Lozano Helena, Catalán Gustau, Esteve Jaume, Domingo Neus, Murillo Gonzalo
Instituto de Microelectrónica de Barcelona, Bellaterra 08193, Spain.
Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain.
Nanotechnology. 2021 Jan 8;32(2):025202. doi: 10.1088/1361-6528/abb972.
Zinc oxide (ZnO) nanowires (NWs) as semiconductor piezoelectric nanostructures have emerged as material of interest for applications in energy harvesting, photonics, sensing, biomedical science, actuators or spintronics. The expression for the piezoelectric properties in semiconductor materials is concealed by the screening effect of the available carriers and the piezotronic effect, leading to complex nanoscale piezoresponse signals. Here, we have developed a metal-semiconductor-metal model to simulate the piezoresponse of single ZnO NWs, demonstrating that the apparent non-linearity in the piezoelectric coefficient arises from the asymmetry created by the forward and reversed biased Schottky barriers at the semiconductor-metal junctions. By directly measuring the experimental I-V characteristics of ZnO NWs with conductive atomic force microscope together with the piezoelectric vertical coefficient by piezoresponse force microscopy, and comparing them with the numerical calculations for our model, effective piezoelectric coefficients in the range d ∼ 8.6 pm V-12.3 pm V have been extracted for ZnO NWs. We have further demonstrated via simulations the dependence between the effective piezoelectric coefficient d and the geometry and physical dimensions of the NW (radius to length ratio), revealing that the higher d is obtained for thin and long NWs due to the tensor nature proportionality between electric fields and deformation in NW geometries. Moreover, the non-linearity of the piezoresponse also leads to multiharmonic electromechanical response observed at the second and higher harmonics that indeed is not restricted to piezoelectric semiconductor materials but can be generalized to any type of asymmetric voltage drops on a piezoelectric structure as well as leaky wide band-gap semiconductor ferroelectrics.
氧化锌（ZnO）纳米线作为半导体压电纳米结构，已成为能量收集、光子学、传感、生物医学、致动器或自旋电子学等应用领域中备受关注的材料。半导体材料中压电特性的表达式被可用载流子的屏蔽效应和压电电子效应所掩盖，导致复杂的纳米级压电响应信号。在此，我们开发了一种金属 - 半导体 - 金属模型来模拟单个ZnO纳米线的压电响应，证明压电系数中明显的非线性源于半导体 - 金属结处正向和反向偏置肖特基势垒产生的不对称性。通过使用导电原子力显微镜直接测量ZnO纳米线的实验I - V特性，并结合压电响应力显微镜测量压电垂直系数，然后将它们与我们模型的数值计算结果进行比较，提取出ZnO纳米线的有效压电系数范围为d ∼ 8.6 pm V⁻¹ - 12.3 pm V。我们还通过模拟进一步证明了有效压电系数d与纳米线的几何形状和物理尺寸（半径与长度比）之间的依赖性，揭示出由于纳米线几何形状中电场与变形之间的张量性质比例关系，细而长的纳米线具有更高的d值。此外，压电响应的非线性还导致在二次及更高次谐波处观察到多谐波机电响应，实际上这种现象不仅限于压电半导体材料，还可推广到压电结构上任何类型的不对称电压降以及宽带隙半导体铁电体。
