10$^{16}$ cm$^{-3}$ under O-rich condition. While future epitaxial growth development is still needed, this study establishes In$_{2}$X$_{2}$O$_{7}$ as promising n-type wide-gap semiconductors for power electronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Recurrent Neural Networks for Prediction of Electronic Excitation Dynamics

We demonstrate a machine learning based approach which can learn the time-dependent electronic excitation dynamics of small molecules subjected to ion irradiation. Ensembles of recurrent neural networks are trained on data generated by time-dependent density functional theory to relate atomic positions to occupations of molecular orbitals. New data is incrementally and efficiently added to the training data using an active learning process, thereby improving model accuracy. Predicted changes in orbital occupations made by the recurrent neural network ensemble are found to have errors and one standard deviation uncertainties which are two orders of magnitude smaller than the typical values of the orbital occupation numbers. The trained recurrent neural network ensembles demonstrate a limited ability to generalize to molecules not used to train the models. In such cases, the models are able to identify key qualitative features, but struggle to match the quantitative values. The machine learning procedure developed here is potentially broadly applicable and has the potential to enable study of broad ranges of both materials and dynamical processes by drastically lowering the computational cost and providing surrogate model for multiscale simulations.

physics.chem-ph↗

arXiv2024

Combinatorial synthesis and characterization of thin film Al1-xRExN (RE = Pr3+, Tb3+) heterostructural alloys

The potential impact of cation-substituted AlN-based materials, such as Al1-xScxN, Al1-xGaxN, and Al1-xBxN, with exceptional electronic, electromechanical, and dielectric properties has spurred research into this broad family of materials. Rare earth (RE) cations are particularly appealing as they could additionally impart optoelectronic or magnetic functionality. However, success in incorporating a significant level of RE cations into AlN has been limited so far because it is thermodynamically challenging to stabilize such heterostructural alloys. Using combinatorial co-sputtering, we synthesized Al1-xRExN (RE = Pr, Tb) thin films and performed a rapid survey of the composition-structure-property relationships as a function of RE alloying. Under our growth conditions, we observe that Al1-xPrxN maintains a phase-pure wurtzite structure until transitioning to amorphous for x>0.22. Al1-xTbxN exhibits a phase-pure wurtzite structure until x<0.15, then exhibits mixed wurtzite and rocksalt phases for 0.16<x<0.28, and finally becomes amorphous beyond that. Ellipsometry measurements reveal that the absorption onset decreases with increasing rare earth incorporation and has a strong dependence on the phases present. We observe the characteristic cathodoluminescence emission of Pr3+ and Tb3+, respectively. Using this synthesis approach, we have demonstrated incorporation of Pr and Tb into the AlN wurtzite structure up to higher compositions levels than previously reported and made the first measurements of corresponding structural and optoelectronic properties.

cond-mat.mtrl-sci↗

arXiv2024

From Prediction to Experimental Realization of Ferroelectric Wurtzite Al$_{1-x}$Gd$_{x}$N Alloys

AlN-based alloys find widespread application in high-power microelectronics, optoelectronics, and electromechanics. The realization of ferroelectricity in wurtzite AlN-based heterostructural alloys has opened up the possibility of directly integrating ferroelectrics with conventional microelectronics based on tetrahedral semiconductors such as Si, SiC and III-Vs, enabling compute-in-memory architectures, high-density data storage, and more. The discovery of AlN-based wurtzite ferroelectrics has been driven to date by chemical intuition and empirical explorations. Here, we demonstrate the computationally-guided discovery and experimental demonstration of new ferroelectric wurtzite Al$_{1-x}$Gd$_x$N alloys. First-principles calculations indicate that the minimum energy pathway for switching changes from a collective to an individual switching process with a lower overall energy barrier, at a rare-earth fraction $x$ of $x>$ 0.10$-$0.15. Experimentally, ferroelectric switching is observed at room temperature in Al$_{1-x}$Gd$_x$N films with $x>$ 0.12, which strongly supports the switching mechanisms in wurtzite ferroelectrics proposed previously (Lee et al., $\textit{Science Advances}$ 10, eadl0848, 2024). This is also the first demonstration of ferroelectricity in an AlN-based alloy with a magnetic rare-earth element, which could pave the way for additional functionalities such as multiferroicity and opto-ferroelectricity in this exciting class of AlN-based materials.

