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Postdoc positions in big-data materials science available!

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People currently working in our group:

Juliane Mörsel
Markus Scheidgen
Archana Manoharan
Christian Vorwerk
Claudia Draxl
Benedikt Maurer
Helen Jurscha
Hannah Kleine
Maria Troppenz
Le Fang
Olga Turkina
Pasquale Pavone
Santiago Rigamonti
Benedikt Hoock
Sven Lubeck
Julian Graupner
Tim Bechtel
Jungho Shin
Konstantin Lion
Martin Kuban
Sebastian Tillack
Axel Hübner
Victoria Coors
Daniel Speckhard
Mao Yang
Cecilia Vona
Maximilian Schebeck
Simon Gabaj
Peter Weber
Ignacio Gonzalez
  • Juliane Mörsel
  • Markus Scheidgen
  • Archana Manoharan
  • Christian Vorwerk
  • Claudia Draxl
  • Benedikt Maurer
  • Helen Jurscha
  • Hannah Kleine
  • Maria Troppenz
  • Le Fang
  • Olga Turkina
  • Pasquale Pavone
  • Santiago Rigamonti
  • Benedikt Hoock
  • Sven Lubeck
  • Julian Graupner
  • Tim Bechtel
  • Jungho Shin
  • Konstantin Lion
  • Martin Kuban
  • Sebastian Tillack
  • Axel Hübner
  • Victoria Coors
  • Daniel Speckhard
  • Mao Yang
  • Cecilia Vona
  • Maximilian Schebeck
  • Simon Gabaj
  • Peter Weber
  • Ignacio Gonzalez

GraFOx

We are a main partner in the Leibniz ScienceCampus Growth and Fundamentals of Oxides (GraFOx) for electronic applications. Our projects are focused on the ab initio description of physical and optical properties of complex oxides using density-functional theory (DFT), molecular dynamics (MD), and many-body perturbation theory. Oxides pose a noticeable challenge to density functional theory as standard treatments of the exchange-correlation interaction suffer from several severe shortcomings, not only due to their structural complexity.

Below, we sketch the GraFOx related topics that we address:

Optical excitations in M2O3 (M=Ga, In, Al)

We explore the optical and core-level excitations by means of many-body perturbation theory. More recently, we have developed a novel approach to resonant inelastic x-ray (RIXS) scattering (PhD thesis of Christian Vorwerk). Moreover, we investigate the impact of lattice screening on exciton binding energies (master thesis Max Schebek).

  • Involved people: Christian Vorwerk (PhD student), Max Schebek (master student), Dmitrii Nabok, Fabio Caruso

Surface structure, stability, and energy of Ga2O3 surfaces

While the bulk properties of Ga2O3 have been extensively studied in the past, the relaxations and reconstructions of their surfaces is largely unexplored. The surface structure is controlled by the composition, pressure, and temperature of the environment. Also the doping (intentionally or unintentionally) of the bulk results in the formation of a mesoscopic space-charge layer with significant influence on the surface energies and stabilities. Within this project, we compute surface structure, stability, and energy of Ga2O3 surfaces as a function of doping levels, O2 pressure, and temperature.

  • Involved people: Konstantin Lion (PhD student); collaboration with the GraFOx team of the FHI (Scheffler's group)

Electron-phonon interactions and polarons in group-III oxides

Scattering processes including phonons as well as the formation of polarons, i.e. phonon-dressed charge carriers, heavily impact optoelectronic and transport properties of materials. I order to study such phenomena from first principles, we employ many-body perturbation theory. By solving the corresponding Dyson equation, we compute both electronic and phononic self energies. This allows for the calculation of polaron life-times, spectral functions and band-structure renormalization as a function of temperature.

  • Involved people: Sebastian Tillack (PhD student), Fabio Caruso, Pasquale Pavone

 

Publications

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J. E. N. Swallow, et al.
Influence of Polymorphism on the Electronic Structure of β-Ga2O3
Chem. Mater. 32, 8460 (2020).

The search for new wide-band-gap materials is intensifying to satisfy the need for more advanced and energy-efficient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase.
Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure–electronic structure relationships. Valence and conduction electronic structure as well as semicore and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provides detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.

 

C. Vorwerk, F. Sottile, and C. Draxl
Excitation Pathways in Resonant Inelastic X-ray Scattering of Solids
Phys. Rev. Research 2, 042003(R) (2020).

