Transient absorption spectroscopy with attosecond EUV pulses on binary and ternary solar cell semiconductor systems for the investigations of their charge carrier dynamics
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Final Report Abstract
Semiconductors are driving our lives being used in e.g. solar cells, light emitters or optoelectronics. While the improvements of the most used semiconductor silicon has come close to an end after decades of research, binary systems like GaAs or ternary systems like AlGaAs are still topic of research. Their demand on overcoming lattice mismatching during epitaxial growth limits their stacking capability (in terms of the number of different materials) when being used in multi-junction systems. The realization of multi-junction systems is the key requirement for complex solar cells and electronics. The intensive research on semiconductors aims for ever faster electronics, brighter emitters, more efficient solar cells for renewable energy and ever smaller devices. A new type of binary 2D semiconductors, like transition metal dichalcogenides (TMDCs) has emerged the past years to fulfill this aim in the future. These atomic thin 2D layers can be stacked to complex van der Waals hetero-structures. Molybdenum disulfide (MoS2) is the most prominent one of this material family. Investigations of other binary semiconductors like iron oxide (Fe2O3) are important, since these are materials for use in solar water splitting or artificial photosynthesis. This project aimed to answer fundamental questions of the fastest electronic processes in binary semiconductors to contribute to the very important field of renewable energies. What type of (electronic) processes happen on very short timescales and how could they probably be manipulated? Attosecond Transmission Absorption Spectroscopy (ATAS) in the extreme ultraviolet (EUV) photon range has been utilized to track the fastest carrier dynamics in these systems. ATAS in the EUV is a very powerful technique as it is element specific and oxidation state specific combined with a high temporal attosecond resolution. The project could show that this temporal resolution and spectral sensitivity is necessary to gain insight into binary semiconductor systems. Various different carrier dynamics like population transfer, hot electron thermalization, long-lived core-excitons, coherent quantum dynamics, coherent strain waves or spin-orbit split induced core-level transition delays could be observed and extracted from the different transient absorption measurements on different types of material systems. New fundamental physics of ultrafast processes in matter could be revealed. These revealed carrier dynamics offer new tuning capabilities for optimizing these systems for future applications. For example, the first dynamic measurements of strongly bound long-lived core-excitons in MoS2, a new type of binary 2D semiconductor, in conjunction with ab-initio theory calculations draw a new physical picture of this new material type. The time-resolved evolution of this core-level quasiparticle adds important carrier information which allows further optimizing binary 2D semiconductors and may open a new field of EUV/soft-X-ray excitonics. Technically it was not possible to realize the proposed binary and ternary material systems on a 30 nm thin membrane, the prerequisite for ATAS measurements in the EUV. As a consequence wafers have been realized and studied in reflection geometry. Their poor spectral characteristics (e.g. reflectivity, absorption edge contrast) in the EUV hindered the investigations of dynamics due to high harmonic flux reasons. As project solution two realizable binary semiconductor systems Fe2O3 and MoS2 have been studied using ATAS. The electron dynamics have been studied with attosecond resolution. While the measured population transfer in Fe2O3 was expected the detection of long-lived core-excitons together with coherent quantum dynamics in MoS2 was surprising. After the project the experimental analysis combined with theory collaborations draws a complete understood physical picture about this exotic electronic dynamics and opens a new research door for this type of binary 2D semiconductor.
Publications
- “Electron dynamics in transition metal dichalcogenides utilizing attosecond transient absorption spectroscopy,” CLEO: QELS Fundamental Science, FF1P.4 (2018)
A. Guggenmos, H.-T. Chang, M. Zürch, D. Y. Qiu, R. Geneaux, Y.-C. Chen, X. Wei, C.-M. Jiang, Y. Liang, F. H. da Jornada, A. Schwartzberg, D. Prendergast, V. C. Tung, S. G. Louie, D. M. Neumark, and S. R. Leone
- “Transient absorption spectroscopy using High Harmonic Generation: A review of ultrafast X-ray dynamics in molecules and solids,” Phil. Trans. R. Soc. A
R. Geneaux, H. J. B. Marroux, A. Guggenmos, D. M. Neumark, S. R. Leone
(See online at https://doi.org/10.1098/rsta.2017.0463)
