arXiv subjects

Walter R. L. Lambrecht

Publications and source records attributed to Walter R. L. Lambrecht.

At least 19 recordsLinked to original sources

arXiv2026

Interface Doping Mechanism of p-Type Conductivity in Mist-CVD-Grown LiGa5O8 Thin Films

LiGa5O8 is an ultrawide-bandgap oxide semiconductor that has recently been experimentally demonstrated to exhibit p-type conductivity; however, the origin of the observed p-type transport remains poorly understood. In this work, we investigate the origin of p-type conductivity by systematically comparing the electrical, compositional, and structural properties of mist chemical vapor deposition (mist-CVD) grown LiGa5O8 thin films exhibiting either p-type conductivity or insulating behavior under different growth conditions. The p-conductive films exhibit room-temperature hole concentrations on the order of 10^18 cm^-3. The emergence of p-type conductivity is found to be strongly correlated with the elemental composition of the films, with X-ray photoelectron spectroscopy (XPS) revealing a reduced Li/Ga ratio in the p-conductive films, indicative of a Li-deficient composition. Raman spectroscopy and X-ray diffraction (XRD) further reveal signatures of \b{eta}-Ga2O3 in the p-conductive films that are absent in the insulating films, demonstrating the formation of a secondary \b{eta}-Ga2O3 phase. Considering the reported type-II band alignment between LiGa5O8 and \b{eta}-Ga2O3, together with theoretically predicted acceptor levels associated Li vacancies (V_Li) in LiGa5O8, the formation of mixed LiGa5O8/\b{eta}-Ga2O3 phases is proposed to facilitate electron transfer from the \b{eta}-Ga2O3 valence band to acceptor states in LiGa5O8, thereby promoting hole accumulation and p-type conductivity.

cond-mat.mtrl-sci↗

arXiv2026

Electronic and optical properties of arsenic monolayers: from planar honeycomb to the puckered phase

Group-V monolayer materials exhibit intriguing electronic and optical properties, influenced by their unique crystal symmetries and structural phases. In this work, we study arsenic monolayers, investigating their electronic and optical properties across different phases, including planar, and puckered forms, using density functional theory (DFT) and quasi-particle self-consistent $GW$ (QS$GW$) methods, with and without vertex contributions (ladder diagrams) and examine the effects of spin-orbit coupling and the orbital composition of the bands. The Bethe-Salpeter equation (BSE) method is used to study the optical response and the band origin of the low lying excitons is determined. The gradual transformation from the puckered $α$-phase to the flat honeycomb structure is studied under biaxial strain and the evolution of the band structure and optical response is described in terms of band inversions of bands of different orbital character.

cond-mat.mtrl-sci↗

arXiv2026

First-principles study of infrared, Raman, piezoelectric and elastic properties of Mg-IV-N\textsubscript{2} (IV = Ge, Si, Sn)

Mg-IV-N\textsubscript{2} compounds with IV=Si, Ge, Sn are ultra-wide band gap semiconductors with various potential electronic and optoelectronic applications. They share the \textit{Pna}2\textsubscript{1} space group crystal structure. Here we present Density Function Perturbation Theory (DFPT) calculations of the vibrational modes of these materials. We focus on the vibrational modes at the zone center to establish the relation between vibrational modes and their corresponding point-group symmetries, which determine the Raman and infrared spectra but also report the full Brillouin zone phonon dispersions and density of states. We also determine the piezoelectric tensor and the elastic compliance tensor.

cond-mat.mtrl-sci↗

arXiv2026

Electronic band structure and exciton properties of $Pna2_1$ CaSnN$_2$

The electronic band structure of CaSnN$_2$ in the wurtzite-based $Pna2_1$ structure is calculated using the Quasiparticle Self-consistent (QS)GW$^{BSE}$ method, including ladder diagrams in the screened Coulomb interaction W$^{BSE}$ and is found to have a direct gap of 2.59 eV at Γ, which corresponds to blue light wavelength of 478 nm and makes it an attractive candidate for sustainable blue light-emitting diodes (LEDs), avoiding Ga and In. The valence band splitting is analyzed in terms of symmetry labeling, and the effective mass tensor is calculated for several bands at Γ. The valence band maximum has a1 symmetry and gives allowed transitions to the conduction band minimum for light polarized along the {\bf c}-direction. While this is unfavorable for light emission with transverse electric (TE) or s-polarization from the basal plane, this would not be an impediment if another surface other than the basal plane is used. Furthermore, the crystal field splitting between the $a_1$ and $b_1$ states, corresponding to polarizations along {\bf c} and {\bf a} respectively, reverses under an applied uniaxial tensile strain of 3.7% along the {\bf c} direction, which might occur under biaxial compressive strain in the basal plane. The optical dielectric function, including electron-hole interaction effects is also reported, and the excitons are analyzed, including several dark excitons.

