arXiv subjects

Khang Hoang

Publications and source records attributed to Khang Hoang.

At least 19 recordsLinked to original sources

arXiv2026

First-principles identification of optically efficient erbium centers in GaAs

Gallium arsenide (GaAs) doped with erbium (Er), a material of interest for optoelectronics and quantum information, has been studied for decades. Yet the formation of Er luminescence centers in the semiconductor host and their properties are still not well understood. Here we present a systematic investigation of Er-related defects in GaAs, including defect complexes consisting of Er and native point defects or oxygen impurities, using first-principles hybrid-functional defect calculations. We find that these defects have electronic structure and energetics that are generally asymmetric with respect to n- and p-type doping and tend to favor electron trapping. On the basis of the calculated defect levels, formation energies, and nonradiative carrier capture coefficients, we identify Er-related defects that are efficient as trap-assisted nonradiative recombination centers for Er$^{3+}$ excitation under host photoexcitation or via minority carrier injection. Our results provide an understanding for why a particular defect center with Er coupled to two oxygen atoms is most efficient, and for the effects of n- and p-type doping and of the Er/O ratio on the formation of optically active Er centers and on the Er luminescence observed in experiments.

cond-mat.mtrl-sci↗

arXiv2025

Native defects, hydrogen impurities, and metal dopants in CeO$_2$

Ceria (CeO$_2$) is a material of significant technological importance. A detailed understanding of the material's defect physics and chemistry is key to understanding and optimizing its properties. Here, we report a hybrid density-functional study of native point defects, hydrogen impurities, and metal dopants in CeO$_2$. We find that electron polarons ($η_{\rm Ce}^-$) and oxygen vacancies ($V_{\rm O}^{2+}$) are the dominant native defects under conditions ranging from extreme oxidizing to highly reducing. Hydrogen is stable either in the hydroxyl (H$_i^+$) or hydride (H$_{\rm O}^+$) structure but the substitutional H$_{\rm O}^+$ is energetically more favorable than H$_i^+$ only under highly reducing conditions. The interstitial H$_i^+$ is highly mobile in the bulk. Yttrium (Y) is energetically most favorable at the substitutional Ce site. Copper (Cu) and nickel (Ni) can be incorporated at the substitutional site and/or an interstitial site, depending on actual conditions during preparation, and the dopants can exist in different charge and spin states. In light of the results, we discuss electronic and ionic conduction and the effects of metal doping on the formation of electron polarons and oxygen vacancies.

cond-mat.mtrl-sci↗

arXiv2025

First-principles characterization of native defects and oxygen impurities in GaAs

We present a systematic investigation of native point defects and oxygen impurities in GaAs using hybrid functional calculations. Defects are characterized by their structural, electronic, and optical properties. Under thermodynamic equilibrium, dominant native defects are Ga antisites (Ga$_{\rm As}$), As antisites (As$_{\rm Ga}$), and/or Ga vacancies ($V_{\rm Ga}$) in which As$_{\rm Ga}$ and $V_{\rm Ga}$ are charge-compensating defects under As-rich conditions. On the basis of the defect transition levels, the isolated As$_{\rm Ga}$ can be identified with the $EL2$ center reported in experiments. The defect, however, has a negligible nonradiative electron capture cross section and thus cannot be the ``main electron trap'' as commonly believed. We find that GaAs can have multiple O-related defect centers, especially when prepared under As-rich conditions. The quasi-substitutional O impurity (O$_{\rm As}$) and its complex with two As$_{\rm Ga}$ defects (O$_{\rm As}$-2As$_{\rm Ga}$) both have a metastable and paramagnetic middle (neutral) charge state; however, only the latter can be identified with the experimentally observed Ga--O--Ga or ``OX'' center. These two defects have large nonradiative electron capture cross sections and can be effective carrier traps or recombination centers, which has important implications for materials design.

cond-mat.mtrl-sci↗

arXiv2025

Hydrogen defects as probes of band alignment in metal-organic frameworks

Band alignment, namely the prediction of band-edge positions of semiconductors and insulators in aqueous solutions, is an important problem in physics and chemistry. Such a prediction is especially challenging for structurally and chemically complex, multi-component materials. Here we present an approach to align band structure of metal-organic frameworks (MOFs) on an absolute energy scale which can be used for direct comparison with experiments. Hydrogen defects are used as probes into the chemical bonding of the hybrid inorganic-organic materials. An effective hydrogen defect level, defined as the average of the charge-state transition levels of the defects at the secondary building unit and at the linker, is identified as a charge neutrality level to align band structures. This level captures subtle chemical details at both the building blocks and provides results that are in agreement with experiments in a wide range of different MOFs. We also compare with results obtained from using other approaches involving surface calculations and average pore-center electrostatic potentials.

