Exploring piezoelectric properties of III-V nanowires using piezo-response force microscopy

Semiconducting III-V materials exhibiting piezoelectric properties are much sought after due to their potential applications in piezotronic and piezo-phototronic devices. Nanowires (NWs) of III-V semiconductors are particularly interesting in this respect due to the occurrence of the wurtzite (WZ) structure commonly associated with enhanced piezoelectric properties, as opposed to the zinc blende (ZB) structure that is typically observed in the bulk. However, performing direct measurements of the piezoelectric properties of III-V NWs using piezo-response force microscopy (PFM) is challenging, and the analysis and interpretation of such measurements is far from trivial. Here we present detailed finite element simulations of single GaAs NWs, with both WZ and ZB crystalline structure, scanned by an atomic force microscope tip in PFM mode, demonstrating the effect of the non-uniform electric field between the tip and NW, scan direction, as well as NW orientation on the resulting PFM signal. We also report PFM data from single GaAs and InP NWs with both ZB and WZ structure, grown by molecular beam epitaxy, based on a novel non-destructive intermittent contact PFM mode. We explain our experimental data in the framework of our simulations, and for the first time, extract an experimental value for the axial piezoelectric coefficient of WZ InP, d₃₃ = 0.7-1 pm V^-1. The methods and analysis described here are particularly relevant for the investigation of piezoelectric properties in a wide range of semiconducting III-V NW systems.