cond-mat.mtrl-sci↗

arXiv2024

Accelerating Electronic Stopping Power Predictions by 10 Million Times with a Combination of Time-Dependent Density Functional Theory and Machine Learning

Knowing the rate at which particle radiation releases energy in a material, the stopping power, is key to designing nuclear reactors, medical treatments, semiconductor and quantum materials, and many other technologies. While the nuclear contribution to stopping power, i.e., elastic scattering between atoms, is well understood in the literature, the route for gathering data on the electronic contribution has for decades remained costly and reliant on many simplifying assumptions, including that materials are isotropic. We establish a method that combines time-dependent density functional theory (TDDFT) and machine learning to reduce the time to assess new materials to mere hours on a supercomputer and provides valuable data on how atomic details influence electronic stopping. Our approach uses TDDFT to compute the electronic stopping contributions to stopping power from first principles in several directions and then machine learning to interpolate to other directions at a cost of 10 million times fewer core-hours. We demonstrate the combined approach in a study of proton irradiation in aluminum and employ it to predict how the depth of maximum energy deposition, the "Bragg Peak," varies depending on incident angle -- a quantity otherwise inaccessible to modelers. The lack of any experimental information requirement makes our method applicable to most materials, and its speed makes it a prime candidate for enabling quantum-to-continuum models of radiation damage. The prospect of reusing valuable TDDFT data for training the model make our approach appealing for applications in the age of materials data science.

cond-mat.mtrl-sci↗

arXiv2024

Reliable operation of Cr$_2$O$_3$:Mg/ $β$-Ga$_2$O$_3$ p-n heterojunction diodes at 600$^\circ$C

$β$-Ga$_2$O$_3$-based semiconductor heterojunctions have recently demonstrated improved performance at high voltages and elevated temperatures and are thus promising for applications in power electronic devices and harsh-environment sensors. However, the long-term reliability of these ultra-wide band gap (UWBG) semiconductor devices remains barely addressed and may be strongly influenced by chemical reactions at the p-n heterojunction interface. Here, we experimentally demonstrate operation and evaluate the reliability of Cr$_2$O$_3$:Mg/ $β$-Ga$_2$O$_3$ p-n heterojunction diodes at during extended operation at 600$^\circ$C, as well as after 30 repeated cycles between 25-550$^\circ$C. The calculated pO2-temperature phase stability diagram of the Ga-Cr-O material system predicts that Ga$_2$O$_3$ and Cr$_2$O$_3$ should remain thermodynamically stable in contact with each other over a wide range of oxygen pressures and operating temperatures. The fabricated Cr$_2$O$_3$:Mg / $β$-Ga$_2$O$_3$ p-n heterojunction diodes show room-temperature on/off ratios >10$^4$ at $\pm$5V and a breakdown voltage (V$_{Br}$) of -390V. The leakage current increases with increasing temperature up to 600$^\circ$C, which is attributed to Poole-Frenkel emission with a trap barrier height of 0.19 eV. Over the course of a 140-hour thermal soak at 600$^\circ$C, both the device turn-on voltage and on-state resistance increase from 1.08V and 5.34 m$Ω$-cm$^2$ to 1.59V and 7.1 m$Ω$-cm$^2$ respectively. This increase is attributed to the accumulation of Mg and MgO at the Cr$_2$O$_3$/Ga$_2$O$_3$ interface as observed from TOF-SIMS analysis. These findings inform future design strategies of UWBG semiconductor devices for harsh environment operation and underscore the need for further reliability assessments for $β$-Ga$_2$O$_3$ based devices.

cond-mat.mtrl-sci↗

arXiv2024

NiGa$_{2}$O$_{4}$ interfacial layers in NiO/Ga$_{2}$O$_{3}$ heterojunction diodes at high temperature

