Development of Electrostatic Force Microscope with Spatial Resolution For Elementary Charge

静电力显微镜（EFM）提供了一个新的机会，以测量各种静电特性的表面上的亚微米尺度，这是由于不均匀的电荷分布和表面功函数的变化。例如，电位法、接触带电电荷的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的横向分辨率还不够。这是由于以下两个主要原因：（i）很难测量具有良好信噪比（S/N）的静电力的弱距离依赖性。(ii)静电力和货车德瓦耳斯力的分离是一个非常困难的问题，本课题基于频率调制（FM）检测方法，提出了一种同时检测货车德瓦耳斯力和静电力相互作用的新方法。GaAs（110）表面上原子级点缺陷处的表面结构和表面电荷以真正的原子分辨率被清楚地分辨。点缺陷处的静电力图像对比度在样品正偏压区呈单调变化，而在样品负偏压区变化较为复杂。从这种偏置电压的依赖性，我们可以验证，在点缺陷的原子分辨的表面电荷的符号是积极的。此外，我们还研究了用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 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.