cond-mat.mtrl-sci↗

arXiv2026

Symmetry breaking structural relaxation and optical transitions of native defects and carbon impurities in LiGa$_5$O$_8$

LiGa$_5$O$_8$ in a spinel type structure has recently been claimed to be an unintentional p-type ultra-wide-band-gap oxide semiconductor. While previous computational work did not yet identify the origin of p-type doping and in fact predicted insulating behavior by compensation of deep acceptors by shallow donors, defect characterization in terms of its optical signatures remains important. Rather than focusing on thermodynamics transition levels, as in earlier work, this present paper focuses on the vertical transitions in a defect configuration diagram of defects in different charge states, representing absorption and emission processes involving carrier capture/emission from/to band edges. In addition, the structural relaxation of several native defects is revisited by allowing for more complex symmetry-breaking distortions in an effort to reconcile conflicting results in the previous literature. Special attention is given to the Li vacancy because it is the shallowest native acceptor. For this defect, the previously reported transition levels are revised on the basis of symmetry-breaking relaxations. The structural relaxations, band structures, and densities of states are compared between the symmetry-broken polaronic and symmetry-conserving non-polaronic states. Finally, we also study carbon impurities, which are likely to originate from growth methods involving organic precursors.

cond-mat.mtrl-sci↗

arXiv2025

Phonon, Infrared and Raman Spectra of LiGa5O8 from Density Functional Perturbation Theory

LiGa$_5$O$_8$ with a cubic spinel type structure was recently reported to be a ultra-wide-band-gap semiconductor with unintentional p-type conduction. While the origin of p-type doping is still unclear, the fundamental properties of this material are of interest. Here we present a first-principles study of the phonons using density functional perturbation theory. The phonon band structures show no unstable modes verifying the stability of the structure. We focus mainly on the phonons at the Brillouin zone center for which a full symmetry analysis is presented. We present the dielectric function contributions from the infrared active modes as well as the Raman spectra and their polarization dependence. The phonon density of states integrated over the Brillouin zone is also presented as this may related to disordered Raman spectra.

cond-mat.mtrl-sci↗

arXiv2025

Optical absorption and luminescence of $α$-LiV$_2$O$_5$ from the Bethe Salpeter Equation

$α$-Li$_x$V$_2$O$_5$ is obtained by intercalating Li between the layers of V$_2$O$_5$. The partial filling of the split-off conduction band by electron donation from Li leads to significant changes in optical properties. Here we study the electronic band structure of $α$-LiV$_2$O$_5$ using quasiparticle self-consistent (QS) $GW$ calculations and the optical dielectric function by means of the Bethe-Salpeter Equation (BSE). The half-filling of the narrow split-off band leads to a spin-splitting and formation of magnetic moments of 0.5 $μ_B$ per V which order antiferromagnetically along the chain or $b$-direction. The imaginary part of the dielectric function shows a very strong optical absorption band near 2 eV for polarization along the $a$-direction. This absorptions stems from a localized transition between the occupied V-$d_{xy}$ derived band, which is odd with respect to the $a$-mirrorplane to the higher lying empty band formed from the same V-$d_{xy}$ orbitals but even with respect to that mirror-plane, which explains its polarization and large oscillator strength. We relate this to a recent experimental study of cathodoluminescence (CL) in which a suppression of the lowest CL peak was observed upon addition of Li. The lowest CL peak near 1.8 eV, which lies well below the indirect absorption onset of V$_2$O$_5$ is proposed to be related to recombination of a self-trapped electron polaron, resulting from oxygen vacancies, with a hole at the valence band maximum and is suppressed in LiV$_2$O$_5$ by the strong self--absorption from the Li induced occupied band to the higher empty bands at about the same energy.

