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, the capacitance change providing by an alternating bias applied between the tip/sample system under a DC bias to alternately accumulate and deplete carriers within the semiconductor underneath the local tip is dependent on the local carrier concentration of the semiconductor. In SSRM, the local resistivity is determined via the resistance measurement at the tip/sample system allowing the determination of the doping concentration. These two techniques need of an accurate calibration method for a quantitative doping analysis.

In this communication, we present first a calibration method based on cross-sectional scanning of multilayers samples with different Ga doping concentration allowing the quantitative measurement of n-type ZnO doping by SCM and SSRM. Then, to study ZnO NWs, we have developed a methodology of sample preparation, based on dip-coating filling of NWs field. 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 and polishing process.  

One important results has been to be able to measure using SCM and SSRM,  the non-intentionally n-type doping (nid)  of the ZnO nanowires, well estimated at 210¹⁸cm^-3, explaining  the difficulty to turn these NWs  into p-type  during p-type doping experiments, a crucial problematic in ZnO.  Using antimony (Sb) doping in nid ZnO core/ Sb ZnO shell NW structures, we have successfully determined the decrease of carrier concentration with respect to the nid core ZnO, which can be ascribed to the formation of Sb-related acceptors compensating the native donors. The understanding of this electric compensation mechanism is the clear signature of  p-type Sb doping  feasibility in ZnO NW.  This important result opens the way to succeed in the p-type doping in ZnO.

The generalization of this doping profiling methodology to other semiconductors NW could be pointed out.

Figures:

Figure 1 : (a) height AFM topography and ( b) SCM data image of the ZnO Ga doped multilayer sample with VAC=1000 mV, VDC=-1 V. Smaller SCM data indicate higher carrier densities; (c) Averaged profile of image (c) plotted together with [Ga] profile of sample B measured by SIMS. Surface is to the left for both samples; (d) Corresponding doping /SCM signal calibration curve.

Figure 2: SCM and SSRM results on a NW with ZnO nid/ZnO:Sb core-shell structure. (a) SCM data image at VDC =0 V; (b) SCM data profile along the line marked in (a); (c) SSRM resistance image at tip force of ~1.5 μN and at bias voltage of -3 V (d) SSRM resistance profile of the arrowed line indicated in (c).

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EMC Abstracts - https://emc-proceedings.com/abstract/assessment-of-doping-profiles-in-semiconductor-nanowires-by-scanning-probe-microscopy-study-of-p-type-doping-in-zno-nanowires/