A complete study based on advanced atomic force microscopy (AFM) electrical mode called scanning spreading resistance microscopy (SSRM) is carried out on a series of samples of zinc oxide (ZnO) nanowires grown by chemical bath deposition (CBD) with different doping concentrations using gallium (Ga). The concentration of free charge carriers determined through SSRM signal calibration with a specific molecular beam epitaxy (MBE) grown multilayer structure with variation in each layer of electrically active Ga doping ranges from 1×10 17 to 7×10 20 at/cm 3 . The concentration of free charge carriers found changes in every nanowire sample with a different ratio of the doping precursor. It increases from 3×10 18 at/cm 3 in non-intentionally doped (NiD) nanowires to 7.6×10 19 at/cm 3 in samples grown with a doping precursor concentration [Ga(NO3)3]/[Zn(NO3)2] of more than 2%, which makes it possible to gradually dope the nanowires with more accurate regulation of the precursor concentration. A similar electrical activity for aluminum (Al) doped nanowires is found. Piezoresponse force microscopy (PFM) in dualfrequency resonance tracking (DFRT) mode reveals a stable piezoelectric activity of highly doped nanowires that is presumably attributed to the increased surface trap density causing a Fermi level pinning when ZnO nanowires are grown at a high pH value favorable for the intentional doping.It also shows the degradation of piezoelectric properties caused by the "screening effect," which directly correlates with the increase of free charge carrier concentration in nanowires. PFM in DFRT mode is eventually proposed as an original direct method for analyzing the electrical properties of a single piezoelectric nanowire.

The realization of practical semiconductor nanowire optoelectronic devices requires controlling their electrical transport properties, which are affected by their large surface/volume ratio value and potentially inhomogeneous electrical dopant distribution. In this article, the local carrier density in Si-doped and Mg-doped GaN nanowires grown catalyst-free by molecular beam epitaxy was quantitatively measured using scanning spreading resistance microscopy. A conductive shell surrounding a more resistive core was observed in Mg-doped, p-type GaN nanowires, balancing the formation of a depleted layer associated with sidewall surface states. The formation of this conductive layer is assigned to the peripheral accumulation of Mg dopants up to values in the 1020 /cm3 range, as determined by quantitative energy dispersive x ray spectroscopy measurements. By contrast, Si-doped n-type GaN nanowires exhibit a resistive shell, consistent with the formation of a depleted layer, and a conductive core exhibiting a decreasing resistivity for increasing Si doping level.