In-depth characterization of spectroscopic features is a key element for the application of materials in opto-electronic devices. Ab initio theory can provide such analysis in terms of methods based in many-body perturbation theory (MBPT). While the GW approach is the state-of-the-art tool for describing the electronic structure as (typically) compared to ARPES measurements, the solution of the Bethe-Salpeter equation (BSE) provides optical spectra including excitonic effects. The latter are crucial in semiconductors, and in particular in wide band-gap systems.
On the methodology side, oxides challenge state-of-the art theory not only due to their structural complexity, i.e. large unit cells. Approaches based on MBPT may suffer from the so-called starting-point dependence. This means that the usual procedure of applying the GW approach in a single-shot manner in first-order perturbation theory leads to different results, depending on the used xc functional of the underlying DFT calculation. This shortcoming may then be carried over to the BSE step. This issue will be addressed by either exploring different xc starting points or applying a self-consistent GW procedure.
While from the theory point of view Ga2O3 has been quite well (though not fully quantitatively) investigated in terms of optical and core-level spectroscopy, In2O3 is less explored mainly due to its structural complexity. On the other hand, it is most interesting for its optical gap being larger than the fundamental one. This points to a peculiar interplay of selection rules of optical transitions, determined by crystal symmetry and band character, with excitonic effects. This at the same time provides a handle for tuning the spectra as strain, defects, and dimensionality break symmetry, thus changing absorption onset and spectral features.
Systematic investigation of optical and core excitations, will be performed to address these aspects and will include analysis of excitonic binding energies, and oscillator strength, and the nature of excitons in terms of degree of spatial extension and atomic origin.
Major accomplishments expected:
Collaboration with partners in the project:
For quantitative comparison with experiments, knowledge on the impact of sample quality, doping, surface effects, etc. on the measured spectra is required.
Christian Vorwerk obtained his BSc in 2013, and his MSc in the spring of 2016, both at Humboldt University; Berlin. During his master's studies, he spent one year at the University of Washington in Seattle, USA. He has worked on the topic of ab initio theoretical spectroscopy in his master's thesis, focusing on the description of x-ray absorption spectra. He is excited to develop new methodologies to determine the optical and core spectra of the group-III oxides.
If you have queries about the project, please contact the PI:
Claudia Draxl, Humboldt Universität zu Berlin
Leibniz-Insitut im Forschungsverbund Berlin e.V.
10117 Berlin, Germany
The Leibniz ScienceCampus GraFOx is a network of two Leibniz institutes, two universities and one institute of the Max Planck Society. The Network is based in Berlin, Germany.