Showing 1–15 of 15 results for author: Skachkov, D

Charge redistribution at metal-ZrO2 interfaces: A combined DFT and continuum electrostatic study

Authors: Ximeng Wang, Yongfeng Zhang, Dmitry Skachkov, Arnab Das, Junliang Liu, Alexander Kvit, Jennifer T. Choy, Adrien Couet

Abstract: Nanoscale metallic inclusions (NMIs) are commonly observed within oxide scales formed during high-temperature oxidation, revealing the existence of chemical and electronic heterogeneity beyond conventional corrosion theories that assume homogeneous, fully oxidized films. Using tetragonal zirconia (tZrO2) facing a series of face-centered cubic (fcc) metals as the model system, this work investigate… ▽ More Nanoscale metallic inclusions (NMIs) are commonly observed within oxide scales formed during high-temperature oxidation, revealing the existence of chemical and electronic heterogeneity beyond conventional corrosion theories that assume homogeneous, fully oxidized films. Using tetragonal zirconia (tZrO2) facing a series of face-centered cubic (fcc) metals as the model system, this work investigates the short-range and long-range charge redistributions across metal-oxide interfaces by coupling density functional theory (DFT) calculations with continuum modeling. We show that metal-oxide contact induces a short-range charge redistribution confined to a few atomic layers and a long-range redistribution of space charge that can extend over macroscopic distances within weakly doped oxides. DFT calculations show that the short-range redistribution is dominated by metal induced gap states (MIGS) in tZrO2 facing noble metals like Au and Ag, and by chemical bonding in tZrO2 facing active metals like Al. DFT-informed continuum theoretical analysis shows that the range of space-charge redistribution is governed by the doping level of tZrO2, and that the Schottky barrier height (SBH) exhibits a stronger dependence on the metal work function than the doping level. Both the short-range and long-range charge redistributions can alter the transport of charge carriers via their associated electric fields, extending several nm to hundreds of nm from the interface, depending on the doping concentrations, suggesting possible heterogeneous oxide growth caused by NMIs. △ Less

Submitted 27 January, 2026; originally announced January 2026.

On-chip high-order parametric downconversion in the excitonic Mott insulator Nb$_3$Cl$_8$ for programmable multiphoton entangled states

Authors: Dmitry Skachkov, Dirk R. Englund, Michael N. Leuenberger

Abstract: Spontaneous parametric downconversion (SPDC) and four-wave mixing in $χ^{(2)}$ and $χ^{(3)}$ media underpin most entangled-photon sources, but direct generation of higher-order entangled multiphoton states by $n$-th order parametric downconversion remains extremely challenging because conventional materials exhibit tiny high-order nonlinearities. Here we show that single-layer Nb$_3$Cl$_8$, an exc… ▽ More Spontaneous parametric downconversion (SPDC) and four-wave mixing in $χ^{(2)}$ and $χ^{(3)}$ media underpin most entangled-photon sources, but direct generation of higher-order entangled multiphoton states by $n$-th order parametric downconversion remains extremely challenging because conventional materials exhibit tiny high-order nonlinearities. Here we show that single-layer Nb$_3$Cl$_8$, an excitonic Mott insulator on a breathing Kagome lattice, supports exceptionally large nonlinear susceptibilities up to seventh order. Many-body GW--Bethe--Salpeter and time-dependent BSE / Kadanoff--Baym simulations yield resonant $χ^{(2)}$--$χ^{(7)}$ for monolayer Nb$_3$Cl$_8$, with $|χ^{(4)}|$ and $|χ^{(5)}|$ surpassing values in prototypical transition metal dichalcogenides by 5--9 orders of magnitude. We trace this enhancement to flat bands and strongly bound Frenkel excitons with ferroelectrically aligned out-of-plane dipoles. Building on experimentally demonstrated 1$\times N$ integrated beam splitters with arbitrary power ratios, we propose an on-chip architecture where each output arm hosts an Nb$_3$Cl$_8$ patch, optionally gated by graphene to tune the complex $n$-photon amplitudes. Using the ab-initio $χ^{(3)}$ and $χ^{(4)}$ values, we predict that three-photon GHZ$_3$ and four-photon cluster-state sources in this platform can achieve $n$-photon generation rates up to $\sim 10^8$ and $\sim 10^6$ times larger, respectively, than silica-fiber- and MoS$_2$-based implementations with comparable geometry. We derive the quantum Hamiltonian and explicit $n$-photon generation rates for this platform, and show how suitable interferometric networks enable electrically and spectrally tunable GHZ, $W$, and cluster states based on genuine high-order nonlinear processes in a 2D excitonic Mott insulator. △ Less

