Measurement Condition for Atomic Resolution imaging of Elementary Charge Using Electrostatic Force Microscope

静电力显微镜(EFM)为在亚微米尺度上测量表面的各种静电性质提供了新的机会，这些性质是由于电荷分布不均匀和表面功函数的变化造成的。例如，电位法、接触带电电荷的成像及其在绝缘表面的耗散过程等。不幸的是，到目前为止，EFM的横向分辨率还不够。这主要是由于以下两个原因：(1)用良好的信噪比(S/N)很难测量静电力的弱距离相关性。(Ii)静电力和范德华力的分离非常困难。在本项目中，我们展示了一种基于超高压调频(FM)检测方法的同时检测范德华力和静电力相互作用的新方法。首次以真原子分辨率清晰地分辨出了(110)面上原子级点缺陷的表面结构和表面电荷。点缺陷处的静电力图像对比度在正偏压区表现为单调变化，而在负偏压区变化较为复杂。从这个偏置电压依赖关系中，我们可以验证在点缺陷处原子分辨表面电荷的符号是正的。此外，我们还研究了利用EFM对电荷进行原子分辨率成像的测量条件。

The electrostatic force microscope (EFM) offers new opportunity to measure a variety of electrostatic properties on the surface on a sub-micron scale, which result from nonuniform charge distribution and variations in surface work function. For example, potentiometry, imaging of contact-electrified charge and its dissipation process on insulating surfaces and so on have been demonstrated. Unfortunately, so far, the lateral resolution of the EFM has been insufficient. This is due to the following two main reasons : (i) It is rather difficult to measure weak distance dependence of the electrostatic force with a good signal-to-noise (S/N) ratio. (ii) It is much difficult to separate the electrostatic force from the van der Waals force.In this project, we demonstrated a novel method to detect the van der Waals and the electrostatic force interactions simultaneously, which was based on the Frequency modulation (FM) detection method in UHV. For the first time, the surface structure and the surface charge at atomic-scale point defects on the GaAs (110) surface were clearly resolved with true atomic resolution. The contrast of the electrostatic force image at a point defect showed monotonous change in the positive sample bias region, while it complicatedly changed in the negative sample bias region. From this bias voltage dependence, we could verify that the sign of the atomically resolved surface charge at the point defect was positive. Furthermore, we investigated the measurement condition of the atomic resolution imaging of the charge using the EFM.