arXiv subjects

Emmanouil Kioupakis

Publications and source records attributed to Emmanouil Kioupakis.

At least 19 recordsLinked to original sources

arXiv2026

Optical properties of Ag, Au, and Cu from first principles

We present a comprehensive framework for investigating the optical response of metals from first principles that combines density functional theory, many-body perturbation theory, and efficient interpolation techniques based on maximally localized Wannier functions, and apply it to analyze the optical properties of silver (Ag), gold (Au), and copper (Cu). We evaluate the optical properties of these metallic materials considering both single-particle direct and phonon-assisted excitations, as well as the resistive Drude contribution. We find an overall excellent agreement with experimental optical measurements for these materials, and show that both single-particle and collective excitations are important in capturing their optical response in the infrared. Our methodology provides fundamental understanding of the optical response of metals and is generally applicable to investigate the optoelectronic properties of emerging metallic materials.

cond-mat.mtrl-sci↗

arXiv2026

Crystal Orbital Guided Iteration to Atomic Orbitals: A Pathway to Chemically Adaptive Atomic Orbitals from DFT

Atomic orbitals underpin our understanding of electronic structure, providing intuitive descriptions of bonding, charge transfer, magnetism, and correlation effects. Despite their utility, an atomic basis that is adaptable, strictly localized on atomic centers, and enables accurate tight-binding interpolation has remained elusive. Here, we introduce Crystal Orbital Guided Iteration To atomic-Orbitals (COGITO), a framework that constructs an optimal atomic orbital basis by identifying and resolving key mathematical obstacles inherent to nonorthogonal bases -- particularly uncontrolled orbital mixing, and the fixed-overlap constraint between orbitals. We demonstrate that COGITO enables tight-binding models as accurate as MLWF-based approaches, while preserving the ability of tight-binding parameters to represent the projected atomic basis -- an essential feature lost in schemes that enforce orbital orthogonality or maximal localization. By creating accurate and chemically interpretable models of electronic structure, COGITO reveals the orbital-resolved covalent bonds and charge transfer that is encoded in the Kohn-Sham wavefunctions of DFT. Our method thus offers a powerful tool for any physics- or chemistry-based application that relies on a faithful description of local electronic structure.

cond-mat.mtrl-sci↗

arXiv2026

Thermodynamic and electronic properties of rutile Sn$_{1-x}$Ge$_x$O$_2$ alloys from first principles

Rutile Sn$_{1-x}$Ge$_x$O$_{2}$ alloys are promising materials for high-power electronic applications due to their dopability and tunable ultra-wide band gaps. We use first-principles density functional theory and statistical mechanics to investigate the crystallographic, electronic, and thermodynamic properties of rutile $\text{Sn}_{1-x}\text{Ge}_x\text{O}_2$ alloys. We predict that the lattice parameters follow Vegard's law, while band gaps calculated with the hybrid HSE06 functional exhibit strong bowing, consistent with experiment. We also predict that the disordered phase has a large positive mixing enthalpy and a slight tendency for Ge-Sn clustering, indicated by weakly negative short-range order parameters. This large positive mixing enthalpy produces a miscibility gap with a critical temperature above 2300 K, implying that the high Ge and Sn solubilities observed in thin-film synthesis cannot be explained by the incoherent phase diagram alone. We demonstrate that coherency strain during epitaxial growth substantially alters phase stability. Calculations of the coherent spinodal show significant suppression of the miscibility gap, reducing the critical temperature to $\approx 900$ K. These coherent phase boundaries account for the experimentally observed high solubilities at typical growth temperatures. Our results indicate that coherency strain stabilizes these metastable alloys and enables bandgap engineering in this ultrawide-bandgap material system.

cond-mat.mtrl-sci↗

arXiv2025

Origin of shallow n-type doping in AlN and Al-rich AlGaN

Achieving efficient n-type doping in AlN, a representative ultrawide bandgap (UWBG) semiconductor, remains a longstanding challenge that limits its application in high-power electronics and deep-ultraviolet optoelectronics. Conventional dopants in AlN often introduce deep levels or form compensating complexes, leading to low free-carrier concentrations. In this work, we combine first-principles defect calculations with a structural search method tailored to explore metastable configurations to systematically investigate donor-type defects in AlN. Our results reveal that the aluminum interstitial ($Al_i$) can exhibit shallow-donor behavior in specific metastable configurations that were previously overlooked. This discovery expands the understanding of n-type dopability in AlN, and highlights the critical role of metastable defects in modulating electronic properties.

