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.

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.

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.

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 L_2,3 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.

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.