cond-mat.mtrl-sci↗

arXiv2025

Metal-organic chemical vapor deposition of MgGeN2 films on GaN and sapphire

MgGeN2 films were synthesized using metal-organic chemical vapor deposition on GaN/c-sapphire templates and c-plane sapphire substrates. Energy-dispersive X-ray spectroscopy was used to estimate the cation composition ratios. To mitigate magnesium evaporation, the films were grown at pyrometer temperature 745 °C with a wafer rotation speed of 1000 rpm. Growth rates were determined by fitting energy-dispersive X-ray spectroscopy spectra to film thicknesses using NIST DTSA-II software. The thickness estimates determined by this method were consistent with scanning transmission electron microscopy measurements done for selected samples. Scanning electron microscopy images revealed faceted surfaces indicative of a tendency toward three-dimensional growth. X-ray diffraction spectra confirmed that the films were highly crystalline and exhibited preferential orientation in alignment with the substrate. Atomic force microscopy measurements show that film thicknesses are consistent across samples grown on both GaN templates and sapphire substrates, with typical roughnesses around 10 nm. Transmittance spectra of films grown on double-side-polished sapphire substrates yielded band gaps of 4.28 +- 0.06 eV for samples exhibiting close-to-ideal stoichiometry. Comparison of the measured spectra with ab initio calculations are in good agreement both near the band gap and at higher energies where excitation is into higher-lying bands. These findings provide insight into the growth and characterization of MgGeN2, contributing to the development of this material for potential applications in optoelectronics and power electronics.

cond-mat.mtrl-sci↗

arXiv2024

Quasiparticle band structure and excitonic optical response in V2O5 bulk and monolayer

The electronic band structure of V$_2$O$_5$ is calculated using an all-electron quasiparticle self-consistent (QS) $GW$ method, including electron-hole ladder diagrams in the screening of $W$. The optical dielectric function calculated with the Bethe-Salpeter equation exhibits excitons with large binding energy, consistent with spectroscopic ellipsometry data and other recent calculations. Sharp peaks in the direction perpendicular to the layers at high energy are found to be an artifact of the truncation of the numbers of bands included in the BSE calculation of the macroscopic dielectric function. The $\varepsilon_1(ω=0)$ gives indices of refraction in good agreement with experiment. The excitons are charge transfer excitons with the hole primarily on oxygen and electrons on vanadium, but depending on which exciton, the distribution over different oxygens changes. The exciton wave functions have a spread of about 5-15Å, with asymmetric character for the electron distribution around the hole depending on which oxygen the hole is fixed at. The monolayer quasiparticle gap increases inversely proportional to interlayer distance once the initial interlayer covalent couplings are removed which is thanks to the long-range nature of the self-energy and the reduced screening in a 2D system. The optical gap on the other hand is relatively independent of interlayer spacing because of the compensation between the self-energy gap shift and the exciton binding energy, both of which are proportional to the screened Coulomb interaction $\hat{W}$. Recent experimental results on very thin layer V$_2$O$_5$ obtained by chemical exfoliation provide experimental support for an increase in gap.

cond-mat.mtrl-sci↗

arXiv2024

Band structure and excitonic properties of WSe$_2$ in the isolated monolayer limit in an all-electron approach

A study is presented of the electronic band structure and optical absorption spectrum of monolayer WSe$_2$ using an all-electron quasiparticle self-consistent $GW$ approach, QS$G\hat W$, in which the screened Coulomb interaction $\hat W$ is calculated including ladder diagrams representing electron-hole interaction. The Bethe-Salpeter Equation is used to calculate both the screened Coulomb interaction $\hat W$ in the quasiparticle band structure and the imaginary part of the macroscopic dielectric function. The convergence of the quasiparticle band gap and lowest exciton peak position is studied as function of the separation of the monolayers when using periodic boundary conditions. The quasiparticle gap is found to vary as $1/d$ with $d$ the size of the vacuum separation, while the excitonic lowest peak reaches convergence much faster. The nature of the exciton spectrum is analyzed and shows several excitonic peaks below the quasiparticle gap when a sufficient number of $\textbf{k}$ points is used. They are found to be in good agreement with prior work and experiment after adding spin-orbit coupling corrections and can be explained in the context of the Wannier-Mott theory adapted to 2D.

cond-mat.mtrl-sci↗

arXiv2024

Native defects and their complexes in spinel LiGa5O8: the puzzle of p-type doping