NiO/Ga$_{2}$O$_{3}$ heterojunction diodes have attracted attention for high-power applications, but their high-temperature performance and reliability remain underexplored. Here we report on the time evolution of the static electrical properties in the widely studied p-NiO/n-Ga$_{2}$O$_{3}$heterojunction diodes and the formation of NiGa$_{2}$O$_{4}$ interfacial layers when operated at $550^{\circ}$C. Results of our thermal cycling experiment show an initial leakage current increase which stabilizes after sustained thermal load, due to reactions at the NiO-Ga$_{2}$O$_{3}$ interface. High-resolution TEM microstructure analysis of the devices after thermal cycling indicates that the NiO-Ga$_{2}$O$_{3}$ interface forms ternary compounds at high temperatures, and thermodynamic calculations suggest the formation of the spinel NiGa$_{2}$O$_{4}$ layer between NiO and Ga$_{2}$O$_{3}$. First-principles defect calculations find that NiGa$_{2}$O$_{4}$ shows low p-type intrinsic doping, and hence can also serve to limit electric field crowding at the interface. Vertical NiO/Ga$_{2}$O$_{3}$ diodes with intentionally grown 5 nm thin spinel-type NiGa$_{2}$O$_{4}$ interfacial layers show excellent device ON/OFF ratio of > 10$^{10}$($\pm$3 V), V$_{ON}$ of ~1.9 V, and breakdown voltage of ~ 1.2 kV for an initial unoptimized 300-micron diameter device. These p-n heterojunction diodes are promising for high-voltage, high-temperature applications.

cond-mat.mtrl-sci↗

arXiv2023

Computational insights into phase equilibria between wide-gap semiconductors and contact materials

Novel wide-band-gap semiconductors are needed for next-generation power electronic but there is a gap between a promising material and a functional device. Finding stable contacts is one of the major challenges, which is currently dealt with mainly via trial and error. Herein, we computationally investigate the thermochemistry and phase co-existence at the junction between three wide gap semiconductors, $β$-Ga$_{2}$O$_{3}$, GeO$_2$, and GaN, and possible contact materials. The pool of possible contacts includes 47 elemental metals and 4 common $n$-type transparent conducting oxides (ZnO, TiO$_2$, SnO$_2$, and In$_2$O$_3$). We use first-principles thermodynamics to model the Gibbs free energies of chemical reactions as a function of the gas pressure (p$_{\mathrm{O}_2}$/p$_{\mathrm{N}_2}$) and equilibrium temperature. We deduce whether a semiconductor/contact interface will be stable at relevant conditions, possibly influencing the long-term reliability and performance of devices. We generally find that most elemental metals tend to oxidize or nitridize and form various interface oxide/nitride layers. Exceptions include select late- and post-transition metals, and in case of GaN also the alkali metals, which are predicted to exhibit stable coexistence, although in many cases at relatively low gas partial pressures. Similar is true for the transparent conducting oxides, for which in most cases we predict a preference toward forming ternary oxides when in contact with $β$-Ga$_{2}$O$_{3}$ and GeO$_{2}$. The only exception is SnO$_2$, which can form stable contacts with both oxides. Finally, we show how the same approach can be used to predict gas partial pressure vs. temperature phase diagrams to help direct synthesis of ternary compounds. We believe these results provide a valuable guidance in selecting contact materials to wide-gap semiconductors and suitable growth conditions.

cond-mat.mtrl-sci↗

arXiv2023

Defects and Oxygen Impurities in Ferroelectric Wurtzite Al$_{1-x}$Sc$_x$N Alloys

III-nitrides and related alloys are widely used for optoelectronics and as acoustic resonators. Ferroelectric wurtzite nitrides are of particular interest because of their potential for direct integration with Si and wide bandgap semiconductors, and unique polarization switching characteristics; such interest has taken off since the first report of ferroelectric Al$_{1-x}$Sc$_x$N alloys. However, the coercive fields needed to switch polarization are on the order of MV/cm, which is 1-2 orders of magnitude larger than oxide perovskite ferroelectrics. Atomic-scale point defects are known to impact the dielectric properties, including breakdown fields and leakage currents, as well as ferroelectric switching. However, very little is known about the native defects and impurities in Al$_{1-x}$Sc$_x$N, and their effect on the dielectric properties. In this study, we use first-principles calculations to determine the formation energetics of native defects and unintentional oxygen incorporation in Al$_{1-x}$Sc$_x$N. We find that nitrogen vacancies are the dominant native defects, and that they introduce multiple mid-gap states that can lead to premature dielectric breakdown in ferroelectrics and carrier recombination in optoelectronics. Growth under N-rich conditions will reduce the concentration of these deep defects. We also investigate unintentional oxygen incorporation on the nitrogen site and find that the substitutional defect is present in high concentrations, which can contribute to increased temperature-activated leakage currents. Our findings provide fundamental understanding of the defect physics in Al$_{1-x}$Sc$_x$N alloys, which is critical for future deployment of ferroelectric devices.

