Electronic excitations and their real-time dynamics are critical for how we use optical materials in applications and how we probe carrier thermalization and energy transfer between electrons and ions. Recent experimental advances allow us to do so with unprecedented accuracy and time resolution, however, their interpretation relies on detailed theoretical understanding. This can be provided by first-principles theoretical-spectroscopy, based on many-body perturbation theory and time-dependent density functional theory.
This talk will briefly discuss how we use quantum-mechanical first-principles simulations, based on the GW+BSE approach, to provide an accurate description of optical properties for oxide materials. I will then show how different dielectric screening contributions affect this description: More specifically, we compare screening due to free carriers for doped systems, electronic interband screening, and lattice polarizability. The first effect can be modeled using a Thomas-Fermi description of free electrons, and the latter can be described using the Froehlich model. Incorporating these into our simulations, allows us to quantify how exciton binding energies are reduced by additional screening and how optical and excitonic properties of various transparent conducting oxides are impacted.
Finally, we explore carrier dynamics emerging in ZnO after an initial optical excitation by imposing occupation numbers on electron and hole Kohn-Sham states. The resulting temperature-dependent optical spectra allow us to draw conclusions about contributions from band-gap renormalization, Burstein-Moss shift, and inter-band transitions in excellent agreement with experiment.
First-principles simulations of excited states, using density-functional and many-body perturbation theory, are now capable of accurately predicting electronic and optical properties of complex oxides, enabling unprecedented understanding and computational materials design. After briefly discussing these techniques and their numerical implementation, I will focus on their application to Ga2O3. This material is interesting for transparent electronics in the semiconductor industry since it conducts electrical current while being transparent at the same time.
For Ga2O3 I will provide an overview of recent efforts by several groups to understand the optical absorption in terms of quasiparticle electronic structure and optical transition-matrix elements. These studies provide a clear quantitative picture of the optical anisotropy. I will then show how excitonic effects influence the spectrum close to the absorption onset and at high photon energies. In order to achieve an accurate computational description of excitons, the electron-hole interaction needs to be taken into account. To this end it is mandatory to understand the influence of dielectric screening, which is a long-standing problem in computational materials science. I will explain how the presence of free carriers and of lattice polarization contributes to dielectric screening, impacting the electron-hole interaction, with consequences for optical spectra.
Finally, I will also allude to the computational infrastructure needed to compute these highly accurate theoretical spectra for Ga2O3, since, more generally, combining numerical approaches with cutting-edge computation allows to further develop computational materials science and to perform highly accurate theoretical spectroscopy for modern, complex materials that drive societal progress.
Titanium dioxide is a versatile material with ubiquitous applications, many of which are critically linked to either light absorption or transparency in the visible spectral range in addition to electrical conductivity. Doping is a well-known way to influence those properties in order to bring them into a desired range. Working towards a comprehensive understanding of the electronic and optical properties of TiO2 (as well as of the link between them) we review and summarize electronicstructure results that we obtained using cutting-edge theoretical spectroscopy techniques. We focus on the formation of electron and hole polarons and we elucidate the influence of doping on the optical properties of TiO2. In addition, we present new results for the reflectivity of pure TiO2.
In order to facilitate the development of next-generation display devices or modern solar cells, material performance is critically important. A combination of high transparency in the optical spectral range and high electrical conductivity under ambient conditions is attractive, if not crucial, for many applications. While the doping-induced presence of free electrons in the conduction bands of CdO can increase the conductivity up to values desired for technological applications, it is, however, expected to impact the optical properties at the same time. More specifically, variations of the band gap, effective electron mass, and optical-absorption onset have been reported. In this work recent results from modern theoretical spectroscopy techniques are compared to experimental values for the optical band gap in order to discuss the different effects that are relevant for an accurate understanding of the absorption edge in the presence of free electrons with different concentrations.
The continuously increasing power of modern supercomputers renders the application of more and more accurate parameterfree
models to systems of increasing complexity feasible. Consequently, it becomes possible to even treat different realstructure
effects such as alloying or n-doping in systems like the technologically important transparent conducting oxides.
In this paper we outline how we previously used a combination of quasiparticle calculations and a cluster expansion scheme
to calculate the fundamental band gap of MgxZn1-xO and CdxZn1-xO alloys. We discuss the results in comparison to
values for In2O3, SnO2, SnO, and SiO2. In addition, we discuss our extension of the Bethe-Salpeter approach that has
been used to study the interplay of excitonic effects and doping in n-type ZnO. The dependence of the Burstein-Moss shift
on the free-carrier concentration is analyzed.
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