cond-mat.mtrl-sci↗

arXiv2022

Design of battery materials via defects and doping

This chapter illustrates the use of defect physics as a conceptual and theoretical framework for understanding and designing battery materials. It starts with a methodology for first-principles studies of defects in complex transition-metal oxides. The chapter then considers defects that are activated in a cathode material during synthesis, during measurements, and during battery use. Through these cases, it discusses possible defect landscapes in the material and their implications, guidelines for materials design via defect-controlled synthesis, mechanisms for electronic and ionic conduction and for electrochemical extraction and (re-)insertion, and effects of doping. Although specific examples are taken from studies of battery cathode materials, the computational approach and discussions are general and applicable to any ionic, electronic, or mixed ionic-electronic conducting materials.

cond-mat.mtrl-sci↗

arXiv2022

Defects and persistent luminescence in Eu-doped SrAl$_2$O$_4$

We investigate native point defects and rare-earth (co)dopants in SrAl$_2$O$_4$ using hybrid density-functional defect calculations. Europium (Eu) and dysprosium (Dy) are found to be mixed valence and energetically most favorable at the Sr lattice sites. However, unlike Eu where both Eu$^{2+}$ and Eu$^{3+}$ can be realized in synthesis, Dy is stable predominantly as Dy$^{3+}$, and the divalent Dy$^{2+}$ may only be photogenerated under irradiation. On the basis of an analysis of Eu-related band-defect (including charge-transfer) and interconfigurational $5d$-$4f$ optical transitions, we assign the characteristic broad blue (445 nm) and green (520 nm) emission bands in Eu$^{2+}$-doped SrAl$_2$O$_4$ to the $4f^65d^1$ $\rightarrow$ $4f^7$ transition in Eu$^{2+}$ incorporated at the Sr1 and Sr2 sites, respectively. Strontium interstitials (not oxygen vacancies, in contrast to what is commonly believed) and Dy$_{\rm Sr}$ can act as efficient electron traps for room-temperature persistent luminescence. This work calls for a re-assessment of certain assumptions regarding specific carrier trapping centers made in all mechanisms previously proposed for the persistent luminescence in Eu- and (Eu,Dy)-doped SrAl$_2$O$_4$. It also serves as a methodological template for the understanding and design of rare-earth doped phosphors.

cond-mat.mtrl-sci↗

arXiv2022

Rare-earth defects in GaN: A systematic investigation of the lanthanide series

Rare-earth (RE) doped GaN is of interest for optoelectronics and spintronics and potentially for quantum applications. A fundamental understanding of the interaction between RE dopants and the semiconductor host is key to realizing the material's full potential. This work reports an investigation of lanthanide ($Ln$) defects in GaN using hybrid density-functional defect calculations. We find that all the $Ln$ dopants incorporated at the Ga lattice site, $Ln_{\rm Ga}$ ($Ln$ = La--Lu), are stable as trivalent ions, but Eu and Yb can also be stabilized as divalent and Ce, Pr, and Tb as tetravalent. The location of $Ln$-related defect levels and the $Ln$ $4f$ states in the energy spectrum of the host material is determined from first principles. We elucidate the interplay between defect formation and electronic structure, including the $Ln$--N interaction, and the effect of doping on the local lattice environment. Optical properties are investigated by considering possible defect-to-band and band-to-defect transitions involving $Ln_{\rm Ga}$ defects with in-gap energy levels, including broad "charge-transfer" transitions. These defects can also act as carrier traps and mediate energy transfer from the host into the $4f$-electron core of the $Ln$ ion which leads to sharp intra-$f$ luminescence.

cond-mat.mtrl-sci↗

arXiv2022

Why is it so difficult to realize Dy$^{4+}$ in as-synthesized BaZrO$_3$?

