Time-resolved nanospectroscopy on Si-doped GaAs-InGaAs core-shell nanowires

High-quality epitaxial nanowires (NWs) based on III–V semiconductors such as (In)GaAs offer the possibility to fabricate ultrafast optical devices due to their direct bandgap and the high electron mobility. Contactless investigation of the average charge carrier concentration and mobility in NWs is enabled by terahertz time domain spectroscopy [1]. The determination of these properties locally on individual NWs can be carried out by scattering type scanning near-field optical microscopy (s-SNOM), which provides spatial resolution far beyond the diffraction limit. In optical-pump THz-probe experiments the response of photoexcited carriers has been investigated with 10 nm and 10 fs spatial and temporal resolution [2].
Time-resolved studies are still missing in both far-field and near-field spectroscopy for doped nanowires excited by THz radiation via intraband excitation. Here we report on THz-pump MIR-probe s SNOM studies on highly-doped GaAs/InGaAs core-shell NWs utilizing intense narrowband THz radiation from the free-electron laser (FEL) FELBE.
The samples under study are Si-doped GaAs-InGaAs core-shell NWs grown by molecular beam epitaxy. They consist of a 25-nm-thick GaAs core and an 80-nm-thick In0.44Ga0.56As shell that is homogeneously doped with Si at a concentration of 9 × 1018 cm-3. For s-SNOM studies, these NWs are transferred to a (100) Si substrate and dispersed randomly over the substrate.
The experiment was carried out with an s-SNOM setup from Neaspec GmbH equipped with a broadband difference-frequency generation (DFG) source (5 – 15 µm; 20 – 60 THz). For the pump-probe measurements the laser oscillator of the DFG source was synchronized to the FEL and the time delay between the pulses was varied by an optical delay line. A low-pass filter suppresses the scattered THz FEL radiation from the nano-FTIR unit (Fig 1a).
In the unpumped case, a sharp plasma edge around 130 meV is observed. Upon intraband pumping with 13THz FEL radiation (pulse duration 2 – 5 ps, average power 15 mW), the near-field response of the plasma resonance changes dramatically. The spectrally integrated pump-probe signal exhibits a small negative component followed by a stronger positive signal that decays with the time constant (1/e) of ≈ 7 ps (Fig. 1b, insert). The nano FTIR studies reveal strong red shift (black curve) and then flattening (red curve) of the plasma resonance (Fig. 1b). We assign this effect to a substantial heating of the electrons in the conduction band and subsequent increase of the effective mass in the nonparabolic Γ-valley due to high peak electric field strengths up to several 10 kV cm−1 of pulsed FEL radiation [3]. Power-dependent nanoimaging pump-probe studies will be performed to conclude the nature of observed effects. In particular, the experiments should reveal if there is a contribution of carrier transfer to side valleys at high excitation fields.
References
[1] P. Parkinson, et al., Nano Lett. 7, 2162 (2007).
[2] M. Eisele, et al., Nature Photon. 8, 841 (2014).
[3] D. Lang, et al., Nanotechnology 30, 084003 (2019).