On-demand strain-induced recombination dynamics in semiconductor quantum wells

• Applicants: Dr. Peter Gaal; Privatdozentin Dr. Fariba Hatami
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Synthesis and Properties of Functional Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 471105220

This project aims at manipulating electronic states in low-dimensional quantum systems, in particular in quantum wells (QWs) via a transient strain and deformation of the surrounding crystal lattice. The deformation is generated by absorption of ultrashort light pulses with a duration of approximately 100 femtoseconds in a few nanometer thin transducer. The charge recombination dynamics is measured by time-resolved photoluminescence. We are investigating optical transitions in GaAs/AlAs QWs. This material system is highly developed and already used technological applications, e.g., in optical communication technology. In particular, the influence of static strain in nanostructures which is generated by a defined lattice mismatch is studied profoundly and well understood. At a thickness of approximately less than 4 nm the energy of the electronic X-state in the AlAs barrier can be decreased below the level of the Gamma-state in GaAs. This results in carrier diffusion from the QW ground state in GaAs into the barrier which decreases the recombination probability of the now indirect transition. We will grow GaAs/AlAs QWs with properties near the above mentioned equilibrium working point to modulate the carrier recombination probability. By inducing lattice deformations optically the working point is dynamically shifted towards either a direct or indirect transition.A prerequisite for a successful implementation of our project is the ability to generate and control transient lattice deformations. Recently the applicants of this proposal have made tremendous progress in this area. One possibility is the generation of longitudinal coherent acoustic phonon wavepackets by absorption of a temporal series of laser pulses. Spectrum and wavevector of the phonon wavepacket can be controlled by via the temporal shape of the excitation pattern. Another possibility are so-called surface acoustic waves. In addition to control via the temporal shape, these excitations can also be controlled by the spatial excitation pattern. We will use both strategies in this project.To optimize the deformation amplitude which is responsible for the energetic shift of the electronic states in the QW, our samples will be processed in phonon resonator structures. The mechanical resonance leads to an enhancement of the structural deformation after optical excitation. The resonant vibration can again be controlled by the spatiotemporal series of excitation pulses.This project builds on the expertise of the collaborators for sample growth and structuring and for photoacoustic and time-resolved photoluminescence measurements. If successful, we will develop a new method for transient modulation of electronic and optical properties in semiconductor nanostructures. The interaction of strain and electronic properties of the solid is expected to play a major role in novel nano-/micromechanical devices in quantum technology.

• DFG Programme: Research Grants