Recently, LiGa$_5$O$_8$ was identified as a cubic spinel type ultra-wide-band-gap semiconductor with a gap of about 5.36 eV and reported to be unintentionally p-type. Here we present first-principles calculations of the native defects and various of their complexes to try to explain the occurrence of p-type doping. Although we find Li-vacancies to be somewhat shallower acceptors than in LiGaO$_2$, and becoming slightly shallower in complexes with donors such as V$_{\rm O}$ and Ga$_{\rm Li}$ antisites, these $V_{\rm Li}$ based defects are not sufficiently shallow to explain p-type doping. The dominant defects are donors and in equilibrium the Fermi level would be determined by compensation between donors and acceptors, and pinned deep in the gap.

cond-mat.mtrl-sci↗

arXiv2024

Electronic structure and magnetism in P4/nmm KCoO2

KCoO2 has been found in 1975 to exist in a unique structure with P4/nmm spacegroup with Co in a square pyramidal coordination with the Co atoms in the plane linked by O in a square arrangement reminiscent of the cuprates but its electronic structure has not been studied until now. Unlike Co atoms in LiCoO2 and NaCoO2 in octahedral coordination, which are non-magnetic band structure insulators, the unusual coordination of d6 Co^{3+} in KCoO2 is here shown to lead to a magnetic stabilization of an insulating structure with high magnetic moments of 4μB per Co. The electronic band structure is calculated using the quasiparticle self-consistent (QS)GW method and the basic formation of magnetic moments is explained in terms of the orbital decomposition of the bands. The optical dielectric function is calculated using the Bethe-Salpeter equation including only transitions between equal spin bands. The magnetic moments are shown to prefer an antiferromagnetic ordering along the [110] direction. Exchange interactions are calculated from the transverse spin susceptibility and a rigid spin approximation. The Néel temperature is estimated using the mean-field and Tyablikov methods and found to be between approximately 100 and 250 K. The band structure in the AFM ordering can be related to the FM ordering by band folding effects. The optical spectra are similar in both structures and show evidence of excitonic features below the quasiparticle gap of about 4 eV.

cond-mat.str-el↗

arXiv2024

Comparison of interband related optical transitions and excitons in ZnGeN$_2$ and GaN

The optical dielectric function of ZnGeN$_2$ is calculated from the interband transitions using the energy bands calculated in the quasiparticle self-consistent (QS)$G\hat W$ method using two different levels of approximation: the independent particle approximation (IPA) and the Bethe-Salpeter Equation (BSE) approach. The first allows us to relate peaks in $\varepsilon_2(ω)$ to specific bands and {\bf k}-points but does not include excitonic effects, while the latter does. The corresponding changes in the shape of $\varepsilon_2(ω)$ are found to be similar to those in GaN. The screened Coulomb interaction $\hat W$ is here calculated including electron-hole interactions in the polarization function and gives a band structure already going beyond the random phase approximation. The static dielectric constants including only electronic screening, commonly referred to as $\varepsilon^\infty$, were calculated separately by extrapolating the wave vector dependent macroscopic $\varepsilon_M({\bf q},ω=0)$ for ${\bf q}\rightarrow0$. Below the quasiparticle gap, we find three bound excitons optically active for different polarization. The convergence of these bound excitons with respect to the density of the {\bf k}-mesh used in the BSE is studied and found to require a fine mesh. It is also found that these bound excitons originate from only the lowest conduction band and the top three valence bands. To incorporate the lattice screening, we include a scaling factor $(\varepsilon^\infty/\varepsilon^0)^2$, which allows us to obtain exciton binding energies of the correct order of magnitude similar to those in GaN. The excitons are related to each of the three fold split valence bands and the splittings of the latter are also studied as function of strain. Finally, a relation between the anisotropic effective masses and the valence band splitting is pointed out and explained.

cond-mat.mtrl-sci↗

arXiv2024

Spinel LiGa$_5$O$_8$ prospects as ultra-wideband-gap semiconductor: band structure, optical properties and doping

LiGa$_5$O$_8$ in the spinel type structure is investigated as a potential ultra-wide-band-gap semiconductor. The band structure is determined using the quasiparticle self-consistent $GW$ method and the optical properties are calculated at the Bethe Salpeter Equation level including electron-hole interaction effects. The optical gap including exciton effects and an estimate of the zero-point motion electron phonon coupling renormalizations is estimated to be about 5.2$\pm0.1$ eV with an exciton binding energy of about 0.4 eV. Si doping as potential n-type dopant is investigated and found to be a promising shallow donor.

