2, GeePs, Gif sur Yvette, , France
3, LPICM, Palaiseau, , France
Semiconductor nanostructures have attracted a great deal of research interest because of their nanoscale properties that offer a great potential for improving performances in existing devices. Nanowire arrays with radial junctions are one of the most promising nanostructures for photovoltaic (PV) applications due to their light trapping and carrier collection properties that are purposely combined for enhancing solar efficiency with respect to conventional planar structures. The efficiency of nanowire solar cells may be limited by damaged nanowire junctions in the array, nevertheless efficiencies up to 9,6 % have been already reached for silicon nanowire (SiNW) radial junctions based on Si thin-film technology . The characterization of such structures remains a critical issue and notably the possibility to characterize the photoelectrical performances of individual nanowires.
In this study, Kelvin Probe Force Microscopy (KPFM) technique has been used to evaluate the possibility to measure the local open-circuit voltage (VOC) of individual SiNW radial junctions based on thin-film technology . All the photoelectrical measurements were performed in the KPFM platform (AIST-NT “Trios” SPM setup) that allows illumination of the sample from different directions (top, side and bottom) and with different laser wavelengths. In addition, a micropositioner has been coupled to this platform in order to electrically probe the “macroscopic” current-voltage (I-V) properties of completed SiNW devices under the same illumination conditions that will be used for KPFM analysis.
KPFM measurements were first performed on completed SiNW devices at the dark and under illumination conditions. The extracted surface photovoltage (SPV) that corresponds to the local VOC exhibits values over 650 mV, and shows for different illumination conditions a good agreement with the macroscopic values with a relative difference below 5%. Next KPFM measurements performed on individual SiNWs (uncompleted devices without ITO top contact) reveal a strong shading effect coming from the AFM tip when the tip is illuminated from the top. Side illumination greatly minimizes the previous shading effect with a local VOC that varies logarithmically with the incident power, however, the VOC remains 25 % below the values acquired on completed devices. Many issues can explain the following discrepancy notably, an angular dependence of light trapping, a change of the illumination area, a residual AFM tip shading, …. Despite these different issues we show the great interest of the KPFM technique for VOC imaging at the nanoscale on completed SiNW PV devices with good perspectives to characterize individual SiNW.
 S. Misra, L. Yu, M. Foldyna, P. Roca I Cabarrocas, IEEE vol.5 p.40-45 (2014) 14820286
 S. Misra, L. Yu, W. Chen, M. Foldyna and P. Roca i Cabarrocas, J. Phys. D: Appl. Phys. 47 (2014) 393001