Surface acoustic wave-driven carrier dynamics as a contact-less probe for mobilities of photogenerated carriers in undoped nanowires

This paper presents a surface acoustic wave (SAW) technique that provides a contact-less and massively parallel probe for the intrinsic mobilities of electrons and holes in otherwise undoped surface-passivated GaAs/AlGaAs core-shell NWs. To resolve the SAW-driven carrier dynamics, stroboscopic time-correlated single photon counting (s-TCSPC) of the photoluminescence (PL) emission of individual NWs was performed. When subject to a SAW (fSAW = 194 MHz, PSAW = +5 dBm), a characteristic beating in the PL transient (blue), matching T_SAW = 5.1 ns is observed. As expected, this beating shifts in time by T_SAW/2 as the local acoustic phase is tuned by 180° (green). This observation is a direct fingerprint of the charge carrier dynamics and acoustic charge conveyance within the SAW-induced type-II band edge modulation. Electrons transfer from the position of the excitation laser (x₀) into the energetically favourable stable points at the minimum of the conduction band. In contrast to electrons, holes remain stationary at the point of photogeneration due to their reduced mobility. T_SAW/2 later these electrons are conveyed to the position of the holes giving rise to the observed beating.