cond-mat.mtrl-sci↗

arXiv2025

Cubic BeB$_2$: A metastable $p$-type conductive material from first principles

Boron forms a wide variety of compounds with alkaline earth elements due to its unique bonding characteristics. Among these, binary compounds of Be and B display particularly rich structural diversity, attributed to the small atomic size of Be. Cubic BeB$_2$ is a particularly interesting phase, where Be donates electrons to stabilize a diamond-like boron network under high pressure. In this work, we employ \textit{ab initio} methods to conduct a detailed investigation of cubic BeB$_2$ and its functional properties. We show that this metastable phase is dynamically stable under ambient conditions, and its lattice match to existing substrate materials suggests possible epitaxial stabilization via thin-film growth routes. Through a comprehensive characterization of its electronic, transport, and superconductivity properties, we demonstrate that cubic BeB$_2$ exhibits high hole concentrations and high hole mobility, making it a potential candidate for efficient $p$-type transport. In addition, cubic BeB$_2$ is found to exhibit low-temperature superconductivity at degenerate doping levels, similar to several other doped covalent semiconductors such as diamond, Si, and SiC.

cond-mat.mtrl-sci↗

arXiv2025

Efficient First-Principles Framework for Overdamped Phonon Dynamics and Anharmonic Electron-Phonon Coupling in Superionic Materials

Relying on the anharmonic special displacement method, we introduce an ab initio quasistatic polymorphous framework to describe local disorder, anharmonicity, and electron-phonon coupling in superionic conductors. Using the example of cubic Cu2Se, we show that positional polymorphism yields extremely overdamped anharmonic vibrations while preserving transverse acoustic phonons, consistent with experiments. We also demonstrate well-defined electronic band structures with large band gap openings due to polymorphism of 1.0 eV and calculate anharmonic electron-phonon renormalization, yielding band gap narrowing with increasing temperature in agreement with previous measurements. Our approach opens the way for efficient ab initio electronic structure calculations in superionic crystals to elucidate their compelling high figure-of-merit.

cond-mat.mtrl-sci↗

arXiv2025

Electronic structure and defect properties of Bi-doped GaN: origins of photoluminescence and optical absorption

Extreme lattice-mismatched III-V nitrides, such as Bi-incorporated GaN, have been realized experimentally thanks to recent advances in epitaxial growth and characterization techniques. However, theoretical insights into defect-related optical absorption and emission phenomena in these materials remain scarce. Here, we apply hybrid density functional theory to systematically explore the role of substitutional bismuth atoms on both cationic $\mathrm{Bi_{Ga}}$ and anionic $\mathrm{Bi_{N}}$ sites in Bi-incorporated GaN, as well as their complexes with native vacancies. Our calculations reveal that the charge-compensated defect complexes $(\mathrm{Bi_{N}} + \mathrm{V_{Ga}})^{3-}$ and $(\mathrm{Bi_{N}} + \mathrm{V_{Ga}})^{3+}$ stabilize anionic bismuth incorporation, accounting for the experimentally observed absorption peaks at ~1.11 eV and ~3.17 eV. We further uncover the origins of the reported band-edge emissions near 2.0 eV and 2.5 eV by examining various charge states of $\mathrm{Bi_{Ga}}$ and $\mathrm{Bi_{N}}$ centers. Our findings elucidate the defect-level physics of Bi-doped GaN and provide practical guidelines for controlling the incorporation of Bi into GaN.