Rare-earth doped barium zirconate (BaZrO$_3$) ceramics are of interest as proton-conducting and luminescent materials. Here, we report a study of dysprosium (Dy) and other relevant point defects in BaZrO$_3$ using hybrid density-functional defect calculations. The tetravalent Dy$^{4+}$ is found to be structurally and electronically stable at the Zr lattice site (i.e., as Dy$_{\rm Zr}^0$), but most often energetically less favorable than the trivalent Dy$^{3+}$ (i.e., Dy$_{\rm Zr}^-$) in as-synthesized BaZrO$_3$, due to the formation of low-energy, positively charged oxygen vacancies and the mixed-site occupancy of Dy in the host lattice. The Dy$^{4+}$/Dy$^{3+}$ ratio can, in principle, be increased by preparing the material under highly oxidizing and Ba-rich conditions and co-doping with acceptor-like impurities; however, post-synthesis treatment may still be needed to realize a non-negligible Dy$^{4+}$ concentration. We also find that certain unoccupied Dy $4f$ states and the O $2p$ states are {\it strongly hybridized}, a feature not often seen in rare-earth-containing materials, and that the isolated Dy$_{\rm Zr}$ defect might be the source of a broad blue emission in band-to-defect ("charge-transfer") luminescence.

cond-mat.mtrl-sci↗

arXiv2021

Rare-earth defects and defect-related luminescence in ZnS

Structure and energetics of rare-earth (RE) defects and luminescence of RE and related defects in zincblende zinc sulfide (ZnS) are investigated using hybrid density-functional defect calculations. We find that europium (Eu) is stable predominantly as the divalent Eu$^{2+}$ ion in bulk ZnS. The trivalent Eu$^{3+}$ is structurally and electronically stable, but energetically unfavorable compared to Eu$^{2+}$ due to the presence of low-energy native defects and Eu$^{2+}$-related defect complexes. Other RE dopants, dysprosium (Dy) and erbium (Er), are stable only as Dy$^{3+}$ and Er$^{3+}$, respectively. These results provide an explanation why it is difficult to realize Eu$^{3+}$ in bulk ZnS. A non-negligible Eu$^{3+}$/Eu$^{2+}$ ratio might be achieved with Li co-doping under S-rich (and probably non-equilibrium) synthesis conditions. Optically, Eu-related defects can act as carrier traps for band-to-defect transitions and emit light in the visible range. To assist with experimental optical characterization of the RE defects, we include band-to-defect luminescence involving native defects (Zn vacancies) and/or non-RE impurities (Cu, Cl, and Al) that may also be present in Eu-doped ZnS samples, and assign luminescence centers often observed in experiments to specific defect configurations.

cond-mat.mtrl-sci↗

arXiv2021

Tuning the valence and concentration of europium and luminescence centers in GaN through co-doping and defect association

Defect physics of europium (Eu) doped GaN is investigated using first-principles hybrid density-functional defect calculations. This includes the interaction between the rare-earth dopant and native point defects (Ga and N vacancies) and other impurities (O, Si, C, H, and Mg) unintentionally present or intentionally incorporated into the host material. While the trivalent Eu$^{3+}$ ion is often found to be predominant when Eu is incorporated at the Ga site in wurtzite GaN, the divalent Eu$^{2+}$ is also stable and found to be predominant in a small range of Fermi-level values in the band-gap region. The Eu$^{2+}$/Eu$^{3+}$ ratio can be tuned by tuning the position of Fermi level and through defect association. We find co-doping with oxygen can facilitate the incorporation of Eu into the lattice. The unassociated Eu$_{\rm Ga}$ is an electrically and optically active defect center and its behavior is profoundly impacted by local defect--defect interaction. Defect complexes such as Eu$_{\rm Ga}$-O$_{\rm N}$, Eu$_{\rm Ga}$-Si$_{\rm Ga}$, Eu$_{\rm Ga}$-H$_i$, Eu$_{\rm Ga}$-Mg$_{\rm Ga}$, and Eu$_{\rm Ga}$-O$_{\rm N}$-Mg$_{\rm Ga}$ can efficiently act as deep carrier traps and mediate energy transfer from the host into the Eu$^{3+}$ $4f$-electron core which then leads to sharp red intra-$f$ luminescence. Eu-related defects can also give rise to defect-to-band luminescence. The unassociated Eu$_{\rm Ga}$, for example, is identified as a possible source of the broad blue emission observed in n-type, Eu$^{2+}$-containing GaN. This work calls for a re-assessment of certain assumptions regarding specific defect configurations previously made for Eu-doped GaN and further investigation into the origin of the photoluminescence hysteresis observed in (Eu,Mg)-doped samples.