Submitted 1 December, 2025; originally announced December 2025.

Comments: 16 pages, 6 figures

Tuning the Magnetic Anisotropy Energy of MoS$_2$-supported Mn$_{12}$ complexes by Electric Field: A First-Principles Study

Authors: Shuanglong Liu, Adam V. Bruce, Dmitry Skachkov, James N. Fry, Hai-Ping Cheng

Abstract: In this work, we examine low-energy adsorption configurations of four dodecanuclear manganese single-molecule magnets [Mn$_{12}$O$_{12}$(O$_2$CR)$_{16}$(H$_2$O)$_4$] (Mn$_{12}$), where the ligand R being H, CH$_3$, CHCl$_2$ or C$_6$H$_5$, on a molybdenum disulfide (MoS$_2$) monolayer using force field and density functional theory calculations. The van der Waals interaction is shown to be crucial… ▽ More In this work, we examine low-energy adsorption configurations of four dodecanuclear manganese single-molecule magnets [Mn$_{12}$O$_{12}$(O$_2$CR)$_{16}$(H$_2$O)$_4$] (Mn$_{12}$), where the ligand R being H, CH$_3$, CHCl$_2$ or C$_6$H$_5$, on a molybdenum disulfide (MoS$_2$) monolayer using force field and density functional theory calculations. The van der Waals interaction is shown to be crucial for determining the adsorption energy. Some electrons transfer from the substrate to the molecules upon surface adsorption, resulting in a reduction of the magnetic anisotropy energy of Mn$_{12}$. Since the lowest unoccupied molecular orbital of Mn$_{12}$ is close to the valence band of MoS$_2$, a negative electric field is more effective in modulating charge transfer and energy band alignment, and thus altering the magnetic anisotropy energy, compared with a positive electric field. A significant increase in the magnetic anisotropy energy of Mn$_{12}$ with the ligand R=CHCl$_2$ or R=C$_6$H$_5$ under a sufficiently high electric field has been predicted. Our calculations show that the molecules remain intact on the surface both before and after the electric field is applied. Finally, a two-level system formed by different adsorption configurations is evaluated, and the tunability of its energy barrier under an electric field is demonstrated. Our study sheds light on tuning the properties of single-molecule magnets using an electric field, when the molecules are supported on a surface. △ Less

Submitted 15 September, 2025; originally announced September 2025.

Multiferroic Dark Excitonic Mott Insulator in the Breathing-Kagome Lattice Material Nb$_3$Cl$_8$

Authors: Mahtab A. Khan, Naseem Ud Din, Dmitry Skachkov, Dirk R. Englund, Michael N. Leuenberger

Abstract: Flat electronic bands strongly enhance Coulomb interactions and can stabilize unconventional insulating states. Motivated by the recent discovery of flat bands in breathing Kagome lattices, we use first-principles GW--Bethe--Salpeter theory to investigate the excitonic spectrum of single-layer Nb$_3$Cl$_8$. We find a dark spin-triplet Frenkel exciton whose spectral peak lies at negative energy (… ▽ More Flat electronic bands strongly enhance Coulomb interactions and can stabilize unconventional insulating states. Motivated by the recent discovery of flat bands in breathing Kagome lattices, we use first-principles GW--Bethe--Salpeter theory to investigate the excitonic spectrum of single-layer Nb$_3$Cl$_8$. We find a dark spin-triplet Frenkel exciton whose spectral peak lies at negative energy ($-0.14$~eV) relative to the quasiparticle gap, directly signaling a preformed bound state and an excitonic Mott insulating phase potentially stable at room temperature. Bright excitons appear at $0.94$~eV and $1.21$~eV, with ultra-large binding energies of $2.05$~eV and $1.77$~eV. By mapping the low-energy dynamics onto a spin-1 Hubbard model on a triangular lattice, we show that frustrated antiferromagnetic and ferroelectric tendencies naturally emerge. These results identify Nb$_3$Cl$_8$ as a candidate multiferroic dark excitonic insulator, opening a pathway to correlated quantum phases in two dimensions. △ Less

