arXiv subjects

Marianne Etzelmüller Bathen

Publications and source records attributed to Marianne Etzelmüller Bathen.

15 recordsLinked to original sources

arXiv2026

Device contacts as spin-state selectors for silicon vacancies in 4H-SiC

Optically addressable defect spins in semiconductors are promising building blocks for scalable quantum technologies. Yet, the consequences of conventional contact schemes used in semiconductor device integration for the quantum spin environment remain largely unexplored. Using the silicon vacancy (V$_\mathrm{Si}$) in silicon carbide (SiC) as a model system, we show that a widely used contact metal, nickel (Ni), intrinsically perturbs the defect spin state and quenches the characteristic emission from the spin-quartet channel of V$_\mathrm{Si}$. Low-energy muon spin rotation further reveals that Ni contacts create a magnetically contaminated region extending at least $\sim$110 nm into the SiC, in stark contrast to non-magnetic contacts such as Ti and Al. Moreover, cross-sectional cathodoluminescence measurements on samples with high defect density demonstrate a suppressed quartet-state luminescence from the V$_\mathrm{Si}$ up to 500 nm beneath the Ni contact. This drastic influence on the defect's magnetic environment is accompanied by the appearance of a signature consistent with photoluminescence from the spin-doublet state of V$_\mathrm{Si}$ under the Ni layer, which was absent in other material stacks. These results establish that standard device configurations can drive quantum defects into unwanted charge and spin configurations, underscoring the necessity of precise design to preserve quantum-grade spin environments in semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2026

Identification of carbon-related color centers in hBN by high-resolution ab initio photoluminescence modeling

Identification of optically active defects in hexagonal boron nitride (hBN) remains challenging because different defect candidates can produce similar calculated photoluminescence (PL) lineshapes. Here, we demonstrate that force-constant embedding methodology can be extended to calculate high-resolution, vibrationally resolved PL lineshapes in the polar layered material hBN. Benchmarking against the carbon dimer defect, responsible for the 4.1 eV emission, we show that the methodology accurately reproduces the low-energy acoustic phonons and fine spectral features that are inaccessible when using direct supercell calculations. Applying the methodology to neutral carbon trimers ($\mathrm{C_2 C_B}$ and $\mathrm{C_2 C_N}$), we find that the calculated PL lineshape and excitation energy of $\mathrm{C_2 C_N}$ closely match the low-temperature spectrum of the emitter with a zero-phonon line at 1.54 eV. These results establish high-resolution phonon-sideband modeling as a powerful tool for discriminating among candidate defects in polar materials.

cond-mat.mtrl-sci↗

arXiv2026

Fine structure of the M-center in Si

Color centers in silicon offer great possibilities for scalable quantum technologies. The M-center, proposed to originate from a carbon-hydrogen complex, offers telecommunications-band emission and a paramagnetic ground state similar to the T-center. Here, we report on photoluminescence lines in the vicinity of the M-center and investigate their properties, including their dependence on temperature, implantation fluence, implantation isotope, and annealing temperature. Three emission lines are observed that are blue-shifted by 1.1, 2.8 and 3.6 meV relative to the 761 meV zero phonon line of the M center, where the 2.8 meV line exhibits negative thermal quenching and is therefore proposed to originate from a second excited state of the M-center. The remaining blue-shifted emission lines, together with two additional red-shifted (4.7 and 6.4 meV) emission lines display normal thermal quenching, and are unaffected by an isotope shift of the implanted carbon atom ($^{12}$C versus $^{13}$C implantation) and implantation fluence. Thus, they likely arise either from other defects with similar emission energies or from a perturbed configuration of the M-center.

cond-mat.mtrl-sci↗

arXiv2026

Impact of strain on electron-phonon coupling of quantum emitters

Defects in semiconductors acting as optically active spin qubits are intriguing objects of fundamental study and future technological developments. These defect-based color centers are of particular interest for detection and response to physical variations such as pressure and strain, or conversely -- as we demonstrate the possibility of herein -- pressure and strain can be utilized to manipulate quantum emitter properties. To investigate how strain can alter the fundamental electron-phonon interaction of quantum defects, we employ the negatively charged silicon vacancy ($\mathrm{V_{Si}^{-}}$) in 4H-SiC as a use-case and study its vibrational structure under applied tensile and compressive uniaxial strain using first-principles calculations. We show that the strain variations of the emission spectrum can be explained by differing responses of bulk-like and quasi-localized vibrational modes. Importantly, the $\mathrm{V_{Si}^{-}}$ defect exhibits a strain-induced enhancement of the Debye-Waller factor under uniaxial tensile strain applied along the $a$-axis of 4H-SiC, thereby improving its performance as a quantum emitter. The strain-dependent changes in the phonon sideband enable distinguishing between compressive and tensile strain, opening up the possibility of magnetic-field-free strain detection using only spin-conserving transitions of solid-state quantum emitters.

