Assessment of doping profiles in semiconductor nanowires by scanning probe microscopy

Methods to measure and quantitatively determine the doping profile in semiconductor nanowires ( NW) are strongly requested for understanding the doping incorporation in such one-dimensional structures and so for developing technology using them. In the last two decades, scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) based on atomic force microscopy, has emerged as promising tools for two-dimensional high resolution carrier/dopant profiling. In SCM, an alternating bias is applied between the tip/sample system under a DC bias to alternately accumulate and deplete carriers within the semiconductor underneath the tip, changing the capacitance of the structure. This capacitance change is dependent on the local carrier concentration of the semiconductor, thanks to the spatial resolution of the AFM tip. In SSRM, the local resistivity is determined via the resistance measurement at the tip/sample system allowing a more quantitative determination of the doping concentration. In this communication the advantages and limitations of these techniques to make measurements on nanowires are discussed. We present through examples of cross-sectional scanning of multilayers samples with different doping concentration, the possibility to have a calibration approach of such doping measurement by these electrical techniques. Using ZnO nanowires, we show a methodology of sample preparation, based on dip-coating filling of NWs field, capable to allow the doping characterization of NW by these scanning probe techniques. The dip-coating parameters as coating solution, removal rate and NW field morphology have been controlled by SEM, ellipsometry and atomic force microscopy topography in order to optimize the filling process. After filling and polishing, we show that SCM measurements give rise to signal contrast in good agreement with the electrical activity of doping in such ZnO NWs. The generalization of this doping profiling methodology to other semiconductors NW will be pointed out.