Submitted 1 November, 2025; v1 submitted 17 December, 2024; originally announced December 2024.

Comments: 11 pages, 8 figures

Linear and nonlinear optical response based on many-body GW-Bethe-Salpeter and Kadanoff-Baym approaches for two-dimensional layered semiconductors

Authors: Dmitry Skachkov, Dirk R. Englund, Michael N. Leuenberger

Abstract: The family of 2D layered semiconductors, including transition metal chalcogenides (TMCs) of the form MX (M=Ga, In; X=S, Se, Te) exhibit exceptional nonlinear optical properties. The energetically most favorable crystal ordering for nonlinear response is the AB layer stacking, which breaks central inversion symmetry for an arbitrary number of layers, resulting in non-zero off-diagonal elements of t… ▽ More The family of 2D layered semiconductors, including transition metal chalcogenides (TMCs) of the form MX (M=Ga, In; X=S, Se, Te) exhibit exceptional nonlinear optical properties. The energetically most favorable crystal ordering for nonlinear response is the AB layer stacking, which breaks central inversion symmetry for an arbitrary number of layers, resulting in non-zero off-diagonal elements of the $χ^{(2n')}$ tensor, $n'$ being a positive integer, for arbitrary thickness of the materials. We perform first-principles many-body calculations of bandstructures and linear and nonlinear optical responses of monolayer (ML) and bulk TMC crystals based on $GW$-Bethe-Salpeter and Kadanoff-Baym approaches in and out of equilibrium, respectively, while taking many-body band gap renormalization and excitonic effects into account. We develop a detailed analysis of the linear and nonlinear optical selection rules by means of group and representation theory. We observe the general trend that the lowest-energy excitons are dark in 2D ML TMCs whereas they are bright or mixed bright-dark in 3D bulk TMCs, which we attribute to the difference between spatially dependent screening in 2D and constant screening in 3D. In particular, we derive general formulas for the nonlinear optical response based on exciton states in semiconductor materials. We find anti-bound excitons in ML GaS, which we attribute to dominant exciton exchange interaction. We show that by choosing elements with larger mass and by reducing the detuning energy it is possible to increase the nonlinear response not only for $χ^{(2)}$ and $χ^{(3)}$, responsible for SHG and third harmonic generation (THG), but also in general for $χ^{(n)}$ nonlinear response with $n>3$, giving rise to high harmonic generation (HHG) in 2D semiconductor materials. △ Less

Submitted 8 October, 2024; originally announced October 2024.

Comments: 24 pages, 16 figures

Learning-Augmented Online Caching: New Upper Bounds

Authors: Daniel Skachkov, Denis Ponomaryov, Yuri Dorn, Alexander Demin

Abstract: We address the problem of learning-augmented online caching in the scenario when each request is accompanied by a prediction of the next occurrence of the requested page. We improve currently known bounds on the competitive ratio of the BlindOracle algorithm, which evicts a page predicted to be requested last. We also prove a lower bound on the competitive ratio of any randomized algorithm and sho… ▽ More We address the problem of learning-augmented online caching in the scenario when each request is accompanied by a prediction of the next occurrence of the requested page. We improve currently known bounds on the competitive ratio of the BlindOracle algorithm, which evicts a page predicted to be requested last. We also prove a lower bound on the competitive ratio of any randomized algorithm and show that a combination of the BlindOracle with the Marker algorithm achieves a competitive ratio that is optimal up to some constant. △ Less

Submitted 28 July, 2025; v1 submitted 2 October, 2024; originally announced October 2024.

