Semiconductor Optoelectronics

This research group primarily deals with the integration of alternative semiconductor materials, especially group IV (germanium, GeSn and SiGeSn), into silicon technology, of which the material properties appear superior to those of silicon towards their application in the field of photonics, sensor and THz technology. In addition, silicon-based quantum materials are evaluated and optimized with respect to their suitability for novel quantum technologies.

High-level control of the material properties targeted for such new material systems plays a central role to further increase device performances. An example is given by the innovative SiGe(Sn) material system, which is being intensively researched for future group IV semiconductor optoelectronics. In addition to the growth of SiGe(Sn) layer systems of appropriate quality on silicon, their detailed material characterization from the macro-​ to the nano-​scale is a major challenge.

Main targets

  • development of CMOS-compatible materials and their processing for photonics, sensors or quantum technologies
  • development and optimization of new characterization methods for material physics of complex Si-based material structures

Research topics

  • development and optimization of group IV heterostructures for quantum computing and CMOS compatible THz sources
  • advanced structural, chemical, optical and electrical material characterization of semiconductors
  • optical characterization of plasmonic structures and optoelectronic devices
  • Raman thermometry for Si-based thermoelectrics
  • surface and interface physics of group IV semiconductors and their compounds
  • theoretical material physics taking into account real material parameters

Research results

Surface Science on GeSn Surfaces and Interfaces for Si CMOS Compatible Photonics and Electronics

Our investigations started with the growth of pure Sn on Ge(001) at room temperature, aimed towards establishing a first fundamental understanding of the Sn/Ge(001) electronic structure and growth mechanism. By angle-​resolved photoelectron spectroscopy (ARPES), the modification of the Ge(001) surface electronic structure upon Sn deposition was studied. We observed, that Sn induces an upward band bending on the Ge(001) surface, indicating the formation of a Schottky barrier. The k-space resolved valence band structure revealed an increase of the heavy hole band effective mass after Sn deposition. Furthermore, we attribute the disappearance of a Ge(001) surface state after Sn deposition to the broken periodicity of the surface after Sn deposition. In fact, we were able to detail the growth mechanism of Sn on the Ge(001) surface at the atomic scale by scanning tunneling microscopy (STM). Two processes compete during the self-organized growth at room temperature: Sn incorporation into the surface and ad-​dimer formation. The latter results in Sn-lines on the surface, oriented along the (110) direction. The combination of both - ARPES and STM – allowed us to correlate the appearance of a Sn induced surface state to a Sn ad-dimer configuration within the Sn-lines on the surface. Our latest results are a first step towards future investigations, studying the influence of temperature on the Sn/Ge(001) interface, as well as structural and electronical properties of the GeSn alloy.

Reduction of threading dislocation density beyond the saturation limit by optimized reverse grading

The threading dislocation density (TDD) in plastically relaxed Ge/Si(001) heteroepitaxial films is commonly observed to decrease progressively with their thickness due to mutual annihilation. However, there exists a saturation limit, known as the geometrical limit, beyond which a further decrease of the TDD in the Ge film is hindered. Recently, we have shown that such a limit can be overcome in SiGe/Ge/Si heterostructures thanks to the beneficial role of the second interface [1]. Indeed, we have achieved Si0.06Ge0.94/Ge/Si(001) films, which display a TDD remarkably lower than the saturation limit of Ge/Si(001). Such a result is interpreted with the help of dislocation dynamics simulations. The reduction of TDD is attributed to the enhanced mobility acquired by preexisting threading dislocations after bending at the new interface to release the strain in the upper layer. Importantly, we demonstrate that the low TDD achieved in Si0.06Ge0.94/Ge/Si layers is also preserved when a second, relaxed Ge layer is subsequently deposited. This also makes the present reverse-​grading technique of direct interest for achieving a low TDD in pure-​Ge films.

[1]  Skibitzki, Oliver, et al. "Reduction of threading dislocation density beyond the saturation limit by optimized reverse grading." Physical Review Materials 4.10 (2020): 103403.

Dr. Wolfgang Klesse

Im Technologiepark 25
15236 Frankfurt (Oder)

Phone: +49 335 5625 424
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