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。举例说明了电位法、接触带电电荷成像及其在绝缘表面上的耗散过程等。然而，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：(i)在良好的信噪比（S/N）下，很难测量静电力的弱距离依赖性。(2)静电力与范德华力很难分离。因此，需要进行技术改进以提高EFM的横向分辨率。在本项目中，我们展示了一种在原子尺度上同时检测范德华力和电磁力相互作用的新方法，该方法基于调频（FM）检测方法。在GaAs（110）表面的非接触AFM图像中，可以清晰地分辨出矩形晶格和原子尺度的点缺陷。另一方面，在EFM图像中，在正偏置电压下，点缺陷相对于背景显示为亮，而在负偏置电压下，点缺陷相对于背景显示为暗。因此，EFM图像的对比度取决于样品偏置电压v，这意味着EFM图像实际上是由GaAs（110）表面点缺陷处的电荷引起的。从而首次实现了静电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.