A quadratic-order problem kernel for the traveling salesman problem parameterized by the vertex cover number

Authors: René van Bevern, Daniel A. Skachkov

Abstract: The NP-hard graphical traveling salesman problem (GTSP) is to find a closed walk of total minimum weight that visits each vertex in an undirected edge-weighted and not necessarily complete graph. We present a problem kernel with $τ^2+τ$ vertices for GTSP, where $τ$ is the vertex cover number of the input graph. Any $α$-approximate solution for the problem kernel also gives an $α$-approximate solut… ▽ More The NP-hard graphical traveling salesman problem (GTSP) is to find a closed walk of total minimum weight that visits each vertex in an undirected edge-weighted and not necessarily complete graph. We present a problem kernel with $τ^2+τ$ vertices for GTSP, where $τ$ is the vertex cover number of the input graph. Any $α$-approximate solution for the problem kernel also gives an $α$-approximate solution for the original instance, for any $α\geq1$. △ Less

Submitted 11 October, 2023; v1 submitted 18 July, 2022; originally announced July 2022.

Comments: Much shorter alternative proof compared to previous versions

MSC Class: 05C85 ACM Class: F.2.2; G.2.2

Real space representation of the quasiparticle self-consistent $GW$ self-energy and its application to defect calculations

Authors: Ozan Dernek, Dmitry Skachkov, Walter R. L. Lambrecht, Mark van Schilfgaarde

Abstract: The quasiparticle self-consistent QS$GW$ approach incorporates the corrections of the quasiparticle energies from their Kohn-Sham density functional theory (DFT) eigenvalues by means of an energy independent and Hermitian self-energy matrix usually given in the basis set of the DFT eigenstates. By expanding these into an atom-centered basis set (specifically here the linearized muffin-tin orbitals… ▽ More The quasiparticle self-consistent QS$GW$ approach incorporates the corrections of the quasiparticle energies from their Kohn-Sham density functional theory (DFT) eigenvalues by means of an energy independent and Hermitian self-energy matrix usually given in the basis set of the DFT eigenstates. By expanding these into an atom-centered basis set (specifically here the linearized muffin-tin orbitals) a real space representation of the self-energy corrections becomes possible. We show that this representation is relatively short-ranged. This offers new opportunities to construct the self-energy of a complex system from parts of the system by a cut-and-paste method. Specifically for a point defect, represented in a large supercell, the self-eneregy can be constructed from those of the host and a smaller defect containing cell. The self-energy of the periodic host can be constructed simply from a $GW$ calculation for the primitive cell. We show for the case of the As$_\mathrm{Ga}$ in GaAs that the defect part can already be well represented by a minimal 8 atom cell and allows us to construct the self-energy for a 64 cell in good agreement with direct QS$GW$ calculations for the large cell. Using this approach to an even larger 216 atom cell shows the defect band approaches an isolated defect level. The calculations also allow to identify a second defect band which appears as a resonance near the conduction band minimum. The results on the extracted defect levels agree well with Green's function calculations for an isolated defect and with experimental data. △ Less

Submitted 18 February, 2022; originally announced February 2022.

Comments: 14 pages, 13 figures

Serial and parallel kernelization of Multiple Hitting Set parameterized by the Dilworth number, implemented on the GPU

Authors: René van Bevern, Artem M. Kirilin, Daniel A. Skachkov, Pavel V. Smirnov, Oxana Yu. Tsidulko

Abstract: The NP-hard Multiple Hitting Set problem is finding a minimum-cardinality set intersecting each of the sets in a given input collection a given number of times. Generalizing a well-known data reduction algorithm due to Weihe, we show a problem kernel for Multiple Hitting Set parameterized by the Dilworth number, a graph parameter introduced by Foldes and Hammer in 1978 yet seemingly so far unexplo… ▽ More The NP-hard Multiple Hitting Set problem is finding a minimum-cardinality set intersecting each of the sets in a given input collection a given number of times. Generalizing a well-known data reduction algorithm due to Weihe, we show a problem kernel for Multiple Hitting Set parameterized by the Dilworth number, a graph parameter introduced by Foldes and Hammer in 1978 yet seemingly so far unexplored in the context of parameterized complexity theory. Using matrix multiplication, we speed up the algorithm to quadratic sequential time and logarithmic parallel time. We experimentally evaluate our algorithms. By implementing our algorithm on GPUs, we show the feasability of realizing kernelization algorithms on SIMD (Single Instruction, Multiple Date) architectures. △ Less

Submitted 8 July, 2023; v1 submitted 13 September, 2021; originally announced September 2021.

