Nanoscale mapping of carrier collection in single nanowire solar cells using X-ray beam induced current

Nanofocused X-ray beam induced current (XBIC) is used to quantitatively map the spatially dependent carrier collection probability within single nanowires.


S1. Sample preparation
The investigated nanowires in this study are InP and In0.56Ga0.44P, which were synthesized by the vaporliquid-solid (VLS) growth mechanism with Au seed particles. The growth temperature is 440 °C and the growth parameters are included in Tab. S1 and S2. The nanowire length was monitored in situ by use of optical reflectometry (Heurlin et al., 2015, Otnes et al., 2017. Hydrogen chloride (HCl) was applied during the syntheses to prevent a lateral growth (Borgström et al., 2010).
For the nanowire device fabrication, The nanowires were transferred to suspended silicon nitride (Si3N4) membranes of 1 µm thickness (Silson), which were patterned with metal bond pads. Electron beam lithography (EBL) in combination with metal evaporation was used to defined metal contacts to individual nanowires. A Pd/Zn/Pd/Au (~10/10/10/170 nm) metal combination was used (Bruce et al., 1990). The substrate was mounted and wedge bonded on a carrier chip, which is the platform of the device to connect to the other equipment.
The X-ray source was the ID16B beamline at the European synchrotron radiation facility (ESRF), France. The beam size was about 60 nm vertically and 50 nm horizontally. The X-ray was a pink beam (dE/E ≈ 10 -2 ) with an energy of 17.5 keV. The flux of this X-ray beam was controlled by a set of metal filters. The XBIC and the XRF signals were simultaneously collected. The scanning was performed by moving the sample with piezo motors along the normal plane of the optical axis.
A voltage was applied to one end of the nanowire whereas the current was monitored by a current amplifier (Stanford research systems SR570) on the other end. For most measurements, we used a sensitivity of 10 or 100 pA/V, at which the amplifier has noise of 5 and 10 fA, respectively. We checked the noise and stability by making hundreds of repeated measurements at identical conditions, and we found that the variation was indeed 5-10 fA with the X-ray beam on and a few fA with the beam off.
We also checked the sensitivity to ambient light. Since the microscope lamp gave a strong response it was turned off, while the ambient room light was found not to affect the measurements.

S2. Simulation with Comsol
Comsol Multiphysics (version 5.2, COMSOL AB, Stockholm,Sweden) is a commercial software that can solve the Poisson's equation for electrostatic potential, and the continuity transport equations for the charge carrier concentrations. Therefore, it is a robust tool to study the band structure and the charge carrier transport within semiconductors. We assumed radial symmetry within the nanowires, and used the 1D analysis system with the Semiconductor module. The simulation model of the nanowire is a single line in which the junctions and metal contacts were assign to points in the nanowire. The evenly photoexcited area was defined in a region between two positions (60 nm apart), and then these positions were shifted along the nanowire line to simulate the scanning of the photoexcitation over the nanowire.
All of the parameters used in this simulation are presented in Tab. S3.