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Exciton-Phonon Coupling from First Principles

Periodic Reporting for period 1 - EXPHON (Exciton-Phonon Coupling from First Principles)

Reporting period: 2017-10-01 to 2019-09-30

Today’s technological innovation is driven by a deep understanding of the fundamental properties of matter at the nanoscale and below, where the quantum character of the electrons and the many-body effects of their correlated dynamics determine the properties of a material. This understanding is twofold: On the one hand, modern spectroscopy techniques probe these many-body quantum excitations. On the other hand, theoretical ab-initio methods, that rely solely on the basic laws of quantum physics and do not make model assumptions, predict the micro- and macroscopic properties of a material and explain experimental findings.

In particular, the electronic and optical properties of a material are most relevant both for a better understanding of the fundamental materials physics with applications in state-of-the-art characterization techniques (e.g. ellipsometry and photoluminescence) and the design of new materials systems and devices for photovoltaics (as photovoltaic absorber or transparent conducting semiconductor in photovoltaic cells) or optoelectronics (such as light-emitting diodes, semiconductor lasers or novel display technologies). An accurate theoretical description of the optical properties requires to take the lattice degrees of freedom of a material (e.g. static lattice strain, lattice deformation by defects, lattice dynamics due to phonons) and its coupling to the electronic motion into account, without relying on models that necessitate empirical input parameters. Up to now, the available theoretical ab-initio tools largely disregarded the impact of the atomic lattice.

The present project aimed at developing theoretical methods and ab-initio numerical simulation tools for a quantitatively correct prediction of optical materials properties focusing in particular on the impact of electron-lattice coupling. Further, the optical properties of a number of simple semiconductors and some more complex materials of interest for technological applications should be calculated with these new tools to benchmark the developed methods with known data, to make predictions for future experiments, and to identify novel materials that are of potential technological interest.

In addition to advancing fundamental science, the project contributes to the EU Societal Challenges for “secure, clean, and efficient energy”, “smart, green, and integrated transport”, and “climate action, environment, resource efficiency and raw materials” by its direct impact in photovoltaics and optoelectronics and its utility in the search for novel materials with technologically interesting properties.
The structural, elastic, electronic, and optical properties of hexagonal Ge in the lonsdaleite structure have been calculated using density-functional theory. Hexagonal Ge turns out to have a direct band that is not optically active. It could be shown that, due to structural modification by applying uniaxial lattice strain or alloying with hexagonal Si, the lowest optical transitions of hexagonal Ge become optically allowed. It has been demonstrated that strained hexagonal Ge and hexagonal SiGe alloys become efficient light emitters that are potential candidates for a Si-compatible nanolaser in a future Si-compatible photonics that incorporates microelectronics and optoelectronics. These theoretical predictions have been experimentally verified by now.

Since the major method development originally foreseen in the project (inclusion of lattice-screening contributions in the framework of the Bethe-Salpeter equation for the calculation of exciton binding energies) had been completed by others in the meantime, the focus in method development shifted towards a more accurate prediction of the electronic band gap which is the crucial property that determines the absolute position of the optical absorption onset. In the framework of many-body perturbation theory, a new scheme for a nonlocality-induced vertex correction beyond the established GW method was developed, implemented, and tested for Si. More tests for important semiconductors are underway.

Furthermore, the electronic and optical properties of various phases of CuI have been characterized theoretically. CuI with its band gap of about 3 eV and very small hole effective masses is a promising candidate for a p-type transparent semiconductor that would have various applications in optoelectronics, for instance as transparent electrode. The Bethe-Salpeter equation has been solved to calculate the optical spectrum including excitonic effects for CuI and the impact of defects and doping on the absorption properties has been evaluated. The work on this topic is still in progress and research on this materials system will be continued in the future.

The results obtained throughout the duration of the project have been published in peer-reviewed journals or are currently under review at peer-reviewed journals. More manuscripts are in preparation. A post-processing software tool to calculate radiative lifetimes has been developed. A greater software toolbox that allows to calculate various optical properties including lifetimes, emission and absorption spectra for thermally excited and degenerately pumped semiconductors is under development. The results of the project have been presented as invited and contributed talks or posters at several international conferences and workshops.
The research conducted in this project has impact for both fundamental science and technological applications. The development of a new scheme to include vertex corrections beyond the established GW method in many-body perturbation theory fosters the fundamental understanding of the problem of electronic correlation as such and provides a tool that can straightforwardly be used for a potentially more precise prediction of fundamental band gaps of semiconductors. It also offers a new perspective on the wealth of literature results obtained with different flavors of the GW method.

The software tool under development to predict absorption and emission properties of thermally excited and degenerately pumped semiconductors can be applied to a plethora of semiconducting systems and used to calculate their optoelectronic properties. Due to the ab-initio character of the approach, fields of applications are abundant and quantitative agreement (without adjustable parameters) can be obtained. This will help to analyze the results of various state-of-the art spectroscopy techniques, such as photoluminescence or pump-probe experiments.

On the materials side, the achieved characterization of the electronic and optical properties of hexagonal Ge (with and without lattice strain) and hexagonal SiGe alloys opens routes towards the development of a Si-technology compatible nanolaser which may potentially allow for the integration of optoelectronics into microelectronics. CuI, whose optical properties have been characterized for various phases, in presence of defects and additional charge carriers is currently discussed as potential p-type transparent conductor which may become a key material in future transparent optoelectronics with various applications (e.g. in photovoltaic cells or as thermoelectric material).