cond-mat.mtrl-sci↗

arXiv2019

Defect energy levels and persistent luminescence in Cu-doped ZnS

Zinc sulfide (ZnS) based materials are widely used in many applications. Yet, due to a lack of detailed knowledge of defect energy levels, the electrical properties and luminescence mechanisms in the materials still give rise to debate. Here, we report a first-principles study of native point defects and impurities in zincblende ZnS using hybrid density-functional calculations. We find that cation and anion vacancies and antisite defects introduce deep defect levels in the band gap and can act as donors or acceptors depending on the position of the Fermi level. The substitutional impurity Cu$_{\rm Zn}$ acts as a deep acceptor and thus does not contribute to p-type conductivity. Substitutional impurities Al$_{\rm Zn}$ and Cl$_{\rm S}$, on the other hand, are shallow donors. More importantly, we identify the isolated (i.e., unassociated) Cu$_{\rm Zn}$ as a source of the green luminescence observed in ZnS-based phosphors and Cu$_{\rm Zn}$$-$Al$_{\rm Zn}$ and Cu$_{\rm Zn}$$-$Cl$_{\rm S}$ defect complexes as sources of blue luminescence. The materials may have both green and blue emissions with the relative intensity dependent on the ratio between the unassociated defect and defect complex concentrations, which is also consistent with experimental observations.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure and properties of lithium-rich complex oxides

Lithium-rich complex transition-metal oxides Li$_2$MoO$_3$, Li$_2$RuO$_3$, Li$_3$RuO$_4$, Li$_3$NbO$_4$, Li$_5$FeO$_4$, Li$_5$MnO$_4$ and their derivatives are of interest for high-capacity battery electrodes. Here, we report a first-principles density-functional theory study of the atomic and electronic structure of these materials using the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional which treats all orbitals in the materials on equal footing. Dimerization of the transition-metal ions is found to occur in layered Li$_2$MoO$_3$, in both fully lithiated and partially delithiated compounds. The Ru--Ru dimerization does not occur in fully lithiated Li$_2$RuO$_3$, in contrast to what is commonly believed; Ru--Ru dimers are, however, found to occur in the presence of lithium vacancies caused by lithium loss during synthesis and/or lithium removal during use. We also analyze the electronic structure of the complex oxides and discuss the delithiation mechanism in these battery electrode materials.

cond-mat.mtrl-sci↗

arXiv2018

Defect physics in complex energy materials

Understanding the physics of structurally and chemically complex transition-metal oxide and polyanionic materials such as those used for battery electrodes is challenging, even at the level of pristine compounds. Yet these materials are also prone to and their properties and performance are strongly affected or even determined by crystallographic point defects. In this review, we highlight recent advances in the study of defects and doping in such materials using first-principles calculations. The emphasis is on describing a theoretical and computational approach that has the ability to predict defect landscapes under various synthesis conditions, provide guidelines for defect characterization and defect-controlled synthesis, uncover the mechanisms for electronic and ionic conduction and electrochemical extraction and (re-)insertion, and provide an understanding of the effects of doping. Though applied to battery materials here, the approach is general and applicable to any materials in which the defect physics plays a role or drives the properties of interest. Thus, this work is intended as an in-depth review of defect physics in particular classes of materials, but also as a methodological template for the understanding and design of complex functional materials.

cond-mat.mtrl-sci↗

arXiv2017

Doping Li-rich cathode material Li$_2$MnO$_3$: Interplay between lattice site preference, electronic structure, and delithiation mechanism

We report a detailed first-principles study of doping in Li$_2$MnO$_3$, in both the dilute doping limit and heavy doping, using hybrid density-functional calculations. We find that Al, Fe, Mo, and Ru impurities are energetically most favorable when incorporated into Li$_2$MnO$_3$ at the Mn site, whereas Mg is most favorable when doped at the Li sites. Ni, on the other hand, can be incorporated at the Li site and/or the Mn site, and the distribution of Ni over the lattice sites can be tuned by tuning the materials preparation conditions. There is a strong interplay between the lattice site preference and charge and spin states of the dopant, the electronic structure of the doped material, and the delithiation mechanism. The calculated electronic structure and voltage profile indicate that, in Ni-, Mo-, or Ru-doped Li$_2$MnO$_3$, oxidation occurs on the electrochemically active transition-metal ion(s) before it does on oxygen during the delithiation process. The role of the dopants is to provide charge-compensation and bulk electronic conduction mechanisms in the initial stages of delithiation, hence enabling the oxidation of the lattice oxygen in the later stages. This work thus illustrates how the oxygen-oxidation mechanism can be used in combination with the conventional mechanism involving transition-metal cations in design of high-capacity battery cathode materials.

