Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The combination of near-field microscopy with spectral resolution in the mid-infrared and terahertz range can address a large number of specific elementary and collective excitations locally. This is of interest for a variety of materials such as semiconductors, strongly correlated materials and biomolecules.
The free-electron laser FELBE provides a quasi-cw beam of radiation tunable in the range from 4 – 230 µm (photon energy 310 – 5 meV) for a scattering near-field microscope (sSNOM) based on a home-built atomic-force microscope. We demonstrate its potential to obtain quantitative information on buried semiconductor structures by determining the carrier concentration of boron-implanted stripes in a silicon matrix [1]. To this end, FELBE is tuned in the region of the plasma resonance of the holes in the implanted regions and the contrast of the sSNIM images is analyzed. Next we present a study of spectroscopy on exciting nanostructures, namely self-assembled InAs quantum dots. Due to their three-dimensional confinement, the dots exhibit atom-like states. However, many of the key features such as extremely sharp optical transition lines (at low temperature) are masked by the variation of energy levels from dot-to-dot due to differences in size and composition. Therefore in recent years many techniques have been developed to study single quantum dots. We address doped InAs quantum dots overgrown with 70 nm of GaAs by tuning the wavelength to intersublevel transitions of conduction band states. In Fig. 1 sSNIM images are shown for the photon energies of 86 meV and 91 meV. While for the first one, which corresponds to the p-d electronic resonance in the dots, individual quantum dots are clearly resolved, the contrast vanishes for the slightly higher photon energy. A similar behavior is observed for photon energies around 122 meV, corresponding to the s-d transition in the dots. The resonances of individual quantum dots measured with the sSNIM exhibit a linewidth of about 5 meV. This is about 4 times smaller than the inhomogeneously broadened linewidth obtained at room temperature from an ensemble of quantum dots.

This work was performed in collaboration with R. Jacob, J. Bhattacharyya, D. Stehr, H. Schneider, M. Helm (HZDR), M. T. Wenzel, H.-G. von Ribbeck, L. M. Eng (TU Dresden), S. C. Kehr (Univ. St. Andrews, St. Andrews, UK) P. Atkinson, A. Rastelli, O. G. Schmidt (IFW Dresden).

References
[1] R. Jacob, S. Winnerl, H. Schneider, M. Helm, M. T. Wenzel, H.-G. von Ribbeck, L. M. Eng, and S. C. Kehr, Opt. Express 18, 26206 (2010).