Development of Electrostatic Force Microscope with Spatial Resolution For Elementary Charge

静电力显微镜（EFM）为在亚微米尺度上测量由电荷分布不均匀和表面功函数变化引起的表面各种静电特性提供了新的机会。例如，电位法、接触带电电荷成像及其在绝缘表面上的耗散过程等。不幸的是，到目前为止，EFM的横向解决方案还不够。这主要是由于以下两个原因：(1)在良好的信噪比（S/N）下，很难测量出静电力的弱距离依赖性。(2)静电力与范德华力很难分离。本课题基于超高压调频（FM）检测方法，提出了一种同时检测范德华力和静电力相互作用的新方法。首次以真正的原子分辨率清晰地解析了GaAs（110）表面原子尺度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显示，样品正偏压区变化单调，样品负偏压区变化复杂。从这种偏置电压依赖性中，我们可以验证点缺陷处原子分解的表面电荷的符号是正的。此外，我们还研究了用EFM原子分辨成像电荷的测量条件。

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 developed 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.