Unveiling the Influence of Surface Fermi Level Pinning on the Piezoelectric Response of Semiconducting Nanowires

ZnO nanowires (NWs) are excellent candidates for the integration of energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. However, ZnO NWs are usually nonintentionally n-doped during their growth. Thus, it can be expected theoretically that their piezoelectric response be degraded and mostly geometry-independent as a result of strong screening effects by free carriers, while experimentally many NW-based piezoelectric transducers demonstrate quite reasonable performance. In this paper, this apparent contradiction is explained by surface Fermi level pinning (SFLP). Simulations based on the finite element method are developed to investigate the influence of SFLP on the electrical response of ZnO piezogenerators based on arrays of vertical ZnO NWs (the so-called VING) operated in compression or bending mode. The results show that the nonsymmetrical response with respect to loading direction, which has been observed experimentally when the VING is bent, can be explained when account is taken of the reverse piezoelectric effect associated with SFLP. Influence of geometry and comparison to thin film are also discussed. It is found that the performance improvement, which is observed experimentally at scales much larger than predicted by ab initio models when NWs are compared to thin films, can be partly related to SFLP effects rather than larger piezoelectric coefficients.