Investigation of Measurement Condition to Detect Single Charge Using Atomic 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特性提供了新的机会。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。然而，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：(I)很难用良好的信噪比(S/N)来测量静电力的弱距离相关性。(2)很难将静电力和范德华力分开。在这个项目中，我们展示了一种在原子尺度上同时检测范德华相互作用和静电力相互作用的新方法，该方法基于调频(FM)检测方法。在GaAs(110)表面上测量的非接触AFM图像中，矩形晶格和原子尺度的点缺陷可以被清晰地和可重复地分辨。另一方面，在EFM图像中，点缺陷在正偏压下相对于背景表现为明亮的对比度，而在负偏压下表现为暗对比度，因此EFM图像的对比度取决于样品的偏置电压V，这意味着EFM图像实际上是由点缺陷处的电荷引起的。从而首次实现了对静电力测量的原子分辨率成像。

The electrostatic force microscope (EFM) offered new opportunity to measure a variety of electrostatic properties on the surface on a sub-micron scale. For example, potentiometry, imaging of contact-electrified charge and its dissipation process on insulating surface and so on have been demonstrated. However, so far, the lateral resolution of the EFM has been insufficient. This is due to the following two main reasons : (i) It is much 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. Thus, the technical improvements have been required to increase the lateral resolution of the EFM.In this project, we demonstrated a novel method to detect the van der Waals and the electroatatic force interactions simultaneously on atomic scale, which is based on frequency modulation (FM) detection method. In the noncontact AFM images measured on the GaAs (110) surface, the rectangular lattice and the atomicscale point defects can be clearly and reproducibly resolved. On the other hand, in the EFM images, point defect appeared as bright respect to the background at positive bias voltage, while it appeared dark contrast at negative bias voltage.Thus, the contrast of the EFM images depends on the sample bias voltage V.This means that the EFM images really resulted from the charge at the point defect on the GaAs (110) surface. Thus, for the first time, the atomic resolution imaging has been achieved for the electrostatic force measurement.