cond-mat.mtrl-sci↗

arXiv2022

Transition metal impurities in Silicon: Computational search for a semiconductor qubit

Semiconductors offer a promising platform for physical implementation of qubits, but their broad adoption is presently hindered by limited scalability and/or very low operating temperatures. Learning from the nitrogen-vacancy centers in diamond, our goal is to find equivalent optically active point defect centers in crystalline silicon, which could be advantageous for their scalability and integration with classical devices. Transition metal (TM) impurities in silicon are common paramagnetic deep defects, but a comprehensive theoretical study of the whole 3$d$ series that considers generalized Koopmans' condition is missing. We apply the HSE06(+U) method to examine their potential as optically active spin qubits and identify seven TM impurities that have optically allowed triplet-triplet transitions within the silicon band gap. These results provide the first step toward silicon-based qubits with higher operating temperatures for quantum sensing. Additionally, these point defects could lead to spin-photon interfaces in silicon-based qubits and devices for mid-infrared free-space communications.

cond-mat.mtrl-sci↗

arXiv2022

Electron dynamics in extended systems within real-time time-dependent density functional theory

Due to a beneficial balance of computational cost and accuracy, real-time time-dependent density functional theory has emerged as a promising first-principles framework to describe electron real-time dynamics. Here we discuss recent implementations around this approach, in particular in the context of complex, extended systems. Results include an analysis of the computational cost associated with numerical propagation and when using absorbing boundary conditions. We extensively explore the shortcomings for describing electron-electron scattering in real time and compare to many-body perturbation theory. Modern improvements of the description of exchange and correlation are reviewed. In this work, we specifically focus on the Qb@ll code, which we have mainly used for these types of simulations over the last years, and we conclude by pointing to further progress needed going forward.

cond-mat.mtrl-sci↗

arXiv2022

Ternary Wide Band Gap Oxides for High-Power Electronics Identified Computationally

As electricity grids become more renewable energy-compliant, there will be a need for novel semiconductors that can withstand high power, high voltage, and high temperatures. Wide band gap (WBG) semiconductors tend to exhibit large breakdown field, allowing high operating voltages. Currently explored WBG materials for power electronics are costly (GaN), difficult to synthesize as high-quality single crystals (SiC) and at scale (diamond, BN), have low thermal conductivity ($β$-Ga$_2$O$_3$), or cannot be suitably doped (AlN). We conduct a computational search for novel semiconductors across 1,340 known metal-oxides using first-principles calculations and existing transport models. We calculate the Baliga figure of merit (BFOM) and lattice thermal conductivity ($κ_L$) to identify top candidates for n-type power electronics. We find 40 mostly ternary oxides that have higher $κ_L$ than $β$-Ga$_2$O$_3$ and higher n-type BFOM than SiC and GaN. Among these, several material classes emerge, including 2-2-7 stoichiometry thortveitites and pyrochlores, II-IV spinels, and calcite-type borates. Within these classes, we propose In$_2$Ge$_2$O$_7$, Mg$_2$GeO$_4$, and InBO$_3$ as they are the most favorable for n-type doping based on our preliminary evaluation and could be grown as single crystals or thin film heterostructures. These materials could help advance power electronic devices for the future grid.

cond-mat.mtrl-sci↗

arXiv2021

Real-time exciton dynamics with time-dependent density-functional theory

Linear-response time-dependent density-functional theory (TDDFT) can describe excitonic features in the optical spectra of insulators and semiconductors, using exchange-correlation (xc) kernels behaving as $-1/k^{2}$ to leading order. We show how excitons can be modeled in real-time TDDFT, using an xc vector potential constructed from approximate, long-range corrected xc kernels. We demonstrate for various materials that this real-time approach is consistent with frequency-dependent linear response, gives access to femtosecond exciton dynamics following short-pulse excitations, and can be extended with some caution into the nonlinear regime.