cond-mat.mtrl-sci↗

arXiv2023

Improved quasiparticle self-consistent electronic band structure and excitons in $β$-LiGaO$_2$

The band structure of $β$-LiGaO$_2$ is calculated using the quasiparticle self-consistent QS$G\hat W$ method where the screened Coulomb interaction $\hat W$ is evaluated including electron-hole interaction ladder diagrams and $G$ is the one-electron Green's function. Improved convergence compared to previous calculations leads to a significantly larger band gap of about 7.0 eV. However, exciton binding energies are found to be large and lead to an exciton gap of about 6.0 eV if also a zero-point-motion correction of about $-0.4$ eV is included. These results are in excellent agreement with recent experimental results on the onset of absorption. Besides the excitons observed thus far, the calculations indicate the existence of a Rydberg-like series of exciton excited states, which is however modified from the classical Wannier exciton model by the anisotropies of the material and the more complex mixing of Bloch states in the excitons resulting from the Bethe-Salpeter equation. The exciton fine structure and the exciton wave functions are visualized and analyzed in various ways.

cond-mat.mtrl-sci↗

arXiv2023

Computational study of defect complexes in $β$-LiGaO$_2$ and their relation to the donor-acceptor-pair recombination

Hybrid functional calculations are presented for defects in LiGaO$_2$ with the fraction of non-local exchange adjusted to reproduce the recently reported exciton gap of 6.0 eV. We study how the defect transition levels of the main native defects change with respect to the band edges compared to earlier calculations which assumed a smaller band gap near 5.1 eV. In addition, we consider defect complexes formed by combining the main native donor Ga$_\mathrm{Li}$ with the main acceptors, $V_\mathrm{Li}$ and Li$_\mathrm{Ga}$ antisites as function of their relative position. These results are used to tentatively identify the photoluminescence bands previous assigned to donor-acceptor-pair recombination.

cond-mat.mtrl-sci↗

arXiv2023

Robustness of electronic screening effects in electron spectroscopies: example of V$_2$O$_5$

In bulk and low-dimensional extended systems, the screening of excitations by the electron cloud is a key feature governing spectroscopic properties. Widely used computational approaches, especially in the framework of many-body perturbation theory, such as the GW approximation and the resulting approximate Bethe-Salpeter equation, are explicitly formulated in terms of the screened Coulomb interaction. In the present work we explore the effect of screening in absorption and electron energy loss spectroscopy, concentrating on the effect of local distortions on the screening and elucidating the resulting changes in the various spectra. Using the layered bulk oxide V$_2$O$_5$ as prototype material, we show in which way local distortions affect the screening, and in which way changes in the screening impact electron energy loss and absorption spectra including excitons. We highlight cancellations that make many-body effects in the spectra very robust with respect to structural modifications, while the band structure undergoes significant changes and the nature of the excitations may also be affected. This yields insight concerning the structure-properties relations that are crucial for the use of V$_2$O$_5$ as energy storage material, and more generally, that may be used to optimize the analysis and the calculation of electronic spectra in complex materials.

cond-mat.mtrl-sci↗

arXiv2022

Quaternary MgSiN_2-GaN alloy semiconductors for deep UV applications

Ultra-wide direct band gap semiconductors hold great promise for deep ultraviolet opto-electronic applications. Here we evaluate the potential of MgSiN$_2$-GaN alloys for this purpose. Although MgSiN$_2$ itself has an indirect gap $\sim$0.4 eV below its direct gap of $\sim$6.5 eV, its different sign lattice mismatch from GaN in two different basal plane directions could avoid the tensile strain which limits Al$_x$Ga$_{1-x}$N on GaN for high $x$. Two octet-rule preserving structures (with space groups $Pmn2_1$ and $P1n1$) of a 50% alloy of MgSiN$_2$ and GaN are investigated and are both found to have gaps larger than 4.75 eV using quasiparticle self-consistent (QS) $GW$ calculations. Both are nearly direct gap in the sense that the indirect gap is less than 0.1 eV lower than the direct gap. Their mixing energies are positive yet small, with values of 8 (31) meV/atom for $Pmn2_1$ ($P1n1$) indicating only a small driving force toward phase separation.

cond-mat.mtrl-sci↗