Spatially resolved photovoltaic performance of axial GaAs nanowire pn-diodes

In this work the authors have investigated the photovoltaic properties of GaAs nanowires with an axial pn-junction with high spatial resolution in order to quantify the volume of absorption and the efficiency obtained with the available growth technology. Spatially resolved photocurrent spectroscopy was used to investigate mechanism of carrier photo generation. The I-V characteristics of nanowire pn-diode were measured, while nanowires were locally illuminated by focused CW laser (λ = 532 nm) at different positions. The short circuit current is maximal when the diode is illuminated at the position of the pn-junction. No photocurrent was detected either in the vicinity of contacts or in the p- and n-diode parts. This demonstrates that charge separation by an electric field takes place only in the vicinity of the depletion region, while p- and n-nanowire regions are field free and contacts are well ohmic. The charge separation efficiency at the pn-junction was estimated to η_cc = 42 % and was limited by the recombination of carriers at the nanowire surface traps.