Motivation
In initial courses on solid state physics, the concept of an ideal crystal is introduced in which each atom occupies a certain place. This, however, is an idealization that is very far from reality. There are no ideal crystals.
Already in the early stages of the solid state research, it was established that defects often have a profound effect on their physical properties. The most impressive examples of this kind can be found in physics of semiconductors. The very existence of semiconductor electronics is based on the ability to control both the magnitude and type of electrical conductivity by introducing small additions of chemical impurities that have the properties of small donors or acceptors.
The study of defects in semiconductors therefore became an area of active research immediately after semiconductors emerged as important technological materials. Moreover, the development of nanoelectronics has led to a significant increase in the vulnerability of devices to unwanted defects arising during material processing. At the same time, due to the development of new devices and technological processes, the requirements for precision control of material properties through the introduction of desired defects have increased.
For a long time, our group has been studying the properties of defects in semiconductors ranging from "classical" Si, Ge and GaAs to transparent conducting oxides such as ZnO, SnO2, TiO2, etc. The main research method is optical spectroscopy, including photoluminescence, IR absorption, Raman scattering and photoconductivity. Additional methods include various types of capacitance spectroscopy (DLTS, minority carrier spectroscopy, etc.) and the Hall effect. In addition, we closely collaborate with various theoretical groups. The combination of various experimental methods and in-depth theoretical analysis allows to achieve significant progress in understanding the properties of defects in semiconductors and, consequently, in the development of new semiconductor technologies.
Current projects
Shallow dopants and compensating centers in antimony chalcogenides (2026-2029, Deutsche Forschungsgemeinschaft, LA 1397/21-1)
The urgent need for high-efficiency, low-cost solar cells motivates researchers to look for new absorber materials for thin-film photovoltaics. Antimony triselenide (Sb2Se3) and antimony trisulfide (Sb2S3) have attracted immense research interest as new absorber for highly efficient, environmentally friendly, stable, and cost-effective thin-film solar cells. Defects and impurities have a decisive impact on the performance of all semiconductor devices. At present, the physics of defects in Sb2Se3 and Sb2S3 remains unexplored and it is not
ambiguously clear which of native defects, impurities, or defect complexes and to what extend introduce charge carriers and act as detrimental trap sites or recombination centers. The central goal of our project is to study fundamental properties of shallow dopants and compensating centers in single-crystalline Sb2Se3 and Sb2S3. We will focus on technologically important impurities such as chlorine, oxygen, and hydrogen. Optical and electrical spectroscopy will be employed to get insight into microscopic structure, electrical activity, thermal stability, diffusion mechanisms, evolution kinetics, and formation of complexes between extrinsic dopants and native defects. The project will contribute to a better understanding of defects in Sb2Se3 and Sb2S3 and, thus, support efforts in bringing these materials to a broader technological usage in photovoltaics, photoelectrochemical cells, or other solar-driven applications.
Cryogenic Vibrational Spectroscopy of Site-Specific Hydrogen Adsorption in Si–Ti Mixed Oxides
(2026, Deutsche Forschungsgemeinschaft, LA 1397/27)
This project establishes quantitative structure-property relationships for hydrogen interaction in nanostructured Ti-containing oxides using low-temperature vibrational spectroscopy. Materials include SiO2-TiO2 mixed oxides, their amorphous/crystalline phases, and Au-containing composites. Sol-gel synthesis enables controlled variation of Ti coordination and oxide reducibility in catalyst-relevant frameworks.
Despite industrial use of Si-Ti networks, tetrahedral Ti4+, Si-O-Ti linkages, and Ti-O-Ti domains remain insufficiently resolved. Molecular hydrogen serves as a probe (15-300 K), where H-H stretching shifts reflect local electrostatic fields and adsorption sites.
This study tests whether (i) H2 interaction is governed by local Ti coordination and affected by host crystallinity (ii) in Au systems, activation and spillover occur at Au-Ti interfacial sites in connected TiO2-like environments; (iii) Au nanoparticles act as plasmonic/electronic probes enhancing defect- and interface-related vibrational modes.
Preliminary results show composition-dependent structural evolution in Si-Ti systems and site-specific H2 adsorption linked to cations and silanol groups. Porous frameworks show cryogenic physisorption and significant H2 uptake. In Au systems, adsorption at low-coordinated Au sites and metal-support interfaces, including size effects and spillover, modifies vibrational and plasmonic response.
The project delivers systematic spectroscopic datasets across material classes, overcoming literature fragmentation and enabling predictive understanding of hydrogen–surface interactions for catalysis, hydrogen storage, and plasmon-assisted process.