Comments: Added experiments on one more data set

MSC Class: 05C65 ACM Class: F.2.2

Journal ref: Journal of Computer and System Sciences 139:103479, 2024

First-Principles Theory for Schottky Barrier Physics

Authors: Dmitry Skachkov, Shuang-Long Liu, Yan Wang, Xiao-Guang Zhang, Hai-Ping Cheng

Abstract: We develop a first-principles theory for Schottky barrier physics. The Poisson equation is solved completely self-consistently with the electrostatic charge density and outside the normal density functional theory (DFT) electronic structure iteration loop, allowing computation of a Schottky barrier entirely from DFT involving thousands of atomic layers in the semiconductor. The induced charge in t… ▽ More We develop a first-principles theory for Schottky barrier physics. The Poisson equation is solved completely self-consistently with the electrostatic charge density and outside the normal density functional theory (DFT) electronic structure iteration loop, allowing computation of a Schottky barrier entirely from DFT involving thousands of atomic layers in the semiconductor. The induced charge in the bulk consists of conduction and valence band charges from doping and band bending, as well as charge from the evanescent states in the gap of the semiconductor. The Schottky barrier height is determined when the induced charge density and the induced electrostatic potential reach self-consistency. △ Less

Submitted 3 May, 2021; v1 submitted 2 January, 2020; originally announced January 2020.

Journal ref: Phys. Rev. B 104, 045429 (2021)

Computational Study of Electron Paramagnetic Resonance Spectra for Li and Ga Vacancies in LiGaO2

Authors: Dmitry Skachkov, Walter Lambrecht, Klichchupong Dabsamut, Adisak Boonchun

Abstract: A computational study of the Electron Paramagnetic Resonance (EPR) $g$-tensors and hyperfine tensors of Li and Ga vacancies in LiGaO$_2$ is presented. Density Functinal Theory (DFT) calculations are carried out of the Ga and Li vacancies using the DFT+U approach in the charge states which carry an unpaired spin. In both vacancies the hole is located on one oxygen $p$-orbital adjacent to the vacanc… ▽ More A computational study of the Electron Paramagnetic Resonance (EPR) $g$-tensors and hyperfine tensors of Li and Ga vacancies in LiGaO$_2$ is presented. Density Functinal Theory (DFT) calculations are carried out of the Ga and Li vacancies using the DFT+U approach in the charge states which carry an unpaired spin. In both vacancies the hole is located on one oxygen $p$-orbital adjacent to the vacancy. Apical and different basal plane O are considered. The magnetic resonance parameters of the defects are determined using the Gauge Including Projector Augmented Wave (GIPAW) method. The EPR spectra of $V_\mathrm{Ga}^{2-}$ is characterized by a quasi-isotropic superhyperfine (SHF) interaction with one Ga nucleus and for the apical O spin gives a $g$-tensor with maximum oriented along the bond direction from that O to its other Ga neighbor. For $V_\mathrm{Li}^0$ there is a quasi-isotropic SHF interaction with two Ga nuclei and the $g$-tensor maximum is along ${\bf c}$ for the basal plane O spin. Both of these are in agreement with experiment but we predict also the $g$-tensors for the other possible localization of the spins as well as the small hyperfine splittings (as yet not observed) on Li. The energies of formation and transition levels of the corresponding defects provide insight into the conditions required to activate these EPR spectra. △ Less

Submitted 8 November, 2019; v1 submitted 5 November, 2019; originally announced November 2019.