cond-mat.mtrl-sci↗

arXiv2017

First-principles identification of defect levels in Er-doped GaN

Erbium (Er) doped GaN has been studied extensively for optoelectronic applications, yet its defect physics is still not well understood. In this work, we report a first-principles hybrid density functional study of the structure, energetics, and thermodynamic transition levels of Er-related defect complexes in GaN. We discover for the first time that Er$_{\rm Ga}$-C$_{\rm N}$-$V_{\rm N}$, a defect complex of Er, a C impurity, and an N vacancy, and Er$_{\rm Ga}$-O$_{\rm N}$-$V_{\rm N}$, a complex of Er, an O impurity, and an N vacancy, form defect levels at 0.18 and 0.46 eV below the conduction band, respectively. Together with Er$_{\rm Ga}$-$V_{\rm N}$, a defect complex of Er and an N vacancy which has recently been found to produce a donor level at 0.61 eV, these defect complexes provide explanation for the Er-related defect levels observed in experiments. The role of these defects in optical excitation of the luminescent Er center is also discussed.

cond-mat.mtrl-sci↗

arXiv2017

Polaron formation, native defects, and electronic conduction in metal tungstates

Iron tungstate (FeWO$_4$) and manganese tungstate (MnWO$_4$) belong to a family of wolframite-type materials that has applications in various areas, including supercapacitors, batteries, and multiferroics. A detailed understanding of bulk properties and defect physics in these transition-metal tungstates has been lacking, however, impeding possible improvement of their functional properties. Here, we report a first-principles study of FeWO$_4$ and MnWO$_4$ using screened hybrid density-functional calculations. We find that in both compounds the electronic structure near the band edges are predominantly the highly localized transition-metal $d$ states, which allows for the formation of both hole polarons at the Fe (Mn) sites and electron polarons at the W sites. The dominant native point defects in FeWO$_4$ (MnWO$_4$) under realistic synthesis conditions are, however, the hole polarons at the Fe (Mn) sites and negatively charged Fe (Mn) vacancies. The presence of low-energy and highly mobile polarons provides explanation for the good p-type conductivity observed in experiments and the ability of the materials to store energy via a pseudocapacitive mechanism.

cond-mat.mtrl-sci↗

arXiv2017

First-principles theory of doping in layered oxide electrode materials

Doping lithium-ion battery electrode materials LiMO$_2$ (M = Co, Ni, Mn) with impurities has been shown to be an effective way to optimize their electrochemical properties. Here, we report a detailed first-principles study of layered oxides LiCoO$_2$, LiNiO$_2$, and LiMnO$_2$ lightly doped with transition-metal (Fe, Co, Ni, Mn) and non-transition-metal (Mg, Al) impurities using hybrid-density-functional defect calculations. We find that the lattice site preference is dependent on both the dopant's charge and spin states, which are coupled strongly to the local lattice environment and can be affected by the presence of co-dopant(s), and the relative abundance of the host compound's constituting elements in the synthesis environment. On the basis of the structure and energetics of the impurities and their complexes with intrinsic point defects, we determine all possible low-energy impurity-related defect complexes, thus providing defect models for further analyses of the materials. From a materials modeling perspective, these lightly doped compounds also serve as model systems for understanding the more complex, mixed-metal, LiMO$_2$-based battery cathode materials.

cond-mat.mtrl-sci↗

arXiv2015

Hybrid density functional study of optically active Er$^{3+}$ centers in GaN

Understanding the luminescence of GaN doped with erbium (Er) requires a detailed knowledge of the interaction between the rare-earth dopant and the nitride host, including intrinsic defects and other impurities that may be present in the host material. We address this problem through a first-principles hybrid density functional study of the structure, energetics, and transition levels of the Er impurity and its complexes with N and Ga vacancies, substitutional C and O impurities, and H interstitials in wurtzite GaN. We find that, in the interior of the material, Er$_{\rm Ga}$ is the dominant Er$^{3+}$ center with a formation energy of 1.55 eV; Er$_{\rm Ga}$-$V_{\rm N}$ possesses a deep donor level at 0.61 eV which can assist in the transfer of energy to the 4$f$-electron core. Multiple optically active Er$^{3+}$ centers are possible in Er-doped GaN.

cond-mat.mtrl-sci↗