cond-mat.mtrl-sci↗

arXiv2025

Electron mobility in AlN from first principles

Aluminum nitride is a promising ultra-wide band gap semiconductor for optoelectronics and power electronics. However, its practical applications have been limited by challenges with doping and achieving high electrical conductivity. Recent advances in crystal quality and defect control have led to improvements in experimentally measured mobilities. In this work, we apply first-principles calculations to determine the upper limits of the electron mobility in AlN as a function of temperature, doping, and crystallographic orientation. We account for the combined effects of electron scattering by phonons and ionized impurity to model doped systems, and examine both full and partial ionization conditions. Our results show that the piezoelectric interaction from the long-range component of the acoustic modes is the dominant source of electron-phonon scattering at room temperature. Ionized-impurity scattering starts to dominate scattering at dopant concentrations above $10^{16}$ cm$^{-3}$, reducing the mobility by more than an order of magnitude in the high doping regime. Our calculated Hall mobility values are in good agreement with experimental data for samples with comparable dopant concentrations. We also find that electron mobilities as high as $956$ cm$^2$/V$\cdot$s could be achievable at lower dopant concentrations.

cond-mat.mtrl-sci↗

arXiv2025

Extreme-Band-Gap Semiconductors with Shallow Dopants and Mobile Carriers

The conventional distinction between semiconductors and insulators is often based on the magnitude of the band gap, with materials exhibiting gaps wider than 3 eV typically classified as insulators. However, the emergence of ultra-wide-band-gap (UWBG) semiconductors such as AlGaN, diamond, BN, and Ga2O3 challenges this paradigm for materials classification and raises fundamental questions about the upper bound of band gaps compatible with semiconducting behavior. Here we develop a computational-discovery strategy to identify semiconductors with band gaps exceeding that of AlN (6.2 eV), while retaining essential semiconducting properties such as shallow dopants and mobile charge carriers. We discover that materials composed of light elements in densely packed crystal structures exhibit wide band gaps and light carrier effective masses that enable shallow dopants, high mobility, and weak polaron binding. By applying the hydrogenic Bohr model and first-principles defect calculations - validated against available experimental data - to screen for materials with shallow dopants, we identify dopable compounds with gaps as wide as 9.5 eV that nonetheless host mobile charge carriers. Our findings demonstrate that semiconducting behavior persists even at extreme band gaps, far beyond conventional upper bounds traditionally associated with semiconductor materials.

cond-mat.mtrl-sci↗

arXiv2025

Enhancing light emission with electric fields in polar nitride semiconductors

Significant effort has been devoted to mitigating polarization fields in nitride LEDs, as these fields are traditionally viewed as detrimental to light emission, particularly for red emission. Contrary to this prevailing notion, we demonstrate that strong polarization fields can enhance the optical-transition strength of AlInGaN quantum wells emitting in the red, which has been historically challenging to achieve. By leveraging machine-learning surrogate models trained on multi-scale quantum-mechanical simulations, we globally explore the heterostructure design space and uncover that larger fields correlate with higher electron-hole overlap. This relation arises from the quantum-confined Stark effect, which enables thinner wells without requiring higher indium compositions, thus overcoming a key limitation in nitride epitaxy. Structural and compositional engineering of internal fields offers a unique dimension for designing polychromatic nitride LEDs, crucial for miniaturizing LED pixels to the micron scale for extended-reality and biomedical applications. Broadly, our work demonstrates how machine learning can uncover unexpected paradigms for semiconductor design.

physics.optics↗

arXiv2025

Ab initio thermal conductivity of Ge$_x$Sn$_{1-x}$O$_2$ alloys

Rutile GeO2 is an emerging ultra-wide band gap semiconductor (UWBG) that has demonstrated excellent potential for applications in power electronic devices. Alloys of rutile SnO2, a well-established UWBG semiconducting oxide, with GeO2 are promising for tuning the material properties for applications. The thermal conductivity, in particular, is a key property which is significantly impacted by alloy disorder, but which is also essential in assessing the operation and degradation of materials in high-power electronic applications. Here, we present first-principles calculations of the thermal conductivity of rutile GeO2, SnO2, and their alloys, and quantify the effects of scattering by alloy disorder, temperature, and isotope mass distribution. We show that the relatively high thermal conductivity of the binary compounds is reduced by alloying, grain boundaries, and isotope disorder. However, we also find that the room-temperature thermal conductivity of the alloys is still comparable to or surpasses the values for beta-Ga2O3, an established UWBG semiconducting oxide. Our findings provide a roadmap for the codesign of the thermal properties of rutile GexSn1-xO2 alloys for electronic device applications.