Computational study of Electron Paramagnetic Resonance parameters for Mg and Zn impurities in $β$-Ga$_2$O$_3$

Authors: Dmitry Skachkov, Walter R. L. Lambrecht

Abstract: A computational study of the electron paramagnetic resonance (EPR) $g$-tensors and hyperfine tensors in Mg and Zn doped $β$-Ga$_2$O$_3$ is presented. While Mg has been found previously to prefer the octahedral site, we find here that Zn prefers the tetrahedral substitutional site. The EPR signatures are found to be distinct for the two sites. Good agreement with experiment is found for the $g$-ten… ▽ More A computational study of the electron paramagnetic resonance (EPR) $g$-tensors and hyperfine tensors in Mg and Zn doped $β$-Ga$_2$O$_3$ is presented. While Mg has been found previously to prefer the octahedral site, we find here that Zn prefers the tetrahedral substitutional site. The EPR signatures are found to be distinct for the two sites. Good agreement with experiment is found for the $g$-tensor and hyperfine interaction for Mg$_\mathrm{Ga2}$ and predictions are made for the Zn case. △ Less

Submitted 10 April, 2019; originally announced April 2019.

Proton Irradiation Induced Defects in \b{eta}-Ga2O3: a combined EPR and Theory Study

Authors: Hans Jürgen von Bardeleben, Shengqiang Zhou, Uwe Gerstmann, Dmitry Skachkov, Walter R. L. Lambrecht, QuocDuy Ho, Peter Deák

Abstract: Proton irradiation of both n-type and semi-insulating bulk samples of \b{eta}-Ga2O3 leads to the formation of one paramagnetic defect with spin S=1/2, monoclinic point symmetry, a g-tensor with principal values of gb=2.0313, gc=2.0079, ga*= 2.0025 and quasi isotropic superhyperfine interaction of 13G with two equivalent Ga neigbours. Its high introduction rate indicates it to be a primary irradiat… ▽ More Proton irradiation of both n-type and semi-insulating bulk samples of \b{eta}-Ga2O3 leads to the formation of one paramagnetic defect with spin S=1/2, monoclinic point symmetry, a g-tensor with principal values of gb=2.0313, gc=2.0079, ga*= 2.0025 and quasi isotropic superhyperfine interaction of 13G with two equivalent Ga neigbours. Its high introduction rate indicates it to be a primary irradiation induced defect. At low temperature, photoexcitation transforms this defect into a different metastable S=1/2 center with principal g-values of gb=2.0064, gc=2.0464, ga*= 2.0024 and a reduced hyperfine interaction of 9G. This metastable defect is stable up to T=100K, when it switches back to the previous configuration. Density functional theory calculations of the Spin Hamiltonian parameters of various intrinsic defects are carried out using the Gauge Including Projector Augmented Wave method in order to determine the microscopic structure of these defects.Our results do not support the intuitive model of the isolated octahedral or tetrahedral gallium vacancy, VGa2-, but favor the model of a gallium vacancy complex VGa-Gai-VGa. △ Less

Submitted 13 November, 2018; originally announced November 2018.

Comments: 12 figures

Computational identification of Ga-vacancy related electron paramagnetic resonance centers in $β$-Ga$_2$O$_3$

Authors: Dmitry Skachkov, Walter R. L. Lambrecht, Hans Jürgen von Bardeleben, Uwe Gerstmann, Quoc Duy Ho, Peter Deák

