Charge carrier dynamics under the influence of extreme strain gradients realized in bent semiconductor nanowires

• Applicants: Dr. Lutz Geelhaar; Professor Dr. Ullrich Pietsch
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 434114264

Strain engineering is an established approach to tailor the electronic structure of semiconductors. In conventional planar thin film heterostructures, a lattice mismatch leads to homogeneous biaxial strain. A generalization of this concept is inhomogeneous strain. Such strain fields form in three-dimensional nanostructures with strain gradients. However, there are still many open questions regarding their influence on semiconductor physics. The overall goal of this project is to gain a deep understanding of the charge carrier dynamics under the influence of extreme strain gradients.To realize such gradients, we will bend free-standing semiconductor nanowires in a controlled way close to the elastic limit. We focus on nanowires consisting of the direct semiconductor GaAs, for which we have already demonstrated such a bending. For this purpose, a lattice-mismatched shell is deposited on only one side of the nanowires.Strain gradients lead to various phenomena that influence the dynamics of excited charge carriers. First, the deformation potential interaction leads to a gradient in the band gap that acts as a quasi-electric field. Second, strain induces a piezoelectric polarization. Third, the strain gradient causes a flexoelectric polarization. The influence of the latter effect on charge carrier dynamics has so far been largely ignored. We aim to unravel the interplay of these phenomena and to determine the hitherto unknown relevant flexoelectric coefficient of GaAs. To systematically study the charge carrier dynamics, we will perform photo- and cathodoluminescence experiments on dedicated sample series, supplemented by k·p simulations. For the analysis of the luminescence data, precise information about strain and structure of the nanowires is crucial. These parameters will be determined by nano x-ray diffraction. X-ray diffraction on strongly bent crystals, however, represents uncharted territory, so that we will first develop a corresponding methodology. This achievement will be of scientific value in its own right, independent of the study of charge carrier dynamics.Finally, by combining nano x-ray analysis with synchronized laser pulses, we will investigate how a change in charge carrier density influences the bending of nanowires via the converse piezo- and flexoelectric effects. In addition, we will excite nanowires to strong mechanical oscillations and thus bend them dynamically. In this way, we will generate a continuous variation of the bending radius for further experiments.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Oliver Brandt