Evidence of Piezoelectric Potential and Screening Effect in Single Highly Doped ZnO:Ga and ZnO:Al Nanowires by Advanced Scanning Probe Microscopy

A complete study based on advanced atomic force microscopy 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 with different doping concentrations using gallium (Ga). The concentration of free charge carriers determined through SSRM signal calibration with a specific molecular beam epitaxy-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 . It was found that the concentration of free charge carriers changed 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 nanowires to 7.6 × 10 19 at./cm 3 in samples grown with a doping precursor concentration [Ga(NO 3 ) 3 ]/[Zn(NO 3 ) 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 dual-frequency 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 the DFRT mode is eventually proposed as an original direct method for analyzing the electrical properties of a single piezoelectric nanowire.