Abstract: A combined experimental/theoretical study of the EPR in irradiated $β$-Ga$_2$O$_3$ is presented. Four EPR spectra, two $S=1/2$ and two $S=1$, are observed after high-energy proton or electron irradiation. One of the S=1/2 spectra (EPR1) can be observed at room temperature and below and is characterized by the spin Hamiltonian parameters $g_b=2.0313$, $g_c=2.0079$, $g_{a*}=2.0025$ and a quasi isotr… ▽ More A combined experimental/theoretical study of the EPR in irradiated $β$-Ga$_2$O$_3$ is presented. Four EPR spectra, two $S=1/2$ and two $S=1$, are observed after high-energy proton or electron irradiation. One of the S=1/2 spectra (EPR1) can be observed at room temperature and below and is characterized by the spin Hamiltonian parameters $g_b=2.0313$, $g_c=2.0079$, $g_{a*}=2.0025$ and a quasi isotropic hyperfine interaction with two equivalent Ga neighbors of $~\sim$14 G on $^{69}$Ga. The second (EPR2) is observed after photoexcitation (with threshold 2.8 eV) at low temperature and is characterized by $g_b=2.0064$, $g_c=2.0464$, $g_{a*}=2.0024$ and a quasi isotropic hyperfine interaction with two equivalent Ga neighbors of 10 G. A spin $S=1$ spectrum with a similar g-tensor and a 50\% reduced hyperfine splitting accompanies each of these, which is indicative of a defect of two weakly coupled $S=1/2$ centers. DFT calculations of the magnetic resonance fingerprint of a wide variety of native defect models are carried out to identify these EPR centers in terms of specific defect configurations. The EPR1 center is proposed to correspond to a complex of two tetrahedral $V_\mathrm{Ga1}$ with an interstitial Ga in between them. This model was previously shown to have lower energy than the simple tetrahedral Ga vacancy and has a $2-/3-$ transition level higher than other $V_\mathrm{Ga}$ related models, which would explain why the other ones are already in their diamagnetic $3-$ state and are thus not observed if the Fermi level is pinned approximately at this level. The EPR2 spectra are proposed to correspond to the octahedral $V_\mathrm{Ga2}$. Models based on self-trapped holes and oxygen interstitials are ruled out because they would have hyperfine interaction with more than two Ga nuclei and because they can not support a corresponding $S=1$ center. △ Less

Submitted 25 February, 2019; v1 submitted 16 August, 2018; originally announced August 2018.

Journal ref: Journal of Applied Physics 125 (18), 185701, 2019

Native interstitial defects in ZnGeN$_2$

Authors: Dmitry Skachkov, Walter R. L. Lambrecht

Abstract: A density functional study is presented of the interstitial Zn$_i$, Ge$_i$, and N$_i$ in ZnGeN$_2$. Corrections to the band gap are included by means of the LDA+U method. The Zn and Ge interstitials are both found to strongly prefer the larger octahedral site compared to the two types of tetrahedral sites. The Zn interstitial is found to be a shallow double donor but has higher energy than pre… ▽ More A density functional study is presented of the interstitial Zn$_i$, Ge$_i$, and N$_i$ in ZnGeN$_2$. Corrections to the band gap are included by means of the LDA+U method. The Zn and Ge interstitials are both found to strongly prefer the larger octahedral site compared to the two types of tetrahedral sites. The Zn interstitial is found to be a shallow double donor but has higher energy than previously studied antisite defects. It has a resonance in the conduction band which is Zn-$s$ like. The Ge interstitial is an even higher energy of formation defect and also behaves as a shallow double donor but has also a deep level in the gap, corresponding to a Ge-$s$ orbital character while the Ge-$p$ forms a resonance in the conduction band. The nitrogen interstitial forms a split-interstitial configuration, as also occurs in GaN. Its electronic levels can be related to that of a N$_2$ molecule. The defect levels in the gap correspond to the $π_g$-like lowest unoccupied molecular orbital (LUMO) of the molecule, which here becomes filled with 3 electrons in the defect's neutral charge state. They are found to prefer a high-spin configuration in the $q=+1$ state. The corresponding transition levels are obtained and show that this is an amphoteric trap level occurring in $+2$, $+1$, 0 and $-1$ charge states. The two possible sites for this split interstitial, on top of Zn or on top of Ge differ slightly in N$_2$ bond-length. While the N$_i$ defects have the lowest formation energy among the interstitials, it is still higher than that of the antisites. Hence they are not expected to occur in sufficient concentration to affect the intrinsic Fermi level position. In particular, they do not contribute to the unintentional n-type background doping. △ Less

Submitted 6 October, 2017; v1 submitted 11 August, 2017; originally announced August 2017.

Comments: 8 pages, 10 figures

Journal ref: Phys. Rev. Materials 1, 054604 (2017)