cond-mat.mtrl-sci↗

arXiv2025

Impact of Quantum Well Thickness on Efficiency Loss in InGaN/GaN LEDs: Challenges for Thin-Well Designs

We investigate the impact of quantum well (QW) thickness on efficiency loss in c-plane InGaN/GaN LEDs using a small-signal electroluminescence (SSEL) technique. Multiple mechanisms related to efficiency loss are independently examined, including injection efficiency, carrier density vs. current density relationship, phase space filling (PSF), quantum confined stark effect (QCSE), and Coulomb enhancement. An optimal QW thickness of around 2.7 nm in these InGaN/GaN LEDs was determined for quantum wells having constant In composition. Despite better control of deep-level defects and lower carrier density at a given current density, LEDs with thin QWs still suffer from an imbalance of enhancement effects on the radiative and intrinsic Auger-Meitner recombination coefficients. The imbalance of enhancement effects results in a decline in internal quantum efficiency (IQE) and radiative efficiency with decreasing QW thickness at low current density in LEDs with QW thicknesses below 2.7 nm. We also investigate how LED modulation bandwidth varies with quantum well thickness, identifying the key trends and their implications for device performance.

physics.app-ph↗

arXiv2024

Selective incorporation of antimony into gallium nitride

Dilute concentrations of antimony (Sb) incorporation into GaN induce strong band-gap bowing and tunable room-temperature photoluminescence from the UV to the green spectral regions. However, the atomistic details of the incorporation of Sb into the GaN host remain unclear. In this work, we use first-principles calculations to understand the thermodynamics of Sb substitution into GaN, and its effect on the optical and Raman spectra. Although it is empirically considered that Sb is preferentially incorporated as an anion ($\mathrm{Sb^{3-}}$) into the N sublattice, we demonstrate that Sb can also be incorporated as a cation ($\mathrm{Sb^{3+}}$, $\mathrm{Sb^{5+}}$) into the metal sublattice. Our thermodynamic analysis demonstrates that $\mathrm{Sb_N^0}$, $\mathrm{Sb_{Ga}^{2+}}$, and $\mathrm{Sb_{Ga}^0}$ can co-exist under Ga-rich conditions in n-type samples. We further confirm the dual incorporation of Sb by calculating the vibrational frequencies of different anionic and cation substitutions to explain the origins of experimentally observed additional Raman peaks of Sb-doped GaN. Moreover, the calculated band structures of different Sb substitutions into GaN explain the experimental photoluminescence and optical absorption spectra. Overall, our analysis suggests that the coexistence of $\mathrm{Sb^{3-}}$, $\mathrm{Sb^{3+}}$, and $\mathrm{Sb^{5+}}$ substitutions into GaN explains the totality of experimental measurements. Our results demonstrate that the selective incorporation of Sb into GaN (and potentially other group-V elements such as As, P, or Bi) by tuning the growth conditions can drastically modify the electronic properties, for applications in visible light emitters and photocatalysis.

cond-mat.mtrl-sci↗

arXiv2024

Phonon screening of excitons in atomically thin semiconductors

Atomically thin semiconductors, encompassing both 2D materials and quantum wells, exhibit a pronounced enhancement of excitonic effects due to geometric confinement. Consequently, these materials have become foundational platforms for the exploration and utilization of excitons. Recent ab initio studies have demonstrated that phonons can substantially screen electron-hole interactions in bulk semiconductors and strongly modify the properties of excitons. While excitonic properties of atomically thin semiconductors have been the subject of extensive theoretical investigations, the role of phonon screening on excitons in atomically thin structures remains unexplored. In this work, we demonstrate via ab initio GW-Bethe-Salpeter equation calculations that phonon screening can have a significant impact on optical excitations in atomically thin semiconductors. We further show that the degree of phonon screening can be tuned by structural engineering. We focus on atomically thin GaN quantum wells embedded in AlN and identify specific phonons in the surrounding material, AlN, that dramatically alter the lowest-lying exciton in monolayer GaN via screening. Our studies provide new intuition beyond standard models into the interplay among structural properties, phonon characteristics, and exciton properties in atomically thin semiconductors, and have implications for future experiments.

