Experimental Physics/Electron- and Ion-Nanooptics

Improvement of S/TEM-based methods for reliable and accurate measurement of strain and electric potential

Funded by German Research Foundation (DFG)
Funding reference number: DFG NI 2221/1-1
PI: Dr. Laura Niermann

Present day lighting technology from general purpose up to dedicated applications like optical data transmission is based on light emitting diodes (LED) and laser diodes built from semiconductor heterostructures, which allow shaping the electrostatic potential landscape by means of semiconductor doping in combination with different material properties. Accurate knowledge of the strain induced by material differences and the knowledge of the actual electric potential in these structures is essential for further understanding the underlying physics and hence opening doors for precise modeling and further development of these devices.

Within this project AlGaN based LEDs emitting in the ultraviolet wavelength range will be investigated. In the Nitrides the electric potential landscape does not only result from doping, but also from electrical polarization. In the devices under investigation the polarization influences the light emitting region of the LED and is also exploited to optimize tunnel junctions, which are used as highly efficient electrical p-contact.

Scanning-/ transmission electron microscope (S/TEM) based methods, in principle, allow the rather unique possibility of a direct and spatially resolved measurement of strain and electric potential. However, these methods are far away from providing reliable quantitative data.

This project has the goal to develop reliable and quantitative methods within the S/TEM for measurements of strain and electrostatic potentials in nanostructured devices. A special focus of the project is the effect of the interfaces between the materials on these measurements.

Knowledge will be gained about the interplay of strain, limited spatial resolution, and dynamical diffraction effects in QW-layers of small size and their effect on potential measurements by off-axis electron holography (EH) and differential phase contrast (DPC). Therefor comparative measurements of strain are done by using TEM-based techniques like dark-field electronholography (DFEH) and 4D-STEM based techniques like convergent beam electron diffraction (CBED). This measurements are also compared to multislice simulations and macroscopic CV-measurements.

Electrical in-situ contacted specimen will be used for defined electrical boundary conditions. Furthermore, scanning transmission electron beam induced curent (STEBIC) measurements will be performed.