Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Optical pump – Terahertz (THz) probe (OPTP) spectroscopy has proven efficacy for contactless probing of electronic transport in semiconductor NWs [1]. Particularly in III-V NWs, scattering rates of charge carriers, as well as their plasmonic resonances for typical doping levels, are located in the THz range. The analysis of the optical conductivity spectra using the localized surface plasmon model allows estimating the carrier lifetime and the carrier mobility.
Here, OPTP spectroscopy is employed to study two unique phenomena in GaAs/(In,Al,Ga)As core/shell nanowires. First, it is demonstrated that the mobility of electrons in the hydrostaticallystrained GaAs core (owing to the lattice mismatch between the core and the shell [2]) exceeds the mobility in bulk GaAs by 30-50% [3]. The role of the various scattering mechanisms is analyzed as a function of strain and temperature. Depending on the density of NWs in the probed sample, some of them can form bundles or touch each other, leading to an inhomogeneous broadening of the plasmon resonance. We discuss the role of this effect and its impact on the estimation of carrier mobility [3, 4]. Second, we demonstrate a strong THz nonlinearity using single-cycle intense THz pulses with peak electric fields reaching up to 0.6 MV/cm. With the increase of the driving THz field, we observe a systematic redshift of the plasmon frequency, accompanied by a gradual suppression of the spectral weight. Remarkably, the spectral weight does not remain proportional to the square of the plasmon
frequency when the driving electric field exceeds 0.4 MV/cm, indicating an onset of a spatially inhomogeneous carrier distribution across the NW. The observed behavior can be ascribed to nonlinear effects caused by the scattering of electrons from the Gamma- to L-valley occurring in the high electric field regime. However, in contrast to bulk semiconductors, this effect initially sets in at hot spots of the NW, where the local electric field is enhanced by the plasmonic resonance [5].
All in all, our findings provide important guidelines for the exploitation of nanowires in high-frequency electronics, but also underline the unique strengths of OPTP spectroscopy for the study of electronic transport in nanowires.

[1] H. J. Joyce et al., Semicond. Sci. Technol. 31, 103003 (2016).
[2] L. Balaghi et al., Nat. Commun. 10, 2793 (2019).
[3] L. Balaghi et al., Nat. Commun. 12, 6642 (2021).
[4] I. Fotev et al., Nanotechnology 30, 244004 (2019).
[5] R. Rana et al., Nano Lett. 20, 3225 (2020).