cond-mat.mtrl-sci↗

arXiv2024

Electron mobility of SnO2 from first principles

The transparent conducting oxide SnO2 is a wide bandgap semiconductor that is easily n-type doped and widely used in various electronic and optoelectronic applications. Experimental reports of the electron mobility of this material vary widely depending on the growth conditions and doping concentrations. In this work, we calculate the electron mobility of SnO2 from first principles to examine the temperature- and doping-concentration dependence, and to elucidate the scattering mechanisms that limit transport. We include both electron-phonon scattering and electron-ionized impurity scattering to accurately model scattering in a doped semiconductor. We find a strongly anisotropic mobility that favors transport in the direction parallel to the c-axis. At room temperature and intrinsic carrier concentrations, the low-energy polar-optical phonon modes dominate scattering, while ionized-impurity scattering dominates above 10^18 cm^-3.

cond-mat.mtrl-sci↗

arXiv2024

Carrier confinement and alloy disorder exacerbate Auger-Meitner recombination in AlGaN ultraviolet light-emitting diodes

The quantum efficiency of AlGaN ultraviolet light-emitting diodes (LEDs) declines (droops) at increasing operating powers due to Auger-Meitner recombination (AMR). Using first-principles density-functional theory, we show that indirect AMR mediated by electron-phonon coupling and alloy disorder can induce bulk $C$ coefficients as large as $\sim10^{-31}$ cm$^6$/s. Furthermore, we find that the confinement of carriers by polarization fields within quantum wells severely relaxes crystal-momentum conservation, which exacerbates the rate of AMR over radiative recombination by an order of magnitude relative to the bulk. This results in a striking decrease in quantum efficiency at high power. Suppressing polarization fields and jointly increasing the well width would greatly mitigate AMR and efficiency droop.

cond-mat.mtrl-sci↗

arXiv2024

Electric-Field-Induced Domain Walls in Wurtzite Ferroelectrics

Wurtzite ferroelectrics possess transformative potential for next-generation microelectronics. A comprehensive understanding of their ferroelectric properties and domain energetics is crucial for tailoring their ferroelectric characteristics and exploiting their functional properties in practical devices. Despite burgeoning interest, the exact configurations, and electronic structures of the domain walls in wurtzite ferroelectrics remain elusive. In this work, we elucidate the atomic configurations and electronic properties of electric-field-induced domain walls in ferroelectric ScGaN. By combining transmission electron microscopy and theoretical calculations, a novel charged domain wall with a buckled two-dimensional hexagonal phase is revealed. The dangling bonds associated with these domain walls give rise to unprecedented metallic-like mid-gap states within the forbidden band. Quantitative analysis further unveils a universal charge-compensation mechanism stabilizing antipolar domain walls in ferroelectric materials, wherein the polarization discontinuity at the 180° domain wall is compensated by the dangling bond electrons. Furthermore, the reconfigurable conductivity of these domain walls is experimentally demonstrated, showcasing their potential for ultra-scaled device applications. Our findings represent a pivotal advancement in understanding the structural and electronic properties of wurtzite ferroelectric domain walls and lay the groundwork for fundamental physics studies and device applications.

cond-mat.mtrl-sci↗

arXiv2023

Strain Effects on Auger-Meitner Recombination in Silicon

We study the effects of compressive and tensile biaxial strain on direct and phonon-assisted Auger-Meitner recombination (AMR) in silicon using first-principles calculations. We find that the application of strain has a non-trivial effect on the AMR rate. For most AMR processes, the application of strain increases the AMR rate. However, the recombination rate for the AMR process involving two holes and one electron is suppressed by 38% under tensile strain. We further analyze the specific phonon contributions that mediate the phonon-assisted AMR mechanism, demonstrating the increased anisotropy under strain. Our results indicate that the application of tensile strain increases the lifetime of minority electron carriers in p-type silicon, and can be leveraged to improve the efficiency of silicon devices.

cond-mat.mtrl-sci↗