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Time-resolved spectroscopy methods for the examination of light-matter interaction in semiconductors and semiconductor structures.

Light-matter interaction in semiconductors and semiconductor nanostructures have traditionally been studied in the workgroup ‘experimental semiconductor physics’. With pulsed laser beams, the electronic states, optical transitions, and, with suitable methods, time dynamics of a semiconductor system can be studied. Along with this comes a whole series of time-resolved spectroscopic methods that can be used, including pump-probe experiments, four-wave mixing, and time-resolved luminescence measurements.

_{Figure 1: Photoluminescence experiment with time-resolved spectroscopy.}

The investigated material systems include binary, ternary and quaternary III-V and II-VI semiconductors as well as semiconductor quantum wells based on these materials. For the experiments, various femtosecond laser systems, a high-performance amplifier system for the generation of strong fields, and different analysis systems are all used. With streak cameras, luminescence can be time-resolved in the near-infrared and ultraviolet ranges as well as the visible range.

_{Figure 2: Time-resolved spectrum of differential absorption obtained in a pump-probe experiment. The plot shows charge carrier dynamics in a semiconductor system.}

For many years we have worked very successfully with the theoretical physicists in our department, who have simulated existing experiments or inspired new ones. This has enabled a successful merging of theoretical models with real experimental data, as well as a deeper understanding of light-material interactions in novel systems.

^{Figure 3: Schematic drawing of an ultra-fast spectroscopy setup.}

^{In many of these experiments we expose the samples additionally to strong THz pulses in order to study the interaction of these high-frequency alternating fields with multi-particle systems.}
^{You may find short descriptions of planned experiments in the Bachelor/Master Theses section.}

Beyond time-resolved spectroscopy, our team also performs a time-independent/steady-state (continuous-wave) analysis of fluorescence/photoluminescence, absorption, and photocurrent spectra. Such investigations give fundamental insight into the properties of new samples and material systems. Thus, these essential techniques are also utilized as a part of our research activities.

In the new Collaborative Research Centre “Structure and Dynamics of Buried Interfaces” (http://www.dfg.de/en/service/press/press_releases/2013/press_release_no_16/index.html) of the German Research Foundation (DFG) we explore the building and decay dynamic of boundary excitons, i. e. bound electron-hole pairs, which are created at inner boundaries. These boundary states are important for the separation of charge in solar cells before the charge carriers flow towards the contacts and an energy consumer can be operated with the solar cell. Inner boundaries exist in almost all electrical and optoelectronic components, for example in batteries, storage batteries, semiconductor lasers, semiconductor-diodes and -transistors and the already mentioned solar cells. The charge carriers must always flow through the boundary hence the component is operational.

The main question, which is to be answered by the special research field (SRF), is how the morphology of the boundary influences the charge carrier transport. For example: do the charge carriers move better through the boundary if it is less rough or abrupt.

The well defined and characterized semiconductor probes for our experiments are provided by WZMW (scientific centre for material science) in Marburg.

The already mentioned boundary excitons should react sensitively to the morphology of the boundary and may provide a model system to answer our questions.

Literature:

Dynamics of charge-transfer excitons in type-II semiconductor heterostructures
M. Stein, C. Lammers, P.-H. Richter, C. Fuchs, W. Stolz, M. Koch, O. Vänskä, M.J. Weseloh, M. Kira, and S. W. Koch
Phys. Rev. B 97, 125306 (2018)

Enhanced absorption by linewidth narrowing in optically excited type-II semiconductor heterostructures
M. Stein, C. Lammers, M. J. Drexler, C. Fuchs, W. Stolz, and M. Koch
Phys. Rev. Lett. 121, 017401 (2018)

Time-Resolved Charge-Transfer State Emission in Organic Solar Cells: Temperature and Blend Composition Dependences of Interfacial Traps
A. Arndt, M. Gerhard, A. Quintilla, I.A. Howard, M. Koch, U. Lemmer
J. Phys. Chem C. 119, 13516 (2015)

Temperature- and Energy-Dependent Separation of Charge-Transfer States in PTB7-Based Organic Solar Cells
M. Gerhard, A.P. Arndt, I.A. Howard, A. Rahimi-Iman, U. Lemmer and M. Koch
J. Phys. Chem C. 119, 28309 (2015)

Evidence for Anisotropic Electronic Coupling of Charge Transfer States in Weakly Interacting Organic Semiconductor Mixtures
V. Belova, P. Beyer, E. Meister, T. Linderl, M.-U. Halbich, M. Gerhard, Marina; S. Schmidt, T. Zechel, T. Meisel, A. Generalov, A.S. Anselmo, R. Scholz, O. Konovalov, A. Gerlach, M. Koch, A. Hinderhofer, A. Opitz, W. Brutting, F. Schreiber
J. Am. Chem. Soc. 139, 8474 (2017)

If you are interested in our exciting experiments with short pulses, strong THz fields, functionalized material systems, and semiconductor structures, and if you consider doing your Bachelor/Master/PhD project research in this field, feel free to contact us.

Contact: Prof. Dr. Martin Koch