Walter Schottky Institut and Physics Department, Technische Universität München , 85748 Garching, Germany.
Nano Lett. 2017 Jun 14;17(6):3582-3590. doi: 10.1021/acs.nanolett.7b00741. Epub 2017 May 31.
Group III-nitride materials such as GaN nanowires are characterized by a spontaneous polarization within the crystal. The sign of the resulting sheet charge at the top and bottom facet of a GaN nanowire is determined by the orientation of the wurtzite bilayer of the different atomic species, called N and Ga polarity. We investigate the polarity distribution of heteroepitaxial GaN nanowires on different substrates and demonstrate polarity control of GaN nanowires on diamond. Kelvin Probe Force Microscopy is used to determine the polarity of individual selective area-grown and self-assembled nanowires over a large scale. At standard growth conditions, mixed polarity occurs for selective GaN nanowires on various substrates, namely on silicon, on sapphire and on diamond. To obtain control over the growth orientation on diamond, the substrate surface is modified by nitrogen and oxygen plasma exposure prior to growth, and the growth parameters are adjusted simultaneously. We find that the surface chemistry and the substrate temperature are the decisive factors for obtaining control of up to 93% for both polarity types, whereas the growth mode, namely selective area or self-assembled growth, does not influence the polarity distribution significantly. The experimental results are discussed by a model based on the interfacial bonds between the GaN nanowires, the termination layer, and the substrate.
III 族氮化物材料（如 GaN 纳米线）的晶体具有自发极化特性。GaN 纳米线的顶面和底面的面电荷的符号取决于不同原子种类的纤锌矿双层的取向，称为 N 极性和 Ga 极性。我们研究了不同衬底上异质外延 GaN 纳米线的极性分布，并证明了在金刚石上 GaN 纳米线的极性控制。我们使用 Kelvin 探针力显微镜在大范围内确定了单个选择性区域生长和自组装纳米线的极性。在标准生长条件下，各种衬底上的选择性 GaN 纳米线都存在混合极性，即在硅、蓝宝石和金刚石上。为了在金刚石上获得生长方向的控制，在生长前通过氮和氧等离子体暴露对衬底表面进行改性，并同时调整生长参数。我们发现，表面化学和衬底温度是获得高达 93%的两种极性类型控制的决定性因素，而生长模式（即选择性区域或自组装生长）对极性分布的影响不大。实验结果通过基于 GaN 纳米线、终止层和衬底之间的界面键的模型进行了讨论。