cond-mat.mtrl-sci↗

arXiv2026

Seeing the forbidden: overcoming optical selection rules through nanophotonic integration

Optically addressable spin defects in silicon carbide, including the neutral divacancy (VV$^0$) and the negative nitrogen-vacancy (NV$^-$), are among leading building blocks of solid-state quantum technologies. Integrating these defects into photonic structures such as nanopillars improves photon collection efficiency, but the consequences extend further. We show that the sub-wavelength geometry of nanopillars drastically modifies the local electromagnetic environment, providing optical access to defect transitions that are otherwise suppressed by selection rules in bulk material. Using low-temperature photoluminescence spectroscopy, we observe that emission from the PL3 divacancy, which is nearly absent in planar devices, becomes pronounced in nanopillars owing to a polarization transformation of the excitation field within the pillar. We further leverage the orientation-dependent collection of nanopillars to resolve the origin of previously ambiguous spectral lines. In particular, the NV4$'$ feature displays the signal enhancement expected for axially oriented NV$^-$ centres, consistent with assignment to a higher excited state of the $kh$ defect configuration. Our results establish nanophotonic integration as a symmetry-sensitive probe that can both activate nominally dark transitions and identify the dipole character of poorly understood defect states.

physics.optics↗

arXiv2026

Unraveling the electronic structure of silicon vacancy centers in 4H-SiC

Point defects in silicon carbide (SiC), particularly the negatively-charged silicon vacancy ($\mathrm{V_{Si}^{-}}$) in 4H-SiC, are leading candidates for scalable quantum technologies due to their favorable spin-optical properties and compatibility with industrial semiconductor fabrication processes. Comprehensive knowledge of a defect's electronic structure is essential for interpreting spin-optical dynamics and for the reliable design and optimization of defect-based quantum devices. Despite extensive study, our knowledge of the electronic structure of $\mathrm{V_{Si}^{-}}$\ is limited since key excited-state manifolds have remained inaccessible to conventional steady-state spectroscopy. In this study, transient absorption spectroscopy is utilized to probe non-equilibrium electronic transitions of $\mathrm{V_{Si}^{-}}$\ and to uncover previously unobserved excited states. The first direct observation of the elusive V2' quartet transition is presented, with its broad spectral signature attributed to nonadiabatic vibronic coupling. Within the spin-doublet manifold, which is central to optically detected magnetic resonance (ODMR) but has remained unresolved spectroscopically, multiple optical transitions are identified. The complete electronic level structure in the relevant energy range is elucidated by combining polarization-resolved spectroscopy, group-theoretical analysis, quantum embedding calculations and first-principles optical lineshape modeling. Collectively, these results provide a microscopic understanding of the $\mathrm{V_{Si}^{-}}$\ electronic structure. Our approach also establishes a general framework for resolving and understanding complex excited-state manifolds in wide-bandgap color centers.

cond-mat.mtrl-sci↗

arXiv2024

Theory of the divacancy in 4H-SiC: Impact of Jahn-Teller effect on optical properties

Understanding the optical properties of color centers in silicon carbide is essential for their use in quantum technologies, such as single-photon emission and spin-based qubits. In this work, first-principles calculations were employed using the r2SCAN density functional to investigate the electronic and vibrational properties of neutral divacancy configurations in 4H-SiC. Our approach addresses the dynamical Jahn-Teller effect in the excited states of axial divacancies. By explicitly solving the multimode dynamical Jahn-Teller problem, we compute emission and absorption lineshapes for axial divacancy configurations, providing insights into the complex interplay between electronic and vibrational degrees of freedom. The results show strong alignment with experimental data, underscoring the predictive power of the methodologies. Our calculations predict spontaneous symmetry breaking due to the pseudo Jahn-Teller effect in the excited state of the $kh$ divacancy, accompanied by the lowest electron-phonon coupling among the four configurations and distinct polarizability. These unique properties facilitate its selective excitation, setting it apart from other divacancy configurations, and highlight its potential utility in quantum technology applications. These findings underscore the critical role of electron-phonon interactions and optical properties in spin defects with pronounced Jahn-Teller effects, offering valuable insights for the design and integration of quantum emitters for quantum technologies.

cond-mat.mtrl-sci↗

arXiv2024

Optical lineshapes of the C-center in silicon from ab initio calculations: Interplay of localized modes and bulk phonons