Resonant inelastic x-ray scattering (RIXS) is a powerful spectroscopic technique that offers an elemental- and orbital-selective probe of the electronic excitations over a huge energy range. We present a many-body approach to determine RIXS spectra in solids, yielding an intuitive expression for the RIXS cross section in terms of pathways between intermediate many-body states containing a core hole, and final many-body states containing a valence hole. Explicit excited many-body states are obtained from the diagonalization of the Bethe-Salpeter equation in an all-electron framework. For the paradigmatic example of the fluorine K edge of LiF, we show how the excitation pathways determine the spectral shape of the emission, and demonstrate the nontrivial role of electron-hole correlation in the RIXS spectra.

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S. Tillack, A. Gulans, and C. Draxl
Maximally localized Wannier functions within the (L)APW+LO method
Phys. Rev. B 101, 235102 (2020).

We present a robust algorithm that computes (maximally localized) Wannier functions without the need of providing an initial guess. Instead, a suitable starting point is constructed automatically from so-called local orbitals which are fundamental building blocks of the basis set within (linearized) augmented plane-wave methods.
Our approach is applied to a vast variety of materials such as (semi)metals, bulk and low-dimensional semiconductors, and complex inorganic-organic hybrid interfaces. For the interpolation of electronic single-particle energies, an accuracy in the meV range can be easily achieved. We exemplify the capabilities of our method by the calculation of the joint density of states in aluminum, (generalized) Kohn-Sham and quasiparticle band structures in various semiconductors, and the electronic structure of β−Ga2O3, including electron and hole effective masses.

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R. Schewski, et al.
Step-flow growth in homoepitaxy of β-Ga2O3 (100) – the influence of the miscut direction and faceting
APL Materials 7, 022515 (2019).

In a combined experimental and theoretical work, we present a systematic study on the influence of the miscut orientation on structural and electronic properties in the homoepitaxial growth on off-oriented β-Ga2O3 (100) substrates by metalorganic chemical vapour phase epitaxy.
By using high-resolution scanning transmission electron microscopy and atomic force microscopy, we find significant differences in the surface morphologies of the substrates after annealing and of the layers in dependence on their miscut direction. While substrates with 6° miscuts toward [001] exhibit monolayer steps terminated by (201) facets, mainly bilayer steps are found for 6° miscuts toward [001]. Epitaxial growth on both substrates occurs in step-flow mode. However, while layers on substrates with a miscut toward [001] are free of structural defects, those on substrates with a miscut toward [001] are completely twinned with respect to the substrate and show stacking mismatch boundaries. This twinning is promoted at step edges by transformation of the (001)-B facets into (201) facets.Density functional theory calculations of stoichiometric low index surfaces show that the (201) facet has the lowest surface energy following the (100) surface. We conclude that facet transformation at the step edges is driven by surface energy minimization for the two kinds of crystallographically inequivalent miscut orientations in the monoclinic lattice of β-Ga2O3.

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C. Vorwerk, C. Cocchi, and C. Draxl
Addressing electron-hole correlation in core excitations of solids: An all-electron many-body approach from first principles
Phys. Rev. B 95, 155121 (2017).

We present an ab initio study of core excitations of solid-state materials focusing on the role of electron-hole correlation. In the framework of an all-electron implementation of many-body perturbation theory into the exciting code, we investigate three different absorption edges of three materials, spanning a broad energy window, with transition energies between a few hundred to thousands of eV.
Specifically, we consider excitations from the Ti K edge in rutile and anatase TiO2, from the Pb M4 edge in PbI2, and from the Ca L2,3 edge in CaO. We show that the electron-hole attraction rules x-ray absorption for deep core states when local fields play a minor role. On the other hand, the local-field effects introduced by the exchange interaction between the excited electron and the hole dominate excitation processes from shallower core levels, separated by a spin-orbit splitting of a few eV. Our approach yields absorption spectra in good agreement with available experimental data and allows for an in-depth analysis of the results, revealing the electronic contributions to the excitations, as well as their spatial distribution.

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C. Cocchi, et al.
Atomic signatures of local environment from core-level spectroscopy in β-Ga2O3
Phys. Rev. B 94, 075147 (2016).

We present a joint theoretical and experimental study on core-level excitations from the oxygen K edge of β-Ga2O3. A detailed analysis of the electronic structure reveals the importance of O-Ga hybridization effects in the conduction region.
The spectrum from O 1s core electrons is dominated by excitonic effects, which overall redshift the absorption onset by 0.5 eV, and significantly redistribute the intensity to lower energies. Analysis of the spectra obtained within many-body perturbation theory reveals atomic fingerprints of the inequivalent O atoms. From the comparison of energy-loss near-edge fine-structure (ELNES) spectra computed with respect to different crystal planes, with measurements recorded under the corresponding diffraction conditions, we show how the spectral contributions of specific O atoms can be enhanced while quenching others. These results suggest ELNES, combined with ab initio many-body theory, as a very powerful technique to characterize complex systems, with sensitivity to individual atomic species and to their local environment.