cond-mat.mtrl-sci↗

arXiv2020

Multi-scale simulations of electron and ion dynamics in self-irradiated silicon

The interaction of energetic ions with the electronic and ionic system of target materials is an interesting but challenging multi-scale problem and understanding of the early stages after impact of heavy, initially charged ions is particularly poor. At the same time, energy deposition during these early stages determines later formation of damage cascades. We address the multi-scale character by combining real-time time-dependent density functional theory for electron dynamics with molecular dynamics simulations of damage cascades. Our first-principles simulations prove that core electrons affect electronic stopping and have an unexpected influence on the charge state of the projectile. We show that this effect is absent for light projectiles, but dominates the stopping physics for heavy projectiles. By parameterizing an inelastic energy loss friction term in the molecular dynamics simulations using our first-principles results, we also show a qualitative influence of electronic stopping physics on radiation-damage cascades.

cond-mat.mtrl-sci↗

arXiv2018

Hot-electron mediated ion diffusion in proton-irradiated magnesium oxide

Highly energetic ions that impact materials have applications from semiconductor industry to medicine, and are fundamentally interesting as they trigger multi-length and time-scale processes. In particular, they excite electrons into non-thermalized energy distributions with subsequent non-equilibrium electron-electron and electron-ion dynamics. In order to achieve a quantitative description of these, we propose a general first-principles framework that bridges time scales from ultrafast electron dynamics directly after impact, to ion diffusion over migration barriers in semiconductors. We apply it to magnesium oxide under proton irradiation and discover a diffusion mechanism that is mediated by hot electrons. Our quantitative simulations show that this mechanism strongly depends on the projectile-ion velocity. This indicates that it may occur only at a specific penetration depth in the target and that it can be triggered by varying the kinetic energy of the particle radiation. Either of these predictions should facilitate direct experimental observation of this effect and significantly advances current understanding of non-equilibrium electron-ion dynamics.

cond-mat.mtrl-sci↗

arXiv2018

Electronic stopping and proton dynamics in InP, GaP, and In$_{0.5}$Ga$_{0.5}$P from first principles

The phosphide-based III-V semiconductors InP, GaP, and In$_{0.5}$Ga$_{0.5}$P are promising materials for solar panels in outer space and radioisotope batteries, for which lifetime is a major issue. In order to understand high radiation tolerance of these materials and improve it further, it is necessary to describe the early stages of radiation damage on fast time and short length scales. In particular, the influence of atomic ordering, as observed e.g. in In$_{0.5}$Ga$_{0.5}$P, on electronic stopping is unknown.We use real-time time-dependent density functional theory and the adiabatic local density approximation to simulate electronic stopping of protons in InP, GaP, and the CuAu-I ordered phase of In$_{0.5}$Ga$_{0.5}$P across a large kinetic energy range.These results are compared to SRIM and we investigate the dependence on the channel of the projectile through the target.We show that stopping can be enhanced or reduced in In$_{0.5}$Ga$_{0.5}$P and explain this using the electron-density distribution. By comparing Ehrenfest and Born-Oppenheimer molecular dynamics, we illustrate the intricate dynamics of a proton on a channeling trajectory.

cond-mat.mtrl-sci↗

arXiv2013

Replacement Paths via Row Minima of Concise Matrices

Matrix $M$ is {\em $k$-concise} if the finite entries of each column of $M$ consist of $k$ or less intervals of identical numbers. We give an $O(n+m)$-time algorithm to compute the row minima of any $O(1)$-concise $n\times m$ matrix. Our algorithm yields the first $O(n+m)$-time reductions from the replacement-paths problem on an $n$-node $m$-edge undirected graph (respectively, directed acyclic graph) to the single-source shortest-paths problem on an $O(n)$-node $O(m)$-edge undirected graph (respectively, directed acyclic graph). That is, we prove that the replacement-paths problem is no harder than the single-source shortest-paths problem on undirected graphs and directed acyclic graphs. Moreover, our linear-time reductions lead to the first $O(n+m)$-time algorithms for the replacement-paths problem on the following classes of $n$-node $m$-edge graphs (1) undirected graphs in the word-RAM model of computation, (2) undirected planar graphs, (3) undirected minor-closed graphs, and (4) directed acyclic graphs.

cs.DS↗