In this work, we present a first-principles density functional theory (DFT) computational investigation of the luminescence and absorption lineshapes associated with the neutral carbon-oxygen interstitial pair (CiOi) defect in silicon. We obtain the lineshapes of the defect in the dilute limit using a computational methodology that constructs dynamical matrices of supercells containing tens of thousands of atoms, utilizing systems directly accessible through DFT. Both perturbed bulk phonons and localized vibrations contribute to the phonon sideband. We achieve excellent agreement with experimental luminescence data. Our findings further reinforce the attribution of the well-known C-line in silicon to the neutral CiOi complex.

cond-mat.mtrl-sci↗

arXiv2024

Surface Structuring of Patterned 4H-SiC Surfaces Using a SiC/Si/SiC Sandwich Approach

Mesa- and trench-patterned surfaces of 4H-SiC(0001) 4{\textdegree}off wafers were structured in macrosteps using Si melting in a SiC-Si-SiC sandwich configuration. Si spreading difficulties were observed in the case of trench-patterned samples while the attempts on mesa-patterned ones were more successful. In the latter case, parallel macrosteps were formed on both the dry-etched and unetched areas though these macrosteps rarely cross the patterns edges. The proposed mechanism involved preferential etching at Si-C bilayer step edges and fast lateral propagation along the [1120] direction.

cond-mat.mtrl-sci↗

arXiv2022

Investigation of the SiO2-SiC interface using low energy muon spin rotation spectroscopy

Using positive muons as local probes implanted at low energy enables gathering information about the material of interest with nanometer depth resolution (low energy muon spin rotation spectroscopy (LE-$μ$SR). In this work, we leverage the capabilities of LE-$μ$SR to perform an investigation of the SiO$_\text{2}$-SiC interface. Thermally oxidized samples are investigated before and after annealing in nitric oxide (NO) and argon (Ar) ambience. Thermal oxidation is found to result in structural changes both in the SiC crystal close to the interface and at the interface itself. Annealing in NO environment is known to passivate the defects leading to a reduction of the density of interface traps (D$_{it}$); LE-$μ$SR further reveals that the NO annealing results in a thin layer of high carrier concentration in SiC, extending to more than 50 nm depending on the annealing conditions. We also see indications of Si vacancy (V$_{Si}$) formation in SiC after thermal oxidation. Following NO annealing, nitrogen occupies the V$_{Si}$ sites, leading to the reduction in D$_{it}$ and at the same time, creating a charge-carrier-rich region near the interface. By comparing the LE-$μ$SR data from a sample with known doping density, we perform a high-resolution quantification of the free carrier concentration near the interface after NO annealing and discuss the origin of observed near-surface variations. Finally, the depletion of carriers in a MOS capacitor in the region below the interface is shown using LE-$μ$SR. The NO annealed sample shows the narrowest depletion region, likely due to the reduced D$_{it}$ and charge-carrier-rich region near the interface. Our findings demonstrate the many benefits of utilizing LE-$μ$SR to study critical regions of semiconductor devices that have been inaccessible with other techniques while retaining nanoscale depth resolution and a non-destructive approach.

cond-mat.mtrl-sci↗

arXiv2022

Predicting Solid State Material Platforms for Quantum Technologies

Semiconductor materials provide a compelling platform for quantum technologies (QT), and the properties of a vast amount of materials can be found in databases containing information from both experimental and theoretical explorations. However, searching these databases to find promising candidate materials for quantum technology applications is a major challenge. Therefore, we have developed a framework for the automated discovery of semiconductor host platforms for QT using material informatics and machine learning methods, resulting in a dataset consisting of over $25.000$ materials and nearly $5000$ physics-informed features. Three approaches were devised, named the Ferrenti, extended Ferrenti and the empirical approach, to label data for the supervised machine learning (ML) methods logistic regression, decision trees, random forests and gradient boosting. We find that of the three, the empirical approach relying exclusively on findings from the literature predicted substantially fewer candidates than the other two approaches with a clear distinction between suitable and unsuitable candidates when comparing the two largest eigenvalues in the covariance matrix. In contrast to expectations from the literature and that found for the Ferrenti and extended Ferrenti approaches focusing on band gap and ionic character, the ML methods from the empirical approach highlighted features related to symmetry and crystal structure, including bond length, orientation and radial distribution, as influential when predicting a material as suitable for QT. All three approaches and all four ML methods agreed on a subset of $47$ eligible candidates %(to a probability of $>50 \ \%$) of $8$ elemental, $29$ binary, and $10$ tertiary compounds, and provide a basis for further material explorations towards quantum technology.

cond-mat.mtrl-sci↗

arXiv2021

Formation of carbon interstitial-related defect levels by thermal injection of carbon into $n$-type 4$H$-SiC

