Detecting defects in semiconductors at the atomic level

Modern solar cells work with thin layers of semiconductors that convert sunlight into electrical energy. The key to increasing their efficiency even further lies in the composition and structure of the material. Due to the way the material is manufactured, it can have defects that have a disruptive effect. Scientists at the University of Leipzig have now designed and built a measuring station that allows them to better examine semiconductor materials for such defects at the atomic level. The new measurement capability can also be useful in researching materials for other applications, such as LEDs or in telecommunications.

At the PETRA III synchrotron of the German Electron Synchrotron (DESY) in Hamburg, a new measurement setup will be available to the user community from January 2023. It will make it possible to investigate electronic and structural properties of semiconductor thin films and other materials - simultaneously and element-specifically. This provides information on how structural defects affect the electronic properties of the material. The measurement setup is the result of the research project -Identification of defects by element-specific excitation of optical luminescence- led by Claudia S. Schnohr at the Felix Bloch Institute for Solid State Physics at the University of Leipzig.

-Defects are disturbances in the structure of a material, for example when certain atoms are not located in the crystal lattice where they are supposed to be-, Claudia Schnohr explains. -They often occur unintentionally during the production of the samples. For example, foreign atoms may get into the sample or the crystal lattice may not grow perfectly.- There are often theoretical predictions about the nature of these defects, -but experimental confirmation of these predictions is often difficult,- says the physicist.

Measurement setup uses effects of high-brilliance X-rays

The measurement setup combines two methods: X-ray absorption spectroscopy (XAS) and X-ray excited optical luminescence (XEOL). The researchers take advantage of two effects that occur simultaneously when certain materials are excited with high-brilliance X-rays such as those provided by PETRA III: First, the sample absorbs some of the X-rays. This causes the material itself to emit X-rays again. Both the radiation emitted by the sample and the non-absorbed radiation that passes directly through the sample are measured. From how strongly the material absorbs the X-rays at different energies - i.e., at different wavelengths - one can derive structural information about the material, such as the mutual spacing of atoms in the material. -The special thing here is that you can apply this to specific chemical elements in the material at a time, even if several elements occupy the same lattice site in the crystal structure, as is often desired in thin-film solar cells,- says Claudia Schnohr. -Unlike other characterization methods, the XAS method is therefore element- and thus often site-specific. This is important for the investigation of structural defects in the thin-film material.

On the other hand, the material begins to glow in a specific way after excitation by X-rays in the visible or near-ultraviolet range. -This luminescence provides clues to the electronic properties of the material-, the professor said. To this end, the team, together with Edmund Welter and his team at DESY, has set up an XEOL detection unit that analyzes the respective luminescence activity of the samples via a spectrograph simultaneously with the measurement of the X-ray absorption. -There, this light is decomposed into the individual wavelengths. Different defects become visible through their luminescence at different wavelengths-, Schnohr said.

Finding structural causes for electronic defects

By simultaneously combining the two methods, scientists can therefore relate information about defects in the lattice structure to electronic properties of these defects. -In the best case, we can see what the structural causes of certain electronic defects are.

If defects are not desired in thin-film solar cells or even LEDs, this can be quite different in other applications. Examples of this are luminescent materials or certain optical materials that are supposed to glow in a specific color. Here, targeted defects in the material are one way to achieve the desired effect, explains Claudia Schnohr.

Three semiconductor materials under test

The group demonstrated the functionality and performance of the setup using three semiconductor materials: Copper indium diselenide (CulnSe2) is an absorber material for high-efficiency thin-film solar cells. Zinc oxide (ZnO) is a transparent semiconductor used in light-emitting diodes, liquid crystals or other optoelectronic devices. Gallium nitride (GaN) is a key material for blue light-emitting diodes. The samples were provided by the research groups of Susanne Siebentritt from the Laboratory for Photovoltaics at the University of Luxembourg and Marius Grundmann from the Felix Bloch Institute for Solid State Physics at the University of Leipzig, with whom the team is collaborating on this project.

-The materials differ on the one hand in their chemical elements and thus in the energy of the synchrotron X-ray beam required for the investigation, and on the other hand in the wavelength of the emitted light-, says Professor Claudia Schnohr. -The investigation of these three materials thus demonstrates the broad application possibilities of our setup, even though our own research is currently focused on thin-film solar cells.-.

Only few similar measurement facilities in Europe

The new measurement infrastructure is one of only a few of its kind in Europe. The project was funded over three years by the German Federal Ministry of Education and Research (BMBF) within its program -Research on Universe and Matter- (ErUM) with a sum of about 570,000 Euro (grant number 05K19OL1). The setup and demonstration of the functionality was published by the research group in the -Journal of Synchrotron Radiation-. The paper was selected for the cover page.

Original title of the publication in the -Journal of Synchrotron Radiation-:
High-resolution XEOL spectroscopy setup at the X-ray absorption spectroscopy beamline P65 of PETRA III , doi: 10.1107/S1600577522007287

Birgit Pfeiffer