Electrical properties of point defects in 4$H$-SiC have been studied extensively, but those related to carbon interstitials (C$_{i}$) have remained surprisingly elusive until now. Indeed, when introduced via ion irradiation or implantation, signatures related to C$_{i}$ observed by deep level transient spectroscopy (DLTS) tend to overlap with those of other primary defects, making the direct identification of C$_{i}$-related levels difficult. Recent literature has suggested to assign the so-called M center, often found in as-irradiated 4H-SiC, to charge state transitions of the C$_{i}$ defect in different configurations. In this work, we have introduced excess carbon into low-doped n-type 150 μm thick 4$H$-SiC epilayers by thermal annealing, with a pyrolyzed carbon cap on the sample surface acting as a carbon source. Because the layers exhibited initially low concentrations of carbon vacancies ([V$_{C}$] = 10$^{11}$ cm$^{-3}$), this enabled us to study the case of complete V$_{C}$ annihilation, and formation of defects due to excess carbon, i.e. carbon interstitials C$_{i}$ and their higher-order complexes. We report on the occurrence of several new levels upon C injection which are likely C$_{i}$-related. Their properties are different from those found for the M center, which points towards a different structural identity of the detected levels. This suggests the existence of a rich variety of C$_{i}$-related defects. The study will also help generating new insights into the microscopic process of V$_{C}$ annihilation during carbon injection processes.

cond-mat.mtrl-sci↗

arXiv2020

Conversion pathways of primary defects by annealing in proton-irradiated n-type 4H-SiC

The development of defect populations after proton irradiation of n-type 4H-SiC and subsequent annealing experiments is studied by means of deep level transient (DLTS) and photoluminescence (PL) spectroscopy. A comprehensive model is suggested describing the evolution and interconversion of irradiation-induced point defects during annealing below 1000°C. The model proposes the EH4 and EH5 traps frequently found by DLTS to originate from the (+/0) charge transition level belonging to different configurations of the carbon antisite-carbon vacancy (CAV) complex. Furthermore, we show that the transformation channel between the silicon vacancy (VSi) and CAV is effectively blocked under n-type conditions, but becomes available in samples where the Fermi level has moved towards the center of the band gap due to irradiation-induced donor compensation. The annealing of VSi and the carbon vacancy (VC) is shown to be dominated by recombination with residual self-interstitials at temperatures of up to 400°C. Going to higher temperatures, a decay of the CAV pair density is reported which is closely correlated to a renewed increase of VC concentration. A conceivable explanation for this process is the dissociation of the CAV pair into separate carbon anitisites and VC defects. Lastly, the presented data supports the claim that the removal of free carriers in irradiated SiC is due to introduced compensating defects and not passivation of shallow nitrogen donors.

cond-mat.mtrl-sci↗

arXiv2017

Spin Seebeck effect and thermoelectric phenomena in superconducting hybrids with magnetic textures or spin-orbit coupling

We theoretically consider the spin Seebeck effect, the charge Seebeck coefficient, and the thermoelectric figure of merit in superconducting hybrid structures including either magnetic textures or intrinsic spin-orbit coupling. We demonstrate that large magnitudes for all these quantities are obtainable in Josephson-based systems with either zero or a small externally applied magnetic field. This provides an alternative to the thermoelectric effects generated in high-field ($\sim 1$ T) superconducting hybrid systems, which were recently experimentally demonstrated. The systems studied contain either textured ferromagnets, spin-active interfaces, or spin-orbit coupling. We present a framework for calculating the linear thermoelectric response for both spin and charge of a system upon applying temperature and voltage gradients based on quasiclassical theory which allows for arbitrary spin-dependent textures and fields to be conveniently incorporated.

cond-mat.mes-hall↗

arXiv2016

Spin caloritronics with superconductors: Enhanced thermoelectric effects, generalized Onsager response-matrix, and thermal spin currents

It has recently been proposed and experimentally demonstrated that it is possible to generate large thermoelectric effects in ferromagnet/superconductor structures due to a spin-dependent particle-hole asymmetry. Here, we theoretically show that quasiparticle tunneling between two spin-split superconductors enhances the thermoelectric response manyfold compared to when only one such superconductor is used, generating Seebeck coefficients ($\mathcal{S} > 1$ mV/K) and figures of merit ($ZT \simeq 40$) far exceeding the best bulk thermoelectric materials, and also becomes more resilient toward inelastic scattering processes. We present a generalized Onsager response-matrix which takes into account spin-dependent voltage and temperature gradients. Moreover, we show that thermally induced spin currents created in such junctions, even in the absence of a polarized tunneling barrier, also become largest in the case where a spin-dependent particle-hole asymmetry exists on both sides of the barrier. We determine how these thermal spin currents can be tuned both in magnitude and sign by several parameters, including the external field, temperature, and the superconducting phase-difference.

cond-mat.supr-con↗